Plasma membranes

Question 1

Does the effect of cholesterol depend on temperature?

Answer 1

Yes

Question 2

At low temperatures what affect does cholesterol have on the fluidity of the membrane?

Answer 2

increases fluidity

Question 3

At high temperatures what affect does cholesterol have on the fluidity of the membrane?

Answer 3

decreases fluidity

Question 4

Describe the fluid mosaic model of membranes

Answer 4

Fluid: phospholipid bilayer in which individual phospholipids can move = membrane has flexible shape
Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded

Question 5

Explain the role of cholesterol and glycolipids in membranes

Answer 5

Cholesterol: steroid molecule in some plasma membranes; connects phospholipids and reduces fluidity to make bilayer more stable
Glycolipids: cell signalling and cell recognition

Question 6

Explain the functions of extrinsic and transmembrane proteins in membranes

Answer 6

• binding sites/receptors e.g. for hormones and drugs
• antigens (glycoproteins)
• bind cells together
• involved in cell signalling

Question 7

Explain the functions of intrinsic transmembrane proteins in membranes

Answer 7

• electron carriers (respiration/photosynthesis)
• channel proteins (facilitated diffusion)
• carrier proteins (facilitated diffusion/active transport)

Question 8

Explain the functions of membranes within cells

Answer 8

• provide internal transport system
• selectively permeable to regulate passage of molecules into/out of organelles or within organelles
• provide reaction surface
• isolate organelles from cytoplasm for specific metabolic reactions

Question 9

Explain the functions of the cell-surface membrane

Answer 9

• isolates cytoplasm from extracellular environment
• selectively permeable to regulate transport of substances
• involved in cell signalling/cell recognition

Question 10

Name and explain 3 factors that affect membrane permeability

Answer 10

• temperature: high temperature denatures membrane proteins/phospholipid molecules have more kinetic energy and move further apart
• pH: changes tertiary structure of membrane proteins
• use of a solvent: may dissolve membrane

Question 11

Outline how colorimetry could be used to investigate membrane permeability

Answer 11

1, Use plant tissue with soluble pigment in vacuole. Tonoplast and cell-surface membrane disrupted = increased permeability = pigment diffuses into solution
2, Select colorimeter filter with complementary colour
3, Use distilled water to set colorimeter to 0. Measure absorbance/ % transmission value of solution
4, High absorbance/low transmission = more pigment in solution

Question 12

Define osmosis

Answer 12

Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established

Question 13

What is water potential?

Answer 13

• pressure created by water molecules measured in kPa
• water potential of pure water at 25 degrees C and 100kPa is 0
• more solute = water potential more negative

Question 14

How does osmosis affect plant and animal cells?

Answer 14

osmosis INTO cell:
plant: protoplast swells = cell turgid
animal: lysis

osmosis OUT of cell:
plant: protoplast shrinks = cell flaccid
animal: crenation

Question 15

Define simple diffusion

Answer 15

Passive process requires no energy from ATP hydrolysis.
Net movement of small, lipid-soluble molecules directly through the bilayer from an area of high concentration to an area of lower concentration.

Question 16

Define facilitated diffusion

Answer 16

Passive process. Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules/ions (not soluble in hydrophobic phospholipid tail) down concentration gradient

Question 17

Explain how channel and carrier proteins work

Answer 17

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

Question 18

Define active transport

Answer 18

Active process: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape.
Specific carrier protein transports molecules/ions from area of low concentration to area of higher concentration

Question 19

Define exocytosis and endocytosis

Answer 19

• active process
• involved in bulk transport and transporting large particles
• vesicles fuse with cell surface phospholipid membrane

Question 20

Name 5 factors that affect the rate of diffusion

Answer 20

• temperature
• diffusion distance
• surface area
• size of molecule
• difference in concentration (how steep the concentration gradient is)