Cellular Respiration: Pathways and Mechanisms

Types of Nutrition

Autotrophic Nutrition

Autotrophs are organisms that produce their own organic molecules from inorganic substances in the environment. They are the primary producers in ecosystems and form the base of most food chains.

• Photoautotrophs: Use light energy to synthesize organic compounds (e.g., plants, algae, cyanobacteria).

• Chemoautotrophs: Obtain energy by oxidizing inorganic substances (e.g., some bacteria in extreme environments).

• Special Case: Some organisms, like the emerald green sea slug, can retain chloroplasts from algae they consume and perform photosynthesis.

• Key Example: Cyanobacteria are important photoautotrophs in aquatic environments.

Heterotrophic Nutrition

Heterotrophs obtain organic molecules by consuming other organisms or their byproducts. They are consumers and decomposers in ecosystems.

• Consumers: Animals, fungi, and many bacteria that ingest or absorb organic matter.

• Decomposers: Organisms that break down dead organic material, recycling nutrients back into the ecosystem.

Chemical Cycling and Energy Exchange

Energy flows through ecosystems, while chemical elements are recycled. Photosynthesis and cellular respiration are central to these processes.

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

• Cellular Respiration: Breaks down glucose in the presence of O2 to produce ATP, CO2, and H2O.

• Energy Flow: Sunlight energy enters, ATP drives cellular work, and heat energy exits the ecosystem.

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that convert biochemical energy from nutrients into ATP, releasing waste products.

• Aerobic Respiration: Requires oxygen and yields the most ATP.

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

• Electron Carriers: Molecules like NAD+ and FAD temporarily hold electrons during cellular respiration.

Stages of Cellular Respiration

Cellular respiration occurs in three main stages: glycolysis, pyruvate oxidation and the citric acid cycle, and the electron transport chain.

• 1. Glycolysis (cytoplasm): Glucose is split into two molecules of pyruvate.

• 2. Pyruvate Oxidation & Citric Acid Cycle (mitochondrial matrix): Pyruvate is converted to acetyl-CoA, which enters the cycle to produce electron carriers.

• 3. Electron Transport Chain (inner mitochondrial membrane): Electrons from NADH and FADH2 are used to generate a proton gradient, driving ATP synthesis.

Glycolysis

Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm. It breaks down one glucose molecule into two pyruvate molecules, producing a small amount of ATP and NADH.

• Energy Investment Phase: 2 ATP are used to phosphorylate glucose intermediates.

• Energy Harvesting Phase: 4 ATP and 2 NADH are produced.

• Net Yield: 2 ATP, 2 NADH, and 2 pyruvate per glucose.

Substrate	Product
1 glucose	2 pyruvate
2 ATP (used)	4 ATP (produced)
2 NAD+	2 NADH

Pyruvate Oxidation & Citric Acid Cycle (Krebs/TCA Cycle)

Pyruvate from glycolysis is transported into the mitochondrion, where it is oxidized to acetyl-CoA. The citric acid cycle completes the oxidation of glucose derivatives, generating electron carriers and ATP.

• Pyruvate Oxidation: Each pyruvate loses a carbon (as CO2), is converted to acetyl-CoA, and produces NADH.

• Citric Acid Cycle: Acetyl-CoA enters the cycle, producing NADH, FADH2, ATP (or GTP), and CO2.

• Key Steps: Multiple redox reactions transfer electrons to NAD+ and FAD.

Stage	NADH Produced	FADH2 Produced	ATP Produced
Glycolysis	2	0	2
Pyruvate Oxidation	2	0	0
Citric Acid Cycle	6	2	2

Electron Transport Chain (ETC) and Chemiosmosis

The ETC is located in the inner mitochondrial membrane. Electrons from NADH and FADH2 are transferred through protein complexes, powering the pumping of H+ ions to create a proton gradient. ATP synthase uses this gradient to produce ATP.

• Electron Flow: Electrons move from NADH/FADH2 to O2, forming H2O.

• Proton Gradient: H+ ions are pumped into the intermembrane space, creating potential energy.

• ATP Synthesis: H+ flows back through ATP synthase, driving the phosphorylation of ADP to ATP.

• Total ATP Yield: About 32 ATP per glucose molecule (varies by cell type).

Anaerobic Pathways and Fermentation

When oxygen is not available, cells can generate ATP through anaerobic pathways such as fermentation. These processes yield less ATP than aerobic respiration.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate, regenerating NAD+ for glycolysis. Occurs in muscle cells and some bacteria.

• Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2, regenerating NAD+. Common in yeast and some plants.

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

Pathway	End Product	ATP Yield
Lactic Acid Fermentation	Lactate	2
Alcoholic Fermentation	Ethanol + CO2	2

Feedback Inhibition and Regulation

Cellular respiration is tightly regulated by feedback inhibition. The end product of a metabolic pathway can inhibit an enzyme involved earlier in the pathway, preventing overproduction of ATP and intermediates.

• Example: High levels of ATP inhibit phosphofructokinase, a key enzyme in glycolysis.

• Purpose: Maintains energy balance and resource efficiency in the cell.