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Biology Answer the following questions in detail

Photosynthesis · Maharashtra Board STD 11 Science (MSBSHSE - English Medium). 7 questions.

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Answer the following questions in detail

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7 questions · self-marked practice — reveal the answer and mark yourself.

Question 14 Marks
Give reason. Why $C _4$ plants are favoured in tropical regions?
Answer
1. $C_4$ plants are favoured in tropical regions as they require 30 ATP to produce 1 molecule of glucose.
2. High temperature in tropical regions leads to closure of stomata to reduce rate of transpiration. Due to this availability of $CO _2$ decreases.
3. PEP carboxylase present in mesophyll cells can fix $CO _2$ even at low concentration. This helps the plant in efficient assimilation of atmospheric carbon dioxide.
4. $C_4$ plants contain a special leaf anatomy called Kranz anatomy which minimizes the losses due to photorespiration.
5. It helps $C_4$ plants to survive in conditions of high daytime temperatures, intense sunlight and low moisture.
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Question 24 Marks
Give significance of $C _4$ pathway.
Answer
$1. \ C_4$ plants have special type of leaf anatomy called Kranz anatomy.
$2.$ In $C_4$ plants, $CO_2$ fixation occurs twice.
$3.$ In these plants, chloroplasts of mesophyll cells contain enzyme $\text{PEP}$ carboxylase which fixes atmospheric $CO_2.$ Thus, first $CO_2$ fixation occurs in mesophyll cells.
$4.$ Decarboxylation of malic acid in bundle sheath cells results in increase in $CO_2$ concentration.
$5.$ Thus, $RuBisCO$ acts as carboxylase and brings about carboxylation of $RuBP.$
$6.$ Due to this oxygenation of $RuBP$ and photorespiration is prevented.
$7.$ Thus, despite of having less number of bundle sheath cells carrying out Calvin cycle, $C_4$ plants are highly productive.
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Question 34 Marks
Describe Calvin cycle and its significance.
Answer
Significance:
$1.$ Carboxylation: $RuBisCO$ is the most abundant enzyme in the world. It is responsible for fixing carbon in the form of $CO_2$ into sugar. As a result of Carboxylation, the first stable product of carbon fixation i.e. $3- \text{PGA}$ is synthesized.
$2.$ Reduction/Glycolytic reversal: $\text{NADPH2}$ donates electrons to $1, 3-$Bisphoshoglycerate to form $3-$ phosphoglyceraldehyde molecules. During this process $\text{ADP}$ and $\text{NADP}$ are generated which are used in light reaction.
$3.$ Regeneration of $RuBP:$ Some $3-$phosphoglyceraldehyde molecules are involved in production of glucose while others are recycled to regenerate the $5-$carbon compound $RuBP$ which used to accept new carbon molecules. Thus, regeneration of $RuBP$ is required for Calvin cycle to run continuously.
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Question 44 Marks
Explain how light and dark reactions of photosynthesis are interdependent.
Answer
Link between light-dependent and dark reactions:
1. The light reaction gives rise to two important products, a reducing agent $NADPH _2$ and an energy-rich compound ATP. Both these are utilized in the dark phase of photosynthesis.
2. ATP and $NADPH _2$ molecules function as vehicles for transfer of energy of sunlight into dark reaction leaving to carbon fixation. In this reaction $CO _2$ is reduced to carbohydrate.
3. During dark reaction, ATP and $NADPH _2$ are transformed into ADP, iP and NADP which are transferred to the grana in which light reaction takes place.
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Question 54 Marks
What are the internal factors which affects photosynthesis?
Answer
Internal factors which affects photosynthesis are as follows:

1. Chlorophyll:
a. Though presence of chlorophyll is essential for photosynthesis but rate of photosynthesis is proportional to the quantity of chlorophyll present.
b. It is because of the fact that chlorophyll merely acts as a biocatalyst and hence a small quantity is quite enough to maintain the large bulk of the reacting substances.

2. Sugar:
The final product in the photosynthesis reaction is sugar and its accumulation in the cells slow down the process of photosynthesis.

3. Internal structures:
The thickness of cuticle and epidermis of the leaf, the size and distribution of intercellular spaces and the distribution of the stomata and the development of chlorenchyma and other tissues also affects the rate of photosynthesis.
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Question 74 Marks
Explain diagrammatically the ultrastructure of chloroplast.
Answer
  1. The chloroplasts are discoid and lens shaped in higher plants. Chloroplast is bounded by a double membrane.
  2. System of chlorophyll bearing a double-membrane sac is present inside the stroma. These are stacked one above the other to form grana.
  3. Individual sacs in each granum is are known as thylakoid.
  4. All the pigments chlorophylls, carotenes and xanthophylls are located in thylakoid membranes.
  5. These pigments are fat soluble and are present in lipid part of membrane also they absorb light of specific spectrum in the visible regions.
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