S2) Membrane Proteins & Cell Signalling

Question 1

The movement of ions and hydrophilic molecules across a membrane is mediated.

Provide reasons for this

Answer 1

• Maintenance of intracellular pH
• Maintenance of intracellular ionic composition
• Regulation of cell volume
• Extrusion of waste products of metabolism

Question 2

What is passive transport?

Answer 2

Passive transport is a movement of molecules across a cell membrane, down a concentration gradient, without energy input

Question 3

When does passive transport occur?

Answer 3

Occurs when non-polar molecules can enter and, therefore, diffuse across the hydrophobic domain of lipid bilayers

Question 4

How does the rate of passive transport change with the concentration gradient?

Answer 4

The rate of passive transport increases linearly with increasing concentration gradient

Question 5

What is osmosis?

Answer 5

Osmosis is a process by which molecules of water tend pass across a semipermeable membrane from a less concentrated solution into a more concentrated one

Question 6

What are permeability coefficients?

Answer 6

- Permeability coefficient is the speed at which any substance transverses a lipid bilayer

• Low for ions and hydrophilic molecules, higher for water molecules

Question 7

What is facilitated transport?

Answer 7

Facilitated transport is a saturable process of spontaneous passive transport of molecules or ions across a cell’s membrane via specific transmembrane integral proteins

Question 8

What is active transport?

Answer 8

Active transport is the movement of ions or molecules, against a concentration gradient (low-high) across a cell membrane, requiring energy

Question 9

What is secondary active transport?

Answer 9

- Secondary active transport is when the transport of one substance is linked to the concentration gradient for another substance via a co-transporter

• The primary energy source, e.g. hydrolysis of ATP, is used indirectly

Question 10

Identify and describe two examples of secondary active transport

Answer 10

• Na⁺- Ca²⁺-exchange – inward flow of sodium down its concentration gradient drives outward flow of Ca²⁺ up its concentration gradient (antiport)
• Na⁺- H⁺- exchange – inward flow of sodium down its concentration gradient leads to cell alkalization by removing H⁺ (antiport)

Question 11

Active transport is employed to overcome unfavourable chemical or electrical gradients.

How does it work?

Answer 11

• Movement of the transported ion or molecule must be coupled to a thermodynamically favourable reaction
• Free energy to drive active transport can come either directly or indirectly from the hydrolysis of ATP, electron transport or light

Question 12

What is uniport?

Answer 12

Uniport: one solute molecule alone is transported from one side of the membrane to the other

Question 13

Symport and antiport are two forms of co-transport.

Distinguish between them

Answer 13

• Symport: the transfer of one solute molecule depends on the simultaneous transfer of a second solute in the same direction
• Antiport: the transfer of one solute molecule depends on the simultaneous transfer of a second solute in the opposite direction