Transport Across Cell Membranes

Question 1

Why is the cell surface membrane model said to be fluid and mosaic?

Answer 1

Molecules free to move laterally in phospholipid bilayer
Many components -phospholipids proteins glycoproteins and glycolipids

Question 2

Describe the structure of a phospholipid

Answer 2

Hydrophilic head, which is a attracted to water
Hydrophobic tail which is repelled by water

Question 3

Describe the function of phospholipids

Answer 3

Allows lipid soluble substances to enter/leave
Prevent water soluble substances entering/leaving
Makes membrane flexible and self healing

Question 4

Describe extrinsic proteins

Answer 4

Occur on the surface of the bilayer and don’t extend across it

Question 5

Describe the structure of intrinsic proteins

Answer 5

Channel for carrier proteins

Question 6

Describe the function of extrinsic proteins

Answer 6

Provide structural support
Act as channel supporting water soluble substances
Allow active transport

Question 7

What is the function of intrinsic proteins?

Answer 7

Cell surface receptors identify cells
Helps cell adhere together
Acts as receptors, e.g. hormones

Question 8

Describe the structure of cholesterol

Answer 8

Very hydrophobic molecule that pulls fatty acid tails off the phospholipids together

Question 9

What is the function of a cholesterol?

Answer 9

Reduces lateral movement of other molecules, including phospholipid
makes membrane decrease fluid at increase in temperature
prevents leakage of water and dissolved ions from cells

Question 10

What is the structure of glycolipid?

Answer 10

Made up of carbohydrate bonded with a lipid the carbohydrate extends from the bilayer

Question 11

What is the function of glycolipid?

Answer 11

Recognition sites, maintain stability, help cells attach to form tissues

Question 12

What is the structure of glycoprotein?

Answer 12

Carbohydrates attached to an extrinsic protein

Question 13

What is the function of a glycoprotein?

Answer 13

Recognition sites
maintain stability
allow cells to recognise one another

Question 14

Explain the arrangement of phospholipids in a cell membrane

Answer 14

Bilayer with water present on either side
Hydrophobic fatty acid tails repelled from water so point away from water
Hydrophilic phosphate heads attracted to water so point to water

Question 15

Explain the role of cholesterol in cell membranes

Answer 15

Restricts movement of other molecules making up membrane

So decreases fluidity and permeability and increases rigidity

Question 16

Suggest how cell membranes are adapted for other functions

Answer 16

Phospholipid bilayer is fluid -membrane can bend for vesicle formation/phagocytosis
Glycoproteins/ glycolipids act as receptors/ antigens-involved in cell signalling/ recognition

Question 17

Describe how movement across membranes occurs by simple diffusion

Answer 17

Lipid soluble(non polar) or very small substances for example oxygen
Move from an area of higher conc to an areas of lower con down a conc grad
Across phospholipid bilayer
Passive- doesn’t require energy from atp/ respiration (only Ek of substances)

Question 18

What are the limitations imposed by the nature of the phospholipid bilayer?

Answer 18

Restricts movement of water soluble (polar) and larger substances eg sodium/ glucose
Due to hydrophobic fatty acid tails in interior of bilayer

Question 19

Describe movement across membranes occurs by facilitated diffusion

Answer 19

Water soluble (polar) slightly larger substances
Moves down a concentration gradient
Three specific channel/carrier protein
Passive – doesn’t require energy from ATP/respiration

Question 20

Explain the role of carrier and channel proteins in facilitated diffusion

Answer 20

Shape/charge of protein determines which substances move
Channel
protein facilitate diffusion of water soluble substances
-hydrophilic pore filled with water
-may be gated can open/close
Carrier proteins facilitate diffusion of slightly larger substances
– complementary substance attaches to binding site
-Protein changes shape to transport substances

Question 21

Describe how movement across membranes occurs via osmosis

Answer 21

Water diffusers/move
From an area of high to no water potential/down the water potential gradient
Through a partially permeable membrane
Passive – doesn’t require energy from ATP/respiration (only kinetic energy of substances )

Question 22

Describe how movement across membranes occurs by active transport

Answer 22

Substances move from an area of lower to higher concentration/against a conc grad
Requiring hydrolysis of ATP and specific carrier proteins

Question 23

Describe the role of carrier proteins and the importance of hydrolysis of atp in active transport

Answer 23

Complementary substance binds to specific carrier protein

ATP binds, hydrolysed into ADP plus inorganic phosphate group releasing energy

carrier proteins change shape releasing substances on side of higher concentration

in organic phosphate group released – protein returns to original shape