The Membrane

Selective Permeability

The plasma membrane exhibits selective permeability, allowing some substances to cross more easily than others. This property is essential for maintaining the internal environment of the cell.

• Selective permeability: Only certain molecules can pass through the membrane freely.

• Example: Water and small nonpolar molecules can cross easily, while ions and large polar molecules require transport proteins.

Phospholipids and Membrane Structure

Phospholipids are the most abundant lipids in the plasma membrane. They are amphipathic molecules, containing both hydrophilic (water-loving) heads and hydrophobic (water-fearing) tails.

• Hydrophilic head: Faces outward toward the aqueous environment.

• Hydrophobic tail: Faces inward, away from water.

• Fluid mosaic model: Describes the membrane as a mosaic of proteins floating in or on the fluid lipid bilayer. Cholesterol is present in animal cell membranes, affecting fluidity.

• Phospholipid movement: Phospholipids move laterally within the bilayer, contributing to membrane fluidity.

• Unsaturated fatty acids: Have hydrocarbon tails with kinks (double bonds), increasing fluidity.

• Saturated fatty acids: Have straight tails (no double bonds), making the membrane less fluid.

Membrane Proteins

Proteins are embedded in the membrane and perform various functions.

• Peripheral proteins: Bound to the surface of the membrane.

• Integral proteins: Penetrate the hydrophobic core; some span the entire membrane (transmembrane proteins).

• N-terminus: The amino end of a protein chain.

• C-terminus: The carboxyl end of a protein chain.

Functions of Membrane Proteins

• Transport: Move substances across the membrane.

• Enzymatic activity: Catalyze chemical reactions.

• Signal transduction: Relay signals from outside to inside the cell.

• Cell-cell recognition: Allow cells to identify each other, often via carbohydrates on the membrane.

• Intercellular joining: Connect adjacent cells.

• Attachment to cytoskeleton and extracellular matrix (ECM): Maintain cell shape and stabilize protein location.

Cell Recognition

• Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane.

Vesicle Transport Pathway

• Vesicles travel through the cell in the following order: Rough Endoplasmic Reticulum (RER) → Golgi Apparatus → Vesicles → Cell membrane or Lysosome.

Transport Proteins

• Transport proteins: Allow passage of hydrophilic substances across the membrane.

• Aquaporins: Channel proteins that facilitate water movement.

• Carrier proteins: Bind to specific molecules and change shape to shuttle them across the membrane.

Diffusion and Equilibrium

• Diffusion: The tendency for molecules to spread out evenly into available space, moving from higher to lower concentration.

• Example: Carbonation in soda dispersing when opened.

• Equilibrium: State where concentrations are balanced; no net movement of molecules.

• Concentration gradient: Difference in concentration between two areas.

• Moving down the gradient: High to low concentration (no energy required).

• Moving up the gradient: Low to high concentration (requires energy).

Passive Transport

• Movement of molecules across a membrane without energy input.

• Example: Diffusion of oxygen into cells.

Osmosis and Tonicity

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Example: Raisins swelling in water.

• Water moves from lower solute concentration to higher solute concentration.

• Tonicity: The ability of a solution to cause a cell to gain or lose water.

• Osmoregulation: Control of water balance in cells and organisms.

Facilitated Diffusion

• Transport proteins speed the passive movement of molecules across the plasma membrane.

• Channel proteins: Provide corridors for specific molecules or ions (e.g., aquaporins for water, ion channels for ions).

• Carrier proteins: Bind and transport specific molecules by changing shape.

Active Transport

• Movement of substances against their concentration gradient, requiring energy (usually ATP).

• Sodium-potassium pump: Actively transports Na+ out and K+ into animal cells.

Equation for sodium-potassium pump:

Membrane Potential and Electrochemical Gradients

• Membrane potential: Voltage difference across a membrane due to ion distribution.

• Electrochemical gradient: Combination of chemical and electrical forces driving ion movement.

• Electrogenic pump: Transport protein that generates voltage across a membrane (e.g., sodium-potassium pump in animals, proton pump in plants, fungi, and bacteria).

Bulk Transport: Endocytosis and Exocytosis

• Phagocytosis: Cell engulfs a particle in a vacuole, which fuses with a lysosome for digestion.

• Pinocytosis: Cell "gulps" extracellular fluid into tiny vesicles.

Additional info: The notes above expand on the original outline by providing definitions, examples, and context for each concept. Table entries for plant cells are inferred based on standard biology knowledge.