Respiration and gas exchange in plants

Respiration and gas exchange in plants

In plain words: respiration is the cell’s way of spending food slowly and carefully, like changing a large note into coins. Instead of burning glucose in one flash, the cell breaks it down in many small, controlled steps and catches the energy in small packets of ATP that it can spend wherever it needs them.

Respiration and respiratory substrates

• Respiration supplies usable energy by oxidising food. Photosynthetic cells also capture light energy directly to make ATP and reducing power. Green plants and cyanobacteria make their own food by photosynthesis, but even in green plants only the cells with chloroplasts photosynthesise, so food must be translocated to all the non-green parts (NCERT page 153).
• NCERT says all respired food ultimately comes from photosynthesis (page 154). This is a broad ecological simplification: its own Chapter 2 describes chemosynthetic autotrophs that build organic matter using chemical energy (page 13).
• In eukaryotes, photosynthesis takes place in the chloroplasts, while the breakdown of food takes place in the cytoplasm and the mitochondria (page 154).
• The compounds oxidised in respiration are the respiratory substrates. Usually these are carbohydrates, but proteins, fats and even organic acids can be used in some plants, under certain conditions (page 154).

ATP as the energy currency

• The energy in respiratory substrates is not released all at once. It comes out in a series of slow, step-wise reactions controlled by enzymes, and is trapped as chemical energy in ATP (NCERT page 154).
• The energy released by oxidation cannot be used directly. It is used to make ATP, and ATP is broken down whenever and wherever energy is needed. So ATP acts as the energy currency of the cell (page 154).
• Respiration also supplies carbon skeletons that the cell uses as precursors to build other molecules (page 154).

The complete combustion of glucose gives CO_₂ and water, and most of its energy comes out as heat (NCERT page 155):

C_₆H_₁₂O_₆ + 6O_₂ → 6CO_₂ + 6H_₂O + energy

The cell cuts this waste by oxidising glucose in several small steps, some of them just large enough for the energy released to be coupled to ATP synthesis: “How this is done is, essentially, the story of respiration” (page 155).

Stomata and lenticels

Do plants breathe? Yes, in the sense that they need O_₂ for respiration and give out CO_₂. But plants, unlike animals, have no specialised organs for gaseous exchange; they have stomata and lenticels for this purpose (NCERT page 154; see stomata).

Gas exchange without respiratory organs

NCERT gives three general reasons why plants manage without lungs or gills (pages 154–155). These describe typical diffusion arrangements, not a claim that every plant cell is directly exposed to outside air:

1. Each plant part takes care of its own gas-exchange needs; very little gas is transported from one part to another.
2. Plants do not demand much gas exchange. Roots, stems and leaves respire at rates far lower than animals do. Large volumes of gas are exchanged only during photosynthesis, when each leaf looks after its own needs, and O_₂ is not a problem because it is released inside the photosynthesising cells.
3. The distance gases must diffuse is short, even in large, bulky plants: each living cell lies quite close to the surface. In woody stems, the living cells form thin layers inside and beneath the bark, which has openings called lenticels, while many mature xylem conducting/supporting cells are dead. Living xylem parenchyma also occurs within sapwood, so the whole interior is not dead (see NCERT Chapter 6 tissue types). The loose packing of parenchyma in leaves, stems and roots gives an interconnected network of air spaces.

Aerobic and anaerobic organisms

• Aerobic respiration of carbohydrate uses oxygen and releases carbon dioxide, water and energy; the combustion reaction needs oxygen (NCERT page 155).
• But some cells live where oxygen may or may not be available. There are good reasons to believe that the first cells on Earth lived in an atmosphere that lacked oxygen (page 155).
• Many organisms today are adapted to anaerobic conditions: some are facultative anaerobes, and for others the anaerobic requirement is obligate (page 155).
• Glycolysis partially oxidises glucose to pyruvate without requiring O_₂. NCERT calls it universal (page 155), but the classical EMP pathway has exceptions and variants among microbes (Baron, Medical Microbiology, “Bacterial Metabolism”).

Solved examples

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Quick revision summary

• Respiration breaks C–C bonds by oxidation inside cells, releasing considerable energy; the compounds oxidised are respiratory substrates, usually carbohydrates.
• The energy is released step-wise and trapped in ATP, the energy currency of the cell; carbon skeletons from respiration feed biosynthesis.
• Plants need O_₂ and give out CO_₂, but have no respiratory organs: stomata and lenticels serve, each part manages its own needs, demand is low, and living cells are near the surface.
• Some organisms are facultative or obligate anaerobes. Glycolysis needs no O_₂, and microbes also have alternative metabolic pathways.

Questions students ask

Why do plants need respiration if they make their own food?

Plants need respiration because photosynthesis stores energy in food, and respiration supplies ATP for cellular work, including in tissues without photosynthetic chloroplasts. NCERT explains that photosynthesis converts light energy into chemical energy stored in the bonds of carbohydrates like glucose, sucrose and starch, and that only cells containing chloroplasts photosynthesise, so food has to be translocated to non-photosynthetic parts, such as roots, which need it for oxidation (page 153). Respiration also yields the carbon skeletons that cells use as precursors to build other molecules (NCERT page 154).

Do plants take in oxygen and give out carbon dioxide at night?

Yes, at night most plants take in O_₂ and give out CO_₂, because respiration goes on in every living cell while photosynthesis, which needs light, stops. In daylight respiration continues, but photosynthesis exchanges much larger volumes of gas, and the O_₂ it releases inside the leaf cells is available to them (NCERT pages 154–155), so in bright light a leaf gives out O_₂ and takes in CO_₂ overall. How the two processes differ is compared on the requirements for photosynthesis page.

What is the difference between respiration in plants and animals?

Respiration inside the cells is the same process in plants and animals, the step-wise oxidation of food that traps energy as ATP, but the way the two exchange gases differs. Most animals breathe through respiratory organs, such as gills in fishes and lungs in amphibians, reptiles, birds and mammals, and in humans the gases are carried by the blood (NCERT Chapter 14, pages 183 and 185), while plants have no specialised organs for gaseous exchange: stomata and lenticels let gases in and out by diffusion, and there is very little transport of gases from one plant part to another (NCERT pages 154 and 164). Plant roots, stems and leaves also respire at rates far lower than animals do (page 154).

What is the difference between facultative and obligate anaerobes?

Facultative anaerobes can live with or without oxygen, while for obligate anaerobes the absence of oxygen is a requirement; NCERT names both among the present-day organisms adapted to anaerobic conditions (page 155). Beyond the NCERT text, oxygen inhibits growth of obligate anaerobes, but their ability to survive exposure varies; absence of growth is not the same as instant death. Many are poorly equipped to remove reactive oxygen products, while facultative anaerobes such as Escherichia coli grow either way, respiring aerobically when they have oxygen and falling back on anaerobic respiration or fermentation when they do not (StatPearls, ‘Anaerobic Infections’).

Is Respiration in Plants in the NEET (UG) 2026 syllabus?

Yes, Respiration in Plants is in the NEET (UG) 2026 syllabus: Unit 4, Plant Physiology, of the NMC syllabus lists “Respiration: Exchange gases; Cellular respiration-glycolysis, fermentation (anaerobic), TCA cycle and electron transport system (aerobic); Energy relations - Number of ATP molecules generated; Amphibolic pathways; Respiratory quotient.” These match NCERT Class 11 Biology Chapter 12, Sections 12.1 to 12.7, from gas exchange to the respiratory quotient, so all seven sections of the chapter are covered.

Practice questions

Try each question first, then open the answer to check your working.

1. Q1. What are respiratory substrates?

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   The compounds oxidised during respiration, usually carbohydrates (NCERT page 154).

2. Q2. In which parts of a eukaryotic cell are food molecules broken down to yield energy?

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   The cytoplasm and the mitochondria (NCERT page 154).

3. Q3. Which openings in woody stems allow gas exchange?

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   Lenticels (NCERT page 155).