Membrane Structure and Function

The Fluid Mosaic Model

The plasma membrane is described by the fluid mosaic model, which emphasizes its dynamic and flexible nature. The membrane is composed of a sea of phospholipids in which proteins, cholesterol, and carbohydrates are embedded and move laterally. This structure allows for selective permeability and adaptability in cellular environments.

• Phospholipid Bilayer: Two layers of phospholipids arranged tail-to-tail, with hydrophilic heads facing outward and hydrophobic tails inward, forming a selectively permeable barrier.

• Key Membrane Components:

  • Phospholipids: Structural backbone of the membrane.

  • Cholesterol: Stabilizes membrane fluidity.

  • Proteins: Involved in transport, signaling, and recognition.

  • Carbohydrates: Provide cell identity and communication.

Membrane Fluidity & Protein Functions

Membrane fluidity is regulated by temperature, cholesterol, and the saturation of fatty acids. Proteins embedded in the membrane serve various functions, including transport, communication, recognition, and cell attachment.

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Cholesterol: Acts as a buffer, preventing excessive rigidity or fluidity.

• Unsaturated Fatty Acids: Increase fluidity due to kinked tails.

Protein Functions

• Transport: Channel and carrier proteins move substances across the membrane that cannot cross on their own, such as ions and large polar molecules.

• Communication: Receptor proteins bind signaling molecules and trigger responses inside the cell.

• Recognition: Glycoproteins act as cellular ID tags, helping the immune system distinguish self from non-self.

• Cell Attachment: Adhesion proteins anchor cells to the extracellular matrix and neighboring cells, maintaining tissue integrity.

Membrane Transport

Selective Permeability & Passive Transport

The plasma membrane is selectively permeable, allowing only certain substances to cross. Small, nonpolar molecules cross easily, while large or charged molecules require assistance.

• Simple Diffusion: Movement of small, nonpolar substances down their concentration gradient without energy or protein assistance. Examples: O2, CO2, ethanol.

• Facilitated Diffusion: Movement of polar or charged substances down their gradient with the help of transport proteins. No ATP required. Examples: glucose, Na+, K+.

• Osmosis: Diffusion of water across a selectively permeable membrane, moving toward the more concentrated (hypertonic) solution.

Hypertonic, Hypotonic & Isotonic Solutions

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell, causing it to shrivel.

• Hypotonic: Lower solute concentration outside; water enters the cell, causing it to swell.

• Isotonic: Equal solute concentrations; no net water movement, cell maintains normal shape.

Active Transport & Bulk Transport

Active transport moves substances against their concentration gradient and requires ATP. Bulk transport involves vesicle-mediated movement of large molecules.

• Sodium-Potassium Pump (Na+/K+ ATPase): Uses 1 ATP to move 3 Na+ out and 2 K+ in per cycle, maintaining electrochemical gradients.

• Coupled Transport: Uses ion gradients to move other substances against their gradient (secondary active transport).

Endocytosis & Exocytosis

• Endocytosis: Cell membrane folds inward to form a vesicle that engulfs material.

• Exocytosis: Intracellular vesicle fuses with the membrane to release contents outside.

Types of Endocytosis

• Phagocytosis: "Cell eating"; pseudopods engulf large particles into a food vacuole.

• Pinocytosis: "Cell drinking"; membrane folds inward to bring in extracellular fluid and dissolved solutes.

• Receptor-Mediated Endocytosis: Highly specific; receptor proteins bind target molecules and bring them in via coated vesicles.

Energy Fundamentals

Types of Energy

All cellular work depends on energy, which exists in two main forms:

• Kinetic Energy: Energy of motion (heat, light, moving electrons).

• Potential Energy: Stored energy based on position or structure (chemical bonds, concentration gradients).

Oxidation & Reduction (Redox)

Redox reactions involve the transfer of electrons:

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• These processes always occur together. OIL RIG: Oxidation Is Loss, Reduction Is Gain.

Laws of Thermodynamics

• 1st Law: Energy cannot be created or destroyed, only converted.

• 2nd Law: Every energy conversion increases entropy (disorder); some energy is always lost as heat.

Entropy

• Entropy: Measure of disorder or randomness. Living organisms maintain low internal entropy but increase the entropy of their surroundings.

Free Energy, ATP & Metabolic Pathways

Free Energy (G)

• Exergonic Reactions: Release free energy (ΔG < 0), spontaneous. Example: cellular respiration.

• Endergonic Reactions: Require energy input (ΔG > 0), non-spontaneous. Example: protein synthesis.

ATP Hydrolysis

• ATP → ADP + Pi: Releases ~7.3 kcal/mol of free energy, powering endergonic reactions via phosphorylation.

Metabolic Pathways

• Cells organize reactions into sequential pathways for efficient energy use and control.

Allosteric Regulation

• Allosteric proteins change shape when a regulatory molecule binds to a site other than the active site, allowing real-time regulation of metabolic activity.

Enzymes: Catalysts of Life

Enzyme Function

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy (Ea). They are highly specific and are not consumed in the reaction.

• Active Site: Specifically shaped region where substrate binds.

• Induced Fit Model: Enzyme changes shape upon substrate binding to optimize the reaction.

• Enzyme Specificity: One enzyme, one substrate (or class of substrates).

Factors Affecting Enzyme Activity

• Temperature: Activity increases with heat up to an optimal point; excessive heat causes denaturation.

• pH: Each enzyme has an optimal pH; deviations disrupt active site shape.

• Inhibitors:

  • Competitive: Block the active site.

  • Noncompetitive: Bind elsewhere and alter enzyme shape.

Quick-Reference: Key Terms

• Substrate: Reactant that binds to enzyme.

• Active site: Enzyme's binding region.

• Cofactor: Inorganic helper (e.g., Zn2+, Fe2+).

• Coenzyme: Organic helper (e.g., vitamins).

• Denaturation: Loss of enzyme shape/function.

• Allosteric site: Regulatory binding site.

• Feedback inhibition: End product inhibits its own pathway.

Key Equations

• ATP Hydrolysis:

• Free Energy Change:

Additional info: Academic context was added to clarify the functions of membrane proteins, types of transport, and enzyme regulation.