Membranes. compartments and transport 1 compontents, fluidity, permeabilty

Membranes define cellular boundaries and create internal compartments in eukaryotic cells.
Compartmentalization allows specialized functions to occur within cells.
Membranes play a critical role in the cell's ability to harvest energy and materials from the environment.

Membranes consist of a double layer of lipids (phospholipids) and proteins.
The thickness of the membrane is approximately five nanometers.
Membrane lipids are amphipathic, possessing distinct polar (hydrophilic) and nonpolar (hydrophobic) regions:
- Polar head groups (phosphate and choline).
- Two fatty acid tails determining the membrane's hydrophobic character.

Fluidity refers to the flexibility of membranes, influenced by:
- Temperature: Higher temperatures increase fluidity (lipids act more like oil), while lower temperatures decrease fluidity (lipids act more like butter).
- Saturation of fatty acids: Saturated fats lead to tighter packing and reduced fluidity; unsaturated fats, with bends, allow for increased fluidity.
- Length of fatty acid tails: Longer tails increase intermolecular forces (IMFs) and decrease fluidity.
Proper membrane fluidity is essential for maintaining barrier functions.

The amphipathic nature of phospholipids allows for spontaneous formation of bilayers in an aqueous environment without energy input.
Hydrophobic tails cluster away from water, while polar heads interact with the surrounding aqueous environment, maximizing interactions through hydrogen bonding and minimizing exposure to water.
London Dispersion Forces (LDFs) between fatty acid tails inversely correlate with membrane fluidity—more LDFs = less fluidity.

Cells adapt their lipid composition in response to temperature changes:
- During temperature increases, cells may increase saturation or length of fatty acids to decrease fluidity.
Cholesterol, another membrane lipid, helps regulate fluidity in animal cells and is essential for maintaining stability, despite its often negative reputation in health contexts.

Membrane proteins serve various functions, including transport, and exhibit amphipathic properties to interact with the hydrophobic core of the membrane and the aqueous environments.
The environment of a membrane protein differs significantly from that of proteins folding in the cytosol due to the hydrophobic nonpolar tails present in the membrane.

Membranes are selectively permeable—some molecules can cross easily while others cannot.
- Nonpolar and small molecules (e.g., gases like O2, CO2) cross membranes efficiently.
- Charged or large molecules, and polar molecules (except water) generally cannot cross without assistance.
- Transport of water, glucose, amino acids, and ions requires membrane proteins.

Membranes are essential for cellular organization and function.
Major components include phospholipids, cholesterol, and membrane proteins.
Membrane fluidity is influenced by temperature, saturation, and length of fatty acid tails, and cells can adjust lipid profiles accordingly.
The amphipathic nature of lipids and proteins is crucial for membrane structure and function.