Compensation Point and Gas Exchange in Plants | Class 10

Quick answer: The compensation point is the light intensity at which a plant's rate of photosynthesis equals its rate of respiration, so its net exchange of oxygen and carbon dioxide is zero. It is usually reached twice a day, around dawn and dusk. In bright daylight, plants take in carbon dioxide and give out oxygen overall. At night, with no photosynthesis, they take in oxygen and give out carbon dioxide.

Green plants carry out two opposite processes. Photosynthesis uses sunlight to make glucose, taking in carbon dioxide and giving out oxygen. Respiration breaks down glucose to release energy, taking in oxygen and giving out carbon dioxide. Respiration goes on all the time, day and night, in every living cell, but photosynthesis happens only when there is light.

When light is dim, such as early in the morning or late in the evening, there is a point where photosynthesis and respiration happen at exactly the same rate. This balance point is called the compensation point.

In CBSE Class 10 Science, Chapter 5 (Life Processes), gaseous exchange in plants during the day and night is a frequently asked topic, often together with the question why we should not sleep under a tree at night.

Syllabus note: The NCERT textbook explains how gas exchange in plants differs between day and night. The term "compensation point" itself goes slightly beyond the textbook, but it is a useful way to understand and explain the same idea.

What You Will Learn

• What the compensation point is
• The equations for photosynthesis and respiration, and how they balance
• Why the compensation point is reached around dawn and dusk
• How gas exchange in plants differs between day and night
• The reason behind the advice not to sleep under a tree at night

1. Photosynthesis vs Respiration

Photosynthesis:

$6\mathrm{CO_2} + 12\mathrm{H_2O} \xrightarrow{\text{sunlight, chlorophyll}} \mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} + 6\mathrm{H_2O}$

Aerobic respiration:

$\mathrm{C_6H_{12}O_6} + 6\mathrm{O_2} \longrightarrow 6\mathrm{CO_2} + 6\mathrm{H_2O} + \text{energy}$

2. What Is the Compensation Point?

Definition: The compensation point is the light intensity at which the rate of photosynthesis equals the rate of respiration, so there is no net exchange of oxygen or carbon dioxide between the plant and the air.

At the compensation point:

• All the carbon dioxide released by respiration is used up in photosynthesis.
• All the oxygen released by photosynthesis is used up in respiration.
• The plant takes in no extra gas from the air and gives out no extra gas.

Both processes are still happening; they simply cancel each other out.

When Does It Happen?

Light intensity changes gradually through the day, so the compensation point is usually reached twice a day:

1. In the early morning, as the light gets brighter.
2. In the evening, as the light fades.

 Rate
  ^          photosynthesis
  |             * * *
  |           *       *
  | - - - - * - - - - -*- - - respiration
  |       *              *
  +------*----------------*----> time
       dawn              dusk
  (compensation points where the lines cross)

Extra fact: Scientists also describe a carbon dioxide compensation point, the carbon dioxide level at which photosynthesis and respiration balance. For Class 10, the light-based explanation above is enough.

3. Gas Exchange During the Day and Night

During the Day

• Photosynthesis is much faster than respiration.
• The carbon dioxide released by respiration is used up in photosynthesis, and the plant also takes in extra carbon dioxide from the air.
• The plant produces more oxygen than it needs, so oxygen release is the major event during the day.

During the Night

• There is no light, so photosynthesis stops.
• Respiration continues.
• The plant takes in oxygen and gives out carbon dioxide, so carbon dioxide release is the major event at night.

4. Why Should We Not Sleep Under a Tree at Night?

Common board question: It is often advised not to sleep under a tree at night. Give the scientific reason.

Answer:

1. During the day, trees carry out photosynthesis and release oxygen.
2. At night, there is no light, so photosynthesis stops, but respiration continues.
3. So at night, trees take in oxygen and give out carbon dioxide.
4. The air close to a large tree at night may therefore have slightly less oxygen and more carbon dioxide, which is why it is advised not to sleep under a tree at night.

Note: This is the expected exam answer. In open air, the actual change in gas levels is small, because the air keeps mixing, but the reasoning about photosynthesis and respiration is what examiners are looking for.

5. Summary Table

6. Exam Tips and Common Mistakes

Exam Tip: When you define the compensation point, mention both gases and say clearly that the net gas exchange is zero.

Common Mistake: Saying that plants respire only at night. Plants respire all the time; it is photosynthesis that happens only in light.

Common Mistake: Saying that plant processes stop at the compensation point. Both photosynthesis and respiration are still happening, at equal rates.

Frequently Asked Questions

What is the compensation point in plants?

It is the light intensity at which the rate of photosynthesis equals the rate of respiration, so there is no net exchange of oxygen or carbon dioxide.

When is the compensation point reached?

It is usually reached twice a day, in the early morning and in the evening, when the light is dim.

What is the major gas exchange in plants during the day?

During the day, photosynthesis is faster than respiration, so the plant takes in carbon dioxide and gives out oxygen.

What is the major gas exchange in plants at night?

At night there is no photosynthesis, so the plant takes in oxygen and gives out carbon dioxide due to respiration.

Why should we not sleep under a tree at night?

At night, trees only respire, taking in oxygen and giving out carbon dioxide, so the air near them may have slightly more carbon dioxide and less oxygen.

Do plants respire during the day?

Yes. Plants respire all the time. During the day, the carbon dioxide from respiration is used in photosynthesis.