Plants make food by photosynthesis, but their cells must also release energy from food. Cellular respiration is the controlled breakdown of respiratory substrates, usually glucose, to release energy that the cell can capture as ATP. Plant cells respire both during the day and at night.
Table of Contents
Do Plants Exchange Gases?
Plants do not have specialised breathing organs like animals. Oxygen and carbon dioxide move mainly by diffusion. Leaves exchange gases through stomata, woody stems through lenticels, and roots exchange gases with air spaces in the soil.
Photosynthesis and respiration are different processes. Photosynthesis stores energy in organic compounds; respiration releases usable energy from those compounds.
A simplified equation for aerobic respiration is:
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy
The energy is released through many enzyme-controlled steps, and some of it is captured in ATP.
Main Stages of Cellular Respiration
| Stage | Main location in a plant cell | What happens |
|---|---|---|
| Glycolysis | Cytoplasm | Glucose is split into two pyruvate molecules |
| Pyruvate oxidation and TCA cycle | Mitochondrial matrix | Carbon compounds are oxidised; CO₂ and electron carriers are produced |
| Electron transport and oxidative phosphorylation | Inner mitochondrial membrane | Electron energy helps produce most of the ATP |
Glycolysis
Glycolysis occurs in the cytoplasm and does not directly require oxygen. One six-carbon glucose molecule is converted into two three-carbon pyruvate molecules.
For each glucose molecule, glycolysis:
- Uses 2 ATP in its early steps.
- Produces 4 ATP in later steps, giving a net gain of 2 ATP.
- Produces 2 NADH.
What happens to pyruvate next depends on oxygen availability and the organism.
Fermentation
When oxygen is unavailable, some organisms use fermentation after glycolysis. It allows NADH to be converted back to NAD⁺ so glycolysis can continue.
In alcoholic fermentation, pyruvate is converted to ethanol and CO₂. Yeast is a familiar example. In lactic acid fermentation, the end product is lactic acid.
Fermentation breaks down glucose only partially. Its net ATP yield is 2 ATP per glucose, obtained during glycolysis.
Aerobic Respiration
When oxygen is available, pyruvate enters the mitochondrion. There it is converted to acetyl-CoA, releasing CO₂ and producing NADH.
TCA Cycle or Krebs Cycle
Acetyl-CoA enters the tricarboxylic acid (TCA) cycle, also called the Krebs cycle, in the mitochondrial matrix. The cycle releases CO₂ and produces NADH and FADH₂. These molecules carry high-energy electrons to the electron transport system. A small amount of ATP or its equivalent is also formed directly.
Electron Transport System
The electron transport system (ETS) is located in the inner mitochondrial membrane. Electrons from NADH and FADH₂ pass through a series of carriers. The released energy helps create a proton gradient.
As protons flow back through ATP synthase, ATP is produced. This process is called oxidative phosphorylation. Oxygen is the final electron acceptor and combines with electrons and protons to form water.
How Much ATP Is Produced?
The current NCERT chapter presents 38 ATP per glucose as a theoretical net yield for aerobic respiration under a set of simplifying assumptions. In a living cell, pathways operate together and intermediates may be used for other purposes, so this figure should not be treated as a fixed yield for every cell.
For revision, remember the central comparison: fermentation gives a net 2 ATP per glucose, while aerobic respiration can yield much more ATP.
| Feature | Fermentation | Aerobic respiration |
|---|---|---|
| Oxygen needed | No | Yes |
| Breakdown of glucose | Partial | More complete |
| ATP yield | Net 2 ATP per glucose | Much higher; NCERT’s theoretical balance sheet gives 38 |
| Main end products | Depend on pathway; ethanol and CO₂ or lactic acid | CO₂ and water |
Why Is Respiration an Amphibolic Pathway?
Respiratory pathways break down food molecules to release energy. Their intermediate compounds can also be used to build other molecules. Because the pathways participate in both breakdown (catabolism) and synthesis (anabolism), they are called amphibolic.
Respiratory Quotient (RQ)
The respiratory quotient compares carbon dioxide released with oxygen consumed during aerobic respiration:
RQ = Volume of CO₂ released ÷ Volume of O₂ consumed
The value depends on the respiratory substrate:
- Carbohydrates: RQ is usually 1 when completely oxidised.
- Fats: RQ is generally less than 1.
- Proteins: RQ is about 0.9 in the example discussed by NCERT.
Quick Revision
- Glycolysis: Glucose → two pyruvate molecules; occurs in the cytoplasm.
- Fermentation: Partial breakdown without oxygen; net gain of 2 ATP per glucose.
- TCA cycle: Occurs in the mitochondrial matrix.
- ETS: Occurs on the inner mitochondrial membrane.
- Oxygen: Final electron acceptor in aerobic respiration.
- Amphibolic pathway: Involved in both breakdown and synthesis.
- RQ: CO₂ released divided by O₂ consumed.
Check Your Understanding
- Where does glycolysis occur?
- What is the net ATP gain from glycolysis per glucose molecule?
- Which molecule is the final electron acceptor in aerobic respiration?
- Why is the respiratory pathway called amphibolic?
- What is the RQ for complete oxidation of a carbohydrate?
Answers: (1) Cytoplasm. (2) Two ATP. (3) Oxygen. (4) Its intermediates take part in both breakdown and synthesis. (5) One.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What is cellular respiration?
It is the breakdown of organic molecules to release energy that cells capture mainly as ATP.
What is glycolysis?
It is the pathway that converts one glucose molecule into two pyruvate molecules in the cytoplasm.
Does glycolysis require oxygen directly?
No; glycolysis itself does not directly use oxygen.
What happens to pyruvate under aerobic conditions?
It enters mitochondria, is converted to acetyl-CoA, and is further oxidised.
What is fermentation?
It is an anaerobic pathway that regenerates NAD⁺ and produces products such as ethanol or lactate.
Where does the TCA cycle occur in eukaryotic cells?
It occurs in the mitochondrial matrix.
What is another name for the TCA cycle?
It is also called the citric acid cycle or Krebs cycle.
Where is the electron transport chain located?
In eukaryotic cells, it is located in the inner mitochondrial membrane.
What is the role of oxygen in aerobic respiration?
Oxygen is the final electron acceptor in the electron transport chain and forms water.
What is oxidative phosphorylation?
It is ATP formation coupled to electron transport and the resulting proton gradient.
Why can total ATP yield vary?
It depends on shuttle systems, energy costs and how efficiently the cell couples electron transport to ATP synthesis.
What is the respiratory quotient (RQ)?
RQ is the ratio of CO₂ released to O₂ consumed during respiration.
What is the RQ for complete oxidation of glucose?
It is 1 because equal numbers of CO₂ and O₂ molecules are involved.
Why is respiration called an amphibolic pathway?
Its intermediates are used both in breakdown reactions and in building other molecules.
How do plants exchange respiratory gases?
Gases diffuse through structures such as stomata and lenticels and across cell surfaces.