Plasma Membranes and Membrane Permeability

Definition: Plasma membranes are biological membranes that separate and protect the internal cell environment from the external environment.
Key Structures:
- Phospholipid Bilayer: The fundamental structure of the plasma membrane.
- Phospholipid Molecule: Comprises a hydrophilic head (which is attracted to water) and two hydrophobic tails (repellant to water).
- Amphipathic Nature: Phospholipids possess both hydrophilic and hydrophobic regions, allowing them to form a bilayer in aqueous environments.
  - Hydrophilic Heads: Face outward towards the aqueous environment (both intracellular and extracellular fluid).
  - Hydrophobic Tails: Face inward, away from water, thereby creating a non-polar interior.
- Proteins: Integral and peripheral proteins are embedded within or associated with the membrane.
- Integral Proteins: Embedded within the lipid bilayer; can form channels or transport proteins.
  - Transmembrane Proteins: A type of integral protein that spans the membrane entirely.
  - Structure: Often contain an alpha-helix configuration and are also amphipathic.
- Peripheral Proteins: Loosely bonded to the surface of the membrane; not embedded within the lipid bilayer.
Membrane Carbohydrates: Use for cell recognition and communication.
- Glycolipids: Carbohydrates attached to lipids.
- Glycoproteins: Carbohydrates attached to proteins; among the most abundant.

Selective Permeability: The property of plasma membranes to regulate the movement of substances in and out of the cell.
- Hydrophilic Substances: Will generally require channels or transport proteins to pass through as they cannot easily penetrate the lipid bilayer.
Fluid Mosaic Model: Describes the structural characteristics of cell membranes.
- Fluid Nature: Membranes are flexible due to weak hydrophobic interactions; thus, components can move laterally.
- Temperature Influence: Membrane fluidity is affected by temperature.
  - Unsaturated Hydrocarbon Tails: Prevent dense packing of phospholipids, maintaining fluidity at lower temperatures.
  - Cholesterol: Stabilizes membrane structure at both high and low temperatures.
  - At High Temperatures: Reduces the movement of phospholipids, preventing excessive fluidity.
  - At Low Temperatures: Prevents tight packing of phospholipids, maintaining fluidity.

Cell Wall: Unique to plant cells, provides additional structure and protection beyond the plasma membrane.
- Composition: Primarily made of cellulose.
- Functionality:
- Provides shape and structure to the cell.
- Protects the cell from physical damage and regulates water intake.
- Contains Plasmodesmata: Channels that allow for communication and transport between adjacent cells, filled with cytosol.

Channichthyidae Fish Study: In the context of the adaptations of the Channichthyidae fish (crocodile ice fish) living in cold waters.
- Expected Membrane Lipid Composition: Investigate the phospholipid composition expected in such cold-adapted organisms and their justifications.
- Justifications: Molecules are likely to include a higher proportion of unsaturated fatty acids to enhance fluidity at low temperatures, ensuring proper membrane function regardless of external environmental conditions.