Question

Cell Biology Tutorial: Membrane Transport

Answer 1

\({\bf{Transporters:}}\) : integral membrane proteins with the ability to act as channels/pumps/shuffles for transport
- will often couple energetically unfavorable rxns w/ favorable ones
- contain alpha helices that allow hydrophilic substances to move

\({\bf{Types~of~Transporters:}}\)
- channels: transport water, ions, small molecules down concentration gradient, can be gated or nongated
- transporters: slower than channels, can be uniporters/antiporters/symporters depending on direction of transprt and concentration gradient
- ATP pumps: ATpases use ATP hydrolysis to move molecules against concentration gradient/electrical potential

\({\bf{Molecules~by~Transport~Type:}}\)
- gases, hormones, drugs (simple diffusion)
- glucose (facilitated transport through uniporters, ex GLUT1, or cotransport)
- ions (facilitated transport through channels, or active transport)
- water: (facilitated transport through channel)
- hydrophilic molecules, lipids (ATP pumps)
- amino acids: (facilitated transport through uniporters or cotransport)

\({\bf{Formulas:}}\)
- Fick's Law: Q = DA(C1-C2)/L
diffusion rate (Q), diffusion coefficient (D), C1 and C2 are the concentrations of the substance at two locations, L = distancec
- osmotic pressure: Pi = concentration * gas constant * temp
Pi_inside < Pi_outside = hypertonic (wrt cell), shrinkage
Pi_outside < Pi_inside = hypertonic (wrt cell), swelling

Answer 2

\({\bf{Subtypes~of~ATP~Pumps:}}\)
- P class: membranes of plants, fungi, or other eukaryotes
> can be Ca2+, Na+, K+, H+,
> 2 alpha (atp binding) + 2 beta (regulatory)

- V class: only protons
> generates low pH gradient
- vacuolar membranes in yeast/plants/other fungi
- endosomal/lysosmal membranes in animal cells
- plasma membranes in osteoclasts/kidney tubules

- F class: uses proton gradient to make ATP
> bacterial membranes
> innter mitochondrial membrane
> thylakoid membranes

> ABC superfamily:
- bacterial/mammalian plasma membranes
- amino acids, sugar, peptide, phospholipids
- highly specfic to a sinlg esubstrate
- two transmembrane (T) + two ATP-binding domains (A)

Answer 3

\({\bf{Ca2+~In~Muscle~Cells:}}\)
- P-class pump, has two conformations E1 and E2
- 2 Ca2+ binding sites, accessible from both sides of membrane
- aspartate and atp binding site on the cytosolic side
- Ca2+ binding sites within the pump

Mechanism of Action:
1. E1 binds calcium/ATP
2. phosphorylation of aspartate
3. release of ADP
4. conformational change to E2 form
5. calcium release
6. dephosphorylation
7. conformational change back to E1

Na+/K+ almost works the same way:
1. E1 form binds Na+ and ATP
2. phosphorylation of aspartate
3. release of ADP
4. conformational change to E2
5. dephosphorylation
6. conformation change back to E1
7. release of K+

Answer 4

other transport systems:

\({\bf{Cl^{-}~and~HCO3^{-}~Antiporter:}}\)
systemic capillaries: CO2 in, gets converted to HCO3-, O2 out
pulmonary: works in opposite direction
AE1 protein acts as antiporter between Cl- and CCO3-

\({\bf{Blood~Lumen~Transport:}}\)
- Atpase in basolateral surface: generates Na+ and K+
- outward movement of K+, reduces membrane potential
- drives uptake of glucose from lumen via symporter
- glucose exits the cell via facilitated transport/GLUT2

\({\bf{Acidification~of~Stomach~Lining:}}\)
- apical membrane of parietal calls have H+/K+ ATPase, Cl- and K+ channels
- outward K+ transport
- Cl and HCO3 antiporter similar to the one in the capillaries
- keeps lumen acidic, while cytosol neutral

\({\bf{Osteoclast::}}\)
- combined action of HCO3-, Cl- ions, keeps the internal space acidified, keeps cytosol neutral

Answer 5

Anyway, that's the end of my tutorial, I hope it was a helpful resource. Source material is Chapter 11 of Molecular Cell Biology, Eighth Edition, Lodish, et. al.