1.3 Phospholipid bilayer (plasma membrane)

Question 1

Why is the plasma membrane described as a fluid mosaic model?

Answer 1

Fluid: the phospholipids and proteins are continually moving relative to each other.

Mosaic: the proteins are randomly arranged within the membrane.

Question 2

Why do the phospholipids arrange themselves in a bilayer?

Answer 2

The outer phosphate heads are attracted to the water (in the extracellular and intracellular fluid.)
The lipid tails are hydrophobic and are repelled by water.

Question 3

Why is cholesterol a critical component of the plasma membrane?

Answer 3

Cholesterol controls membrane fluidity in different environmental conditions

Hot conditions - holds phospholipids
together preventing leaky membranes

Cold conditions - prevents tight packing of phospholipids which would stop small molecules such as gases diffusing through

Question 4

What are glycoproteins?

Answer 4

Proteins with carbohydrate groups attached that protrude into extracellular fluid.

Question 5

What is the main function of glycoproteins?

Answer 5

Signalling molecules such as antigens

Question 6

What is the glycocalyx?

Answer 6

Coating of molecules external to the cell, made of sugars (glycoproteins)

Question 7

What factors affect the permeability of the plasma membrane?

Answer 7

Temperature
Organic solvents

Question 8

How does temperature affect the permeability of the cell membrane?

Answer 8

Increasing temperature = increased kinetic energy of cell components in the cell membrane, ∴ increasing fluidity and permeability.
Increasing above the optimum causes proteins to denature and the cell membrane will be disrupted

Question 9

How do organic solvents affect cell membrane permeability?

Answer 9

Adding organic solvents (alcohol) dissolves lipids the in membrane ∴ creating pores and increasing permeability

Question 10

What factors will affect the rate of diffusion through the plasma membrane?

Answer 10

1. Surface area
2. Concentration gradient
3. Temperature
4. Size of molecule
5. Lipid solubility (polarity)

Question 11

What types of molecules are able to diffuse through the lipid bilayer?

Answer 11

Small, non-polar (lipid-soluble) molecules
e.g. gases, vitamins and water

Question 12

What are intrinsic proteins?

Answer 12

Proteins that span the lipid bilayer
- channel and carrier proteins

Question 13

What is an extrinsic protein?

Answer 13

A protein found on either side of the membrane

Question 14

What determines the position of a protein within a plasma membrane?

Answer 14

It’s Polarity

Question 15

What is the polarity of side chains in a channel protein?

Answer 15

Side chains on the outside are non-polar (hydrophobic)
Side chains within the channel are polar (hydrophilic)

Question 16

How do molecules pass across the plasma membrane?

Answer 16

• Simple diffusion
• Facilitated diffusion - via intrinsic protein (channel or carrier protein)
• Active transport

Question 17

What type of diffusion occurs when channel and carrier proteins are used?

Answer 17

Facilitated diffusion

Question 18

What type of molecule uses a carrier protein?

Answer 18

Larger, polar molecules such as glucose
and amino acids.
Protein changes shape to allow passage of
molecule

Question 19

functions of the proteins within plasma membrane

Answer 19

• transporting substances
• maintaining cell shape
• act as receptors(cell signalling)

Question 20

functions of plasma membrane

Answer 20

• selectively permeable
• controls uptake of nutrients
• allows waste products to leave cell
• secretes enzymes + glycoproteins
• cell recognition