Energy, Enzymes, and Metabolism: Study Notes for Anatomy & Physiology

Energy in Biological Systems

Properties of Living Organisms

Living organisms exhibit several key properties that distinguish them from non-living matter. These include organization into cells, metabolism, responsiveness, growth, development, reproduction, homeostasis, and adaptability.

• Organization: Cells are the basic unit of life, forming tissues, organs, and systems.

• Metabolism: The sum of all chemical reactions in the body.

• Homeostasis: Maintenance of a stable internal environment.

Energy Capture and Storage in Plants vs. Animals

Plants and animals differ in how they capture and store energy.

• Plants: Capture light energy via photosynthesis and store it as chemical energy in organic molecules.

• Animals: Consume organic molecules and use cellular respiration to convert them to ATP.

Energy Is Used to Perform Work

Energy is essential for performing work in biological systems.

• Definition of Energy: The capacity to do work.

• Types of Work:

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

  • Transport work: Moving ions or molecules across membranes (e.g., sodium-potassium pump).

  • Mechanical work: Movement (e.g., muscle contraction).

• Concentration Gradient: A difference in concentration of a substance between two regions.

Forms of Energy: Kinetic and Potential

Energy exists in two main forms in biological systems.

• Kinetic Energy: Energy of motion (e.g., molecules moving, muscle contraction).

• Potential Energy: Stored energy (e.g., chemical bonds in ATP, gradients).

• Example: ATP stores potential energy in its phosphate bonds; when hydrolyzed, this energy is released as kinetic energy.

Energy Conversion

Energy can be converted from one form to another, such as from potential to kinetic energy.

• First Law of Thermodynamics: Energy is conserved; it cannot be created or destroyed.

• Second Law of Thermodynamics: Processes move toward disorder; entropy always increases.

• Entropy: The degree of disorder or randomness in a system.

Chemical Reactions in Biology

Energy Transfer During Reactions

Chemical reactions involve the transfer of energy between molecules.

• Reactants: Substances that enter a reaction to form products.

• Products: Molecules formed from reactants.

• Substrate: A reactant that binds to an enzyme's active site.

• Reaction Rate: Measured by the rate of substrate disappearance or product formation.

• Free Energy and Chemical Bonds: Free energy is stored in chemical bonds; stronger bonds hold more free energy.

Activation Energy

Activation energy is the minimum energy required to initiate a chemical reaction.

• Activation Energy ($E_a$): The energy barrier that must be overcome for reactants to be converted into products.

Endergonic vs. Exergonic Reactions

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

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

• Example: ATP hydrolysis is exergonic; protein synthesis is endergonic.

Coupling Reactions

Cells couple exergonic and endergonic reactions to drive essential processes.

• Example: ATP hydrolysis (exergonic) is coupled to active transport (endergonic).

Reversible and Irreversible Reactions

• Reversible Reactions: Can proceed in both directions depending on energy changes.

• Irreversible Reactions: Release so much energy that they cannot reverse.

• Most biological reactions are reversible.

Enzymes

Definition and Function

• Enzymes: Protein catalysts that speed up chemical reactions by lowering activation energy.

• Substrates: Reactants that bind to enzyme active sites.

Isozymes and Medical Diagnosis

• Isozymes: Enzyme variants that catalyze the same reaction but differ in structure or regulation.

• Medical Use: Isozymes serve as diagnostic markers (e.g., cardiac isozymes in heart attack diagnosis).

Coenzymes and Vitamins

• Coenzymes: Organic cofactors derived from vitamins (e.g., NADH, FADH).

• Vitamins: Precursors to coenzymes, essential for enzyme function.

Enzyme Inactivation

• Inhibitors: Molecules that reduce enzyme activity (competitive or noncompetitive).

• Denaturation: Loss of enzyme structure due to heat or pH changes.

Enzyme Mechanism

• Lowering Activation Energy: Enzymes stabilize transition states and orient substrates properly.

Enzymatic Reaction Categories

• Oxidation-Reduction: Transfer of electrons (e.g., NADH → NAD+).

• Hydrolysis-Dehydration: Split or join molecules with water.

• Addition-Subtraction-Exchange: Transfer groups between molecules.

• Ligation: Join molecules using ATP (e.g., ligases).

Metabolism

Metabolic Pathways and Intermediates

• Metabolic Pathway: A series of enzymatic reactions.

• Intermediates: Molecules formed along the pathway.

• Energy Measurement: 1 kilocalorie (kcal) raises 1 kg of water by 1°C.

Regulation of Metabolic Pathways

• Methods: Controlling enzyme amounts, allosteric modulation, feedback inhibition, compartmentalization, reversible reactions.

Feedback Inhibition

• Definition: End product of a pathway inhibits an earlier enzyme to prevent overproduction.

Reversible Reactions and Enzyme Regulation

• Advantage: Separate enzymes allow independent regulation of forward and reverse reactions.

Compartmentalization

• Advantage: Allows specialization and prevents interference between pathways.

ATP/ADP Ratio

• Role: High ATP inhibits catabolism; low ATP stimulates it.

ATP Structure and Function

• ATP: Adenosine triphosphate, the main energy currency of the cell.

Catabolic Pathways and ATP Production

• Aerobic Pathways: Require oxygen, yield 30-32 ATP per glucose.

• Anaerobic Pathways: Do not require oxygen, yield 2 ATP per glucose.

Pathway	Key Substrate(s)	Key Product(s)	Oxygen Required?
Glycolysis	Glucose	Pyruvate, ATP, NADH	No
Citric Acid Cycle	Acetyl CoA	CO2, NADH, FADH2, ATP	No (but requires O2 for downstream steps)
Electron Transport System (ETS)	NADH, FADH2, O2	ATP, H2O	Yes

Oxidative Phosphorylation and Chemiosmotic Theory

• Oxidative Phosphorylation: Synthesis of ATP using the proton gradient generated by the electron transport chain.

• Chemiosmotic Theory: Protons flow back into the mitochondrial matrix through ATP synthase, driving ATP production.

ATP Yield from Glucose

• Net Yield: 30-32 ATP per glucose, depending on shuttle systems and tissue type.

Anaerobic Metabolism

• Pyruvate Branch Point: Pyruvate can enter mitochondria (aerobic) or convert to lactate (anaerobic).

• Net Energy Yield: Conversion of glucose to lactate yields 2 ATP.

Proteins and Gene Expression

Genes and Regulation

• Gene: A segment of DNA that codes for a protein.

• Constitutively Active Genes: Always expressed.

• Regulated Genes: Expressed only when needed.

Protein Synthesis

• Transcription: DNA is transcribed into mRNA by RNA polymerase, starting at the promoter region.

• mRNA Processing: Includes splicing (removal of introns), addition of 5' cap and poly-A tail.

• Alternative Splicing: Allows multiple proteins from one gene by varying exon combinations.

• RNA Interference: Small RNAs can regulate gene expression post-transcriptionally.

Translation

• Process: Ribosomes translate mRNA into a polypeptide chain using tRNA and amino acids.

• Key Terms: Codon (mRNA triplet), Anticodon (tRNA triplet), Peptide bond (links amino acids), Termination (end of translation).

Post-Translational Modifications

• Protein Folding: Achieves functional 3D structure.

• Cross-linkage: Covalent bonds between protein chains.

• Denaturation: Loss of structure due to heat, pH, or chemicals.

• Glycosylation: Addition of carbohydrate groups.

• Phosphorylation: Addition of phosphate groups.

• Assembly: Formation of polymeric proteins from subunits.