Biological membranes

Question 1

What are the key parts of the cell surface membrane and what do they do?

Answer 1

Phospholipids - These are what make up the layer. Hydrophobic/ philic.
Glycoproteins - Cell signalling
Glycolipids - Cell signalling
Protein channel pore - Allows substances to pass through the membrane
Cholesterol - Adds strength to the membrane and decreases fluidity
Extrinsic proteins

Question 2

Why is the membrane known as the fluid mosaic model?

Answer 2

Because it contains many different protein molecules and is flexible with it’s components moving throughout the membrane.

Question 3

What factors effect the fluidity of the membrane?

Answer 3

Temperature - Higher temperature = More fluid
Solvents - Many organic solvents will dissolve membranes, disrupting cells. Thus, the fluidity of the membrane increases.

Question 4

What are the two factors that affect rate of diffusion?

Answer 4

Temperature - Higher temp = Higher rate as particles have more kinetic energy to move with
Concentration gradient - The greater the difference in concentration between regions, the faster the rate of diffusion.

Question 5

What are the types of transport across membranes? Describe them.

Answer 5

Diffusion - The overall movement of particles from an area higher concentration to a region of lower concentration
Active transport - Movement of molecules of ions from an area of lower concentration to an area of higher concentration using ATP. Must be done across a carrier protein.
Osmosis - The diffusion of water across a partially permeable membrane

Question 6

What is fick’s law and how does it describe effects on diffusion?

Answer 6

Rate of diffusion = Surface area x Conc gradient
——————————————
Thickness of exchange surface

This law states that the rate is increased as surface area and gradient do, but is decreased by thicker exchange surfaces.

Question 7

Why are membranes partially permeable?

Answer 7

Non-polar substances and very small molecules can easily pass through, however polar molecules can only diffuse through very slowly and ions cannot diffuse through passively at all.

Question 8

What is facilitated diffusion?

Answer 8

Diffusion across a membrane through a protein channel and/or using a carrier protein.

Question 9

What is a carrier protein?

Answer 9

A protein that changes shape when a specific molecule binds to it.

Question 10

What are the 6 steps of active transport?

Answer 10

1. The molecule or ion to be transported binds to the receptors in the channel of the carrier protein on the outside of the cell.
2. On the inside of the cell, ATP binds to the carrier protein and is hydrolysed to ADP and phosphate.
3. Binding of the phosphate molecule to the carrier protein causes the protein to change shape - opening up to the inside of the cell.
4. The molecule or ion is released to the inside of the cell.
5. The phosphate molecule is released from the carrier protein and recombines with ADP to form ATP.
6. The carrier protein returns to it’s original shape.

Question 11

What is bulk transport?

Answer 11

A form of active transport, where large molecules or whole cells are too large to move through a channel, so are moved in/out through a vesicle.

Question 12

How does endocytosis work?

Answer 12

1. The membrane folds inwards around the material when it comes into contact with it.
2. The membrane enfolds the material until it fuses, releasing a vesicle.
3. This vesicle pinches off and moves into the cytoplasm to transfer it’s materials for further processing, i.e. moving bacteria to the lysosomes for digestion.

Question 13

What is exocytosis?

Answer 13

When a cell forms a vesicle of material on the inside and fuses with the cell membrane, releasing the material.

Question 14

What is water potential and what is it measured in?

Answer 14

The pressure exerted by water molecules as they collide with a membrane. It is measured in kPa.

Question 15

What is the water potential of pure water?

Answer 15

0 kPa

Question 16

How does osmosis move water with regards to water potential?

Answer 16

Osmosis is the movement of water from an area of higher water potential to an area of lower water potential until an equilibrium is reached.

Question 17

What is hydrostatic pressure?

Answer 17

The pressure caused by water moving into a solution, increasing volume.

Question 18

What happens to animal cells if they are placed in a solution with:
A very high water potential?
An equal water potential?
A very low water potential?

Answer 18

High = Water will flood into the cell, causing it to become too full and burst in a process called cytolysis.
Equal = No osmosis will occur 
Low = The cell will shrink, as water leaves the cell to enter the solution. This is known as crenation.

Question 19

What happens to plant cells if they are placed in a solution with:
A very high water potential?
An equal water potential?
A very low water potential?

Answer 19

High = Water will enter the cell, causing it to swell and become turgid. However, the cell wall will prevent bursting.
Equal = No osmosis will occur.
Low = Water will leave the cell, causing the inside of the cell to shrink and pull away from the cell wall. This is called plasmolysis.