Respiration in Plants

In this chapter: gas exchange in plants; glycolysis; fermentation; link reaction; Krebs (TCA) cycle; electron transport system and oxidative phosphorylation; ATP balance sheet; amphibolic pathway; respiratory quotient.

Do plants breathe?

Plants have no special respiratory organs. Each part takes care of its own gas exchange through stomata and lenticels. This works because plants have low respiration rates compared with animals, most living cells are close to the surface, and leaves release oxygen during photosynthesis.

Respiration is the breakdown of C-C bonds of complex molecules by oxidation, releasing energy that is trapped as ATP, the energy currency of the cell. Glucose is the usual respiratory substrate.

Glycolysis (EMP pathway)

The EMP pathway is named after Gustav Embden, Otto Meyerhof and J. Parnas. Glycolysis takes place in the cytoplasm of all living organisms, and is the only process in anaerobic organisms. Glucose (6C) is partially oxidised to two molecules of pyruvic acid (3C) in ten enzyme-controlled steps.

• Glucose is phosphorylated to glucose-6-phosphate by hexokinase (uses ATP). Fructose-6-phosphate is phosphorylated to fructose-1,6-bisphosphate (uses ATP).
• Fructose-1,6-bisphosphate splits into two 3C molecules: dihydroxyacetone phosphate (DHAP) and 3-phosphoglyceraldehyde (PGAL).
• PGAL is oxidised to 1,3-bisphosphoglycerate (BPGA); NADH is formed.
• BPGA to 3-phosphoglyceric acid (PGA) and PEP to pyruvic acid are the two ATP-yielding steps.

Per glucose: 2 ATP used, 4 ATP made, so net 2 ATP; plus 2 NADH and 2 pyruvic acid.

Fermentation (anaerobic respiration)

• Yeast: pyruvic acid is converted to CO₂ and ethanol by pyruvic acid decarboxylase and alcohol dehydrogenase.
• Some bacteria, and animal muscles during vigorous exercise (when oxygen is short): pyruvic acid is reduced to lactic acid by lactate dehydrogenase.
• In both, NADH is reoxidised to NAD⁺, so glycolysis can continue.
• Less than 7% of the energy in glucose is released, and not all of it as ATP. The products are hazardous: yeasts poison themselves when alcohol reaches about 13%.

Aerobic respiration

Aerobic respiration takes place in the mitochondria. Pyruvate is completely oxidised to CO₂ (in the matrix), and electrons removed as hydrogen are passed to O₂, forming water with ATP synthesis (on the inner membrane).

Link reaction

Pyruvic acid enters the matrix and undergoes oxidative decarboxylation by the pyruvate dehydrogenase complex (needs NAD⁺, coenzyme A and Mg²⁺):

Pyruvic acid + CoA + NAD⁺ → Acetyl CoA + CO₂ + NADH + H⁺

Two NADH are formed per glucose (one per pyruvate).

Krebs cycle (TCA cycle, citric acid cycle)

Discovered by Hans Krebs. It takes place in the mitochondrial matrix.

1. Acetyl CoA (2C) condenses with oxaloacetic acid (OAA, 4C) and water to form citric acid (6C), catalysed by citrate synthase.
2. Citrate is isomerised to isocitrate, then two successive decarboxylations give α-ketoglutaric acid and then succinyl-CoA.
3. Succinyl-CoA is converted to succinic acid, with GTP formed (substrate-level phosphorylation; GTP gives ATP).
4. Succinic acid is oxidised to fumarate (FADH₂ formed), then malate, then OAA (NADH formed). OAA is regenerated, so the cycle continues.

Per pyruvate (link reaction + one Krebs turn): 3 CO₂, 4 NADH, 1 FADH₂, 1 ATP (via GTP).
Per glucose: double this.

Electron transport system and oxidative phosphorylation

The ETS is located in the inner mitochondrial membrane.

Electron flow pumps protons into the intermembrane space. As protons flow back into the matrix through F₀, F₁ makes ATP. For each ATP produced, 2 H⁺ pass through F₀. Oxidation of one NADH gives 3 ATP; one FADH₂ gives 2 ATP.

Role of oxygen: it is the final hydrogen (electron) acceptor. It acts only at the end, but it drives the whole process by removing hydrogen from the system. Unlike photophosphorylation, where light creates the proton gradient, here the energy of oxidation-reduction is used, which is why the process is called oxidative phosphorylation.

The respiratory balance sheet

Theoretically, 38 ATP are gained per glucose in aerobic respiration. This assumes that the pathway runs in sequence, NADH from glycolysis enters the mitochondria for oxidative phosphorylation, no intermediates are used to make other compounds, and only glucose is respired. In a living cell these assumptions do not all hold, so 38 is a theoretical figure worked out on paper.

Amphibolic pathway

Respiration is usually called catabolic, but it is better described as amphibolic (both catabolic and anabolic). Fats are broken into glycerol (enters as PGAL) and fatty acids (enter as acetyl CoA); proteins are broken into amino acids, which are deaminated and enter at pyruvate, acetyl CoA or within the Krebs cycle. When the organism needs to make fatty acids, acetyl CoA is withdrawn from the pathway. The same intermediates serve both breakdown and synthesis.

Respiratory quotient (RQ)

RQ = volume of CO₂ evolved ÷ volume of O₂ consumed

• Carbohydrates: RQ = 1 (equal CO₂ and O₂).
• Fats: RQ is less than 1. For tripalmitin: 2(C₅₁H₉₈O₆) + 145 O₂ → 102 CO₂ + 98 H₂O, so RQ = 102/145 = 0.7.
• Proteins: RQ about 0.9.

In living organisms, respiratory substrates are often a mixture; pure proteins or fats are rarely used.

Common traps: (1) Glycolysis is in the cytoplasm; Krebs cycle in the matrix; ETS on the inner membrane. (2) Net glycolysis yield is 2 ATP, not 4. (3) Cytochrome c is mobile; complex IV contains copper. (4) Fats have RQ below 1, not above.

NEET focus

• Steps of glycolysis where ATP is used and made; net products.
• Fermentation products and enzymes; 13% alcohol limit.
• Link reaction enzyme and cofactors; products per pyruvate in the Krebs cycle.
• ETS complexes in order; F₀ and F₁; role of oxygen.
• 38 ATP assumptions; amphibolic pathway; RQ values.

Practice questions