Enzymes and Cellular Respiration: Key Concepts in General Biology

Concept 8.4 - Enzymes

Definition and Function of Enzymes

Enzymes are biological catalysts, typically proteins, that accelerate chemical reactions in living organisms by lowering the activation energy barrier. They do not alter the free energy change ($\Delta G$) of a reaction, but increase the rate at which equilibrium is reached.

• Enzymes: Catalytic proteins that speed up biochemical reactions.

• Activation Energy ($E_a$): The energy required to initiate a chemical reaction. Enzymes lower $E_a$.

• Free Energy ($\Delta G$): The difference in energy between reactants and products; unaffected by enzymes.

Example: The graphs show that the course of a reaction with an enzyme has a lower activation energy than without an enzyme, but the overall free energy change remains the same.

Enzyme-Substrate Interaction

Enzymes act on specific molecules called substrates. The region where the substrate binds is known as the active site. The binding is highly specific and often involves an induced fit, where the enzyme changes shape to better accommodate the substrate, enhancing its catalytic ability.

• Substrate: The reactant molecule upon which an enzyme acts.

• Active Site: The specific region of the enzyme where substrate binding occurs.

• Enzyme-Substrate Complex: The temporary association formed when an enzyme binds its substrate.

• Induced Fit: The process by which enzyme binding alters its shape to fit the substrate more closely.

Example: Diagrams show the enzyme before and after substrate binding, highlighting the active site and the formation of the enzyme-substrate complex.

Cellular Respiration and Fermentation

Overview of Energy Conversion in Cells

Cells convert energy from food into usable forms through cellular respiration and fermentation. Photosynthesis in plants captures light energy to produce organic molecules and oxygen, which are then used in cellular respiration to generate ATP, the energy currency of the cell.

• Photosynthesis: Converts light energy, CO2, and H2O into organic molecules and O2.

• Cellular Respiration: Breaks down organic molecules to produce ATP and heat.

• ATP: Adenosine triphosphate, the main energy carrier in cells.

Example: Diagram shows the flow of energy from sunlight to ATP production in plant and animal cells.

Cellular Respiration

Terminology and General Descriptions

Cellular respiration is a set of metabolic pathways that break down proteins, carbohydrates, and lipids to release energy. It involves electron transfer and can occur via aerobic respiration or fermentation.

• Aerobic Respiration: Requires oxygen; complete breakdown of organic compounds.

• Fermentation: Occurs in the absence of oxygen; partial breakdown of sugars with end products such as alcohols or acids.

Process	Oxygen Required?	End Products
Aerobic Respiration	Yes	CO2, H2O, Energy (ATP)
Fermentation	No	Alcohols, Acids, Energy (less ATP)

Electron Transfer, Oxidation, and Reduction

Cellular respiration involves redox reactions, where electrons are transferred from one molecule to another. Oxidation is the loss of electrons, while reduction is the gain of electrons. The molecule that loses electrons is the reducing agent; the one that gains electrons is the oxidizing agent.

• Oxidation: Loss of electrons (and often hydrogen).

• Reduction: Gain of electrons (and often hydrogen).

• Reducing Agent: Donates electrons and becomes oxidized.

• Oxidizing Agent: Accepts electrons and becomes reduced.

Example: In the oxidation of methane ($CH_4$), oxygen acts as the oxidizing agent:

$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O + \text{Energy}$

Electron Carriers and Coenzymes

Electron carriers such as NAD+ (nicotinamide adenine dinucleotide) play a crucial role in cellular respiration. NAD+ acts as a coenzyme, cycling between oxidized (NAD+) and reduced (NADH) forms to shuttle electrons between reactions.

• NAD+: Oxidized form; accepts electrons to become NADH.

• NADH: Reduced form; donates electrons to the electron transport chain.

• Coenzyme: Non-protein molecule that assists enzyme function.

Example: The reaction catalyzed by dehydrogenase:

$\text{C-OH} + NAD^+ \xrightarrow{\text{Dehydrogenase}} \text{C=O} + NADH + H^+$

ATP Production: Substrate-Level and Oxidative Phosphorylation

ATP is produced in cellular respiration by two main mechanisms: substrate-level phosphorylation and oxidative phosphorylation.

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP from a substrate.

• Oxidative Phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis; accounts for most ATP produced.

Main Stages of Cellular Respiration

Cellular respiration consists of three main stages:

1. Glycolysis: Occurs in the cytosol; breaks down glucose into two molecules of pyruvate.

2. Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; completes the breakdown of glucose by oxidizing acetyl CoA to CO2.

3. Electron Transport Chain and Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; uses electrons from NADH and FADH2 to generate ATP.

Summary Table: Aerobic Respiration vs Fermentation

Feature	Aerobic Respiration	Fermentation
Oxygen Required	Yes	No
ATP Yield (per glucose)	~30-32	2
End Products	CO2, H2O	Alcohols, Acids

Additional info:

• Enzymes are highly specific for their substrates due to the unique shape of their active sites.

• Cellular respiration is essential for energy production in both plant and animal cells.

• Fermentation allows cells to produce ATP when oxygen is scarce, but yields much less energy than aerobic respiration.