transport across membranes

Question 1

Describe the basic structure of the cell membrane.

Answer 1

Phospholipid bilayer. Contains binding sites (receptors) for chemical recognition. Flexible due to FA’s but can rupture if over stretched. The hydrophobic inner portion of the membrane provides insulation.

Question 2

What are the 2 classes of membrane proteins?

Answer 2

Integral and peripheral. Integral cannot be removed without disrupting the membrane. Peripheral can be removed without major disruption.

Question 3

What can integral membrane proteins act as?

Answer 3

Carriers, ion channels, enzymes and receptors.

Question 4

What are peripheral membrane proteins important for? Give 1 example.

Answer 4

Cell shape and motility. Contact IMP’s in cytoplasmic surface and have enzymatic function. Example: dystrophin - peripheral membrane protein of skeletal muscle. In DMD, muscles degenerate until they fail.

Question 5

Are membrane carbohydrates intracellular or extracellular?

Answer 5

All extracellular.

Question 6

What is the protein content of myelin, plasma membranes and mitochondrial membranes?

Answer 6

Myelin: low, 18%. Plasma membrane: 50%. Mitochondrial: 75% as it is involved in energy transduction.

Question 7

What is the permeability co-efficient?

Answer 7

A measure of the ease at which a molecule can pass through a given membrane.

Question 8

Define osmolarity.

Answer 8

Total number of solute particles.

Question 9

What does an isosmotic solution contain?

Answer 9

An isosmotic soultion contains the same number of solute particles as plasma (ECF). Fewer - hyposmotic. Greater - hyperosmotic.

Question 10

What determines the tonicity of a solution?

Answer 10

The number of non-penetrating solute particles. In the ECF this is Na+. In the ICF this is K+. Isotonic? Same as plasma. Fewer? Hypotonic. Greater? Hypertonic.

Question 11

What 3 characteristics must a molecule have to diffuse through the lipid bilayer?

Answer 11

Small, lipophilic and not charged. O, N, CO2 and urea diffuse easily across.

Question 12

Why do ions go through the lipid bilayer a lot quicker than they should?

Answer 12

They go through protein channels. These provide an aqueous route for ions to get across the membrane.

Question 13

How does H2O pass through the lipid membrane?

Answer 13

Through aquaporins which are ubiquitous.

Question 14

What are the 2 types of ion channel?

Answer 14

Voltage-gated (Na+ channels) or ligand-gated (ACh).

Question 15

When is electrochemical equilibrium reached?

Answer 15

When chemical and electrical gradients are in balance.

Question 16

What is a carrier-mediated transport system?

Answer 16

A molecule, such as glucose, binds to a receptor. The receptor undergoes a conformational change which exposes the binding site on the other side of the membrane. Glucose diffuses through.

Question 17

What are the 2 types of carrier-mediated transport system?

Answer 17

Facilitated diffusion (glucose) and active transport (Na+/K+ ATPase).

Question 18

If a molecule cannot cross the cell membrane, what happens when its concentration changes?

Answer 18

Osmotic flux - water moves to even out the concentrations.

Question 19

What is the osmolarity of normal human plasma?

Answer 19

285 mOsmoles/L (300).

Question 20

What is the safest fluid?

Answer 20

NaCl - isotonic saline.

Question 21

What happens in vivo in patients with uraemia?

Answer 21

Non-penetrating NaCl in the ECF prevents water following urea into cells and so haemolysis does not occur. Plasma is hyperosmotic but isotonic. In vitro, cells suspended in a solution of urea will burst as urea is not a non-penetrating solute.

Question 22

Which factors favour diffusion through the membrane?

Answer 22

A large surface area, high permeability and high concentration gradient.