[5 Mark Question Answer]

Question 15 Marks

The diagram below shows two test-tubes A and B. Test-tube A contains a green water plant. Test-tube B contains both a green water plant and a snail. Both test-tubes are kept in sunlight. Answer the questions that follow:

(a) Name the physiological process that releases the bubbles of oxygen.
(b) Explain the physiological process as mentioned above in (a).
(c) What is the purpose of keeping a snail in test-tube B?
(d) Why does test-tube B have more bubbles of oxygen?
(e) Give an example of a water plant that can be used in the above experiment.
(f) Write the overall chemical equation for the above process.

Answer

(a) Photosynthesis releases bubbles of oxygen.
(b) Photosynthesis is a physiological process by which plant cells containing chlorophyll produce food in the form of carbohydrates by using carbon dioxide, water and light energy. Oxygen is released as a by-product.
(c) Carbon dioxide released by the snail during respiration is used by the plant for photosynthesis. This increases the rate of photosynthesis in the plant placed in test tube B. This also suggests that both respiration and photosynthesis are complementary processes to maintain the concentration of oxygen and carbon dioxide in the atmosphere.
(d) A plant and a snail are kept in test tube B. The plant in test tube B has more concentration of $CO+2$ available because the snail releases $CO_2$ during respiration. This increases the rate of photosynthesis in the plant placed in test tube B which leads to the release of more amount of oxygen.
(e)Hydrilla
(f) Chemical equation for photosynthesis:
$\ce {6CO_2 + 12H_2O ->[Light energy][Chlorophyll] C_6H_12O_6 + 6H_2O + 6O_2 ↑ }$

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Question 25 Marks

Describe the main chemical changes which occur during photosynthesis in:
Dark reaction

Answer

The reactions in this phase does not require light energy and occur simultaneously with the light reaction. The time gap between the light and dark reaction is less than one thousandth of a second. In the dark reaction, ATP and NADPH molecules (produced during light reaction) are used to produce glucose $(C_6H_{12} O_6)$ from carbon dioxide. Fixation and reduction of carbon dioxide occurs in the stroma of the chloroplast through a series of reactions. The glucose produced is either immediately used up by the cells or stored in the form of starch.

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Question 35 Marks

Describe the main chemical changes which occur during photosynthesis in:
Light reaction

Answer

The light reaction occurs in two main steps:
(1) Activation of chlorophyll - On exposure to light energy, chlorophyll becomes activated by absorbing photons.
(2) Splitting of water - The absorbed energy is used in splitting the water molecule into hydrogen and oxygen, releasing energy. This reaction is known as photolysis of water.
$2 H _2 O \xrightarrow{\text { energy of } 4 \text { photons }} 4 H ^{+}+4 e ^{-}+ O _2$
The fate of $H ^{+}, e ^{-}$and $( O )$ component are as follows:
The hydrogen ions $\left( H ^{+}\right)$obtained from above are picked up by a compound NADP (Nicotinamide adenine dinucleotide phosphate) to form NADPH.
$NADP ^{+}+ e ^{-}+ H ^{+} \xrightarrow{\text { enzyme }} NADPH$
The oxygen $( O )$ component is given out as molecular oxygen $\left( O _2\right)$.
$'2"O" -> "O"_2'$
The electrons ( $e ^{-}$) are used in converting ADP into energy rich ATP by adding one inorganic phosphate group $P _{ i }$.
$"ADP" + "P" -> "ATP"`$
This process is called photophosphorylation.

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Question 45 Marks

How would you demonstrate that green plants release oxygen when exposed to light?

Answer

Place hydrilla plant (a water plant) in a beaker containing pond water and cover it by a short-stemmed funnel. (Make sure the level of water in the beaker is above the level of the stem of the funnel)

Invert a test tube full of water over the stem of the funnel.
Place the set up in the sun light for a few hours.

Observation:

Bubbles appear in the stem which rise and are collected in the test tube. When sufficient gas gets collected, a glowing splinter will be introduced in the test tube, which will burst into flames.
Inference:

The splinter glows due the presence of oxygen in the test tube which proves that the gas collected in the test is released by hydrilla during photosynthesis.

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Biology [5 Mark Question Answer]

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