Metabolism, Cellular Respiration, and Photosynthesis: Comprehensive Study Notes

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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 transformation.

Definition: The sum of all chemical reactions in an organism, including both energy-releasing (catabolic) and energy-consuming (anabolic) pathways.
Example: Cellular respiration (catabolic) and protein synthesis (anabolic).

Catabolic and Anabolic Pathways

Metabolic pathways are classified based on their role in energy transformation and molecule synthesis or breakdown.

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: Types and Forms

Energy is the capacity to cause change or do work. It exists in various forms relevant to biological systems.

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.

First Law of Thermodynamics

The First Law states that energy cannot be created or destroyed, only transformed from one form to another. The total energy of the universe remains constant.

Second Law of Thermodynamics

Every energy transfer or transformation increases the entropy (disorder) of the universe. Organisms must constantly obtain energy to maintain order and counteract entropy.

Free Energy and Spontaneity of Reactions

Free energy ($\Delta G$) determines if a reaction is spontaneous. A negative $\Delta G$ indicates a spontaneous, energy-releasing reaction; a positive $\Delta G$ indicates a non-spontaneous (endergonic) reaction that requires energy input.

$\Delta G = G_{final} - G_{initial}$

Exergonic vs. Endergonic Reactions

Reactions are classified by their energy changes.

Exergonic Reaction: Releases energy; spontaneous ($\Delta G < 0$).
Endergonic Reaction: Requires energy input; non-spontaneous ($\Delta G > 0$).

ATP: Structure and Function

ATP (adenosine triphosphate) is the cell’s main energy currency, storing energy in high-energy phosphate bonds.

Structure: Adenine base, ribose sugar, and three phosphate groups.
Function: Hydrolysis of ATP releases energy for cellular work.
Equation: $\text{ATP} + \text{H}_2\text{O} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

Enzymes and Activation Energy

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

Activation Energy ($E_A$): Minimum energy required to start a reaction.
Active Site: Region on enzyme where substrate binds.
Induced Fit Model: Enzyme changes shape to fit substrate, enhancing catalysis.
Cofactors: Non-protein helpers (metal ions).
Coenzymes: Organic helpers (vitamins).

Enzyme Regulation

Enzyme activity is regulated by inhibitors and feedback mechanisms.

Inhibitors: Competitive or noncompetitive; block enzyme activity.
Feedback Inhibition: End product inhibits an earlier enzyme to regulate pathway.

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 ($\Delta G$)	Energy available to do work in a system.
Exergonic Reaction	Releases energy; spontaneous ($\Delta G < 0$).
Endergonic Reaction	Requires energy input; nonspontaneous ($\Delta G > 0$).
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 ($E_A$)	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	Enzyme product inhibits an earlier enzyme to regulate pathway.

Chapter 7: Cellular Respiration and Fermentation

Purpose and Overview of Cellular Respiration

Cellular respiration extracts energy stored in food molecules (e.g., glucose) and converts it into ATP, the cell’s main energy currency.

Overall Equation: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Energy}$ (about 36–38 ATP)

Oxidation and Reduction in Respiration

Cellular respiration involves redox reactions, where electrons are transferred from glucose to oxygen, releasing energy.

Oxidation: Loss of electrons or hydrogen atoms.
Reduction: Gain of electrons or hydrogen atoms.

Electron Carriers: NAD+ and FAD

NAD+ and FAD are electron carriers that shuttle electrons during cellular respiration, becoming NADH and FADH2.

Main Stages of Cellular Respiration

Cellular respiration occurs in several stages, each with specific locations and products.

Glycolysis: Cytoplasm; glucose → 2 pyruvate + 2 ATP + 2 NADH.
Pyruvate Oxidation: Mitochondrial matrix; 2 pyruvate → 2 acetyl-CoA + 2 CO2 + 2 NADH.
Citric Acid Cycle (Krebs Cycle): Matrix; 2 acetyl-CoA → 4 CO2 + 2 ATP + 6 NADH + 2 FADH2.
Electron Transport Chain (ETC): 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: 34 ATP (oxidative phosphorylation)
Total: 36–38 ATP per glucose

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).

Electron Transport Chain and Chemiosmosis

The ETC uses electron carriers to pump H+ ions, creating a proton motive force that powers ATP synthesis via ATP synthase.

Chemiosmosis: Flow of protons back into the matrix powers ATP production.

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.
NAD+ / NADH	Electron carrier that shuttles electrons to the ETC.
FAD / FADH2	Electron carrier that shuttles electrons to the ETC.
Glycolysis	Conversion of glucose to pyruvate, producing ATP and 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 electrons through protein complexes, creating gradient.
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 of Photosynthesis

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

Overall Equation: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{Light Energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Structures Involved in Photosynthesis

Photosynthesis occurs in chloroplasts, which contain thylakoids and stroma.

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

Pigments in Photosynthesis

Pigments absorb light energy for photosynthesis.

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

Stages of Photosynthesis

Photosynthesis has two main stages: light reactions and the Calvin cycle.

Light Reactions: Occur in thylakoid membranes; use light energy to produce ATP and NADPH.
Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into glucose.

Light Reactions: Details

Light reactions use photosystems to capture light and transfer electrons.

Photosystem II (PSII): Splits water, releases O2, and transfers electrons.
Photosystem I (PSI): Energizes electrons for NADPH production.
Electron Transport Chain (ETC): Pumps protons, creates gradient for ATP synthesis.
ATP Synthase: Enzyme that synthesizes ATP using proton motive force.

Calvin Cycle: Details

The Calvin cycle uses ATP and NADPH to fix CO2 into glucose.

Key Steps: Carbon fixation, reduction, regeneration of RuBP.
Enzyme: Rubisco catalyzes CO2 fixation.
Product: G3P (glyceraldehyde-3-phosphate), a three-carbon sugar.

Photorespiration and Adaptations

Photorespiration is a wasteful process where Rubisco binds O2 instead of CO2, reducing photosynthetic efficiency.

C4 Plants: Minimize photorespiration by fixing CO2 in mesophyll cells, then transferring to bundle sheath cells.
CAM Plants: Fix CO2 at night to minimize water loss.

Comparison: Photosynthesis vs. Cellular Respiration

Photosynthesis and cellular respiration are complementary processes in energy transformation.

Feature	Photosynthesis	Cellular Respiration
Overall Purpose	Stores energy in glucose	Releases energy from glucose
Energy Source	Sunlight	Chemical bonds in glucose
Products	Glucose, O2	CO2, H2O
Final Electron Acceptor	NADP+	Oxygen (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	Light-harvesting complex that gathers photons and transfers energy.
Photosystem I (PSI)	First photosystem; splits water, releases O2 and electrons.
Photosystem II (PSII)	Second photosystem; energizes electrons for NADPH production.
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 at 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.