Topic 2---B: Cell membranes- 1. Cell membranes- The basics

Question 1

What are cell-surface membranes?

Answer 1

• They surround cells
• They are a barrier between the cell and it’s environment
• They control which substances enter and leave the cell
• They’re partially permeable so they let some molecules through but not others
• Sometimes called the plasma membrane

Question 2

How can substances move across the cell-surface membrane?

Answer 2

By
- Diffusion
- Osmosis
- Active transport

Question 3

How do membranes around organelles divide the cell into different compartments?

Answer 3

• They act as a barrier between the organelle and the cytoplasm
  E.g. The substances needed for respiration (like enzymes) are kept together inside a mitochondrion by the membrane surrounding the mitochondrion.

Question 4

What components are in a membrane?

Answer 4

• Phospholipids (7nm thick)
• Proteins
• Carbohydrates
• Cholesterol

Question 5

What was the ‘Fluid Mosaic Model’?

Answer 5

• In 1972 Singer and Nicholson suggested it to describe the arrangement of molecules in the membrane.
• In the model phospholipids molecules form a continuous, double layer (bilayer) which is ‘fluid’ because the phospholipids are constantly moving.
• Proteins are scattered through the bilayer like tiles in a ‘mosaic’ (including channel and carrier proteins).

Question 6

What do channel and carrier proteins do?

Answer 6

They allow large molecules and ions to pass through the membrane.

Question 7

Phospholipids

Answer 7

• Phospholipid molecules form a barrier to dissolved (water-soluble) substances.
• They have a head (hydrophilic) which attracts water
• They have a tail (hydrophobic) which repels water
• Heads face out towards the water on either side of the membrane
• Centre of the bilayer is hydrophobic so the membrane doesn’t allow water-soluble substances to diffuse through it
  e.g (ions& polar molecules)
• Small, non-polar substances can diffuse through the membrane
  e.g (carbon dioxide and water)

Question 8

Cholesterol

Answer 8

• Present in all cell membranes apart from bacterial cell membranes
• It gives the membrane stability
• Cholesterol fits between the phospholipids
• Cholesterol binds to the hydrophobic tails of the phospholipids causing them to pack more closely together
• This restricts movement of the phospholipids making the membrane less fluid and more rigid
• Cholesterol has hydrophobic regions so it’s able to create a further barrier to polar substances moving through the membrane

Question 9

What does cholesterol help with?

Answer 9

It helps to maintain the shape of animal cells that don’t have cell walls
e.g (red blood cells)

Question 10

Protein

Answer 10

• Scattered through the bilayer
• They allow molecules and ions to pass through the membrane
• Receptor proteins on cell-surface membrane allow the cell- surface membrane to detect chemicals released from other cells

Question 11

Carbohydrates

Answer 11

• Glycoprotein (attatched to a protein)
• Glycolipid (attatched to a lipid)

Question 12

Functions (protein)

Answer 12

• Act as a hydrophilic pore where water soluble substances can pass in or out of the cell

Question 13

Phospholipid bilayer (function)

Answer 13

• Allows water to pass through with relative ease
• Acts as a barrier to the passage of water-soluble substances

Question 14

Carbohydrate function

Answer 14

• Helps stabilize the membrane structure by forming hydrogen bonds with water molecules outside the membrane

Question 15

How does temperature affect membranes?

Answer 15

• It affects how much the phospholipids in the bilayer can move
• Affecting membrane structure and permeability

Question 16

Temperatures below 0 degrees

Answer 16

• Phospholipids don’t have much energy so they can’t move very much
• They’re packed closely together and the membrane is more rigid
• Channel and carrier proteins in the membrane denature (lose structure and function) so this increases the permeability of the membrane
• Ice crystals may form and pierce the membrane making it highly permeable when it thaws

Question 17

Temperatures between 0 and 45 degrees

Answer 17

• Phospholipids can move around and aren’t packed as tightly together
• Membrane is partially permeable
• As the temperature increases the phospholipids move more as they have more energy which increases the permeability of the membrane

Question 18

Temperatures above 45 degrees

Answer 18

• Phospholipid bilayer starts to melt and the membrane becomes more permeable
• Water inside the cell expands putting pressure on the membrane
• Channel and carrier proteins in the membrane denature so they can’t control what enters or leaves the cell
• This increases the permeability of the membrane