Respiration in Plants

Cellular respiration is the process by which cells break down organic molecules (mainly glucose) to release energy in the form of ATP. Unlike photosynthesis, respiration occurs in ALL living cells and at ALL times.

Overall equation for aerobic respiration:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (approx. 686 kcal/mol or 2880 kJ/mol)

Types of Respiration

Aerobic respiration: Oxygen is the final electron acceptor; complete oxidation of glucose; produces ~36-38 ATP per glucose
Anaerobic respiration (fermentation): No oxygen; incomplete oxidation; produces only 2 ATP per glucose

Glycolysis (Embden-Meyerhof-Parnas Pathway)

Occurs in the cytoplasm (cytosol). Present in ALL organisms. Does NOT require oxygen.

Steps summary:
Glucose (6C) is phosphorylated using 2 ATP → Fructose-1,6-bisphosphate
Splits into two molecules of PGAL (Glyceraldehyde-3-phosphate) (3C each)
Each PGAL is oxidised to pyruvate (3C)
Net yield per glucose: 2 ATP (4 produced - 2 used), 2 NADH, 2 pyruvate

Anaerobic Respiration (Fermentation)

When oxygen is absent, pyruvate is NOT sent to the mitochondria.
In yeast: pyruvate → acetaldehyde → ethanol + CO2 (alcoholic fermentation); enzyme: pyruvate decarboxylase and alcohol dehydrogenase
In animal muscle (during intense exercise): pyruvate → lactic acid (lactic acid fermentation); enzyme: lactate dehydrogenase; causes muscle fatigue/cramps

Purpose: regenerate NAD+ from NADH so glycolysis can continue.
Net ATP from fermentation: only 2 ATP per glucose.

Aerobic Respiration

Pyruvate (formed in cytoplasm) is transported into the mitochondrial matrix.

Pyruvate Oxidation (Link Reaction):
Pyruvate (3C) → Acetyl CoA (2C) + CO2 + NADH
Enzyme: pyruvate dehydrogenase complex
Per glucose: 2 pyruvate → 2 Acetyl CoA + 2 CO2 + 2 NADH

Krebs Cycle (TCA Cycle / Citric Acid Cycle) — in Mitochondrial Matrix:
Acetyl CoA (2C) + Oxaloacetate (4C) → Citrate (6C)
Per turn (per Acetyl CoA):
2 CO2 released (complete oxidation)
3 NADH produced
1 FADH2 produced
1 GTP (= 1 ATP equivalent) produced
Per glucose (2 turns): 4 CO2, 6 NADH, 2 FADH2, 2 GTP

Electron Transport Chain (ETC) and Oxidative Phosphorylation — Inner Mitochondrial Membrane:
Electrons from NADH and FADH2 are passed through a series of protein complexes (Complex I → II → III → IV).
Final electron acceptor: O2 → combines with H+ and electrons → H2O
Energy released drives chemiosmosis: H+ pumped into intermembrane space → flows back through ATP synthase → ATP produced.

ATP yield:
1 NADH → ~2.5 ATP (sometimes approximated as 3)
1 FADH2 → ~1.5 ATP (sometimes approximated as 2)
Total per glucose (aerobic): ~30-32 ATP (older textbooks use 36-38)
NCERT Class 11 commonly refers to approximately 36 ATP per glucose

Summary

Glycolysis: 0 CO2
Link reaction: 2 CO2
Krebs cycle (2 turns): 4 CO2
Total: 6 CO2 per glucose (matches the equation)

Respiratory Quotient (RQ)

RQ = Volume of CO2 released / Volume of O2 consumed
Carbohydrates: RQ = 1 (equal volumes of CO2 and O2)
Fats: RQ < 1 (approximately 0.7) — more O2 needed per CO2
Proteins: RQ ≈ 0.9
Organic acids (e.g., succinate): RQ > 1

Amphibolic Pathways

Respiratory pathways are amphibolic — both catabolic (breaking down substrates for energy) and anabolic (providing intermediates for biosynthesis). For example:
Acetyl CoA → fatty acid synthesis
Alpha-ketoglutarate → amino acids
Oxaloacetate → amino acids (via transamination)

Common mistakes

Confusing the location: Glycolysis is in the cytoplasm; Krebs cycle in the mitochondrial matrix; ETC on the inner mitochondrial membrane.
Forgetting that each glucose produces 2 pyruvates → 2 turns of the Krebs cycle.
RQ of fat < 1 (NOT > 1) because fat has low oxygen content and more O2 is needed per carbon.
Anaerobic respiration yields only 2 ATP, not 36-38.