cell membrane

Question 1

describe the fluid mosaic model of membranes .

Answer 1

• made up of bilayer of phospholipids with proteins + cholesterol interspersed throughout structure .
• fluid = individual phospholipids constantly moving - flexible shape membrane
• mosaic = protein molecules scattered = extrinsic + intrinsic proteins of diff sizes embedded

Question 2

explain the role of cholesterol and glycolipids in membranes .

Answer 2

• cholesterol = lipid which slots in between phospholipid tails - pushing them closer together = regulates stability + fluidity of plasma membrane
• glycolipids = phospholipids with sugar molecules attached = act as recognition sites + antigens = increase membrane stability by forming hydrogen bonds with water molecules

Question 3

explain the functions of extrinsic and intrinsic proteins in membranes .

Answer 3

EXTRINSIC
- found on surface of plasma membrane - function as enzymes and catalyse chemical reactions.
- binding sites / receptors
- involved in cell signaling
- bind cells together

INTRINSIC
- span both bolsters of plasma membrane - act as channel or carrier proteins to transport water soluble molecules .
- electron carriers

Question 4

explain the functions of membranes within cells.

Answer 4

• provide internal transport system.
• selectively permeable to regulate passage of molecules into / out of organelles
• provide reaction surface
• isolate organelles from cytoplasm - specific metabolic reactions .

Question 5

explain functions of the cell-surface membrane .

Answer 5

• isolates cytoplasm from extracellular environment.
• selectively permeable to regulate transport of substances
• involved in cell signalling/recognition.

Question 6

explain the function of phospholipids .

Answer 6

• consists of hydrophilic head group faces intracellular/ extracellular fluid + 2 hydrophobic tails which point towards each other away from water.
• form a barrier to anything which is not lipid soluble (ions/glucose )

Question 7

name and explain 3 factors that affect membrane permeability

Answer 7

• temperature = high temp denatured membrane proteins / phospholipid molecules have more kinetic energy + move further apart
• pH = changes tertiary structure of membrane proteins.
• use of solvent = may dissolve membrane

Question 8

explain the link between temp and membrane permeability

Answer 8

• temp below freezing - permeability of cell membrane increases as proteins in membrane unfold + become deformed
• become closely packed together so rigid membranes
• 0-45 degrees = membranes are partially permeable . components in membrane gain kinetic energy and move around more.
• exceed 45 - permeability increases rapidly as proteins in membrane become denatured + starts unravelling .

Question 9

outline how calorimetry used to investigate membrane permeability .

Answer 9

1) use plant tissues with soluble pigment in vacuole - tonoplast + cell surface membrane disrupted = high permeability —> pigments diffuses in solution

2) select calorimeter filter with complementary colour

3) use distilled water to set calorimeter to 0. measure absorbance

4) high absorbance / low transmission = more pigment in solution.

Question 10

define osmosis.

Answer 10

• water diffuses across semi/permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established .

Question 11

what is water potential ? (ψ)

Answer 11

• the tendency of water to leave a solution + measure in kPa
• ψ of pure water at 25 degrees + 100kPa : 0
• more solute = ψ more negative .
• addition of solute decreases water protection + applying pressure increases water potential

Question 12

how does osmosis affect plant + animal cells ?

Answer 12

osmosis INTO cell =
- plant : protoplast swells = cell turgid
- animal : lysis

osmosis OUT of cell =
- plant : protoplast shrinks = cell flaccid
- animal : crenation

Question 13

suggest how a student could produce a desired concentration of solution from a stock solution?

Answer 13

• volume of stock solution = required concentration x final vol needed / concentration of stock solution
• vol of distilled water = final volume needed - volume of stock solution

Question 14

define simple diffusion .

Answer 14

• passive process requires no energy from ATP hydrolysis
• net movement of small, lipid-soluble molecules directly through phospholipid bilayer from area of high concentration to area of lower concentration (down conc gradient )

Question 15

what does the rate of simple diffusion depend on?

Answer 15

• the concentration gradient = the steeper the gradient, the faster the rate of diffusion
• the thickness of the exchange surface = thicker exchange surfaces mean a longer diffusion distance
• surface area = larger surface areas mean more space for diffusion to take place = adaptions like microvilli in small intestine increases SA

Question 16

define voltage-gated channels .

Answer 16

• channels that respond to voltage changes
• other channel may respond to other signals (arrival of particular hormone at cel surface membrane .)

Question 17

define gated channels .

Answer 17

• channels that can be open or closed
• in nerve cells = changes in electrical charge make sodium ion channels open/close

Question 18

define facilitated diffusion .

Answer 18

• passive process.
• specific channel or carrier proteins with complementary binding sites transport large / polar molecules/ions (not soluble in hydrophobic phospholipid tail) down conc gradient .

Question 19

give example of a facilitated diffusion

Answer 19

• movement of glucose molecules into liver cells through glucose transporter proteins embedded in plasma membrane .

Question 20

what does the rate of facilitated diffusion depend on?

Answer 20

• the conc gradient - the steeper the gradient , the faster the rate of diffusion.
• no. channel/carrier proteins - more transport proteins , more faster substances can be moved across membrane .

Question 21

explain how channel and carrier proteins work.

Answer 21

• channel: hydrophilic channels bind to specific ions = one side of protein closes + other opens
• carrier: binds to complementary molecule = conformational change releases molecule on other side of membrane : in facilitated diffusion , passive process: in active transport. needs energy from atp hydrolysis

Question 22

name 5 factors that affect the rate of diffusion.

Answer 22

• temperature
• diffusion distance
• surface area
• size of molecule
• difference in concentration (how steep conc gradient is)

Question 23

name the factors affecting osmosis.

Answer 23

• water potential gradient - the steeper the gradient, the faster the rate of osmosis.
• the surface area - the larger the surface area, the more space there is for osmosis to take place .
• thickness of exchange surface - thinner surfaces mean a shorter diffusion distance

Question 24

how are cells adapted to maximise rate of transport across their membranes ?

Answer 24

• many carrier/channel proteins
• folded membrane increases surface area

Question 25

explain the difference between the shape of a graph of conc (x-axis) against rate (y-axis) for simple vs facilitated diffusion

Answer 25

• simple diffusion = straight diagonal line : rate of diffusion increases proportionally as conc increases.
• facilitated diffusion = straight diagonal line layer levels off when all channels/carrier proteins saturated

Question 26

define active transport.

Answer 26

• ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape
• specific carrier protein transport molecules from area low conc to area higher conc (against conc gradient)

Question 27

what does the rate of active transport depend on?

Answer 27

• no. carrier proteins in membrane - more proteins mean faster rate transport
• speed of each carrier protein - faster carrier proteins work, the faster rate of active transport
• the rate of respiration - faster respiration taking place, more ATP generated . inhibition of respiration also inhibit active transport

Question 28

compare + contrast active transport + facilitated diffusion .

Answer 28

• both may involve carrier proteins.
• active transport requires energy from ATP hydrolysis : facilitated diffusion is a passive process
• facilitated diffusion may involve channel proteins

Question 29

define co-transport .

Answer 29

• movement of substance against concentration gradient is with movement of another substance down its conc gradient.
• substances bind to complementary intrinsic protein:
  —> symport = transport substance in same direction
  —> antiport = transport substances in opposite direction (sodium potassium pump)

Question 30

explain how co-transport is involved in absorption of glucose in small intestine.

Answer 30

to maximise no. glucose molecules , gut replies on active transport
1) in epithelial cells lining, sodium potassium ion pump actively transport sodium ions out of cell

2) creates sodium ion gradient between ileum cell + intestinal lumen

3) co-transporter protein uses sodium ion gradient to transport glucose against its gradient
—> picks both molecules in lumen side + transport both into ileum cell

4) glucose move into bloodstream by facilitated diffusion via glucose channel