What is respiration in plants class 11

Respiration in Plants

When we think of plants, we often associate them with the process of photosynthesis—creating food using sunlight. But just like all other living organisms, plants also need energy to survive and carry out vital life processes. This energy comes from a process called respiration.

Respiration in plants is a complex, biochemical process through which plants break down food (mainly glucose) to release energy in the form of ATP (Adenosine Triphosphate). Unlike animals, plants do not have specialized respiratory organs. Instead, each living cell in a plant respires independently, making this process both unique and fascinating.

This chapter explores:

The fundamental steps of cellular respiration
How glucose is broken down during glycolysis, the Krebs cycle, and the electron transport chain
The differences between aerobic and anaerobic respiration
The concept of Respiratory Quotient (RQ)
The importance of mitochondria, also called the "powerhouse of the cell"

Why Do Plants Respire?

Just like animals, plants also need energy to perform various life-sustaining functions such as:

Cell division and growth
Repair of damaged tissues
Active transport of minerals and water
Opening and closing of stomata
Biosynthesis of organic compounds

Although plants produce glucose during photosynthesis, they cannot use this energy directly. Instead, they must break down glucose molecules through the process of cellular respiration to release usable energy in the form of ATP (Adenosine Triphosphate).

So, why do plants respire?

Because every living cell in a plant needs energy—even at night when photosynthesis stops. Respiration ensures a constant supply of ATP for all cellular activities, making it an essential process for the plant's survival, growth, and development.

Types of Respiration in Plants

Respiration is the process by which plants break down food (mainly glucose) to release energy. But did you know that plants can respire in different ways depending on the availability of oxygen? Yes, the environment plays a key role in determining how respiration occurs.

There are two main types of respiration in plants:

1. Aerobic Respiration – Takes place in the presence of oxygen
2. Anaerobic Respiration – Takes place in the absence of oxygen

Difference Between Aerobic and Anaerobic Respiration

Feature	Aerobic Respiration	Anaerobic Respiration
Oxygen Requirement	Requires oxygen	Does not require oxygen
End Products	Carbon dioxide and water	Alcohol (ethanol) and carbon dioxide (in plants)
Energy Yield (ATP)	High (about 36-38 ATP per glucose molecule)	Low (about 2 ATP per glucose molecule)
Location in Cell	Occurs in mitochondria	Occurs in cytoplasm
Occurrence	Most of the time in all plant cells	In certain conditions (e.g., germinating seeds, low oxygen)
Example	Normal plant respiration during the day/night	Fermentation in germinating seeds in waterlogged soil

In most cases, plants respire aerobically, breaking down glucose completely into carbon dioxide and water, and releasing a large amount of energy (ATP). However, in oxygen-deficient conditions (like in waterlogged soils), anaerobic respiration may occur, leading to the formation of ethanol or lactic acid with less ATP.

Glycolysis – Occurs in Cytoplasm

Glycolysis is the first step of cellular respiration and occurs in the cytoplasm of all living cells, including plant cells. The term "glycolysis" literally means “splitting of glucose.”
In this process, one molecule of glucose (a 6-carbon compound) is broken down into two molecules of pyruvate (a 3-carbon compound). This step does not require oxygen, so glycolysis occurs in both aerobic and anaerobic respiration.

During glycolysis:
A small amount of energy is released
2 ATP molecules are used, and 4 ATP molecules are produced (net gain = 2 ATP)
NADH, an energy-rich molecule, is also formed

Point: Glycolysis is a universal pathway—it occurs in almost all organisms, from bacteria to humans, highlighting its evolutionary importance.

Fermentation (Anaerobic Respiration)

When oxygen is not available, cells still need a way to release energy from food. In such conditions, plants (and some microorganisms) use a process called fermentation, also known as anaerobic respiration.
Fermentation is an incomplete breakdown of glucose that occurs without oxygen, typically in the cytoplasm. Although this process produces far less energy than aerobic respiration, it allows cells to survive and continue functioning under anaerobic conditions—like in waterlogged soils or during rapid growth.

There are two common types of fermentation:

1. Alcoholic Fermentation – Produces ethanol and CO₂ (seen in yeast and some plant tissues)

2. Lactic Acid Fermentation – Produces lactic acid (common in animal muscles, but rare in plants)

Aerobic Respiration

Aerobic respiration is the most common and efficient way by which plants (and most organisms) release energy from food. This process takes place in the presence of oxygen and results in the complete breakdown of glucose into carbon dioxide and water, with a high yield of energy.

Features of Aerobic Respiration:

Occurs in mitochondria (after glycolysis)
Produces a large amount of ATP (up to 36–38 ATP molecules per glucose)

Involves three main stages:

1. Glycolysis (in cytoplasm)
2. Krebs Cycle (in mitochondrial matrix)
3. Electron Transport Chain (ETC) (on inner mitochondrial membrane)

Glycolysis: The First Step of Energy Production (Takes Place in Cytoplasm)

Glycolysis is the first and most essential step in the process of breaking down glucose to release energy. This step happens in the cytoplasm of the cell and does not require oxygen, which means it is an anaerobic process.

What Happens in Glycolysis?

One molecule of glucose (6-carbon) is broken down into two molecules of pyruvate (3-carbon each).
Along the way, the cell gains:

2 ATP (net energy gain)
2 NADH (electron carriers for later use)

Why is Glycolysis Important?

It provides a quick burst of energy.
It prepares the cell for further steps in aerobic respiration (Krebs cycle, Electron Transport Chain) if oxygen is available.
In the absence of oxygen, the pyruvate can undergo fermentation to keep producing ATP.

In plants, aerobic respiration is continuous, even when photosynthesis is not happening (e.g., at night), as it supplies the energy needed for cellular processes like growth, transport, and repair.

Glycolysis Equation:

Glucose (C_{6} H_{12} O_{6}) + 2 NAD^{+} + 2 ADP + 2 P_{i} \to 2 Pyruvate (C_{3} H_{4} O_{3}) + 2 NADH + 2 ATP + 2 H_{2} O + 2 H^{+}

Krebs Cycle – Occurs in Mitochondrial Matrix

The Krebs Cycle, also known as the Citric Acid Cycle or TCA Cycle (Tricarboxylic Acid Cycle), is the second stage of aerobic respiration. This cycle plays a central role in the complete oxidation of glucose and the release of usable energy in the form of ATP.

Where does it happen?

It occurs in the mitochondrial matrix – the inner space of mitochondria – often called the cell’s powerhouse because this is where most of the energy is generated.

What happens in this cycle?

Before entering the Krebs Cycle, pyruvate (from glycolysis) is converted into Acetyl-CoA. Acetyl-CoA then enters the cycle and undergoes a series of enzyme-controlled reactions that:

Release CO₂ as a waste product
Generate high-energy molecules: NADH and FADH₂
Produce a small amount of ATP

Though the ATP yield is modest in this stage, NADH and FADH₂ produced here are crucial for the Electron Transport Chain, where the majority of ATP is formed.

Electron Transport Chain (ETC) – Inner Mitochondrial Membrane

The Electron Transport Chain (ETC) is the final and most energy-yielding stage of aerobic respiration. It takes place in the inner mitochondrial membrane (also called the cristae), where a series of protein complexes work together to produce large amounts of ATP – the energy currency of the cell.

What is ETC?

The ETC is a chain of protein complexes and electron carriers that receive high-energy electrons from NADH and FADH₂, which are produced during glycolysis and the Krebs cycle.

What Happens in ETC?

1. Electrons are passed from NADH and FADH₂ through a series of membrane-bound complexes.
2. As electrons move along the chain, protons (H⁺ ions) are pumped into the intermembrane space, creating a proton gradient.
3. This gradient powers ATP synthase, an enzyme that produces ATP from ADP and inorganic phosphate.
4. At the end of the chain, oxygen acts as the final electron acceptor and combines with protons and electrons to form water (H₂O).

Important Points for NEET

Glycolysis occurs in cytoplasm, not mitochondria.
Pyruvate is the key intermediate in respiration.
NADH and FADH₂ are electron carriers.
Oxygen is needed only in ETC, not in glycolysis or Krebs cycle.

Anaerobic respiration produces ethanol in plants/yeast, lactic acid in animals.

It produces about 32–34 ATP molecules per glucose, making it the most energy-efficient stage.
It requires oxygen, which is why it's part of aerobic respiration.

Energy Yield

One of the most important outcomes of cellular respiration is the production of energy in the form of ATP (Adenosine Triphosphate) — the universal energy currency of the cell. The efficiency of respiration is measured by how much ATP is generated from the complete breakdown of one glucose molecule.

In aerobic respiration, glucose is fully oxidized into carbon dioxide and water through three main stages:

1. Glycolysis (in cytoplasm)
2. Krebs Cycle (in mitochondrial matrix)
3. Electron Transport Chain (ETC) (on inner mitochondrial membrane)

Each stage contributes a specific number of ATP molecules, either directly or indirectly through energy carriers like NADH and FADH₂. These carriers later release energy in the ETC to form additional ATP molecules.

Why Study Energy Yield?

To understand how efficiently cells produce energy
To compare aerobic vs anaerobic respiration
To know how each step contributes to the total ATP production
To prepare for conceptual and numerical NEET questions

Respiratory Quotient (RQ)

The Respiratory Quotient (RQ) is an important concept in plant respiration that helps us understand which type of substrate (carbohydrate, fat, or protein) a plant or organism is using to produce energy.

What is Respiratory Quotient?

The Respiratory Quotient (RQ) is defined as the ratio of the volume of carbon dioxide (CO₂) evolved to the volume of oxygen (O₂) consumed during respiration.