Amphipathic Molecules

• Definition: Molecules having both hydrophilic (water-attracting) and hydrophobic (water-repelling) parts.

• Phospholipids Structure:

• Head: Contains phosphate group (negative charge), hydrophilic.

• Tail: Composed of hydrocarbon chains, hydrophobic.

• Phosphatidylcholine as an example of a phospholipid.

• Forms bilayers in aqueous environments with heads facing water and tails hidden from water.

Fluid Mosaic Model

• Components: Lipids and proteins are arranged in a mosaic-like pattern.

• Fluid Nature:

• Lipid bilayer allows lateral movement of proteins and lipids.

• Membranes differ in protein to lipid ratios and may have cholesterol or carbohydrates.

• Protein Types:

• Integral: Spanning the membrane, can be transmembrane.

• Peripheral: Loosely attached to the membrane surface.

• Anchored: Covalently bonded to lipids within the bilayer.

Membrane Fluidity

• Factors Affecting Fluidity:

• Lipid Composition: Chain length and degree of saturation.

  • Shorter, unsaturated tails increase fluidity.

  • Longer, saturated tails reduce fluidity.

• Cholesterol: High levels decrease fluidity, acting as a buffer.

• Temperature: Higher temperatures increase movement, while lower temperatures decrease it.

• Adaptations: Cold-adapted organisms may alter lipid composition for fluidity.

Membrane Proteins

• Integral Proteins: Embedded in the bilayer with hydrophilic and hydrophobic segments.

• Peripheral Proteins: Not embedded; interact with integral proteins or phospholipid heads.

• Movement and Distribution:

• Membrane proteins are not evenly distributed and may be confined to specific regions.

• Cytoskeleton Interaction: Can restrict movement of membrane proteins.

Membrane Carbohydrates

• Functions: Serve as recognition sites, involved in cell cell adhesion.

• Types:

• Glycolipids: Carbohydrates attached to lipids.

• Glycoproteins: Carbohydrates attached to proteins.

• Proteoglycans: Heavily glycosylated proteins, longer carbohydrate chains.

Cell Junctions

• Types:

• Tight Junctions: Prevent leakage between cells, maintaining tissue integrity.

• Desmosomes: Anchor cells together like rivets for mechanical strength.

• Gap Junctions: Allow communication and transport between adjacent cells.

Homeostasis and Membrane Regulation

• Homeostasis: Maintaining stable internal conditions despite external changes, e.g., ion and molecule concentrations.

• Selective Permeability: Membranes allow specific substances to pass, using mechanisms like passive and active transport.

Transport Mechanisms

• Passive Transport: Movement along concentration gradients; does not require energy.

• Active Transport: Movement against concentration gradients; requires energy (e.g., ATP).

• Facilitated Diffusion: Uses transport proteins for molecules that cannot freely cross the membrane (e.g., glucose).

Endocytosis and Exocytosis

• Endocytosis: Process of engulfing extracellular material, leading to vesicle formation.

• Types: Phagocytosis (large particles), Pinocytosis (fluids), Receptor-mediated endocytosis (specific molecules).

• Exocytosis: Vesicles fuse with the cell membrane to release contents outside the cell, important for secretion of hormones or neurotransmitters.

Tonicity of Solutions

• Isotonic: No net movement of water; solute concentrations are equal on both sides of the membrane.

• Hypotonic: Causes cells to swell; water moves into cells.

• Hypertonic: Causes cells to shrink; water moves out of cells.

Summary of Transport Mechanisms**

• Facilitated vs. Active Transport:

• Facilitated: Passive, no energy required, via channels or carriers.

• Active: Requires ATP, moving substances against their gradient.*

• Na+/K+ Pump: An example of active transport; moves sodium out and potassium into cells, critical for maintaining membrane potential.