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Chapter: 11th Botany : Chapter 13 : Photosynthesis

Answer the following questions

Botany : Photosynthesis - Book Back and Important Questions Answers - Brief Questions Answers, Short Questions Answers

Plant Physiology (Functional Organisation)



6. Two groups (A & B) of bean plants of similar size and same leaf area were placed in identical conditions. Group A was exposed to light of wavelength 400-450nm & Group B to light of wavelength of 500-550nm. Compare the photosynthetic rate of the 2 groups giving reasons.


‘A’group of plants exposed to light of 400 - 450nm. Chlorophyll a shows maximum absorption peak at 450nm (blue region). Hence rate of photosynthesis was high.

‘B’ group of plants exposed to light o f 500 - 550nm. This wavelength refers to green region of the spectrum. Chlorophyll does not absorb light in the green region but reflects green. So plants appear green rate of photosynthesis was negligible in these plants.


7. A tree is believed to be releasing oxygen during night time. Do you believe the truthfulness of this statement? Justify your answer by giving reasons?


It is a belief that some trees such as Aloe vera Peepal tree, some palm varieties grown as indoor plants release oxygen during night time. There is no scientific evidence for this So it is not true. This is because oxygen is evolved during the light reaction of photosynthesis only. This reaction occurs only in the presence of light. Therefore oxygen cannot be released during night time. 


8. Grasses have an adaptive mechanism to compensate photorespiratory losses-Name and describe the mechanism.


The photo respiratory losses are checked by certain grasses by having physiological adaptation. The process of photosynthesis occurs in mesophyll cells and bundle sheath cells.

Mesophyll cells:

a) Initially CO2 is taken up by Phosphoenol pyruvate (PEPA) (3C) and changed to oxaloaccetate (4C) in the presence of PEP carboxylase.

b) Oxaloccetate is reduced to Malate/Aspartate. The product formed reaches the bundle sheath.

Bundle Sheath:

a) The oxidation of Malate and Aspartate occurs with release of carbon di oxide and formation of Pyruvate (3C)

b) Due to increased CO2 concentration RUBISCO, functions as carboxylase and not as Oxygenase.

c) The photosynthetic losses are prevented.

d) RUBP operates now under calvin cycle and pyruvate transported back to Mesophyll cells is changed into Phosphoenol pyruvate to keep the cycle going.


9. In Botany class, teacher explains, Synthesis of one glucose requires 30 ATPs in C4 plants and only 18 ATPs in C3 plants. The same teacher explains C4 plants are more advantageous than C3 plants. Can you identify the reason for this contradiction?

Answer: C4 Plants are more advandageous than C3 plants because of the following reasons:

C3 Plants

• CO2 fixation occurs in mesophyll cells only

• RUBP is the only CO2 acceptor

• Fixation of CO2 occurs if the atmospheric concentration of CO2 is 50 ppm only

• Optimum temperature is 20° to 25°C

• RUBP carboxylase enzyme also functions as oxygenase if the O2 concentration is higher than carbon dioxide

• Higher rate of photorespiration and hence rate of photosynthesis is reduced.

C3 Plants

• CO2 fixation occurs in mesophyll and bundle sheath cells

• PEPA Phosphoenol pyruvate in mesophyll is the acceptor in the first phase

• It can fix carbon di oxide even if the atmospheric concentration of CO2 is below l0ppm

• Optimum temperature is 30° to 45°C and is thus effective in tropical regions.

• PEP carboxylase enzyme functions even at low carbon – di - oxide concentrations.

• Minimal rate of photorespiration is seen is C4 plants.


10. When there is plenty of light and higher concentration of O2, what kind of pathway does the plant undergo? Analyse the reasons.


Photorespiration is the excess respiration taking place in photosynthetic cells due to absence of CO2 and increase of O2. This condition changes the carboxylase role of RUBISCO (RUBP carboxylase oxygenase) enzyme into oxygenase. C2 cycle or photorespiration, begins and operates in the chloroplast, Peroxisome and Mitochondria.

It protects, photosynthetic cells from photooxidative damage.

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