https://cbseacademic.nic.in/web_material/Manuals/Science_PracticeBook23_C7.pdf
Theme 1: Nutrition in Plants
Prior Knowledge
It is recommended that you revise the following topics before you start working on these questions.
Mode of nutrition in plants
Photosynthesis - food-making process in plants
Products of photosynthesis
Autotrophs and heterotrophs
The World Before Photosynthesis
Our planet is 4.5 billion years old. The earliest undisputed evidence of life comes from around 3.4 billion years ago. At that time, our planet looked nothing like what it does today. It was hot and barren. Our atmosphere was filled with toxic gases and water vapour condensed in this thick atmosphere to form massive clouds and rained acid rain upon the Earth to create the first seas. So, how did life, as we know it now, come to be?
We have to thank a bunch of bluish-green cyanobacteria who figured out how to trap
sunlight and carbon dioxide from the atmosphere, and convert it into nutrition for
themselves. In the process of making their food, these autotrophic (creatures that make
their own food) organisms release oxygen. As there was plenty of carbon dioxide and
sunlight during this time, the population of these cyanobacteria grew so much that within a
million years, they had changed the composition of our planet’s atmosphere. The Earth's
atmosphere now had oxygen that these bacteria had given out as they produced their own
food. Plants, like the ones we see now, as well as animals, found the planet as just the right
place to grow and thrive, and so here we are.
Even today, the tiny cyanobacteria are responsible for producing most of the oxygen on our planet.
Case Study A - How did we discover Photosynthesis?
Around 1772, British scientist Joseph Priestley tried a set of experiments to study the role of air in photosynthesis.
He used two airtight glass containers. In one container (Container A), he placed a candle and a mouse. In another (Container B), he placed a mint plant, a candle and a mouse together. He observed that the mouse died within seconds in Container A but in Container B, the mouse survived for a few minutes before its death.
At the end of the experiment, Priestley concluded that the candle and the mouse “spend” air while the mint plant “revives” the air.
Another scientist named Jan Ingenhousz repeated Priestley’s experiment, using a similar set-up, but also proved that plants need sunlight to “revive” the air.
It was only by 1905 that scientists had figured out the details of photosynthesis, and the role of chlorophyll.
Question 1 What might have been the observation made by Priestley that led him to conclude that the candle and the mouse “spent” the air in Container A?
a. After some time, the candle and mouse heated the air in Container A.
b. After some time, the candle did not extinguish and the mouse was active in Container A.
c. After some time, the candle extinguished and the mouse died.
d. It is not clear what Priestley meant by “spending” of air.
Answer : c. After some time, the candle extinguished and the mouse died
Question 2 Priestley’s conclusion that the mint plant “revived” the air was based on the observation that the state of the candle and the mouse in Container B did not change after some time. What explains this observation?
a. The plant provided food for the mouse and fuel for the candle.
b. The plant provided oxygen for the mouse to breathe and the candle to stay lit.
c. The plant provided carbon dioxide and used up oxygen for respiration.
d. The mouse respired and gave oxygen to the candle and the plant absorbed the carbon dioxide emitted by the candle to keep the air ‘pure’
Answer : b. The plant provided oxygen for the mouse to breathe and the candle to stay lit.
Question 3 Asha replicated Priestley’s experiment, but without the mouse. She placed a lit candle in Container A and a candle and a plant in Container B. She noticed that the candle in Container B lasted only a few minutes more than the candle in Container A. She had expected the Container B candle to burn until it ran out of wax. Why did it get extinguished before it ran out of wax?
Answer:
The plant does not produce oxygen at the same rate at which the candle consumes oxygen. Therefore, the candle eventually runs out of oxygen.Question 4 : Jan Ingenhousz repeated Joseph Priestley’s experiment to see whether sunlight had a role to play in photosynthesis. Which of the following could have been a suitable design for his experiment?
Place a lit candle in Container A. Place a plant and a lit candle in Container B. Keep both containers in sunlight.
Place a lit candle in Container A and keep it in a dark place. Place a plant and a lit candle in Container B and keep it under the Sun.
Place a plant and a lit candle in each container. Keep one container in a dark room and another in sunlight.
Place a plant and a lit candle in each container. Keep both containers first in the dark and then in sunlight.
Answer: c. Place a plant and a lit candle in each container. Keep one container in a dark room and
another in sunlight
Question 5
Based on your answer to Question 4, predict what the outcome of Jan Ingenhousz’s
version of the experiment was.
Answer: The outcome was that the candle in the container, which was in the sunlight, stayed lit for longer because sunlight was needed for photosynthesis and the plant produced oxygen, which was needed for the wick to burn.
Question 6
Case Study B - Parasites
Several plants do not have chlorophyll (green pigment) and cannot perform photosynthesis. They obtain nutrition from a host plant and are called parasites. A parasite gives no benefit to its host. Instead, some parasites cause extreme damage to their hosts.
Witchweed or Striga is one such parasite. It is a small plant with pretty flowers. It needs a host plant in order to germinate and grow into a young plant. Once it grows, it can create its own food. However, as a sapling, it attaches itself to the roots of its host plant and uses all its nutrition. In the process, the host plant either cannot grow fully or dies completely. Striga especially affects farmers in Asia, Africa, America and Australia as they attach themselves to crops like sugarcane, rice and corn. They can reduce the yield or destroy the entire crop.
Question 1:
i. Which of the plants shown in Fig 1.6 are parasites?
Answer:. (i) Bird’s Nest Orchid
ii. What factors about the plant did you consider while answering Part i.?
a. Colour b. Shape c. Type d. Size
Answer a. Colour
Question2 :
A group of scientists in Kenya are trying to help farmers whose crops are affected by Striga/witchweed. They have found 3 chemical solutions that prevent Striga seeds from germinating in the roots of the host plant. They are called Nij1, GR24 and MP1.
To test the effect of the solutions, they apply the solutions to a collection of four groups of Striga infested rice plant pots. They observe the plants to see whether they are effective in controlling the growth of the parasite or not. Here are their results:
Answer the following questions based on the above results:
I. Look at Graph (a). Apart from the three solutions Nij1, GR24 and MP1, why did the experimenters also look at the impact of just water?
Answer: The experimenters have included water to see how many Strigas would grow in a pot if no other solution is applied to the rice plant. That way the scientists can infer if adding the other chemical solutions has any impact on the germination of Striga plants
ii. As a farmer, which solution should I use to prevent Striga growth?
a. Water b. Nij1 c. GR24 d. MP1
Answer d. MP1 Explanation: MP1 is most effective in controlling the growth of the parasitic plants and it also enables the rice plant to reach its optimal height.
iii.Which of the following shows that Striga has a negative effect on the growth of its host - the rice plant?
a. Graph (a) b. Graph (b)
c. Both Graph (a) and Graph (b) d. No such conclusion can be drawn from either of
the two graphs.
Answer bGraph (b)
Case Study C - Photosynthesis - CO2
In this activity, one leaf is exposed to potassium hydroxide, which will absorb carbon dioxide from the surrounding air. It is then exposed to light and tested for starch (carbohydrate). The experiment is designed to test whether carbon dioxide is needed for the plant to produce starch/carbohydrates.
Question 1 Adil was surprised to see that the plant grew by a few cm even when it was kept in the dark. He concluded that photosynthesis was possible even in the absence of light. Rehana countered his conclusion by giving the following statements:
Assertion(A): In the dark, plants use up the food stored in the form of starch to grow.
Reason(R): Food cannot be prepared in the absence of sunlight.
What is your opinion about this debate between the two?
a. Adil is right.
B. Adil is wrong and Rehana gave the correct explanation for the observed growth.
c. Both Adil and Rehana made incorrect conclusions.
D. Adil is right and Rehana's assertion is incorrect though her reason is right.
b. Adil is wrong and Rehana gave the correct explanation for the observed growth.
Question 2 Rehana performed the experiment described in Case Study C herself. After making the leaves colourless, when she put iodine, she observed that none of the leaves turned blue-black. What can be the possible source of error?
The leaves were not de-starched properly initially, when the plant was placed in the dark room.
The leaf chosen as Control could not receive proper sunlight.
The leaves were not boiled in water for a sufficient period of time to kill the cells.
There was a hole in the Test bag through which CO2 (carbon dioxide) could enter
Question 3 The experiment described in Case Study C was repeated two more times, without any error. In one of the repetitions, the plant was kept in the dark room for 24 hours and in the other one, the plant was kept in the dark room for 120 hours. The same process was followed afterwards to test if CO2 (carbon dioxide) is required to produce starch. The test results are shown in Table 1.1.
What accounts for the starch detected in the Test leaf of the plant that was kept in the dark room for 24 hours?
Starch was already present in the Test leaf before the leaf was enclosed in the potassium hydroxide bag.
Starch was prepared by the Test leaf in Step 2 in the absence of CO2 2. (carbon dioxide).
1 only b. 2 only
c. Both 1 and 2 d. An error made in the later stages of the activity.
Question 4. CO2 concentration in air is indicated in terms of parts per million or ppm. 100 ppm would mean that 100 particles of CO2 are present in 1,000,000 air particles. The graph in Fig. 1.8 shows the effect of CO2 levels on the plant growth rate.
I. On the basis of the activity given in Case Study C and Fig. 1.8, which of the following inferences are correct? Select all that apply.
CO2 is necessary for photosynthesis.
Plant growth increases with increase in atmospheric CO2 , only up to a certain level
A continuous increase in atmospheric CO2 after a certain level can have a negative effect on plant growth. \
CO2 is not necessary for photosynthesis but it enhances plant growth.
Answer a. CO2 is necessary for photosynthesis.b. Plants growth increases with increase in atmospheric CO2, only up to a certain level.c. A continuous increase in atmospheric CO2after a certain level can have a negativeeffect on plant growth.
Ii. Over the past 60 years, CO2 in the Earth’s atmosphere has increased by nearly 100 ppm. It stands at 420 ppm at present. How do you think it might have affected plant life on the planet?
Answer: The rate of photosynthesis has increased for plants on Earth
1. Photosynthesis is an important process for life on earth because
(a) it is the primary source of all food on earth
(b) it is responsible for the release of oxygen
(c) it is the only natural process responsible for the utilisation of sunlight
(d) All of the above
2. The main purpose of photosynthesis is to
(a) consume CO2
(b) produce ATP
(c) convert light energy into Chemical energy
(d) produce starch
3. The process of digestion in our digestive system involves
(a) conversation of complex substances into simple form
(b) absorption of monomers by the body
(c) conversion of monomers into polymers
(d) absorption of water and food
4. Digestion in the digestive system of humans is accomplished by
(a) mechanical and Chemical processes
(b) chemical processes only
(c) mechanical processes only
(d) none of the above
5. A baby boy aged two years is admitted to play school and passed through a dental checkup . The dentist observed that the boy had twenty teeth. Which teeth were absent.
(a) Incisors
(b) Canines
(c) Premolars
(d) Molars
6. The primary dentition in human differs from permanent dentition not having one of the following type of teeth.
(a) Canine
(b) Premolars
(c) Molar
(d) incisor
