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Flip-flopping in plasma membrane

Flip-flopping in plasma membrane



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Flip-flopping of lipids (and in proteins it is impossible) in plasma membrane is rare due to high energy barrier (video ref). However, it is an important mechanism since it allows asymmetric distribution of lipids in cell membrane.

Question: If a specific lipid is flipped E.g. phosphatidylinositol from cytoplasm side to extra-cellular side (E.g. by flippase), what mechanism can the cell do to ensure it will stay towards the extra-cellular fluid side but not flip-flop AGAIN (i.e. establish the asymmetry & enable its function)?


The Plasma Membrane

An intact membrane is essential to a cell. If the plasma membrane is disrupted, the cell loses its content and dies. The membrane is very important two vital functions are:

The regulation of the entrance and exit of molecules: The interior and exterior of the cell is mainly fluid. The membrane functions to keep the intracellular fluid constant despite molecules such as nutrients and waste constantly moving in and out.

Communication: The components of a membrane signal other cells as to what type of cell it is. It may also serve as receptors for various signal molecules that affect the cell’s metabolism.

Membrane Models:

At the beginning of the last century, scientist noted that lipid-soluble molecules entered the cells more rapidly than water-soluble molecules. This caused them to think that lipids were a component of the plasma membrane.

Later it was discovered that it consists of phospholipids and proteins. Phospholipids are lipids in which one of the fatty acid groups is replaced by H3PO4. The phosphoric acid is hydrophillic, the rest of the molecule is hydrophobic.

The ‘Fluid’ Membrane

A membrane is held together by weak hydrophobic interactions. Most membrane lipids are able to drift laterally within the membrane and occasionally flip vertically, known as ‘flip-flopping’. Phospholipids move quickly along the membrane plane, where as the proteins move relatively slowly.

Unsaturated hydrocarbon tails enhance membrane fluidity because the kinks at the carbon-carbon double bonds hinder close packing of the phospholipids. Membranes solidify at the critical temperature. This is lower in a membrane with a higher concentration of C=C bonds.

Cholesterol found in the plasma membranes of eukaryotes modulates membrane fluidity by keeping the membrane fluid in cold environments and solid in hot temperatures. Cells may also the concentration of unsaturated fats to better suit their environment.

Integral proteins, which are inserted into the membrane have hydrophobic regions, surrounded by the hydrophobic areas of the phhospholipids. Their hydrophillic ends are exposed at both sides of the membrane.

The proteins in the plasma membrane may provide a variety of major cell functions:

  • Transport
  • Intercellular joining
  • Enzymatic activity
  • Cell-cell recognition
  • Signal transduction
  • Attachment to the cytoskeleton and extracellular fliud.

Substances can be moved through the membrane via: Active Transport Diffusion and Osmosis.

Active Transport is the movement of molecules from an area of low solute concentration to an area of high solute concentration, against the concentration gradient, in a process that requires energy.

Diffusion is the passive movement of molecules from an area of high concentration to an area of low concentration.

Osmosis is the movement of water molecules from a region of high water concentration to a region of low water concentration through a semi-permeable membrane.

Diffusion is influenced by:

  • The permeability of the membrane
  • The shape and size of the molecule to be transported
  • Number of proteins on the cell surface
  • Concentration of molecules on either side of the membrane
  • Surface area of the membrane

Facilitated diffusion is a method of diffusion that uses proteins to transport substances that find it difficult to pass through a membrane. e.g. polar molecules. The proteins are known as carrier proteins.

Rate of diffusion is proportional to the surface area multiplied by the difference in concentration, all divided by the thickness of the membrane.

Rate of diffusion (fish) S urface area x Concentration gradient

Osmosis – Thermodynamics

1 molecule of water will move quickly if heat is applied, or if the water concentration in a solution is high.

If water molecules are moving from left to right, then the potential energy is greater on the left than on the right. The potential energy is known, in Biology, as water potential.

Water diffuses from an area of high water potential to a region of low water potential through a semi permeable membrane. Water potential can be regarded as the tendency of water to leave a solution.

If solute molecules are present, they always slow down the movement of the water molecules in a solution. The tendency of the water to leave the solution is reduced because water is always attracted to the solute.

Water Potential Gradient

Low Concentrated solution = -1000 kPa

Water potential is never positive. When the potential is more negative, water will flow into the cell.

Isotonic – two solutions are of the same water concentration, and as such there is no net movement of water.

Hypotonic – The water potential outside of the cell is greater than the intracellular potential. As such, there is a net inflow of water. The inside of the cell is more negative.

Hypertonic – The water potential inside of the cell is greater then the extracellular potential. As such, there is a net outflow of water. There inside of the cell is less negative.

Active Transport

Active transport requires energy in the form of ATP. it trasports molecules and ions in a direction that is not natural to the normal flow. This means that there will be many mitochondria present.

The following use ATP to transport molecules and ions:

1. Membrane pumps

  • An active transport mechanism that moves ions in order to obtain polarisation
  • For active transport two factors need to be considered: concentration and electrical charge.
  • Ions generally diffuse to form an area of high concentration to an area of low concentration and are attracted to regions with an opposite charge. Therefore we take into consideration both the concentration and elecrtochemical gradient.
  • Cells maintain a potential difference across the membrane. Many studies have shown that the inside of a cell is -ve and therefore cations are attracted and anions repulsed.
  • however, their relative concentrations inside and outside the of the cells helps to decide which way they move.
  • Three common ions to be transported are K+, Na+ and Cl-

1. Sodium Potassium pump

  • Cell surface membranes have pumps that are intrinsic proteins that span the membrane. The sodium pump removes Na+ from the cell. K+ is taken into the cell and so is coupled with the Na+ pump. It is therefore known as the Na+/P+ pump.
  • The pump requires more than one third of the ATP produced by a resting animal. It is very important.
  • The pump is essential for:
  1. controlling cell volume (osmoregulation)
  2. Maintaining electrical activity in nerve and muscle cells
  3. Driving active transport of other substances (e.g. sugars and amino acids.)

Active transport in the intestine:

Soon after feeding there is a high concentration of food in the gut. Absorption is mainly due to diffusion but it is very slow and so it is coupled with the active transport and the movement of Na+. As the sodium is actively transported out by the Na+/K+ pump, it will start to diffuse back in. A membrane rquires both Na+ and glucose and so another pump is used that transports glucose at the same time as Na+.


Flip-flopping in plasma membrane - Biology

1. Particles move across membranes by simple diffusion, facilitated diffusion, osmosis and active transport.

Concentration gradient: Molecules can diffuse across membranes from areas of higher to lower concentration by:

  • Simple diffusion: traveling directly through the membrane if they are small and uncharged, thus avoiding repulsion by the hydrophobic, non-polar tails of phospholipids in the middle of the membrane.
  • Osmosis = the passive movement of water molecules, across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.
    • hypertonic = higher solute concentration
    • hypotonic = lower solute concentration
    • isotonic = equal solute concentrations

    2. Application: Structure and function of potassium channels for facilitated diffusion in axons.

    • Facilitated diffusion: traveling through special transport proteins, if they match the shape and charge requirements to fit through the channels provided by the transport proteins.

    3. Application: Structure and function of sodium–potassium pumps for active transport and potassium channels for facilitated diffusion in axons.

    Against the concentration gradient: Moves substance from an area where it is in lower concentration to an area where it is in higher concentration.

    • Usually provided by ATP
    • Often by phosphorylating the protein pump as ATP is hydrolyzed

    4. Vesicles move materials within cells.

    • Protein synthesis: rER produces proteins which travel through the lumen of the ER
    • Transport in vesicles: Membranes produced by the rER flows in the form of transport vesicles to the Golgi, carrying proteins within the vesicles
    • Modification: Golgi apparatus modifies proteins produced in rER
    • Transport to membrane: Golgi pinches off vesicles that contain modified proteins and travel to plasma membrane
    • Exocytosis: Vesicles then fuse with plasma membrane, releasing their contents by

    5. The fluidity of membranes allows materials to be taken into cells by endocytosis or released by exocytosis.

    • Lipids move laterally in a membrane, but flip-flopping across the membrane is rare.
    • Unsaturated hydrocarbon tail of phospholipids have kinks that keep the molecules from packing together, enhancing membrane fluidity.
    • Cholesterol reduces membrane fluidity by reducing phospholipid movement at moderate temperatures but it also hinders solidification at low temperatures.

    6. Application: Tissues or organs to be used in medical procedures must be bathed in a solution with the same osmolarity as the cytoplasm to prevent


    Figure 1

    Figure 1. Lipid mixtures of DPPC/DLiPC/CHOL (a,d), DPPC/DOPC/CHOL (b,e), and DPPC/DOPC/DLiPC/CHOL (c,f). (a–c) Top and side views of the membrane organization after 30 μs of simulation without any restrictions on CHOL. DPPC, blue DOPC, purple DLiPC, red CHOL, yellow. The phospholipid headgroups are omitted for clarity. (d–f) Distributions of the Pearson correlation of the CHOL densities of the two leaflets evaluated for the last 20 μs of the simulations. Snapshots were taken every 500 ps without averaging.

    DPPC/DLiPC/CHOLDPPC/DOPC/CHOL
    w/o flip-flop w/o flip-flop
    w/flip-flopfb3.5fb1.6w/flip-flopfb3.5fb1.6
    average APL (nm 2 )b0.7360.7370.7380.6590.6600.659
    average area compressibility (mN/m)b399 ± 5407 ± 5411 ± 7389 ± 4392 ± 6391 ± 7
    average tail order DPPCc0.6340.6260.6390.5330.5340.535
    average tail order DLiPC/DOPCc0.2440.2450.2410.3800.3820.382
    average bilayer thickness (nm)c4.0714.0754.0614.1914.1904.190
    CHOL flip-flop rate (10 6 s –1 )b5.45 ± 0.080.00.01.77 ± 0.050.00.0

    All errors are standard errors and were omitted if ≤0.002.

    Averaged over the last 10 μs.

    Averaged over the last 2 μs.


    1.4 IB Bio

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    08 August 2014

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    By Mathieu Pinot , Stefano Vanni , Sophie Pagnotta , Sandra Lacas-Gervais , Laurie-Anne Payet , Thierry Ferreira , Romain Gautier , Bruno Goud , Bruno Antonny , Hélène Barelli

    Science 08 Aug 2014 : 693-697

    Certain membrane lipids adapt their conformation to membrane curvature, facilitating membrane deformation and fission.


    A Non-Hormonal Prenylation Activity of Farnesol/FLS

    Farnesol/FLS with a role as a hormone like in insects starts acting at the extracellular side of cells, at the contact site between the blood and the plasma membrane. Next it may diffuse into the intracellular membrane system (De Loof et al., 2014 De Loof, 2015, 2017). Yet, there is an equally important other possible mechanism of action, namely at the border between the cytoplasm and the plasma membrane with its numerous embedded helix bundle transmembrane proteins, in particular the GPCRs and their associated G-proteins. Here prenylation is the mechanism involved. Indeed, farnesyl- that is intracellularly synthesized in the mevalonate pathway, also has non-hormonal activity, as illustrated by its role in Ca 2+ -homeostasis (ੲ). GPCRs are key cell-surface proteins that transduce external environmental cues into biochemical signals across the membrane (Thal et al., 2018). They are intrinsically allosteric proteins that interact via spatially distinct yet conformationally linked domains with both endogenous and exogenous proteins, nutrients, metabolites, hormones, small molecules, and biological agents (Bondke Persson, 2013). This explains why they play such an important role in cell physiology and in endocrinology. Yet, their possible link with the mevalonate pathway is seldom mentioned in the literature.

    This paper advances arguments in favor of the view that such link may help to clarify how allosteric changes in a GPCR may finally result in activation of the two possible downstream pathways (see later and Figure 5). The influx of relatively larger amounts of Ca 2+ through canonical Ca 2+ channels is a major event, with important physiological impact. However, in addition to such Ca 2+ channels, there are also numerous transmembrane proteins in which an intramolecular microchannel exists, that upon being stretched by, e.g., ligand binding-dependent allosteric changes, allows some Ca 2+ or/and H + to enter the cytoplasm. Thus, in order to keep [Ca 2+ ]i low, such micro-channels must also be kept closed as much as possible.


    Egg Yolk

    Egg yolk is one of the common components of most semen cryopreservation extenders for domestic animals. Chicken egg yolk consistently improved the survival and the maintenance of the fertilizing ability of spermatozoa when included in the conservation medium (Trimeche et al., 1997). The fatty substances of the yolk are mostly triglycerides (true fat) 65.5%, phospholipids 28.3%, and cholesterol 5.2% (USDA, 2000). Egg yolk contains phospholipids, cholesterol and low density lipoproteins which are the factors that provide protection to sperm against cold shock during the freeze-thaw process (Kulaksız et al., 2010). The best quality gametes after cryopreservation are enriched in lipid, triacylglycerol and phospholipid (Cerolini et al., 2001).


    The most detailed picture yet of biomembrane asymmetry

    Living cells and many of their internal organelles are bounded by a membrane composed predominantly of lipids. These are molecules with a water-soluble head region and a water-insoluble tail region. As a result, they form a bilayer structure, with the tail regions facing each other and the head regions facing the water. Lipids come in a variety of types, and the lipid composition differs between the two sides of a biomembrane.

    Fig 1: Asymmetric lipid membrane, with a more ordered lipid species in the upper leaflet, and a more disordered lipid species in the lower one.

    While this asymmetry has been known and characterized to some extent since the 1970s, a recent paper published in Nature Chemical Biology by Lorent et al. takes this to a new level, offering detailed insight into the composition and asymmetry of cellular plasma membranes—the membranes surrounding cells.

    A single type of lipid is sufficient to make a lipid membrane. Given that, it is puzzling that cells build their biomembranes from mixtures of hundreds of different types of lipids. It is not clear why they do this. Moreover, membranes artificially created in the laboratory contain the same composition of lipids on its two sides, but biomembranes are often distinctly asymmetric in that regard. Again, the reason for this is not known. This structural complexity of cellular membranes is one of the big mysteries of cell biology, and one of the first steps of solving it is to gain a better understanding of this complexity.

    In the past, lipid molecules from membranes were identified using chromatography. This is similar to the following well-known experiment: place an ink drop at the bottom of a filter paper strip if one lets water wick through the strip, the drop is spread into multiple different colors, thereby revealing its composition. An analogous technique was used to separate different lipid types, but this becomes increasingly difficult for a large number of similar molecules. In the case of plasma membranes, this number is in the hundreds. Lorent et al. instead use the much more sensitive technique of mass spectrometry, which differentiates lipids by accelerating them through electric and magnetic fields.

    It has been known for several decades that the outer leaflet of the cell’s plasma membrane has a large number of lipids whose hydrocarbon tails do not contain any double bonds. Such lipids pack very well into ordered arrangements. In contrast, the inner leaflet of the plasma membrane hosts a much larger fraction of lipids which do have double bonds. These lipids tend to form much more disordered arrangements. Lorent et al. confirm this finding, but with the ability to distinguish lipid species with much finer granularity ( e.g. , tail lengths, number of double bonds, head types, etc .) than could previously be accomplished. This allows them to present a “lipidomic barcode”, which succinctly characterizes the precise composition of each leaflet of the membrane.

    Fig. 2: Lipid membrane with an embedded protein. Notice that the part of the protein embedded in the upper ordered bilayer is thinner than the part embedded in the lower disordered bilayer.

    In addition to lipids, biomembranes contain a myriad of proteins that either bind to the membrane surface (“peripheral”) or insert into it, passing through the bilayer from one side to the other (“transmembrane”). It was observed previously that the leaflet asymmetry of the plasma membrane is mirrored in the shape of the membrane-spanning portions of transmembrane proteins.

    Lorent et al. go significantly beyond this finding: since the shape of that transmembrane region can be readily inferred from the protein sequence, one can infer the asymmetry of biomembranes belonging to intracellular organelles that are difficult or impossible to isolate and probe directly. For instance, their paper shows that the asymmetry of endosomal or lysosomal membranes very closely resembles that of the plasma membrane. In contrast, other organelles, such as the endoplasmic reticulum or the Golgi apparatus, have proteins with symmetric transmembrane regions, suggesting that their host membranes are symmetric as well.

    Even more interestingly: Lorent et al. applied the same analysis to a wide variety of different organisms, such as amphibians, worms, and even fungi—a broad survey of “eukarya”, one of the three big domains of life. The plasma membranes in all these organisms were found to be asymmetric in fact, they closely matched the asymmetry of our own cells. This leads to a deep conclusion: while we do not yet know the ultimate reason or purpose for biomembrane asymmetry, the fact that it is so widely conserved across such a broad spectrum of organisms implies that it must be fundamentally important.

    This is an exciting result about the biological importance of membrane asymmetry, which underscores its ubiquity and poses a host of new questions. It also happens at a time where membrane biophysicists have devised several new experimental methods for artificially creating and examining asymmetric membranes. These provide a simplified test system for investigating the many ways in which asymmetric membranes differ from their symmetric counterparts, and how these differences could point us to the physiological purpose of asymmetry. For instance, it has been speculated that the higher degree of disorder in the inner leaflet of the plasma membrane makes it ideal for protein rearrangements and signaling, but at the same time the widely studied “lipid rafts”, believed to be signaling platforms, are formed in the outer leaflet. Uncovering how these connections work is important for understanding many human diseases that arise if such signaling goes wrong, which shows that exploring the physiological basis for asymmetry could help combat such diseases.


    Membrane structure and function. overview: life at the edge plasma membrane -boundary that separates.

    Concept 7.1: Cellular membranes are fluid mosaics of lipids and proteinsPhospholipids are the most abundant lipid in the plasma membranePhospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regionsThe fluid mosaic model states that a membrane is a fluid structure with a mosaic of various proteins embedded in it

    Figure 7.2Hydrophilic headHydrophobic tailWATERWATER

    Figure 7.3Phospholipid bilayerHydrophobic regions of proteinHydrophilic regions of protein

    In 1972, S. J. Singer and G. Nicolson proposed that the membrane is a mosaic of proteins dispersed within the bilayer, with only the hydrophilic regions exposed to water

    Freeze-fracture studies of the plasma membrane supported the fluid mosaic model

    Figure 7.4KnifePlasma membraneCytoplasmic layerProteinsExtracellular layerInside of extracellular layerInside of cytoplasmic layerTECHNIQUERESULTS

    The Fluidity of MembranesPhospholipids in the plasma membrane can move within the bilayerMost of the lipids, and some proteins, drift laterallyRarely does a molecule flip-flop transversely across the membrane

    Figure 7.6Lateral movement occurs 107 times per second.Flip-flopping across the membrane is rare ( once per month).

    Membranes- Temperature DependentAs temperatures cool, membranes switch from a fluid state to a solid stateThe temperature at which a membrane solidifies depends on the types of lipidsMembranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty acids

    Cholesterol is a BufferThe steroid cholesterol has different effects on membrane fluidity at different temperaturesAt warm temperatures (such as 37C), cholesterol restrains movement of phospholipidsAt cool temperatures, it maintains fluidity by preventing tight packing

    Figure 7.8FluidUnsaturated hydrocarbon tailsViscousSaturated hydrocarbon tails(a) Unsaturated versus saturated hydrocarbon tails(b) Cholesterol within the animal cell membraneCholesterol

    Evolution of Differences in Membrane Lipid CompositionVariations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditionsAbility to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary

    Membrane Proteins and Their FunctionsProteins determine most of the membranes specific functionsPeripheral proteins are bound to the surface of the membraneIntegral proteins penetrate the hydrophobic core a.k.a.-transmembrane proteins

    Figure 7.9N-terminus helixC-terminusEXTRACELLULAR SIDECYTOPLASMIC SIDE

    Six major functions of membrane proteinsTransportEnzymatic activitySignal transductionCell-cell recognitionIntercellular joiningAttachment to the cytoskeleton and extracellular matrix (ECM)

    Figure 7.10EnzymesSignaling moleculeReceptorSignal transductionGlyco- proteinATP(a) Transport(b) Enzymatic activity(c) Signal transduction(d) Cell-cell recognition(e) Intercellular joining(f) Attachment to the cytoskeleton and extracellular matrix (ECM)

    Concept 7.2: Membrane structure results in selective permeabilityA cell must exchange materials with its surroundings, a process controlled by the plasma membranePlasma membranes are selectively permeable, regulating the cells molecular traffic

    The Permeability of the Lipid BilayerHydrophobic (nonpolar) molecules can dissolve in the lipid bilayer and pass through the membrane rapidlyex. O2, CO2

    Polar molecules, do not cross the membrane easilyEx. Glucose, water

    Concept 7.3: Passive transport is diffusion across a membrane --no energy investmentDiffusion is the tendency for molecules to spread out evenly into the available spaceSubstances diffuse down their concentration gradient, ie- higher to lower concentrationCalled-passive transport because no energy is expended by the cell to make it happen

    Figure 7.13aMolecules of dyeMembrane (cross section)WATER(a) Diffusion of one soluteNet diffusionNet diffusionEquilibrium

    Facilitated Diffusion: Passive Transport Aided by ProteinsIn facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membraneChannel proteins includeAquaporins, for facilitated diffusion of waterIon channels that open or close in response to a stimulus (gated channels)

    Transport ProteinsTransport proteins allow passage of hydrophilic substances across the membraneChannel proteins- act like a tunnelEx. aquaporins facilitate the passage of water

    Carrier proteins, bind to molecules and change shape to shuttle them across the membrane- specific for molecule

    Figure 7.17EXTRACELLULAR FLUIDCYTOPLASMChannel proteinSoluteSoluteCarrier protein(a) A channel protein(b) A carrier protein

    Effects of Osmosis on Water BalanceOsmosis is the diffusion of water across a selectively permeable membrane

    Figure 7.14Lower concentration of solute (sugar)Higher concentration of soluteSugar moleculeH2OSame concentration of soluteSelectively permeable membraneOsmosis

    Water Balance of Cells Without WallsTonicity is the ability of a surrounding solution to cause a cell to gain or lose waterIsotonic solution: Solute concentration is the same as that inside the cell no net water movement across the plasma membraneHypertonic solution: Solute concentration is greater than that inside the cell cell loses waterHypotonic solution: Solute concentration is less than that inside the cell cell gains water

    Figure 7.15Hypotonic solutionOsmosisIsotonic solutionHypertonic solution(a) Animal cell(b) Plant cellH2OH2OH2OH2OH2OH2OH2OH2OCell wallLysedNormalShriveledTurgid (normal)FlaccidPlasmolyzed

    Osmoregulation, the control of solute concentrations and water balance, is a necessary adaptation for life in such environments

    The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump

    Figure 7.16Contractile vacuole50 m

    Concept 7.4: Active transport uses energy to move solutes against their gradientsActive transport moves substances against their concentration gradients

    Active transport requires energy, usually in the form of ATP

    Active transport is performed by specific proteins embedded in the membranes

    Sodium-Potassium Pump[Na+] higher outside cell[K+ ] higher inside cellThe sodium-potassium pump is one type of active transport system- maintains this concentration gradient

    Figure 7.18-6EXTRACELLULAR FLUID[Na] high[K] low[Na] low[K] highCYTOPLASMNaNaNaNaNaNaNaNaNaKKKKKKPPPP iATPADP

    Figure 7.19Passive transportActive transportDiffusionFacilitated diffusionATP

    How Ion Pumps Maintain Membrane PotentialMembrane potential is the voltage difference across a membraneVoltage is created by differences in the distribution of positive and negative ions across a membrane

    Two combined forces, collectively called the electrochemical gradient, drive the diffusion of ions across a membraneA chemical force (the ions concentration gradient)An electrical force (the effect of the membrane potential on the ions movement)

    An electrogenic pump is a transport protein that generates voltage across a membraneThe sodium-potassium pump is the major electrogenic pump of animal cellsThe main electrogenic pump of plants, fungi, and bacteria is a proton pumpElectrogenic pumps help store energy that can be used for cellular work

    Figure 7.20CYTOPLASMATPEXTRACELLULAR FLUIDProton pumpHHHHHH

    Cotransport: Coupled Transport by a Membrane ProteinCotransport occurs when active transport of a solute indirectly drives transport of other solutes Plants commonly use the gradient of hydrogen ions generated by proton pumps to drive active transport of nutrients into the cell

    Figure 7.21ATPHHHHHHHHProton pumpSucrose-H cotransporterSucroseSucroseDiffusion of H

    Concept 7.5: Bulk transport across the plasma membrane occurs by exocytosis and endocytosisSmall molecules and water enter or leave the cell through the lipid bilayer or via transport proteinsLarge molecules, such as polysaccharides and proteins, cross the membrane in bulk via vesiclesBulk transport requires energy

    ExocytosisIn exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contentsMany secretory cells use exocytosis to export their products

    EndocytosisIn endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membraneEndocytosis is a reversal of exocytosis, involving different proteinsThere are three types of endocytosisPhagocytosis (cellular eating)Pinocytosis (cellular drinking)Receptor-mediated endocytosis

    Figure 7.22SolutesPseudopodiumFood or other particleFood vacuoleCYTOPLASMPlasma membraneVesicleReceptorLigandCoat proteinsCoated pitCoated vesicleEXTRACELLULAR FLUIDPhagocytosisPinocytosisReceptor-Mediated Endocytosis

    **For the Cell Biology Video Structure of the Cell Membrane, go to Animation and Video Files.

    *Figure 7.2 Phospholipid bilayer (cross section).*Figure 7.3 The original fluid mosaic model for membranes.**Figure 7.4 Research Method: Freeze-fracture**Figure 7.6 The movement of phospholipids.***Figure 7.8 Factors that affect membrane fluidity.***Figure 7.9 The