Membrane Transport and Cell Signaling

Diffusion: Free movement of solutes from high concentration to low concentration.
Osmosis: A type of passive movement where water moves across a semipermeable or selectively permeable membrane. The key distinction from diffusion is that water can move, but solutes cannot.

Cholesterol's Role: Acts as a buffer for membrane fluidity.
- Hot temperatures: Makes the membrane more viscous by bringing lipids closer together, preventing them from falling apart.
- Cold temperatures: Keeps the membrane from locking up, making it less viscous (prevents crystallization).
Designing Membranes for Different Environments:
- Arctic organism: Would need a membrane that resists locking up in the cold. Adding cholesterol helps. Another crucial factor is the saturation level of fatty acid tails.
  - Kinks (double bonds, unsaturated fatty acids): Increase fluidity (less viscous), preventing locking up in cold conditions.
- Desert organism (hot environment): Would need a membrane that resists becoming too fluid. High saturation (no kinks, saturated fatty acids) would make it more viscous, and cholesterol would help prevent it from falling apart.
Key takeaway: Go beyond definitions; apply concepts to novel situations.

Integral vs. Peripheral Proteins:
- Integral proteins: Are integrated into the lipid membrane and possess hydrophobic regions that interact with the hydrophobic fatty acid tails of the phospholipids.
- Peripheral proteins: Lack hydrophobic regions and are typically bound to the surface of the membrane or to integral proteins.
Transmembrane Proteins: A subset of integral proteins that extend all the way through the lipid bilayer.
- Not all integral proteins are transmembrane (some only penetrate partway into the bilayer).
Channel vs. Carrier Proteins:
- Channel proteins: Form a hydrophilic pore (a