Membrane Structure and Composition

Membranes define boundaries of cells and organelles, separating interior and exterior environments.
Intracellular membranes: compartmentalize functions within cells.
Serve as permeability barriers due to hydrophobic interiors.

Boundary and Permeability Barrier: Maintain distinct environments inside and outside cells.
Organization and Localization of Function: Specific proteins associated with membranes enable diverse cellular functions.
Transport Processes: Regulate movement of substances (e.g., ions, nutrients).
Signal Detection: Membranes detect and respond to chemical signals from outside.
Cell-to-Cell Interactions: Facilitate communication and interaction between cells.

Consist of lipids, proteins, and carbohydrates.
Lipids: ~50% protein by mass, as they weigh less than proteins.
- Types of lipids: Phospholipids, glycolipids, and sterols (cholesterol, phytosterols, ergosterol).

Phospholipid Bilayer: Composed of two leaflets (extracellular and cytosolic) with polar heads and nonpolar tails.
Fluid Mosaic Model: Proposed by Singer and Nicolson (1972); describes membranes as fluid with lipids and proteins moving laterally.
Membrane organization is asymmetrical, with specific lipids like glycolipids found primarily in the outer leaflet.

Phospholipids: Most abundant membrane lipids, aiding bilayer formation; include glycerol-based phosphoglycerides and sphingosine-based sphingolipids.
Glycolipids: Lipids with carbohydrates attached; critical in cell recognition and signaling.
Sterols (Cholesterol): Stabilize membrane structure, affecting fluidity and permeability; act as fluidity buffers by preventing tight packing of phospholipids.

Depends on:
- Saturation of fatty acid chains (unsaturated increases fluidity).
- Length of fatty acid chains (shorter chains increase fluidity).
- Presence of sterols (cholesterol decreases fluidity but stabilizes structure).
Transition Temperature (Tm): Defines the temperature for phase transition (gel->fluid); below Tm, membrane functions are impaired.

Lipids are unevenly distributed in bilayers, established during synthesis.
Movement Types:
- Lateral Diffusion: Occurs rapidly within one monolayer.
- Rotational Movement: Lipids can rotate around their axes.
- Transverse Movements (less common): Mediated by enzymes like scramblase, flippase, and floppase, with energy differences.

Selective Permeability: Distinct substances cross membranes at different rates; affected by fatty acid saturation, chain length, sterols, and temperature.
Important for maintaining ionic gradients crucial for cellular processes.

Diffusion: Movement from high to low concentration.
Facilitated Diffusion: Utilizes channel and transporter proteins for polar or ionic substances to traverse the bilayer without energy use.
Active Transport: Requires ATP to move substances against their concentration gradient (e.g., ion pumps in nerve cells).

Channel Proteins: Facilitate passive transport and are specific for certain ions/molecules; have polar amino acids lining the pore.
Transporter Proteins: Bind solutes, undergo conformational changes and are slower than channels, allowing selective transport across the membrane.