Transport Across Cell Membranes

Question 1

What is the role of the cell surface membrane?

Answer 1

It controls the entry and exit of substances in and out the cell

Question 2

What is the role of cholesterol in the cell surface membrane?

Answer 2

It decreases the permeability and increases the stability of the membrane. Different types of cells have different amounts of cholesterol.

Question 3

What is the role of glycolipids in the cell surface membrane?

Answer 3

They are composed of carbohydrates attached to a phospholipid within the membrane. They are also important for cell recognition.

Question 4

What is the role of glycoproteins in the cell surface membrane?

Answer 4

Composed of carbohydrates and a protein which are on the outer surface of the membrane and are important in cell recognition, sometimes acting as antigens

Question 5

What is the role of receptors in the cell surface membrane?

Answer 5

Specific receptors detect hormones like insulin which attach to them and allow the cell to respond

Question 6

What is the role of carrier proteins in the cell surface membrane?

Answer 6

They aid in the transport of ions and polar molecules (e.g. glucose) by facilitated diffusion

Question 7

What is the role of channel proteins in the cell surface membrane?

Answer 7

Ions and polar molecules can pass through these.

Question 8

What is the role of phospholipids in the cell surface membrane?

Answer 8

They are arranged into a bilayer with the non polar tails on the inside and polar heads on the outside.
They allow lipid soluble molecules to pass through but restrict ions and polar molecules

Question 9

What is the role of aquaporins in the cell surface membrane?

Answer 9

Channels that allow for the passage of water in and out the cell

Question 10

What are the 4 types of movement across a cell membrane?

Answer 10

• Diffusion
• Facilitated diffusion
• Osmosis
• Active transport

Question 11

What is diffusion?

Answer 11

Diffusion is the movement of substances down a concentration gradient. It doesn’t require ATP and allows lipid soluble molecules to pass.

Question 12

What is facilitated diffusion?

Answer 12

When substances diffuse through carrier or channel proteins down a concentration gradient.

Question 13

What is active transport?

Answer 13

When molecules move across a carrier protein against the concentration gradient. Requires ATP (used to change the shape of the carrier protein to allow movement).

Question 14

What is co transport?

Answer 14

The process by glucose or amino acids are absorbed into the epithelial lining of the small intestine.

Question 15

What happens to active transport if ATP production is inhibited?

Answer 15

It cannot occur and sodium ions wont be transported so it will build up in the blood. This will stop facilitated diffusion in the lumen so glucose cannot enter cells.

Question 15

What happens to diffusion if the tertiary structure of carrier proteins changes?

Answer 15

Facilitated diffusion of glucose cannot occur and it will build up in the cell.
Sodium absorption would stop

Question 16

What is the process of co-transport?

Answer 16

1. Na+ ions are actively transported out of the cell into the blood. This causes a concentration gradient
2. Na+ diffuses into the cell through the cotransporter protein, coupled with glucose. This increases the concentration of glucose within the cell
3. Glucose diffuses into the blood, by facilitated diffusion, through a channel protein into the blood

Question 17

How do you find the percentage change?

Answer 17

(Final mass - initial mass) / Initial mass X 100

Question 18

How do you make serial dilutions?

Answer 18

1. Add 10cm3 of the 1M sucrose solution into the first test tube and in the other four test tubes add 5 cm3 of distilled water
2. Using a graduated pipette measure 5 cm3 of the sucrose solution from the first test tube and add to the second tube. You now have a 0.5M solution of sucrose
3. Repeat the process from the second to the third test tube to give a 0.25M solution.
4. Repeat the process for the next two tubes to produce a 0.125M and a 0.0625M

Question 19

Some substances can cross the cell-surface membrane of a cell by simple diffusion through the phospholipid bilayer. Describe other ways by which substances cross this membrane. (5)

Answer 19

By osmosis (no mark)
1. From a high water potential to a low water potential / down a water potential gradient;
2. Through aquaporins / water channels;
By facilitated diffusion (no mark)
3. Channel / carrier protein;
4. Down concentration gradient;
By active transport (no mark)
5. Carrier protein / protein pumps;
6. Against concentration gradient;
7. Using ATP / energy (from respiration);
[Co-transport subsumed into mark scheme for active transport and facilitated diffusion]
By phagocytosis / endocytosis (no mark)
8. Engulfing by cell surface membrane to form vesicle / vacuole;
By exocytosis / role of Golgi vesicles (no mark)
9. Fusion of vesicle with cell surface membrane;

Question 20

Some substances pass through the plasma membrane of a milk-producing cell by diffusion. Describe the structure of a plasma membrane and explain how different substances are able to pass through the membrane by diffusion. (6)

Answer 20

1 Phospholipids forming bilayer/two layers;
2 Details of arrangement with “heads” on the outside;
3 Two types of protein specified; e.g. passing right through or confined to one layer / extrinsic or intrinsic / channel proteins and carrier proteins / two functional types
4 Reference to other molecule e.g. cholesterol or glycoprotein;
5 Substances move down concentration gradient/from high to low concentration;
Reject references to across or along a gradient
6 Water/ions through channel proteins/pores;
7 Small/lipid soluble molecules/examples pass between phospholipids/through phospholipid layer;
8 Carrier proteins involved with facilitated diffusion;