Chapter 7: Membrane Structure and Function Flashcards
selective permeability
some substances cross plasma membrane more easily
amphipathic
hydrophilic region and hydrophobic region
fluid mosaic model
membrane is a fluid structure with a mosaic of proteins (hydrophobic regions inside phospholipid bilayer with hydrophilic regions protruding out)
freeze fracture
cell is frozen and fractured with a knife. splits the center of phospholipid bilayer into two separated layers. Each membrane protein goes wholly into one of the layers.
membrane movement
membrane held together by hydrophobic interactions so most lipids (10^7 times per second) and some proteins shift laterally (not transversely).
membrane fluidity
membrane solidifies as temperature decreases. fluidity depends on type of lipids (saturated solidifies more easily than unsaturated, which has kinks in tails)
cholesterol
steroid that acts as membrane fluidity buffer. at high temps, restrains phospholipid movement. at low temps, prevent phospholipids from packing too close together.
effects of membrane fluidity
solidified membrane has different permeability and enzymes may become inactive. too fluid membranes cannot support protein function.
integral proteins
penetrate hydrophobic interior of lipid bilayer. majority are transmembrane. hydrophobic regions consist of 1+ stretches of nonpolar amino acids coiled into α helices.
transmembrane protein
span the membrane
peripheral proteins
loosely bound to surface of membrane (not embedded in lipid bilayer), often to exposed parts of integral proteins.
membrane protein attachments
cytoplasmic side: attach to cytoskeleton
extracellular side: attach to ECM
functions of membrane proteins
transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, attachment to cytoskeleton and ECM (coordinate extra and intra cellular changes)
glycolipids
lipids with covalently bound carbohydrates
glycoproteins
proteins with covalently bound carbohydrates
HIV infection
HIV binds to CD4 receptor on cell and also needs to bind to CCR5 coreceptor in order to infect cell
permeability of lipid bilayer
hydrophobic (nonpolar) molecules like O2, CO2, hydrocarbons, diffuse easily. hydrophilic (ions + polar) molecules diffuse slowly.
transport proteins
help hydrophilic substances cross lipid membrane
channel proteins
have hydrophilic channel that molecules/ions use as tunnel.
aquaporins
channel proteins for water. (3 * 10^9 water molecules per second)
carrier protein
bind to and change shape to shuttle molecule/ion across membrane
diffusion
movement of molecule so it spreads out evenly
concentration gradient
region along which density of substance increases/decreases
passive transport
energy is not expended
osmosis
diffusion of free water across selectively permeable membrane
tonicity
ability of surrounding solution to gain/lose water. depends on partly on its concentration of nonpenetrating solutes
isotonic
same tonicity. no net movement of water across plasma membrane
hypertonic
more nonpenetrating solutes. cell loses water
hypotonic
less nonpenetrating solutes. cell gains water
osmoregulation
control of solute concentration and water balance
turgor pressure
back pressure from rigid cell wall (plant cell) that opposes further water uptake
turgid
very firm. needs hypotonic solution
flaccid
limp. caused by hypertonic/isotonic solution
plasmolysis
as plant cell loses water, plasma membrane shrinks away from cell wall. causes plant to wilt.
facilitated diffusion
proteins assist with passive diffusion of impermeable substances
ion channels
channel proteins that transport ions
gated channels
channels that open or close in response to stimulus.
active transport
expends energy. enables cells to maintain internal concentrations of solutes.
cystinuria
absence of carrier protein that transports cysteine and other amino acids across membranes of kidney cells. causes painful kidney stones
sodium potassium pump
exchanges Na+ for K+ across the plasma membrane.
Steps:
1. 3 Na+ bind, triggers phosphorylation by ATP (reduces affinity for Na+, increases affinity for K+)
2. Na+ released outside the cell. 2 K+ bind, causing phosphate group to be released
3. 2 K+ released inside the cell, and because the protein shape is restored the cycle repeats.
voltage
electrical potential energy; separation of opposite charges
membrane potential
voltage across a membrane. negative, because inside of the cell is more negative than the outside
electrochemical gradient
combination of chemical force (concentration gradient) and electrical force (membrane potential) acting on an ion
electrogenic pump
transport protein that generates voltage across a membrane
animals: sodium-potassium pump
plants, fungi, bacteria: proton pump
proton pump
actively transports protons (H+) outside the cell
cotransport
active transport driven by a concentration gradient.
sucrose-H+ cotransporter
couples downhill diffusion of H+ into a plant cell to active transport of sucrose into the cell
exocytosis
transport vesicles fuse with plasma membrane to secrete molecules outside the cell
endocytosis
cell takes in substances by forming new vesicles from plasma membrane
Low-density lipoproteins
complex of lipids and proteins that transport cholesterol. binds to LDL receptors on plasma membranes and enter cell through endocytosis
familial hypercholesterolemia
high level of cholesterol in the blood because of defective LDL receptors
phagocytosis
cell engulfs particle by wrapping pseudopodia around it and forming a food vacuole (which fuses with a lysosome)
pinocytosis
cell “gulps” droplets of extracellular fluid (with molecules) into vesicles. nonspecific transport
receptor-mediated endocytosis
allows for bulk transport of specific substances
1. proteins with receptor sites exposed to extracellular fluid are embedded in the plasma membrane.
2. Ligands (for receptors) bind
3. Receptor proteins cluster in coated pits (lined with fuzzy proteins on cytoplasmic side)
4. after receptors are emptied, the vesicle returns them to plasma membrane
