6. Biological Membranes

Question 1

What is the function of plasma (cell surface) membranes?

Answer 1

Regulates transport into and out of the cell.
Contains enzymes involved in metabolic pathways.
Contains antigens that the body recognises as ‘self’ so it does not attack.
Contains receptors for chemical signal, so is a site for cell communication.

Question 2

What is the fluid mosaic model?

Answer 2

Proposed that the fabric of the membrane consists of a phospholipid bilayer with proteins floating in it. The lipids and some proteins can move, giving it fluidity.

Question 3

What structures are present in the phospholipid bilayer?

Answer 3

phospholipids, glycoproteins, carrier proteins, channel proteins, cholesterol, glycolipids, protein receptor sites

Question 4

What are the different proteins within membranes?

Answer 4

Some have pores and act as channels to allow ions to pass through.
Some proteins are carriers and, by changing their shape, carry specific molecules across the membrane.

Question 5

What is the function of cholesterol in a cell surface membrane?

Answer 5

Helps to regulate fluidity, maintain stability and resist the effects of temperature on the membrane’s structure.

Question 6

What is the total thickness of a cell membrane?

Answer 6

Between 5 and 10 nm

Question 7

What is the distribution of proteins in neurones?

Answer 7

A myelin sheath covering the axon formed by flattened cells contains protein.
This allows the conduction of electrical impulses along its length.

Question 8

What is the function of proteins in white blood cells?

Answer 8

Protein receptors in their plasma membrane enable them to recognise antigens on foreign cells.

Question 9

What is the distribution of proteins in root hair cells?

Answer 9

They have many carrier proteins to actively transport nitrate ions from the soil into cells.

Question 10

What is diffusion?

Answer 10

When molecules move down the concentration gradient until they reach equilibrium. It may or may not be across a membrane. It does not involve metabolic energy (ATP)

Question 11

How is a concentration gradient maintained in cells?

Answer 11

The molecules are used for metabolic reactions which keeps more of the molecules entering the cell.

Question 12

How does temperature affect the rate of diffusion?

Answer 12

As temperature increases, molecules have more kinetic energy, so their rate of diffusion will increase and vise versa.

Question 13

How does diffusion distance affect the rate of diffusion?

Answer 13

The thicker the membrane across which the molecules have to diffuse, the slower the rate of diffusion.

Question 14

How does surface area affect the rate of diffusion?

Answer 14

More diffusion can take place across a larger surface area.

Question 15

How does the size of diffusing molecule affect the rate of diffusion?

Answer 15

Smaller ions or molecules diffuse more rapidly than larger ones.

Question 16

How does concentration gradient affect the rate of diffusion?

Answer 16

The steeper the gradient, the faster the diffusion to the side where there are fewer molecules, down the gradient.

Question 17

What is facilitated diffusion?

Answer 17

The movement of molecules down the concentration gradient across a partially permeable membrane via protein channels or carriers.
It does not involve metabolic energy (ATP)

Question 18

What is a solution?

Answer 18

Solute molecules dissolved in a solvent. Water is the solvent in aqueous solutions.

Question 19

What is osmosis?

Answer 19

The passage of water molecules down their water potential gradient, across a partially permeable membrane.

Question 20

What is water potential?

Answer 20

Measure of the tendency of water molecules to diffuse from one region to another.
It is represented by Ψ (psi) and measured in kPa.
Pure water has the highest water potential of 0kPa.
Dissolved solutes lower the water potential.

Question 21

What does hypotonic mean?

Answer 21

High water potential (more water than solute)

Question 22

What does hypertonic mean?

Answer 22

Low water potential (more solute than water)

Question 23

What does isotonic mean?

Answer 23

When a solution has the same concentration of water and solute.

Question 24

What happens when animal cells are placed in a solution with a higher water potential?

Answer 24

Water molecules move by osmosis into the cell, the cell will swell and burst as the plasma membrane breaks.
This is cytosis.

Question 25

What happens when plant cells are placed in a solution with a higher water potential?

Answer 25

The cellulose cell wall will prevent bursting.
The cell will swell and is described as turgid.
This helps to support non-woody plants.

Question 26

What happens when plant cells are placed in a solution with a higher water potential?

Answer 26

The cytoplasm of plant cells shrinks and the membrane pulls away from the cellulose cell wall.
The cells are described as plasmolysed.
The plant tissue is described as flaccid.

Question 27

What is active transport?

Answer 27

The movement of substances against their concentration gradient across a cell membrane, using ATP as protein carriers.

Question 28

How do carrier proteins help active transport?

Answer 28

They have specific regions that combine with certain solute molecules or ions.
They have a region that allows the hydrolysis of ATP, to release energy.
The energy helps the carrier protein to change its conformation cand carry the ion across the membrane.

Question 29

How does the sodium potassium pump work?

Answer 29

3 sodium ions bind to a specific site on the sodium-potassium pump protein.
ATP binds to its site and is hydrolysed to ADP+P releasing energy.
2 potassium ions bind to their specific site.
The energy from ATP enables the pump protein to change its shape.
The 3 sodium ions are now on the outside of the cell and the 2 potassium ions are inside.

Question 30

What is endocytosis?

Answer 30

Bulk transport of molecules too large to pass through the cell membrane, into a cell.
A segment of the plasma membrane surrounds and encloses the particle and brings it into the cell, enclosed in a vesicle.

Question 31

What is endocytosis of solids called?

Answer 31

phagocytosis

Question 32

What is endocytosis of liquids called?

Answer 32

pino(endo)cytosis

Question 33

What is exocytosis?

Answer 33

Bulk transport of molecules too large to pass through the cell membrane, out of a cell.
A vesicle containing them fuses with the plasma membrane and releases them into the external environment.

Question 34

Why is ATP needed in endocytosis and exocytosis?

Answer 34

To provide energy to form vesicles and move them using molecular motor proteins along cytoskeleton threads.
To fuse the membranes together and move the vesicles.