[2 Mark Questions]

Question 12 Marks

Name a plant growth hormone synthesized at the shoot tip. Explain its effect on the growth of a plant in response to light.

Answer

- Auxin
- When light is coming from one side of the plant, Auxin located at shoot tip diffuses towards the shaded side of the shoot.
- Concentration of Auxin in the shaded region stimulates the cells to grow longer as compared to the region exposed to light. So the plant tends to bend towards the light.

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Question 22 Marks

What is meant by trophic level in a food chain? Construct a terrestrial food chain with four trophic levels. The energy flow in a food chain is always unidirectional. Why?

Answer

Each step in a food chain through which the transfer of food takes place constitutes a trophic level.
Grass $\rightarrow$ Insect $\rightarrow$ Frog $\rightarrow$ Snake
The flow of energy in food chains is unidirectional. The energy which passes to the herbivores does not come back to the autotrophs. The energy that is captured by the autotrophs does not revert back to the Sun. Therefore, in the food chain, the energy moves progressively through various trophic levels. This energy is no longer available to the previous trophic level. Thus, the flow of energy in a food chain is unidirectional.

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Question 32 Marks

a. Construct a food chain of four trophic levels operating in a grassland. If the energy available to the organisms of $4^{\text {th }}$ trophic level is 75 joules, how much energy was available with the organisms of the $2^{\text {nd }}$ trophic level for transfer to the next trophic level? Justify your answer.
b. Why is the flow of energy unidirectional in a food chain?

Answer

a. The energy at the 4th trophic level is given 75J. According to the $10 \%$ law, only $10 \%$ of the energy from one trophic level is transferred to the next trophic level. Energy at the 3rd level (Snake $75 \mathrm{~J} / 10 \%=75 \mathrm{~J} / 0.1=750 \mathrm{~J}$.
b. The flow of energy is unidirectional in a food chain. The energy that is captured by the autotrophs does not revert back to the solar input and the energy which passes to the herbivores does not come back to autotrophs. As it moves progressively through the various trophic levels it is no longer available to the pervious tropic level.

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Question 42 Marks

Define the term pollination. Differentiate between self-pollination and cross-pollination. What is the significance of pollination?

Answer

Pollination is the act of transferring pollen grains from the male anther of a flower to the female stigma. The goal of every living organism, including plants, is to create offspring for the next generation. One of the ways that plants can produce offspring is by making seeds. Seeds contain the genetic information to produce a new plant.
Self-pollination:
i. It is the transfer of pollen grains from the anther of the stamen to the stigma of the carpel of the same flower or different flower of the same plant.
ii. It does not require external agents.
iii. It does not introduce variation.
iv. It is a sure and economical method.
v. It avoids mixing or dilution of the original characters.
vi. The progeny becomes weaker after every generation
Cross-pollination:
i. It is the transfer of pollen grains from the anther to the stigma of different flowers of different plants.
ii. It requires external agents like wind, insects, etc. to carry out pollination.
iii. It introduces variation and adaptability.
iv. It is not economical and is highly wasteful.
v. Good characters of a race can be diluted.
vi. The progeny becomes healthier and better fitted for existence.

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