Chapter 8. Membrane Structure and Function

• Membrane Structure.

– Membrane models have evolved to fit new data.

– Membranes are Fluid.

– Membranes are mosaics of structure and function.

– Membrane carbohydrates are important for cell-cell interactions.

• Traffic Across Membranes.

– Membranes are selectively permeable.

– Passive transport is diffusion across a membrane.

– Osmosis is the passive transport of water.

– Cell survival depends on balancing water uptake and loss.

– Specific proteins can facilitate passive transport.

– Active transport is pumping of solutes against their concentration gradient.

– Some ion pumps generate voltage across membranes.

– In cotransport, a membrane protein couples transport of two solutes.

– Exocytosis and endocytosis transport large molecules.

• Membrane Structure.

– The plasma membrane is the edge of life and like all membranes it is selectively permeable, that is it allows some substances to cross more easily than others.

– Staple ingredients of a membrane are lipids (phospholipids) and proteins.

• Phospholipids are amphipathic meaning it has a hydrophobic portion and a hydrophilic portion.

• Proteins and lipids interact in the fluid mosaic model.

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• Membranes are abundant and essential.

• Isolate cellular environments from the outside and from each other.

• The hydrophobic bilayer makes them good barriers (Fig 8.1).

• Membranes do more than define compartments they process material energy and information.

– Including communication with the extracellular environment and other cells.

– Membrane models have evolved to fit new data.

• Data from many experimenters have shown us that membranes are a bilayer (Fig 8.1) allowing for them to be barriers and that proteins are embedded in the membrane (Fig 8.3).

• All of the information has led us to our current fluid mosaic model.

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» The proteins have hydrophobic domains that pass through the membrane and hydrophilic regions on both ends.

– Membranes are fluid.

• Membranes are not static sheets locked rigidly into place.

• Lipids and proteins can flow laterally quite easily even occasionally flipping (Fig 8.4).

– Why would flipping only occur rarely?

– Protein movement was demonstrated by a cell fusion experiment (Fig 8.5).

• Unsaturated fatty acids in a membrane decrease its ability to packed easily (by temp) where saturated hydrocarbons pack more easily.

• Cholesterol is an important part of animal membranes it keeps them fluid at low temperatures and yet provides some rigidity.

– Helps counter saturated hydrocarbons.

• Cell fusion experiment:

– Two cells are fused (one from a human one from a mouse) in this experiment and the membrane proteins are analyzed to see if they will mix with the proteins of the other cell.

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– Some proteins do not move about the membrane because they are anchored to the cytoskeleton.

– Other proteins seem to have directed movement depending on chemical signals.

– Membranes are mosaic of structure and function.

• A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer.

• There are two types of membrane proteins: integral and peripheral.

– Integral membrane proteins penetrate the phospholipid bilayer, most are transmembrane meaning they pass from one side of the membrane to the other.

– Peripheral membrane proteins are not embedded in the bilayer, but are attached to exposed parts of integral membrane proteins or phospholipids.

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– Example: Internal membranes of the mitochondria and chloroplast have specialized proteins for energy production.

• The two surfaces of a membrane have different properties due to the asymmetric distribution of transmembrane proteins.

– The different domains of a protein have different properties and transmembrane proteins can have different domains on either side of the membrane.

– Peripheral proteins are either on one side or another of the membrane.

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• There are also regional differences.

• What decides if a membrane protein is peripheral or integral?

– Amino acid side chains have different properties: some are hydrophobic or hydrophilic.

– Integral membrane proteins often have long a-helical sections that are hydrophobic.

» These areas are perfect for lying in a membrane.

» Hydrophilic and hydrophobic forces keep the integral membrane protein in place.

– How does the integral membrane protein get in the membrane?

» Specialized sequences during translation signal for the protein to be inserted in the membrane.

– Some peripheral membrane proteins have specific sequences for the addition of specialized lipids that get inserted into the membrane to keep the protein peripheral to the membrane.

– Membrane carbohydrates are important for cell to cell recognition.

• Human red blood cell plasma membrane consists of (by weight) 40% lipid, 52 percent protein and 8 percent carbohydrates.

• The carbohydrates are on the outer surface of the membrane and serve as recognition sites.

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• Carbohydrates can recognize specific extracellular compounds.

– These carbohydrates are attached to a protein or lipid.

– The carbohydrates protrude from the lipid or protein into the extracellular environment.

• Carbohydrates are bound to lipids and proteins as glycolipids and glycoproteins respectively.

• Glycolipids are important in recognizing cancerous cells for destruction by the body.

– They change their components and are recognized by white blood cells for phagocytosis.

• Most of the carbohydrate is bound to proteins.

– They are bound as oligosaccharide side chains.

– Added to the proteins inside the ER and modified in the Golgi.

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– Great variety of carbohydrates due to the variety of monomers.

• Traffic across Membranes.

– A membrane’s molecular organization results in selective permeability.

• Cells must maintain a constant movement of material back and forth across the membrane.

• Not all things move across the membrane equally, selective permeability.

• Hydrophobic molecules (CO₂ and O₂) easily pass through the membrane.

• Transport of ions or polar molecules (hydrophilic) is tightly regulated.

• Certain proteins (transport proteins) play a vital role in controlling the movement of that which cannot pass the hydrophobic barrier.

– Transport proteins may actively help molecules across the membrane or form a simple pore for things to diffuse through.

– Movement through the membrane can be an active or passive process.

– Passive transport is diffusion across a membrane.

• Diffusion is the tendency for molecules to move from an area of high concentration to an area of low concentration.

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– This is considered down the concentration gradient.

– This requires no work and increases entropy (random mixture).

• Even though the movement is random the net movement of particles is directional until equilibrium is reached.

• Diffusion of a substance across a biological membrane is called passive diffusion.

• The passive transport includes: Simple diffusion and facilitated diffusion (uses proteins).

– Osmosis is passive transport (diffusion) of water.

• The movement of water depends on the amount of solute particles present (not the type).

• Osmotic potential is based in large part on solute concentration (Fig 8.11).

• If two solutions have identical osmotic potentials they are isostonic no matter their chemical composition.

• If the two solutions are not isosmotic then the solution with a high solute concentration is hypertonic to the other solution which is hypotonic.

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• Hyper/hypoosmotic solutions can cause cells to shrink or expand (sometimes burst) respectively (Fig 8.12).

– Cell survival depends on balancing water uptake and loss.

• Movement of water across a membrane and the balance between water and the environment is very important.

• Organisms without cell walls have the ability to control water balance, osmoregulation.

• Cells with cell walls have an ability to withstand bursting when placed in a hypoosmotic solution due to turgor pressure.

– Driving force for growth in plant cells.

• Plant cells can go from turgid (normal) all the way to flaccid (limp) (Isotonic) and still survive due to their cell wall.

• Plants can die in a hypertonic solution due to their PM coming apart from the cell wall, plasmolysis.

– Specific proteins facilitate passive transport of water and selected solutes.

• Many polar molecules need help in order to diffuse.

• Passive diffusion with the help of a transport protein is called facilitated diffusion.

– Aquaporins are channel proteins involved in helping water (highly polar) across the membrane.

• Some channel proteins form simple pores for polar molecules to move through while others act as gated channels.

– Gated channels respond to a stimulus to open and close (no energy directly involved).

– In fact binding of a solute molecule may cause the channel to change conformation allowing the solute to pass (8.14 b)).

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• Facilitated diffusion can saturate, when there is more solute than available carriers.

– Active Membrane Transport is the pumping of solutes against their concentration gradient.

• Passive transport always moves molecules from areas of high concentration to areas of low concentration.

• Active transport can move molecules from low à high (“uphill”).

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• Includes specific membrane transporters as well as systems involved in membrane fusion.

• Extremely important in keeping cellular concentrations of small molecules (ions) different inside the cell than outside.

• One common example is the sodium-potassium pump which expels sodium and brings potassium inside the cell (Fig 8.15).

• The sodium-potassium pump, an integral membrane glycoprotein found in all animal cells.

– The breakdown of ATP into ADP and Phosphate (P_i) is coupled to the movement of 3Na⁺ ions out of the cell and 2K⁺ ions into the cell.

– The movement of Na⁺ out and K⁺ in by this protein classifies it as an antiporter.

– Different pumps are responsible for the transport of several other ions but only cations are transported by primary active transport.

– Other solutes are transported by secondary active transport.

– Some ion pumps generate voltage across membranes.

• Movement of anions and cations set up a membrane potential across the membrane.

– The inside of the cell is negatively charged when compared to the outside and supplies a potential of –50 to –200 millivolts (negative because the inside of the cell is negative).

• Since the cytoplasm is negative cations are more likely to diffuse into the cell and anions out of the cell.

• Thus there are two forces acting on an ion the concentration gradient and the chemical gradient this combination is called the electrochemical gradient.

• Electrogenic pumps are those involved in generating voltage across a membrane (Ex: Na/K pump).

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– Cotransport results from a membrane protein coupling the transport of two solutes.

• A single ATP-powered pump that transports a specific solute can indirectly drive the active transport of several other solutes.

– Analogous to water being pumped up a hill then doing work as it flows back down.

– Ex: Proton pump that drives sucrose across a membrane as it moves back across the membrane.

– Exocytosis and endocytosis transport large molecules.

• Exocytosis is the process by which materials packaged in vesicles are secreted from the cell.

– The vesicle membrane fuses with the PM (from the Golgi) the phospholipids of the two membranes merge and opening to the extracellular environment develops.

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– Used for the secretion of cell waste, enzymes, neurotransmitters and other cellular secretions.

• There are three forms of endocytosis: phagocytosis, pinocytosis and receptor-mediated endocytosis.

– All three involve invagination (infolding) of the plasma-membrane making a small pocket.

– This deepening forms a vesicle containing contents from the extracellular environment.

• Phagocytosis is a feeding process found in unicellular protists and white blood cells (often the engulfing of an entire cell.

– The phagosome that is formed often fuses with a lysosome.

• Pinocytosis (cell drinking) uses smaller vesicles and brings in dissolved particles.

• Receptor-mediated endocytosis involves specialized reactions that take place at the membrane and trigger uptake.

– Receptor-mediated endocytosis is highly specific.

» Similar to other endocytosis events except receptors on the PM bind to specific substances in the extracellular environment.

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– These coated pits are depressions of the PM with receptors on the extracellular surface and coated on the cytoplasmic surface with the protein clathrin.

– After the receptor(s) bind the appropriate substrate the coated pit invaginates and forms a clathrin coated vesicle.

» Clathrin may act to strengthen and stabilize the vesicle.

– Once inside the cell the vesicle may lose its coat and fuse with other vesicles.

– Receptor-mediated endocytosis is much more rapid and efficient method of taking up substances out of the cellular environment.

• One example of receptor-mediated endocytosis is how cholesterol gets into mammalian cells.

– Cholesterol is synthesized in the liver and transported in the blood as a lipoprotein.

– This low-density lipoprotein or LDL is taken up into cells by attachment to a specific receptor in coated pits.

– These LDL particles are engulfed, the receptor is recycled and the LDL containing vesicle fused to a lysosome.

– The LDL particle is digested and the cholesterol is made available to the cell.

– A deficient receptor for LDL leads to dangerously high levels of cholesterol in the blood, hypercholesterolemia.