2B Cell Membranes

Question 1

What are cell membranes used for?

Answer 1

Compartmentalisation and control of substances

Question 2

How do substances move in and out of membranes and by which processes?

Answer 2

Membranes are partially permeable so they allow movement of substances by diffusion, osmosis or active transport

Question 3

What are membranes usually made out of?

Answer 3

Lipids (primarily phospholipids), proteins and carbohydrates

Question 4

What model explains membranes?

Answer 4

The fluid mosaic model

Question 5

What do the phospholipids in a membrane form?

Answer 5

A fluid, double layer called a bilayer

Question 6

What creates the mosaic structure of a membrane?

Answer 6

Proteins that are found in the phospholipid bilayer

Question 7

What are the several types of protein in the bilayer?

Answer 7

Channel and carrier proteins, receptor proteins and glycoproteins

Question 8

What do channel and carrier proteins within a membrane do?

Answer 8

They allow molecules and ions to pass through the membrane

Question 9

What do receptor proteins within a membrane do?

Answer 9

They allow the cell to detect chemicals released by other cells

Question 10

What do glycoproteins combine with within a membrane?

Answer 10

They are combined with sugars

Question 11

What are glycolipids and what are they involved with within membranes?

Answer 11

They are lipids combines with sugars and they are involved in cell recognition and immune response

Question 12

What substance found within membranes is responsible for the flexibility of the membrane?

Answer 12

Cholesterol, it spread phospholipids apart to create flexibility

Question 13

How do phospholipids form a barrier to water-soluble compounds?

Answer 13

• the hydrophilic heads and hydrophobic tails arrange into a bilayer
• the centre of the molecule is hydrophobic so water-soluble substances can’t cross bilayer (including ions and polar molecules)
• small, non-polar molecules can still pass through

Question 14

Explain how cholesterol adds stability to membranes

Answer 14

It fits in the gaps between the phospholipids and binds to the hydrophobic tails which forces them closer together and restricts the movement of the phospholipid, therefore making it more rigid

Question 15

What is cholesterol’s added stability especially important for within membranes?

Answer 15

Maintaining the shape of animal cells, especially in free-floating cells eg. Red blood cell

Question 16

What is the difference between glycolipids and glycoproteins?

Answer 16

Glycolipids are lipids attached to carbohydrates whereas glycoproteins are proteins attached to carbohydrates

Question 17

What are the uses for glycolipids and glycoproteins?

Answer 17

They are involved in cell recognition- some are antigens.
Glycolipids are a receptor for hormones

Question 18

Why do cold water fish have more cholesterol in their cell membranes than warm water fish?

Answer 18

More cholesterol in membranes leads to more flexibility as cold membranes would be more rigid which isn’t beneficial

Question 19

Define diffusion

Answer 19

The net movement of particles from an area of high concentration to an area of low concentration

Question 20

Describe diffusion and what it involves

Answer 20

A passive process that involves molecules or ions

Question 21

What factors can affect the rate of diffusion?

Answer 21

• concentration gradient: bigger difference = faster
• thickness of exchange surface: thinner = faster
• surface area of exchange surface: bigger = faster
• temperature: warmer = faster

Question 22

What is facilitated diffusion?

Answer 22

They way that large or charged (polar) molecules diffuse through membranes. They diffuse through channel or carrier proteins along conc. gradients. Passive process

Question 23

What is a channel protein’s role in facilitated diffusion?

Answer 23

Forms pores in membrane which allows charged particles to diffuse through them

Question 24

What is a carrier protein’s role in facilitated diffusion?

Answer 24

Moves large molecules through membrane. The molecule attaches on one side, changes the shape of the protein and is forced out the other side

Question 25

What factors affect the rate of facilitated diffusion?

Answer 25

• concentration gradient: bigger difference = faster
• number of channel/carrier proteins: more = faster

Question 26

When would the rate of facilitated diffusion be limited?

Answer 26

If all the carrier/channel proteins became saturated, the rate of facilitated diffusion couldn’t increase

Question 27

Define osmosis

Answer 27

The net movement of water molecules from an area of higher water potential to and area of lower water potential across a partially permeable membrane

Question 28

What is the value of water potential in pure water?

Answer 28

0

Question 29

When adding solutes to pure water, what happens to the value of water potential?

Answer 29

It become negative

Question 30

Where does water move to in osmosis in terms of values of water potential?

Answer 30

The most negative value

Question 31

What does isotonic mean in terms of membranes?

Answer 31

2 concentrations are the same either side of the membrane

Question 32

What does hypotonic mean in terms of membranes?

Answer 32

solution with a higher water potential than inside the cell (cell will swell)

Question 33

What does hypertonic mean in terms of membranes?

Answer 33

solution with a lower water potential than inside the cell (cell will shrivel)

Question 34

What factors can affect the rate of osmosis?

Answer 34

• water potential gradient: bigger difference = faster
• thickness of exchange surface: thinner = faster
• surface area of exchange surface: bigger = faster

Question 35

What is a co-transporter?

Answer 35

A type of carrier protein that binds to 2 molecules at once

Question 36

What happens in active co-transport?

Answer 36

The conc. gradient of one molecule is used to move the other molecule across the membrane

Question 37

Give an example of active co-transport

Answer 37

Sodium-glucose co-transport mechanism:
In the ileum (small intestine) the conc of glucose is too low for diffusion so is absorbed alongside sodium