Cell Membranes

Question 1

Plasma membrane

Answer 1

• Phospholipid bilayer
• Cholesterol
• Extrinsic proteins
• Intrinsic proteins
• Glycolipids and Glycoproteins

Question 2

Factors affecting membrane fluidity

Answer 2

• More unsaturated fatty acid tails the more fluid the membrane as tails are bent and fit loosely
• Temperature increase gives phospholipids and proteins more energy, increased fluidity

Question 3

Phospholipids

Answer 3

• Allows lipid soluble substances in

- Doesn’t allow water soluble substances

Question 4

Cholesterol

Answer 4

Lie alongside phospholipids to stabilise the bilayer and regulate fluidity

Question 5

Extrinsic proteins

Answer 5

• Structural support

- Cell receptors

Question 6

Intrinsic proteins

Answer 6

• Span through bilayer

- Some are carriers and form hydrophilic channels

Question 7

Cell membrane proteins

Answer 7

• Transport proteins: provide hydrophilic channels for specific ions
• Glycoproteins: Form hydrogen bonds with water molecules and stabilise membrane
• Receptor molecules
• Antigens
• Enzymes

Question 8

Glycolipids

Answer 8

• Short branched carbohydrate chains attached to phosphate head
• Act as receptors like glycoproteins

Question 9

Fluid Mosaic Model

Answer 9

• “Fluid” as the individual phospholipid molecules are in constant motion
• “Mosaic” due to embedded proteins scattered throughout varying in shape, size and pattern

Question 10

Membrane permeability

Answer 10

• Lipid soluble substances freely permeable e.g Oxygen and CO2
• Water soluble substances not freely permeable e.g glucose and amino acids (Polar molecules)

Question 11

Factors affecting membrane permeability

Answer 11

• Organic solvents dissolve membrane

- Temperature increases fluidity

Question 12

Types of transport

Answer 12

• Simple diffusion
• Facilitated diffusion
• Osmosis
• Active transport (+Co transport)
• Bulk transport (Endocytosis and Exocytosis)

Question 13

Simple diffusion

Answer 13

The net movement of molecules or ions down a concentration gradient, from a region of higher concentration to a region of lower concentration

-Uses channel proteins ONLY

Question 14

Factors affecting the rate of diffusion

Answer 14

• Concentration gradient (large): Faster net rate of diffusion
• Surface area (large): Increases with larger surface area (Surface are x Difference in concentration)/Length of diffusion path
• Diffusion distance (small): Faster with a smaller distance
• Size of molecule: Large molecules diffuse slower
• Temperature: Faster due to more kinetic energy
• Nature of membrane: Number and composition of pores

Question 15

Facilitated diffusion

Answer 15

• The passive transfer of molecules or ions down a concentration gradient across a membrane, by carrier proteins in the membrane
• Uses BOTH channel and carrier proteins

Question 16

Facilitated diffusion (Channel proteins)

Answer 16

• Pores lined with polar groups allowing water soluble ions to pass through
• Open rigid

Question 17

Facilitated diffusion (Carrier proteins)

Answer 17

• Allow diffusion of larger polar molecules e.g amino acids
• Change shape when a specific molecule fits into them to allow it to pass through
• Passive process

Question 18

Facilitated diffusion (Co-transport)

Answer 18

Brings molecules and ions into cells together on the same transport protein

Question 19

Osmosis

Answer 19

The diffusion of water molecules from a region of higher water potential to a region of lower water potential through a selectively permeable membrane

-Water molecules pass through phospholipids and aquaporins

Question 20

Water potential

Answer 20

The tendency of water molecules to move into/out of a cell or solution

Question 21

Solute potential

Answer 21

The reduction in water potential due to the presence of solute molecules

Question 22

Pressure potential

Answer 22

The pressure exerted on the cell contents by the cell wall

Question 23

Hypotonic, Hypertonic and Isotonic solutions

Answer 23

• Hypotonic: Low solute potential
• Hypertonic: High solute potential
• Isotonic: Equal solute potential

Question 24

Active Transport (Na+/K+ pump)

Answer 24

• Movement of molecules across a membrane against a concentration gradient using energy
• Uses intrinsic carrier proteins ONLY

Na+/K+ pump:

1. Na+ binds to the specific receptor site on the specific carrier protein
2. ATP binds to the carrier protein and is hydrolysed, leaving only a phosphate still bound (Phosphorylation)
3. Carrier protein changes shape allowing Na+ to be released out on the other side
4. K+ then binds to the carrier protein from outside which causes another change in shape
5. The bound phosphate detaches itself which allows the K+ to be released into the cytoplasm
6. The carrier protein returns to its original shape

• Cyanide is a respiratory inhibitor, so when it is added the rate of active transport is slowed or unable to occur
• Rate affected by number of carrier proteins available

Question 25

Co-transport (Na+/Glucose)

Answer 25

-Type of facilitated diffusion

• Na+ and glucose bind to the co-transporter carrier protein
• The carrier protein then changes shape
• The Na+ molecule moves down its concentration gradient into the cell
• The glucose molecule moves against its concentration gradient into the cell
• Concentration gradient of Na+ is maintained by the active transport of Na+ into the blood by the Na+/K+ pump

Question 26

Bulk transport

Answer 26

Endocytosis (Entering):

• The engulfing of material by the infolding of the plasma membrane, bringing it into the cell enclosed within a vesicle.
• Phagocytosis: Bulk uptake of solid
• Pinocytosis: Bulk uptake of fluid
• Decreases surface area of membrane

Exocytosis (Leaving):

• The secretion of materials through the fusing of vesicles with the membrane
• Increases surface area of membrane