cell membranes.

Question 1

Three key roles of plasma (cell surface) membranes.

Answer 1

• Partially permeable barriers — control which substances enter or leave the cell.
• Site of chemical reactions.
• Contain receptors where hormones and drugs can bind.

Question 2

Three roles of membranes within cells.

Answer 2

• Control which substances enter or leave organelles.
• Site of chemical reactions.
• Compartmentalisation.

Question 3

Function of phospholipids in a cell membrane.

Answer 3

• Form the bilayer.
• Acts as a barrier to water soluble substances.

Question 4

Function of cholesterol in a cell membrane.

Answer 4

• Adds stability to the membrane — less fluid and more rigid.

Question 5

Function of glycolipids in a cell membrane.

Answer 5

• Stabilise the membrane by forming H bonds with water.
• Receptors.
• Antigens.

Question 6

Function of proteins in a cell membrane.

Answer 6

• Channel proteins for facilitated diffusion.
• Carrier proteins for active transport.
• Receptors.

Question 7

Function of glycoproteins in a cell membrane.

Answer 7

• Stabilise the membrane by forming H bonds with water.
• Receptors.
• Antigens.

Question 8

Describe the fluid mosaic model of membrane structure.

Answer 8

• Phospholipids form a bilayer.
• Hydrophobic tails on the inside and hydrophilic heads on the outside.
• Proteins scattered through the bilayer.

Question 9

Explain cell signalling.

Answer 9

• Cells communicating with each other.
• One cell releases a messenger molecule.
• Detected by a receptor in a different cell.
• Receptor has a complementary shape to the messenger.

Question 10

Describe and explain the effect of changing temperature on membrane structure and permeability.

Answer 10

• As the temperature increases, the permeability of the membrane increases.
• The phospholipids gain kinetic energy.
• Membrane becomes more fluid.
• At high temperatures the phospholipids start to melt.
• Proteins denature.
• Membrane becomes freely permeable.

Question 11

Describe and explain the effect of increasing solvent concentration on membrane structure and permeability.

Answer 11

• As the solvent concentration increases, the permeability of the membrane increases.
• The lipids in the membrane dissolve in the solvent.
• The membrane loses its structure.

Question 12

Describe how to investigate the effect of temperature on membrane structure and permeability.

Answer 12

• Cut 5 cubes of beetroot all the same size and therefore with the same surface area.
• Add each one to a different test tube.
• Cover with 5cm cubed of distilled water.
• Place the test tubes in 5 different water baths, 10, 20, 30, 40 and 50 degrees.
• Leave for 10 minutes.
• Remove the beetroot from the test tubes, leaving the purple solution.
• Set up a colorimeter on blue filter.
• Zero the colorimeter with a cuvette of distilled water.
• Take absorbance reading for the solution from each test tube.
• The more permeable the membrane, the more pigment will diffuse from the beetroot cells.
• Therefore, the higher the absorbance reading.