Metabolism, Cellular Respiration, and Photosynthesis: Core Concepts and Comparisons

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Chapter 6: Introduction to Metabolism

What is Metabolism?

Metabolism encompasses all chemical reactions occurring within an organism, enabling life by managing energy and matter. These reactions are organized into pathways that either release or consume energy.

Catabolic Pathways: Break down complex molecules into simpler ones, releasing energy (e.g., cellular respiration).
Anabolic Pathways: Build complex molecules from simpler ones, consuming energy (e.g., protein synthesis).

Energy: Forms and Transfer

Energy is the capacity to cause change or do work. It exists in various forms and can be transferred or transformed.

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

Thermodynamics and Biological Systems

Thermodynamics is the study of energy transformations in matter. Biological systems obey the laws of thermodynamics:

First Law: Energy cannot be created or destroyed, only transformed.
Second Law: Every energy transfer increases the entropy (disorder) of the universe.

Free Energy and Spontaneity of Reactions

Free energy () is the portion of a system's energy available to do work. The change in free energy () determines if a reaction is spontaneous.

Exergonic Reaction: Releases energy; spontaneous ().
Endergonic Reaction: Requires energy input; nonspontaneous ().

ATP: Structure and Function

ATP (adenosine triphosphate) is the cell's main energy currency. Its energy is stored in high-energy phosphate bonds, which are released upon hydrolysis.

ATP Hydrolysis:
Drives cellular work by coupling exergonic and endergonic reactions.

Enzymes and Activation Energy

Enzymes are biological catalysts that speed up chemical reactions by lowering activation energy ().

Activation Energy (): Minimum energy required to start a reaction.
Active Site: Region on the enzyme where the substrate binds.
Induced Fit Model: Enzyme changes shape to fit the substrate, enhancing catalysis.
Cofactors: Non-protein helpers (metal ions).
Coenzymes: Organic helpers (vitamin derivatives).
Enzyme Regulation: Includes inhibitors (competitive/noncompetitive) and feedback inhibition.

Key Terms and Definitions

Term	Definition
Metabolism	All chemical reactions occurring within an organism.
Catabolic Pathway	Pathway that breaks down complex molecules, releasing energy.
Anabolic Pathway	Pathway that builds complex molecules, consuming energy.
Kinetic Energy	Energy of motion.
Potential Energy	Stored energy due to position or structure.
Chemical Energy	Potential energy stored in chemical bonds.
Thermodynamics	Study of energy transformations in matter.
Entropy	Measure of disorder or randomness in a system.
Free Energy ()	Energy available to do work in a system.
Exergonic Reaction	Releases energy; spontaneous ().
Endergonic Reaction	Requires energy input; nonspontaneous ().
ATP	Cell's energy currency; drives cellular work.
Phosphorylation	Transfer of a phosphate group to another molecule.
Enzyme	Protein catalyst that lowers activation energy.
Activation Energy ()	Energy required to start a reaction.
Active Site	Region of enzyme that binds substrate.
Induced Fit	Enzyme changes shape slightly when binding substrate.
Cofactor	Non-protein enzyme helper (metal ion).
Coenzyme	Organic enzyme helper (vitamin derivative).
Feedback Inhibition	End product inhibits an earlier enzyme to regulate pathway.

Chapter 7: Cellular Respiration and Fermentation

Purpose and Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy stored in food molecules (e.g., glucose) and convert it into ATP. It involves a series of redox reactions and occurs in several stages.

Overall Equation:

Redox Reactions in Respiration

Redox reactions involve the transfer of electrons. Oxidation is the loss of electrons, while reduction is the gain of electrons. These reactions drive the flow of energy in respiration.

Oxidation: Loss of electrons or hydrogen atoms; molecule loses energy.
Reduction: Gain of electrons or hydrogen atoms; molecule gains energy.

Main Stages of Cellular Respiration

Cellular respiration consists of four main stages, each occurring in specific locations within the cell:

Glycolysis: Cytoplasm; Glucose → 2 Pyruvate + 2 ATP + 2 NADH.
Pyruvate Oxidation: Mitochondrial matrix; 2 Pyruvate → 2 Acetyl-CoA + 2 CO2 + 2 NADH.
Krebs Cycle (Citric Acid Cycle): Mitochondrial matrix; 2 Acetyl-CoA → 4 CO2 + 2 ATP + 6 NADH + 2 FADH2.
Electron Transport Chain (ETC) & Chemiosmosis: Inner mitochondrial membrane; Electrons from NADH/FADH2 power ATP synthase to make 34 ATP.

ATP Yield from Glucose

Glycolysis: 2 ATP + 2 NADH
Pyruvate Oxidation: 2 NADH
Krebs Cycle: 2 ATP + 6 NADH + 2 FADH2
ETC/Chemiosmosis: ~34 ATP
Total: 36-38 ATP per glucose (aerobic respiration)

Fermentation and Anaerobic Respiration

When oxygen is unavailable, cells use fermentation to regenerate NAD+ and allow glycolysis to continue.

Lactic Acid Fermentation: Pyruvate → Lactate (muscle cells)
Alcohol Fermentation: Pyruvate → Ethanol + CO2 (yeast)

Key Terms and Definitions

Term	Definition
Oxidation	Loss of electrons or hydrogen atoms; molecule loses energy.
Reduction	Gain of electrons or hydrogen atoms; molecule gains energy.
Oxidizing Agent	Accepts electrons and becomes reduced.
Reducing Agent	Donates electrons and becomes oxidized.
NAD+ / NADH	Electron carrier that shuttles electrons to the ETC.
FAD / FADH2	Electron carrier that transfers electrons to the ETC after NADH.
Glycolysis	Glucose breakdown in cytoplasm, producing pyruvate, ATP, NADH.
Substrate-level phosphorylation	Direct ATP formation using enzyme-catalyzed phosphate transfer.
Citric Acid Cycle (Krebs Cycle)	Cycle oxidizing Acetyl-CoA, producing ATP, NADH, FADH2.
Electron Transport Chain (ETC)	Movement of proteins through which ATP synthesis occurs.
ATP synthase	Enzyme that synthesizes ATP using proton motive force.
Anaerobic respiration	Uses alternative molecules instead of oxygen.
Fermentation	Anaerobic process regenerating NAD+ to allow glycolysis to continue.
Lactic acid fermentation	Produces lactate and regenerates NAD+ (muscle cells).
Alcohol fermentation	Produces ethanol and CO2 (yeast).
Proton motive force	Electrochemical gradient driving ATP synthesis.

Chapter 8: Photosynthesis

Overview and Equation

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose. It consists of two main stages: the light reactions and the Calvin cycle.

Overall Equation:

Structures Involved in Photosynthesis

Chloroplast: Organelle where photosynthesis occurs; contains thylakoids and stroma.
Thylakoid: Membranous sac where light reactions take place.
Stroma: Fluid space surrounding thylakoids; site of Calvin cycle.

Pigments and Light Absorption

Chlorophyll a: Main pigment absorbing blue and red light.
Accessory Pigments: Chlorophyll b, carotenoids; expand range of light absorption.

Stages of Photosynthesis

Light Reactions: Occur in thylakoid membranes; convert light energy to chemical energy (ATP and NADPH).
Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix carbon into glucose.

Light Reactions: Details

Photosystems I (PSI) and II (PSII) absorb light and transfer electrons.
Electron transport chain pumps protons, creating a gradient for ATP synthesis.
Products: ATP, NADPH, O2

Calvin Cycle: Details

Uses ATP and NADPH to fix CO2 into glucose.
Key enzyme: Rubisco.
For every 3 CO2 molecules, the cycle produces 1 G3P (glyceraldehyde-3-phosphate).

Photorespiration and Adaptations

Photorespiration occurs when Rubisco binds O2 instead of CO2, producing a carbon compound that must be recycled. This process consumes ATP and releases CO2 without producing sugar, making it energetically wasteful.

C4 Plants: Fix CO2 into 4C compounds to reduce photorespiration.
CAM Plants: Fix CO2 at night to minimize water loss.

Comparison: Photosynthesis vs. Cellular Respiration

Feature	Photosynthesis	Cellular Respiration
Overall Purpose	Stores energy in glucose	Releases energy from glucose
Energy Source	Sunlight	Chemical bonds in glucose
Products	Glucose (C6H12O6)	CO2 and H2O
Final Electron Acceptor	NADP+	O2
Location	Chloroplasts	Mitochondria
Type of Process	Anabolic, endergonic	Catabolic, exergonic

Key Terms and Definitions

Term	Definition
Chloroplast	Organelle where photosynthesis occurs, containing thylakoids and stroma.
Thylakoid	Membranous sac where light reactions take place.
Stroma	Fluid space surrounding thylakoids; site of Calvin cycle.
Chlorophyll	Main pigment that captures light energy.
Photosystem I (PSI)	Main photosystem; splits water, releasing O2 and electrons.
Photosystem II (PSII)	Second photosystem; energizes electrons for ATP synthesis.
Electron Transport Chain (ETC)	Series of proteins transferring electrons and pumping protons.
Chemiosmosis	Proton gradient driving ATP synthesis via ATP synthase.
ATP synthase	Enzyme that synthesizes ATP using proton motive force.
NADPH	Electron carrier providing reducing power for the Calvin cycle.
Calvin Cycle	Cycle that fixes CO2 into glucose using ATP and NADPH.
Rubisco	Enzyme that catalyzes CO2 fixation to RuBP.
G3P (glyceraldehyde-3-phosphate)	Three-carbon sugar product of the Calvin cycle.
Photorespiration	Wasteful process where Rubisco binds O2 instead of CO2.
C4 Plants	Plants that fix CO2 into 4C compounds to reduce photorespiration.
CAM Plants	Plants that fix CO2 at night to minimize water loss.
Z-scheme	Diagram of electron flow from H2O → PSII → PSI → NADP+.
Light reactions	Stage converting light energy to ATP and NADPH.
Carbon fixation	Incorporation of CO2 into organic molecules.