C2.1 Membrane Transport Flashcards
what is the structure + function of the cell membrane
Lipid bilayers basis of membrane and other amphipathic molecules
serves as a border between cell and environment controls passage of substances.
What do phospholipid molecules contain
Phosphate “head”, two hydrocarbon “tails”
Why does the membrane core have low permeability to hydrophilic particles, ions, polar molecules?
The core is formed through interactions between hydrophobic tails, hence why low permeability.
Why may substances be drawn back to aqueous solution outside when reaching the core?
Hydrophobic hydrocarbon core doesn’t repel solutes, they are more attracted to each other.
solutes more attractedd to water.
How does molecular size affect membrane permeability
larger the molecule, lower ermeability.
What is diffusion?
Spreading out of particles in liquids and gases, particles are in random motion.
Net movement from high to low concentration, down the concentration gradient.
Why does diffusion occur across membranes?
Can only happen as phospholipid bilayer is permeable to particles.
Non polar particles can diffuse through easily, e.g if higher concentration outside will passively diffuse inside.
Why can ions/polar molecules cannot easily diffuse through
polar molecules have partial charges, so diffuse at low rates, ions cannot easily diffuse through due to hydrophobic membrane core.
What are the two groups of membrane proteins?
Peripheral and Integral proteins.
proteins oriented in a way so that they can carry out function
What are integral proteins?
hydrophobic on surface, embedded in hydrocarbon chains in centre of membrane.
may fit in one of two phospholipid layers/extend across both (transmembrane, many are)
hydrophilic parts project through regions of phosphate heads.
what are peripheral proteins?
hydrophilic on surface, not embedded in membrane.
most attached to surface of integral proteins, attachment reversible.
some have hcarbon chain attached, inserted to membrane, acting as an anchor.
How does protein content of membrane vary?
variable as the function of membrane also varies.
More active a membrane, the higher its protein content.
Why does osmosis occur?
- due to differences in concentration of substances dissolved in water. passive
how does a solute dissolve in water?
forms intermolecular bonds.
Bonds restrict movement of water molecules.
Higher solute concentrations have lower concentration of water molecules that are free to move than lower solute concentration. Hence why net movement of low to high solute concentration.
Why can osmosis occur in all cells?
Water molecules small enough to pass through bilayer.
Some cells have aquaporin (Water channel, integral protein) greatly increasing membrane permeability to water, e.g kidneys, facilitates rapid movement of water.
What can help ions and polar molecules to pass through phospholipids?
Diffusion is possible with channel proteins, they are integral and transmembrane.
Protein with pore that connects cyto to aq solution outside.
diameter and chemical properties of its sides ensure only one type of particles passes through.
can pass either direction, no energy expended, high to low
facilitated diffusion
channel proteins required for diffusion to occur.
cells can select which substances diffuse in and out by types of channels synthesised
some closed/open, so permeability can be changed.
What are pump proteins?
carry out transport tasks, substance can be absorbed against concentration gradient/pump substances out
cells have many diff pumps, each transfer one specific type, allows cell to control content of cyto
how do pump proteins differ from channel proteins?
use energy, carry out active transport, diffusion passive.
can only move particles in one direction, pump proteins both
move against/down concentration gradient, whereas channel is down concentration gradient.
What are the different conformations that pump proteins can interconvert to
1) Transported particle enters pump from one side to reach central chamber/binding site.
2) Next conformation, ion/molecule passes out in opposite side of the membrane.
energy used to change from more stable to less stable conformation.
reverse change, no energy.
What is a semi-permeable membrane?
Membrane allows passage of certain small solutes. freely permeable to solvent.
what are properties of a cell membrane?
More selective and has variable permeability to water. Described as partially permeable, both semi and selectively permeable.
selectively permeable membrane?
allows passage of particular particles, facilitated diffusion+active transport allow for selective permeability, as they are specific.
What is a glycoprotein
conjugated proteins, carb as non-polypeptide component.
component of p membrane, protein part embedded and carb projected to external environment.
what are glycolipids
molecules consisiting of carbs linked to lipids, carb part is a monosaccharide. lipid part contains one/two hcarbon chains, fit into hydrophobic core of membranes.
carb projected outwards to extracellular environment.
role of glycoprotein/lipid
uses in cell to cell recognition, pathogens and foreign tissue can be destroyed
both form a carb rich layer on outer surface of plasma membrane of animal cells.
aq solution between gaps, this is glycocalyx,
What is the function of glycoalyx?
Glycoalyx of adjacent cells can fuse binding clels together, preventing tissue from falling apart.
What is the fluid mosaic model of membrane structure?
bilayer of phospholipids with proteins in variety of positions.
peripheral proteins, attached to inner/outer surface
integral proteins, embedded into bilayer
as phospholipid molecules are free to move laterally, proteins can also move.
Why is fluidity of membrane reduced due to saturated fatty acids?
Straight chains, pack together tightly, giving high density, reduces flexibility and permeability.
How unsaturated fatty acids differ from saturated fatty acids?
have one/more kinks in hcarbon chain, pack loosely, membrane more fluid, flexible+permeable.
how do membranes attain its specific required properties?
relative amounts of sat/unsat fatty acids regulated.
Must remain fluid, but be strong enough to avoid become perforated.
must be permeable but not too porous
ideal ratio depends on temp.
what is cholesterol?
makes up 20-40% of lipids in p membrane
steroid, most of molecule is hydrophobic.
attracted to centre
one end (-OH group) hydrophilic, attracted to phosphate heads, faces outwards
molecules positioned between phospholipids
intercalates (insert) between saturated.
helps maintain orderly arrangement of lipids, stabilises membrane at higher temps, maintaining impermeability to hydrophilic particles
ensures sat acids do not solidify at low temps.
what happens if membrane is too fluid?
less able to control what substances pass through. Thus fluidity must be controlled
what happens if too viscous/inflexible (membrane)
cell movement restricted+more lilely to burst, membranes do not correspond to any of the states of matter. They are in a ‘liquid-ordered phase’.
what is a vesicle?
small sac of membrane with droplet of fluid inside.
spherical
cells continuously make
transport their contents and then unmaking
possible due to fluidity of membrane, allows structures surrounded to change shape and move
how to make a vesicle
small region of membrane pulled and pinched off
membrane proteins carry out process using ATP
if vesicle made from plasma membrane, will contain material out of cell.
endocytosis
taking materials into the cell using vesicles.
Contain water+solutes out of cell that cannot pass across membrane
pathogens are taken in by endocytosis by white blood cells.
uses of a vesicle
used to move materials around cells, contents within.
or used to move proteins in the membrane of vesicle.
e.g protein synthesised by ribosomes on rER vesicles containing them bud off and carry to Golgi
what happens when vesicles reach their destination?
vesicles fuse with target membrane and disappear in the process.
transferring all contents of vesicle
if fuse with p membrane, contents expelled from cell (exocytosis)
exocytosis
discharging waste products/unwanted materials.
e.g expulsion of water, through contractile vacuole.
protein synthesis
difference between secretion and expulsion
secretion, useful substance released
expulsion useless
in a growing cell how does the area of a plasma membrane increase?
phospholipids synthesised and then inserted into rER membrane.
ribosomes on rER synthesise membrane proteins which are added to the membrane
vesicles bud off rER and move to p membrane, fuse + increase area by small amount
same method used to increase size of organelles
What are ion channels?
allow specific ions across membrane, in either direction.
higher to lower concentration
facilitated diffusion
able to open/close reversibly, allowing diffusion off/on
e.g in nerve fibres
What does a nerve impulse in neurons involve?
rapid movements of sodium and potassium ions across membrane.
occur by facilitated diffusion, Na and K channels voltage gated.
why are there voltages across a membrane? (neurons as example)
imbalance of pos/neg charges across.
neg voltages, more + charges outside
e.g if below -50mV, Na + K channels closed, if above, channels open, allowing Na diffusing in, voltage rises.
if reaches +40mV K channels open, K ions diffuse out.
what does gating mechanism of K and Na Channels involve?
Reversible conformational changes with subunits.
either in open position with narrow pore between, allowing for entry or closed position with no pore.
what are subunits
protein subunit is a polypeptide chain or single protein molecule that assembles (or “coassembles”) with others to form a protein complex.
structure of K channel?
4 subunits and extra globular protein (resembles ball) attached by flexible chain of aminos.
when in open conformation, ball can fit inside open pore and does so after pore opens.
ball remains in place until K channel returns to original closed state.
Why must Na an K channels be specific?
both carry + charge but unable to fit in different channel. K channels do not allow Na+ ions, pore is 0,.3nm wide, ions in K channel are slightly smaller than 0.3nm
- however, when bonded to shell of water molecules, too large to pass through.
- to pass, bonds between ion and water break and bonds form between ion and amino acids in narrow part of pore.
after passing it can be associated with shell of water
- na ions too small to form bonds with narrowest part of pore, so cannot shed their shell.
What are nicotinic acetylcholine receptors?
Acetylcholine, Neurotransmitter in many synapses. Many receptors for acetylcholine, nicotine + acetyl… bind to receptors.
structure of nicotinic acetylcholine receptors?
5 transmembrane subunits arranged symmetrically, binding site between two of subunits for acetylcholine.
binding causes conformation change, opening pore between subunits, cations then can pass (reversible)
diffusing into postsynaptic neuron, changing its voltage, causing voltage gated Na channel to open.
when dissociates, conformational change is reversed and pore is closed.
What is required for a nerve impulse to be conveyed by neuron?
concentration gradients of Na+ and K+ across membrane.
generated by active transport, using pump proteins.
follows a set of steps, where 3 na+ is pumped out, and two k+ is pumped in
requires 1 ATP
Why is the pump an example of a exchange transporter?
transports ions in different directions across membranes.
1) Pump is open to the inside, 3 Na+ can enter. and attach to sites,
2) ATP transfers phosphate group causing conformational change and closes pump
3) opens to outside, Na+ can exit
4) 2 k+ enters from outside, attached to sites,
5) binding causes release of phosphate, causing conformational change, closes pump
6) pump opens to the inside, iosn exit, increase k+ conc, More Na+ can enter.
what is the purpose of Na-Glucose cotransporter proteins?
transfer na+ and glucose together across p membrane into cell.
glucose, against concentration gradient, as na+ moves down
energy released by movement of na+ > energy needed to move glucose
e.g used in proximal tubule in the kidney, cells reabsorb glucose
what does glucose absorption depend on?
Na+ concentration outside > inside.
gradient maintained by active transport of na+ out of cell
na-k pumps in plasma membrane on inner side of cell, transfer, Na+ out, K+ in.
requires energy from ATP
why do glucose transporters use atypical active transport?
energy us not used directly by the cotransporter, indirect/secondary active transport.
how are cells in tissue linked by cell-to cell junctions?
junctions depend on cell adhesion molecules in plasma membranes of adjacent cells.
usually proteins with domains embedded in bilayer, and others portruding outwards.
junction formed by binding of CAMs in adjacent cells binding together their extracellular domains.
CAMs can be same/different, if different asymmetrical functioned produced.
How does cell adhesion help architecture of tissues/organs
maintains architecture, needed for functional relationships between adjacent cells.
some types of junction prevent extracellular movement in tissue, other types facilitate it.
has major roles in immune system. e.g tumours, prevents separation of cells, forming secondary tumours.
