5.1 - 5.2 - Structure and Function of Membranes

Question 1

What does the fluid mosaic model explain?

Answer 1

• passive and active movement between cells and their surroundings
• cell to cell interactions
• cell signalling

Question 2

Why is it called the fluid mosaic model?

Answer 2

The fluid mosaic model describes cell membranes as fluid because
• the phospholipids and proteins can move around via diffusion
• the phospholipids mainly move sideways within their own layers
• many different types of proteins interspersed throughout the bilayer move about within it

Question 3

Why does the fluid mosaic model describe cell membranes as mosaics?

Answer 3

The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above

Question 4

What are the functions of cell surface membrane?

Answer 4

• partially permeable barriers between the cell & outside environment, between organelles and three cytoplasm & within organelles
• controls which substances enter and leave the cells
• membranes allow recognition by other cells eg. Cells of the immune system
• sites of cell communication

Question 5

Describe the structure of the cell surface membrane

Answer 5

The cell surface membrane is a bilayer of phospholipids with embedded proteins

Question 6

What are the functions of membranes within cells?

Answer 6

• separate the contents from the cytoplasm
• can form vesicles to transport substances eg. Golgi apparatus
• allowing selected molecules to move in and out of the cell
• isolating organelles from the rest of the cytoplasm allowing cellular processes to occur separately
• a site of biochemical reactions
• allowing a cell to change shape
• provide attachment sites for enzymes

Question 7

What parts of the phospholipids are hydrophilic and hydrophobic?

Answer 7

Head is hydrophilic
Tail is hydrophobic

Question 8

What happens when water and lipids mix?

Answer 8

• these two substances do not mix
• water = polar molecule- oxygen end is slightly more negative and hydrogen more positive
• fats are non polar and do not form hydrogen bonds with water
• fats are hydrophobic and lie on the surface of the water - reduce the surface area in contact between the fat and the water

Question 9

What are the two types of protein in the plasma membrane?

Answer 9

Intrinsic and extrinsic

Question 10

What are some examples of intrinsic proteins ?

Answer 10

• channel proteins
• carrier proteins
• glycoproteins

Question 11

What is an example of extrinsic proteins?

Answer 11

Peripheral proteins

Question 12

What are channel proteins?

Answer 12

Bind specific molecules and transport these molecules and ions across the membrane by active transport and facilitated diffusion

Question 13

What are carrier proteins?

Answer 13

• move larger molecules ( including polar molecules and ions ) into or out of the cell
  - down their concentration gradient
• carrier proteins change shape when a spec molecule binds

Question 14

What are glycoproteins?

Answer 14

• proteins act as receptors eg. Hormones in cell signalling
• when a molecule binds to the protein a chemical reaction is triggered inside the cell
• intrinsic proteins with an starched carbohydrate chain
• stabilise the membrane by firming hydrogen bonds with surrounding water molecules
• cell adhesion
• site where drugs, hormones and antibodies bind - cell recognition

Question 15

What are glycolipids?

Answer 15

• lipids with an attached carbohydrate chain
• act as receptor molecules
• act as cell markers for antigens
• can be recognised by the cells of the immune system as self or non self

Question 16

What is cholesterol?

Answer 16

• type of lipid
• molecules fit between phospholipids
• makes the membrane less fluid and more rigid stabilising the membrane
• increases mechanical strength and stability of membranes

Question 17

Name and explain 3 factors that affect membrane permeability?

Answer 17

1. Temperature- high temperature = denatures membrane proteins/ phospholipid molecules = have more kinetic energy = move further apart
2. pH - changes tertiary structure of membrane proteins
3. Use of a solvent- may dissolve membrane

Question 18

What are 3 factors affecting fluidity?

Answer 18

1. Length of fatty acid side chains - longer the chain = lower the fluidity
2. Proportion of the fatty acids that are saturated - higher the percentage of saturated fats = lower the fluidity
3. Steroid content - higher the cholesterol= lower the fluidity

Question 19

What is cell signalling?

Answer 19

How cells communicate with each other

Question 20

Why is cell signalling important?

Answer 20

• control processes inside the body and respond to changes in the environment eg. Low blood glucose levels
• communicate with each other using messenger molecules :
- one cell releases a messenger molecule eg. A hormone
- this molecule travels in the bloodstream to another target cell
- this messenger molecule binds to a receptor ( called a membrane bound receptor with a complimentary shape to the messenger molecule) on the cell surface membrane of the target cell
• a response is triggered eg. Uptake of glucose in the liver

Question 21

What are membrane bound receptors?

Answer 21

Proteins act as receptors for messenger molecules
- membrane bound receptors have a specific shape so only messenger molecules with a complimentary shape can bind
- a cell that can respond to a particular messenger molecule is called a target cell