Metabolism, Cellular Respiration, and Photosynthesis

Overview

This study guide covers the fundamental concepts of metabolism, cellular respiration, and photosynthesis, focusing on energy transformations, ATP production, and the regulation of metabolic pathways. Understanding these processes is essential for grasping how cells obtain, use, and store energy.

Metabolism and Energy Transformations

Metabolism and Metabolic Pathways

Metabolism is the sum of all chemical reactions occurring in an organism. These reactions are organized into metabolic pathways, which are sequences of reactions, each catalyzed by a specific enzyme.

• Catabolic pathways: Break down large molecules into smaller ones, releasing energy. Example: Breakdown of glucose during cellular respiration.

• Anabolic pathways: Build larger molecules from smaller ones, requiring energy. Example: Synthesis of proteins from amino acids.

Key Point: Catabolism breaks down; anabolism builds up.

Types of Energy

• Kinetic energy: Energy of motion (e.g., moving objects).

• Potential energy: Stored energy due to position or structure.

• Chemical energy: A form of potential energy stored in molecular bonds.

Organisms as Open Systems

Organisms exchange matter and energy with their surroundings (e.g., intake of food and oxygen, release of CO2 and heat).

Laws of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transferred or transformed.

• Second Law: Every energy transformation increases the entropy (disorder) of the universe.

Entropy describes the dispersal of energy and the number of possible arrangements of matter.

Free Energy and Spontaneous Reactions

Gibbs Free Energy

Gibbs free energy (G) is the portion of a system's energy available to perform work under specified conditions.

Equation:

• = change in free energy

• = change in enthalpy

• = temperature in Kelvin

• = change in entropy

Exergonic vs. Endergonic Reactions

At equilibrium: (no net change).

Important: Spontaneous does not mean fast; high activation energy can slow reactions.

ATP and Cellular Work

Structure and Function of ATP

ATP (adenosine triphosphate) is a nucleotide composed of adenine, ribose, and three phosphate groups. It is the cell's main immediate energy-transfer molecule.

ATP Hydrolysis and Energy Coupling

ATP hydrolysis:

• Exergonic reaction; energy released is used to drive endergonic processes.

• Phosphorylation: Transfer of a phosphate group to another molecule, increasing its reactivity or changing protein shape.

Types of Cellular Work Powered by ATP

Enzymes and Activation Energy

Enzyme Function

Enzymes are biological catalysts (usually proteins) that speed up chemical reactions by lowering activation energy (), without being consumed.

• Do not change or reaction equilibrium.

• Enzyme + Substrate → Enzyme–Substrate Complex → Enzyme + Product

• Active site: Region where substrate binds.

• Induced fit: Active site changes shape to facilitate catalysis.

Factors Affecting Enzyme Activity

• Temperature: Increases rate up to optimum; excessive heat denatures enzymes.

• pH: Each enzyme has an optimal pH.

• Substrate concentration: Rate increases until enzymes are saturated.

• Cofactors: Nonprotein helpers; organic cofactors are called coenzymes.

Enzyme Inhibitors

Regulation of Enzyme Activity

Allosteric Regulation

Regulatory molecules bind to sites other than the active site, changing enzyme activity.

• Allosteric activator: Stabilizes active form, increases activity.

• Allosteric inhibitor: Stabilizes inactive form, decreases activity.

Cooperativity

Binding of a molecule to one subunit affects binding to other subunits. Example: Hemoglobin and oxygen.

Feedback Inhibition

Final product of a pathway inhibits an enzyme earlier in the pathway, preventing overproduction.

Example: A → B → C → D → Final Product; final product inhibits an early enzyme.

Cellular Respiration and Fermentation

Cellular Respiration Overview

Cellular respiration breaks down organic molecules to harvest energy and produce ATP.

Equation:

Redox Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• OIL RIG: Oxidation Is Loss, Reduction Is Gain.

NAD+: Electron carrier; accepts electrons to become NADH, which transfers electrons to the electron transport chain (ETC).

ATP Production Mechanisms

Glycolysis

Location and Process

Occurs in the cytosol; does not require oxygen.

• Glucose (6C) → 2 pyruvate (3C each)

• Energy investment phase: 2 ATP used

• Energy payoff phase: 4 ATP and 2 NADH produced

Pyruvate Oxidation and Citric Acid Cycle

Pyruvate Oxidation

Pyruvate is converted to acetyl-CoA in the mitochondrial matrix.

• Per pyruvate: 1 acetyl-CoA, 1 NADH, 1 CO2

• Occurs twice per glucose

Citric Acid Cycle (Krebs Cycle)

Location: Mitochondrial matrix. Acetyl-CoA enters, carbon released as CO2.

Cycle turns twice per glucose; oxaloacetate is regenerated.

Oxidative Phosphorylation and Chemiosmosis

Electron Transport Chain (ETC)

Location: Inner mitochondrial membrane. NADH and FADH2 donate electrons; energy used to pump H+ into intermembrane space. Oxygen is the final electron acceptor, forming water.

Chemiosmosis

Proton gradient drives ATP synthase, producing ATP from ADP and inorganic phosphate.

Electron flow: NADH/FADH2 → ETC → H+ gradient → ATP synthase → ATP

ATP Yield per Glucose

About 34% of glucose's energy is transferred to ATP; the rest is released as heat.

Fermentation and Anaerobic Respiration

Fermentation

Regenerates NAD+ so glycolysis can continue when oxygen is unavailable. Net ATP yield is 2 per glucose (from glycolysis).

Aerobic vs. Anaerobic Respiration vs. Fermentation

Other Fuels and Regulation of Respiration

Alternative Fuels

• Carbohydrates: Broken into sugars for glycolysis.

• Fats: Glycerol enters glycolysis; fatty acids undergo beta oxidation to produce acetyl-CoA.

• Proteins: Amino acids enter pathways after deamination.

Intermediates can be used for anabolic pathways.

Regulation of ATP Production

• Low ATP: Respiration speeds up.

• High ATP: Respiration slows down.

• Phosphofructokinase (PFK): Key regulatory enzyme; inhibited by high ATP (feedback regulation).

Photosynthesis

Photosynthesis Overview

Photosynthetic organisms use light energy to produce organic molecules from inorganic materials.

Autotrophs vs. Heterotrophs

Chloroplasts and the Overall Process

Structure of Chloroplasts

• Stomata: Openings for gas exchange.

• Stroma: Fluid interior; site of Calvin cycle.

• Thylakoids: Membrane sacs with chlorophyll; site of light reactions.

• Grana: Stacks of thylakoids.

Photosynthesis Equation

• Water is oxidized; CO2 is reduced; light supplies energy.

• O2 released comes from water.

Stages of Photosynthesis

Light Reactions: Light, Pigments, and ATP Production

Light and Pigments

• Light: Electromagnetic energy; visible range ~380–740 nm.

• Photon: Packet of light energy; shorter wavelength = more energy.

• Pigments: Absorb specific wavelengths; chlorophyll a (primary), chlorophyll b and carotenoids (accessory).

Absorption spectrum: Wavelengths absorbed by pigment. Action spectrum: Wavelengths most effective for photosynthesis.

Photosystems and Electron Flow

Linear Electron Flow

Sequence: H2O → PSII → ETC → PSI → NADP+ → NADPH

• Light excites electrons in PSII; water split to replace electrons, releasing H+ and O2.

• Electrons travel through ETC, establishing proton gradient.

• ATP synthase produces ATP; PSI produces NADPH.

Cyclic Electron Flow

Uses PSI only; electrons cycle back, producing additional ATP (no NADPH or O2).

Chemiosmosis in Chloroplasts

H+ accumulates in thylakoid space; flows through ATP synthase to stroma, producing ATP (photophosphorylation).

The Calvin Cycle

Purpose and Phases

The Calvin cycle uses ATP and NADPH to incorporate CO2 into organic molecules, producing G3P. Occurs in the stroma.

• Phase 1 — Carbon fixation: CO2 combines with RuBP (via rubisco), forming 3-PGA.

• Phase 2 — Reduction: ATP and NADPH convert 3-PGA to G3P.

• Phase 3 — Regeneration: Most G3P is used to regenerate RuBP (using ATP).

Carbon Accounting (Three Turns)

• 3 CO2 fixed → 6 G3P formed

• 1 G3P is net output; 5 G3P regenerate RuBP

• 9 ATP and 6 NADPH consumed

Two net G3P molecules are needed to build one glucose molecule.

Connecting Metabolism, Respiration, and Photosynthesis

Key Relationships

• Energy cannot be created or destroyed; it changes forms.

• Catabolic pathways break down molecules; anabolic pathways build molecules.

• Exergonic reactions: negative; endergonic: $\Delta G$ positive.

• Enzymes lower activation energy, not .

• ATP couples energy-releasing reactions to energy-requiring work.

• Oxidation = loss of electrons; reduction = gain of electrons.

• Glycolysis: 2 pyruvate, 2 net ATP, 2 NADH per glucose.

• Oxygen is the final electron acceptor in aerobic respiration.

• Most ATP in aerobic respiration is made via oxidative phosphorylation.

• Fermentation regenerates NAD+ for glycolysis.

• Light reactions produce ATP, NADPH, and O2.

• Calvin cycle uses CO2, ATP, NADPH to produce G3P.

• Photosynthesis and respiration both involve electron transfer and energy transformations.

Study Tip

For every pathway, ask yourself: Where does it happen? What enters? What leaves? Why does the cell need it?