2B - Cell Membranes (Diffusion, Osmosis etc.)

Question 1

What 3 ways can substances move across the cell-surface membrane?

Answer 1

• diffusion
• osmosis
• active transport

Question 2

What are cell membranes composed of?

Answer 2

• lipids
• proteins
• carbohydrates

Question 3

What is the ‘fluid mosaic’ structure of cell membranes?

Answer 3

• phospholipids form bilayer; constantly moving
• channel and carrier proteins are scattered through the bilayer
• receptor proteins on cell surface membrane allow chemical detection from the cell
• some proteins move sideways
• glycoproteins and glycolipids are also present

Question 4

What are the different roles of phospholipids in cell membranes?

Answer 4

• head is hydrophilic, tail = hydrophobic
• arrange themselves into a bilayer; heads face outwards
• centre is hydrophobic; no water soluble substances can pass through it

Question 5

What are the different roles of cholesterol?

Answer 5

• gives membrane stability
• bind to the tails of phospholipids
• membrane is more rigid

Question 6

What is diffusion?

Answer 6

• movement of particles from an area of high concentration to low concentration

Question 7

Why is diffusion considered a passive process?

Answer 7

• no energy is required for it to happen

Question 8

What is simple diffusion?

Answer 8

• molecules diffuse directly through cell membrane

Question 9

What is meant by facilitated diffusion?

Answer 9

• larger molecules diffuse through proteins in the membrane

Question 10

What do channel proteins and carrier proteins do during facilitated diffusion?

Answer 10

• CHANNEL form pores in membrane for charged molecules to diffuse
• CARRIER move large molecules across membranes

Question 11

What affects the rate of simple diffusion?

Answer 11

• concentration gradient
• thickness of the exchange surface
• surface area (microvilli increases SA, diffusion is faster)

Question 12

What affects the rate of facilitated diffusion?

Answer 12

• concentration gradient

- no. of carrier and channel proteins (aquaporins are special channel proteins)

Question 13

What is osmosis?

Answer 13

• diffusion of water molecules from an area of high water potential to low water potential

Question 14

What affects the rate of osmosis?

Answer 14

• water potential gradient (higher, faster the rate)
• thickness of exchange surface
• surface area

Question 15

What is meant by active transport?

Answer 15

• uses energy to move molecules across membranes

- usually against a concentration gradient

Question 16

Which protein is used to active transport?

Answer 16

• carrier

Question 17

What are the carrier proteins, ‘co-transporters’?

Answer 17

• bind two molecules at a time

- gradient of one molecule is used to move the other molecule against its own concentration gradient

Question 18

What affects the rate of active transport?

Answer 18

• speed of individual carrier proteins
• no. of carrier proteins present
• rate of respiration in the cell + ATP availability