1
Q
amphipathic
A
having both a hydrophilic and a hydrophobic region
2
Q
fluid mosaic model
A
The currently accepted model of cell membrane structure, which envisions the membrane as a mosaic of protein molecules drifting laterally in a fluid bilayer of phospholipids.
3
Q
lateral movement of phospholipids
A
- adjacent phospholipids switch positions about 10^7 times per second
- a phospholipid can travel about 2 nm in 1 second
- flip-flopping of phospholipids directly across from each other occurs only about once per month
4
Q
integral proteins
A
- penetrate the hydrophobic interior of the lipid bilayer
- majority are transmembrane proteins (span the membrane); others extend only partway into the hydrophobic interior
- the hydrophobic regions of an integral protein consist of 1 or more stretches of nonpolar amino acids, usually coiled into ALPHA HELICES
- some proteins have a hydrophilic channel thru their center that allows passage of hydrophilic substances
5
Q
peripheral proteins
A
- not embedded in lipid bilayer at all
- they are appendages loosely bound to the surface, often to exposed parts of integral proteins
6
Q
membrane carbohydrates
A
- usually short, branched chains of <15 sugar units
- some are covalently bonded to lipids, forming glycolipids
- most are covalently bonded to proteins (glycoproteins)
7
Q
channel proteins
A
- hydrophilic channels
- ex: aquaporins – each one allows entry of up to 3 billion water molecules per second, passing single file thru its central channel, which fits 10 at a time
8
Q
carrier proteins
A
- transport proteins that hold onto their passengers and change shape in a way that shuttles them across the membrane
- undergo subtle change in shape that somehow translocates the solute-binding site across the membrane
- such a change may be triggered by the binding and release of the transported protein
- PASSIVE TRANSPORT! DOWN concentration gradient!
9
Q
tonicity
A
- The ability of a solution surrounding a cell to cause that cell to gain or lose water.
- depends in part on its concentration on nonpenetrating solutes (can’t cross membrane) relative to that inside the cell
10
Q
isotonic
A
same concentration of nonpenetrating solutes inside and outside of the cell
11
Q
cholesterol and membrane fluidity
A
- high temps: cholesterol makes the membrane less fluid by restraining phospholipid movement
- b/c cholesterol also hinders the close packing of phospholipids, it lowers the temp required for the membrane to solidify
- cholesterol is a “fluidity buffer” – resisting changes in membrane fluidity that can be caused by changes in temp
12
Q
osmoregulation
A
Regulation of solute concentrations and water balance by a cell or organism.
13
Q
water balance of cells w/ walls
A
- hypotonic environment: plant cell swells as water enters by osmosis. the inelastic wall will expand only so much before it exerts a back pressure on the cell, called TURGOR PRESSURE, that opposes further water uptake. at this point, the cell is TURGID (very firm; healthy state). nonwoody plants depend on this mechanism for mechanical support
- isotonic environment: no net tendency for water to enter –> cells become flaccid
- hypertonic: cell will lose water to surroundings and shrink; as the cell shrinks, the plasma membane pulls away from the wall. this phenomenon, called plasmolysis, causes wilting and maybe death. (the walled cells of bacteria and fungi also plasmolyze in hypertonic environments).
14
Q
ion channels
A
- channel proteins that transport ions
- many ion channels function as GATED CHANNELS
- PASSIVE TRANSPORT! DOWN concentration gradient!
15
Q
gated channels
A
- open/close in response to a stimulus
stimuli: - electrical
- when a specific substance other than the one to be transported binds to the channel
16
Q
facilitated diffusion
A
The passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure. (SPEEDS transport; DOESNT ALTER DIRECTION)
17
Q
cystinuria
A
- human disease characterized by the absence of a carrier protein that transports cysteine and some other amino acids across the membranes of kidney cells
- kidney cells normally reabsorb these amino acids from the urine and return them to the blood, but an affected individual develops painful stones from amino acids that accumulate and crystallize in the kidneys
18
Q
active transport
A
- the transport proteins involved are all CARRIER, not channel
- ATP supplies the energy for most active transport
- one way: ATP transfers its terminal phosphate group directly to the transport protein. this can induce the protein do change its in a manner that translocates a solute bound to the protein across the membrane
19
Q
Na-K pump
A
- 3 Na out for every 2 K in
- compared w/ its surroundings, an animal cell has a much higher concentration of K+ and a much lower concentration of Na+
20
Q
electrogenic pump
A
- a transport protein that generates voltage across a membrane
- the Na-K pump appears to be the major electrogenic pump of aminal cells
- of plants, fungi, bacteria: PROTON PUMP, which actively transports protons (H+) out of the cell. this transfers positive charge from the cytoplasm to the extracellular solution.
- by generating voltage across membranes, electrogenic pumps help store energy that can be tapped for cellular work.
21
Q
membrane potential
A
- the voltage across a membrane
- ranges from -50 to -200 millivolts (mV)
22
Q
cotransport
A
The coupling of the “downhill” diffusion of one substance to the “uphill” transport of another against its own concentration gradient. (a single ATP-powered pump that transports a specific solute that can indirectly drive the active transport of several other solutes)
23
Q
diarrhea and cotransport
A
- normally, Na in waste is reabsorbed in the colon, maintaining constant levels in the body, but diarrhea expels waste so rapidly that reabsorption isn’t possible, and Na levels fall precipitously
- patients are given a sol’n to drink containing high concentrations of NaCl and glucose; the solutes are taken up by Na-glucose cotransporters on the surface of intestinal cells and passed thru the cells into the blood
24
Q
transport of small particles vs transport of large particles
A
- water and small solutes enter and leave the cell by diffusing thru the lipid bilayer of the plasma membrane OR by being pumped/moved across the membrane by transport proteins
- large molecules (ex: proteins, polysaccharides) cross the membrane in bulk by mechanisms that involve packaging in vesicles. these processes require energy.
25
exocytosis
- many secretory cells use this process to export products
- a transport vesicle that has budded fro the Golgi moves along microtubules of the cytoskeleton to the plasma membrane
- when vesicle membrane and plasma membrane come into contact, specific proteins rearrange the lipid molecules of the 2 bilayers so that the 2 membranes fuse
- the contents of the vesicle spill to the outside of the cell; the vesicle membrane becomes part of the plasma membrane
26
endocytosis
3 types: phagocytosis, piniocytosis, receptor-mediated endocytosis
27
endocytosis example
- human cells use receptor-mediated endocytosis to take in cholesterol for membrane synthesis and the synthesis of other steroids
- cholesterol travels in the blood in particles called low-density lipoproteins (LDLs), each a complex of lipids and a protein
- LDLs bind to LDL receptors on plasma membranes and then enter the cells by endocytosis
28
ligand
- any molecule that binds specifically to a receptor site on another molecule, usually a larger one
29
hypercholesterolemia
- inherited disease
- very high level of cholesterol in the blood
- LDLs can't enter cells b/c LDL receptor proteins are defective or missing, so cholesterol accumulates in the blood, where it contributes to early atherosclerosis
