L1: Membrane Transport

Question 1

Describe intracellular and extracellular distributions of sodium, potassium, calcium and chloride?

Answer 1

• Na: higher in ECF
• K: higher in ICF
• Ca: higher in ECF
• Cl: higher in ECF

Question 2

Types of membrane transport. Describe

Answer 2

1. ) Non-vesicular:
   a. ) Passive: no energy, diffusion through membrane or channels/carriers (facilitated) – moving substances down [] gradient
   b. ) Active: energy, primary uses pumps directly driven by ATPase, while secondary uses gradients indirectly set up by ATPase – for moving substances against concentration gradient

2.) Vesicular: energy, involves lipid fusion for endo- and exo-cytosis

Question 3

Types of secondary active transporters

Answer 3

1. Symporter (co-transporter)

2. Antiporter

Question 4

Factors that affect simple ROD through cell membrane

Answer 4

• Think Fick’s Law: ROD proportional to SA.[] gradient.permeability/membrane thickness
• Membrane permeability=lipid solubility/molecule size
• Therefore factors:
  a. ) lipid solubility
  b. ) molecule size
  c. ) surface area of membrane
  d. ) cell membrane thickness
  e. ) concentration gradient
  f. ) composition of lipid layer

Question 5

What molecules pass through the phospholipid bilayer of a cell through simple diffusion?

Answer 5

• CO2, N2, o2, ethanol are permeable

- Slightly permeable = urea and water

Question 6

What mediates water permeability in a cell membrane?

Answer 6

• Aquaporins

Question 7

Nephrogenic diabetes insipidus. Describe pathophysiology

Answer 7

• Mutations in aquaporin-2 gene resulting in excretion of large amounts of diluted urine leading to excessive thirst
• ADH (aka vasopressin) usually binds to medullary collecting duct cells in kidney signaling upregulation of aquaporins for reabsorption of water. D/t mutation in gene, this signal is not transduced and channels are not put up on cell surface, therefore no / decreased water is reabsorbed back into circulation.

Question 8

Types of ion channels. Is energy required to operate these?

Answer 8

1. ) Voltage gated
2. ) Ligand gated
3. ) Mechanically gated
   * No energy, facilitated diffusion – these are uniporters

Question 9

What types of membrane transport uses a uniporter?

Answer 9

• Facilitated diffusion, no energy

Question 10

Does glucose transport require energy?

Answer 10

• No, via GLUT uniporter via facilitated diffusion

Question 11

Factors that control transport rate via use of transporter protein?

Answer 11

• Concentration gradient
• Transporter number: doubling # of transporters, doubles rate
• Affinity of binding: substrate to transporter – higher Km, lower the affinity

Question 12

Clinical importance of ABC family of proteins

Answer 12

• ABC = ATP binding cassette, all proteins in this family have a common ATP-binding domain. These are primary active transporters.
• 1.) CFTR: cystic fibrosis transmembrane conductance regulator mutation in 2/3rd CF cases
• 2.) MDR-proteins are over-expressed in some tumors causing resistance to cancer drug therapy

Question 13

Describe secondary active transport

Answer 13

• Transport of substance up its concentration gradient is coupled with downhill movement of another substance on its concentration gradient. Requires indirect ATP usage

Question 14

Examples of symporters

Answer 14

1. ) Sodium/glucose symporter: 2x Na+in down its gradient, 1 glucose in up its gradient. High sodium outside cell maintained by Na/K ATPase
2. ) Na/K/Cl symporter: all into cell
3. ) Cl/K symporter: out from cell
4. ) Na/NT symporter: into cell

Question 15

Examples of antiporters

Answer 15

1. ) Na/Ca antiporters: Na in, Ca out

2. ) Na/H antiporters: Na in, H out

Question 16

Do facilitated diffusion (uniporter), symporters and antiporters move proteins or lipids?

Answer 16

• No, requires vesicular transport

Question 17

Types of endocytosis

Answer 17

1. ) Phagocytosis: macrophages, neutrophils
2. ) Pinocytosis: by all cells – called fluid-phase endocytosis by some texts
3. ) Fluid-phase endocytosis: uptake of material dissolved in ECF with material not bound to receptors, some utilize clathrin. Non-specific.
4. ) Receptor-mediated endocytosis: clathrin-dependent involving receptor specific for material being endocytosed
5. ) Caveolae endocytosis: similar to clathrin-mediated but process uses caveolae and caveolin as coating protein.

Question 18

Describe receptor-mediated endocytosis

Answer 18

1. ) Receptor protein binds specific material
2. ) Receptor protein tails bind to adaptins, which associate with clathrin
3. ) Clathrin cages the coated pits and the receptors with substance are included forming endocytic vessel
4. ) Vesicle is uncoated and is ready to fuse with internal destination

Question 19

What is the pathophysiology of familial hypercholesterolemia?

Answer 19

• high plasma LDL levels d/t defect in receptors used for endocytosis of LDL
• individuals predisposed to atherosclerosis and heart disease