Membranes

Question 1

Roles of membranes

Answer 1

• Physical barriers - they separate intracellular environments from extracellular environments
• They regulate the exchange of surfaces in and out of a cell; membranes are partially permeable (i.e. they allow only certain particles to pass through)
• Compartmentalisation - i.e. membranes enclose and isolate organelles, enabling them to maintain specific environments for chemical reactions
• Support for the cytoskeleton
• Sites of chemical reactions
• Sites of cell communication (e.g. cell signalling)
• Formation of vesicles - used in bulk transport

Question 2

Membrane structure

Answer 2

• Phospholipid bilayer - hydrophobic core, hydrophilic phosphate heads
• Various proteins are distributed within the bilayer (including glycoproteins)
• Cholesterol
• Glycolipids
• Membrane-bound receptors - sites where hormones and drugs can bind

Question 3

What is a glycoprotein?

Answer 3

The branching carbohydrate portion of a protein which acts as a recognition site for chemicals, e.g. hormones

Question 4

What is a glycolipid?

Answer 4

Acts as a recognition site e.g. for cholera toxins

Question 5

Components of a phospholipid

Answer 5

Hydrophilic head, hydrophobic tails

Question 6

Role of channel proteins

Answer 6

Facilitated diffusion

Question 7

Roles of carrier proteins

Answer 7

Facilitated diffusion and active transport

Question 8

Role of glycoproteins

Answer 8

Receptors (e.g. for neurotransmitters, peptide hormones and drugs)

Question 9

Role of glycolipids

Answer 9

Cell recognition (i.e. they act as antigens)

Question 10

What regulates the fluidity of membranes?

Answer 10

Cholesterol = inserted between phospholipids, effect of the molecule is temperature dependent - at low temperatures, it maintains fluidity by widening the gaps between phospholipids. At high temperatures, it prevents membranes from becoming too fluid. This is done by attracting phospholipids and limiting their movement (maintains stability!)

Question 11

How does temperature affect membrane structure?

Answer 11

• Decreased temperature reduces fluidity (phospholipids move less due to lower kinetic energy)
• Increased temperature causes greater fluidity and therefore increases permeability, but a membrane will lose its structure (become disrupted/destroyed) and break apart if temperature continues to rise
• Membranes do NOT denature, the proteins within the phospholipid bilayer do

Question 12

How do solvents affect membrane structure?

Answer 12

Organic, weakly polar (e.g. ethanol) or non-polar (e.g. benzene) solvents disrupt or dissolve membranes, making them more permeable

Question 13

Diffusion

Answer 13

• Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration
• It is a passive process (i.e. not requiring energy from ATP)

Question 14

Which particles can diffuse through the bilayer, between phospholipid molecules?

Answer 14

• Large lipid-soluble molecules (e.g. steroid hormones)
• Non-polar molecules (e.g. oxygen)
• Very small polar molecules (e.g. water)

Question 15

Facilitated diffusion

Answer 15

• The process of particles passing through transmembrane proteins
• Channel proteins and carrier proteins are used for this process
• This is carried out by ions and large polar molecules (e.g. glucose and amino acids)

Question 16

Channel proteins

Answer 16

(Intrinsic protein) Pores, which can be gated (i.e. opened and closed), allowing the diffusion of ions

Question 17

Carrier proteins

Answer 17

Intrinsic proteins, they have shapes that allow only the passageway of specific molecules or ions

Question 18

Factors affecting diffusion rates

Answer 18

• Higher temperatures - provide particles with more kinetic energy
• Steeper concentration gradient - the greater the difference in concentration either side of a membrane, the greater the diffusion rate
• Shorter diffusion pathway - thin membranes reduce the distance particles have to move
• Greater surface area
• High concentration of carrier proteins - increases the rate of facilitated diffusion

Question 19

Active transport

Answer 19

• Energy is required for active transport because particles are being moved against a concentration gradient (i.e. from a region of lower concentration to a region of higher concentration)
• The energy is in the form of ATP
• Active transport can use carrier proteins as pumps or take the form of bulk transport

Question 20

How do carrier proteins act as pumps?

Answer 20

• Carrier proteins change shape, thereby allowing particles to pass through them due to ATP hydrolysis (which produces phosphate ions)
  1. Molecules bind to carrier protein and ATP attaches to the membrane protein on the inside of the cell
  2. Binding of phosphate ion to protein causes the protein to change shape so that access for the molecules is open to the inside of the membrane but closed to the outside

Question 21

Bulk transport

Answer 21

The movement of large molecules (e.g. enzymes and hormones) relies on bulk transport, which is the movement in or out of cells, of particles within vesicles

Question 22

Endocytosis

Answer 22

Endocytosis is bulk transport into cells. Vesicles are formed by the plasma membrane being pinched off. Two forms exist:

1. Pinocytosis: a cell engulfs liquid and small dissolved particles
2. Phagocytosis: a cell engulfs large solid material (e.g. a white blood cell engulfing bacterium)

Question 23

Exocytosis

Answer 23

Exocytosis is bulk transport out of cells. In many cases, vesicles are formed by the Golgi apparatus and fuse with the cell surface membrane, releasing their contents (e.g. hormones)

Question 24

Osmosis

Answer 24

Diffusion of water through a partially permeable membrane down a water potential gradient

Question 25

Water potential

Answer 25

The direction in which water diffuses across a membrane is determined by water potential, which is measured in kilopascals (kPa)

Question 26

If there are more solute particles, what happens to the water potential?

Answer 26

It decreases

Question 27

Water potential of:

1. Pure water
2. Dilute glucose solution (low glucose conc.)
3. Concentrated glucose solution (high glucose conc.)

Answer 27

1. Highest water potential = 0 kPa
2. High water potential = -20 kPa
3. Low water potential = -400 kPa

Question 28

Higher water potential of the surrounding cell (less solute in the surrounding solution)

Answer 28

• Net water movement = enters cell
• Effect on animal cell = swells and bursts (i.e. the cell undergoes lysis)
• Effect on plant cell = swells and becomes turgid (i.e. the cell is full of water and the membrane is being pushed against the cell wall)

Question 29

Equal water potential of the surrounding cell (the same solute concentrations in the cell and surrounding solution)

Answer 29

• Net water movement = water leaves and enters the cell, but at equal rates
• Effect on animal cell/plant cell = no change

Question 30

Lower water potential of the surrounding cell (more solute in the surrounding solution)

Answer 30

• Net water movement = Leaves cell
• Effect of animal cell = shrinks (i.e. the cell undergoes crenation)
• Effect on plant cell = plasmolysis (membrane pulls away from the cell wall)