Membrane Structure and Function

Fluid Mosaic Model of the Plasma Membrane

The fluid-mosaic model describes the structure of the plasma membrane as a dynamic arrangement of phospholipids, proteins, and carbohydrates. The membrane is flexible, with components able to move laterally within the layer, resembling a mosaic.

• Macromolecules in the membrane: Phospholipids (form the bilayer), proteins (integral and peripheral), cholesterol (in animal cells), and carbohydrates (attached to proteins and lipids).

• Function: Provides a semi-permeable barrier, supports cell signaling, and maintains homeostasis.

• Example: Glycoproteins act as cell recognition markers.

Selective Permeability

The plasma membrane is selectively permeable, allowing some substances to cross more easily than others.

• Small, nonpolar molecules (e.g., O2, CO2) pass freely.

• Large or charged molecules (e.g., ions, glucose) require transport proteins.

Functions of Membrane Proteins

• Transport: Channel and carrier proteins move substances across the membrane.

• Enzymatic activity: Some proteins catalyze reactions at the membrane surface.

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

• Cell-cell recognition: Glycoproteins serve as identification tags.

• Intercellular joining: Proteins help cells adhere to each other.

• Attachment: Proteins anchor the membrane to the cytoskeleton and extracellular matrix.

Diffusion, Facilitated Diffusion, and Concentration Gradient

• Diffusion: Movement of molecules from high to low concentration, down their concentration gradient.

• Facilitated diffusion: Passive transport of substances via specific transport proteins.

• Concentration gradient: The difference in concentration of a substance across a space or membrane.

• Equation: Fick's Law of Diffusion:

Passive vs. Active Transport

• Passive transport: Movement of substances without energy input (e.g., diffusion, osmosis, facilitated diffusion).

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

• Key difference: Active transport requires energy; passive transport does not.

• Example: Sodium-potassium pump (Na+/K+ ATPase) is an active transporter.

Transport Proteins

• Definition: Proteins that assist in moving substances across the membrane.

• Importance: Enable selective passage of ions and molecules that cannot diffuse freely.

• Types: Channel proteins (form pores), carrier proteins (change shape to move substances).

Osmosis

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

• Direction: Water moves from areas of low solute concentration to high solute concentration.

• Example: Water uptake by plant roots.

Isotonic, Hypotonic, and Hypertonic Solutions

• Isotonic: Solute concentration is equal inside and outside the cell; no net water movement.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell, which may swell or burst.

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

Plant and Animal Cell Responses to Tonicity

• Plant cells: Prefer hypotonic environments (turgid state); in hypertonic solutions, they undergo plasmolysis.

• Animal cells: Prefer isotonic environments; in hypotonic solutions, they may lyse; in hypertonic solutions, they crenate.

• Scientific terms: Turgid (firm), flaccid (limp), plasmolysis (cell membrane pulls away from the wall).

Endocytosis and Exocytosis

• Endocytosis: A Process by which cells take in large molecules by engulfing them in vesicles.

• Exocytosis: A Process by which cells expel materials in vesicles that fuse with the plasma membrane.

• Example: Secretion of neurotransmitters by nerve cells (exocytosis).

Phagocytosis

• Definition: A type of endocytosis where cells engulf large particles or other cells.

• Example: White blood cells ingesting bacteria.

Cellular Energy and Metabolism

Types of Energy

• Kinetic energy: Energy of motion (e.g., movement of molecules).

• Potential energy: Stored energy (e.g., chemical bonds in glucose).

• Example: ATP stores potential energy used for cellular work.

Thermodynamics and Biological Systems

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe.

• Application: Cells transform energy from food into usable forms, releasing heat.

Exergonic vs. Endergonic Reactions

• Exergonic reactions: Release energy; products have less free energy than reactants (e.g., cellular respiration).

• Endergonic reactions: Require energy input; products have more free energy than reactants (e.g., photosynthesis).

• Equation:

Metabolism, Metabolic Pathways, and Energy Coupling

• Metabolism: All chemical reactions in a cell.

• Metabolic pathway: Series of enzyme-catalyzed reactions leading to a product.

• Energy coupling: Using energy released from exergonic reactions to drive endergonic reactions (often via ATP).

ATP: Structure and Function

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• Structure: Adenine base, ribose sugar, and three phosphate groups.

• Function: Provides energy for cellular work (e.g., muscle contraction, active transport, biosynthesis).

• Equation:

ATP Cycle

• ATP is regenerated from ADP and inorganic phosphate using energy from catabolic reactions.

• Cycle: ATP hydrolysis releases energy; ATP synthesis stores energy.

Enzymes and Activation Energy

• Enzyme: A Biological catalyst that speeds up reactions by lowering activation energy.

• Activation energy (Ea): Minimum energy required to start a reaction.

• Effect: Enzymes increase reaction rates without being consumed.

• Equation: (activation energy)

Enzyme-Substrate Complex and Catalysis

• Substrate: The reactant an enzyme acts upon.

• Active site: Region on the enzyme where the substrate binds.

• Enzyme-substrate complex: Temporary association during catalysis.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature.

• pH: Each enzyme has an optimal pH range.

• Cofactors: Non-protein helpers (e.g., metal ions, vitamins).

• Coenzymes: Organic cofactors (e.g., NAD+).

• Inhibitors: Competitive (bind active site) and non-competitive (bind elsewhere, change shape).

Negative Feedback Inhibition

• Definition: The End product of a metabolic pathway inhibits an earlier step, regulating pathway activity.

• Example: Isoleucine inhibits threonine deaminase in amino acid biosynthesis.

Summary Table: Types of Membrane Transport