Transport Across Cell Membranes

Question 1

How do phospholipids form a bilayer in the membrane? What type of molecules can pass?

Answer 1

Hydrophilic phosphate ‘head’ attracted to water, hydrophobic f.acid ‘tail’ repulsed. Aqueous environment inside and outside of cell causes phospholipids to form a bilayer.

• non polar
• small
• lipid soluble

Question 2

How do protein channels and carrier proteins work?

Answer 2

Channels - water filled tubes to allow water soluble ions to diffuse across
Carriers - bind to ions/mol’s and change shape moving them across membrane

Question 3

Role of cholesterol molecules in the membrane

Answer 3

• add strength to membrane w/o making it too rigid
• v. hydrophobic preventing loss of water + dissolved ions

Question 4

Which molecules act as receptors on cell membranes?

Answer 4

Glycolipids - specific chemicals

Glycoproteins - hormones, neurotransmitters

Question 5

State Fick’s law

Answer 5

Rate of diffusion = SA * diff. in conc / length of diffusion path

Question 6

What happens to animal cells when placed in solutions of different concentrations?

Answer 6

hypotonic - osmosis in, cell bursts (lysis)
isotonic - no change
hypertonic - osmosis out, cell crenated (wrinkly)

Question 7

What happens to plant cells when placed in solutions of different concentrations?

Answer 7

hypotonic - osmosis in, turgid
isotonic - cell flaccid
hypertonic - osmosis out, cell plasmolysed as membranes pulls away from cell wall

Question 8

Sodium potassium pump

Answer 8

• found in every cell
• active transport of 3 Na+ ions out, 2 K+ in
• ATP -> ADP + P + energy

Question 9

Co-transport of glucose and sodium

Answer 9

1. Na+ K+ pump creates low Na+ conc. in epithelial cells
2. Facilitated diffusion of Na+ down conc. gradient from lumen into cell
3. No glucose gradient so glucose uses energy inherent in sodium gradient + couples w/ Na+ to move into cells
4. Facilitated diffusion of glucose into blood