Energy and Respiration
The breakdown of organic molecules like glucose to synthesise ATP for metabolic processes.
Cellular respiration is the fundamental process by which living organisms release chemical energy stored in complex organic molecules, such as glucose, and use it to synthesise adenosine triphosphate (ATP). ATP is the universal energy currency of the cell, powering nearly all metabolic activities. The overall equation for aerobic respiration is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy (ATP + heat)
This process occurs in four main stages:
1. Glycolysis
This is the initial stage and occurs in the cytoplasm of the cell. It is an anaerobic process, meaning it does not require oxygen. In glycolysis, one molecule of glucose (a 6-carbon sugar) is broken down into two molecules of pyruvate (a 3-carbon compound).
* Phosphorylation: The process begins with the phosphorylation of glucose using two ATP molecules, making it more reactive. This forms fructose-1,6-bisphosphate.
* Lysis: The 6-carbon sugar then splits into two 3-carbon molecules of triose phosphate.
* Oxidation: Each triose phosphate molecule is oxidised. Hydrogen is removed and transferred to the coenzyme NAD+, forming reduced NAD (NADH). During this conversion to pyruvate, enough energy is released to synthesise four ATP molecules.
The net gain from glycolysis per glucose molecule is 2 ATP (via substrate-level phosphorylation) and 2 reduced NAD.
2. The Link Reaction
If oxygen is present, the pyruvate molecules from glycolysis are actively transported from the cytoplasm into the matrix of the mitochondria. Here, each pyruvate molecule undergoes:
* Decarboxylation: A carboxyl group is removed, releasing a molecule of carbon dioxide (CO₂) .
* Oxidation: The remaining 2-carbon acetate is oxidised, and the removed hydrogen is accepted by NAD+ to form reduced NAD.
* The acetate then combines with coenzyme A to form acetyl coenzyme A (acetyl CoA).
For each glucose molecule, this process happens twice, yielding 2 acetyl CoA, 2 CO₂, and 2 reduced NAD.
3. The Krebs Cycle (Citric Acid Cycle)
This cycle also takes place in the mitochondrial matrix.
* The 2-carbon acetyl CoA combines with a 4-carbon molecule (oxaloacetate) to form a 6-carbon molecule (citrate).
* In a series of enzyme-controlled reactions, the citrate molecule is gradually broken back down into the 4-carbon oxaloacetate, which is then ready to accept another acetyl CoA molecule.
* During this cycle, decarboxylation occurs twice, releasing two molecules of CO₂. Dehydrogenation (oxidation) occurs at four points, generating 3 reduced NAD and 1 reduced FAD (another coenzyme). One molecule of ATP is also produced directly by substrate-level phosphorylation.
Since two acetyl CoA molecules are produced from one glucose molecule, the Krebs cycle turns twice. The total yield per glucose molecule is 2 ATP, 6 reduced NAD, 2 reduced FAD, and 4 CO₂.
4. Oxidative Phosphorylation
This is the final stage and the primary source of ATP. It occurs on the inner mitochondrial membrane (cristae), which is folded to increase surface area. It involves two linked processes: the electron transport chain and chemiosmosis.
* Electron Transport Chain (ETC): The reduced coenzymes (reduced NAD and reduced FAD) produced in the earlier stages release their hydrogen atoms, which split into protons (H+) and high-energy electrons (e-). The electrons are passed along a series of protein carriers embedded in the membrane. As they move from one carrier to the next, they release energy.
* Chemiosmosis: This energy is used by the carriers to actively pump protons (H+) from the mitochondrial matrix into the intermembrane space. This creates a high concentration of protons in the intermembrane space, generating a steep electrochemical gradient (also known as a proton-motive force).
* Protons cannot diffuse back across the impermeable inner membrane. They flow down their gradient through a channel protein called ATP synthase. The kinetic energy from this proton flow drives the enzyme to synthesise ATP from ADP and inorganic phosphate (Pi).
* Oxygen acts as the final electron acceptor. At the end of the chain, it combines with electrons and protons to form water (H₂O), preventing the chain from becoming blocked.
Anaerobic Respiration
In the absence of oxygen, oxidative phosphorylation cannot occur because there is no final electron acceptor. Glycolysis can still happen, but the pyruvate has a different fate. The primary purpose of anaerobic respiration is to regenerate the NAD+ needed for glycolysis to continue producing a small amount of ATP.
* Lactate Fermentation (in animals): Pyruvate is directly reduced by reduced NAD to form lactate (lactic acid). This regenerates NAD+. This process is reversible.
* Alcoholic Fermentation (in yeast, plants): Pyruvate is first decarboxylated to ethanal, releasing CO₂. Ethanal is then reduced by reduced NAD to form ethanol. This also regenerates NAD+ but is an irreversible process.
Key Points to Remember
- 1Cellular respiration breaks down glucose to produce ATP, the cell's energy currency.
- 2Glycolysis occurs in the cytoplasm, splitting one glucose into two pyruvate, with a net gain of 2 ATP and 2 reduced NAD.
- 3The link reaction and Krebs cycle occur in the mitochondrial matrix, producing CO₂, ATP, and loading coenzymes NAD+ and FAD with hydrogen.
- 4Oxidative phosphorylation is the main ATP-producing stage, occurring on the inner mitochondrial membrane.
- 5The electron transport chain uses energy from electrons to pump protons, creating a proton-motive force.
- 6Chemiosmosis harnesses this force, as protons flow through ATP synthase to generate large amounts of ATP.
- 7Oxygen is the final electron acceptor in aerobic respiration, forming water.
- 8Anaerobic respiration regenerates NAD+ in the absence of oxygen, producing lactate (animals) or ethanol and CO₂ (yeast), with a much lower ATP yield.
Pakistan Example
High-Altitude Trekking in the Karakoram
Mountaineers and trekkers in Pakistan's northern mountain ranges, like the Karakoram, face low atmospheric oxygen levels. During strenuous climbing, the demand for ATP in muscle cells exceeds the oxygen supply for aerobic respiration. To compensate, muscle cells rely heavily on anaerobic respiration (lactate fermentation) to produce ATP rapidly. This leads to an accumulation of lactate, contributing to muscle fatigue and cramps—a common challenge for climbers tackling peaks like K2 or Broad Peak, demonstrating the body's physiological shift to a less efficient energy pathway under hypoxic conditions.
Quick Revision Infographic
Biology — Quick Revision
Energy and Respiration
Key Concepts
High-Altitude Trekking in the Karakoram
Mountaineers and trekkers in Pakistan's northern mountain ranges, like the Karakoram, face low atmospheric oxygen levels. During strenuous climbing, the demand for ATP in muscle cells exceeds the oxygen supply for aerobic respiration. To compensate, muscle cells rely heavily on anaerobic respiration (lactate fermentation) to produce ATP rapidly. This leads to an accumulation of lactate, contributing to muscle fatigue and cramps—a common challenge for climbers tackling peaks like K2 or Broad Peak, demonstrating the body's physiological shift to a less efficient energy pathway under hypoxic conditions.