2B Cell Membranes

Question 1

What is the function of cell surface membranes?

Answer 1

> Act as barriers between the cell and the external environment
Controls which substances can enter/leave the cell

Question 2

What is the cell surface membrane also known as?

Answer 2

The plasma membrane

Question 3

What processes carry substances across membranes?

Answer 3

Diffusion, osmosis and active transport

Question 4

What is the function of membranes within cells?

Answer 4

> Divide the cell into its orgnelles and acts as a barrier between the organelles and cytoplasm
Control what enters/leaves the organelles

Question 5

Suggest a function of a membrane surrounding a chloroplast.

Answer 5

To compartmentalise photosynthetic reactions so that they are more efficient

Question 6

Suggest a function of a membrane surrounding a bacterial cell.

Answer 6

To control what substances can enter/leave the cell (as it is partially permeable)

Question 7

What are cell membranes composed of?

Answer 7

Phospholipids, proteins, carbohydrates

Question 8

Describe the fluid mosaic model.

Answer 8

> PHOSPHOLIPIDS form a BILAYER = FLUID because phospholipids are constantly moving
It is scattered with PROTEINS = CHANNEL, CARRIER and RECEPTOR proteins
Some proteins move sideways through the structure whereas others are fixed
Contains GLYCOPROTEINS = proteins w/ carbs attached
Contains GLYCOLIPIDS = lipids w/ carbs attached
Contains CHOLESTEROL

Question 9

Why is the phospholipid bilayer described as fluid?

Answer 9

Because the PHOSPHOLIPIDS are constantly moving

Question 10

Describe the movement of proteins within the bilayer.

Answer 10

> Some are in a fixed position

> Some move sideways

Question 11

Explain why a cell membrane is an effective barrier against water-soluble substances.

Answer 11

The centre contains the HYDROPHOBIC TAILS of the PHOSPHOLIPIDS meaning that water-soluble substances can’t pass through

Question 12

How does the cell surface membrane control what enters/leaves the cell?

Answer 12

Contains proteins which allow some large/charged molecules to enter/leave the cell that wouldn’t be able to cross the membrane otherwise

Question 13

Describe the role of cholesterol in a cell membrane.

Answer 13

> Makes the membrane LESS FLUID and MORE STABLE
Maintains the SHAPE of ANIMAL cells
Creates a BARRIER to POLAR substances

Question 14

Describe how you could investigate the effect of temperature on membrane permeability.

Answer 14

> Cut equal sized beetroot cubes and place each one in a test tube
Add the same volume of water to each tube and then put each tube into a different water bath set at different temperatures and leave them there for the same length of time
Remove the beetroot cubes from each tube and use a colorimeter to measure the absorbance of the remaining liquid
This will indicate how much pigment has been released by each cube and therefore how permeable the membrane is at the temperature tested

Question 15

What is the structure of a phospholipid?

Answer 15

Glycerol, phosphate group & fatty-acid tail (2 fatty acid chains)

Question 16

What CAN pass through the phospholipid bilayer?

Answer 16

SMALL, NON-POLAR MOLECULES and water (by OSMOSIS)

Question 17

What CAN’T pass through the phospholipid bilayer?

Answer 17

WATER-SOLUBLE substances (Eg. ions and polar molecules)

> Water is a polar molecule so can’t pass through unless it diffuses by osmosis

Question 18

What is a polar molecule?

Answer 18

A molecule with one end that has a positive charge and one with a negative charge

Question 19

How does cholesterol provide membrane stability?

Answer 19

It fits between and BINDS to the HYDROPHOBIC TAILS of the phospholipids which causes them to move CLOSER TOGETHER and causes the membrane to become more RIGID

Question 20

What type of molecule is cholesterol?

Answer 20

A lipid

Question 21

What cell membranes can cholesterol be found in?

Answer 21

All cell membranes except bacterial membranes

Question 22

Why is cholesterol important in animal cells?

Answer 22

To provide stability and maintain their shape as they don’t have cell walls

Question 23

How do temperature below 0 degrees affect membranes?

Answer 23

> there is LITTLE ENERGY so LESS MOVEMENT occurs
Molecules are packs more closely and are RIGID
CHANNEL/CARRIER PROTEINS DENATURE - they lose their structure and function making the membrane MORE PERMEABLE
ICE CRYSTALS can form and PIERCE the membrane which makes it HIGHLY PERMEABLE when it thaws

Question 24

How do temperatures between 0 and 45 degrees affect membranes?

Answer 24

> Phospholipids can move and aren’t packed as tightly
Greater temp = more movement which INCREASES PERMEABILITY
However, the membrane remains PARTIALLY PERMEABLE

Question 25

How do temperatures above 45 degrees affect membranes?

Answer 25

> The phosphopipid bilayer BREAKS DOWN making it MORE PERMEABLE
Water inside the cell EXPANDS which applies PRESSURE to the membrane
CHANNEL/CARRIER PROTEINS DENATURE - there is therefore no contorl over what enters/leaves the cell and PERMEABILITY INCREASES