Topic 4 - Transport Across Cell Membranes

Question 1

What are the functions of the cell surface membrane?

Answer 1

• Allows different conditions to be established inside and outside of the cell.
• Selectively permeable.
• Cell signalling and recognition.

Question 2

What is the difference between intrinsic and extrinsic proteins?

Answer 2

Intrinsic = span whole width of membrane, most carrier molecules/channels that help transport substances that can’t diffuse across membrane.

Extrinsic = inner or outer of surface membrane, act as receptors for hormones/neurotransmitters, involved in cell recognition.

Question 3

What are the functions of glycoproteins?

Answer 3

• Act as receptors.
• Act as recognition sites.
• Help cells attach to each other.
• Allows cells to recognise one another.

Question 4

What are the functions of glycolipids?

Answer 4

• Acts as cell surface receptor for specific chemicals.
• Maintain stability of membrane.
• Helps cells to attach to another.

Question 5

What are the functions of cholesterol?

Answer 5

• Add strength to membranes.
• Very hydrophobic.
• Pull together fatty acid tails of phospholipid molecules.

Question 6

State why a membrane is known as a Fluid-Mosaic Model?

Answer 6

Fluid = Phospholipids and proteins can move around via diffusion.

Mosaic = Scattered pattern produced by proteins look similar to a mosaic when viewed from above.

Question 7

What is diffusion? What sort of process is it?

Answer 7

• Diffusion is the net movement of molecules or ions from a region of high concentration to low concentration, until an equilibrium is achieved.
• Passive process = depends upon the kinetic energy of molecules/ions.

Question 8

How is the rate of diffusion affected by:
- Temperature
- Concentration gradient
- Surface area.
- Lenght of diffusion pathway

Answer 8

• Increasing temp = increases kinetic energy = speeds up R.O.D
• The larger the difference in concentration = the faster the ROD.
• Larger = faster diffusion will occur as greater area.
• The thinner the exchange surface, the faster the ROD.

Question 9

What is Fick’s Law?

Answer 9

Rate of diffusion = (surface area x difference in concentration) / length of diffusion pathway

Question 10

What is facilitated diffusion?

Answer 10

Some large molecules/ions diffuse across membrane faster than their conc. gradient allows. Their movement is facilitated by protein channels in plasma membrane.

Question 11

How do channel proteins help control the entry and exit of molecules/ions?

Answer 11

• Form open pores in membrane which allows small molecules/ions to pass through the lipid bilayer.

Question 12

What happens to the carrier protein when a molecule that is trying to cross the membrane binds to it?

Answer 12

The carrier protein will change shape to allow the molecule to move to the opposite side of the membrane - they help large non-polar molecules across.

Question 13

What is osmosis?

Answer 13

Water molecules move from an area of high water potential to an area of low water potential through a partially permeable membrane, down their concentration gradient.

Question 14

What are the main features of water potential?

Answer 14

• Rarely pure.
• Usually dissolved solutes in it.

Question 15

What is active transport?

Answer 15

Movement of molecules/ions into/out of a cell from a region of low concentration to a region of high concentration using energy released from hydrolysis of ATP and carrier protein molecules. Against conc. gradient. Active process.

Question 16

What is the sodium-potassium pump?

Answer 16

The process of moving sodium and potassium ions across the plasma membrane, involving the hydrolysis of ATP to ADP+Pi.
- This process is responsible for maintaining the large excess of Na+ ions outside the cell and the large excess of K+ ions on the inside.

1) Sodium ions attach.
2) ATP hydrolyse.
3) Protein changes shape, releases Na+
4) Leaves, returns to original shape.
5) Releases potassium ions.

Question 17

What is exocytosis?

Answer 17

the contents of a cell vacuole are released to the exterior through fusion of the vacuole membrane with the cell membrane.

Question 18

What is endocytosis?

Answer 18

the process of actively transporting molecules into the cell by engulfing it with its membrane.

Question 19

What is digestion?

Answer 19

The hydrolysis of large insoluble molecules, into smaller soluble ones.

Question 20

How do these properties of villi help maximise the exchange of materials across their surface:
:Large surface area.
Thin wall.
Good supply of blood vessels

Answer 20

• Allows area for more carrier proteins, rate of diffusion/active transport increases.
• Short diffusion pathway.
• Concentration gradient maintained.

Question 21

How does co transport work to extract glucose in the human body?

Answer 21

1) Active transport of sodium ions out from epithelial cells, into the blood.
2) Lowers the conc. of Na+ in cell, sets up a conc. gradient.
3) Na+ diffuse down their conc. gradient through co-transport proteins in epithelial cell membrane.
4) Carrier proteins transport glucose molecules and Na+ together into the cell.
5) Raises the conc. of glucose molecules inside epithelial cell, which then diffuse down conc. gradient into blood capillaries through channel proteins, via facilitated diffusion.
6) Moves glucose molecules against their conc. gradient from the lumen of the small intestine into the cell, and down their conc. gradient out of the cell into blood.