Cellular Respiration, Photosynthesis, and Enzyme Function: Core Concepts in Bioenergetics

Bioenergetics: Energy Flow in Biological Systems

Thermodynamics in Biology

Biological systems obey the laws of thermodynamics, which govern energy transformations and the direction of metabolic processes.

• First Law of Thermodynamics: Energy cannot be created or destroyed, only transformed from one form to another.

• Second Law of Thermodynamics: Every energy transfer increases the entropy (disorder) of the universe; some energy is lost as heat.

• Entropy (S): A measure of disorder; systems tend to move toward higher entropy.

• Free Energy (G): The portion of a system's energy that can perform work. Changes in free energy ($\Delta G$) determine whether a reaction is spontaneous.

Equation:

$\Delta G = \Delta H - T\Delta S$

• Exergonic Reaction: Releases energy ($\Delta G < 0$), spontaneous (e.g., cellular respiration).

• Endergonic Reaction: Requires energy input ($\Delta G > 0$), non-spontaneous (e.g., photosynthesis).

Types of Energy

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

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

• Chemical Energy: Potential energy available for release in a chemical reaction (e.g., ATP hydrolysis).

Example: A ball at the top of a hill has potential energy; as it rolls down, potential is converted to kinetic energy.

Enzymes and Metabolic Pathways

Enzyme Structure and Function

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

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

• Substrate: The reactant molecule upon which an enzyme acts.

• Enzyme-Substrate Complex: Temporary association between enzyme and substrate during catalysis.

Enzyme Action: Enzymes do not change the overall free energy ($\Delta G$) of a reaction; they only lower the activation energy.

Figure Reference: Free energy diagram shows lower activation energy with enzyme present.

Factors Affecting Enzyme Activity

• Temperature: Each enzyme has an optimal temperature; too high or low can denature the enzyme.

• pH: Each enzyme has an optimal pH range.

• Substrate Concentration: Increasing substrate increases reaction rate until enzyme is saturated.

• Enzyme Concentration: Increasing enzyme increases reaction rate if substrate is not limiting.

Enzyme Inhibition and Regulation

• Competitive Inhibitor: Binds to the active site, blocking substrate binding.

• Noncompetitive Inhibitor: Binds to a different site, changing enzyme shape and reducing activity.

• Feedback Inhibition: End product of a pathway inhibits an earlier step, regulating pathway activity.

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) stores and transfers energy for cellular work.

• Energy Source: Energy is released when ATP is hydrolyzed to ADP and inorganic phosphate ($\text{ATP} \rightarrow \text{ADP} + \text{P}_i$).

• Importance: ATP powers cellular processes such as muscle contraction, active transport, and biosynthesis.

Redox Reactions in Metabolism

Oxidation and Reduction

• Oxidation: Loss of electrons (often loss of H).

• Reduction: Gain of electrons (often gain of H).

• Redox in Respiration: Glucose is oxidized; oxygen is reduced.

• Redox in Photosynthesis: Water is oxidized; carbon dioxide is reduced.

Cellular Respiration

Overview and Equation

Cellular respiration is a catabolic, exergonic process that converts glucose and oxygen into ATP, CO2, and H2O.

Overall Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Stages of Cellular Respiration

Stage	Location	Inputs	Outputs	Main Events
Glycolysis	Cytosol	Glucose, 2 ATP, 2 NAD+	2 Pyruvate, 4 ATP (net 2), 2 NADH	Splitting glucose
Pyruvate Oxidation	Mitochondrial matrix	2 Pyruvate, 2 NAD+	2 Acetyl-CoA, 2 CO2, 2 NADH	Links glycolysis to Krebs cycle
Krebs Cycle (Citric Acid Cycle)	Mitochondrial matrix	2 Acetyl-CoA, 6 NAD+, 2 FAD	4 CO2, 6 NADH, 2 FADH2, 2 ATP	Completes glucose oxidation
Electron Transport Chain & Chemiosmosis	Inner mitochondrial membrane	NADH, FADH2, O2	H2O, ~26-28 ATP	Oxidative phosphorylation

Electron Carriers

• NAD+ / NADH and FAD / FADH2 shuttle electrons to the electron transport chain.

Electron Transport Chain (ETC) and Chemiosmosis

• Located in the inner mitochondrial membrane.

• Electrons from NADH and FADH2 pass through protein complexes, releasing energy to pump H+ into the intermembrane space.

• Oxygen is the final electron acceptor, forming water.

• H+ gradient drives ATP synthesis via ATP synthase (chemiosmosis).

Aerobic vs Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen Required?	Yes	No
Final Electron Acceptor	O2	Other molecules (e.g., sulfate, nitrate)
ATP Yield	~30-32 ATP/nglucose	Less ATP (varies)
Stages	Glycolysis, Krebs, ETC	Glycolysis, modified ETC or fermentation

Fermentation: Occurs when oxygen is absent; regenerates NAD+ for glycolysis. Types include lactic acid fermentation (produces lactate) and alcoholic fermentation (produces ethanol and CO2).

Purpose of Oxygen and CO2 Production

• Oxygen: Final electron acceptor in ETC; forms water.

• CO2: Produced during pyruvate oxidation and Krebs cycle; exhaled as waste.

Photosynthesis

Overview and Equation

Photosynthesis is an anabolic, endergonic process converting light energy into chemical energy in plants, algae, and some bacteria.

Overall Equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Primary Producers

• Organisms that produce organic molecules from CO2 using light or chemical energy (e.g., plants, algae).

Photosynthetic Pigments

• Chlorophyll a: Main pigment; absorbs blue-violet and red light, reflects green.

• Accessory Pigments: Chlorophyll b, carotenoids; broaden absorption spectrum.

• Absorption vs Reflection: Pigments absorb specific wavelengths; reflected wavelengths determine color.

Stages of Photosynthesis

Stage	Location	Inputs	Outputs	Main Purpose
Light Reactions	Thylakoid membrane	Light, H2O, NADP+, ADP	O2, ATP, NADPH	Convert light to chemical energy
Calvin Cycle	Stroma	CO2, ATP, NADPH	Glucose (G3P), NADP+, ADP	Carbon fixation

Photosystems and Electron Flow

• Photosystem II (PSII): Absorbs light, splits water to release O2, donates electrons to ETC.

• Photosystem I (PSI): Receives electrons from ETC, uses light energy to reduce NADP+ to NADPH.

• H+ Gradient: Established across thylakoid membrane; drives ATP synthesis (chemiosmosis).

RUBISCO and Carbon Fixation

• RUBISCO: Enzyme that catalyzes the first step of the Calvin cycle, fixing CO2 to RuBP.

Photorespiration, C4, and CAM Pathways

• Photorespiration: Occurs when RUBISCO fixes O2 instead of CO2, reducing photosynthetic efficiency.

• C4 Plants: Spatial separation of carbon fixation and Calvin cycle; adapted to hot, dry environments.

• CAM Plants: Temporal separation; fix CO2 at night, Calvin cycle during day; conserves water.

Summary Table: Key Comparisons

Process	Type of Reaction	Energy Flow	Main Purpose
Cellular Respiration	Catabolic, exergonic	Glucose to ATP	Energy extraction
Photosynthesis	Anabolic, endergonic	Light to glucose	Energy storage

Additional info: The above notes integrate foundational concepts from metabolism, enzyme function, cellular respiration, and photosynthesis, as covered in introductory college biology courses. Diagrams referenced (e.g., free energy with/without enzyme) are standard in textbooks and can be found in most biology resources.