Membrane Transport

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Membrane Transport
Small molecules Large molecules
Passive Active
Facilitated
Non-facilitated
Phospholipid bilayer Proteins
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What about polar molecules?




-

• Ions - Na, K, Mg, Ca, Cl, H
• Other charged molecules - amino acids
• Uncharged polar - glucose
• Require protein transporters

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Protein transporters
Selective- Gated Channels
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Gated Channels (Pores)

• Integral membrane proteins - span membrane
• Selective
• Open only to one side of the membrane at a time
• Conformational change

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Facilitated Transport

• Passive
• High concentration to low concentration
• No energy required
• Active
• Low concentration to high concentration
• Energy required - ATP

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Active transport

• From a low to high concentration
• Energy required - ATP

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How ATP works
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Active Transport Summary

• Transport from low to high concentration
• Energy required - ATP
• ATP hydrolyzed
• Phosphate covalently bound to transporter
• Conformational change

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Sodium-potassium ATPase

• Transports 3 Na ions out
• Transports 2 K ions in
• Hydrolyzes 1 ATP
• Generates a voltage across the membrane -
  Electrogenic
• Found in almost all animal cells

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Why is it needed?
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Electrogenic pumps

• Voltage across membrane membrane potential

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An Electrogenic Pump
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Cotransport
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Transport of large molecules

• Exocytosis

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Transport of large molecules

• Exocytosis
• Endocytosis
• Phagocytosis - particle
• Pinocytosis -liquid
• Receptor-mediated endocytosis -
• selective

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Receptor-mediated endocytosis
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Ligands bind to receptors
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Receptors
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Selectivity

• Enzymes
• Facilitated transport
• Receptor-mediated endocytosis

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Receptors are trapped in coated pits.
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Coated pits become coated vesicles.
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Coated vesicles fuse with lysosomes.
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Ligand is digested
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Receptor recycled to surface
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Receptor-mediated endocytosis

• Ligand binds to receptor
• Ligand-receptor complex is trapped in a coated
  pit
• The coated pit becomes a coated vesicle
• which fuses with a lysosome
• The ligand is digested and
• the receptor is recycled to the surface.
• New pits form.

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What happens if something goes wrong?

• Familial hypercholesteremia
• Genetic defect in cholesterol-LDL receptors -
• Cholesterol is not taken up
• Heart attacks and death at a young age.

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Receptors lack tails
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Binding sites altered
Wrong shape - ligand does not bind