Biological Membranes

Question 1

Roles of membrane

Answer 1

• partially membrane barrier bw cell+environment/bw cytoplasm+organelles
• sites of cell communication=cell signaling
• cell recognition=w antigens
• site of chemical reaction
• holding proteins/enzymes of metabolic pathways in place
• regulating transport of materials in/out of cells

Question 2

The fluid mosaic model (1972): Singer+Nicholson

Answer 2

• describes structure where various proteins are floating in the phospholipid bilayer
  role of components
• phospholipids=form basic structure+selectively permeable
• cholesterol=fits bw fatty acid tails+helps w membrane stability+makes membrane more of a barrier to water+ions
• channel proteins=allow movement of ions by facilitated diffusion
• carrier proteins=use ATP to move substances across the membrane/move large molecules by facilitated diffusion
• glycoproteins=cell recognition, hormone receptors+bind cells tgt
• glycolipids=cell recognition+hormone receptors

Question 3

Factors affecting membrane permeability

Answer 3

• temp
  low temp=phospholipid x much Ek=packed closely tgt=rigid cell membrane=down permeability
  medium temp=temp up=phospholipid more Ek=move faster+not closely packed=up permeability
  high temp=phospholipid bilayer breaks down+channel/carrier proteins denature=x control what enters/leaves cell=up permeability
• solvents
  In some solvents eg. Ethanol, phospholipids dissolve=more fluid membrane=disrupts membrane structure=more permeable
  Up conc. of solvent=up permeability cell membrane
• cholesterol
  high temp=phospholipid have more Ek=move further apart=cholesterol make it tightly packed=less permeability
  low temp=phospholipids have less Ek=tightly packed=cholesterol creates space bw them=up permeability

Question 4

Diffusion definition

Answer 4

Net movement of particles from a region of high concentration to a region of low concentration, down a conc gradient

Question 5

Types of diffusion

Answer 5

Simple
Facilitated

Question 6

Simple diffusion

Answer 6

Some molecules diffuse directly across cell membranes
- particles need to be small=pass through pores in bilayer+non-polar=dissolve through hydrophobic cell membrane core eg oxygen+CO2

Question 7

Facilitated diffusion

Answer 7

• transports large/polar molecules
• uses proteins=carrier/channel proteins=specific (allow 1/2 types of molecules)
• carrier proteins
  Large molecule attaches to carrier protein=carrier protein changes shape=carrier protein releases molecule on opposite side of membrane
• channel proteins
  Create pores in membrane which ions travel through

Question 8

Active transport definition

Answer 8

Net movement of particles against a concentration gradient using energy from respiration (ATP)

Question 9

How are carrier proteins used in active transport?

Answer 9

• act as pumps+complementary shape to molecule they carry
• large molecule/ion binds to carrier protein
• ATP binds to carrier protein
• ATP hydrolysis to ADP+Pi=carrier protein changes shape (the phosphate)=releases large molecule/ion on opposite side of membrane
• Pi released from carrier protein=returns to original shape

Question 10

Factors that affect rate of active transport

Answer 10

Temperature
- high temp=particles more EK=move faster
- Respiration also increases with temperature
- Very high temp=denature carrier proteins, decreasing the rate of active transport
Thickness of membrane
- thin membrane=short distance=move faster
Number of carrier proteins
- more proteins=faster the rate of active transport
Rate of respiration
- more respiration=more ATP to be used

Question 11

Bulk transport definition

Answer 11

Another form of active transport that transports large quantities of material are moved in/out of cells

Question 12

Types of bulk transport

Answer 12

Endocytosis=transports material into cells eg insulin taken to a cell
Exocytosis=transports material out of cells eg vesicles from golgi

Question 13

Types of endocytosis

Answer 13

Phagocytosis=solid materials
Pinocytosis=liquid materials

Question 14

Osmosis definition

Answer 14

Net movement of water molecules across a selectively permeable membrane down a water potential gradient

Question 15

Factors affecting rate of osmosis

Answer 15

Temperature
Higher temp=water molecules more EK=diffuse faster
Water potential gradient
steeper the gradient=faster rate of osmosis
thickness of membrane
Thin membrane=short distance=faster diffusion
surface area
Large SA=more molecules cross membrane at once=faster osmosis

Question 16

How osmosis affects cells

Answer 16

animal cells
hypotonic solution=higher Ψ than cell=water moves into cell=cell swell+may burst
isotonic solution=same Ψ as cell=x net movement of water=cell stays the same
hypertonic solution=lower Ψ than cell=water moves out of cells eg insulin=cell shrinks

plant cells
hypotonic solution=higher Ψ than cell=water moves into cell=cell swell+becomes turgid
isotonic solution=same Ψ as cell=x net movement of water=cell stays the same
hypertonic solutions=lower Ψ than cell=water moves moves out of cells eg insulin=cell shrinks+becomes flaccid+plasmalyses