Membrane bound protiens

Membrane Proteins and Membrane Transport

• Membrane proteins have various functions, including transport of molecules and ions across the cell membrane
• There are three mechanisms of membrane transport: passive, active, and secondary active transport
• Ion channels and pumps are involved in membrane transport and generate ion gradients that power secondary active transport
• Water can also move across membranes through osmosis

Integral Membrane Proteins

• Have alpha helices that span the membrane with hydrophilic residues facing the inside of the protein and hydrophobic residues facing the lipid bilayer
• Involved in various functions such as cell adhesions and cancer metastasis

Functions of Membrane Proteins

• Cell adhesions and cancer metastasis
• Enable cells to drive cancer cell invasion
• Involved in fibroblast-driven cancer cell invasion

Membrane Transport

• Permeability through the lipid bilayer: some molecules can pass freely, but many biologically important molecules cannot pass through at rates required by cells
• Proteins have evolved to allow cells to move these molecules in a fast and controlled manner
• Three mechanisms of membrane transport:
  • Passive transport: molecules move down their concentration gradient; spontaneous and requires no energy input
  • Active transport: ions move against their concentration/charge gradients; non-spontaneous and requires energy input
  • Secondary active transport: uses ion gradients generated by active transport to energize the transport of another ion/solute

Passive Transport

• Two types:
  • Simple diffusion: molecules move through the bilayer itself
  • Facilitated diffusion: molecules move through transport proteins (channels/carriers)
• Osmosis: movement of water down its concentration gradient
• Aquaporins: proteins that facilitate water transport across the cell membrane

Active Transport

• Ions move against their concentration/charge gradients
• Energy input required
• Examples:
  • Na+/K+ ATPase
  • H+ ATPase
  • Ca2+ ATPase

Secondary Active Transport

• Uses ion gradients generated by active transport to energize the transport of another ion/solute
• Example: Na+/glucose symporter in the kidney, which can move glucose against a 30,000-fold concentration gradient