Cellular Energetics: Metabolism, Photosynthesis, and Cellular Respiration

Unit 3: Cellular Energetics

Topic 3.1: Enzyme Structure and Function

Enzymes are biological catalysts that speed up chemical reactions in cells by lowering activation energy. They are essential for metabolism, growth, and cellular regulation.

• Enzyme Structure: Enzymes are proteins with a specific three-dimensional shape, including an active site where substrates bind.

• Function: Enzymes facilitate reactions by stabilizing the transition state and reducing activation energy.

• Specificity: Each enzyme is specific to its substrate due to the shape of its active site (lock-and-key model).

• Induced Fit Model: The active site changes shape slightly to fit the substrate more closely.

• Factors Affecting Enzyme Activity: Temperature, pH, and substrate concentration can affect enzyme function.

Example: Amylase breaks down starch into sugars in the digestive system.

Topic 3.2: Enzyme Catalysis

Enzyme catalysis involves the acceleration of chemical reactions by enzymes. The rate of reaction depends on several factors and can be regulated by inhibitors and activators.

• Activation Energy: The energy required to start a reaction; enzymes lower this barrier.

• Substrate Concentration: Increasing substrate concentration increases reaction rate until enzymes are saturated.

• Inhibitors: Molecules that decrease enzyme activity. Competitive inhibitors bind to the active site, while noncompetitive inhibitors bind elsewhere.

• Allosteric Regulation: Enzyme activity can be increased or decreased by molecules binding to sites other than the active site.

Equation:

$E + S \rightarrow ES \rightarrow E + P$

Where E is enzyme, S is substrate, ES is enzyme-substrate complex, and P is product.

Topic 3.3: Cellular Energy

Cells require energy to perform work, grow, and maintain homeostasis. Energy is stored and transferred in the form of ATP.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell.

• Energy Coupling: Cells use exergonic reactions to drive endergonic processes via ATP hydrolysis.

• Metabolic Pathways: Series of enzyme-catalyzed reactions that transform molecules and transfer energy.

Equation:

$ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

Example: Muscle contraction uses ATP for movement.

Topic 3.4: Photosynthesis

Photosynthesis is the process by which plants, algae, and some bacteria convert light energy into chemical energy stored in glucose.

• Light Reactions: Occur in the thylakoid membranes; convert solar energy to chemical energy (ATP and NADPH).

• Calvin Cycle: Occurs in the stroma; uses ATP and NADPH to fix carbon dioxide into glucose.

• Chlorophyll: The pigment responsible for capturing light energy.

Equation:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Example: Green plants use photosynthesis to produce food and oxygen.

Topic 3.5: Cellular Respiration

Cellular respiration is the process by which cells break down glucose to produce ATP, the energy currency of the cell. It occurs in three main stages: glycolysis, the Krebs cycle, and oxidative phosphorylation.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate, producing ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Occurs in the mitochondria; processes pyruvate to produce ATP, NADH, and FADH2.

• Oxidative Phosphorylation: Electron transport chain and chemiosmosis produce the majority of ATP.

• Aerobic vs. Anaerobic Respiration: Aerobic uses oxygen, anaerobic does not (fermentation).

Equation:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

Example: Human cells use aerobic respiration to generate energy for daily activities.

Comparison Table: Photosynthesis vs. Cellular Respiration

Process	Location	Reactants	Products	Energy Source
Photosynthesis	Chloroplasts	CO2, H2O, Light	Glucose, O2	Light energy
Cellular Respiration	Mitochondria	Glucose, O2	CO2, H2O, ATP	Chemical energy (glucose)

Additional info: These notes expand on the original outline by providing definitions, equations, and examples for each topic, ensuring a comprehensive review for exam preparation.