Membrane Transport

Passive Diffusion (Osmosis)

Passive diffusion is the movement of molecules across a cell membrane without the input of cellular energy. Osmosis is a specific type of passive diffusion involving water molecules.

• Passive Diffusion: Movement of substances from an area of higher concentration to an area of lower concentration, down their concentration gradient.

• Osmosis: The diffusion of water across a selectively permeable membrane.

• Example: Water entering a plant cell, causing it to become turgid.

Active Ion Transport

Active transport requires energy (usually from ATP) to move ions or molecules against their concentration gradient.

• Active Transport: Movement of substances from low to high concentration, requiring energy input.

• Example: The sodium-potassium pump in animal cells, which maintains electrochemical gradients.

• Equation:

Vesicular Transport – Exocytosis and Endocytosis

Vesicular transport involves the movement of large particles or volumes of substances via vesicles.

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

• Endocytosis: The process by which cells take in materials by engulfing them in vesicles.

• Types of Endocytosis: Phagocytosis ("cell eating"), Pinocytosis ("cell drinking"), and Receptor-mediated endocytosis.

Types of Solutions and Effects on Cells

The tonicity of a solution affects the movement of water into or out of cells, impacting cell volume and function.

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell.

• Isotonic Solution: Equal solute concentration; no net water movement.

• Hypertonic Solution: Higher solute concentration outside; water leaves the cell.

Forms of Energy

Cells use and transform energy to perform work. Energy exists in different forms relevant to biological systems.

• Kinetic Energy: Energy of motion. Thermal energy is a form of kinetic energy due to the movement of molecules.

• Potential Energy: Stored energy due to position or structure. Chemical energy is potential energy stored in chemical bonds.

• Example: Glucose contains chemical energy that cells can convert to ATP.

Thermodynamics

Thermodynamics describes the principles governing energy transformations in biological systems.

• First Law: Energy cannot be created or destroyed, only transformed (Law of Conservation of Energy).

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

• Third Law: As temperature approaches absolute zero, the entropy of a system approaches a constant minimum.

• Entropy (S): A measure of disorder or randomness in a system.

• Exergonic Reactions: Release energy; products have less free energy than reactants.

• Endergonic Reactions: Require energy input; products have more free energy than reactants.

Enzyme Functions

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

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

• Enzyme-Substrate Complex: The temporary association between an enzyme and its substrate(s).

• Active Site: The region on the enzyme where the substrate binds.

• Induced Fit Hypothesis: The enzyme changes shape slightly to fit the substrate more snugly, enhancing catalysis.

• Equation: Where E = enzyme, S = substrate, ES = enzyme-substrate complex, P = product.

• Example: Sucrase catalyzing the hydrolysis of sucrose into glucose and fructose.