Cell Membranes

Question 1

Describe the fluid mosaic model of membranes.

Answer 1

Fluid: phospholipid bilayer in which individual phospholipids can move giving the membrane a flexible shape.
Mosaic: extrinsic and intrinsic proteins of different sizes are embedded

Question 2

Explain the role of cholesterol and glycolipids in membranes.

Answer 2

Steroid molecule ( cholesterol) - connects phospholipids and reduces fluidity to make bilayer more stable
Glycolipids - cell signalling and cell recognition

Question 3

Explain the function of extrinsic proteins in membranes.

Answer 3

-Binding sites/ receptors e.g for hormones and drugs
-Antigens (glycoproteins)
-Bind cells together
-Involved in cell signalling

Question 4

Explain the functions of intrinsic transmembrane proteins in membranes.

Answer 4

-Channel proteins
(facilitated diffusion)
-Carrier proteins (facilitated diffusion/ active transport)

Question 5

Explain the functions of membranes within cells.

Answer 5

• Selectively permeable to regulate passage of molecules into/ out of organelles
• Sites of chemical reactions
• Sites of cell communication (cell signalling)
  -Compartmentalising

Question 6

Explain the function of the cell surface membrane

Answer 6

• Isolates cytoplasm from extracellular environment
• Selectively permeable to regulate transport of substances
• Involved in cell signalling/ recognition
• Binding site for hormones/ toxins

Question 7

Name and explain three factors that effect membrane permeability

Answer 7

Temperature - high temperature denatures membrane proteins/ phospholipid molecules have more kinetic energy and move further apart which increases membrane permeability
Solvents - dissolve the lipids in the membrane creating more space so the membrane becomes more permeable.
Detergents - soap molecules knock the heads off the phospholipids which creates gaps in the membrane increasing permeability.

Question 8

Define osmosis.

Answer 8

Osmosis is the net movement of water across a semi permeable membrane from an area of higher water potential to an area of lower water potential down a water potential gradient.

Question 9

What is water potential?

Answer 9

The measure of the ability of water molecules to move freely in solution.
Pure water has the highest water potential.

Question 10

How does osmosis affect plant and animal cells?

Answer 10

Water moves into the cell:
- plant: protoplast swells = turgid cell
- Animal: lysis (cell bursts)
Water moves out of cell:
- plant: protoplast shrinks = flaccid cell
- Animal: crenation

Question 11

Define simple diffusion.

Answer 11

Net movement of small, lipid soluble ( able to dissolve in the lipid portion of the membrane) molecules from an area of high concentration to an area of lower concentration down a concentration gradient

Question 12

Define facilitated diffusion.

Answer 12

Specific channel or carrier proteins with complimentary binding sites transport large or polar molecules/ions down a concentration gradient.

Question 13

Explain how channel and carrier proteins work?

Answer 13

Channel: hydrophilic channels bind to specific ions = one side of the protein closes and the other opens
Carrier: binds to complimentary molecule = conformational change releases molecule on other side of membrane, in facilitated diffusion: passive process; in active transport: requires energy from ATP hydrolysis.

Question 14

Define active transport.

Answer 14

Active process: ATP hydrolysis releases a phosphate ion which binds to carrier protein changing its shape.
A specific carrier protein transports molecules/ ions from an area of low concentration to an area of high concentration. (AGAINST A CONCENTRATION GRADIENT)

Question 15

Define exocytosis and endocytosis.

Answer 15

Endocytosis = A cell can surround a substance with a section of its plasma membrane. The membrane can then pinch off to form a vesicle inside the cell containing the ingested substance.
Exocytosis = Vesicles containing substances which need to be released from the cell e.g. digestive enzymes or lipids pinch off from the sacs of the golgi apparatus and move towards the plasma membrane where they fuse with the plasma membrane and release their contents outside the cell.

Question 16

Name 5 factors that affect rate of diffusion.

Answer 16

-Temperature
-Diffusion distance
-Surface area
-Size of molecule
-Steepness of concentration gradient

Question 17

Practical: investigating the permeability of the cell membrane

Answer 17

• Cut 5 equal sized pieces of beetroot and rinse them to remove any pigment released during cutting
• Place the 5 pieces in 5 different test tubes, each with 5cm cubed of water
• Place each test tube in a water bath at a different temperature e.g. 10, 20, 30, 40 50, degrees celcius etc for the same length of time
• Remove the pieces of beetroot from the tubes leaving just the coloured liquid
• Now you need to use a colorimeter to measure how much light is absorbed. The higher the permeability of the membrane the more pigment released so the higher the absorbance of the liquid

Question 18

Describe the phospholipid bilayer and explain its role within the membrane.

Answer 18

• Hydrophobic tail repels water
• Hydrophilic head attracts water
• Molecules arrange themselves in a bilayer, heads facing outwards towards the water on either side of the membrane
• The centre of the membrane is hydrophobic and doesnt allow the water-soluble substances through.
• Therefore the role of a phospholipid bilayer is to act as a barrier to water soluble substances

Question 19

Practical: investigate the factors affecting the rate of diffusion.

Answer 19

• Make up some agar jelly with phenolpthalein and dilute sodium hydroxide which will turn the jelly pink
• Fill a beaker with some dilute hydrochloric acid
• Using a scalpel cut out a few cubes from the jelly and put them in the beaker of acid
• If you leave the cubes for awhile they will eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide.
• Change the factor and repeat e.g concentration of HCL, surface area to volume ratio of agar jelly

Question 20

What would happen if you put cells in an isotonic solution?

Answer 20

An isotonic solution is one where the solution has the same water potential as the inside of the cell so there would be no net movement

Question 21

What would happen if you put animal cells in a hypotonic solution?

Answer 21

Hypotonic solution means the solution has a higher water potential than the inside of the cell so there would be a net movement of water into the cell and as there is no cell wall the cell bursts and lysis occurs

Question 22

What happens when you put plant cells in a hypotonic solution?

Answer 22

Water enters the plant cell and the vacuole begins to swell. The contents of the cell push up against the cell wall making the cell turgid. However, the cell wall stops the cell from bursting.

Question 23

What would happen if you put animal cells in a hypertonic solution?

Answer 23

Water leaves the animal cell and the cell shrinks then crenates.

Question 24

What would happen if you put a plant cell in a hypertonic solution?

Answer 24

Water leaves the cell and the vacuole. The cell membrane pulls away from the cell wall and plasmolysis occurs. Then the cell becomes flaccid.

Question 25

Name the passive methods of movement across membranes.

Answer 25

Diffusion and facilitated diffusion.

Question 26

Name the active methods of movement across membranes.

Answer 26

Active transport, endocytosis and exocytosis

Question 27

What are the differences between active and passive methods of movement across membranes?

Answer 27

The active ones require ATP as a immediate source of energy.