Respiration Class 10 Science — Complete NCERT Notes
Respiration is one of the most important life processes because every living cell needs energy to carry out its activities. From walking and growing to repairing cells and transporting substances, almost everything your body does requires energy.
But where does this energy come from? The answer begins with food and ends with the release of usable energy inside our cells.
📚 Table of Contents — Click to Expand
- What Is Respiration?
- Why Do Living Organisms Need Respiration?
- Respiration Is a Cellular Process
- How Is Energy Released from Glucose?
- Aerobic Respiration
- Anaerobic Respiration
- Aerobic vs Anaerobic Respiration
- Anaerobic Respiration in Yeast
- Respiration in Our Muscles
- ATP — The Energy Currency of the Cell
- Respiration in Plants
- Respiration in Human Beings
- Breathing vs Respiration
- Exchange of Gases
- Common Mistakes
- Exam Focus
- Practice Questions
- Answers with Explanations
- Frequently Asked Questions
- One-Page Revision Sheet
- Flashcards
- Final Takeaway
1. What Is Respiration?
Technical definition: Respiration is the process in which food substances are broken down in cells to release energy.
In simple words: The food we eat contains stored chemical energy. During respiration, this food is broken down and the stored energy is released in a form that the cells can use.
| What enters | What happens | What is obtained |
|---|---|---|
| Food, especially glucose | Glucose is broken down | Energy is released |
⭐ CORE CONCEPT
Respiration is not simply breathing. Respiration is a chemical process occurring inside cells, while breathing is the physical process of taking in oxygen and giving out carbon dioxide.
2. Why Do Living Organisms Need Respiration?
All living organisms perform various activities. These activities require energy.
- Growth requires energy.
- Movement requires energy.
- Repair of damaged cells requires energy.
- Transport of substances requires energy.
- Reproduction requires energy.
- Maintenance of normal body functions requires energy.
This energy is obtained by breaking down food during respiration.
| Food | Respiration | Energy |
|---|---|---|
| Contains stored chemical energy | Breaks down food molecules | Becomes available for cellular activities |
🔥 IMPORTANT
Without a continuous supply of usable energy, cells cannot carry out their normal life processes.
3. Respiration Is a Cellular Process
Respiration takes place inside cells. This is why it is often described as a cellular process.
Food such as carbohydrates is ultimately converted into simpler substances such as glucose. The glucose molecule can then be broken down to release energy.
One important stage of glucose breakdown occurs in the cytoplasm, where glucose is converted into a three-carbon compound called pyruvate.
📌 REMEMBER
Glucose → Pyruvate
This initial breakdown takes place in the cytoplasm.
4. How Is Energy Released from Glucose?
The complete breakdown of glucose does not necessarily occur in one single step. The process begins with the formation of pyruvate.
What happens to pyruvate next depends on the availability of oxygen and the organism or cell involved.
| Condition | Major pathway | Products |
|---|---|---|
| Oxygen available | Aerobic respiration | Carbon dioxide + water + energy |
| Oxygen not available | Anaerobic respiration | Different products depending on the organism |
5. Aerobic Respiration
Aerobic respiration is respiration in which glucose is broken down in the presence of oxygen, releasing a large amount of energy.
The overall process can be represented as:
| Glucose + Oxygen → Carbon dioxide + Water + Energy |
In aerobic respiration, pyruvate is further broken down in the mitochondria. Carbon dioxide and water are formed and energy is released.
🎯 EXAM FOCUS
For Class 10, remember this sequence:
Glucose → Pyruvate → Further breakdown in mitochondria → Carbon dioxide + Water + Energy
Why Is Aerobic Respiration Efficient?
Because glucose is broken down more completely in the presence of oxygen, a larger amount of energy becomes available to the cell.
6. Anaerobic Respiration
Anaerobic respiration occurs when glucose is broken down without using oxygen.
It releases less energy than aerobic respiration.
The fate of pyruvate under anaerobic conditions can differ between organisms.
In Yeast
In yeast, pyruvate is converted into ethanol and carbon dioxide, with the release of some energy.
This process is called fermentation.
| Starting substance | Products |
|---|---|
| Pyruvate | Ethanol + Carbon dioxide + Energy |
7. Anaerobic Respiration in Yeast
Yeast is a useful example for understanding anaerobic respiration.
When oxygen is unavailable, yeast can obtain energy through fermentation. The products formed include ethanol and carbon dioxide.
This ability of yeast is useful in several industries, including baking and the production of alcoholic beverages.
🧠 MEMORY TRICK
Yeast without oxygen → Alcohol + CO₂
8. What Happens in Our Muscles During Heavy Exercise?
During vigorous physical activity, our muscles need a large amount of energy. Sometimes the supply of oxygen to muscle cells is not sufficient for their energy requirement.
Under such conditions, pyruvate can be converted into lactic acid.
This anaerobic pathway releases less energy.
🔥 VERY IMPORTANT
In muscle cells under oxygen deficiency:
Pyruvate → Lactic acid + Energy
The accumulation of lactic acid is associated with muscle cramps during strenuous physical activity.
9. ATP — The Energy Currency of the Cell
The energy released during respiration is used to form a molecule called ATP.
ATP stands for adenosine triphosphate.
ATP stores energy in a form that can be readily used by the cell for different activities.
| Term | Meaning |
|---|---|
| ATP | Adenosine triphosphate |
| Role | Provides usable energy for cellular activities |
For example, energy supplied through ATP can be used for muscle movement and other energy-requiring cellular processes.
📌 MUST REMEMBER
ATP is commonly described as the energy currency of the cell.
10. Respiration in Plants
Plants also need energy for their life processes. Therefore, plants respire continuously.
Plants exchange gases through different structures depending on the plant part.
| Plant part | Gas exchange structure |
|---|---|
| Leaves | Stomata |
| Stems | Lenticels, where present |
| Roots | Air spaces in the soil and root surfaces |
Roots obtain oxygen from the air present in the spaces between soil particles.
❌ Common Mistake
Plants do not respire only at night. Respiration occurs continuously, both during the day and at night.
11. Respiration in Human Beings
In humans, oxygen needed for aerobic respiration is obtained from the air through the respiratory system.
The pathway of air can be represented as:
| Nostrils → Nasal passage → Pharynx → Larynx → Trachea → Bronchi → Bronchioles → Alveoli |
Nasal Passage
Air enters through the nostrils. The nasal passage helps filter, warm and moisten the incoming air.
Trachea
The trachea is the windpipe through which air passes towards the lungs.
Bronchi and Bronchioles
The trachea divides into two main bronchi, one leading into each lung. These branches divide further into smaller tubes called bronchioles.
Alveoli — The Main Site of Gas Exchange
At the ends of the smallest branches are tiny balloon-like structures called alveoli.
Alveoli provide a large surface area for exchange of gases between the air and the blood.
⭐ CORE CONCEPT
The alveoli are adapted for efficient gas exchange because they provide a large surface area and have a rich blood supply.
12. Breathing vs Respiration
These two terms are related, but they are not identical.
| Breathing | Respiration |
|---|---|
| Physical process of taking in and giving out air | Biochemical process that releases energy from food |
| Involves respiratory organs | Occurs inside cells |
| Supplies oxygen and removes carbon dioxide | Uses food molecules to release usable energy |
| Does not itself release cellular energy from glucose | Releases energy from food |
🎯 EXAM TRAP
Do not write “breathing is respiration” in an examination. Breathing helps supply oxygen, whereas respiration is the cellular process through which energy is released from food.
13. Exchange of Gases in the Lungs
When we inhale, air containing oxygen reaches the alveoli.
The oxygen passes from the alveolar air into the blood. At the same time, carbon dioxide moves from the blood into the alveoli and is eventually exhaled.
| Gas | Direction |
|---|---|
| Oxygen | Alveoli → Blood |
| Carbon dioxide | Blood → Alveoli |
14. Aerobic vs Anaerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Oxygen | Required | Not required |
| Breakdown of glucose | More complete | Incomplete |
| Energy released | More | Less |
| Products | Carbon dioxide and water | Depend on organism/pathway |
| Example | Human cells under normal conditions | Yeast; muscles during oxygen deficiency |
15. Common Mistakes Students Make
| ❌ Common Mistake | ✅ Correct Understanding |
|---|---|
| Respiration and breathing are the same. | Breathing is physical; respiration is cellular and biochemical. |
| Plants only respire at night. | Plants respire continuously. |
| Anaerobic respiration releases more energy. | Anaerobic respiration releases less energy than aerobic respiration. |
| All anaerobic respiration produces alcohol. | Products depend on the organism and pathway. Yeast produces ethanol and CO₂; muscle cells can form lactic acid. |
| Respiration occurs only in the lungs. | Gas exchange occurs in the lungs, but cellular respiration occurs inside cells. |
16. Exam Focus — What You Must Know
🎯 CLASS 10 EXAM CHECKLIST
- Definition of respiration
- Difference between breathing and respiration
- Formation of pyruvate from glucose
- Aerobic respiration
- Anaerobic respiration
- Fermentation in yeast
- Lactic acid formation in muscles
- Role of ATP
- Respiration in plants
- Human respiratory pathway
- Structure and importance of alveoli
- Comparison between aerobic and anaerobic respiration
17. Quick Concept Map
| GLUCOSE |
| ↓ |
| PYRUVATE |
| ↙ ↘ |
| Aerobic pathway Anaerobic pathway |
| Oxygen available Oxygen unavailable |
| CO₂ + H₂O + more energy Different products + less energy |
18. Practice Questions
A. Very Short Answer Questions
- What is respiration?
- Where does the initial breakdown of glucose occur?
- What is pyruvate?
- What is ATP?
- Name the site of gas exchange in the lungs.
- What type of respiration occurs in yeast in the absence of oxygen?
- Which substance is formed in muscles during anaerobic respiration?
- Name the gas required for aerobic respiration.
B. MCQs
-
Respiration mainly helps cells to:
A. Produce sunlight
B. Release usable energy from food
C. Absorb minerals only
D. Produce water only -
The initial breakdown of glucose into pyruvate occurs in the:
A. Nucleus
B. Cytoplasm
C. Ribosome
D. Cell wall -
In yeast, anaerobic respiration produces:
A. Lactic acid only
B. Ethanol and carbon dioxide
C. Oxygen and water
D. Glucose and oxygen -
The structures where gaseous exchange occurs in human lungs are:
A. Trachea
B. Bronchi
C. Alveoli
D. Diaphragm -
Which generally releases more energy?
A. Aerobic respiration
B. Anaerobic respiration
C. Both always release exactly the same amount
D. Neither releases energy
C. Short Answer Questions
- Why is respiration necessary for living organisms?
- Differentiate between aerobic and anaerobic respiration.
- Why is ATP called the energy currency of the cell?
- What happens to pyruvate in yeast when oxygen is absent?
- Why may muscle cramps occur during vigorous exercise?
- Why are alveoli well suited for gas exchange?
- Why do plants need to respire even though they make food by photosynthesis?
D. Assertion and Reason
Assertion: Aerobic respiration releases more energy than anaerobic respiration.
Reason: Glucose undergoes a more complete breakdown during aerobic respiration.
Choose:
- A. Both Assertion and Reason are true, and Reason correctly explains Assertion.
- B. Both are true, but Reason does not explain Assertion.
- C. Assertion is true but Reason is false.
- D. Assertion is false but Reason is true.
E. HOTS / Application Questions
- A student runs very fast for several minutes and then experiences muscle cramps. Explain the possible connection between vigorous exercise, oxygen supply and respiration.
- Yeast is kept in conditions where oxygen is unavailable. Which pathway can it use and what products are formed?
- A student says, “Plants do not respire during the daytime because they are photosynthesising.” Do you agree? Give a reason.
- Why would damage to the alveoli seriously affect the supply of oxygen to body tissues?
19. Answers with Explanations
Very Short Answers
- Respiration: It is the cellular process in which food substances are broken down to release energy.
- Cytoplasm: The initial breakdown of glucose into pyruvate occurs in the cytoplasm.
- Pyruvate: It is a three-carbon compound formed during the initial breakdown of glucose.
- ATP: Adenosine triphosphate, a molecule used by cells as a readily usable source of energy.
- Alveoli: Gas exchange occurs across the walls of the alveoli.
- Fermentation: Yeast can carry out anaerobic respiration/fermentation in the absence of oxygen.
- Lactic acid: Muscle cells can form lactic acid under oxygen-deficient conditions.
- Oxygen: Aerobic respiration requires oxygen.
MCQ Answers
| Question | Answer | Explanation |
|---|---|---|
| 1 | B | Respiration releases usable energy from food. |
| 2 | B | The initial breakdown of glucose occurs in the cytoplasm. |
| 3 | B | Yeast can convert pyruvate into ethanol and carbon dioxide. |
| 4 | C | Alveoli provide the surface for exchange of oxygen and carbon dioxide. |
| 5 | A | Aerobic respiration generally releases much more energy because glucose is more completely broken down. |
Assertion and Reason
Answer: A
Both statements are true. Aerobic respiration involves a more complete breakdown of glucose, so more energy is released.
HOTS Answers
1. During vigorous exercise, muscles require a large amount of energy. If oxygen supply becomes insufficient, muscle cells may temporarily use an anaerobic pathway in which pyruvate is converted into lactic acid. This is associated with muscle cramps.
2. Yeast can use anaerobic respiration. Pyruvate is converted into ethanol and carbon dioxide, releasing some energy.
3. No. Plants respire continuously. Photosynthesis and respiration are different processes. Photosynthesis stores energy in food, whereas respiration releases usable energy from food.
4. Alveoli provide the surface through which oxygen enters the blood and carbon dioxide leaves it. Damage to alveoli can reduce the efficiency of gas exchange and therefore affect oxygen supply.
20. Frequently Asked Questions
Is breathing the same as respiration?
No. Breathing is the physical movement of air into and out of the respiratory system. Respiration is the cellular process through which energy is released from food.
Where does glucose breakdown begin?
The initial breakdown of glucose takes place in the cytoplasm and produces pyruvate.
Why does aerobic respiration release more energy?
Because glucose undergoes a more complete breakdown in the presence of oxygen.
Do plants respire during the day?
Yes. Plants respire continuously. During daylight, photosynthesis may also occur in green parts of plants.
Why is ATP important?
ATP provides energy in a readily usable form for many cellular activities.
Why are alveoli numerous and tiny?
The large number of alveoli provides a large surface area for efficient exchange of gases.
21. One-Page Revision Sheet
| Point | Remember |
|---|---|
| Respiration | Cellular process that releases energy from food |
| Initial glucose breakdown | Glucose → Pyruvate |
| Location of initial breakdown | Cytoplasm |
| Aerobic respiration | Uses oxygen; releases more energy |
| Aerobic products | Carbon dioxide + water |
| Yeast | Ethanol + carbon dioxide under anaerobic conditions |
| Muscle cells | Lactic acid may form when oxygen is insufficient |
| ATP | Energy currency of the cell |
| Gas exchange | Occurs at alveoli |
| Plants | Respire continuously |
🧠 FIVE-SECOND RECALL
Food → Glucose → Pyruvate → Respiration → Energy
22. Flashcards
| Question | Answer |
|---|---|
| What is respiration? | Cellular breakdown of food to release energy. |
| What is pyruvate? | A three-carbon compound formed during the initial breakdown of glucose. |
| Where is pyruvate formed? | In the cytoplasm during the initial breakdown of glucose. |
| What is aerobic respiration? | Respiration using oxygen. |
| What are major products of aerobic respiration? | Carbon dioxide and water, with energy released. |
| What happens to pyruvate in yeast without oxygen? | It is converted into ethanol and carbon dioxide. |
| What can form in muscles during oxygen deficiency? | Lactic acid. |
| What is ATP? | Adenosine triphosphate. |
| Why is ATP called the energy currency? | It provides energy in a readily usable form for cellular activities. |
| Where does gas exchange occur in the lungs? | At the alveoli. |
| Do plants respire during daytime? | Yes, respiration occurs continuously. |
| What is the key difference between breathing and respiration? | Breathing is physical gas movement; respiration is cellular energy-releasing chemistry. |
23. Final Takeaway
🌟 REMEMBER THESE CORE IDEAS
- Respiration releases usable energy from food.
- The initial breakdown of glucose produces pyruvate in the cytoplasm.
- With oxygen, aerobic respiration releases more energy.
- Without sufficient oxygen, anaerobic pathways may occur.
- Yeast can produce ethanol and carbon dioxide during fermentation.
- Muscle cells may produce lactic acid when oxygen is insufficient.
- ATP acts as the energy currency of the cell.
- Plants also respire continuously.
- Breathing and respiration are not the same process.
- Alveoli are the major sites of gaseous exchange in human lungs.
📚 Understand the process, remember the pathway, and respiration becomes much easier to master.
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