Bio Lecture 01/27

Structure of Cell Membranes

• Membrane Functionality

  • Keeps certain substances in while allowing others to exit.

  • Membranes create compositions that can speed up reactions (e.g. transcription) by concentrating reactants in one compartment.

  • They act as selective barriers, controlling what enters and exits the cell.

• Phospholipid Bilayer

  • Comprised of polar (hydrophilic) heads facing the aqueous environment and non-polar (hydrophobic) tails facing inward, away from water.

  • Small nonpolar molecules can easily diffuse across the membrane.

  • Ions and larger polar molecules require assistance to pass through due to their charge or size.

Factors Influencing Membrane Permeability

• Size and Charge

  • Size: Smaller molecules can diffuse through more easily.

  • Charge: Fully charged ions have difficulty crossing the membrane without assistance.

  • Transport mechanisms are required for ions and larger polar molecules (e.g. sugars).

Active and Passive Transport

• Active Transport

  • Movements against concentration gradients require energy (e.g. ATP).

  • Energy is necessary for the transport proteins to function effectively.

• Passive Transport

  • Movement down concentration gradients without the use of energy.

Concentration Gradients

• Identifying Gradients

  • Gradient refers to differences in concentration between inside and outside the cell.

  • More of a substance outside than inside indicates a concentration gradient.

Specific Cases in Transport Mechanisms

• Use of Protein Channels

  • Specific channels (e.g. potassium channels) facilitate movement through membranes.

  • Channels may vary (e.g. sodium channels) to allow selective ion passage and maintenance of gradients.

Discussion on Toxic Resistance

• Example Case: Snakes and Newts

  • Certain snakes have become resistant to toxins found in newts but face potential trade-offs in fitness in their environments.