Class 7 Science Chapter 10 Notes Life Processes in Plants
→ All living organisms require food that provide energy for their growth and development.
→ Plants use carbon dioxide and water in the presence of sunlight and chlorophyll to produce glucose and oxygen. This process of synthesis of food is known as photosynthesis.
→ Leaves are the ‘food factories’ of a plant.
→ Tiny pores on the surface of leaves, called stomata, help in the exchange of oxygen and carbon dioxide during photosynthesis and respiration.
→ The xylem transports water and minerals from roots, while the phloem carries food from leaves to all parts of the plants.
→ Plants break down glucose and release energy by a process called respiration. They use oxygen and release carbon dioxide in this process.
In Grade 6, we learnt that all living beings grow and need food for their growth. Also, in the previous chapter, we discussed the process through which animals obtain nutrition.
We know that animals eat food to grow, but what about plants? Have you ever seen plants eating food like animals do? As animals grow, their size and weight usually increase, and their bodies undergo various changes. What changes do you notice in plants when they grow?
We learnt that food provides nutrients like carbohydrates, fats, proteins, vitamins, and minerals, which, along with water, are all essential for growth. Let us explore how plants obtain nutrients for their growth.
How Do Plants Grow?
Look around your neighbourhood. Have you observed any changes in a plant during its life span? As a plant grows, new leaves and branches emerge, its height increases, and its stem thickens. What do you think causes these changes? Discuss with your friends and provide your explanation as well.
Let us perform an experiment to test some of these explanations.
Let us test some explanations
- Take three earthen pots (or used bottles/containers) of the same size filled with garden soil. Plant saplings of similar sizes of a fast-growing plant like chilli or tomato in each pot (Figure).
- Label the pots A, B, and C.
- Count the number of leaves on each sapling and record your observations.
- Place pot A in direct sunlight. Keep the soil in this pot slightly moist by adding an adequate amount of water every day (Fig. (a)).
- Place pot B in direct sunlight, without adding water to the soil (Fig. (b)).
- Observe the plants for two weeks and record changes in their height, number of leaves, colour of leaves, and any other changes that may appear.
(a) Pot A kept in direct sunlight, with water
(b) Pot B kept in direct sunlight, without water
(c) Pot C kept in the dark, with water
Fig. Experimental set-up to understand the role of sunlight and water in plant growth
Analyse the observations recorded in Table 10.1, and discuss them with your teacher and friends.
You are likely to find that the plant in Pot A, kept in direct sunlight with adequate water, grows better than the plant in Pot C, which gets adequate water but no sunlight. The plant in Pot B may have died as it did not get water even though it received adequate sunlight.
What do you infer from the observations made in this activity? The results indicate that plants require both sunlight and water for their growth.
Fascinating Facts
“Trees do not produce fruits and flowers merely because they are planted.” This line is from an ancient Indian text named Vrikshayurveda. It records useful observations about plant growth, soil, and agricultural practices to help improve crop health, growth and production. The knowledge in the text seems to be based on practical experiences and patterns seen over time. These ideas were then systematically documented to guide farming practices. For instance, there are references to different methods of organic manure preparation, such as mixing water, barley, and various seeds, like green, black, and horse grams.
How Do Plants Get Food for their Growth?
We know that animals get their food from plants, either directly by eating plants or indirectly by eating animals that in turn eat plants for their nutrition and growth. But how do plants obtain the food they need to grow? Unlike animals, plants do not eat food.
Leaves: the ‘food factories’ of plants
Plants store food in the form of starch, a type of carbohydrate. This starch is produced in the leaves of a plant which, by design, are generally broad and flat. These are mostly green because of the presence of a green pigment called chlorophyll, that helps in capturing sunlight efficiently.
Let us find out the role chlorophyll plays in the preparation of food in the form of starch in plants.
Let us check (demonstration activity)
The teacher may demonstrate this activity.
- Keep a leaf in boiling water for five minutes to soften it.
- Dip this leaf in a test tube containing alcohol.
- Place the test tube in a beaker containing boiling water. Wait until the leaf becomes colourless (Fig. (a)).
- Take out the leaf and place it on a plate.
- Now, put a few drops of diluted iodine solution with the help of a dropper on the decolourised leaf (Fig. (b)). Wait for a few minutes and observe.
- If the colour of the leaf changes to blue-black, it indicates the presence of starch.
Caution – Alcohol should never be placed near a heat source directly, as it is highly flammable and can easily lead to fire and burns.
Did you wonder why we decolourise the leaf in the beginning of this activity?
Decolourisation of a leaf enables us to easily observe colour change and, thus, the presence of starch.
In Activity, we have learnt that water and sunlight are essential for plant growth. In Activity, we have discovered that green leaves store starch as food.
Bhaskar loves gardening during his free time. Being a curious student, he often looks around his garden and wonders how plants produce food. From his experiences, Bhaskar knows that water and sunlight are essential for plant growth. But he wonders if sunlight contributes to the production of food in the form of starch in plants.
Bhaskar took a leaf having both green and non-green patches from each of two similar potted plants—one kept in sunlight and the other kept in the dark for 36 hours. He wanted to compare the leaves before and after the starch test.
He made a sketch of the leaves to record the location of the green and the non-green patches on them with the help of a tracing paper. After that, he performed an iodine test (as shown in Activity 10.2) on the leaves. Bhaskar recorded his observations in Table.
Table: Presence of starch in green and non-green parts of the leaves of plants
In Table, Bhaskar recorded a blue-black colour (indicating the presence of starch) on the green patches of the leaf obtained from the plant placed in sunlight. Bhaskar also recorded that the leaf obtained from the plant kept in the dark does not show a blue-black colour, even on the green patches, indicating that no starch has been produced. Non-green patches of the leaf obtained from the plant placed in sunlight do not turn blue-black. Does it indicate that there is no chlorophyll present in those patches? The non-green patches may not have sufficient chlorophyll to prepare enough starch to be detected using the iodine test.
Fascinating Facts
Some plant leaves appear red, violet, or brown because they contain more of these coloured pigments than the green-coloured chlorophyll. This hides the green colour. Some of these pigments also help in photosynthesis. You can use an iodine test to check for the presence of starch in these leaves, indicating that photosynthesis has indeed taken place.
What do we infer from the observations listed in Table?
As we know, leaves are mostly green because of the presence of chlorophyll. We have also seen that the starch is produced where green patches of the leaf are present. We can infer that chlorophyll helps in preparing starch in the presence of sunlight. In fact, it is essential for the preparation of starch. Hence, the leaves are also called ‘food factories’ of plants.
What else is essential for the preparation of food in plants?
Let us find out.
Role of air in the preparation of food
Let us experiment (demonstration activity)
Fig. Testing the role of chlorophyll and air
The teacher may demonstrate this activity.
- Take a potted green plant and keep it in the dark for two to three days to allow it to destarch (i.e., lose any stored starch). Then, locate one leaf of this plant for this experiment.
- Take a wide-mouthed bottle and pour some caustic soda (sodium hydroxide) into it (caustic soda absorbs carbon dioxide from the air).
Caution – Caustic soda is a strong chemical that can ^ cause skin burns; only teachers should handle it.
- Insert half of the destarched leaf into the bottle through a split cork, leaving the other half of the leaf outside, and place the bottle as shown in Fig. (a).
- Place the set-up in sunlight for a few hours.
- Observe and record the availability of water, sunlight, chlorophyll, and carbon dioxide in Table.
- Remove the leaf and test it for starch using the iodine test, as was done in Activity.
- Record your observations in Table.
We notice that the part of the leaf that was outside the bottle turns blue-black, indicating the presence of starch. However, the part of the leaf inside the bottle does not turn blue-black in colour, indicating that food is not made in that part of the leaf. This is because the caustic soda solution inside the flask absorbs the carbon dioxide present in the air. What does this experiment show?
This experiment shows that carbon dioxide present in the air is essential for plants to prepare starch.
Based on Activities, what do you conclude? Which part of the plant is involved in the synthesis of starch?
Based on our learnings so far, we have found that sunlight, water, chlorophyll and carbon dioxide are essential for the synthesis of food in plants. This process by which plants prepare food in the presence of sunlight and chlorophyll is called photosynthesis. A leaf is the primary site for photosynthesis. Do other green parts of the plant also perform photosynthesis? Yes, other parts of the plants which have chlorophyll also perform photosynthesis.
So far, we have learnt that plants take in carbon dioxide from the air and water, and use sunlight to prepare their food by the process of photosynthesis. But have you ever thought about what more happens during this process? Do plants only take in
Let us explore
- Look at Figure. Compare the two set-ups labelled as A and B, and analyse.
- In Figure, set-up A is placed in sunlight, and set-up B is placed in the dark. What difference do you observe in the two set-ups? Do you observe air bubbles emerging in the inverted test tube in set-up A? The gas produced in this set-up caused bubbles to emerge and get accumulated in the inverted test tube. Which gas is this?
Fig. Activity showing the release of oxygen during photosynthesis
Based on Barkha didi’s experiment, we can conclude that oxygen is released during photosynthesis.
Photosynthesis: in a nutshell
We know that water, sunlight, carbon dioxide from the air, and chlorophyll are necessary to carry out the process of photosynthesis that produces carbohydrates (Figure). During photosynthesis, food is actually produced in the form of glucose, a simple carbohydrate. This glucose not only serves as an instant source of energy but also later gets converted into starch for storage. The word equation of photosynthesis is given below:
Fig. A diagram showing photosynthesis
Know A Scientist
Many scientists across the world contributed to develop an understanding of photosynthesis. In India, Rustom Hormusji Dastur (1896-1961) studied the process of photosynthesis. He was a plant scientist and served as the head of the Botany Department at the Royal Institute of Science, Bombay (now the Institute of Science, Mumbai), from 1921-1935. He studied effects of the amount of water and temperature on photosynthesis. He examined the importance of water, temperature, and the colour of light in the process of photosynthesis.
How do leaves exchange gases during photosynthesis?
We now know that photosynthesis requires carbon dioxide, and oxygen is released in the process. Which part of the plant helps in the exchange of carbon dioxide and oxygen? Let us conduct an activity to understand where the exchange of gases takes place.
The teacher may demonstrate this activity.
- Collect a leaf from a plant such as rhoeo, money plant, onion, hibiscus, coleus, or any grass.
- Put it in a beaker fi lled with water.
- Carefully peel a thin layer from the lower surface of the leaf.
- Place the peel in a watch glass with water.
- Now, take a microscope slide and carefully put a drop of water on it.
- Using forceps, transfer the peel of the leaf from the watch glass to the slide with the help of forceps.
- Put a drop of ink on the leaf peel with the help of a dropper.
- Cover the peel with a coverslip and observe it under a microscope.
What do you observe? Do you notice tiny pores on the peel, as shown in Figure?
These pores are called stomata. Stomata, present on the surface of leaves, help in the exchange of gases.
Transport in Plants
Transport of water and minerals: All living beings need water to grow. Plants use water in the process of photosynthesis. Water, along with minerals present in the soil, is taken up by the roots of a plant. Minerals are important nutrients for the growth of plants. How do water and minerals taken up by the roots move to all parts of the plant? We can study water transport in plants by carrying out an activity. For this activity, we require two glass tumblers, some water, red ink, and twigs of two similar tender plants, preferably with white-coloured flowers (for example, white sadabahar, balsam), as shown in Figure.
Fig. Experiment to check for water transportation in plants
(a) With water plant twigs placed in water with different treatments
(b) With coloured water plant twigs placed in water with different treatments
(c) With water plant twigs after one day
(d) With coloured water plant twigs after one day
(e) Enlarged view of cut end of the twig
Fig. Transport of water and minerals in a plant
Do Plants Respire?
In the Grade 6 Science textbook Curiosity, chapter ‘Living Creatures: Exploring their Characteristics’, you learnt that all living beings respire. Do plants also respire like we do?
Fig. Set-up to test respiration in plants
Let us find out (demonstration activity)
- Soak some moong bean seeds in water overnight.
- Put a layer of cotton in a conical flask (Figure) and moisten the cotton with water to keep it wet.
- Place the soaked seeds over the wet cotton in the conical flask.
- Cover the mouth of the conical flask with a cork having two holes.
- Fit two tubes A and B through the two holes on the cork, as shown in Figure.
- Leave it undisturbed for 24 hours in the dark.
- Take two test tubes and fill them with lime water.
- Cover the mouth of one test tube with a cork having one hole in it.
- Dip one glass tube in the test tube through a hole in the cork.
- Connect the flask and test tube with a rubber pipe as shown in Figure.
Compare both the test tubes for any change in colour. Does the lime water turn milky in both the test tubes? Why does the lime water turn milky in the test tube connected to the flask? Lime water turns milky due to the presence of more carbon dioxide in the flask. But where does this carbon dioxide come from? As we know, carbon dioxide is naturally present in very small quantities in the air. In the flask, additional carbon dioxide is produced by the seeds as they respire.
During respiration, glucose is broken down in the presence of oxygen, releasing carbon dioxide, water, and energy. The word equation for the proccess of respiration, is as follows:
Glucose + Oxygen → Carbon dioxide + Water + Energy
The energy produced during respiration is used by plants for their growth and development. All parts of a plant, green or non-green, carry out respiration. Thus, plants have different mechanisms for synthesising, transporting, and utilising food to get energy.
Class 7 Science Notes
The post Life Processes in Plants Class 7 Notes Science Chapter 10 appeared first on Learn CBSE.