Cell Membranes

Question 1

Fluid mosaic model structure

Answer 1

• Phospholipid molecules form a bilayer
• Bilayer is fluid as the phospholipids are constantly moving
• Channel and carrier proteins scattered through the bilayer allow large molecules and ions to pass through the membrane
• Receptor proteins on the cell-surface membrane detect chemicals released from other cells
• Glycoproteins and glycolipids have a carbohydrate attached
• Cholesterol molecules are also present within the bilayer

Question 2

Phospholipid function

Answer 2

• Form a barrier to dissolved substances
• Head is hydrophilic, tail is hydrophobic
• Phospholipid molecules arrange themselves into a bilayer
• Centre of the bilayer is hydrophobic so water-soluble substances can’t diffuse through it
• Small, non-polar substances and water can diffuse through the membrane

Question 3

Cholesterol function

Answer 3

• Gives the membrane stability (not bacterial cell membranes)
• Fits between the phospholipids and binds to their hydrophobic tails, causing them to pack closely together, making the membrane more rigid
• Helps to maintain the shape of animal cells
• Has hydrophobic regions, creating further barrier to polar substances moving through the membrane

Question 4

How are membranes affected by temperatures below 0°C?

Answer 4

• Phospholipids don’t have much energy, so can’t move much
• Phospholipids packed closely together and the membrane is rigid
• Channel proteins and carrier proteins in the membrane denature, increasing permeability
• Ice crystals may form and pierce the membrane, making it highly permeable when it thaws

Question 5

How are membranes affected by temperatures between 0 and 45°C?

Answer 5

• Phospholipids can move around and aren’t packed as tightly together
• As temperature increases the phospholipids move more because they have more energy, increasing the permeability of the membrane

Question 6

How are membranes affected by temperatures above 45°C?

Answer 6

• Phospholipid bilayer starts to break down, making membrane more permeable
• Water inside the cell expands, putting pressure on the membrane
• Channel proteins and carrier proteins in the membrane denature so they can’t control what enters or leaves the cell, increasing permeability of the membrane

Question 7

Investigating how temperature affects beetroot membrane permeability

Answer 7

• Use a scalpel to cut five equal sized pieces of beetroot
• Add pieces to five different test tubes containing 5cm3 H20
• Place each test tube in a water bath at a different temperatures for the same length of time
• Remove the pieces of beetroot from the tubes, leaving the coloured liquid
• Use a pipette to transfer a sample of the liquid from the first test tube to a cuvette
• Put the cuvette in the colorimeter and record the absorbance of the solution
• Repeat for the liquids in the remaining four test tubes
• Higher the absorbance reading, more pigment released, higher the permeability of the membrane