Respiration in Plants

Respiration is not the physical movement of air (that’s breathing). In plants, respiration is the biochemical breakdown of glucose using oxygen to release energy, producing CO₂ and H₂O. In plants, respiration is the catabolic mechanism through which energy for plant development is produced.

Process of Respiration

Respiration (Latin respirare, breathing) is the inhalation or uptake of O₂ and the expiration or liberation of CO₂, and usage of the O₂ to oxidise the nutrients to obtain energy.

External Respiration

External respiration in plants refers to gaseous exchange (O₂ intake, CO₂ release) via roots, stems, and leaves.

Respiration In Roots

Roots absorb oxygen from the air present in soil spaces between soil particles, which is used for respiration. The absorbed oxygen liberates energy utilised to transport salts and minerals from the soil.

Respiration In Stems

Lenticels present in higher plants or woody plants perform gaseous exchange.

Respiration In Leaves

Gaseous exchange occurs through diffusion via stomata (the tiny pores present in the lower epidermis of the leaf). The turgidity of two guard cells regulates each stomata. Exchange of gases occurs with the closing and opening of the stoma.

Internal Respiration or Cellular Respiration

It is the biochemical reaction where the food is oxidised to liberate CO₂, water and energy. The process occurs in the cytosol and mitochondria of each cell, with some specialities in certain cells. Cellular respiration might be

Aerobic Respiration: oxidation in the presence of free O₂.
Anaerobic Respiration: oxidation in the absence of free O₂.

Aerobic Respiration

It occurs in four steps in the cytosol and mitochondria.

Glycolysis: The process occurs in the cytosol. One molecule of the six-carbon sugar glucose undergoes a series of chemical transformations.

NAD⁺ → NADH
Two molecules of ATP and two molecules of pyruvate (a three-carbon compound).

Pyruvate oxidation: Each pyruvate goes into the mitochondrial matrix and is converted into acetate (a two-carbon compound) that remains bound to coenzyme A.

Acetyl-CoA formed
NADH is generated
CO₂ is released

Kreb’s Cycle (TCA cycle): In the mitochondrial matrix:

Acetyl-CoA combines with a 4-carbon molecule and goes through a cycle of reactions
Acetyl-CoA regenerates the four-carbon starting molecule
ATP, NADH, FADH₂ and CO₂ are released

Oxidative phosphorylation: The final stage of aerobic respiration, where most ATP is produced.

NADH and FADH₂ donate electrons to the electron transport chain (ETC) in the inner mitochondrial membrane.
Electron transfer releases energy. It pumps protons (H⁺) across the membrane and creates a proton gradient.
Protons flow back through ATP synthase. It drives ATP production (chemiosmosis).
Oxygen acts as the final electron acceptor and it combines with H⁺ to form water.

Glycolysis

The steps of glycolysis are in the figure below:

Pyruvate Oxidation

The multienzyme system Pyruvic Dehydrogenase Complex is present within the mitochondria matrix. It catalyses the oxidative decarboxylation of pyruvate. In this reaction, decarboxylation of the 3-carbon compound pyruvate occurs to form a 2-carbon compound. This binds to CoA-SH to form acetyl-CoA.

Kreb’s Cycle

The 2-carbon compound acetyl-CoA combines with the 4-carbon compound oxaloacetic acid to form a 6-carbon compound citric acid. Through a cyclical series of reactions, this citric acid degrades and regenerates oxaloacetic acid and liberates CO₂ and hydrogen atoms.

Oxidative Phosphorylation

A series of proteins and organic molecules is found in the inner membrane of the mitochondria, which is known as the electron transport chain. In a series of redox reactions, electrons are passed from one member of the transport chain to liberate energy. Energy thus released is captured as a proton gradient and is used to make ATP through chemiosmosis. The electron transport chain and chemiosmosis together make up oxidative phosphorylation. Key steps include-

Delivery of electrons by NADH and FADH₂

The reduced electron carriers NADH and FADH₂ (from the previous steps) transfer their electrons to molecules at the beginning of the electron transport chain. As a result, they turn back into NAD⁺ and FAD, which are reused.

Electron transfer and proton pumping

Protons are pumped from the matrix into the intermembrane space. In this way, they create the proton gradient. Electrons move down the chain. Energy is released which pumps H⁺ ions from the mitochondrial matrix into the intermembrane space.

Splitting of oxygen to form water

At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and combines with H⁺ to form water.

Synthesis of ATP.

H⁺ ions, while moving down their gradient, pass through the ATP synthase enzyme, which helps to synthesise ATP.

Energy Yield During Aerobic Respiration

It is estimated that the maximum ATP yield for one molecule of glucose is around 30–32 molecules of ATP. Synthesis of one ATP molecule is powered by four H⁺ ions; electrons from NADH drive the movement of 10 H⁺ ions, so each NADH yields about 2.5 ATP, and similarly, FADH₂ yields about 1.5 ATP. The detailed calculation is mentioned in the table below.

Stage	Direct Products	Ultimate ATP Yield (net)
Glycolysis	2 ATP	2 ATP
Glycolysis	2 NADH	3–5 ATP
Pyruvate Oxidation	2 NADH	5 ATP
Kreb’s Cycle	2 GTP	2 ATP
	6 NADH	15 ATP
	2 FADH₂	3 ATP
Total		30–32 ATP

The equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (~686 Kcal)

Anaerobic Respiration

Anaerobic cellular respiration is a process of oxidation of food without the participation of oxygen, found in the prokaryotes living in low-oxygen environments. The Electron Transport Chain may or may not be present. If present, the final electron acceptor at the end of the electron transport chain might be sulphur, sulphate (SO₄²⁻), nitrate (NO₃⁻), carbon dioxide (CO₂), or some other molecules.

Under anaerobic conditions (like in waterlogged soils), some higher plant tissues (e.g., germinating seeds) can temporarily respire without oxygen. Glucose is broken down to ethanol and CO₂, with 2 ATP released. This is alcoholic fermentation. Prolonged alcohol accumulation is toxic and damages plant cells.

After the process of glycolysis, pyruvate is converted to acetaldehyde due to the action of the enzyme pyruvate decarboxylase and the coenzyme thiamine pyrophosphate.
Alcohol dehydrogenase enzyme transforms acetaldehyde to ethanol (ethyl alcohol).
NADH, produced during glycolysis, results in the production of 2 ATP.

The equation is C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ + 2 ATP

Frequently Asked Questions (FAQs)

Q1. How is respiration in plants different from photosynthesis?

In respiration, glucose breaks down to release energy (ATP). In photosynthesis, sunlight, water, and carbon dioxide make glucose.

Q2. Do plants respire only at night?

Respiration happens all the time. Photosynthesis occurs simultaneously during the day. It can mask respiration.

Q3. Why do some plants switch to anaerobic respiration?

When oxygen is unavailable (e.g., in waterlogged soils), plant tissues may temporarily switch to anaerobic respiration (alcoholic fermentation) to produce small amounts of ATP. However, this can harm cells if prolonged.