Membrane Structure

Plasma Membrane: Acts as a boundary between living cells and their surroundings.
Selective Permeability: The membrane allows some substances to pass more easily than others.

Phospholipids: Most abundant lipid in the plasma membrane.
- Amphipathic Molecules: Contain both hydrophobic (water-repelling) and hydrophilic (water-attracting) regions.
Fluid Mosaic Model: Describes membranes as fluid structures with a diverse mosaic of proteins embedded.

Chemical Analysis: Membranes consist of proteins and lipids, structured as a phospholipid bilayer.

Movement in Bilayer:
- Most lipids and some proteins move laterally; flipping across the membrane is rare.
- Lateral Movement Rate: Approximately 10 million times per second; flip-flop occurs roughly once a month.
Temperature Effects:
- As temperature drops, membranes become more solid.
- Membranes rich in unsaturated fatty acids remain more fluid than those rich in saturated fatty acids.
Cholesterol's Role:
- Restricts phospholipid movement at warm temperatures, while preventing tight packing at cooler temperatures.

Collage of Proteins: Membranes contain diverse proteins embedded in the lipid bilayer.
- Functions of Membrane Proteins: Include transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to cytoskeleton/ECM.
Types of Membrane Proteins:
- Peripheral Proteins: Bound to the surface.
- Integral Proteins: Penetrate the hydrophobic core.
- Transmembrane Proteins: Span the membrane, aiding in transport and function.

Cell Recognition: Cells recognize each other by binding to carbohydrates on the plasma membrane.
- Carbohydrates may be attached to lipids (glycolipids) or proteins (glycoproteins).

Molecular Exchange: Cells exchange materials with their environment controlled by the plasma membrane.
Permeability of the Bilayer:
- Nonpolar molecules (e.g., hydrocarbons) can easily pass through.
- Polar molecules (e.g., sugars) find it hard to cross the membrane.

Facilitating Passage:
- Transport proteins help hydrophilic substances cross the membrane.
- Channel Proteins: Provide hydrophilic channels for molecules/ions.
  - Example: Aquaporins facilitate water transport.
- Carrier Proteins: Bind and change shape to shuttle substances across.

Diffusion: Process by which molecules spread out evenly into available space.
Concentration Gradient: Molecules diffuse down their gradient without energy expenditure.
- Osmosis: Diffusion of water across a selectively permeable membrane.

Tonicity: Ability of a solution to cause a cell to gain or lose water.
- Isotonic: Equal solute concentration; no net movement.
- Hypertonic: Higher solute concentration; cell loses water.
- Hypotonic: Lower solute concentration; cell gains water.

Aided by Proteins:
- Channel proteins provide specific corridors for molecules.
- Carrier proteins undergo conformational changes to transport solutes.

Energy Usage:
- Active transport moves solutes against concentration gradients using energy (typically ATP).
- Sodium-Potassium Pump: A major active transport system in animals.

Membrane Potential: Voltage difference across membranes via distribution of ions.
Electrogenic Pump: Generates voltage; e.g., the sodium-potassium pump.

Coupled Transport:
- Active transport of one solute drives the transport of another.

Exocytosis: Vesicles fuse with membrane to release contents.
Endocytosis: Cells take in macromolecules by forming vesicles.
- Types: Phagocytosis, pinocytosis, and receptor-mediated endocytosis.