Chapter 4: Energy, Enzymes, and Protein Synthesis

Energy in Biological Systems

Energy is fundamental to all biological processes, enabling cells to perform work, grow, and maintain homeostasis.

• Definition of Energy: Energy is the capacity to do work or cause change. In biological systems, it is often stored in chemical bonds.

• Kinds of Work in Biological Systems:

  • Chemical Work: Making and breaking of chemical bonds (e.g., synthesis of proteins).

  • Transport Work: Moving ions or molecules across membranes (e.g., Na+/K+ pump).

  • Mechanical Work: Movement of structures (e.g., muscle contraction).

• Concentration Gradient: A difference in the concentration of a substance between two regions, driving diffusion.

Kinetic and Potential Energy

Cells utilize both kinetic and potential energy to drive biological processes.

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

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

• Example: Glucose has potential energy stored in its bonds; when metabolized, this is converted to kinetic energy.

Activation Energy and Reaction Types

Chemical reactions require an initial input of energy, and can be classified by their energy changes.

• Activation Energy: The minimum energy required to start a chemical reaction.

• Exergonic Reactions: Release energy (e.g., cellular respiration).

• Endergonic Reactions: Require energy input (e.g., synthesis of ATP).

• Coupling of Reactions: In cells, exergonic and endergonic reactions are often coupled so that the energy released from one powers the other.

• Reversible vs. Irreversible Reactions: Reversible reactions can proceed in both directions; irreversible reactions proceed in one direction only.

Enzymes and Catalysis

Enzymes are biological catalysts that speed up chemical reactions without being consumed.

• Enzymes: Proteins that lower activation energy and increase reaction rates.

• Substrates: The reactants upon which enzymes act.

• Factors Affecting Enzyme Activity: Temperature, pH, substrate concentration, and presence of inhibitors or activators.

• Coenzymes: Non-protein organic molecules (often vitamins) that assist enzymes.

• Vitamins: Organic compounds required in small amounts for normal metabolism, often functioning as coenzymes.

• Catabolic Reactions: Break down molecules and release energy.

• Anabolic Reactions: Build complex molecules and require energy.

• Feedback Inhibition: A regulatory mechanism where the end product of a pathway inhibits an earlier step, maintaining homeostasis.

Metabolic Pathways

Cells use metabolic pathways to efficiently manage energy and resources.

• Aerobic Pathways: Require oxygen and produce more ATP (e.g., cellular respiration).

• Anaerobic Pathways: Do not require oxygen and produce less ATP (e.g., fermentation).

Protein Synthesis and Posttranslational Modifications

Protein synthesis involves multiple steps and modifications after translation.

• Signal Sequence: A short peptide that directs the transport of a protein to specific locations in the cell.

• Protein Folding: The process by which a protein assumes its functional shape.

• Cross-Linkage: Formation of covalent bonds between different parts of a protein or between proteins (e.g., disulfide bonds).

• Cleavage: Cutting of a protein to activate or deactivate it.

• Glycosylation and Phosphorylation: Addition of carbohydrate or phosphate groups, affecting protein function and localization.

• Assembly into Polymeric Proteins: Multiple protein subunits combine to form a functional complex (e.g., hemoglobin).

Additional info: Proteins lacking a targeting sequence may remain in the cytosol or be degraded.

Chapter 5: Membrane Transport and Water Balance

Water Distribution and Osmosis

Water is distributed among various body compartments and moves according to osmotic gradients.

• Distribution of Water: Water is found in intracellular fluid (ICF) and extracellular fluid (ECF), including plasma and interstitial fluid.

• Osmosis: The movement of water across a semipermeable membrane from an area of low solute concentration to high solute concentration.

• Osmotic Pressure: The pressure required to prevent osmosis, proportional to solute concentration.

• Osmolarity vs. Molarity: Osmolarity measures total solute particles per liter; molarity measures moles of solute per liter.

Osmotic Terms and Tonicity

Understanding osmotic relationships is crucial for predicting water movement in and out of cells.

• Isosmotic: Solutions with equal osmolarity.

• Hyposmotic: Lower osmolarity compared to another solution.

• Hyperosmotic: Higher osmolarity compared to another solution.

• Tonicity: The ability of a solution to change the shape of cells by altering their water content.

• Isotonic: No net water movement; cell shape remains unchanged.

• Hypotonic: Water enters the cell; cell may swell or burst.

• Hypertonic: Water leaves the cell; cell shrinks.

Membrane Transport Mechanisms

Cells regulate the movement of substances across their membranes using various transport mechanisms.

• Bulk Flow: The movement of large volumes of fluid and solutes together, driven by pressure gradients.

• Permeable vs. Impermeable Substances: Small, nonpolar molecules (e.g., O2, CO2) are typically permeable; large or charged molecules (e.g., proteins, ions) are often impermeable.

• Active Transport: Requires energy (usually ATP) to move substances against their concentration gradient.

• Passive Transport: Does not require energy; substances move down their concentration gradient.

• Diffusion: The net movement of molecules from high to low concentration.

• Fick's Law: The rate of diffusion is proportional to surface area, concentration gradient, and membrane permeability, and inversely proportional to membrane thickness. Equation: where is the flux, is the diffusion coefficient, is the concentration difference, and is the distance.

• Protein-Mediated Transport: Involves specific membrane proteins to move substances.

• Facilitated Diffusion: Passive transport via carrier proteins.

• Active Transport: Movement against a gradient, requiring energy.

Membrane Proteins and Channels

Membrane proteins play key roles in transport and cell signaling.

• Receptor Proteins: Bind signaling molecules and initiate cellular responses.

• Channel Proteins: Form pores for specific ions or molecules to pass through.

• Carrier Proteins: Bind and transport substances across the membrane.

• Open Channels: Always open, allowing continuous movement.

• Gated Channels: Open or close in response to stimuli (e.g., voltage, ligands).

• Facilitated Diffusion vs. Simple Diffusion: Facilitated diffusion requires a protein; simple diffusion does not.

Na+/K+ ATPase Pump

This pump is essential for maintaining cellular ion gradients.

• Structure: Transmembrane protein with binding sites for Na+, K+, and ATP.

• Mechanism: Pumps 3 Na+ ions out and 2 K+ ions into the cell per ATP hydrolyzed.

• Equation:

Vesicular Transport

Cells use vesicles to move large particles and fluids across membranes.

• Phagocytosis: Cell engulfs large particles or microorganisms.

• Exocytosis: Vesicles fuse with the plasma membrane to release contents outside the cell (active process).

• Endocytosis: Cell takes in substances by forming vesicles from the plasma membrane (active process).

• Electrochemical Gradient: The combined effect of concentration and electrical gradients on ion movement.

Additional info: Exocytosis and endocytosis are both forms of active transport, requiring energy.