C2.1 Membrane Transport

Question 1

what is the structure + function of the cell membrane

Answer 1

Lipid bilayers basis of membrane and other amphipathic molecules
serves as a border between cell and environment controls passage of substances.

Question 2

What do phospholipid molecules contain

Answer 2

Phosphate “head”, two hydrocarbon “tails”

Question 3

Why does the membrane core have low permeability to hydrophilic particles, ions, polar molecules?

Answer 3

The core is formed through interactions between hydrophobic tails, hence why low permeability.

Question 4

Why may substances be drawn back to aqueous solution outside when reaching the core?

Answer 4

Hydrophobic hydrocarbon core doesn’t repel solutes, they are more attracted to each other.
solutes more attractedd to water.

Question 5

How does molecular size affect membrane permeability

Answer 5

larger the molecule, lower ermeability.

Question 6

What is diffusion?

Answer 6

Spreading out of particles in liquids and gases, particles are in random motion.
Net movement from high to low concentration, down the concentration gradient.

Question 7

Why does diffusion occur across membranes?

Answer 7

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.

Question 8

Why can ions/polar molecules cannot easily diffuse through

Answer 8

polar molecules have partial charges, so diffuse at low rates, ions cannot easily diffuse through due to hydrophobic membrane core.

Question 9

What are the two groups of membrane proteins?

Answer 9

Peripheral and Integral proteins.
proteins oriented in a way so that they can carry out function

Question 10

What are integral proteins?

Answer 10

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.

Question 11

what are peripheral proteins?

Answer 11

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.

Question 12

How does protein content of membrane vary?

Answer 12

variable as the function of membrane also varies.
More active a membrane, the higher its protein content.

Question 13

Why does osmosis occur?

Answer 13

• due to differences in concentration of substances dissolved in water. passive

Question 14

how does a solute dissolve in water?

Answer 14

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.

Question 15

Why can osmosis occur in all cells?

Answer 15

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.

Question 16

What can help ions and polar molecules to pass through phospholipids?

Answer 16

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

Question 17

facilitated diffusion

Answer 17

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.

Question 18

What are pump proteins?

Answer 18

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

Question 19

how do pump proteins differ from channel proteins?

Answer 19

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.

Question 20

What are the different conformations that pump proteins can interconvert to

Answer 20

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.

Question 21

What is a semi-permeable membrane?

Answer 21

Membrane allows passage of certain small solutes. freely permeable to solvent.

Question 22

what are properties of a cell membrane?

Answer 22

More selective and has variable permeability to water. Described as partially permeable, both semi and selectively permeable.

Question 23

selectively permeable membrane?

Answer 23

allows passage of particular particles, facilitated diffusion+active transport allow for selective permeability, as they are specific.

Question 24

What is a glycoprotein

Answer 24

conjugated proteins, carb as non-polypeptide component.
component of p membrane, protein part embedded and carb projected to external environment.

Question 25

what are glycolipids

Answer 25

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.

Question 26

role of glycoprotein/lipid

Answer 26

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,

Question 27

What is the function of glycoalyx?

Answer 27

Glycoalyx of adjacent cells can fuse binding clels together, preventing tissue from falling apart.

Question 28

What is the fluid mosaic model of membrane structure?

Answer 28

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.

Question 29

Why is fluidity of membrane reduced due to saturated fatty acids?

Answer 29

Straight chains, pack together tightly, giving high density, reduces flexibility and permeability.

Question 30

How unsaturated fatty acids differ from saturated fatty acids?

Answer 30

have one/more kinks in hcarbon chain, pack loosely, membrane more fluid, flexible+permeable.

Question 31

how do membranes attain its specific required properties?

Answer 31

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.

Question 32

what is cholesterol?

Answer 32

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.

Question 33

what happens if membrane is too fluid?

Answer 33

less able to control what substances pass through. Thus fluidity must be controlled

Question 34

what happens if too viscous/inflexible (membrane)

Answer 34

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'.

Question 35

what is a vesicle?

Answer 35

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

Question 36

how to make a vesicle

Answer 36

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.

Question 37

endocytosis

Answer 37

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.

Question 38

uses of a vesicle

Answer 38

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

Question 39

what happens when vesicles reach their destination?

Answer 39

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)

Question 40

exocytosis

Answer 40

discharging waste products/unwanted materials. e.g expulsion of water, through contractile vacuole. protein synthesis

Question 41

difference between secretion and expulsion

Answer 41

secretion, useful substance released expulsion useless

Question 42

in a growing cell how does the area of a plasma membrane increase?

Answer 42

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

Question 43

What are ion channels?

Answer 43

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

Question 44

What does a nerve impulse in neurons involve?

Answer 44

rapid movements of sodium and potassium ions across membrane. occur by facilitated diffusion, Na and K channels voltage gated.

Question 45

why are there voltages across a membrane? (neurons as example)

Answer 45

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.

Question 46

what does gating mechanism of K and Na Channels involve?

Answer 46

Reversible conformational changes with subunits. either in open position with narrow pore between, allowing for entry or closed position with no pore.

Question 47

what are subunits

Answer 47

protein subunit is a polypeptide chain or single protein molecule that assembles (or "coassembles") with others to form a protein complex.

Question 48

structure of K channel?

Answer 48

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.

Question 49

Why must Na an K channels be specific?

Answer 49

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.

Question 50

What are nicotinic acetylcholine receptors?

Answer 50

Acetylcholine, Neurotransmitter in many synapses. Many receptors for acetylcholine, nicotine + acetyl... bind to receptors.

Question 51

structure of nicotinic acetylcholine receptors?

Answer 51

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.

Question 52

What is required for a nerve impulse to be conveyed by neuron?

Answer 52

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

Question 53

Why is the pump an example of a exchange transporter?

Answer 53

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.

Question 54

what is the purpose of Na-Glucose cotransporter proteins?

Answer 54

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

Question 55

what does glucose absorption depend on?

Answer 55

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

Question 56

why do glucose transporters use atypical active transport?

Answer 56

energy us not used directly by the cotransporter, indirect/secondary active transport.

Question 57

how are cells in tissue linked by cell-to cell junctions?

Answer 57

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.

Question 58

How does cell adhesion help architecture of tissues/organs

Answer 58

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.