Transport Across Membranes

Question 1

Describe the fluid mosaic model of membranes.

Answer 1

Fluid: phospholipid bilayer which is flexible and moving
Mosaic: extrinsic & intrinsic proteins embedded throughout

Question 2

Explain the role of cholesterol and glycolipids in membranes.

Answer 2

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

Question 3

Explain the function of extrinsic and transmembrane proteins in membranes.

Answer 3

Extrinsic:
- binding sites/receptors
- antigens
- bind cells together
- involved in cell signalling
Intrinsic:
- electron carriers
- channel proteins
- carrier proteins

Question 4

Explain the functions of membranes within cells.

Answer 4

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

Question 5

Explain the functions of the cell-surface membrane.

Answer 5

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

Question 6

Name and explain 3 factors that affect membrane permeability.

Answer 6

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

Question 7

Define osmosis.

Answer 7

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

Question 8

What is water potential (ψ)?

Answer 8

• pressure created by water molecules measured in kPa
• ψ of pure water at 25 degrees & 100 kPa: 0
• more solute = ψ more negative

Question 9

How does osmosis affect plant and animal cells?

Answer 9

• osmosis INTO cell:
  Plant: protoplast swells = cell turgid
  Animal: lysis
• osmosis OUT of cell?
  Plant: protoplast shrinks = cell flaccid
  Animal: crenation

Question 10

Define simple diffusion.

Answer 10

• passive process requires no energy from ATP hydrolysis
• net movement of small, lipid-soluble molecules from an area of high concentration to an area of low concentration

Question 11

Define facilitated diffusion.

Answer 11

Passive process. Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules/ ions down the conc gradient

Question 12

Explain how channel and carrier proteins work.

Answer 12

Channel: simple tunnel no changing shape
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 13

Name 5 factors that affect the rate diffusion.

Answer 13

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

Question 14

Define active transport.

Answer 14

ATP hydrolysis releases phosphate group that bonds to carrier protein, causing it to change shape.
Specific carrier protein transports molecules from area of low concentration to an area of higher concentration

Question 15

Define co-transport.

Answer 15

Movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient.

Question 16

Explain how co-transport is involved in the absorption of glucose / amino acids in the small intestine.

Answer 16

1. Na+ actively transported out of epithelial cells & into bloodstream
2. Na+ concentration lower in epithelial cells than lumen of gut.
3. Transport of glucose/ amino acids from lumen to epithelial cells is coupled to facilitated diffusion of Na+ down electrochemical gradient