Plasma membranes

Question 1

Explain why phospholipids form a bilayer in plasma membranes. (3 marks)

Answer 1

• hydrophilic phosphate groups face aqueous solutions
• fatty acid tails are non polar and hydrophobic so will hide from aqueous environment
• as both tissue fluid and cytoplasm are aqueous, 2 layers are formed with heads facing out and tails facing in

Question 2

Why is the phospholipid bilayer called the fluid mosaic model?

Answer 2

Fluid- phsopholipids are free to move within the layer
Mosaic- proteins of various shapes and sizes embed the layers

Question 3

What are 5 features of the plasma membrane?

Answer 3

Glycoprotein
Glycolipid
Cholestrol
Extrinsic protein
Intrinsic protein (channel and carrier)

Question 4

What are 4 functions of (plasma) membranes?

Answer 4

• separate cell contents from outside environments
• compartmentalises cell interior into organelles which specific optimal conditions
• controls what enters/exits
• a site for biochemical reactions

Question 5

What is the function of cholestrol?

Answer 5

• binds to FATTY ACID TAILS to cause PACKING (also binds to heads but this does not cause packing)
  High temps:
• converges phospholipids
• fluidity DECREASES
  Low temps:
• diverges phospholipids
• fluidity INCREASES

Question 6

What do channel proteins do?

Answer 6

allow polar molecules/ions into the cell

Question 7

Simple vs facilitated diffusion

Answer 7

Facilitated involves channel/carrier protein

Question 8

Why are non polar molecules able to diffuse across the phospholipid bilayer? What does this make them?

Answer 8

They are able to interact with the hydrophobic fatty acid tails in the core
Which makes them lipid soluble

Question 9

3 ways to inc diffusion across membranes

Answer 9

• inc SA
• dec thickness of surface
• if facilitated, inc protein channels

Question 10

Glycolipids vs Glycoproteins

Answer 10

Both are antigens but glycoproteins also allow cell adhesion and cell signaling by acting as receptors

Question 11

Water potential Ψ (and measured in?)

Answer 11

the pressure exerted by water molecules as they collide with a membrane/container, measured in Pa

Question 12

What is the water potential of pure water?

Answer 12

0 Pa

Question 13

Isotonic, hypertonic and hypotonic solutions, explained in terms of water potential

Answer 13

Iso: same water potential as cytoplasm

Hyper: lower water potential (higher conc) than cytoplasm

Hypo: higher water potential (lower conc) than cytoplasm

Question 14

What happens to an animal/plant cell in low water potential? High water potential? Define turgor

Answer 14

ANIMAL
Crenation
Cytolysis
PLANT
Plasmolysis
Turgor (pressure of membrane against cell wall)

Question 15

Osmosis

Answer 15

the movement of water from an area of high to low water potential across a partially permeable membrane, until an equilibrium is reached

Question 16

Active transport (selective or not?)

Answer 16

pumping of molecules across a membrane through carrier proteins AGAINST a concentration gradient (selective)

Question 17

How is active transport carried out? (3)

Answer 17

• molecules bind to receptors on a carrier protein
• ATP binds to a carrier protein and hydrolyses
• energy causes protein to change shape and release molecules INTO cell

Question 18

2 types of bulk transport (and 2 subtypes)? How do they work?

Answer 18

Exocytosis:
(last steps of protein synthesis) vesicle fuses with membrane & releases

Endocytosis:
phagocytosis (solids) - pinocytosis (liquids)
membrane invaginates and fuses w material to form vesicle (endosome)

Question 19

3 reasons bulk transport is active

Answer 19

energy is needed for:
- movement of vesicles along cytoskeleton
- changing the shape of cells (invagination)
- fusion of membranes