Cellular Energy and Cellular Respiration

Cells Transform Energy

Energy is essential for all cellular processes. It exists in two main forms: kinetic energy (energy of motion) and potential energy (stored energy, including chemical energy). The laws of thermodynamics govern energy transformations:

• First Law: Energy can change form but cannot be created or destroyed.

• Second Law: Energy transfers increase disorder (entropy), with some energy lost as heat.

Chemical Reactions and Energy

Chemical reactions in cells either release or store energy:

• Exergonic reactions: Release energy (e.g., cellular respiration).

• Endergonic reactions: Require energy input and yield products rich in potential energy (e.g., photosynthesis).

• Metabolism: The sum of all chemical reactions in a cell.

ATP: The Energy Currency of the Cell

ATP (adenosine triphosphate) powers nearly all forms of cellular work by transferring a phosphate group to other molecules, driving chemical, transport, and mechanical work.

Enzymes and Metabolic Pathways

Enzymes are biological catalysts that lower the activation energy required for reactions, increasing reaction rates without being consumed. Each enzyme is specific to its substrate, which binds at the enzyme's active site.

• Enzyme inhibition: Can regulate enzyme activity.

• Competitive inhibitors: Block the active site, preventing substrate binding.

• Noncompetitive inhibitors: Bind elsewhere, altering enzyme shape and function.

• Feedback inhibition: End product of a pathway inhibits an upstream enzyme, regulating metabolism.

Enzyme Inhibitors in Medicine and Agriculture

Many drugs, pesticides, and poisons act as enzyme inhibitors. For example, certain pesticides inhibit enzymes in insect nervous systems, while some drugs target enzymes in pathogens or human cells.

Photosynthesis and Cellular Respiration: Energy for Life

Life depends on energy, primarily from the sun. In photosynthesis, plants use sunlight to convert carbon dioxide and water into organic molecules and oxygen. In cellular respiration, cells consume oxygen to break down organic molecules, releasing energy stored as ATP.

Breathing and Cellular Respiration

Respiration refers to the exchange of gases: organisms take in O2 and release CO2. Cellular respiration uses O2 to extract energy from food, linking breathing and energy production.

Cellular Respiration: Overview and Equation

Cellular respiration is an exergonic process that transfers energy from glucose to ATP. About 34% of glucose's energy is captured as ATP; the rest is lost as heat.

The overall equation for cellular respiration is:

Electron Transfer and Energy Capture

Cells extract energy from fuel molecules by transferring electrons. Electrons removed from fuels (oxidation) are transferred to NAD+ (reduction), forming NADH. NADH passes electrons to the electron transport chain, releasing energy as electrons "fall" to oxygen.

Stages of Cellular Respiration

Cellular respiration occurs in four main stages:

1. Glycolysis: Occurs in the cytosol; splits glucose into two pyruvate molecules, producing a net of 2 ATP and 2 NADH.

2. Pyruvate Oxidation (Acetyl CoA Formation): In the mitochondrial matrix; converts pyruvate to acetyl CoA, producing NADH and CO2.

3. Citric Acid Cycle (Krebs Cycle): In the mitochondrial matrix; completes glucose breakdown, producing ATP, NADH, FADH2, and CO2.

4. Oxidative Phosphorylation: Involves the electron transport chain and chemiosmosis; produces most ATP by using energy from electrons to pump H+ and drive ATP synthesis.

Glycolysis: Splitting Glucose

Glycolysis consists of two phases:

• Energy investment phase: Uses 2 ATP to phosphorylate glucose and split it into two three-carbon sugars.

• Energy payoff phase: Produces 4 ATP (net gain of 2 ATP) and 2 NADH by oxidizing the three-carbon sugars to pyruvate.

ATP is formed by substrate-level phosphorylation (direct transfer of a phosphate group to ADP).

Citric Acid Cycle (Krebs Cycle)

Each turn of the cycle:

• Adds two carbons from acetyl CoA.

• Releases 2 CO2.

• Produces 3 NADH, 1 FADH2, and 1 ATP (per acetyl CoA).

Oxidative Phosphorylation and Chemiosmosis

NADH and FADH2 donate electrons to the electron transport chain in the inner mitochondrial membrane. Energy from electron transfer pumps H+ into the intermembrane space. The resulting gradient drives H+ back through ATP synthase, synthesizing ATP. Oxygen is the final electron acceptor, forming water.

ATP Yield from Cellular Respiration

Each glucose molecule can yield up to 32 ATP molecules through substrate-level and oxidative phosphorylation.

Fermentation and Anaerobic Respiration

When oxygen is unavailable, cells can produce ATP by fermentation:

• Strict anaerobes: Survive only in anaerobic environments.

• Facultative anaerobes: Can switch between fermentation and aerobic respiration depending on O2 availability.

Lactic Acid Fermentation

In muscle cells under low oxygen, pyruvate is reduced to lactate, regenerating NAD+ for glycolysis. Accumulation of lactic acid causes muscle soreness, which is relieved as lactic acid is transported to the liver for processing.

Alcohol Fermentation

Yeasts and some bacteria convert pyruvate to ethanol and CO2 in the absence of oxygen. This process is used in brewing and baking industries.

Other Fuels for Cellular Respiration

Cells can use carbohydrates, fats, and proteins as fuel for cellular respiration. These molecules enter the metabolic pathway at various points and are ultimately used to generate ATP.

Biosynthesis and Metabolic Regulation

Intermediates from cellular respiration are used for biosynthesis of other organic molecules. Metabolic pathways are regulated by feedback inhibition to maintain cellular balance.