Lecture 5 Notes: Energy, Chemical Reactions, Enzymes, and ATP
Energy basics and units
Energy is measured in calories. One calorie is the amount of energy required to raise 1 g (1 mL) of water by 1 °C (specific heat of water).
Nutritional calorie (Calorie) is capital C: 1 Cal = 1000 calories = 1 kcal.
Practical takeaway: biochemical energy is often discussed in kcal per mole or per reaction in physiological contexts.
Potential energy and kinetic energy conversion
Potential energy exists in:
Ion gradients: Na+ ions at high concentration possess potential energy that can drive movement down their concentration gradient when channels open or membranes allow flux.
Electrons: Electrons in higher-energy electron shells possess potential energy; when they fall to lower-energy shells, kinetic energy is released.
Movement down gradients or energy-state changes releases kinetic energy that can do work.
Key concepts:
Concentration gradients can store potential energy.
Electron movement from high-energy to low-energy states releases kinetic energy.
Diagrammatic interpretation (conceptual):
Outside vs inside of cell: Na+ moving toward lower concentration releases kinetic energy that can perform cellular work.
High-energy shell → low-energy shell transitions release kinetic energy.
Chemical potential and free energy
Chemical potential is the free energy per mole that drives chemical changes.
General relation (as presented):
Initial Chemical Potential = Final Chemical Potential at equilibrium; no net change.
If Initial FE > Final FE, the transition is spontaneous and downhill (energy available to do work).
If Final FE > Initial FE, the transition requires input of energy (uphill).
Practical takeaway: reactions tend to proceed toward lower free energy unless coupled to energy input.
Chemical potential (formal expression)
Chemical Potential, μ, is given (as per lecture) by:
This simplified form highlights two contributions:
Concentration term (chemical potential associated with molecule concentration, e.g., in M).
Electrical potential term (electrical potential energy due to charge distribution).
Effects of concentration on chemical potential
High chemical potential corresponds to high chemical potential energy (FE).
Example intuition: High Na+ concentration yields a high chemical potential; low Na+ concentration yields a low chemical potential.
Implication: Substances tend to move from regions of high chemical potential to regions of lower chemical potential.
Effects of both concentration and electrical potential on chemical potential
When both concentration and electrical potential are considered, the chemical potential is influenced by:
Concentration gradient: high vs low concentration.
Electrical gradient: charge separation (e.g., Na+, K+) and the membrane potential (inside vs outside).
Notation in slides hints at positive and negative contributions depending on charge and location, illustrating how ions move under combined chemical and electrical forces.
Types of chemical reactions
Synthetic (anabolic) reactions: build larger molecules from smaller units.
Decomposition (catabolic) reactions: break larger molecules into smaller units.
Exchange reactions: parts of molecules are exchanged between reactants.
Reversible reactions: proceed in both directions; equilibrium can be established.
Examples of chemical reactions (with energy context)
Decomposition (hydrolysis):
Example: Sucrose (disaccharide) → Glucose + Fructose
General form: AB → A + B
In the slide, depiction includes hydrolysis steps showing water involvement and hydrogen/ hydroxyl group rearrangements.
Synthesis (dehydration synthesis):
Example: Two amino acids → Dipeptide + H2O
General form: A + B → AB, with water produced as a byproduct when bonds form.
Exchange reaction:
Atoms, molecules, ions, or electrons are exchanged between structures; general form: AB + CA ⇄ CB + CA (illustrative pattern)
Phosphoryl transfer example (energy handling):
Creatine phosphate donates a phosphate to ADP to form ATP; overall: Creatine phosphate + ADP → Creatine + ATP + Pi
This is a common cellular mechanism to quickly replenish ATP.
Irreversible vs reversible reactions
Irreversible: A + B → AB (proceeds in one direction to products with little/slowed reverse reaction under cellular conditions).
Reversible: A + B ⇌ AB (can proceed in both directions depending on conditions such as concentrations and energy state).
Energy changes in chemical reactions
Glucose oxidation example:
Reactants: glucose (C6H12O6) + O2
Products: CO2 + H2O
Energy flow: Energy is released as heat and used to form ATP (exergonic reaction).
Endergonic reactions: require energy input to proceed (e.g., amino acid polymerization into proteins using ATP-derived energy).
Exergonic vs Endergonic distinction is tied to whether energy is released or consumed during the reaction.
Factors determining reaction rate
1) Mass action (concentration effects)
2) Activation energy (enzymes lower the barrier)
3) Temperature (kinetic energy and molecular collisions)
Mass action (concentration effects)
Reaction example: A + B ⇌ C + D
Higher concentrations of reactants favor product formation; higher product concentrations favor the reverse reaction (Le Chatelier’s principle).
Additional illustrative equilibrium: CO2 + H2O ⇌ H2CO3 ⇌ HCO3− + H+ (carbonic acid/bicarbonate buffer system in physiology)
Enzymes and enzyme kinetics (mechanism)
Enzyme concept: biological catalysts that speed up reactions by lowering activation energy
Key steps in enzyme-catalyzed reactions:
1) Enzyme and substrate binding: Sucrase (enzyme) binds sucrose (substrate) at the active site to form the enzyme-substrate complex.
2) Enzyme-substrate complex formation: ES complex forms.
3) Reaction and release: ES → E + P, generating products (glucose and fructose from sucrose).Typical notation:
E + S ⇌ ES → E + P
Example: Sucrase catalyzes the breakdown of sucrose into glucose and fructose.
Energy and enzymes: activation energy and catalysis
Activation energy (Ea) is the energy barrier that must be overcome for a reaction to proceed.
Enzymes lower Ea, increasing the rate of reaction.
Comparison:
Uncatalyzed reaction: higher activation energy, slower rate.
Catalyzed reaction: lower activation energy, faster rate.
Visual interpretation: reaction coordinate with lower peak when enzyme is present; products formed more rapidly.
Michaelis-Menten kinetics (conceptual features in slides)
Parameters introduced:
Vmax (maximum velocity) — the rate when enzyme is saturated with substrate.
Km (Michaelis constant) — represents the substrate concentration at which the reaction rate is half of Vmax; reflects enzyme affinity for the substrate.
Key relationships (as depicted):
Initial rate v is influenced by both enzyme concentration and substrate concentration.
Under catalysis, at very high [S], the rate approaches Vmax.
Notation in slides pertains to:
Substrate saturation and the catalytic efficiency of the enzyme.
Enzyme kinetics—practical notes
Enzyme kinetics depend on: enzyme concentration, substrate concentration, temperature, and pH.
When enzyme is saturated with substrate, increasing [S] does not increase rate beyond Vmax.
Typical KPIs: Vmax, Km, initial rate v, and catalytic efficiency may be discussed in similar contexts.
Temperature and enzymes
Body temperature is tightly regulated in humans; altering temperature affects enzyme kinetics.
Increased kinetic energy can denature enzymes if temperature becomes too high.
Denaturation reduces enzyme activity or destroys structure, impairing function.
pH effects on enzymes
Each enzyme has an optimum pH range (e.g., stomach enzymes like pepsin work best in acidic conditions; salivary amylase works in near-neutral to slightly basic conditions).
pH shift away from optimum decreases enzymatic activity.
The slide shows a pH range from very acidic to very basic, with optimum around physiological pH depending on the enzyme.
Controlling enzyme function and inhibition
Competitive inhibition:
Inhibitor competes with substrate for the active site.
Reduces rate of reaction at low substrate concentrations, but maximum rate (Vmax) can be reached if substrate concentration is high enough to outcompete the inhibitor.
Noncompetitive inhibition:
Inhibitor binds to a site other than the active site (allosteric site).
Reduces the maximum rate of reaction (Vmax) since the enzyme’s activity is altered even if substrate concentration is high.
Practical implications: inhibitors regulate metabolic pathways, drug design, and metabolic control in physiology.
Cellular respiration and ATP
ATP (Adenosine Triphosphate) structure:
Three phosphate groups linked by high-energy bonds (phosphoanhydride bonds).
Adenosine: ribose sugar + adenine base.
ATP is highly dynamic and is present only for short periods in tissues; it is not a long-term energy storage molecule.
Primary role of ATP: energy transfer between reactions rather than long-term storage.
ATP as energy currency couples exergonic and endergonic processes (energy from exergonic reactions drives endergonic ones).
ATP overview and energy transfer
ATP is formed from ADP + Pi (inorganic phosphate) via energy supplied by cellular respiration.
ATP hydrolysis powers cellular work: muscle contraction, ciliary beating, active transport, and biosynthetic reactions.
Energy coupling concept: energy released from exergonic reactions is used to drive endergonic reactions via ATP hydrolysis and phosphate transfer.
Glucose oxidation and energy yield
Overall oxidation of glucose to CO2 and H2O:
Total energy released from this oxidation:
Energy that becomes ATP: about 38 ATP are produced per glucose under the depicted conditions.
Efficiency of energy transfer to ATP:
Each ATP molecule stores about of energy during synthesis (approximate value used in slides).
Total energy stored as ATP:
Proportion of total energy captured as ATP:
Practical interpretation: roughly two-fifths of the energy from glucose oxidation is captured in ATP under the conditions shown; the rest is dissipated as heat or used for other cellular processes.
Applications: ATP is used for processes such as muscle contraction, ciliary beating, active transport, and biosynthesis, illustrating energy partitioning in metabolism.
Quick references and cross-links
Energy units and conversions: calories vs kilocalories; 1 Cal = 1000 cal.
Potential energy sources include ion gradients and electronic energy levels.
Chemical potential is a function of concentration and electrical potential; movement tends to downhill in free energy unless coupled to input or energy currency (e.g., ATP).
Reaction rates are controlled by mass action, activation energy (enzyme-catalyzed lowering), and temperature.
Enzymes increase reaction rates by lowering activation energy; they are sensitive to temperature and pH and can be inhibited competitively or noncompetitively to regulate metabolism.
Cellular respiration converts chemical energy from glucose into ATP, with a sizeable portion released as heat, and with energy storage efficiency around 40% in the example provided.
Note: All formulas printed in LaTeX are included where shown above, for example:
Chemical potential relation:
Energy per ATP:
Total energy from glucose:
ATP yield:
Energy stored as ATP:
Fraction stored as ATP: