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Question
When there is plenty of light and a higher concentration of O2, what kind of pathway does the plant undergo? Analyse the reasons.
Solution
The rate of photosynthesis decreases when there is an increase in oxygen concentration. This inhibitory effect of oxygen was first discovered by Warburg (1920) using green algae, chlorella.
Analysis and Reasons:
- Photorespiration Pathway: In conditions where there is abundant light and high oxygen concentration, the enzyme RuBisCO, which normally fixes carbon dioxide (CO2) during the Calvin cycle in photosynthesis, has a higher affinity for oxygen. Instead of binding to CO2, RuBisCO binds to O2, initiating photorespiration.
- Mechanism: During photorespiration, RuBisCO reacts with oxygen, leading to the formation of a two-carbon compound called phosphoglycolate, which is then converted into CO2 and energy in a series of reactions. However, unlike photosynthesis, photorespiration does not produce glucose or ATP and is considered an energy-wasting process.
- Reasons for Photorespiration:
- High O2 Concentration: When the concentration of oxygen is higher than that of carbon dioxide, RuBisCO is more likely to bind with oxygen.
- High Light Intensity: Increased light intensity boosts the production of oxygen through photosynthesis, further raising the O2 concentration in the leaf.
- Closed Stomata: In hot or dry conditions, plants often close their stomata to reduce water loss, limiting CO2 entry and increasing the relative concentration of O2 within the leaf.
RELATED QUESTIONS
Short answer question.
Which four of the following are needed for photosynthesis in a leaf:
(i) Carbon dioxide:
(ii) Oxygen:
(iii) Nitrates :
(iv) Water:
(v) Chlorophyll:
(vi) Soil:
(vii) Light:
Suppose we compare the leaf with a factory, match the items in Column A with those in Column B.
| Column A | Column B | ||
| Left | Factory | ||
| 1 | Cells in the leaf | a | Raw materials |
| 2 | Chloroplast | b | Power |
| 3 | Sunlight | c | Machinery |
| 4 | Oxygen and water | d | End product |
| 5 | Carbon dioxide and water | e | By product |
| 6 | Glucose | f | Workroom |
State whether the following statement is True or False:
Green plants prepare their food by using two raw materials, oxygen and water.
State whether the following statement is True or False:
The free oxygen in the atmospheric air is the result of photosynthesis.
Long answer question.
In order to carry out photosynthesis, what are the substances that a plant must take in? Also mention their sources.
Long answer question.
Do plants need oxygen? If so, what is its source?
Tick the most appropriate answer.
Exhaled air contains
Tick the most appropriate answer.
Plants have these for the exchange of gases.
Attempt any TWO of the following:
Describe ‘any two’ factors affecting the rate of photosynthesis.
Identify the false statement and rewrite it correctly by changing the first or last word only.
Photosynthesis requires enzymes.
Identify true statement regarding light reaction of photosynthesis.
Draw and label the structure of oxysomes.
Law of limiting factor is
In tropical rain forests, the canopy is thick and plants growing below receive filtered light. How are they able to carry out photosynthesis?
Why does the rate of photosynthesis decrease at higher temperatures?
According to Blackman, respiration of carbohydrates and fats is ______.
The optimum temperature for the process of photosynthesis is ______.
