Respiration in PlantsNEET Botany · Class 11 · NCERT Chapter 14

Introduction to Respiration in Plants

Cellular respiration is the process by which cells break down organic molecules (mainly glucose) to release energy in a form (ATP) that the cell can use. It is the reverse of photosynthesis at the level of overall equation: respiration consumes glucose and O₂ and releases CO₂, water, and energy as ATP.

• All living cells respire day and night. Plants do not "switch" between photosynthesis and respiration; respiration runs continuously, even during the day. (At low light, photosynthesis is masked by respiration.)
• No specialised organ: plants do not have lungs or gills. Each cell exchanges gases independently with the surrounding air via stomata, lenticels, and the cell surface.
• Respiratory substrate: the molecule oxidised. Usually glucose, but can also be fats, proteins, or organic acids depending on availability.
• Respiratory product: the molecule released. CO₂, H₂O, and ATP in aerobic respiration.

The cell does NOT release all of glucose's energy in one big burst (which would damage the cell). Instead, it breaks down glucose in many small steps, capturing energy as ATP at each step. The full process has four phases.

Mitochondria: Structure and Function

The mitochondrion is a double-membrane organelle. Its structure is finely tuned for cellular respiration. Three of the four phases of aerobic respiration happen inside or on the membranes of the mitochondrion (only glycolysis is in the cytoplasm).

Outer Membrane

• Smooth and freely permeable to small molecules and ions (porin proteins).
• Acts as a sieve and the outer boundary of the organelle.

Intermembrane Space

• The narrow space between outer and inner membranes.
• H⁺ ions accumulate here during the ETC, building the proton motive force.
• Cytochrome c (mobile carrier between Complex III and IV) lives here.

Inner Membrane

• Highly selective: only specific transporters allow molecules across.
• Folded into cristae to increase surface area. More cristae = more ATP-making capacity.
• Embedded with the four ETC complexes (I, II, III, IV) and ATP synthase (Complex V).
• Site of the electron transport chain and oxidative phosphorylation.

Mitochondrial Matrix

• The fluid-filled inner compartment.
• Site of the LINK REACTION (pyruvate dehydrogenase) and the KREBS CYCLE (citrate synthase, isocitrate dehydrogenase, etc.).
• Contains mitochondrial DNA (circular, like prokaryotes), 70S ribosomes, dissolved enzymes, metabolites.

Cellular Respiration: The Big Picture

You should be able to picture the whole process at a glance. Here is the standard NEET-style summary:

Total energy account per glucose (NCERT theoretical):

• 2 ATP (glycolysis substrate-level) + 2 ATP (Krebs substrate-level) = 4 ATP
• 10 NADH (2 from glycolysis + 2 from link reaction + 6 from Krebs) × 3 ATP = 30 ATP
• 2 FADH₂ (Krebs only) × 2 ATP = 4 ATP
• Total = 4 + 30 + 4 = 38 ATP per glucose

Glycolysis (EMP Pathway)

Glycolysis is also called the EMP pathway after the three scientists who worked it out: Embden, Meyerhof, and Parnas. It is the first phase of cellular respiration and the only phase that does NOT require mitochondria. All living cells (aerobic, anaerobic, plant, animal, fungal, bacterial) carry out glycolysis.

Site and Conditions

• Site: cytoplasm (cytosol). NOT inside mitochondria.
• Oxygen: NOT required (anaerobic). Glycolysis runs in both aerobic and anaerobic conditions.
• Substrate: glucose (a 6-carbon sugar) or other hexoses.
• Net products per glucose: 2 pyruvate (3C each) + 2 ATP (net) + 2 NADH.

Three Phases of Glycolysis

• Investment phase (steps 1-3): 2 ATP are CONSUMED. Glucose is phosphorylated and primed for splitting.
• Splitting phase (steps 4-5): The 6C molecule is split into two 3C molecules (G3P and DHAP, then both become G3P).
• Payoff phase (steps 6-10): 4 ATP and 2 NADH are PRODUCED (each step happens twice because there are 2 G3P molecules per glucose). Net ATP = 4 - 2 = 2.

Key Reactions and Enzymes

• Step 1 (Hexokinase): Glucose + ATP → Glucose-6-P. Phosphorylation traps glucose in the cell.
• Step 3 (Phosphofructokinase, PFK): Fructose-6-P + ATP → Fructose-1,6-bisP. The COMMITTED step; rate-limiting; allosterically regulated. After this point, the substrate is locked into glycolysis.
• Step 4 (Aldolase): Fructose-1,6-bisP (6C) → DHAP (3C) + G3P (3C). The split.
• Step 6 (G3P dehydrogenase): G3P + NAD⁺ + Pᵢ → 1,3-bisphosphoglycerate + NADH. The ONLY step that produces NADH (twice, so 2 NADH per glucose).
• Step 7 (Phosphoglycerate kinase): 1,3-BPG + ADP → 3-PGA + ATP. Substrate-level phosphorylation #1 (×2 = 2 ATP).
• Step 10 (Pyruvate kinase): PEP + ADP → pyruvate + ATP. Substrate-level phosphorylation #2 (×2 = 2 ATP).

Fate of Pyruvate

After glycolysis, the cell has 2 pyruvate per glucose. What happens to pyruvate next depends on whether oxygen is available and what type of cell it is.

The pyruvate "decision" is one of the most important biochemical branch points. Whether your cell ends up making ethanol, lactate, or CO₂ + water depends entirely on the availability of O₂ at this single moment.

Fermentation: Alcoholic and Lactic Acid

Fermentation is the process of incomplete (anaerobic) breakdown of glucose. The point of fermentation is NOT to produce more ATP (only the 2 ATP from glycolysis are made). The point is to regenerate NAD⁺ from NADH so that glycolysis can keep running.

Why Fermentation is Necessary

• Glycolysis converts 2 NAD⁺ → 2 NADH per glucose (at step 6).
• NAD⁺ supply in the cell is limited. Without recycling, NADH would build up and NAD⁺ would run out.
• If NAD⁺ runs out, glycolysis stops. The cell makes no more ATP. The cell dies.
• Fermentation oxidises NADH back to NAD⁺ by transferring its hydrogen to pyruvate (forming lactate) or to acetaldehyde (forming ethanol). This keeps glycolysis going.

Alcoholic Fermentation (Yeast)

• Organisms: Yeast (Saccharomyces cerevisiae) and some bacteria.
• Step 1: Pyruvate → acetaldehyde + CO₂. Catalysed by pyruvate decarboxylase (uses TPP / vitamin B1 cofactor).
• Step 2: Acetaldehyde + NADH → ethanol + NAD⁺. Catalysed by alcohol dehydrogenase. Regenerates NAD⁺.
• Net per glucose: 2 ethanol + 2 CO₂ + 2 ATP.
• Real-world uses: brewing (beer, wine), baking (CO₂ from yeast makes bread rise), bioethanol fuel.

Lactic Acid Fermentation

• Organisms: Skeletal muscle cells under O₂ stress; Lactobacillus and other lactic acid bacteria (LAB).
• Single step: Pyruvate + NADH → lactate + NAD⁺. Catalysed by lactate dehydrogenase (LDH).
• NO CO₂ released. One-step pathway.
• Net per glucose: 2 lactate + 2 ATP.
• Real-world uses: curd / yoghurt, cheese, sauerkraut, kimchi (all by Lactobacillus). In humans: muscle "burn" during heavy exercise (the lactate is later transported to the liver and converted back to glucose via the Cori cycle).

Aerobic Respiration: Overview

When oxygen is available, the cell does NOT waste pyruvate's energy by reducing it to lactate or ethanol. Instead, pyruvate enters the mitochondrion and is fully oxidised to CO₂ and H₂O. This produces a much higher ATP yield: 38 ATP per glucose vs 2 in fermentation.

Three Mitochondrial Phases

• Link reaction (matrix): Pyruvate → acetyl CoA + CO₂ + NADH. Catalysed by pyruvate dehydrogenase complex.
• Krebs cycle (matrix): Acetyl CoA fully oxidised to 2 CO₂. Each turn produces 3 NADH + 1 FADH₂ + 1 GTP/ATP.
• ETC + ATP synthase (inner membrane): NADH and FADH₂ are oxidised by O₂; the energy is captured as ATP.

The COMBINED output of all three mitochondrial phases is: 6 CO₂ (from link + Krebs) + 6 H₂O (from ETC at Complex IV) + 36 ATP. Add the 2 ATP from glycolysis = 38 ATP total per glucose.

Link Reaction (Oxidative Decarboxylation of Pyruvate)

The link reaction is the bridge between glycolysis (cytoplasm) and the Krebs cycle (mitochondrial matrix). It is sometimes called the "transition step" or "preparatory step".

• Site: mitochondrial matrix.
• Enzyme: pyruvate dehydrogenase complex (PDH); a multi-enzyme complex.
• Cofactors: TPP (thiamine, vitamin B1), lipoic acid, FAD, NAD⁺, CoA.
• Per pyruvate: 1 acetyl CoA + 1 CO₂ + 1 NADH. Per glucose (2 pyruvates): 2 acetyl CoA + 2 CO₂ + 2 NADH.
• Net change in carbon: 3C → 2C (1 carbon released as CO₂).
• NO ATP produced directly in this step.

Krebs Cycle (TCA / Citric Acid Cycle)

Discovered by Sir Hans Krebs in 1937 (Nobel Prize 1953). Also called the citric acid cycle (because the first product, citrate, is citric acid) or the tricarboxylic acid cycle (TCA cycle, because citrate has three carboxyl groups).

Site and Conditions

• Site: mitochondrial matrix.
• Oxygen: not directly used in the cycle, but the cycle stops if the ETC stops (which needs O₂). So functionally aerobic.
• Substrate: acetyl CoA (from link reaction or fatty acid oxidation).
• Per glucose: 2 turns (because 1 glucose → 2 pyruvate → 2 acetyl CoA).

Per Turn (One Acetyl CoA)

The 8 Steps of the Krebs Cycle

1. Citrate synthase: Acetyl CoA (2C) + OAA (4C) → citrate (6C). CoA is released. The cycle starts.
2. Aconitase: Citrate (6C) → isocitrate (6C). Isomerisation, no energy change.
3. Isocitrate dehydrogenase: Isocitrate (6C) → α-ketoglutarate (5C) + CO₂ + NADH. First oxidative decarboxylation.
4. α-KG dehydrogenase: α-ketoglutarate (5C) → succinyl CoA (4C) + CO₂ + NADH. Second oxidative decarboxylation.
5. Succinyl CoA synthetase: Succinyl CoA (4C) → succinate (4C) + GTP (or ATP). Substrate-level phosphorylation.
6. Succinate dehydrogenase: Succinate (4C) → fumarate (4C) + FADH₂. The only Krebs enzyme bound to the inner membrane (also Complex II of the ETC).
7. Fumarase: Fumarate (4C) + H₂O → malate (4C). Hydration.
8. Malate dehydrogenase: Malate (4C) → OAA (4C) + NADH. OAA is regenerated; the cycle is back at the start.

Total Per Glucose (2 Turns)

Electron Transport Chain (ETC)

The ETC is a series of protein complexes embedded in the inner mitochondrial membrane. It transfers electrons from NADH and FADH₂ to oxygen (the final electron acceptor). The energy released is used to pump H⁺ from the matrix to the intermembrane space, building the proton gradient that drives ATP synthase.

The Five Complexes

• Complex I (NADH dehydrogenase): Accepts electrons from NADH. Pumps 4 H⁺. Passes electrons to ubiquinone (CoQ).
• Complex II (Succinate dehydrogenase): Same as Krebs step 6. Accepts electrons from FADH₂. Does NOT pump H⁺. Passes electrons to CoQ.
• Complex III (Cytochrome bc1): Accepts electrons from CoQ (now CoQH₂). Pumps 4 H⁺. Passes electrons to cytochrome c.
• Complex IV (Cytochrome c oxidase): Accepts electrons from cytochrome c. Pumps 2 H⁺. Final acceptor: 4 e⁻ + 4 H⁺ + O₂ → 2 H₂O.
• Complex V (ATP synthase, F0-F1): NOT an electron carrier. The proton channel + catalytic head. H⁺ flow back into matrix drives ATP synthesis.

Mobile Carriers

• Ubiquinone (Coenzyme Q, CoQ): A small lipid-soluble molecule that diffuses within the inner membrane. Carries electrons from Complex I and Complex II to Complex III.
• Cytochrome c: A small soluble protein that diffuses in the intermembrane space. Carries electrons from Complex III to Complex IV.

NADH vs FADH₂

• NADH: Enters at Complex I. Drives proton pumping at I, III, IV. Total ~10 H⁺ per NADH. Yields 3 ATP (NCERT).
• FADH₂: Enters at Complex II. Drives proton pumping at III, IV (skips Complex I). Total ~6 H⁺ per FADH₂. Yields 2 ATP (NCERT).

Oxidative Phosphorylation and Chemiosmosis

Oxidative phosphorylation is the synthesis of ATP coupled to the oxidation of NADH and FADH₂ by the ETC. The mechanism is chemiosmosis, proposed by Peter Mitchell in 1961 (Nobel Prize 1978).

The Chemiosmotic Mechanism

1. Electrons flow through the ETC (NADH → Complex I → CoQ → Complex III → Cyt c → Complex IV → O₂).
2. At Complex I, III, and IV, the released energy is used to pump H⁺ from the matrix to the intermembrane space.
3. This creates a proton gradient: low H⁺ in matrix (high pH), high H⁺ in intermembrane space (low pH). The gradient is the proton motive force.
4. H⁺ flow back DOWN their gradient through the F0 channel of ATP synthase.
5. This proton flow drives the rotation of F1, which catalyses ADP + Pᵢ → ATP on the matrix side.

ATP Synthase (F0-F1 Complex)

• F0: Embedded in the inner mitochondrial membrane. Acts as the proton channel.
• F1: The catalytic "head piece" that protrudes into the matrix. Synthesises ATP from ADP + Pᵢ. The F1 head appears as a "lollipop" or "elementary particle" / oxysome under the electron microscope.
• About 4 H⁺ are needed per ATP synthesised. Combined with the 10 H⁺ pumped per NADH → 3 ATP per NADH (NCERT).

ATP Yield: Total per Glucose

The total ATP per glucose in aerobic respiration depends on whose count you use. NCERT and NEET use the theoretical maximum of 38 ATP. Modern biochemistry textbooks usually say 30 to 32 ATP because of proton leak and the cost of importing cytoplasmic NADH into the mitochondrion (via shuttle systems).

NCERT Value: 38 ATP per Glucose

Why 38 vs 30 to 32?

• Cytoplasmic NADH (from glycolysis) cannot cross the inner mitochondrial membrane directly.
• It is transferred via a "shuttle" system: malate-aspartate shuttle (3 ATP per NADH) or glycerol-phosphate shuttle (2 ATP per NADH, because it gives FADH₂).
• Some H⁺ leak across the inner membrane without driving ATP synthesis.
• Real efficiency is approximately 30 to 32 ATP per glucose. NEET expects 38 (NCERT theoretical).

Respiratory Quotient (RQ)

The respiratory quotient is the ratio of CO₂ released to O₂ consumed during respiration. It depends on the substrate being respired.

RQ for Different Substrates

• Carbohydrates (glucose): C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O. RQ = 6/6 = 1.0. All carbohydrates give RQ = 1.0.
• Fats (e.g. tripalmitin): 2 C₅₁H₉₈O₆ + 145 O₂ → 102 CO₂ + 98 H₂O. RQ = 102/145 ≈ 0.7. Fats need more O₂ to oxidise.
• Proteins: RQ ≈ 0.9.
• Organic acids (e.g. malic acid): 2 C₄H₆O₅ + 6 O₂ → 8 CO₂ + 6 H₂O. RQ = 8/6 ≈ 1.33. Greater than 1 because the molecule is already highly oxygenated.
• Anaerobic respiration (yeast): C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂ (no O₂ used). RQ = 2/0 = infinity.

Measurement

• RQ is measured using a Ganong's respirometer or Kühne's tube.
• The substrate is in a closed container with KOH (which absorbs CO₂). The drop in volume gives O₂ consumption.
• A second container without KOH gives net volume change (= O₂ consumed - CO₂ produced).
• The two readings together give CO₂ produced and O₂ consumed separately.

Respiration is an Amphibolic Pathway

A pathway is amphibolic when it serves both catabolic (breakdown for energy) and anabolic (synthesis of biomolecules) functions. The respiratory pathway is amphibolic because its intermediates are precursors for many other biomolecules.

Examples of Anabolic Roles

• Acetyl CoA: precursor of fatty acids, sterols, ketone bodies, terpenes (in plants).
• α-ketoglutarate: precursor of glutamate (and arginine, proline, glutamine). In plants, also for chlorophyll synthesis.
• Oxaloacetate (OAA): precursor of aspartate (and lysine, threonine, methionine). Also for pyrimidine bases of DNA / RNA.
• Succinyl CoA: precursor of porphyrins (heme in hemoglobin and cytochromes; chlorophyll in plants).
• Pyruvate: precursor of alanine (via transamination).
• Glucose-6-P: precursor of ribose-5-P (for nucleotides) via the pentose phosphate pathway; also of glycogen / starch.

Why This Matters

• The same pathway that breaks down food can also be tapped to build biomolecules.
• For example: when you eat extra carbohydrates, the cell does not throw it all into Krebs. Some glucose is converted to acetyl CoA, which is then used to build fatty acids (= weight gain).
• In plants: respiratory intermediates feed into chlorophyll synthesis, amino acid synthesis, and cell wall component synthesis.

Worked NEET Problems

Cheat Sheet