The cell surface

Question 1

Cell membrane

Answer 1

Separates the intra- and extra-cellular compartments, creating two different environments. Composed of a phospholipid bilayer, which is fluid and dynamic and has proteins embedded within it.

Question 2

Phospholipids

Answer 2

Each phospholipid unit is ampipathic. The hydrophilic phosphate heads face outwards, while the hydrophobic fatty acid tails face inwards.

Question 3

Function of membrane lipids

Answer 3

Lipids within membranes are used to generate signals. Cholesterol ‘intercalates’ between phospholipids, tightening packing within the bilayer which increases membrane rigidity and decreases permeability.

Question 4

Membrane fluidity

Answer 4

• allows signalling lipids and membrane proteins to move around and interact ie. cell signalling
• allows membranes to fuse ie. exocytosis
• ensures membranes are shared equally during cell division

Question 5

Membrane protein functions

Answer 5

a) Transport
b) Enzymatic activity
c) Signal transduction
d) Cell-cell recognition
e) Intercellular joining
f) Attachment to the cytoskeleton and extracellular matrix (ECM)

Question 6

Types of membrane protein

Answer 6

Integral- either single or multi-pass transmembrane proteins.
Peripheral- do not transverse both membrane layers.

Question 7

Transmembrane protein functions

Answer 7

Transporters- required for polar/charged molecules and ions to cross the membrane.
Anchors- link structures to intra/extracellular scaffolding.
Receptors- bind ligands and/or generate signals inside cells.
Signal transduction molecules- pass on and amplify signals.

Question 8

Passive transport

Answer 8

Substances move down their concentration gradient, via simple or facilitated diffusion. No ATP energy is required for this process.

Question 9

Active transport

Answer 9

Substances move against their concentration gradient, requiring energy from ATP.

Question 10

Protein channels

Answer 10

Transmembrane proteins forming hydrophilic pores, with selectivity based on size and charge of the molecule. Most are non-directional, and transport is fast.

Question 11

Uniporter carrier proteins

Answer 11

Possess a specific solute binding site, undergoing a conformational change to transport a solute. Highly selective, and transport is relatively slow.
Eg. glucose transporter (Glut2) in gut epithelia

Question 12

Electrochemical gradients

Answer 12

Combine the concentration gradient and membrane potential. Cells maintain electrochemical gradients to drive transport, to maintain osmotic balance and to balance electrical forces.

Question 13

ATP driven pumps

Answer 13

Expend energy for primary active transport, to move substances against their electrochemical gradients.
Eg. Na+/K+ ATPase in all animal cells

Question 14

Na+/K+ ATPase

Answer 14

1) 3 Na+ bind. Pump hydrolyses ATP.
2) Phosphorylated pump undergoes conformational change to transport Na+ out of the cell.
3) 2 K+ bind. Phosphate group is released.
4) Dephosphorylated pump returns to original conformation to transport K+ into the cell.

Question 15

Coupled transporters

Answer 15

Pair the movement of one solute to the active transport of another. Do not depend on the hydrolysis of ATP.
Eg. Na+/glucose symporter in gut epithelia
Na+/Ca2+ antiporter in cardiac muscle