Cell Membrane Structure and Composition

Study Notes

The plasma membrane separates living cells from their surroundings and regulates the movement of substances.
Exhibits selective permeability, enabling cells to control chemical exchanges essential for life.

Comprised mainly of lipids and proteins, with carbohydrates also playing an important role.
Phospholipids are the most abundant lipids, characterized as amphipathic (having a hydrophilic head and a hydrophobic tail).
The arrangement of phospholipids forms a bilayer, creating a stable boundary between aqueous compartments.

Describes the membrane as a fluid mosaic where molecules drift laterally.
Cholesterol helps maintain membrane fluidity, with different effects at various temperatures.
Fluidity is crucial for proper membrane function, permeability, and protein mobility.

Membrane solidifies as temperature drops; specific lipid types influence solidification temperature.
Organisms can adapt membrane lipid compositions for optimal fluidity under environmental changes.
Examples of adaptations include:
- Fish in cold environments having more unsaturated lipids.
- Plants like winter wheat increasing unsaturated phospholipids in colder months.

Cell-cell recognition is essential for tissue and organ formation and immune responses.
Carbohydrates on the membrane are often short, branched chains and can be attached to lipids (glycolipids) or proteins (glycoproteins).
Variability in these carbohydrates allows for distinction among different cells.

Lipid Bilayer Permeability: Nonpolar molecules (e.g., hydrocarbons, O2) can easily cross, while ions and polar molecules face barriers.
Transport Proteins facilitate movement of specific ions and polar molecules:
- Channel proteins create hydrophilic passages for certain substances.
- Aquaporins greatly enhance water transport across membranes.
- Carrier proteins bind to specific substances and undergo shape changes to shuttle them across membranes.

Proteins are crucial targets for many medications; for example:
- Omeprazole (Prilosec) blocks transport proteins that pump hydrogen ions, reducing heartburn.
- Fluoxetine (Prozac) inhibits serotonin re-uptake, affecting mood regulation.

Membrane structure and dynamics are central to cell function, involving intricate interactions between lipids, proteins, and carbohydrates. Adjustments in composition and fluidity reflect evolutionary adaptations to diverse environments.