Cellular Energetics: Metabolism, Photosynthesis, and Cellular Respiration

Unit 4: Cellular Energetics

Chapter 6: Metabolism & Enzymes

Metabolism encompasses all chemical reactions within a cell, including those that build up and break down molecules. Enzymes are biological catalysts that accelerate metabolic reactions, ensuring efficient energy transfer and cellular function.

• Metabolism: The sum of all chemical reactions in an organism, divided into catabolic (breakdown) and anabolic (biosynthetic) pathways.

• Energy Transformation: Living organisms require constant input of energy for growth, maintenance, and reproduction. Energy is transformed through metabolic pathways.

• Enzymes: Proteins that lower activation energy, increasing the rate of chemical reactions without being consumed.

• ATP (Adenosine Triphosphate): The primary energy currency of the cell, used to power cellular work.

Key Concepts:

• Metabolic pathways are regulated by enzymes, which are specific to substrates and reactions.

• Enzyme activity can be affected by temperature, pH, and inhibitors.

• ATP is generated by catabolic pathways and used in anabolic reactions.

Example: The enzyme hexokinase catalyzes the phosphorylation of glucose in the first step of glycolysis.

Additional info: Enzyme kinetics can be described by the Michaelis-Menten equation: $v = \frac{V_{max}[S]}{K_m + [S]}$

Chapter 8: Photosynthesis

Overview & Purpose

Photosynthesis is the process by which photoautotrophs capture light energy and convert it into chemical energy stored in glucose and other organic molecules. This process sustains most life on Earth by providing energy and oxygen.

• Photoautotrophs: Organisms that use sunlight to synthesize organic compounds from CO2 and H2O.

• Chloroplasts: Organelles in plant cells where photosynthesis occurs.

• Light Reactions: Convert solar energy to chemical energy (ATP and NADPH).

• Calvin Cycle: Uses ATP and NADPH to fix carbon dioxide into glucose.

Key Concepts:

• Photosynthesis consists of two stages: the light-dependent reactions and the Calvin cycle (light-independent reactions).

• Chlorophyll absorbs light energy, initiating electron transport and ATP/NADPH production.

• Oxygen is produced as a byproduct of water splitting during the light reactions.

• Carbon fixation incorporates CO2 into organic molecules.

Example: The Calvin cycle uses the enzyme RuBisCO to fix CO2 into 3-phosphoglycerate.

Additional info: The overall equation for photosynthesis is: $6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Chapter 7: Cellular Respiration

Overview & Purpose

Cellular respiration is the process by which cells extract energy from organic molecules, primarily glucose, to produce ATP. This process occurs in both aerobic and anaerobic conditions and is essential for maintaining cellular activities.

• Heterotrophs: Organisms that obtain energy by consuming organic matter.

• Glycolysis: The breakdown of glucose into pyruvate, producing ATP and NADH.

• Krebs Cycle (Citric Acid Cycle): Completes the oxidation of glucose, generating NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Uses electrons from NADH and FADH2 to create a proton gradient for ATP synthesis.

• Fermentation: Anaerobic process that regenerates NAD+ for glycolysis when oxygen is absent.

Key Concepts:

• Cellular respiration consists of glycolysis, the Krebs cycle, and oxidative phosphorylation.

• ATP is produced by substrate-level phosphorylation and chemiosmosis.

• Oxygen serves as the final electron acceptor in aerobic respiration.

• Fermentation allows ATP production in the absence of oxygen.

Example: In muscle cells, lactic acid fermentation occurs during intense exercise when oxygen is limited.

Additional info: The overall equation for aerobic cellular respiration is: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}$

Comparison Table: Photosynthesis vs. Cellular Respiration

Feature	Photosynthesis	Cellular Respiration
Location	Chloroplasts	Mitochondria
Reactants	CO2, H2O, light energy	Glucose, O2
Products	Glucose, O2	CO2, H2O, ATP
Energy Conversion	Light to chemical energy	Chemical energy to ATP
Electron Carrier	NADP+/NADPH	NAD+/NADH, FAD/FADH2

Additional info: Both processes are essential for the flow of energy in ecosystems, with photosynthesis providing the organic molecules and oxygen required for cellular respiration.