Membrane transport
1. Summary
A. Simple diffusion
1) Diffusion by electrochemical gradient no energy required
2) No channel or carrier (or transporter protein) is needed
B. Passive transport (= Facilitated diffusion)
1) Transports solutes by electrochemical gradient (concentration gradient + electrical
gradient) no energy required
2) Through channel or carrier protein
C. Active transport
1) Transport solutes to cross membrane against their electrochemical gradient.
Need ATP hydrolysis or an ion gradient
2) Through carrier protein
D. Endocytosis & exocytosis
: transport by memb-bounded vesicle (endosome)
2. Permeability of lipid bilayer by simple diffusion
A. By electrochemical gradient : conc. + electrical gradient
B. Diffusion rate
- the size of molecule ↓→ rate↑
- H-bonds ↓→ rate↑
- solubility in oil ↑→ rate↑,
but exception: H2O (h-philic) → highest rate ∵ very small
C. Osmosis & semipermeable memb.: Undiffusable molecule to semipermeable membrane
affect H2O
D. Osmotic pressure: strength of counting to H2O movement across semipermiable memb.
Role: 1) intake of H2O by all cell.
ex) - plant root intake water→stem & leave (thru softer tissue)
- To keep from bursting, wall-less organism pump out of H2O
2) balancing H2O in animal cell by in and out.
3. Channel proteins
A. Need not bind the solute. Instead they form hydrophilic pores that extend across the
lipid bilayer
B. Transport solutes by passive transport only
C. What kinds?
1) Aquaporin: water channel (through which water molecules cross the memb much
more rapidly than they can diffuse through phospholipid bilayer)
Structure of aquaporin
2) Ion channels
a) Characteristics
- transport is extremely rapid (1,000X faster than carrier-mediated transport)
- not permanently open. The opening is regulated by ‘gate’
- highly selective
- K+ cannot pass through Na+ channel, vice versa.
b) What kinds?
- Voltage-gated channel (e.g., change of electric potential along the neuron)
- Mechanically gated channel
- Ligand-gated channel (e.g., neurotransmitter)
Model of an ion channel
4. Carrier protein: in facilitated diffusion, carrier protein does not need energy for functioning
- Bind specific solutes and change conformation to transfer the solute
across the membrane
- Transport solutes by either passive transport (or facilitated diffusion) or
active transport
- Uniport, symport, antiport
What kinds?
a) Na+-K+ pump (Na+-K+ ATPase)
(i)
Anitiport
(ii) Drives 3 Na+ out, 2 K+ in by hydrolysis of one ATP molecule
(iii) Ouabain is inhibitory to the Na+ - K+ pump by competition for K+ binding site
(iv) Consumes ~25% of the ATP of the cell
(v) Maintain osmotic balance and cell volume
∴ high [Na+ & Cl-]outside cell counterbalance the osmosis
b) Ca2+ ATPase
(i) Keep intracellular [Ca2+] low  makes the cell sensitive to small increase in
intracellular Ca2+ level
(i) In the plasma membrane
(ii) In the sarcoplasmic reticulum of muscle cells
• Stimulus  release of Ca2+ from sarcoplasmic reticulum into the cytosol  muscle
contraction Ca2+ ATPase pump Ca2+ from the cytosol into the sarcoplasmic reticulum
Structural of the Ca2+ pump
c) Na+-glucose symport : in epitherial cell of intestine
Driving force of glucose uptake is concentration gradient of the Na+.
This transport is referred to as secondary active transport. Glucose
is moving against concentration gradient
Low [Na+] of the intestinal cell is maintained by this Na+-K+ pump.
Transport need ATP is called primary active transport.
Glucose is moving from higher concentration to lower concentration.
d) H+ ATPase
- Pump H+ into the organelle from the cytosol at the expense of ATP hydrolysis
- Responsible for low pH in the lysosome.
e) Na+-H+ exchanger
- Antiport
- Couples an influx of Na+ (generated by Na+-K+ pump) to an efflux of H+
- Prevents acidification of the cytosol
f) Na+-Ca2+ exchanger
- Ca2+ is exported from the cell not only by the Ca2+ ATPase
- Couples an influx of Na+ (generated by Na+-K+ pump) to an efflux of Ca2+
g) ABC transporter superfamily
(i) The tranporter protein has ATPase activity and pump in or out particular substances
(ii) Over 100 ABC transporters have been described
(iii) Each member contains highly conserved ATP-binding cassette
(iv) Multidrug resistance (MDR) protein (or transporter)
• expressed in cancer cells
- pump cancer cell-killing drug out of cell  one of the biggest problems in cancer therapy
• Expressed in the brain - Protects brain from toxic chemicals
• CFTR (cystic fibrosis transmembrane conductance regulator)
- Cl- channel of the epithelial cell
- Defective  cystic fibrosis
Structure of an ABC transporter
Review
Membrane transport
- Simple diffusion
- Passive transport
- Active transport
Preview
Membrane transport
- Membrane electrical potential
- Endocytosis
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