2.5 Biological Membranes

Question 1

What is the role of membranes?

Answer 1

1. Provides a partially permeable barrier
2. Compartmentalisation
3. Antigens to recognise cells as ‘self’
4. Enzymes in metabolic pathways
5. Releases chemicals for cell signalling
6. Receptors for chemical signals for other cells

Question 2

What are functions of internal membranes?

Answer 2

1. Partially permeable membrane
2. Compartmentalisation - localising regions in an organelle
3. enclose enzymes e.g. in lysosomes
4. transports substances in vesicles

Question 3

What is the fluid mosaic model?

Answer 3

explains how biological molecules are arranged to form cell membranes

Question 4

What are the features of the fluid mosaic model?

Answer 4

Phospholipids
Cholesterol
Glycoproteins and glycolipids
Transport proteins

Question 5

Why are membranes described as ‘fluid’?

Answer 5

1. phospholipids and proteins can move around via diffusion
2. The phospholipids mainly move sideways, within their own layers
3. The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position

Question 6

What are phospholipids?

Answer 6

• Forms the basis of the membrane
• tails form a hydrophobic core and heads form the hydrophilic outer layer

Question 7

How does the phospholipid bilayer act as a barrier?

Answer 7

it acts as a barrier to most water-soluble substances (the non-polar fatty acid tails prevent polar molecules or ions from passing across the membrane)

Question 8

What is the role of cholesterol?

Answer 8

• It binds to the hydrophobic tails of the bilayer
• Cholesterol increases the fluidity of the membrane, stopping it from becoming too rigid at low temperatures - Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
• Increases strength and stability

Question 9

What is the role of glycolipids and glycoproteins?

Answer 9

• Both contains extrinsic carbohydrate chains - acts as receptor molecules

Question 10

What are the types of receptor?

Answer 10

1. Signalling receptors for hormones and neurotransmitters
2. Cell to cell recognition e.g. antigens
3. Cell adhesion and stabilisation
4. Receptors in endocytosis

Question 11

What are the roles of proteins in the membrane?

Answer 11

Transport proteins create hydrophilic channels to allow ions and polar molecules to travel through the membrane
1. Carrier proteins
2. Channel proteins

Question 12

What is the effect of temperature below 0 on membranes?

Answer 12

• Phospholipids have no energy so cannot move
• Rigid membrane structure
• Phospholipids packed closely together
• Channel and carrier proteins denature

Question 13

What is the effect of temperature from 0 - 45 on membranes?

Answer 13

• Phospholipids can move around - membrane is partially permeable
• Increasing temp gives molecules more kinetic energy - means they more more making the membrane more ‘leaky’ and permeable

Question 14

What is the effect of temperature from 45 + on membranes?

Answer 14

• Phospholipids break down and membrane becomes more permeable
• Channel and carrier proteins denature so cannot control what enters or leaves the cell - increased permeability of the membrane

Question 15

How does solvent concentration affect the membrane?

Answer 15

Organic solvents can increase cell membrane permeability as they dissolve the lipids in the membrane, causing the membrane to lose its structure

Question 16

What are examples of passive transport? What does this mean?

Answer 16

No energy is needed:
Osmosis
Simple diffusion
Facilitated diffusion

Question 17

What are examples of active transport? What does this mean?

Answer 17

Energy (ATP) is required:
Active transport
Endocytosis
Exocytosis

Question 18

Which factors affect diffusion rate?

Answer 18

• Concentration gradient
• Temperature
• Surface area
• Properties of molecules / ions

Question 19

How does the concentration gradient affect diffusion rate?

Answer 19

The steeper the gradient, the faster the rate of diffusion.
Diffusion happens from areas of high to low concentration gradient, down the concentration gradient

Question 20

How does the molecule property affect the diffusion rate?

Answer 20

• Larger molecules diffuse more slowly
• Uncharged and non-polar molecules diffuse directly across the membrane
• Non-polar molecules diffuse more quickly than polar molecules , as they are soluble in the non-polar bilayer

Question 21

How does facilitated diffusion take place in channel proteins?

Answer 21

• They allow charged substances (eg. ions) to diffuse through the cell membrane
• The diffusion of these ions does not occur freely, most channel proteins are ‘gated’, meaning that part of the channel protein on the inside surface of the membrane can move in order to close or open the pore
• This allows the channel protein to control the exchange of ions

Question 22

How does facilitated diffusion take place in carrier proteins?

Answer 22

• carrier proteins can switch between two shapes
  –> This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
• The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
• Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient

Question 23

How does Exocytosis take place?

Answer 23

Bulk transport out of cells:
The substances to be released (such as enzymes, hormones or cell wall building materials) are packaged into secretory vesicles formed from the Golgi body
These vesicles then travel to the cell surface membrane
Here they fuse with the cell membrane and release their contents outside of the cell

Question 24

How does Endocytosis take place?

Answer 24

Bulk transport into cells:
Endocytosis is the process by which the cell surface membrane engulfs material, forming a small sac (or ‘endocytic vacuole’) around it

Question 25

How does Active transport take place?

Answer 25

movement of molecules and ions through a cell membrane from a region of lower concentration to a region of higher concentration using energy from respiration
Requires carrier proteins

Question 26

What is meant by cell signalling?

Answer 26

Allows multicellular organisms to co-ordinate and control their bodies in response to environments

Question 27

What is osmosis?

Answer 27

The net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane

Question 28

What is the water potential of pure water? What do water potential values mean?

Answer 28

Pure water - 0kPa
The more negative the value, the lower the water potential
–> i.e. the lower the water concentration, the higher the solute concentration

Question 29

What is a hypertonic solution, hypotonic solution and isotonic solution?

Answer 29

Hypertonic solution: low water potential inside cell
Hypotonic solution: high water potential inside cell
Isotonic solution: Same water potential inside and outside cells

Question 30

What happens if an animal cell is placed in a solution of concentrated solute?

Answer 30

1. The solution has a lower water potential than the cell
2. Water moves out of the cell by osmosis across the partially permeable membrane
3. Causes the cell to crenate - shrink and shrivel up

Question 31

What happens if an animal cell is placed into a solution of very low concentration of solute.

Answer 31

1. There is a higher water potential outside of the cell
2. Water moves into the cell by osmosis - the cell gains so much water that the cell membrane stretches
3. The cell bursts - cytolysis

Question 32

Where does lysis occur?

Answer 32

Hypotonic solutions - solution outside of cell has a lower solute concentration than inside the cell

Question 33

Where does crenation occur?

Answer 33

Hypertonic solutions - solution outside of cell has a higher solute concentration

Question 34

What happens if a plant cell is placed in a solution of lower water potential than the cell

Answer 34

1. Water leaves the plant cell via osmosis - Plasmolysis
2. As water leaves the vacuole of the plant cell, the volume of the plant cell decreases
3. Protoplast shrinks and does not exert pressure on the cell wall
4. Cell fully plasmolyses - the cells protoplast fully shrinks away from the cell wall

Question 35

What happens if a plant cell is placed in a solution of pure water of very low solute concentration?

Answer 35

1. water will enter the plant cell through its partially permeable cell surface membrane by osmosis,
2. Water enters the vacuole so volume of the plant cell increases
3. The expanding protoplast pushes against the cell wall pressure builds up inside the cell
4. Plant cell become turgid - rigid and firm due to being filled with water