Biological membranes

Question 1

Describe the fluid mosaic model of membranes?

Answer 1

fluid = phospholipid bilayer in which individual phospholipids can move (membrane has a flexible shape)

mosaic = extrinsic and intrinsic proteins of different sizes and shapes are embedded

Question 2

Explain the role of cholesterol in membranes?

Answer 2

• steroid molecules in some plasma membranes
• connects phospholipids and reduces fluidity to make bilayer more stable

Question 3

Explain the role of glycolipids in membranes?

Answer 3

• cell signalling and cell recognition

Question 4

Explain the role of extrinsic proteins in membranes?

Answer 4

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

Question 5

Explain the role of intrinsic proteins in membranes?

Answer 5

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

Question 6

Explain the function of membranes within the cell?

Answer 6

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

Question 7

Explain the function of the cell-surface membrane?

Answer 7

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

Question 8

Name and explain 3 factors that affect membrane permeability?

Answer 8

Temperature
- high temp denatures membrane proteins/ phospholipid molecules have more Ke and move further apart

PH
- changes tertiary structure of membrane proteins

Solvent
- may dissolve membrane

Question 9

Explain how colorimetry could be used to investigate membrane permeability?

Answer 9

1) use plant tissue with soluble pigment in vacuole

2) a disrupted tonoplast and cell surface membrane increases membrane permeability and pigment diffuses into solution

3) select colorimeter filter with complementary colour

4) use distilled water to set colorimeter to 0 then measure absorbance of solution

5) high absorbance/ low transmission = more pigment in solution

Question 10

Define osmosis?

Answer 10

the passive diffusion of water across a semi-permeable membrane from an area of higher water potential to an area of lower water potential

Question 11

What is water potential?

Answer 11

• pressure created by a water molecules measured in kPa
• more solute results in a more negative water potential

Question 12

How does osmosis affect plant and animal cells?

Answer 12

into cell:
animal = lysis
plant = cell turgidity

out of cell:
animal = crenation
plant = cell flaccidity

Question 13

Define diffusion?

Answer 13

the passive net movement of molecules down a concentration gradient from an area of high to low concentration

Question 14

Define facilitated diffusion?

Answer 14

process where specific channel and carrier proteins transport large polar molecules/ ions down a concentration gradient

Question 15

Explain how channel proteins work?

Answer 15

• carry small polar molecules
• hydrophilic channels bind to specific ions
• one side of the protein closes and the other opens

Question 16

Explain how carrier proteins work?

Answer 16

• binds to complementary molecule
• conformational change releases molecule on other side of membrane

Question 17

Define active transport?

Answer 17

active process where ions and molecules move from an area of lower concentration to an area of higher concentration (against a concentration gradient) using energy from respiration

Question 18

Define endocytosis?

Answer 18

• active process
• process where the cell surface membrane engulfs material forming a sac/ vesicle round it

Question 19

Define exocytosis?

Answer 19

• active process
• process which materials are removed/ transported out of the cell
• substances to be released are packaged into secretory vesicles formed by the golgi body
• vesicles travel to the cell surface membrane
• they fuse with the membrane and release their contents out of the cell

Question 20

Name 5 factors affecting rate of diffusion?

Answer 20

• temperature
• diffusion distance
• surface area
• size of molecule
• concentration gradient steepness