Cellular Energetics

Introduction to Metabolism

Metabolism encompasses all the chemical reactions that occur within living organisms to transform matter and energy. These reactions are essential for organization, growth, and reproduction. Metabolic pathways are sequences of reactions, each catalyzed by a specific enzyme, that convert substrates into final products.

• Metabolic Pathway: A series of chemical reactions in a cell, each step catalyzed by an enzyme.

• Anabolic reactions: Build larger molecules from smaller ones and require energy (endergonic).

• Catabolic reactions: Break down molecules and release energy (exergonic).

Additional info: Metabolic pathways are often depicted as a series of boxes/arrows, each representing a substrate, intermediate, or product.

Energy and Free Energy in Biological Systems

Free energy (G) is the portion of a system's energy that can perform work. Changes in free energy determine whether a reaction is spontaneous.

• Exergonic reactions: Release energy; ΔG is negative.

• Endergonic reactions: Require energy input; ΔG is positive.

• Cellular respiration: Exergonic; ΔG is negative.

• Photosynthesis: Endergonic; driven by solar energy.

Additional info: Diagrams often show the relationship between free energy, stability, and work capacity.

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) powers cellular work by coupling exergonic reactions to endergonic reactions. The hydrolysis of ATP releases energy by breaking the terminal phosphate bond.

• ATP Structure: Consists of adenine, ribose, and three phosphate groups.

• Hydrolysis: ATP + H2O → ADP + Pi; ΔG is negative (exergonic).

• Phosphorylation: Transfer of a phosphate group to another molecule, making it less stable and more reactive.

Additional info: The breaking of the terminal phosphate bond is the main source of energy release.

Enzymes and Activation Energy

Enzymes are biological catalysts that speed up metabolic reactions by lowering the activation energy (EA) required for a reaction to proceed. They are highly specific due to the shape and charge of their active site.

• Catalyst: A substance that increases the rate of a reaction without being consumed.

• Activation Energy (EA): The energy barrier that must be overcome for a reaction to occur.

• Enzyme Specificity: Determined by the protein structure and the active site.

Additional info: Enzymes lower EA but do not affect ΔG.

Mechanisms of Enzyme Action

Enzymes use several mechanisms to lower activation energy, including orienting substrates, straining substrate bonds, providing a favorable microenvironment, and covalently bonding to substrates.

• Factors affecting enzyme activity: Substrate concentration, pH, temperature.

• Denaturation: Extreme pH or temperature can alter protein structure and enzyme function.

Enzyme Regulation

Enzyme activity is regulated by inhibitors and activators. Competitive inhibitors bind to the active site, while noncompetitive inhibitors bind elsewhere, changing the enzyme's shape.

• Feedback inhibition: End product of a pathway inhibits an earlier step.

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose through a series of catabolic reactions. It includes glycolysis, pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation.

• Summary equation:

• Redox reactions: Involve the transfer of electrons; oxidation is loss, reduction is gain.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytosol. It breaks down glucose into two molecules of pyruvate, producing ATP and NADH.

• Energy investment phase: 2 ATP used.

• Energy payoff phase: 4 ATP and 2 NADH produced.

• Net gain: 2 ATP, 2 NADH, 2 pyruvate per glucose.

Citric Acid Cycle (Krebs Cycle)

After glycolysis, pyruvate is converted to acetyl CoA, which enters the citric acid cycle in the mitochondria. This cycle completes the oxidation of glucose, producing NADH, FADH2, ATP, and CO2.

• Per glucose: 2 ATP, 6 NADH, 2 FADH2, 4 CO2

• Location: Mitochondrial matrix

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation includes the electron transport chain and chemiosmosis. Electrons from NADH and FADH2 are passed through protein complexes, creating a proton gradient that drives ATP synthesis via ATP synthase.

• Electron transport chain: Located in the inner mitochondrial membrane.

• Final electron acceptor: Oxygen (O2).

• ATP synthase: Uses proton flow to synthesize ATP.

Fermentation

Fermentation allows cells to produce ATP without oxygen. It regenerates NAD+ by transferring electrons to organic molecules, resulting in products like ethanol or lactate.

• Alcohol fermentation: Glucose → ethanol + CO2

• Lactic acid fermentation: Glucose → lactate

Photosynthesis

Overview of Photosynthesis

Photosynthesis is the process by which photoautotrophs convert solar energy into chemical energy, producing glucose and oxygen from carbon dioxide and water.

• Summary equation:

• Occurs in: Chloroplasts

Light Reactions

The light reactions occur in the thylakoid membranes and convert solar energy to ATP and NADPH. Water is split, releasing O2.

• Photosystems: Protein complexes that absorb light and transfer electrons.

• Linear electron flow: Produces ATP and NADPH.

Calvin Cycle

The Calvin cycle uses ATP and NADPH to fix CO2 into G3P, a three-carbon sugar. It is an anabolic pathway governed by enzymes, notably rubisco.

• Carbon fixation: CO2 is attached to RuBP by rubisco.

• Reduction: NADPH donates electrons to reduce 1,3-bisphosphoglycerate to G3P.

• Regeneration: RuBP is regenerated to continue the cycle.

Additional info: Three turns of the Calvin cycle are required to net one G3P molecule.