Biological Thermodynamics and Cross-Membrane Transport

Key Vocabulary and Concepts

This section introduces essential terms and concepts related to biological thermodynamics and membrane transport. Understanding these terms is fundamental for grasping how cells manage energy and move substances across membranes.

• Metabolic: Refers to all chemical reactions occurring within a living organism.

• Anabolic: Pathways that build complex molecules from simpler ones; require energy input.

• Catabolic: Pathways that break down complex molecules into simpler ones; release energy.

• Disorder and Order: Refer to the arrangement of molecules; disorder increases with entropy.

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

• Spontaneous vs. Non-spontaneous: Spontaneous reactions occur without external energy input; non-spontaneous require energy.

• Potential Energy: Stored energy due to position or structure (e.g., chemical bonds, concentration gradients).

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

• Hydrophobic: Repels water; nonpolar molecules.

• Hydrophilic: Attracts water; polar molecules.

• Soluble: Able to dissolve in a solvent, typically water.

• Membrane Permeable: Substances that can cross the cell membrane.

• Passive Transport: Movement of substances across membranes without energy input.

• Active Transport: Movement of substances against their concentration gradient, requiring energy.

• Passive Diffusion: Movement of molecules from high to low concentration without assistance.

• Facilitated Diffusion: Passive transport aided by proteins.

• Primary Active Transport: Direct use of ATP to move substances.

• Secondary Active Transport: Uses energy from an ion gradient established by primary active transport.

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

• Ion Gradient: Difference in ion concentration across a membrane.

• Channel Protein: Forms pores for passive transport.

• Gated Channel Protein: Opens/closes in response to stimuli.

• Pump Protein: Actively moves substances across membranes.

• Sodium/Potassium Pump: Maintains ion gradients in animal cells.

• Equilibrium: State where concentrations are equal across a membrane.

• Osmosis: Diffusion of water across a membrane.

• Isotonic: Equal solute concentration inside and outside the cell.

• Hypertonic: Higher solute concentration outside the cell.

• Hypotonic: Lower solute concentration outside the cell.

• Concentration Gradient: Difference in concentration across a space.

• Enzyme: Protein that speeds up chemical reactions.

• Active Site: Region of enzyme where substrate binds.

• ADP (Adenosine Diphosphate): Product of ATP hydrolysis.

• Competitive Inhibition: Inhibitor binds to active site, blocking substrate.

• Energy of Activation (EA): Energy required to start a reaction.

• Feedback Inhibition: End product inhibits pathway.

• Inorganic Phosphate (Pi): Released during ATP hydrolysis.

• Metabolic Pathway: Series of chemical reactions in a cell.

• Non-competitive Inhibition: Inhibitor binds elsewhere, changing enzyme shape.

• Saturation: All enzyme active sites are occupied.

• Substrate: Molecule acted upon by enzyme.

• Product: Result of a chemical reaction.

• Reactant: Starting material in a reaction.

• Transition State: High-energy intermediate during a reaction.

Energy in Biological Systems

Cells use energy to perform work, such as building molecules, transporting substances, and maintaining homeostasis. Energy exists in two main forms: potential and kinetic.

• Potential Energy in cells is stored in chemical bonds (e.g., glucose, ATP), concentration gradients (e.g., ion gradients across membranes), and electrochemical gradients.

• Kinetic Energy in cells includes movement of molecules (diffusion), heat, and mechanical work (e.g., muscle contraction).

• Cells store potential energy in molecules like ATP, and in gradients (e.g., sodium/potassium gradients).

• Cells harness kinetic energy through processes like diffusion and movement of motor proteins.

Free Energy and Chemical Reactions

Free energy (G) is the energy available to do work in a system. The change in free energy () determines whether a reaction is spontaneous.

• Exergonic Reaction: Releases energy; ; occurs spontaneously.

• Endergonic Reaction: Requires energy input; ; non-spontaneous.

Free energy diagrams illustrate the energy changes during reactions:

• Reactants lower than products: Endergonic reaction.

• Reactants higher than products: Exergonic reaction.

Example: ATP hydrolysis is exergonic; synthesis of glucose from CO2 and H2O is endergonic.

Free Energy Diagram

• Exergonic: Reactants start at higher free energy, products at lower free energy.

• Endergonic: Reactants start at lower free energy, products at higher free energy.

Spontaneity: Exergonic reactions are more likely to occur spontaneously; endergonic reactions require energy input.

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics, which govern energy transformations.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed. Cells convert energy from one form to another (e.g., chemical to mechanical).

• Second Law of Thermodynamics: Entropy (disorder) increases in spontaneous processes. Cells must input energy to maintain order and build complex molecules.

Application: Building polymers (anabolism) requires energy input (endergonic), breaking polymers (catabolism) releases energy (exergonic).

Cellular Functions Powered by Chemical Energy

Cells perform various functions that require energy:

• Active transport of ions and molecules

• Synthesis of macromolecules (proteins, nucleic acids)

• Movement (muscle contraction, flagella)

• Cell division

• Signal transduction

These processes are powered by ATP and other energy sources.

Osmosis and Cell Volume Regulation

Maintaining equal levels of dissolved particles (solutes) inside and outside the cell is crucial for cell survival.

• Osmosis: Water moves across membranes to balance solute concentrations.

• Isotonic: No net water movement; cell volume remains stable.

• Hypertonic: Water leaves the cell; animal cells shrink (crenate), plant cells undergo plasmolysis.

• Hypotonic: Water enters the cell; animal cells may burst (lyse), plant cells become turgid (firm).

Difference between animal and plant cells: Plant cells have a rigid cell wall that prevents bursting; animal cells lack this protection.

Membrane Transport Mechanisms

Cells use various mechanisms to move substances across membranes:

• Passive Transport: No energy required; includes diffusion and facilitated diffusion.

• Active Transport: Requires energy (usually ATP); includes primary and secondary active transport.

• Channel Proteins: Allow specific ions/molecules to pass through.

• Pump Proteins: Move substances against gradients (e.g., sodium/potassium pump).

Comparison of Transport Mechanisms]

Enzyme Function and Regulation

Enzymes are biological catalysts that speed up reactions by lowering the energy of activation (EA).

• Active Site: Region where substrate binds.

• Competitive Inhibition: Inhibitor competes with substrate for active site.

• Non-competitive Inhibition: Inhibitor binds elsewhere, altering enzyme shape.

• Feedback Inhibition: End product inhibits pathway to regulate activity.

• Saturation: All active sites occupied; maximum reaction rate.

Example: ATP acts as a feedback inhibitor in glycolysis.

Key Equations

• Change in free energy: Where: = change in free energy, = change in enthalpy, = temperature (K), = change in entropy.

• ATP hydrolysis:

Additional info: This guide covers foundational concepts for understanding cellular energy and transport, which are essential for topics such as metabolism, cell structure, and physiology.