3.2.3 Transport across cell membranes (cell membrane structure)

Question 1

What is the role of cell-surface membranes?

Answer 1

They are a barrier between the cell and its environment and control what enters and leaves the cell.

Question 2

What does ‘partially permeable’ mean?

Answer 2

It means the membrane lets some molecules through but not others.

Question 3

What processes allow substances to move across the cell-surface membrane?

Answer 3

Diffusion osmosis or active transport.

Question 4

What do membranes around organelles do?

Answer 4

They divide the cell into different compartments and act as a barrier between the organelle and the cytoplasm.

Question 5

What is the basic structure of all cell membranes?

Answer 5

They are composed of lipids (mainly phospholipids) proteins and carbohydrates.

Question 6

Who suggested the fluid mosaic model and when?

Answer 6

The fluid mosaic model was suggested in 1972.

Question 7

What is the arrangement of molecules in the fluid mosaic model?

Answer 7

Phospholipids form a bilayer that is constantly moving making the structure fluid.

Question 8

What is the role of cholesterol in membranes?

Answer 8

Cholesterol gives the membrane stability by binding to the phospholipid tails and making them pack more closely together.

Question 9

What are proteins’ roles in membranes?

Answer 9

Proteins form channels or carrier proteins to allow large molecules and ions to pass through the membrane.

Question 10

What do receptor proteins on the cell membrane do?

Answer 10

They allow the cell to detect chemicals released from other cells.

Question 11

What is a glycoprotein?

Answer 11

A protein with a polysaccharide (carbohydrate) chain attached.

Question 12

What is a glycolipid?

Answer 12

A lipid with a polysaccharide (carbohydrate) chain attached.

Question 13

How do phospholipids form a barrier to dissolved substances?

Answer 13

The phospholipid head is hydrophilic and the tail is hydrophobic forming a bilayer where the centre repels water-soluble substances.

Question 14

What makes cholesterol important for membranes without cell walls?

Answer 14

It provides stability particularly for cells like red blood cells that float in blood.

Question 15

How can membrane permeability be investigated in the lab?

Answer 15

By using beetroot cells which contain a pigment that leaks out when the membrane’s permeability increases.

Question 16

What equipment is needed to investigate membrane permeability using beetroot?

Answer 16

A scalpel cutting board test tubes water baths stopwatch measuring cylinder pipette colorimeter and computer with software.

Question 17

What is the relationship between temperature and membrane permeability?

Answer 17

As temperature increases permeability increases because phospholipids gain energy and move more.

Question 18

What happens to membranes below 0°C?

Answer 18

Phospholipids have low energy and the membrane becomes rigid but channel and carrier proteins deform increasing permeability.

Question 19

What happens to membranes between 0°C and 45°C?

Answer 19

Phospholipids move around and are not as tightly packed making the membrane partially permeable.

Question 20

What happens to membranes above 45°C?

Answer 20

The bilayer melts and carrier and channel proteins denature increasing permeability.

Question 21

Why are membranes described as ‘fluid mosaic’?

Answer 21

They are fluid because phospholipids are constantly moving and mosaic because proteins are scattered throughout the bilayer.

Question 22

Describe the arrangement of phospholipids in the cell-surface membrane of animal cells.

Answer 22

(phospholipids arranged in a) bilayer

OR

water is present inside and outside the cell

hydrophobic fatty acid tails are repelled by water

OR

hydrophilic phosphate heads are attracted to water

Question 23

Explain why the structure of the cell-surface membrane is described as fluid and mosaic.

Answer 23

Fluid:

phospholipids/proteins are free to move around (within the membrane)

Mosaic:

proteins are scattered/interspersed between the phospholipids