🦠 Respiration in Plants Study Notes

14.1 Introduction

All energy for life comes from oxidation of food. Respiration = breaking C–C bonds of complex compounds by oxidation, releasing energy trapped as ATP — the energy currency of the cell.

Compounds oxidised = respiratory substrates (usually carbohydrates, sometimes fats, proteins, organic acids). Energy is released in small, enzyme-controlled steps. Photosynthesis happens in chloroplasts; respiration in the cytoplasm and mitochondria.

14.2 Do Plants Breathe?

Plants need O₂ and release CO₂ but have no specialised organs — they use stomata and lenticels for diffusion. Reasons they manage without lungs:

• Each part handles its own gas exchange; little transport between parts.
• Gas-exchange demand is low (roots/stems/leaves respire slowly).
• Diffusion distances are short; living cells lie near the surface.
• Loose parenchyma packing creates connected air spaces.

Cells avoid releasing all energy as heat by oxidising glucose in many small steps coupled to ATP synthesis. Some organisms are facultative or obligate anaerobes; all retain machinery for glycolysis.

14.3 Glycolysis

Glycos (sugar) + lysis (splitting). Scheme by Embden, Meyerhof & Parnas (EMP pathway). Occurs in the cytoplasm of all cells; in anaerobes it is the only respiration.

Glucose (from sucrose via invertase, or storage carbs) is partially oxidised to 2 pyruvic acid through 10 enzyme-controlled steps.

• ATP used at 2 steps: glucose → glucose-6-P; fructose-6-P → fructose-1,6-bisphosphate.
• NADH+H⁺ formed when PGAL → BPGA.
• ATP made at: BPGA → PGA; PEP → pyruvate.
• Net gain: 2 ATP + 2 NADH per glucose.

14.4 Fermentation

Anaerobic, incomplete oxidation of pyruvate:

NADH+H⁺ is reoxidised to NAD⁺. Less than 7% of glucose's energy is released; net only 2 ATP; products are hazardous. Yeast die at ~13% alcohol.

14.5 Aerobic Respiration

In mitochondria, with O₂. Two crucial events: (1) complete oxidation of pyruvate (matrix) and (2) passing electrons to O₂ with ATP synthesis (inner membrane).

Link reaction: pyruvate → acetyl CoA by pyruvate dehydrogenase (needs NAD⁺ & CoA):

14.4.1 Krebs / TCA cycle (Hans Krebs), in the matrix:

After glycolysis + link + Krebs (per glucose): 8 NADH, 2 FADH₂, 2 ATP (from TCA), plus 2 NADH & 2 ATP net from glycolysis.

14.4.2 ETS & oxidative phosphorylation (inner membrane):

• 1 NADH → 3 ATP; 1 FADH₂ → 2 ATP.
• O₂ is the terminal acceptor, reduced to water.
• Oxidative phosphorylation uses redox energy (not light) to build the proton gradient.
• ATP synthase (Complex V): F1 = ATP-making headpiece; F0 = proton channel. 2 H⁺ per ATP flow from intermembrane space to matrix.

14.6 The Respiratory Balance Sheet

Net 38 ATP per glucose (aerobic) — a theoretical figure under assumptions:

• A sequential, orderly pathway functions step after step.
• Glycolytic NADH enters mitochondria for oxidative phosphorylation.
• No intermediates are withdrawn for other syntheses.
• Only glucose is respired (no alternative substrates).

14.7 Amphibolic Pathway

Glucose is the favoured substrate. Others enter at points along the pathway:

• Fats → glycerol (→ PGAL) + fatty acids (→ acetyl CoA).
• Proteins → amino acids (after deamination) → enter at pyruvate, acetyl CoA, or within Krebs.

Because the same intermediates are used in both breakdown (catabolism) and synthesis (anabolism), the respiratory pathway is amphibolic, not merely catabolic.

14.8 Respiratory Quotient

RQ depends on the substrate. In living organisms substrates are usually mixed — pure fats or proteins are never used alone.