Life Processes in PlantsClass 7 Science Notes
Life Processes in Plants
In Grade 6, we learned 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 learned 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?
As a plant grows, new leaves and branches emerge, its height increases, and its stem thickens. Plants require both sunlight and water for their growth.
How Do Plants Get Food for their Growth?
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. 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. Leaves are the 'food factories' of plants.
- 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.
- 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. Wait for a few minutes and observe.
- If the colour of the leaf changes to blue-black, it indicates the presence of starch.
Role of air in the preparation of food
Air plays a role in the process of food preparation in plants. The gas from the air that is essential in the process of food preparation in plants is carbon dioxide.
- 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).
- 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. 10.3a.
- Place the set-up in sunlight for a few hours.
- Observe and record the availability of water, sunlight, chlorophyll, and carbon dioxide in Table 10.3.
- Remove the leaf and test it for starch using the iodine test, as was done in Activity 10.2.
- Record your observations in Table 10.3.
This experiment shows that carbon dioxide present in the air is essential for plants to prepare 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. Other green parts of the plant which have chlorophyll also perform photosynthesis.
Oxygen is released during photosynthesis.
Photosynthesis: in a nutshell
Water, sunlight, carbon dioxide from the air, and chlorophyll are necessary to carry out the process of photosynthesis that produces carbohydrates. 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-
How do leaves exchange gases during photosynthesis?
Stomata, present on the surface of leaves, help in the exchange of gases.
- Collect a leaf from a plant such as rhoeo, money plant, onion, hibiscus, coleus, or any grass.
- Put it in a beaker filled 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.
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. Water and minerals taken up by the roots move to all parts of the plant through the xylem.
- Take two tumblers and label them A and B.
- Fill one-third of each tumbler with water.
- Add a few drops of red ink to tumbler B.
- Obliquely cut the stems of both plants at their base while keeping them inside the water and immediately place one plant in each tumbler.
- Observe these plants the next day.
This is due to the thin tube-like structure called the xylem present in the stem, branches, and leaves of plants. Just like red ink, minerals dissolved in water also move up the stem through the xylem.
Transport of food
The food prepared by plants in the leaves is transported to all parts of the plant. This food is transported through another set of thin tube-like structures called the phloem. The transported food may also be stored in some other parts of a plant, such as seeds and roots.
Do Plants Respire?
All living beings respire. Plants also respire.
- Soak some moong bean seeds in water overnight.
- Put a layer of cotton in a conical flask 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.
- 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. Compare both the test tubes for any change in colour.
Lime water turns milky due to the presence of more carbon dioxide in the flask. 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 process of respiration is as follows-
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.