Respiration in Plants

Cellular Respiration and GlycolysisAerobic Respiration and FermentationRespiratory Quotient and Amphibolic Pathway

Cellular Respiration and Glycolysis

All living cells need energy to work, and they get it by breaking down food molecules — this is respiration. In cellular respiration, glucose is broken down step by step and the energy released is captured in ATP, the energy currency of the cell. The overall aerobic equation is the reverse of photosynthesis:

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy (ATP)

The substance broken down to release energy is the respiratory substrate (usually glucose, but fats and proteins can also be used). The breakdown is not a single explosive reaction; it happens in many small enzyme-controlled steps so the energy is released gradually and can be trapped efficiently.

The first stage, common to all organisms, is glycolysis (meaning "splitting of sugar"). It takes place in the cytoplasm and does not need oxygen. In glycolysis, one molecule of glucose (6 carbons) is broken into two molecules of pyruvic acid (pyruvate) (3 carbons each). The process gives a small net gain of ATP and some NADH. Because glycolysis needs no oxygen, it is the starting point for both aerobic respiration and fermentation.

1
Worked Example
Example 1: Where in the cell does glycolysis occur, and does it need oxygen?
Solution

Recall the first stage.

  • Glycolysis occurs in the cytoplasm.
  • It does not need oxygen (anaerobic step).
2
Worked Example
Example 2: What is the end product of glycolysis, and how many carbons does it have?
Solution

Glucose is split in glycolysis.

  • One glucose (6C) gives two molecules of pyruvic acid.
  • Each pyruvate has 3 carbons.
3
Worked Example
Example 3: Why does respiration release energy in many small steps rather than all at once?
Solution

Think about trapping energy.

  • Small steps release energy gradually.
  • This lets the cell trap the energy efficiently in ATP instead of losing it as heat.

Key Points

    • Respiration: breakdown of food (respiratory substrate) to release energy stored as ATP.
    • Aerobic equation: glucose + O₂ → CO₂ + H₂O + energy (reverse of photosynthesis).
    • Glycolysis (in cytoplasm, no O₂): glucose (6C) → 2 pyruvic acid (3C each) + small ATP + NADH.
    • Released in many small enzyme-controlled steps for efficiency.
✎ Quick Check — 2 questions0 / 2
Q1.Glycolysis takes place in the:
Explanation: Glycolysis occurs in the cytoplasm and needs no oxygen.
Q2.The end product of glycolysis is:
Explanation: Glycolysis splits glucose into two molecules of pyruvic acid.

Aerobic Respiration and Fermentation

What happens to pyruvate next depends on whether oxygen is present.

Aerobic respiration (with oxygen) takes place in the mitochondria and completely breaks pyruvate down to carbon dioxide and water, releasing a large amount of energy. It has two main parts:

  • The Krebs cycle (citric acid cycle / TCA cycle) — in the mitochondrial matrix, pyruvate (after conversion to acetyl-CoA) is oxidised, releasing CO₂ and the electron carriers NADH and FADH₂.
  • The electron transport chain (ETC) and oxidative phosphorylation — on the inner mitochondrial membrane, NADH and FADH₂ pass electrons down the chain; the energy is used to make most of the cell's ATP, and oxygen acts as the final electron acceptor, combining with hydrogen to form water.

Aerobic respiration is highly efficient, yielding about 36–38 ATP per glucose molecule.

Fermentation (anaerobic respiration) happens when oxygen is absent. Pyruvate is only partly broken down in the cytoplasm, so far less energy (only the 2 ATP from glycolysis) is released. There are two common types: in yeast, pyruvate becomes ethanol + CO₂ (alcoholic fermentation, used in baking and brewing); in some muscle and bacterial cells, pyruvate becomes lactic acid. Fermentation lets cells keep making a little ATP when oxygen runs short.

1
Worked Example
Example 1: In which organelle does aerobic respiration take place, and what is the role of oxygen?
Solution

Aerobic respiration completes the breakdown.

  • It occurs in the mitochondria.
  • Oxygen is the final electron acceptor (forms water).
2
Worked Example
Example 2: What are the products of alcoholic fermentation in yeast?
Solution

Yeast ferments without oxygen.

  • Pyruvate becomes ethanol and carbon dioxide.
3
Worked Example
Example 3: Why does aerobic respiration yield much more ATP than fermentation?
Solution

Compare how completely glucose is broken down.

  • Aerobic respiration fully oxidises glucose to CO₂ and water.
  • Fermentation only partly breaks it down, so far less energy is released.

Key Points

    • Aerobic respiration (mitochondria): Krebs cycle (matrix; releases CO₂, NADH, FADH₂) + ETC (inner membrane; makes most ATP, O₂ = final electron acceptor → water).
    • Yields ~36–38 ATP per glucose.
    • Fermentation (no O₂, cytoplasm): yeast → ethanol + CO₂; muscle/bacteria → lactic acid; only ~2 ATP.
✎ Quick Check — 2 questions0 / 2
Q1.Aerobic respiration takes place mainly in the:
Explanation: The Krebs cycle and ETC occur in the mitochondria.
Q2.In alcoholic fermentation, pyruvate is converted to ethanol and:
Explanation: Yeast converts pyruvate to ethanol and carbon dioxide.

Respiratory Quotient and Amphibolic Pathway

The respiratory quotient (RQ) is the ratio of the volume of CO₂ released to the volume of O₂ used in respiration:

RQ = volume of CO₂ evolved ÷ volume of O₂ consumed

The value of RQ depends on what kind of food is being respired:

  • For carbohydrates the RQ is 1 (equal CO₂ out and O₂ in).
  • For fats the RQ is less than 1 (about 0.7), because fats need more oxygen to be oxidised.
  • For proteins the RQ is about 0.8–0.9.
  • In anaerobic respiration (fermentation, where no oxygen is used) the RQ is infinite or very large.

Although we usually picture glucose as the fuel, respiration is closely connected to all of the cell's building and breaking processes. The respiratory pathway is therefore called amphibolic — it works in both directions, breaking molecules down (catabolism) and providing intermediates to build molecules up (anabolism). For example, intermediates of glycolysis and the Krebs cycle can be drawn off to make fatty acids, amino acids and other compounds. So respiration is not just for energy — it is also a hub that links the breakdown of carbohydrates, fats and proteins with the synthesis of new biomolecules.

1
Worked Example
Example 1: Define the respiratory quotient (RQ).
Solution

It is a ratio of gases.

  • RQ = volume of CO₂ released ÷ volume of O₂ used during respiration.
2
Worked Example
Example 2: What is the RQ when carbohydrates are respired, and why?
Solution

Compare CO₂ out and O₂ in.

  • For carbohydrates the RQ is 1.
  • Equal volumes of CO₂ are released and O₂ are used.
3
Worked Example
Example 3: Why is the respiratory pathway called amphibolic?
Solution

It serves two roles.

  • It both breaks molecules down (catabolism) and supplies intermediates to build them up (anabolism).

Key Points

    • RQ = CO₂ released ÷ O₂ used.
    • Carbohydrates RQ = 1; fats < 1 (~0.7, need more O₂); proteins ~0.8–0.9; anaerobic = infinite.
    • The respiratory pathway is amphibolic: both catabolic (breakdown) and anabolic (build-up).
    • Intermediates feed synthesis of fats, amino acids, etc. — a metabolic hub.
✎ Quick Check — 2 questions0 / 2
Q1.The respiratory quotient (RQ) for carbohydrates is:
Explanation: For carbohydrates, CO₂ released equals O₂ used, so RQ = 1.
Q2.A pathway that is both catabolic and anabolic is called:
Explanation: The respiratory pathway is amphibolic — it both breaks down and builds up.