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Photolysis in the Light Reactions of Photosynthesis

Photolysis in the Light Reactions of Photosynthesis



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I'm a bit confused concerning photolysis. During the light reactions, photons are used to excite the chlorophyll molecules so they are passed to the primary electron acceptor. The electrons initially come from a split water molecule. Is water split by the photons?


A catalytic site on a subunit of Photosystem II is responsible for splitting water via photo-oxidation. This site, called the Oxygen-evolving complex (or OEM), uses energy from sunlight (which is converted to electrochemical potential energy in the photosystem's reaction center) to accomplish this task.

Source: Wikimedia

See Barber (2012)1 for an explanation. The abstract (my emphasis):

The oxygen in our atmosphere is derived and maintained by the water-splitting process of photosynthesis. The enzyme that facilitates this reaction and therefore underpins virtually all life on our planet is known as photosystem II (PSII), a multisubunit enzyme embedded in the lipid environment of the thylakoid membranes of plants, algae, and cyanobacteria. During the past 10 years, crystal structures of a 700-kDa cyanobacterial dimeric PSII complex have been reported with ever-increasing improvement in resolution--the latest at 1.9 Å. Thus, the organizational and structural details of its many subunits and cofactors are now well understood. The water-splitting site was revealed as a cluster of four Mn ions and one Ca ion surrounded by amino-acid side chains, of which seven provide ligands to the metals. The metal cluster is organized as a cubane-like structure composed of three Mn ions and the one Ca2+ ion linked by oxo bonds. The fourth Mn is attached to the cubane via one of its bridging oxygens together with another oxo bridge to an Mn ion of the cubane. The overall structure of the catalytic site provides a framework to propose a mechanistic scheme for the water-splitting process and gives a blueprint for the development of catalysts that mimick the reaction in an artificial chemical system as a means to generate solar fuels.

My understanding is that we are still not totally certain how this actually works

  • Cady et al. (2008)2 wrote a review in 2008 of various functional models trying to explain the chemistry of the oxygen-evolving complex of photosystem II.

    • A quick Google Scholar search reveals plenty of proposed mechanisms published since.
  • Most currently proposed mechanisms do, however, incorporate the 4 oxidation steps proposed by Kok et al. (1970)3 (now referred to as the Kok Cycle, or Kok's S-state cycle).

    • From Kern et al. (2013)4:

      The oxidation equivalents generated by the absorption of four photons by the PS II reaction center are stored in the four consecutive redox states of a Mn4CaO5 cluster, known as the Si (i=1 to 4) states. The accumulated energy is used in the concerted oxidation of two molecules of water to form dioxygen (1) returning the catalyst to the most reduced S0 state in the Kok cycle (Scheme 1).

      Scheme 1 from Kern et al. (2013) explaining states of Kok Cycle.

You can learn more here.


Works Cited:

1 Barber, J., 2012, January. Photosystem II: the water-splitting enzyme of photosynthesis. In Cold Spring Harbor symposia on quantitative biology (Vol. 77, pp. 295-307). Cold Spring Harbor Laboratory Press.

2 Cady, C.W., Crabtree, R.H. and Brudvig, G.W., 2008. Functional models for the oxygen-evolving complex of photosystem II. Coordination chemistry reviews, 252(3-4), pp.444-455.

3 Kok, B., Forbush, B. and McGloin, M., 1970. Cooperation of charges in photosynthetic O2 evolution-I. A linear four step mechanism. Photochemistry and photobiology, 11(6), pp.457-475.

4 Kern, J., Alonso-Mori, R., Tran, R., Hattne, J., Gildea, R.J., Echols, N., Glöckner, C., Hellmich, J., Laksmono, H., Sierra, R.G. and Lassalle-Kaiser, B., 2013. Simultaneous femtosecond X-ray spectroscopy and diffraction of photosystem II at room temperature. Science, 340(6131), pp.491-495.


Photolysis in the Light Reactions of Photosynthesis - Biology

All electromagnetic radiation, or light energy, travels at a particular wavelength and carries a certain amount of energy.

Learning Objectives

Explain the difference between short and long wavelengths.

Key Takeaways

Key Points

  • The amount of energy of a wave can be determined by measuring its wavelength, the distance between consecutive points of a wave.
  • Visible light is a type of radiant energy within the electromagnetic spectrum other types of electromagnetic radiation include UV, infrared, gamma, and radio rays as well as X-rays.
  • The difference between wavelengths relates to the amount of energy carried by them short, tight waves carry more energy than long, wide waves.

Key Terms

  • electromagnetic spectrum: the entire range of wavelengths of all known radiations consisting of oscillating electric and magnetic fields, including gamma rays, visible light, infrared, radio waves, and X-rays
  • wavelength: the length of a single cycle of a wave, as measured by the distance between one peak or trough of a wave and the next it corresponds to the velocity of the wave divided by its frequency
  • visible light: the part of the electromagnetic spectrum, between infrared and ultraviolet, that is visible to the human eye

What Is Light Energy?

The sun emits an enormous amount of electromagnetic radiation (solar or light energy). Humans can see only a fraction of this energy, which is referred to as “visible light.” The manner in which solar energy travels is described as waves. Scientists can determine the amount of energy of a wave by measuring its wavelength, the distance between consecutive points of a wave, such as from crest to crest or from trough to trough.

Wavelengths: The wavelength of a single wave is the distance between two consecutive points of similar position (two crests or two troughs) along the wave.

Visible light constitutes only one of many types of electromagnetic radiation emitted from the sun and other stars. The electromagnetic spectrum is the range of all possible frequencies of radiation. The electromagnetic spectrum shows several types of electromagnetic radiation originating from the sun, including X-rays and ultraviolet (UV) rays. The higher-energy waves can penetrate tissues and damage cells and DNA, which explains why both X-rays and UV rays can be harmful to living organisms. Scientists differentiate the various types of radiant energy from the sun within the electromagnetic spectrum.The difference between wavelengths relates to the amount of energy carried by them.

The Electromagnetic Spectrum: The sun emits energy in the form of electromagnetic radiation. This radiation exists at different wavelengths, each of which has its own characteristic energy. All electromagnetic radiation, including visible light, is characterized by its wavelength.

Each type of electromagnetic radiation travels at a particular wavelength. The longer the wavelength, the less energy is carried. Short, tight waves carry the most energy. This may seem illogical, but think of it in terms of a person moving a heavy rope. It takes little effort by a person to move a rope in long, wide waves. To make a rope move in short, tight waves, a person would need to apply significantly more energy.


Show/hide words to know

ATP: adenosine triphosphate. ATP is the energy-carrying molecule of all cells. more

Cellulose: the structural material found in the cell wall in most plants. Cellulose is used to make many products, including paper and cloth. more

Electron transport chain: cell process that uses electrons to generate chemical energy. more

Ion: an atom or molecule that does not have the same number of electrons as it has protons. This gives the atom or molecule a negative or positive charge. more

Light-dependent reaction: the first part of photosynthesis where (sun)light energy is captured and stored by a plant. more

Molecule: a chemical structure that has two or more atoms held together by a chemical bond. Water is a molecule of two hydrogen atoms and one oxygen atom (H2O). more

Protein: a type of molecule found in the cells of living things, made up of special building blocks called amino acids.

Starch: made by all green plants and used to store energy for later use. more

Thylakoid: the disk-shaped parts of a plant cell where light-dependent reactions occur. more


Absorption of Light

Light energy enters the process of photosynthesis when pigments absorb the light. In plants, pigment molecules absorb only visible light for photosynthesis. The visible light seen by humans as white light actually exists in a rainbow of colors. Certain objects, such as a prism or a drop of water, disperse white light to reveal these colors to the human eye. The visible light portion of the electromagnetic spectrum is perceived by the human eye as a rainbow of colors, with violet and blue having shorter wavelengths and, therefore, higher energy. At the other end of the spectrum toward red, the wavelengths are longer and have lower energy.


Redox Transformations

4.1 Photochemistry

Photolysis (also called photodissociation and photodecomposition) is a chemical reaction in which an inorganic chemical (or an organic chemical) is broken down by photons and is the interaction of one or more photons with one target molecule. The photolysis reaction is not limited to the effects of visible light but any photon with sufficient energy can cause the chemical transformation of the inorganic bonds of a chemical. Since the energy of a photon is inversely proportional to the wavelength, electromagnetic waves with the energy of visible light or higher, such as ultraviolet light, X-rays, and gamma rays can also initiate photolysis reactions. In the current context, photolysis should not be confused with photosynthesis which is a two-part process in which natural chemicals (typically organic chemicals) are synthesized by a living organism as part of the life cycle (life chemistry) of the organism.

To begin a photochemical process, an atom or molecule must absorb a quantum of light energy from a photon. This reaction causes an atom or molecule to undergo a transient excited state, consequently changing the physical and chemical properties from those of the original atom or molecule of the substance. When this occurs, the recipient molecule tends to form a new structure, or combines with other molecules, and transfers electrons, atoms, protons, or excitation energy to other molecules, thus causing a prolonged chemical chain reaction.

During the reaction, the energy of the atom or molecule increases above its normal level—at this stage, the atom or molecule is now in an excited (or activated) state. If a quantum of visible or ultraviolet light is absorbed, then an electron in a relatively low energy state of the atom or molecule is excited into a higher energy state. If infrared radiation is absorbed by a molecule, then the excitation energy affects the motions of the nuclei in the molecule. After the initial absorption of a quantum of energy, the excited molecule can undergo a number of primary photochemical processes. A secondary process may occur after the primary step.

The absorption step can be represented by where the molecule M absorbs a quantum of light of appropriate energy to yield the excited molecular, M∗, which can then react further to produce a range of products:

Thus, the general form of photolysis reaction is:

K is the photolysis rate constant for this reaction in units of s −1 .

Thus, photolysis is a chemical process by which chemical bonds are broken as the result of transfer of light energy (direct photolysis) or radiant energy (indirect photolysis) to these bonds. The rate of photolysis depends upon numerous chemical and environmental factors including the light adsorption properties and reactivity of the chemical, and the intensity of solar radiation. In the process, the photochemical mechanism of photolysis is divided into three stages: (1) the adsorption of light which excites electrons in the molecule, (2) the primary photochemical processes which transform or deexcite the excited molecule, and (3) the secondary (“dark”) thermal reactions which transform the intermediates produced in the previous step (step 2).

An example of a secondary photochemical reaction in the atmosphere is the dissociation of a molecule into radicals (unstable fragments of molecules). The secondary process may involve a chain reaction, which is a process in which a reactive radical attacks a molecule to produce another unstable radical. This new radical can now attack another molecule, thereby reforming the original radical, which can now begin a new cycle of events.

A well-known example of a chain reaction is the hydrogen–chlorine reaction in which hydrogen and chlorine gases (in the presence of ultraviolet light) form hydrogen chloride it is given by

Indirect photolysis or sensitized photolysis occurs when the light energy captured (absorbed) by one molecule is transferred to the inorganic molecule of concern. The donor species (the sensitizer) undergoes no net reaction in the process but has an essentially catalytic effect. Moreover, the probability of a sensitized molecule donating its energy to an acceptor molecule is proportional to the concentration of both chemical species. Thus, complex mixtures may, in some cases, produce enhancement of photolysis rates of individual constituents through sensitized reactions.

There are two fundamental principles (sometimes referred to as the first law of photochemistry and the second law of photochemistry) are the foundation for understanding photochemical transformations: (1) light must be absorbed by a compound in order for a photochemical reaction to take place, and (2) for each photon of light absorbed by a chemical system, only one molecule is activated for subsequent reaction, sometime referred to as the photo-equivalence law.

The efficiency with which a photochemical process occurs is given by the quantum yield (Φ) which is the number of moles of a stated reactant disappearing, or the number of moles of a stated product produced, per mole of photons of monochromatic light absorbed:

Since many photochemical reactions are complex, and may compete with unproductive energy loss, the quantum yield is usually specified for a particular reaction. As an example, the irradiation of acetone with 313 nm light (3130 Å) gives a complex mixture of products, as shown in the following diagram:

The quantum yield of these products is less than 0.2, indicating there are radiative (fluorescence and phosphorescence) and nonradiative return pathways. The primary photochemical reaction is the homolytic cleavage of a carbon–carbon bond shown in the top equation in which the asterisk represents an electronic excited state.

The molecule in the excited state (that is, CH3COCH3∗) may return to its initial state according to any of three physical processes: (1) the molecule can release its excitation energy by emitting luminescent radiation through fluorescence or phosphorescence, (2) the molecule may transfer its energy to some other molecule with which it collides, without emitting light, in which the energy transfer process results in a normal molecule and an excited molecule, and (3) within the molecule, as a result of the initial light absorption step, an electron (e − ) may absorb so much energy that it may escape from the molecule, leaving behind a positive molecular ion as a result of a process known as photoionization. If the excited species M∗ (molecule or atom) does react, then it may undergo any of the following chemical processes: photodissociation, intramolecular (or internal) rearrangement, and reaction with another molecule. Photodissociation may result when the excited molecule breaks apart into atomic and/or molecular fragments. A rearrangement reaction (or photoisomerization reaction) involves the conversion of molecule the into an isomer, which has same numbers and types of atoms but with a different structural arrangement of the atoms.

If the primary photochemical process involves the dissociation of a molecule into radicals (unstable fragments of molecules), then the secondary process may involve a chain reaction. A chain reaction is a cyclic process whereby a reactive radical attacks a molecule to produce another unstable radical. This new radical can now attack another molecule, thereby reforming the original radical, which can now begin a new cycle of events. The hydrogen–chlorine reaction is an example of a chain reaction.

The rate-determining step in a photochemical reaction is determined by the number of photons present that can react with the chemical(s). At midday, the sun is at its apex and light has to travel through the least amount of atmosphere to reach the ground. Thus, a lower number of photons will be filtered out by ozone causing more photochemical reactions to occur. In early morning and evening the sun is at an angle, so light must travel through more of the atmosphere causing more photons to be filtered out. Photochemical degradation processes increase with temperature, so the maximum degradation rates will occur at midday.

Some chemicals are not prone to photochemical reactions and, thus, are not degraded by light because the necessary wavelength needed for the reaction is not present. When a molecule is absorbed onto a solid particle in air, the binding of the molecule onto the surface can change the bond strengths within the molecule. This can affect the absorbance wavelength needed for degradation. Particulate matter can have a negative effect upon photolysis. The particles in air can scatter light, preventing photons from reaching molecules which serves to decrease the number of intermediate frees radicals formed and the amount of pesticide that is degraded by light directly.

Photochemical and free radical reactions are major degradation pathways in the atmosphere, so an understanding of the products that are formed is important. The products formed by photochemical reactions may or may not be more toxic than the parent compound. Once a pesticide, for example, has been degraded, a major removal process for chemicals is to precipitate out of air and return to the earth's surface. Another removal process is for the products to be dissolved in rain and fall back to the surface of the earth.


Practical Work for Learning

It is fairly easy to show that plants produce oxygen and starch in photosynthesis. At age 14–16 students may have collected the gas given off by pond weed (for example Elodea) and tested leaves for starch.

It is not quite so easy to demonstrate the other reactions in photosynthesis. For the reduction of carbon dioxide to carbohydrate there must be a source of electrons. In the cell, NADP is the electron acceptor which is reduced in the light-dependent reactions, and which provides electrons and hydrogen for the light-independent reactions.

In this investigation, DCPIP (2,6-dichlorophenol-indophenol), a blue dye, acts as an electron acceptor and becomes colourless when reduced, allowing any reducing agent produced by the chloroplasts to be detected.

Lesson organisation

This investigation depends on working quickly and keeping everything cool. Your students will need to understand all the instructions in advance to be sure that they know what they are doing.

Apparatus and Chemicals

Per student or group of students:

Centrifuge – with RCF between 1500 and 1800g

Fresh green spinach, lettuce or cabbage, 3 leaves (discard the midribs)

Cold pestle and mortar (or blender or food mixer) which has been kept in a freezer compartment for 15–30 minutes (if left too long the extract will freeze)

Muslin or fine nylon mesh

Glass rod or Pasteur pipette

Measuring cylinder, 20 cm 3

Pipettes, 5 cm 3 and 1 cm 3

Bench lamp with 100 W bulb

Waterproof pen to label tubes

Colorimeter and tubes or light sensor and data logger

0.05 M phosphate buffer solution, pH 7.0: Store in a refrigerator at 0–4 °C (Note 1).

Isolation medium (sucrose and KCl in phosphate buffer): Store in a refrigerator at 0–4 °C (Note 2).

Potassium chloride (Low Hazard) (Note 3).

DCPIP solution (Low Hazard): (1 x 10 - 4 M approx.) (Note 4)

Health & Safety and Technical notes

Although DCPIP presents minimal hazard apart from staining, it is best to avoid skin contact in case prolonged contact with the dye causes sensitisation.
Do not handle electric light bulbs with wet hands.
All solutions used are low hazard – refer to relevant CLEAPSS Hazcards and Recipe cards for more information.

1 0.05 M phosphate buffer solution, pH 7.0. Na2HPO4.12H2O, 4.48 g (0.025 M) KH2PO4, 1.70 g (0.025 M). Make up to 500 cm 3 with distilled water and store in a refrigerator at 0–4 °C. Low hazard – refer to CLEAPSS Hazcard 72.

2 Isolation medium. Sucrose 34.23 g (0.4 M) KCl 0.19 g (0.01 M). Dissolve in phosphate buffer solution (pH 7.0) at room temperature and make up to 250 cm 3 with the buffer solution. Store in a refrigerator at 0–4 °C. Low hazard – refer to CLEAPSS Hazcard 40C.

3 Potassium chloride 0.05 M. Dissolve 0.93 g in phosphate buffer solution at room temperature and make up to 250 cm 3 . Store in a refrigerator at 0–4 °C. Use at room temperature.(Note that Potassium chloride is a cofactor for the Hill reaction.) Refer to CLEAPSS Hazcard 47B and Recipe card 51.

4 DCPIP solution DCPIP 0.007–0.01 g, made up to 100 cm 3 with phosphate buffer. Refer to CLEAPSS Hazcard 32 and Recipe card 46.

Procedure

Keep solutions and apparatus cold during the extraction procedure, steps 1–8, to preserve enzyme activity. Carry out the extraction as quickly as possible.

Preparation

a Cut three small green spinach, lettuce or cabbage leaves into small pieces with scissors, but discard the tough midribs and leaf stalks. Place in a cold mortar or blender containing 20 cm 3 of cold isolation medium. (Scale up quantities for blender if necessary.)

b Grind vigorously and rapidly (or blend for about 10 seconds).

c Place four layers of muslin or nylon in a funnel and wet with cold isolation medium.

d Filter the mixture through the funnel into the beaker and pour the filtrate into pre-cooled centrifuge tubes supported in an ice-water-salt bath. Gather the edges of the muslin, wring thoroughly into the beaker, and add filtrate to the centrifuge tubes.

e Check that each centrifuge tube contains about the same volume of filtrate.

f Centrifuge the tubes for sufficient time to get a small pellet of chloroplasts. (10 minutes at high speed should be sufficient.)

g Pour off the liquid (supernatant) into a boiling tube being careful not to lose the pellet. Re-suspend the pellet with about 2 cm 3 of isolation medium, using a glass rod. Squirting in and out of a Pasteur pipette five or six times gives a uniform suspension.

h Store this leaf extract in an ice-water-salt bath and use as soon as possible.

Investigation using the chloroplasts

Read all the instructions before you start. Use the DCPIP solution at room temperature.

i Set up 5 labelled tubes as follows.

j When the DCPIP is added to the extract, shake the tube and note the time. Place tubes 1, 2 and 4 about 12–15 cm from a bright light (100 W). Place tube 3 in darkness.

k Time how long it takes to decolourise the DCPIP in each tube. If the extract is so active that it decolourises within seconds of mixing, dilute it 1:5 with isolation medium and try again.

Teaching notes

Traditionally the production of oxygen and starch are used as evidence for photosynthesis. The light-dependent reactions produce a reducing agent. This normally reduces NADP, but in this experiment the electrons are accepted by the blue dye DCPIP. Reduced DCPIP is colourless. The loss of colour in the DCPIP is due to reducing agent produced by light-dependent reactions in the extracted chloroplasts.

Students must develop a clear understanding of the link between the light-dependent and light-independent reactions to be able to interpret the results. Robert Hill originally completed this investigation in 1938 he concluded that water had been split into hydrogen and oxygen. This is now known as the Hill reaction.

You can examine a drop of the sediment extract with a microscope under high power to see chloroplasts. There will be fewer chloroplasts in the supernatant – which decolourises the DCPIP more slowly, reinforcing the idea that the reduction is the result of chloroplast activity.

Sample results

Using a bench centrifuge

The experimental procedure was followed. A standard lab centrifuge was used to spin down the chloroplasts (Clifton NE 010GT/I) at 2650 RPM, 95 X g for 10 minutes.

The experiment was started within 5 minutes of preparing the chloroplasts. The reaction was followed using an EEL colorimeter with a red filter – readings taken every minute.

Tube 3 (incubated in the dark) gave a reading of 5.4 absorption units after 20 minutes.
Tube 2 (DCPIP with no leaf extract) was 6.2 absorption units.

Using a micro-centrifuge

The experiment was repeated using a micro-centrifuge.

Tube 3 (incubated in the dark) gave a reading of 4.9 absorption units after 10 minutes.

Tube 2 (DCPIP with no leaf extract) was 6.4 absorption Units.

The relative activity of the pellet was higher than when the bench centrifuge was used. The micro-centrifuge tubes were only 1.5 cm 3 capacity – not ideal for this practical. A higher speed bench centrifuge would be better.

In order to check for loss of chloroplast activity, the experiment was repeated using the same chloroplast suspension 1 and 2 hours after preparation. Chloroplast suspension was kept in a salt-ice bath. There was no loss of activity when the extract was kept in ice for up to 2 hours.

Student questions

1 Describe and explain the changes observed in the five tubes. Compare the results and make some concluding comments about what they show.

2 The rate of photosynthesis in intact leaves can be limited by several factors including light, temperature and carbon dioxide. Which of these factors will have little effect on the reducing capacity of the leaf extract?

3 Describe how you might extend this practical to investigate the effect of light intensity on the light-dependent reactions of photosynthesis.

1 Colour change and inferences that can made from the results:
Tube 1 (leaf extract + DCPIP) colour changes until it is the same colour as tube 4 (leaf extract + distilled water).
Tube 2 (isolation medium + DCPIP) no colour change. This shows that the DCPIP does not decolourise when exposed to light.
Tube 3 (leaf extract + DCPIP in the dark) no colour change. It can therefore be inferred that the loss of colour in tube 1 is due to the effect of light on the extract.
Tube 4 (leaf extract + distilled water) no colour change. This shows that the extract does not change colour in the light. It acts as a colour standard for the extract without DCPIP.
Tube 5 (supernatant + DCPIP) no colour change if the supernatant is clear if it is slightly green there may be some decolouring.
The results should indicate that the light-dependent reactions of photosynthesis are restricted to the chloroplasts that have been extracted.

2 Carbon dioxide will have no effect, because it is not involved in the light-dependent reactions.

3 Students should describe a procedure in which light intensity is varied but temperature is controlled.


Electron Transport Chain of Photosynthesis | Plants

The light-driven reaction of photosynthesis also called light reaction (Hill reaction), referred to as electron transport chain, were first propounded by Robert Hill in 1939. The electron transport chain of photosynthesis is initiated by absorption of light by photosystem II (P68o).

When P680 absorbs light, it is excited and its electrons are transferred to an electron acceptor molecule. Therefore, P680 becomes a strong oxidising agent, and splits a molecule of water to release oxygen. This light-dependent splitting of water molecule is called photolysis.

However, manganese, calcium and chloride ions play important roles in photolysis of water. After photolysis of water, electrons are generated, which are then passed to the oxidised P680. Now, the electron deficient P680 (as it had already transferred its electrons to an acceptor molecule) is able to restore its electrons from the water molecule.

After accepting electron from the excited P680, the primary electron acceptor is reduced. The primary electron acceptor in plants is pheophytin. The reduced acceptor which is a strong reducing agent, now donates its electrons to the downstream components of the electron transport chain.

Photosystem I (PS I):

Similar to photosystem II (P680), photosystem I (P700) is excited on absorption of light and gets oxidised, and transfers its electrons to the primary electron acceptor (pheophytin), which, in turn gets reduced. While the oxidised P700 draws electrons from photosystem II, the reduced electron acceptor of photosystem I, transfers electrons to ferredoxin and ferredoxin-NADP reductase to reduce NADP to NADPH2.

NADPH2 is a powerful reducing agent, and is utilised in the reduction of CO2 to carbohydrates in the carbon reaction of photosynthesis. The reduction of CO2 to carbohydrates requires energy in the form of ATP, produced through electron transport chain. Process of ATP formation from ADP in the presence of light in chloroplasts is called photophosphorylation.

The Light Reaction (Hill Reaction):

The light reaction is thought to be responsible for the production of a ‘reducing power’ and oxygen from water as a result of light energy. This is as follows: The light energy, after absorption by chlorophyll, splits H2O.

(i) The (H) combines with an unidentified compound (probably ferredoxin) and is passed from this to NADP.

(ii) The NADPH2 can cause the reduction of phosphoglyceric acid……. Phosphoglyceraldehyde, together with some ATP production.

(iii) The (OH) forms H2O and oxygen:

The light reaction gives rise to two very important productions:

(i) A reducing agent NADPH2 and

(ii) An energy rich compound ATP.

These two products of the light reaction are utilized in the dark phase of photosynthesis.

The energy transformations in photosynthesis are as follow:

(i) The radiant energy of an absorbed quantum is transformed into the energy of an activated pigment molecule

(pigment molecule or activated pigment)

(ii) Now the activated pigment removes an electron from the hydroxyl ion derived from the water molecule. The (OH) represents the ‘free radical’. These are uncharged, but highly reactive forms.

(iii) The free radicals react in many ways the release of oxygen and formation of free radicals of hydrogen takes place.

(iv) The H + ions from water, together with the electron attached to the pigment are transferred to certain molecules, which then carry the reducing power to other reactions.

(v)Another reaction is the recombination of the split products of water into the water molecules itself.

This reaction is strongly energy-releasing. The chloroplast puts this reaction to work by causing it to synthesize energy-rich ATP from a precursor molecule ADP and inorganic phosphate

(6) The energy of the ATP can now be used, in the reduction of CO2 to sugar by the reducing power (NADP.H) generated in the light reaction.

This way, the radiant energy has been converted to the chemical energy of the sugar molecule by passing through a photo-activated pigment, photolyzed water fragments, and ATP. The main function of light energy in photosynthesis is to produce ATP through a complex of reactions called photophosphorylation.

The subsequent reactions leading to the formation of sugar from CO2 can proceed entirely in darkness.

Photophosphorylation:

Photosynthetic phosphorylation:

With the discovery that CO2 can be assimilated in isolated chloroplasts, this came into existence that the chloroplast must contain the enzymes necessary for this assimilation and must be able to produce the ATP (adenosine tri-phosphate) essential for the formation of the main photosynthesis products.

Arnon and his co-workers (1954) demonstrated that the isolated chloroplasts can produce ATP in the presence of light. They gave the name to this process photosynthetic phosphorylation.

This was revealed for the first time that mitochondria are not the only cytoplasmic particles that produce ATP. ATP formation in chloroplasts differs from that in mitochondria in that it is free from respiratory oxidations. During this process the light energy is being converted to ATP. In other words, there is a conversion to light energy of chemical energy.

ATP is only one of the necessary requirements for the reduction of carbon dioxide to the carbohydrate level. A reductant must be formed in photosynthesis that will provide the hydrogens or electrons for this reduction. Arnon (1951) demonstrated that isolated chloroplasts are capable of reducing pyridine nucleotides in light.

The photochemical reaction and an enzyme system are capable of utilizing the reduced pyridine nucleotide as soon as this was formed, Arnon (1957) found that NADP. H2 is the reduced pyridine nucleotide in photosynthesis.

In the presence of H2O. ADP (adenosine di-phosphate) and orthophosphate (P), substrate amounts of NADP (nicotinamide adenine dinucleotide phosphate) were reduced, accompanied by the evolution of oxygen.

The equation is as follow:

As shown by the equation the evolution of one molecule of oxygen is accompanied by the reduction of two molecules of NADP and esterification of two molecules of orthophosphate. Together, ATP and NADPH2 provide the energy requirements for CO2 assimilation. Arnon gave name to this power assimilatory power (i.e., ATP + NADPH2).

According to Arnon (1967), in bacterial photosynthesis NADH2 is utilized of NADPH2.

In the late 1950’s the reduction of NADP + was thought to be associated with a soluble protein factor found in chloroplasts. Arnon et al. (1957) observed that this protein reduced NADP + accompanied by the evolution of oxygen. They termed it the ‘NADP reducing factor.’

Thereafter the NADP reducing factor was purified and called photosynthetic pyridine nucleotide reductase (PPNR), since its catalytic activity was only apparent when chloroplasts were kept in light.

Tagawa and Arnon (1962) recognized that PPNR is one of a family of nonhemenonflavin, iron-containing proteins that is universally present in chloroplasts. These proteins were given a generic name ferredoxin.

When ferredoxin was not discovered, NADP was thought to be the terminal electron acceptor of the photosynthetic light reaction. Arnon (1967) revealed that illuminated chlorophyll reacts directly with ferredoxin and not with NADP + .

The exposition of chlorophyll to light causes a flow of electrons to ferredoxin. Now the reduced ferredoxin causes the reduction of NADP + in an enzyme catalyzed reaction that is independent of light. In other words, ferredoxin is termed as terminal electron acceptor of the photosynthetic light reaction.

The reduction of NADP takes place by ferredoxin. Under normal condition, in photosynthesis ferredoxin reduced by the acceptance of an electron is immediately reoxidized by NADP + . The reduction of NADP by ferredoxin is catalyzed by ferredoxin-NADP reductase. This shows that the mechanism of NADP + reduction in photosynthesis completes in three steps.

(i) Photochemical reduction of ferredoxin

(ii) Reoxidation of ferredoxin by ferredoxin NADP + reductase and

(iii) Reoxidation of ferredoxin-NADP + reductase by NADP + .

According to Arnon there are two types of photophosphorylation:

(i) Non-cyclic photophosphorylation and

(ii) Cyclic photophosphorylation.

Non-Cyclic Photophosphorylation:

This is a result of an interaction of photosystem I (PSI) and photosystem II (PSII). In non-cyclic photophosphorylation, the electron is not returned to the chlorophyll molecule, but is taken up by NADP ± which thereafter reduces to NADPH. Here the electron that returns to the chlorophyll molecule is derived from an outside source which is water.

In this process oxygen is released and both NADPH2 − and ATP are formed. In green plants and many photosynthetic bacteria, however, illumination is known to produce also NADPH2 − which provides hydrogen for the reduction of carbon dioxide in the day.

The electron lost by the excited chlorophyll is accepted by NADP along with a proton resulting in the formation of NADPH2. The light energy is now stored in the NADPH2 molecule. The proton required for the reduction of NADP is released from the dissociation of water molecule by photolysis into hydrogen H ± and hydroxyl ions OH.

The hydroxyl ions react to produce water and molecular oxygen.

The reaction is as follows:

Here the hydroxyl ion also releases an electron that is accepted by the cytochromes of the chloroplast. In turn, the cytochrome donates this electron to the chlorophyll molecule, which already lost an electron earlier. The energy released during this transfer of electron from the cytochrome is utilized in the formation of ATP by the photophosphorylation of ADP.

In water molecule hydrogen is strongly bound to oxygen and this can be cleaved only by the use of energy. This energy is supplied by light. This way, in non-cyclic photophosphorylation light energy takes part in two processes, i.e., the activation of chlorophyll molecule and photolysis (cleavage) of water.

In non-cyclic photophosphorylation one molecule of NADPH2 and one molecule of ATP are produced by the activation of chlorophyll molecule by a photon, while in cyclic photophosphorylation two molecules of ATP are produced for each photon absorbed by chlorophyll.

The overall reaction of photophosphorylation is as follows:

Cyclic Photophosphorylation:

When non-cyclic photophosphorylation is stopped under certain conditions, cyclic photophosphorylation takes place. The non-cyclic photophosphorylation can be stopped by illuminating isolated chloroplasts with light of wavelength greater than 680 nm.

By this way, only photosystem I (PS I) is activated, as it has a maximum absorption at 700 nm, and photosystem II (PS II), which absorbs at 680 nm, remains inactivated.

Due to inactivation of PS II, the electron flow from water to NADP is stopped, and also CO2 fixation is retarded.

When CO2 fixation stops, electrons are not removed from reduced NADPH. Thus, NADPH will not be oxidised and NADP will not be available as an electron acceptor.

Under above-mentioned conditions, cyclic-photophosphorylation occurs.

During cyclic-photophosphorylation, electrons from photosystem I (PS I) are not passed to NADP from the electron acceptor, as NADP is not available in oxidised state to receive electrons.

Hence, the electrons are transferred back to P700.

This type of movement of electrons from an electron acceptor to P700 result in the formation of ATP from ADP, and the process is called cyclic photophosphorylation.

During cyclic photophosphorylation oxygen is not released, as there is no photolysis of water and NADPH2 is also not produced.

In cyclic photophosphorylation the excited electron lost by the chlorophyll is returned to it through vitamin K or FMN (flavin mononucleotide) and cytochromes. The chlorophyll molecule on losing an electron assumes a positive charge and subsequently the electron is transferred to a second acceptor.

This second acceptor is a group of substances collectively known as cytochrome system. All the members of cytochrome system are variants of cytochrome. Ultimately these cytochromes transfer the electron to the chlorophyll molecule from where it was lost initially.

The electromagnetic energy of the light is utilized in the formation of ATP. This means that light energy is being converted into chemical energy. Here the electron after leaving a chlorophyll travels in a cyclic way and ultimately returns to the same molecule from which it initiated, and therefore, this process has been termed by Arnon as cyclic photophosphorylation.

The final electron acceptor and the initial electron donor is the same substance—the chlorophyll. No outside material takes part in the process. During cyclic photophosphorylation, one electron and two ATP molecules are formed.

One ATP molecule is being formed when the electron travels from the cofactor (i.e., vitamin K or FMN) to the cytochromes while the other when it travels from the cytochromes back to the chlorophyll molecule.

Here the light energy is being converted into chemical energy.

In nature both processes of photophosphorylation proceed simultaneously. In green plants the non-cyclic electron transfer is essential for the production of NADPH2 and ATP.

The oxygen is evolved during the process. The cyclic electron transfer fulfils the requirement of the low yield of ATP during non-cyclic process. This way, the complete light phase of photophosphorylation produces ATP and NADPH2 and oxygen is evolved.

NADPH2 is a biological reductant that brings about the reduction of carbon dioxide to carbohydrates in the dark phase of photosynthesis. Here both NADPH2 and ATP provide energy for reduction. The assimilatory power of the cell is constituted by these two components. The energy of these components is derived from visible part of sunlight.

In the dark phase of photosynthesis the energy that is stored in NADPH2 and ATP, is being transferred to the molecules of organic substances and stored there in the form of chemical energy.

During photosynthesis the electromagnetic energy of visible light is being converted into chemical energy. Now this energy is utilized by living cells as the driving force for various vital activities. This act of the conversion of energy is brought about by the photosynthetic cells of green plants or photosynthetic bacteria.

Here the solar energy is trapped by the chlorophyll apparatus. As soon as the light energy is being transformed into chemical energy, it may be used in the formation of carbohydrates, protein synthesis and other important vital activities.

The living are so designed that they can use only chemical energy for various metabolic activities. The light energy cannot be directly used for these vital activities. The light reaction of the higher plants takes place in the grana of the chloroplasts.


Quantum requirements

The quantum requirements of the individual light reactions of photosynthesis are defined as the number of light photons absorbed for the transfer of one electron. The quantum requirement for each light reaction has been found to be approximately one photon. The total number of quanta required, therefore, to transfer the four electrons that result in the formation of one molecule of oxygen via the two light reactions should be four times two, or eight. It appears, however, that additional light is absorbed and used to form ATP by a cyclic photophosphorylation pathway. (The cyclic photophosphorylation pathway is an ATP-forming process in which the excited electron returns to the reaction centre.) The actual quantum requirement, therefore, probably is 9 to 10.


Photolysis in the Light Reactions of Photosynthesis - Biology

IN THIS CHAPTER

Summary: This chapter discusses the basics behind the energy-creation process known as photosynthesis. It also teaches you how plants generate their energy from light. You will learn to differentiate between the two stages&mdashthe light-dependent and the light-independent reactions.

Overall photosynthesis reaction: H2 O + CO2 + light &rarr O2 + glucose + H2 O.

Light-dependent reactions: inputs are water and light products are ATP, NADPH, and O2.

The oxygen produced in photosynthesis comes from the water.

The carbon in the glucose produced in photosynthesis comes from the CO2.

Light-independent reactions (dark reactions): inputs are NADPH, ATP and CO2 products are ADP, NADP + , and sugar.

Introduction

In Chapter 7 , we discussed how human and animal cells generate the energy needed to survive and perform on a day-to-day basis. Now we are going to look at how plants generate their energy from light&mdashthe process of photosynthesis. We stress again in this chapter what we said about respiration&mdashdo not get caught up in the memorization of every fact. Make sure that you understand the basic, overall concepts and the major ideas. Remember that most of plant photosynthesis occurs in the plant’s leaves. The majority of the chloroplasts of a plant are found in mesophyll cells. Remember that there are two stages to photosynthesis: the light-dependent reactions and the light-independent reactions, commonly called the “dark reactions.” The simplified equation of photosynthesis is

The Players in Photosynthesis

The host organelle for photosynthesis is the chloroplast, which is divided into an inner and outer portion. The inner fluid portion is called the stroma, which is surrounded by two outer membranes. In Figure 8.1 , you can see that winding through the stroma is an inner membrane called the thylakoid membrane system. This is where the first stage of photosynthesis occurs. This membrane consists of flattened channels and disks arranged in stacks called grana. We always remember the thylakoid system as resembling stacks of poker chips, where each chip is a single thylakoid. It is within these poker chips that the light-dependent reactions of photosynthesis occur.

Figure 8.1 An overall view of photosynthesis. (From Biology , 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies .)

Before we examine the process of photosynthesis, here are some definitions that will make things a bit easier as you read this chapter.

Autotroph: an organism that is self-nourishing. It obtains carbon and energy without ingesting other organisms. Plants and algae are good examples of autotrophic organisms&mdashthey obtain their energy from carbon dioxide, water, and light. They are the producers of the world.

Bundle sheath cells: cells that are tightly wrapped around the veins of a leaf. They are the site for the Calvin cycle in C4 plants.

C4 plant: plant that has adapted its photosynthetic process to more efficiently handle hot and dry conditions.

Heterotroph: organisms that must consume other organisms to obtain nourishment. They are the consumers of the world.

Mesophyll: interior tissue of a leaf.

Mesophyll cells: cells that contain many chloroplasts and host the majority of photosynthesis.

Photolysis: process by which water is broken up by an enzyme into hydrogen ions and oxygen atoms occurs during the light-dependent reactions of photosynthesis.

Photophosphorylation: process by which ATP is produced during the light-dependent reactions of photosynthesis. It is the chloroplast equivalent of oxidative phosphorylation.

Photorespiration: process by which oxygen competes with carbon dioxide and attaches to RuBP. Plants that experience photorespiration have a lowered capacity for growth.

Photosystem: a cluster of light-trapping pigments involved in the process of photosynthesis. Photosystems vary tremendously in their organization and can possess hundreds of pigments. The two most important are photosystems I and II of the light reactions.

Pigment: a molecule that absorbs light of a particular wavelength. Pigments are vital to the process of photosynthesis and include chlorophyll, carotenoids, and phycobilins.

Rubisco: an enzyme that catalyzes the first step of the Calvin cycle in C3 plants.

Stomata: structure through which CO2 enters a plant and water vapor and O2 leave.

Transpiration: natural process by which plants lose H2 O via evaporation through their leaves.

BIG IDEA 4.C.1

Molecular variation in pigment molecules allows plants to absorb a greater range of wavelengths .

The Reactions of Photosynthesis

BIG IDEA 2.A.1

All living things require input of energy .

The process of photosynthesis can be neatly divided into two sets of reactions: the light-dependent reactions and the light-independent reactions. The light-dependent reactions occur first and require an input of water and light. They produce three things: the oxygen we breathe, NADPH, and ATP. These last two products of the light reactions are then consumed during the second stage of photosynthesis: the dark reactions. These reactions, which need CO2 , NADPH, and ATP as inputs, produce sugar and recycle the NADP + and ADP to be used by the next set of light-dependent reactions. Now, we would be too kind if we left the discussion there. Let’s look at the reactions in more detail. Stop groaning . . . you know we have to go there.

BIG IDEA 2.A.2

Autotrophs capture free energy present in sunlight through photosynthesis .

Light-Dependent Reactions

Light-dependent reactions occur in the thylakoid membrane system. The thylakoid system is composed of the various stacks of poker chip look-alikes located within the stroma of the chloroplast. Within the thylakoid membrane is a photosynthetic participant termed chlorophyll. There are two main types of chlorophyll that you should remember: chlorophyll a and chlorophyll b . Chlorophyll a is the major pigment of photosynthesis, while chlorophyll b is considered to be an accessory pigment. The pigments are very similar structurally, but the minor differences are what account for the variance in their absorption of light. Chlorophyll absorbs light of a particular wavelength, and when it does, one of its electrons is elevated to a higher energy level (it is “excited”). Almost immediately, the excited electron drops back down to the ground state, giving off heat in the process. This energy is passed along until it finds chlorophyll a , which, when excited, passes its electron to the primary electron acceptor then, the light-dependent reactions are under way.

The pigments of the thylakoid space organize themselves into groups called photosystems . These photosystems consist of varying combinations of chlorophylls a , b , and others pigments called phycobilins and another type of pigment called carotenoids. The accessory pigments help pick up light when chlorophyll a cannot do it as effectively. An example is red algae on the ocean bottom. When light is picked up by the accessory pigments, it is fluoresced and altered so that chlorophyll a can use it.

Imagine that the plant represented in Figure 8.2 is struck by light from the sun. This light excites the photosystem of the thylakoid space, which absorbs the photon and transmits the energy from one pigment molecule to another. As this energy is passed along, it loses a bit of energy with each step and eventually reaches chlorophyll a , which proceeds to kick off the process of photosynthesis. It initiates the first step of photosynthesis by passing the electron to the primary electron acceptor.

Figure 8.2 Light-dependent reactions. (From Biology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies.)

Before we continue, there are two major photosystems we want to tell you about&mdashyou might want to get out a pen or pencil here to jot this down, because the names for these photosystems may seem confusing. They are photosystem I and photosystem II. The only difference between these two reaction centers is that the main chlorophyll of photosystem I absorbs light with a wavelength of 700 nm, while the main chlorophyll of photosystem II absorbs light with a wavelength of 680 nm. By interacting with different thylakoid membrane proteins, they are able to absorb light of slightly different wavelengths.

Now let’s get back to the reactions. Let’s go through the rest of Figure 8.2 and talk about the light-dependent reactions. For the sole purpose of confusing you, plants start photosynthesis by using photosystem II before photosystem I. As light strikes photosystem II, the energy is absorbed and passed along until it reaches the P680 chlorophyll. When this chlorophyll is excited, it passes its electrons to the primary electron acceptor. This is where the water molecule comes into play. Photolysis in the thylakoid space takes electrons from H2 O and passes them to P680 to replace the electrons given to the primary acceptor. With this reaction, a lone oxygen atom and a pair of hydrogen ions are formed from the water. The oxygen atom quickly finds another oxygen atom buddy, pairs up with it, and generates the O2 that the plants so graciously put out for us every day. This is the first product of the light reactions.

The light reactions do not stop here, however. We need to consider what happens to the electron that has been passed to the primary electron acceptor. The electron is passed to photosystem I, P700, in a manner reminiscent of the electron transport chain. As the electrons are passed from P680 to P700, the lost energy is used to produce ATP (remember chemiosmosis). This ATP is the second product of the light reactions and is produced in a manner mechanistically similar to the way ATP is produced during oxidative phosphorylation of respiration. In plants, this process of ATP formation is called photophosphorylation.

After the photosystem I electrons are excited, photosystem I passes the energy to its own primary electron acceptor. These electrons are sent down another chain to ferredoxin, which then donates the electrons to NADP + to produce NADPH, the third and final product of the light reactions. (Notice how in photosynthesis, there is NADPH instead of NADH. The symbol P can help you remember that it relates to photosynthesis. )

Remember the following about the light reactions:

1. The light reactions occur in the thylakoid membrane.

2. The inputs to the light reactions are water and light.

3. The light reactions produce three products: ATP, NADPH, and O2 .

4. The oxygen produced in the light reactions comes from H2 O, not CO2 .

Two separate light-dependent pathways occur in plants. What we have just discussed is the noncyclic light reaction pathway. Considering the name of the first one, it is not shocking to discover that there is also a cyclic light reaction pathway (Figure 8.3 ). One key difference between the two is that in the noncyclic pathway, the electrons taken from chlorophyll a are not recycled back down to the ground state. This means that the electrons do not make their way back to the chlorophyll molecule when the reaction is complete. The electrons end up on NADPH. Another key difference between the two is that the cyclic pathway uses only photosystem I photosystem II is not involved. In the cyclic pathway, sunlight hits P700, thus exciting the electrons and passing them from P700 to its primary electron acceptor. It is called the cyclic pathway because these electrons pass down the electron chain and eventually back to P700 to complete the cycle. The energy given off during the passage down the chain is harnessed to produce ATP&mdashthe only product of this pathway. Neither oxygen nor NADPH is produced from these reactions.

Figure 8.3 Cyclic phosphorylation . (From Biology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies .)

A question that might be forming as you read this is: “Why does this pathway continue to exist?” or perhaps you are wondering “Why do they insist on torturing me by writing about all of this photosynthesis stuff?” We will answer the first question and ignore the second one. The cyclic pathway exists because the Calvin cycle, which we discuss next, uses more ATP than it does NADPH. This eventually causes a problem because the light reactions produce equal amounts of ATP and NADPH. The plant compensates for this disparity by dropping into the cyclic phase when needed to produce the ATP necessary to keep the light-independent reactions from grinding to a halt.

Before moving on to the Calvin cycle, it is important to understand how ATP is formed. We know, we know. . . you thought we were finished . . . but we want you to be an expert in the field of photosynthesis. You never know when these facts might come in handy. For example, just the other day one of us was offered $10,000 by a random person on the street to recount the similarities between photosynthesis and respiration. So, this stuff is useful in everyday life. As the electrons are passing from the primary electron acceptor to the next photosystem, hydrogen ions are picked up from outside the membrane and brought back into the thylakoid compartment, creating an H + gradient similar to what we saw in oxidative phosphorylation. During the light-dependent reactions, when hydrogen ions are taken from water during photolysis, the proton gradient grows larger, causing some protons to leave, leading to the formation of ATP.

You’ll notice that this process in plants is a bit different from oxidative phosphorylation of the mitochondria, where the proton gradient is created by pumping protons from the matrix out to the intermembrane space. In the mitochondria, the ATP is produced when the protons move back in . But in plants, photophosphorylation creates the gradient by pumping protons in from the stroma to the thylakoid compartment, and the ATP is produced as the protons move back out . The opposing reactions produce the same happy result&mdashmore ATP for the cells.

Light-Independent Reactions (Calvin Cycle)

After the light reactions have produced the necessary ATP and NADPH, the synthesis phase of photosynthesis is ready to proceed. The inputs into the Calvin cycle are NADPH (which provides hydrogen and electrons), ATP (which provides energy), and CO2 . From here on, just so we don’t drive you insane switching from term to term, we are going to call the dark reactions of photosynthesis the Calvin cycle (Figure 8.4 ). The Calvin cycle occurs in the stroma of the chloroplast, which is the fluid surrounding the thylakoid “poker chips.” (For further distinctions among the cyclic pathway, the noncyclic pathway, and the Calvin cycle, see Figure 8.5 .)

Figure 8.4 The Calvin cycle. (From Biology, 8th ed., by Sylvia S. Mader, © 1985, 1987, 1990, 1993, 1996, 1998, 2001, 2004 by the McGraw Hill Companies, Inc. Reproduced with permission of The McGraw-Hill Companies .)

Figure 8.5 Summary of photosynthesis.

The Calvin cycle begins with a step called carbon fixation. This is a tricky and complex term that makes it sound more confusing than it really is. Basically, carbon fixation is the binding of the carbon from CO2 to a molecule that is able to enter the Calvin cycle. Usually this molecule is ribulose bis-phosphate, a 5-carbon molecule known to its closer friends as RuBP. This reaction is assisted by the enzyme with one of the cooler names in the business: rubisco. The result of this reaction is a 6-carbon molecule that breaks into two 3-carbon molecules named 3-phosphoglycerate (3PG). ATP and NADPH step up at this point and donate a phosphate group and hydrogen electrons, respectively, to (3PG) to form glyceraldehyde 3-phosphate (G3P). Most of the G3P produced is converted back to RuBP so as to fix more carbon. The remaining G3P is converted into a 6-carbon sugar molecule, which is used to build carbohydrates for the plant. This process uses more ATP than it does NADPH. This is the disparity that makes cyclic photophosphorylation necessary in the light-dependent reactions.

We know that for some of you, the preceding discussion contains many difficult scientific names, strangely spelled words, and esoteric acronyms. So, here’s the bottom line&mdashyou should remember the following about the Calvin cycle:

1. The Calvin cycle occurs in the stroma of the chloroplast.

2. The inputs into the Calvin cycle are NADPH, ATP, and CO2 .

3. The products of the Calvin cycle are NADP + , ADP, and a sugar.

4. More ATP is used than NADPH, creating the need for cyclic photophosphorylation to create enough ATP for the reactions.

5. The carbon of the sugar produced in photosynthesis comes from the CO2 of the Calvin cycle.

Types of Photosynthesis

Plants do not always live under ideal photosynthetic conditions. Some plants must make changes to the system in order to successfully use light and produce energy. Plants contain a structure called a stomata, which consists of pores through which oxygen exits and carbon dioxide enters the leaf to be used in photosynthesis. Transpiration is the natural process by which plants lose water by evaporation from their leaves. When the temperature is very high, plants have to worry about excess transpiration. This is a potential problem for plants because they need the water to continue the process of photosynthesis. To combat this evaporation problem, plants must close their stomata to conserve water. But this solution leads to two different problems: (1) how will they bring in the CO2 required for photosynthesis? and (2) what will the plants do with the excess O2 that builds up when the stomata are closed?

When plants close their stomata to protect against water loss, they experience a shortage of CO2 , and the oxygen produced from the light reactions is unable to leave the plant. This excess oxygen competes with the carbon dioxide and attaches to RuBP in a reaction called photorespiration. This results in the formation of one molecule of PGA and one molecule of phosphoglycolate. This is not an ideal reaction because the sugar formed in photosynthesis comes from the PGA, not phosphoglycolate. As a result, plants that experience photorespiration have a lowered capacity for growth. Photorespiration tends to occur on hot, dry days when the stomata of the plant are closed.

A group of plants called C4 plants combat photorespiration by altering the first step of their Calvin cycle. Normally, carbon fixation produces two 3-carbon molecules. In C4 plants, the carbon fixation step produces a 4-carbon molecule called oxaloacetate. This molecule is converted into malate and sent from the mesophyll cells to the bundle sheath cells, where the CO2 is used to build sugar. The mesophyll is the tissue of the interior of the leaf, and mesophyll cells are cells that contain bunches of chloroplasts. Bundle sheath cells are cells that are tightly wrapped around the veins of a leaf. They are the site for the Calvin cycle in C4 plants.

What is the difference between C3 plants and C4 plants? One difference is that C4 plants have two different types of photosynthetic cells: (1) tightly packed bundle sheath cells, which surround the vein of the leaf, and (2) mesophyll cells. Another difference involves the first product of carbon fixation. For C3 plants, it is PGA, for C4 plants, it is oxaloacetate. C4 plants are able to successfully perform photosynthesis in these hot areas because of the presence of an enzyme called PEP (phosphoenolpyruvate ) carboxylase . This enzyme really wants to bind to CO2 and is not tricked by the devious oxygen into using it instead of the necessary CO2 . PEP carboxylase prefers to pair up with CO2 rather than O2 , and this cuts down on photorespiration for C4 plants. The conversion of PEP to oxaloacetate occurs in the mesophyll cells then, after being converted into malate, PEP is shipped to the bundle sheath cells. These cells contain the enzymes of photosynthesis, including our good pal rubisco. The malate releases the CO2 , which is then used by rubisco to perform the reactions of photosynthesis. This process counters the problem of photorespiration because the shuttling of CO2 from the mesophyll cells to the bundle sheath cells keeps the CO2 concentration high enough so that it is not beat out by oxygen for rubisco’s love and attention.

One last variation of photosynthesis that we should look at is the function performed by CAM (Crassulacean acid metabolizing) plants&mdashwater-storing plants, such as cacti, that close their stomata by day and open them by night to avoid transpiration during the hot days, without depleting the plant’s CO2 reserves. The CO2 taken in during the night is stored as organic acids in the vacuoles of mesophyll cells until daybreak when the stomata close. The Calvin cycle is able to proceed during the day because the stored CO2 is released, as needed, from the organic acids to be incorporated into the sugar product of the Calvin cycle.

To sum up these two variations of photosynthesis:

C4 photosynthesis: photosynthetic process that first converts CO2 into a 4-carbon molecule in the mesophyll cells, converts that product to malate, and then shuttles the malate into the bundle sheath cells. There, malate releases CO2 , which reacts with rubisco to produce the carbohydrate product of photosynthesis.

CAM photosynthesis: plants close their stomata during the day, collect CO2 at night, and store the CO2 in the form of acids until it is needed during the day for photosynthesis.

Review Questions

Questions 1–4 refer to the following answer choices&mdashuse each answer only once.

D. Cyclic photophosphorylation

E. Noncyclic photophosphorylation

1 . Plants use this process so that they can open their stomata at night and close their stomata during the day to avoid water loss during the hot days, without depleting the plant’s CO2 reserves.

2 . Uses NADPH, ATP, and CO2 as the inputs to its reactions.

3 . Photosynthetic process that has ATP as its sole product. There is no oxygen and no NADPH produced from these reactions.

4 . The process by which plants lose water via evaporation through their leaves.

5 . The photosynthetic process performed by some plants in an effort to survive the hot and dry conditions of climates such as the desert is called

D. cyclic photophosphorylation.

E. noncyclic photophosphorylation.

6 . Which of the following is the photosynthetic stage that produces oxygen?

A. The light-dependent reactions

7 . Which of the following reactions occur in both cellular respiration and photosynthesis?

8 . Which of the following is not a product of the light-dependent reactions of photosynthesis?

9 . Which of the following is an advantage held by a C4 plant?

A. More efficient light absorption

B. More efficient photolysis

C. More efficient carbon fixation

D. More efficient uptake of carbon dioxide into the stomata

E. More efficient ATP synthesis during chemiosmosis

10 . Carbon dioxide enters the plant through the

11 . Which of the following is the source of the oxygen released during photosynthesis?

12 . Which of the following is an incorrect statement about the Calvin cycle?

A. The main inputs to the reactions are NADPH, ATP, and CO2 .

B. The main outputs of the reactions are NADP + , ADP, and sugar.

C. More NADPH is used than ATP during the Calvin cycle.

D. Carbon fixation is the first step of the process.

E. The reactions occur in the stroma of the chloroplast.

13 . Which of the following is the source of the carbon in sugar produced during photosynthesis?

14 . The light-dependent reactions of photosynthesis occur in the

Answers and Explanations

1 . C &mdashCAM plants open their stomata at night and close their stomata during the day to avoid water loss due to heat. The carbon dioxide taken in during the night is incorporated into organic acids and stored in vacuoles until the next day, when the stomata close and CO2 is needed for the Calvin cycle.

2 . B &mdashThe Calvin cycle uses ATP, NADPH, and CO2 to produce the desired sugar output of photosynthesis.

3 . D &mdashCyclic photophosphorylation occurs because the Calvin cycle uses more ATP than it does NADPH. This is a problem because the light reactions produce an equal amount of ATP and NADPH. The plant compensates for this disparity by dropping into the cyclic phase when needed to produce the ATP necessary to keep the light-independent reactions from grinding to a halt.

4 . A &mdashTranspiration is the process by which plants lose water through their leaves. Not much else to be said about that.

5 . C &mdashOne of the major problems encountered by plants in hot and dry conditions is of photo-respiration. In hot conditions, plants close their stomata to avoid losing water to transpiration. The problem with this is that the plants run low on CO2 and fill with O2 . The oxygen competes with the carbon dioxide and attaches to RuBP, leaving the plant with a lowered capacity for growth. C4 plants cycle CO2 from mesophyll cells to bundle sheath cells, creating a higher concentration of CO2 in that region, thus allowing rubisco to carry out the Calvin cycle without being distracted by the O2 competitor.

6 . A &mdashThe light-dependent reactions are the source of the oxygen given off by plants.

7 . D &mdashChemiosmosis occurs in both photosynthesis and cellular respiration. This is the process by which the formation of ATP is driven by electrochemical gradients in the cell. Hydrogen ions accumulate on one side of a membrane, creating a proton gradient that causes them to move through channels to the other side of that membrane, thus leading, with the assistance of ATP synthase, to the production of ATP.

8 . D &mdashSugar is a product not of the light-dependent reactions of photosynthesis but of the Calvin cycle (the dark reactions). The outputs of the light-dependent reactions are ATP, NADPH, and O2 .

9 . C &mdashC4 plants fix carbon more efficiently than do C3 plants. Please see the explanation for question 5 for a more detailed explanation of this answer.

10 . A &mdashThe stomata is the structure through which the CO2 enters a plant and the oxygen produced in the light-dependent reactions leaves the plant.

11 . B &mdashThe source of the oxygen produced during photosynthesis is the water that is split by the process of photolysis during the light-dependent reactions of photosynthesis. In this reaction, two hydrogen ions and an oxygen atom are formed from the water. The oxygen atom immediately finds and pairs up with another oxygen atom to form the oxygen product of the light-dependent reactions.

12 . C &mdashThis is a trick question. We reversed the two compounds (NADPH and ATP) in this one. More ATP than NADPH is used in the Calvin cycle. It is for this reason that cyclic photophosphorylation exists&mdashto produce ATP to make up for this disparity.

13 . A &mdashThe carbon of CO2 is used to produce the sugar created during the Calvin cycle.

14 . C &mdashThe light-dependent reactions occur in the thylakoid membrane of the chloroplast. Remember, the thylakoid system resembles the various stacks of poker chips located within the stroma of the chloroplast. The light-independent reactions occur in the stroma of the chloroplast.

Rapid Review

The following terms should be thoroughly familiar to you:

Photosynthesis: process by which plants use the energy from light to generate sugar.

• Light reactions (thylakoid)

Autotroph: self-nourishing organism that is also known as a producer (plants).

Heterotroph: organisms that must consume other organisms to obtain energy&mdashconsumers (humans).

Transpiration: loss of water via evaporation through the stomata (natural process).

Photophosphorylation: process by which ATP is made during light reactions.

Photolysis: process by which water is split into hydrogen ions and oxygen atoms (light reactions).

Stomata: structure through which CO2 enters a plant, and water vapor and oxygen leave a plant.

Pigment: molecule that absorbs light of a particular wavelength (chlorophyll, carotenoid, phycobilins).

There are three types of photosynthesis reactions:

(Noncyclic ) light-dependent reactions

• Occur in thylakoid membrane of chloroplast.

• Inputs are light and water.

• Light strikes photosystem II (P680).

• Electrons pass along until they reach primary electron acceptor.

• Photolysis occurs&mdashH2 O is split to H + and O2 .

• Electrons pass down an ETC to P700 (photosystem I), forming ATP by chemiosmosis.

• Electrons of P700 pass down another ETC to produce NADPH.

• Three products of light reactions are NADPH, ATP, and O2 .

• Oxygen produced comes from H2 O.

(Cyclic ) light-dependent reactions

• Occur in thylakoid membrane.

• Only involves photosystem I no photosystem II.

• ATP is the only product of these reactions.

• No NADPH or oxygen are produced.

• These reactions exist because the Calvin cycle uses more ATP than NADPH this is how the difference is made up.

Light-independent reactions (Calvin cycle )

• Occurs in stroma of chloroplast.

• Inputs are NADPH, ATP, and CO2 .

• First step is carbon fixation, which is catalyzed by an enzyme named rubisco.

• A series of reactions lead to the production of NADP + , ADP, and sugar.

• More ATP is used than NADPH, which creates the need for the cyclic light reactions.

• The carbon of the sugar product comes from CO2 .

C4 plants &mdashplants that have adapted their photosynthetic process to more efficiently handle hot and dry conditions.

C4 photosynthesis &mdashprocess that first converts CO2 into a 4-carbon molecule in the mesophyll cells, converts that product to malate, and then shuttles it to the bundle sheath cells, where the malate releases CO2 and rubisco picks it up as if all were normal.

CAM plants &mdashplants that close their stomata during the day, collect CO2 at night, and store the CO2 in the form of acids until it is needed during the day for photosynthesis.

Photosynthesis

1 . Which of the following do CAM and C4 plants have in common?

(A) They are known to survive well in excessively moist environments.

(B) They readily bind carbon dioxide.

(C) They produce sugar more efficiently than do C3 plants.

(D) They produce ATP less efficiently than do C3 plants.

2 . All of the following are directly involved in photosystems EXCEPT

3 . Which of the following processes occurs in both cellular respiration and photosynthesis?

4 . Which of the following processes represents an anaerobic pathway that produces ATP less efficiently than do oxygen-driven processes?

Answers and Explanations

1 . B &mdashBoth plants provide alternatives to carbon fixation and readily attach to carbon dioxide molecules.

2 . D &mdashRubisco is the only choice not directly involved in photosystems. Rubisco is an enzyme that catalyzes the first step of the Calvin cycle in C3 plants.

3 . D &mdashChemiosmosis is the process by which the formation of ATP is driven by electrochemical gradients in the cell. This process occurs in both respiration and photosynthesis.

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Limiting Factors in Photosynthesis

Some factors affect the rate of photosynthesis in plants, as follows

  • Temperature plays a role in affecting the rate of photosynthesis. Enzymes involved in the photosynthetic process are directly affected by the temperature of the organism and its environment
  • Light Intensity is also a limiting factor, if there is no sunlight, then the photolysis of water cannot occur without the light energy required.
  • Carbon Dioxide concentration also plays a factor, due to the supplies of carbon dioxide required in the Calvin cycle stage.

Overall, this is how a plant produces energy which supplies a rich source of glucose for respiration and the building blocks for more complex materials. While animals get their energy FROM food, plants get their energy FROM the sun.

The next tutorial investigates DNA structure and replication…


Watch the video: The simple story of photosynthesis and food - Amanda Ooten (August 2022).