Animals get energy from a respiratory substrate primarily through cellular respiration, a complex metabolic process that breaks down organic molecules to release energy in the form of adenosine triphosphate (ATP).
What is a Respiratory Substrate?
A respiratory substrate is any organic substance that can be broken down by respiration to release energy. The molecule normally used for respiration and the main respiratory substrate is glucose.
When an animal consumes food, it digests complex carbohydrates, fats, and proteins into simpler molecules. These simpler molecules, such as glucose, fatty acids, and amino acids, then become the respiratory substrates that fuel the animal's cells.
The Process: Aerobic Respiration
The primary way animals extract energy from these substrates is through aerobic respiration, a process that requires oxygen. This process occurs in several stages, mainly within the cytoplasm and mitochondria of cells.
Stages of Aerobic Respiration
Aerobic respiration can be broadly divided into four main stages:
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Glycolysis:
- Location: Cytoplasm
- Process: Glucose (a 6-carbon sugar) is broken down into two molecules of pyruvate (a 3-carbon compound).
- Energy Yield: Produces a small amount of ATP (2 net ATP) and NADH (electron carriers).
- Learn More: Read about Glycolysis on Khan Academy.
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Link Reaction (Pyruvate Oxidation):
- Location: Mitochondrial matrix
- Process: Each pyruvate molecule is converted into acetyl-CoA, releasing carbon dioxide and generating more NADH.
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Krebs Cycle (Citric Acid Cycle):
- Location: Mitochondrial matrix
- Process: Acetyl-CoA enters a cyclic series of reactions, leading to the complete breakdown of the original glucose molecule. More carbon dioxide is released, and significant amounts of NADH and FADH₂ (another type of electron carrier) are produced, along with a small amount of ATP.
- Learn More: Explore the Krebs Cycle on Biology LibreTexts.
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Oxidative Phosphorylation (Electron Transport Chain):
- Location: Inner mitochondrial membrane
- Process: This is where the majority of ATP is generated. NADH and FADH₂ donate their high-energy electrons to a series of protein complexes embedded in the membrane. As electrons move down the chain, energy is released to pump protons (H⁺ ions) across the membrane, creating a proton gradient.
- ATP Synthase: Protons flow back across the membrane through an enzyme called ATP synthase, which uses the energy from this flow to synthesize large amounts of ATP from ADP and inorganic phosphate. Oxygen acts as the final electron acceptor, forming water.
- Learn More: Understand Oxidative Phosphorylation on Wikipedia.
Summary of Energy Production
The following table summarizes the key outputs of each stage of aerobic respiration from one glucose molecule:
| Stage | Location | Key Reactants | Key Products (Energy Carriers) | ATP Yield (approx.) |
|---|---|---|---|---|
| Glycolysis | Cytoplasm | Glucose | 2 Pyruvate, 2 NADH | 2 (net) |
| Link Reaction | Mitochondrial Matrix | 2 Pyruvate | 2 Acetyl-CoA, 2 NADH | 0 |
| Krebs Cycle | Mitochondrial Matrix | 2 Acetyl-CoA | 6 NADH, 2 FADH₂, 2 ATP | 2 |
| Oxidative Phosphorylation | Inner Mitochondrial Membrane | 10 NADH, 2 FADH₂ | H₂O | 28-34 |
| Total | CO₂, H₂O | 30-38 |
Role of Other Respiratory Substrates
While glucose is the main respiratory substrate, animals can also derive energy from:
- Fats (Lipids): These are broken down into fatty acids and glycerol. Fatty acids can be converted into acetyl-CoA, entering the Krebs cycle, while glycerol can be converted into an intermediate of glycolysis. Fats are highly energy-dense, providing more ATP per gram than carbohydrates.
- Proteins: Broken down into amino acids. Amino acids can be deaminated (nitrogen group removed) and then converted into pyruvate, acetyl-CoA, or intermediates of the Krebs cycle. Proteins are typically used for energy only when carbohydrate and fat stores are low.
Practical Insights and Examples
- Sustained Activity: During long-duration activities like running a marathon, the body efficiently uses aerobic respiration to break down glucose and fats to produce a steady supply of ATP, preventing rapid fatigue.
- Rest and Recovery: Even at rest, your cells are constantly undergoing aerobic respiration to power essential bodily functions like maintaining body temperature, breathing, and nerve impulses.
- Dietary Energy: The caloric value of food is a direct measure of the potential energy (respiratory substrate) it can provide through these metabolic pathways. Eating a balanced diet ensures a constant supply of diverse substrates.
By breaking down these organic molecules in a controlled, stepwise manner, animals can efficiently capture and store the released energy in the chemical bonds of ATP, which then powers nearly all cellular activities, from muscle contraction to protein synthesis.
