2.5 Compensation Point
In plants, photosynthesis and cellular respiration take place simultaneously during the day. The compensation point marks the specific level of light intensity where the rate of photosynthesis equals the rate of cellular respiration.
Learning Objectives
- Define the compensation point in plants.
- Explain the attainment of the compensation point in relation to light intensity.
- Describe the net exchange of gases and glucose production below, at, and beyond the compensation point.
- Explain the importance of the rate of photosynthesis exceeding the rate of respiration for plant growth, development, and survival.
- Compare and contrast photosynthesis and cellular respiration in plants based on similarities and differences.
Graph of Light Intensity versus Carbon Dioxide Uptake at the Compensation Point
Increasing light intensity changes the net uptake and release of carbon dioxide. The graph identifies the compensation point, the net loss region and the net gain region.
Short Explanation
Definition of Compensation Point
The compensation point is the level of light intensity when the rate of respiration equals the rate of photosynthesis.
At the compensation point
Rate of photosynthesis equals the rate of respiration. All glucose produced in photosynthesis is completely used up in cellular respiration. There is no net gain or net loss in glucose, and no net uptake or net release of carbon dioxide or oxygen between the plant and the atmosphere.
Below the compensation point
Rate of respiration is faster than the rate of photosynthesis. There is a net loss in glucose because glucose consumed in respiration exceeds the glucose produced in photosynthesis. The plant takes in oxygen and releases carbon dioxide.
Beyond the compensation point
Light intensity increases beyond the compensation point, causing the rate of photosynthesis to become faster than the rate of respiration. Carbon dioxide is absorbed from the atmosphere to meet the demand of photosynthesis. Excessive oxygen produced in photosynthesis is released into the atmosphere. The rate of glucose production exceeds the rate of glucose consumption, producing a net gain in glucose. Excess glucose is converted and stored in the form of starch by the plant.
Effects on plant growth
If a plant remains at its compensation point continuously, all products of photosynthesis are fully utilised for cellular respiration. No excess glucose is available for growth, development, or the production of flowers, seeds, and fruits. The plant cannot grow and may eventually die. To ensure growth, development, and reproduction, the rate of photosynthesis must exceed the rate of respiration daily.
| Aspect | Photosynthesis | Cellular Respiration |
|---|---|---|
| Organisms involved | Green plants and photosynthetic bacteria | All living organisms |
| Type of cells | Happens in cells containing chlorophyll | Happens in all cells |
| Type of metabolism | Anabolism process — synthesis of glucose using carbon dioxide and water | Catabolism process — breakdown of glucose to produce energy |
| Site | Chloroplast | Mitochondria |
| Reaction substances | Carbon dioxide and water | Oxygen and glucose |
| Products | Glucose | Energy |
| By-products | Oxygen and water | Carbon dioxide and water |
| Energy involvement | Light energy is absorbed and converted into chemical energy | Chemical energy is converted to ATP and heat energy is released |
| Light requirement | Needs light | Does not need light |
Try to Answer First
Answer in your mind, then press “Check Answer”.
What is the definition of the compensation point?
What happens to the excess glucose produced when the rate of photosynthesis exceeds the rate of respiration beyond the compensation point?
Name the site of photosynthesis and the site of cellular respiration in plant cells.
Common Mistakes
- Stating that photosynthesis and respiration stop at the compensation point, whereas both processes are actively occurring at equal rates.
- Assuming plants only respire at night, when in fact cellular respiration occurs continuously in all living plant cells, day and night.
- Confusing the metabolic nature of the two processes: photosynthesis is an anabolic process (building glucose), whereas respiration is a catabolic process (breaking down glucose).
- Believing that plants can sustain long-term growth at the compensation point, overlooking that net glucose gain is necessary for producing new tissues, flowers, seeds, and fruits.
Concept Misunderstandings
At the compensation point, no gas exchange occurs inside the plant.
Gas exchange still occurs internally between the chloroplasts and mitochondria; carbon dioxide produced in respiration is directly used in photosynthesis, and oxygen produced in photosynthesis is directly used in respiration, so only the net gas exchange with the surrounding atmosphere is zero.
Respiration requires light just like photosynthesis does.
Respiration does not need light and occurs continuously regardless of light availability, whereas photosynthesis specifically requires light energy.
Summary
- The compensation point is the light intensity where the rate of photosynthesis equals the rate of respiration.
- At this point, there is no net gain or loss of glucose, and net gas exchange with the atmosphere is zero.
- Below the compensation point, respiration exceeds photosynthesis, causing a net loss of glucose.
- Beyond the compensation point, photosynthesis exceeds respiration, resulting in a net gain of glucose (stored as starch) and net release of oxygen into the atmosphere.
- For plants to grow, develop, and reproduce, the daily rate of photosynthesis must exceed the rate of respiration.
- Photosynthesis is an anabolic process occurring in chloroplasts that requires light, while respiration is a catabolic process occurring in mitochondria that does not need light.
Short Activity
Objective: check your understanding of the compensation point, gas exchange and the comparison between photosynthesis and respiration.
A. Objective Quiz
What occurs when the light intensity rises beyond the compensation point?
Which of the following is a similarity between photosynthesis and cellular respiration?
B. Fill in the Blanks
At the compensation point, the rate of photosynthesis is the same as the rate of .
Photosynthesis is an anabolism process that synthesises glucose and occurs in the .
C. Matching / Drag and Drop
Match each biological aspect to its corresponding characteristic in photosynthesis. On a phone, tap a choice first, then tap the matching box. Tap the Answer Choices area to return a selected item.
C. Match the Biological Aspect to Its Photosynthesis Characteristic
Write the correct aspect next to each characteristic.
| Photosynthesis Characteristic | Aspect |
|---|---|
| 1. Chloroplast | |
| 2. Anabolism process synthesising glucose | |
| 3. Absorbs light energy and converts it into chemical energy | |
| 4. Needs light to occur |
Answer choices: Site of reaction • Type of metabolism • Energy involvement • Light requirement
