CBSE Class 10 Science Practical Skills – Light is Necessary for Photosynthesis
BASIC BUILDING CONCEPTS
Photosynthesis (Greek word photos: light; synthesis-, putting together) is the process by which the green plants synthesise their own food (glucose) using carbon dioxide and water as raw materials in presence of sunlight and chlorophyll. During this process oxygen is released as a by-product. The overall process of photosynthesis can be represented by the following equation:
To show experimentally that light is necessary for photosynthesis.
A potted plant, black paper strips or Ganongs light screen, clips or cellotape, alcohol, iodine solution, water bath, beaker, burner, water, petri dish, dropper.
Light is one of the requirements for photosynthesis. When a part of the leaf is covered from both upper and lower sides with a black paper strip, light will not reach to covered part. Hence, photosynthesis will not take place in covered part of the leaf. On the other hand, as light will be available to uncovered part of the leaf, cells of this region will carry out photosynthesis actively and hence, glucose will be synthesised in the uncovered part of the leaf. In the leaf a part of this glucose is utilised to carry out various metabolic activities and rest of the amount converts into starch. Presence of starch in the leaf, which can be detected by performing iodine test, indicates occurrence of photosynthesis.
- Select a healthy potted plant and destarch the plant by placing it in complete darkness for at least 48 hours.
- After 48 hours, using clips or cellotape cover a part of the leaf of the plant from both upper and lower sides with black paper strips.
- Now, keep the potted plant in sunlight for at least 3-4 hours.
- After 3-4 hours, pluck the leaf and remove black paper strips.
- Boil the leaf in water for about 2 minutes.
- Now, to decolourise the leaf, transfer the leaf from boiling water to a beaker containing ethanol and boil the leaf in a water bath.
- Remove the leaf from beaker and wash it in water at room temperature.
- Place this leaf in petri dish and add few drops of iodine solution over the decolourised leaf.
- The area of the leaf that was uncovered performed photosynthesis and will have starch, therefore turned blue-black with iodine solution.
- The covered portion of the leaf remains yellow as it does not perform photosynthesis.
[Note: Ganong’s light screen can be used in place of black paper strip]
- The uncovered part of the leaf turns blue-black. This confirms the presence of starch.
- The covered portion of the leaf remains yellow showing that no starch synthesis occured in this region.
This proves that light is necessary for the photosynthesis.
- Use healthy potted plant.
- Plant should be destarched by placing in the dark.
- The black paper strip should be attached tightly on both sides of leaf so that no sunlight enters in covered portion of leaf.
- Always use a water bath for boiling leaf in alcohol as alcohol is inflammable.
- Wash the leaf before performing iodine test.
Name the process which acts as a source of oxygen in atmosphere.
What is photosynthesis?
It is a process by which green plants synthesise their own food from carbon dioxide and water in presence of chlorophyll and sunlight.
Name site of photosynthesis in plants.
Mesophyll cells of green leaves.
Name the two reactions involved in photosynthesis.
Light reaction and dark reaction.
Name the site of light reaction and also mention the products formed during light reaction.
Light reaction occurs in thylakoids of chloroplast and leads to formation of ATP and reducing power (NADPH).
Dark reaction doesn’t require light, then what is the source of energy for dark reactions to occur?
ATP and reducing power synthesised during light reactions are used in dark reactions.
From where does oxygen get released during photosynthesis?
Oxygen gets released by photolysis of water during light reactions.
Glucose is formed during photosynthesis but then why in the above experiment, test for starch was performed?
The glucose formed during photosynthesis is polymerised to form starch, which is more stable. Hence, we perform the test for starch.
Why is it important to place the plant in dark before conducting the experiment?
It is done so that all the starch produced through photosynthesis present in leaves is used up and the plant is destarched.
Why do we boil leaf in alcohol?
To decolourise the leaf and remove all the chlorophyll.
Why do we need to decolourise the leaf?
Leaf needed for the experiment should be decolourised as otherwise the chlorophyll will mask the blue-black colour of iodine test.
A water bath is used for boiling of leaf in alcohol. Why?
Alcohol being inflammable, may catch fire if boiled directly over the flame.
Which wavelength of visible spectrum is used for photosynthesis?
Name the cell organelle where photosynthesis occurs.
Chloroplasts of plant cells.
Why does leaf appear green?
Chlorophyll, the important light absorbing pigment in plants, does not absorb light in the green part of the spectrum. Light in this range of wavelength is reflected. That is why leaves appear green.
Which fraction of VIBGYOR is used during photosynthesis?
Blue and Red fraction.
Why do we cover a portion of leaf with black paper strips?
Black paper is used to cover the leaf as it is a good absorber of light and blocks the entry of light to the leaf and hence no photosynthesis occurs in the covered portion.
Name the form in which food is synthesised in plants and the form in which it is transported in plants.
Food is synthesised in the form of glucose and transported in the form of sucrose.
Name the chemical used to test starch.
Name the factors that influence the rate of photosynthesis.
Factors that affect the rate of photosynthesis are- light, temperature, water and C02 concentration.
What are the features present in green leaves that make it a suitable site for photosynthesis?
- Large surface area for absorbing maximum light
- Presence of numerous stomata
- Presence of chloroplasts in mesophyll cells.
Name the energy currency of the cell.
ATP (Adenosine triphosphate).
NCERT LAB MANUAL QUESTIONS
What is meant by destarching? Why do plants get destarched when kept in continuous darkness for about forty eight hours?
Destarching is removal of all the starch from the plant. Plants when kept in darkness become destarched as there is no photosynthesis occurring in the plant and plant utilises its stored starch.
Will you get the same result if you perform the experiment without destarching the plant? Give reason.
No, the already present starch would interfere with the result.
Why do we warm the leaves in alcohol?
To remove the chlorophyll or decolourise the leaves so that chlorophyll does not interfere in starch test.
Arrange the following steps in correct sequence:
(i) destarching the plant;
(ii) treatment with iodine;
(iii) attaching black paper strips to the leaf;
(iv) keeping the set-up in sunlight;
(i),(iii), (iv), (ii)
Why do we keep the experimental plant in bright sunlight?
For photosynthesis to occur.
Can this experiment be performed with a destarched leaf detached from the plant? Give reasons.
No, because raw materials like H20 are supplied by xylem and detached leaf cannot perform photosynthesis.
PRACTICAL BASED QUESTIONS
Multiple Choice Questions/VSA (1 Mark)
In the experiment to prove that light is necessary for photosynthesis, which one of the following is not required? [Foreign 2010]
When iodine was added to a particular vegetable which had been crushed into a paste, blue-black colour was obtained. This indicates the presence of [Foreign 2010]
A leaf from a destarched potted plant is covered with black paper strip as shown in Figure 1 and plant is kept in sunlight. The starch test is done on the leaf after 8 hours. The results will be as shown in
For the experiment, “light is necessary for photosynthesis”, the potted plant is first kept in darkness for a day. This is to [AT 2009]
(a) deactivate the chloroplasts
(b) destarch leaves
(c) activate chloroplasts
(d) prepare leaves for photosynthesis
Which of the following solvent is used to decolourise the leaf?
(b) Iodine solution
(d) Starch solution
The experiment, “To show light is necessary for photosynthesis”, involves following steps. Select the incorrect step.
(i) After boiling leaf in alcohol and washing in water, place in Fehling’s solution to test for starch.
(ii) Destarch the plant by keeping it in dark for 2-3 days.
(iii) Cover the leaf with black paper strips and expose plant to sunlight for 3-4 hours.
(iv) Remove paper strips from leaf and boil the leaf in alcohol.
The figure depicts one of the steps of the experiment. What does labelling (1) denote?
(a) Starch solution
(c) Water bath
In an experiment to show that sunlight is necessary for photosynthesis, the leaf is boiled in alcohol for a few minutes using a water bath. It is essential because [CBSE 2012, CBSE Sample Paper]
(a) alcohol is highly volatile.
(b) steam from the water bath heats the leaf rapidly.
(c) steam from the water bath dissolves the chlorophyll.
(d) alcohol is flammable.
A portion of each of four destarched leaves of a
plant was covered with paper strips of various kinds. The plant was exposed to sunlight for 5 hours. Thereafter, the strips were removed and the leaves tested for starch in the covered portion. [AI2007]
Which one out of the four leaves gave the starch test in the covered portion?
(a) That covered with black paper strip
(b) That covered with green paper strip
(c) That covered with white paper strip
(d) That covered with a transparent paper strip
Given here is a sketch of a leaf partially covered
with black paper and which is to be used in the experiment to show that light is compulsory for the process of photosynthesis. At the end of the experiment, which one of the leaf parts labelled I, II and III will become blue black when dipped in iodine solution? [Delhi 2008]
(a) I only
(b) II only
(c) I and III
(d) II and III
Before testing the leaf for starch at the end of the experiment, “light is necessary for photosynthesis”, the experimental leaf should be boiled in [Delhi 2008]
(c) KOH solution
(d) hydrochloric acid
In an experiment on photosynthesis, a student fixed a strip of black paper on the dorsal surface of a Bougainvillea leaf in the morning. In the evening she tested the leaf for starch. The result was
(a) the dorsal surface of the leaf was white but the ventral surface turned blue.
(b) both the surface of the covered portion remained white.
(c) the entire leaf turned blue-black.
(d) the entire leaf remained white.
Which one of the following is the combination of relevant materials required for setting up an experiment to show that light is necessary for photosynthesis? [AI 2007]
(a) Destarched leaves, strips of black paper, starch solution and iodine crystals.
(b) A potted plant, strips of coloured paper, starch solution, iodine and potassium iodide.
(c) A potted plant, black strips, water, water bath
(d) A potted plant, black paper strips, alcohol, iodine solution, water bath, water.
In an experiment to test for the presence of starch in a leaf, the leaf is boiled in alcohol for a few minutes using a water bath. This is an essential step in the experiment because alcohol [AI 2007C]
(a) softens the leaf
(b) disinfects the leaf
(c) allows iodine to enter the leaf
(d) dissolves chlorophyll.
A student performed the starch test on a leaf. Some steps involved are shown below.
The correct sequence of steps should be
(a) (iv), (iii), (ii), (i)
(b) (i), (ii), (iii), (iv)
(c) (ii), (iii), (iv), (i)
(d) (i), (iii), (iv), (ii)
Short Answer Questions
What is destarching? Why is this step necessary for performing the experiment to prove that light is necessary for photosynthesis?
Destarching is the removal of starch from the plant. When a plant is kept in dark, photosynthesis does not occur in plant and therefore, it utilises stored starch. In this way, we can destarch a plant.
Destarching is necessary to remove all the starch present in the plant. If the plant is not destarched, starch will interfere with result.
While performing experiment ‘light is necessary for photosynthesis’, the uncovered part of the leaf on treating with iodine solution turns blue- black while the covered portion remains yellow. Why?
During the experiment, the uncovered portion received light and hence photosynthesis took place in this part. Because of this, starch is formed in the uncovered portion. This starch with iodine gives blue-black colour.
Contrary to above, the covered portion of the leaf does not receive light and therefore starch is not formed in the region. Hence, the covered portion remains yellow on treating with iodine solution.
What are the precautions that should be considered while performing the experiment ‘light’ is necessary for photosynthesis’?
Destarch the potted plant by placing it in dark.
The black paper strip should be attached tightly on .both sides of the leaf.
Always use water bath for boiling leaf in alcohol.
Wash the leaf before performing iodine test.
Various steps associated with iodine test are as follows:
- Boil the leaf in water.
- Boil the leaf in alcohol on a water bath.
- Wash the leaf in water.
- Add few drops of iodine solution.
- This step makes the leaf soft, killing the cell membrane so that iodine enters it easily.
- This step decolourises leaf, and destroys chloroplast.
- This step removes alcohol from leaf surface.
- This step tests the presence of starch in leaf.
Mention the importance of steps given above.
- We use water bath for boiling leaf in alcohol.
- We cover a portion of leaf with black paper strip.
- Alcohol being inflammable may catch fire if boiled directly over the flame.
- Black paper is used to cover the leaf as it is a good absorber of light and blocks the entry of light to the leaf and hence no photosynthesis occurs in the covered portion.