Curriculum

Science Grade XI Biology

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Text lesson

Chapter 11: Photosynthesis in Higher Plants

Grade 11 Science | Chapter 12

Respiration in Plants

Respiration releases the energy stored in food. This chapter develops the respiration equation, glycolysis and the mitochondrial stages, aerobic and anaerobic respiration, and the role of ATP.

Core Concepts

Key Principles

Worked Examples

Practice Sets

Contents

Introduction: Releasing Energy

The Respiration Equation

Glycolysis and the Mitochondrial Stages

Aerobic and Anaerobic Respiration

ATP, the Energy Currency

Respiration and Photosynthesis Compared

Key Reasoning (Principles)

Worked Examples (10)

Practice Sets A to D

10.

Summary and Exam Quick-Check

1. Introduction: Releasing Energy

Making food in photosynthesis stores energy, but to use it a cell must release it again. Respiration is the process that breaks down glucose to release its stored energy in a form the cell can use. Every living cell respires, day and night, plant and animal alike. This chapter follows how the energy is released and captured.

Core idea

In respiration, glucose is broken down, usually with oxygen, to release energy. The energy is captured in molecules of ATP, the cell’s energy currency, with carbon dioxide and water as products.

2. The Respiration Equation

Aerobic respiration is the reverse of photosynthesis in summary. Glucose and oxygen react to give carbon dioxide, water and a large amount of energy. The glucose is the fuel, the oxygen lets it be fully broken down, and the energy released is captured to power the cell’s activities.

Diagram 1 – The Respiration Equation

The word equation of aerobic respiration

Fig 1. Glucose and oxygen are broken down to carbon dioxide, water and energy.

3. Glycolysis and the Mitochondrial Stages

Respiration occurs in stages. It begins with glycolysis in the cytoplasm, where glucose is split into smaller molecules, releasing a little energy. These molecules then enter the mitochondrion, where the Krebs cycle and the electron transport chain release most of the energy, in the presence of oxygen. This is why the mitochondrion is the powerhouse of the cell.

Diagram 2 – The Stages of Respiration

Glycolysis in the cytoplasm leading to the mitochondrial stages and ATP

Fig 2. Glycolysis in the cytoplasm feeds the mitochondrion, where most of the ATP is made.

4. Aerobic and Anaerobic Respiration

Respiration can happen with or without oxygen. Aerobic respiration uses oxygen, fully breaks down glucose, and releases a large amount of energy, giving carbon dioxide and water. Anaerobic respiration happens without oxygen, only partly breaks down glucose, and releases far less energy, giving alcohol in plants and yeast, or lactic acid in animal muscle.

Diagram 3 – Aerobic and Anaerobic Respiration

A comparison of aerobic and anaerobic respiration

Fig 3. Aerobic respiration uses oxygen and releases much energy; anaerobic respiration works without oxygen and releases little.

5. ATP, the Energy Currency

The energy released in respiration is not used directly; it is first captured in a molecule called ATP. ATP acts as the cell’s energy currency, storing energy in a form ready to be spent on any task, from building molecules to moving materials. Aerobic respiration makes far more ATP from each glucose than anaerobic respiration can.

6. Respiration and Photosynthesis Compared

Respiration and photosynthesis are opposite in summary but both essential. Photosynthesis builds glucose and stores energy, using carbon dioxide and water and giving out oxygen, and needs light. Respiration breaks glucose down and releases energy, using oxygen and giving out carbon dioxide and water, at all times. Together they cycle carbon and energy through living things.

7. Key Reasoning (Principles)

Principle 1: Respiration releases stored energy

Glucose holds the energy captured in photosynthesis, and respiration breaks it down to release that energy in a usable form for the cell.

Principle 2: Oxygen allows full breakdown

With oxygen, glucose is fully broken down and releases much energy; without it, only a partial breakdown is possible, releasing far less.

Principle 3: ATP carries the energy

The released energy is captured in ATP, a ready to spend currency, so the cell can power its many activities on demand.

8. Worked Examples

Example 1

Q: What is the purpose of respiration?

▶ Show Solution

To release the energy stored in glucose for the cell to use.

Answer: To release stored energy.

Example 2

Q: Write the word equation of aerobic respiration.

▶ Show Solution

Glucose plus oxygen gives carbon dioxide plus water plus energy.

Answer: Glucose + oxygen gives CO2 + water + energy.

Example 3

Q: Where does glycolysis take place?

▶ Show Solution

Glycolysis takes place in the cytoplasm.

Answer: In the cytoplasm.

Example 4

Q: Where is most of the energy released?

▶ Show Solution

In the mitochondrion, in the Krebs cycle and electron transport.

Answer: In the mitochondrion.

Example 5

Q: What is the difference between aerobic and anaerobic respiration?

▶ Show Solution

Aerobic uses oxygen and releases much energy; anaerobic uses none and releases little.

Answer: Use of oxygen and amount of energy.

Example 6

Q: What does anaerobic respiration produce in plants and yeast?

▶ Show Solution

Alcohol (and carbon dioxide).

Answer: Alcohol.

Example 7

Q: What does anaerobic respiration produce in animal muscle?

▶ Show Solution

Lactic acid.

Answer: Lactic acid.

Example 8

Q: What molecule captures the released energy?

▶ Show Solution

ATP captures the energy.

Answer: ATP.

Example 9

Q: Which releases more energy per glucose, aerobic or anaerobic?

▶ Show Solution

Aerobic respiration releases far more energy.

Answer: Aerobic.

Example 10

Q: Do plant cells respire only at night?

▶ Show Solution

No, every living cell respires at all times.

Answer: No, all the time.

9. Practice Sets A to D

Set A – Multiple Choice (Basic)

1. Respiration releases energy from: (a) water (b) glucose (c) oxygen (d) carbon dioxide

2. Glycolysis occurs in the: (a) nucleus (b) cytoplasm (c) mitochondrion (d) chloroplast

3. Most ATP is made in the: (a) cytoplasm (b) mitochondrion (c) wall (d) vacuole

4. Anaerobic respiration in muscle makes: (a) alcohol (b) lactic acid (c) glucose (d) oxygen

5. The energy currency of the cell is: (a) DNA (b) ATP (c) glucose (d) water

▶ Reveal Answers

1. (b) glucose.

2. (b) cytoplasm.

3. (b) mitochondrion.

4. (b) lactic acid.

5. (b) ATP.

Set B – Short Answer (Understanding)

1. State the purpose of respiration.

2. Write the aerobic respiration equation.

3. Name the two main locations of the stages.

4. Give the products of anaerobic respiration in yeast and in muscle.

5. What is the role of ATP?

▶ Reveal Answers

1. To release the energy stored in glucose in a form the cell can use.

2. Glucose plus oxygen gives carbon dioxide plus water plus energy.

3. Glycolysis in the cytoplasm and the later stages in the mitochondrion.

4. Alcohol in yeast and lactic acid in muscle.

5. It captures and carries the released energy, ready to be spent by the cell.

Set C – Application and Reasoning

1. A muscle works hard with little oxygen. Which respiration occurs?

2. Why does aerobic respiration release more energy than anaerobic?

3. Why is the mitochondrion called the powerhouse?

4. Do plants respire during the day?

5. Why is respiration described as almost the reverse of photosynthesis?

▶ Reveal Answers

1. Anaerobic respiration.

2. Because oxygen allows glucose to be fully broken down, releasing much more energy.

3. Because most of the cell’s energy is released there.

4. Yes, plants respire at all times, day and night.

5. Because it uses glucose and oxygen to give carbon dioxide and water, the opposite of what photosynthesis does.

Set D – Higher Order (Challenge)

1. Explain how photosynthesis and respiration together cycle energy and carbon.

2. Explain why a sprinter’s muscles may switch to anaerobic respiration.

3. Explain why ATP is needed rather than using glucose energy directly.

4. A yeast in a sealed jar of sugar produces alcohol. Explain.

5. Compare the energy yield and products of the two kinds of respiration.

▶ Reveal Answers

1. Photosynthesis stores energy in glucose using carbon dioxide and releasing oxygen; respiration releases that energy using oxygen and returning carbon dioxide, so the two cycle energy and carbon.

2. Because hard work outpaces the oxygen supply, so muscles release some energy without oxygen, producing lactic acid.

3. Because ATP carries energy in small, ready to use amounts that can power any task on demand.

4. With no oxygen, the yeast respires anaerobically, breaking sugar down to alcohol and carbon dioxide.

5. Aerobic uses oxygen, gives carbon dioxide and water, and much ATP; anaerobic uses none, gives alcohol or lactic acid, and little ATP.

Chapter Summary

Purpose

Release the energy stored in glucose.

Equation

Glucose plus oxygen gives carbon dioxide, water and energy.

Glycolysis

In the cytoplasm; splits glucose.

Mitochondrial Stages

Krebs cycle and electron transport release most energy.

Aerobic vs Anaerobic

With oxygen, much energy; without, little (alcohol or lactic acid).

ATP

Captures and carries the released energy.

Quantity

Unit

Symbol

Fuel

glucose

–

Most ATP

mitochondrion

–

Energy carrier

ATP

–

8-Point Exam Quick-Check

Respiration releases the energy stored in glucose.

Glucose plus oxygen gives carbon dioxide, water and energy.

Glycolysis is in the cytoplasm; most energy is released in the mitochondrion.

Aerobic respiration uses oxygen and releases much energy.

Anaerobic gives alcohol (yeast) or lactic acid (muscle) and little energy.

Energy is captured in ATP, the cell’s energy currency.

Every living cell respires at all times.

Respiration is almost the reverse of photosynthesis.

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Class 11 Biology Chapter 12: Respiration in Plants, Complete Notes and Practice

This revision guide follows the current NCERT Class 11 Biology syllabus and develops respiration, covering its purpose of releasing energy, the aerobic equation, glycolysis in the cytoplasm and the Krebs cycle and electron transport in the mitochondrion, the difference between aerobic and anaerobic respiration, and the role of ATP as the energy currency, with three diagrams, ten worked examples and graded practice. Visit SchoolRevise.com to revise, practise and excel.