NCERT Grounding
NCERT Class 11 Biology, Chapter 12 (Respiration in Plants), opens Section 12.1 with the direct statement: "Yes, plants require O2 for respiration to occur and they also give out CO2." It immediately qualifies this by explaining that plants, unlike animals, have no specialised respiratory organs; instead, stomata and lenticels serve as the portals for passive gaseous diffusion. The section provides three structural reasons why this suffices — each cell's proximity to a surface, the low metabolic demand relative to animals, and the interconnected intercellular air-space network — and then transitions into the definition of cellular respiration, setting the stage for glycolysis, fermentation, and aerobic respiration in the sections that follow.
"All the energy required for 'life' processes is obtained by oxidation of some macromolecules that we call 'food'."
NCERT Class 11 Biology — Chapter 12, Respiration in Plants
Gas Exchange Without Lungs
The absence of a respiratory organ in plants is not a deficiency — it is a structural solution matched to a low-demand system. Three anatomical arguments from NCERT explain why diffusion alone is adequate.
Structural principle: In plants, each living cell is responsible for its own gas exchange. There is negligible internal transport of respiratory gases from one organ to another.
Each Part Self-Sufficient
Every organ — root, stem, leaf — manages its own O2 uptake and CO2 release independently. No bulk transport of respiratory gases occurs through vascular tissue.
Low Metabolic Demand
Roots, stems and leaves respire at rates far lower than animal tissues. Only during photosynthesis are large gas volumes exchanged, and individual leaves handle that locally.
Short Diffusion Distances
Even in bulky woody stems, living cells form thin layers near the bark. The interior is dead (mechanical support only). Loose parenchyma packing creates interconnected air spaces throughout.
Portals of Gas Exchange
Three structural openings serve as the gateways through which O2 and CO2 move by simple diffusion between the plant and the atmosphere.
| Structure | Location | Controlled by | Primary function |
|---|---|---|---|
| Stomata | Leaves; young green stems | Guard cells (open/close) | Gas exchange + transpiration |
| Lenticels | Bark of woody stems and roots | Permanently open (complementary cells) | Gas exchange through bark |
| General surface | Young roots, stems, fruits, seeds | Not regulated | Passive diffusion across cuticle-free surface |
Roots are a special case: they obtain O2 from air trapped in soil pores. This is why waterlogged conditions deprive roots of O2, forcing anaerobic respiration and ultimately causing root suffocation. Unlike the aerial parts, roots have no stomata, so diffusion through the general root surface and root-hair zone is the principal route.
Day–Night Gas Exchange Cycle
A persistent point of confusion in NEET is the apparent contradiction that plants are said to produce O2 (photosynthesis) yet also consume O2 (respiration). The resolution lies in the compensation point and the relative rates of the two processes.
Daytime
Net O2 OUT
Photosynthesis rate > Respiration rate
- O2 is released by photosynthesis in excess of what respiration consumes.
- CO2 produced by respiration is consumed by photosynthesis.
- In photosynthesising cells, O2 is generated within the cell — no diffusion barrier for respiratory O2 supply.
- Net exchange: plant appears to give out O2 and absorb CO2.
Night-time
Net CO2 OUT
Only respiration occurs
- Photosynthesis ceases in absence of light.
- Respiration continues in all living cells around the clock.
- O2 is absorbed from intercellular spaces; CO2 diffuses outward.
- Net exchange: plant absorbs O2 and releases CO2, same as animals.
The compensation point is the precise light intensity at which the rate of photosynthesis exactly equals the rate of respiration. At this point, net gas exchange is zero: all O2 released by photosynthesis is consumed by respiration, and all CO2 produced by respiration is fixed by photosynthesis. Below the compensation point (or in darkness), the plant is a net consumer of O2; above it, a net producer.
Compensation Point
Light intensity at which Rate of Photosynthesis = Rate of Respiration. Net CO2 exchange = 0. Apparent photosynthesis = 0. True (gross) photosynthesis continues at the same rate as respiration.
Cellular Respiration Defined
NCERT defines cellular respiration as the breaking of C–C bonds of complex compounds through oxidation within the cells, leading to release of a considerable amount of energy. Two distinctions are essential for NEET.
First, the energy is not released in a single combustion step. It is released in a series of slow, stepwise, enzyme-catalysed reactions, enabling some of the energy to be coupled to ATP synthesis rather than being lost entirely as heat. Second, the compounds oxidised are the respiratory substrates — most commonly carbohydrates, but also fats and proteins under specific conditions.
Energy Currency of the Cell
All energy released by oxidation in respiration is first trapped in ATP. It is broken down wherever and whenever energy is needed — never used directly from substrate oxidation.
The overall equation for aerobic cellular respiration of glucose is:
Figure 1. The overall equation for aerobic cellular respiration of glucose. The ratio of CO2 evolved to O2 consumed (RQ) is exactly 1.0 when glucose is the sole substrate.
Three Stages Overview
Aerobic respiration proceeds through three biochemically distinct stages, each localised to a specific cellular compartment. This compartmentalisation is a favourite source of NEET questions on the site of each reaction.
-
Stage 1
Glycolysis
Glucose (6C) is split into 2 pyruvate (3C) molecules via 10 enzyme-controlled steps. Net gain: 2 ATP + 2 NADH.
Site: Cytoplasm -
Stage 2
Krebs Cycle (TCA)
Pyruvate enters the mitochondrial matrix as acetyl CoA. Two turns per glucose yield 6 NADH, 2 FADH2, 2 GTP (= 2 ATP), 6 CO2.
Site: Mitochondrial matrix -
Stage 3
ETS + Oxidative Phosphorylation
NADH and FADH2 donate electrons to the transport chain. O2 is the terminal acceptor, forming H2O. ~34 ATP produced via chemiosmosis.
Site: Inner mitochondrial membrane
Anaerobic respiration (fermentation) branches off after glycolysis when O2 is unavailable. Pyruvate is reduced to either ethanol + CO2 (yeast) or lactic acid (muscles, certain bacteria), regenerating NAD+ so glycolysis can continue. The net ATP yield is only 2 per glucose — far less than the ~38 under aerobic conditions.
Respiratory Substrates and RQ
A respiratory substrate is any organic molecule that is oxidised during respiration to yield energy. Carbohydrates — especially glucose — are the preferred substrate. However, under starvation or in fat-rich seeds, fats and proteins are mobilised.
The Respiratory Quotient (RQ) is the ratio of the volume of CO2 evolved to the volume of O2 consumed. Its value directly reflects the chemical composition of the substrate being oxidised — specifically the H:O ratio — making it a practical diagnostic tool.
Figure 2. RQ values for the three major respiratory substrate classes. Fats have the lowest RQ because their greater degree of reduction demands proportionally more O2 per carbon atom oxidised.
| Substrate | RQ value | Reason | Example / context |
|---|---|---|---|
| Carbohydrates | 1.0 | Equal moles of CO2 and O2 (already partially oxidised — H:O ≈ 2:1 as in water) | Glucose, sucrose; most plant tissues at rest |
| Fats | <1 (≈0.7) | High H:O ratio; more O2 needed per carbon than CO2 produced | Tripalmitin (RQ = 0.7); fat-rich seeds (castor, groundnut) |
| Proteins | ≈0.9 | Intermediate H:O; nitrogen removed as urea/uric acid before oxidation | Protein-rich seeds (pulses); prolonged starvation |
| Organic acids | >1 | Already oxidised (high O content); less O2 needed relative to CO2 produced | Succulent CAM plants oxidising stored malate at night |
The RQ of tripalmitin (a fat) is calculated as follows from its combustion equation: 2C51H98O6 + 145O2 → 102CO2 + 98H2O; RQ = 102/145 ≈ 0.7. This value was directly tested in NEET 2019 and must be memorised.
Worked Examples
A germinating castor seed (fat-rich) is placed in a respirometer. The volume of CO2 released is measured as 70 mL and O2 consumed as 100 mL over the same period. What is the RQ, and what does it indicate about the substrate?
Solution: RQ = Volume CO2 / Volume O2 = 70 / 100 = 0.7. An RQ of 0.7 indicates that fats are being used as the respiratory substrate. This is consistent with castor seeds, which are rich in oils. Fat oxidation requires proportionally more O2 than carbohydrate oxidation because fat molecules are more reduced (higher H:O ratio). NEET trap: students confuse "RQ < 1 = fat" with "RQ > 1 = fat" — remember, fats need more O2, so the CO2/O2 ratio is less than 1.
A plant is kept in a closed chamber. At a certain light intensity, the net gas exchange is zero — no O2 or CO2 enters or leaves. What is the term for this condition, and what is happening biochemically?
Solution: This is the compensation point. At this specific light intensity, the rate of photosynthesis equals the rate of respiration. Biochemically: all CO2 produced by mitochondrial respiration is immediately refixed by the Calvin cycle, and all O2 released by the light reactions is immediately consumed by cellular respiration. The plant is carrying out both processes simultaneously at equal rates, resulting in no net change in the concentrations of O2 or CO2 in the surrounding atmosphere.
NEET 2022 type: When one molecule of glucose is converted to two molecules of pyruvic acid during glycolysis, what is the net gain of ATP?
Solution: Glycolysis consumes 2 ATP (investment phase: glucose → glucose-6-phosphate, fructose-6-phosphate → fructose-1,6-bisphosphate) and produces 4 ATP (two substrate-level phosphorylations per triose unit × 2 triose units). Net gain = 4 − 2 = 2 ATP. Also produced: 2 NADH. The answer is 2 ATP — this was the correct answer in NEET 2022 Q.127 (Answer option 2).
Common Confusion & NEET Traps
Breathing (External Respiration)
Physical process
- Exchange of gases between organism and environment.
- In animals: driven by muscular action (lungs).
- In plants: passive diffusion through stomata/lenticels.
- No ATP is synthesised in this step.
- Not applicable in the strict sense to plants — they have no breathing organs.
Cellular Respiration
Biochemical process
- Oxidative breakdown of organic molecules within cells.
- Occurs in cytoplasm (glycolysis) and mitochondria (Krebs + ETS).
- Produces ATP — the usable energy currency.
- Occurs in ALL living cells — plant, animal, fungal.
- Continuous: day and night, in every metabolically active cell.