What this chapter is about
This chapter explains how plants obtain energy from organic compounds through the process of cellular respiration. While photosynthesis captures light energy and stores it in glucose, respiration releases that stored energy in a controlled manner so cells can use it for growth, repair, transport and other life processes. You will learn that respiration is not the same as breathing; it is a biochemical process occurring inside every living cell.
The chapter covers the complete pathway of aerobic respiration—glycolysis, the link reaction, the citric acid cycle (Krebs cycle) and the electron transport chain—along with fermentation as an alternative when oxygen is absent. Understanding respiration helps you see how the ATP currency of the cell is generated and why living organisms, including plants, need a continuous supply of respiratory substrates.
After studying this chapter you should be able to describe the stages of respiration, locate them in the cell, balance the energy budget of glucose oxidation, distinguish aerobic from anaerobic pathways, and explain how the respiratory quotient (RQ) varies with different substrates.
Key ideas
- Respiration is an oxidation process in which organic molecules (mainly glucose) are broken down stepwise; energy released is trapped in ATP bonds rather than lost as heat all at once.
- Glycolysis occurs in the cytoplasm, does not require oxygen, and converts one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each), producing a net gain of 2 ATP and 2 NADH.
- Pyruvate enters the mitochondrion where, under aerobic conditions, it loses CO₂ and combines with coenzyme A to form acetyl-CoA (the link reaction), releasing NADH.
- The Krebs cycle (citric acid cycle) runs in the mitochondrial matrix; acetyl-CoA combines with oxaloacetate to form citrate, and through a cyclic series of reactions, 2 CO₂, 3 NADH, 1 FADH₂ and 1 ATP (or GTP) are produced per acetyl-CoA.
- The electron transport chain (ETC) is located on the inner mitochondrial membrane; electrons from NADH and FADH₂ pass through protein complexes, and the energy released pumps H⁺ ions to create a proton gradient used by ATP synthase to make ATP (oxidative phosphorylation).
- Fermentation is anaerobic respiration in which pyruvate is converted to ethanol and CO₂ (alcoholic fermentation) or to lactic acid (lactic acid fermentation) without passing through the Krebs cycle; it yields only 2 ATP per glucose.
- Respiratory quotient (RQ) = volume of CO₂ released ÷ volume of O₂ consumed. RQ varies: approximately 1 for carbohydrates, less than 1 for fats, greater than 1 for organic acids.
- Amphibolic nature of respiration: the pathways of respiration not only break down substrates (catabolism) but also supply intermediates for biosynthesis of amino acids, fatty acids and other molecules (anabolism).
Formulas and facts to remember
1. Net ATP from glycolysis = 2 ATP per glucose (after subtracting 2 ATP used in the preparatory phase).
2. Overall equation for aerobic respiration: C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (approximately 30–32 ATP in eukaryotes).
3. Alcoholic fermentation equation: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ + 2 ATP.
4. Lactic acid fermentation equation: C₆H₁₂O₆ → 2 CH₃CHOHCOOH + 2 ATP.
5. RQ formula: RQ = CO₂ evolved / O₂ consumed. For glucose RQ = 6 CO₂ / 6 O₂ = 1.
6. Sites: Glycolysis—cytoplasm; Krebs cycle—mitochondrial matrix; ETC and oxidative phosphorylation—inner mitochondrial membrane.
7. Chemiosmotic hypothesis (Peter Mitchell): the proton gradient across the inner mitochondrial membrane drives ATP synthesis through ATP synthase.
8. Total NADH yield from one glucose (aerobic): 2 (glycolysis) + 2 (link reaction) + 6 (Krebs cycle) = 10 NADH; plus 2 FADH₂ from Krebs cycle.
Worked examples
### Example 1 – Calculating RQ for a fat
A germinating groundnut seed consumes 145 mL of O₂ and releases 102 mL of CO₂ in a respirometer experiment.
Step 1: Write the RQ formula. RQ = CO₂ released / O₂ consumed
Step 2: Substitute values. RQ = 102 mL / 145 mL ≈ 0.70
Interpretation: An RQ less than 1 indicates the respiratory substrate is a fat, which requires more oxygen for complete oxidation than carbohydrates.
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### Example 2 – ATP yield from Krebs cycle
How many ATP-equivalents are produced per turn of the Krebs cycle, and how many turns occur per glucose molecule?
Step 1: One glucose yields 2 acetyl-CoA (via 2 pyruvate molecules), so the cycle turns twice per glucose.
Step 2: Per turn, products are 3 NADH, 1 FADH₂ and 1 ATP (or GTP).
Step 3: In the ETC, each NADH yields about 2.5 ATP; each FADH₂ yields about 1.5 ATP. Energy per turn = (3 × 2.5) + (1 × 1.5) + 1 = 7.5 + 1.5 + 1 = 10 ATP-equivalents.
Step 4: For two turns, total ≈ 20 ATP-equivalents from the Krebs cycle alone.
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### Example 3 – Fermentation in dough
A student kneads wheat flour with yeast and water, then covers the dough and keeps it in a warm place. After one hour the dough has risen.
Explanation: Yeast cells carry out alcoholic fermentation. Glucose in the flour is converted to ethanol and CO₂. The CO₂ bubbles get trapped in the gluten network, causing the dough to expand. This is why bread is soft and porous after baking.
Common mistakes
- Thinking respiration happens only when a plant is in the dark → respiration occurs continuously, day and night, in all living cells.
- Confusing RQ greater than 1 with inefficient respiration → RQ > 1 simply means the substrate (like organic acids) releases more CO₂ relative to O₂ consumed.
- Believing fermentation produces no ATP → fermentation does produce 2 ATP per glucose through substrate-level phosphorylation in glycolysis.
- Assuming all 38 ATP are made per glucose → in eukaryotes, due to the cost of transporting NADH into mitochondria, the practical yield is about 30–32 ATP.
- Forgetting that the Krebs cycle turns twice per glucose → one glucose gives two acetyl-CoA, so every product of the cycle must be doubled.
Quick revision
1. Glycolysis splits glucose (6C) into two pyruvate (3C each), yielding 2 ATP and 2 NADH, and occurs in the cytoplasm without oxygen.
2. Krebs cycle runs in the mitochondrial matrix and produces NADH, FADH₂, ATP and CO₂ from acetyl-CoA.
3. The ETC uses electrons from NADH and FADH₂ to build a proton gradient; ATP synthase then makes most of the cell's ATP.
4. Fermentation regenerates NAD⁺ so glycolysis can continue when oxygen is absent; net gain is only 2 ATP.
5. RQ = 1 for carbohydrates, < 1 for fats, > 1 for organic acids.
6. Respiration is amphibolic—its intermediates serve both energy release and biosynthesis.