A.P. Biology Chapter 7 Membrane Structure and Function
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- -Also called the plasmalemma.
- -Plasma Membrane = Phospholipid Bilayer + Transmembrane Proteins + Supporting Fibers + Glycoproteins and Glycolipids
- -Scientists studying the plasma membrane reasoned that it must be a phospholipid bilayer
- -Glycerol + 2 Fatty Acids + Phosphorylated Alcohol = Phospholipid
- -Hydrophilic or Polar Region = Phosphate
- -Hydrophobic or Nonpolar region = Fatty Acids
Davson-Danielli sandwich model of membrane structure
- -Stated that the membrane was made up of a phospholipid bilayer sandwiched between two protein layers.
- -Was supported by electron microscope pictures of membranes
Singer and Nicolson
- -Proposed that membrane proteins are dispersed and individually inserted into the phospholipid bilayer
Freeze-fracture studies of the plasma membrane
Supported the fluid mosaic model of membrane structure
- -Nonpolar interior prevents passage of water-soluble, polar compounds.
- -Only very small, uncharged molecules like O2 and H2O can enter through the lipid bilayer.
- -Also, allows nonpolar compounds to freely enter.
- -Fluid = Moving, Dynamic.
- -Each lipid can rotate, move laterally
- -Fluidity depends on temperature and type of fatty acid used.
- -Unsaturated fatty acids are more fluid.
- -Fluid Mosaic Model
Fluidity of Membranes
Phospholipids in the plasma membrane can move within the bilayer
The type of hydrocarbon tails in phospholipids affects the fluidity of the plasma membrane
Has different effects on membrane fluidity at different temperatures
Membrane Proteins and Their Functions
A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer
Transmembrane Proteins (Integral Proteins)
- -Part of the protein that extends through the bilayer is nonpolar (several nonpolar amino acids in this region).
- -Usually is an alpha helix or beta barrel.
- -Used to anchor protein in the membrane.
- -Beta-barrels = form a pore and are called a porin protein
- -Penetrate the hydrophobic core of the lipid bilayer
- -Are often transmembrane proteins, completely spanning the membrane
- -Channels = passive transport of molecules across membrane.
- -Carriers = transport of molecules against the gradient.
- -Receptors = transmit information into the cell.
- -Cell Adhesion Proteins = connect cells to each other.
- -Cytoskeleton Attachment Proteins = to attach actin.
Six major function of membrane proteins
- -Enzymatic activity
- -Signal transduction
- -Cell-cell recognition
- -Intercellular joining
- -Attachment to the cytoskeleton and extracellular matrix (ECM)
A protein that spans the membrane may provide a hydrophilic channel across the membrane that is selective for a particular solute. Other transport proteins shuttle a substance from one side to the other by changing shape. Some of these proteins hydrolyze ATP as an energy source to actively pump substances across the membrane
A protein built into the membranemay be an enzyme with its active site exposed tosubstances in the adjacent solution. In some cases,several enzymes in a membrane are organized asa team that carries out sequential steps of ametabolic pathway.
A membrane protein may havea binding site with a specific shape that fits the shapeof a chemical messenger, such as a hormone. Theexternal messenger (signal) may cause aconformational change in the protein (receptor) thatrelays the message to the inside of the cell.
Some glyco-proteins serve as identification tags that are specifically recognized by other cells.
Membrane proteins of adjacent cellsmay hook together in various kinds of junctions, such asgap junctions or tight junctions
Attachment to the cytoskeleton and extracellular matrix(ECM)
Microfilaments or other elements of thecytoskeleton may be bonded to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins. Proteins that adhere to the ECM can coordinate extracellular and intracellular changes
Proteins in the plasma membrane
- - Can drift within the bilayer
- -Experiment: Researchers labeled the plasma membrane proteins of a mouse cell and a human cell with two different markers and fused the cells. Using a microscope, they observed the markers on the hybrid cell.
- -Conclusion: The mixing of the mouse and human membrane proteins indicates that at least some membrane proteins move sideways within the plane of the plasma membrane.
Movement across the membrane
- -Diffusion = random motion of molecules that causes a net movement from areas of high concentration to areas of low concentration.
- -Osmosis = diffusion of water across a selectively permeable membrane
Factors that Affect the Direction of Diffusion
- -The concentration gradient; High to Low.
- -Temperature; High heat to Low Heat.
- -Pressure; High Pressure to Low Pressure
- -The steepness of the gradient
- -The molecular weight of the solute
- -Osmotic concentrations = concentrations of all solutes in a solution.
- -If unequal concentrations…
- -Hyperosmotic = solution with the higher solute concentration.
- -Hypoosmotic = solution with the lower solute concentration.
- -Isosmotic = solutions with the same osmotic or solute concentration
- -If a cell’s cytoplasm is hyperosmotic to the extracellular fluid, then water diffuses into the cell and it swells. Pressure of the cytoplasm pushing out against the membrane- hydrostatic pressure.
- -Osmotic pressure is the pressure needed to stop the osmotic movement of water across a membrane.
Osmotic Balanace in living things
- -Some oceanic eukaryotes adjust internal [solutes]- they are isosmotic.
- -Animals – circulate an isosmotic fluid around their cells. Must constantly monitor the fluid’s [solute] Ex. Humans secrete albumin into the plasma to match the body cells.
- -Protozoa- are hyperosmotic, so use extrusion to remove excess water; may have special organelles-contractile vacuoles.
- -Plants- are hyperosmotic, but do not circulate an isosmotic solution; are usually under osmotic pressure- turgor pressure-presses the plasma membrane against the cell wall.
Water balance in cells with walls
Plant cell. Plant cells are turgid (firm) and generally healthiest ina hypotonic environ-ment, where theuptake of water iseventually balancedby the elastic wallpushing back on thecell.
Bulk movement through membranes
- -Endocytosis- the cytoskeleton extends the membrane outward toward food particles. Bulk transport into cell.
- -Extended membrane encircles the particle, fuses with itself, and contracts.
- -Forms a vesicle around particle.
Three Types of Endocytosis
- -Receptor-mediated endocytosis
- -A cell engulfs a particle by wrapping pseudopodia around it and packaging it within a membrane-enclosed sac large enough to be classified as a vacuole. The particle is digested after the vacuole fuses with a lysosome containing hydrolytic enzymes.
- -“cell eating”- large, amounts of organic material;white blood cells and protists.
- -The cell “gulps” droplets of extracellular fluid into tiny vesicles. It is not the fluid itself that is needed by the cell, but the molecules dissolved in the droplet. Because any and all included solutes are taken into the cell, pinocytosisis nonspecific in the substances it transports.
- -“cell drinking”- liquid material brought into cell; mammalian ova and follicle cells.
- -Enables nables the cell to acquire bulk quantities of specific substances, even though those substances may not be very concentrated in the extracellular fluid. Embedded in the membrane are proteins with specific receptor sites exposed to the extracellular fluid. The receptor proteins are usually already clustered in regions of the membrane called coated pits, which are lined on their cytoplasmic side by a fuzzy layer of coat proteins. Extracellular substances (ligands) bind to these receptors. When binding occurs, the coated pit forms a vesicle containing the ligand molecules. Notice that there are relatively more bound molecules inside the vesicle, other molecules are also present. After this ingested material is liberated from the vesicle, the receptors are recycled to the plasma membrane by the same vesicle.
- -Use receptors in the membrane for specific transport into cell.
- -Have indentations on the plasma membrane.
- -Indentations = are clathrin-coated pits.
- -Pits have receptor proteins on the extracellular side = trigger
- -When receptor binds to target molecule, clathrin proteins on the cytoplasmic side begins endocytosis.
- -Forms a clathrin -coated vesicle.
- -Very specific.
- -The release of material from vesicles at the cell surface.
- -Examples: protists using a contractile vacuole to release water, gland cells secreting hormones, neurons releasing neurotransmitters.
Problems with Bulk Transport
- -Endocytosis and Exocytosis are energy-intensive.
- -Not highly selective.
Selectively Permeable Transport: Channels
- -Proteins in the cell membrane that transport specific ions into and out of the cell.
- -Water-filled pores span the membrane.
- -Ions do not interact with the channel protein.
- -Diffusion is passive, [high] --> [low].
- -Provide corridors that allow a specific molecule or ion to cross the membrane
Selectively Permeable Transport: Carriers
- -Proteins that transport specific ions, sugars, and amino acids into and out of the cell.
- -The proteins facilitate movement by binding to the solute-facilitated diffusion.
- -The proteins bind to the solute on one side of the membrane and release them on the other.
- -Undergo a subtle change in shape that translocates the solute-binding site across the membrane
- -It is specific, only certain molecules transported by a given carrier.
- -It is passive, net movement is [high]-->[low].
- -It may become saturated if all protein carriers are occupied
- -A method of transporting specific ions, sugars, amino acids, nucleotides against conc. gradient.
- -Involves protein carriers in the membrane and energy (ATP).
- -How cells accumulate molecules internally.
- -Is one type of active transport system
- -Active transport of Na+ and K+ ions.
- -Normally, inside the cell: the [Na+] is low and the [K+] is high
- -The cell maintains this by actively pumping Na+ out and K+ in.
- -The protein uses ATP as an energy source for this movement against the gradient [low]--> [high].
- -See Fig. 6.21, p. 135 for how.
- -Uses ~1/3 of all ATP in resting cell.
- -This pump can transport 300 Na+ ions/second.
- -All animals use it.
Active transport driven by a concentration gradient
Cotransport and Countertransport
- -Many amino acids and sugars are transported into the cell through coupled channels.
- -Their active transport is coupled with the movement of Na+ inside the cell.
- -[Na+] high --> [Na+] low into cell.
- -[Amino acid] low--> [Amino acid] high.
Is a transport protein that generates the voltage across a membrane
- -A transmembrane protein that moves H+ against their concentration gradient, from [low] --> [high] outside of cells or organelles.
- -Example: mitochondria move H+ across the inner membrane during electron transport.
- -Energy to power this pump comes from NADH and FADH molecules
- -The proton pump moves H+ out of the matrix, through the inner membrane.
- -ATP synthase channels H+ back into the matrix, DOWN the gradient. PROVIDES EVERGY!
- -ATP synthesis is coupled to H+ movement.
- -Almost all of the energy for cells is made this way.
Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. The rate of diffusion can be greatly increased by transport proteins in the membrane.
Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.
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