Life Processes in Plants | CBSE Class 7 Science Notes

This note covers plant growth, food preparation in leaves, the roles of sunlight, water and air, tests for stored food, gas exchange, transport of water and food, and the release of energy in plants.

How do sunlight and water affect plant growth?

As plants grow, new leaves and branches emerge, their height increases and their stems thicken. Growth requires nutrients, the substances supplied by food that support growth, together with water.

Sunlight and water can be investigated by growing similar saplings, or young plants, under different conditions. Chilli and tomato are examples of fast-growing plants suitable for this activity. Starting with similar plants helps make the comparison meaningful.

How are the three pots arranged?

1. Fill three pots of the same size with garden soil. Plant a sapling of similar size in each pot and label the pots A, B and C. These letters identify the three treatments.
2. Count and record the leaves on each sapling. Record the initial height and leaf colour so that later changes can be compared with the starting condition.
3. Keep pot A in direct sunlight. Add adequate water every day to keep its soil slightly moist.
4. Keep pot B in direct sunlight, but do not add water. Keep pot C in the dark and add adequate water every day to keep its soil slightly moist.
5. Observe the plants for two weeks. Record height, number of leaves, leaf colour and any other changes that appear.

What can the observations show?

You are likely to find that the plant in pot A grows better than the plant in pot C. The plant in pot B may have died because it received no water, even though sunlight was available. The results indicate that plants require both sunlight and water for growth.

Compare A with B to examine the effect of withholding water while both receive sunlight. Compare A with C to examine the effect of darkness while both receive water. Actual heights and leaf counts must come from observations, rather than assumed results.

Why are leaves called the food factories of plants?

Animals obtain food directly from plants or indirectly from animals that eat plants. Plants prepare food instead of eating as animals do.

Chlorophyll is the green pigment, or colouring substance, that helps capture sunlight. Leaves are generally broad and flat, and are mostly green because they contain chlorophyll. Their role in food preparation gives them the name food factories.

Definition: Photosynthesis is food preparation from carbon dioxide and water using sunlight in the presence of chlorophyll. Carbon dioxide is a gas in air. The process produces glucose, a simple carbohydrate, and releases oxygen, a gas used in releasing energy from food. A carbohydrate is an energy-supplying nutrient.

What food is made and stored?

Glucose is a simple carbohydrate produced during photosynthesis. Glucose serves as an instant source of energy and is later converted into starch, a carbohydrate in which plants store food.

Food production and food storage are therefore connected, but glucose and starch should not be treated as interchangeable names. Glucose is the food actually produced during photosynthesis; testing a leaf for starch gives evidence of food stored there.

A leaf is the primary site of photosynthesis. Other parts of plants that have chlorophyll also perform photosynthesis. Calling leaves food factories does not mean that every other plant part is unable to prepare food.

Does the soil provide ready-made food?

Roots take up water and minerals from soil. Minerals are important nutrients for plant growth, but absorbing them is different from preparing glucose. Food preparation also needs carbon dioxide from the air, sunlight and chlorophyll.

This explains why supplying soil and water alone does not establish that all requirements for photosynthesis have been met. The growth experiment and the leaf experiments investigate different parts of the explanation: conditions for growth and conditions for making food.

How does the iodine test detect starch in a leaf?

The iodine test checks whether starch is present. Diluted iodine solution is added to a prepared leaf. If the leaf turns blue-black, the colour change indicates starch. The preparation makes the change easier to see.

What are the steps of the demonstration?

1. Keep the leaf in boiling water for five minutes to soften it. This is the first preparation step, before removing its colour.
2. Put the leaf in a test tube containing alcohol. Stand this tube in a beaker of boiling water and wait until the leaf becomes colourless.
3. Remove the leaf and place it on a plate. The removal of its colour is called decolourisation.
4. Use a dropper to add a few drops of diluted iodine solution to the decolourised leaf. Wait a few minutes before observing it.
5. Look for a blue-black colour. Where this colour appears, starch is present.

Note: This is a teacher demonstration. Alcohol is highly flammable, meaning that it catches fire easily. It must never be placed directly near a heat source because it can cause fire and burns.

The drawing shows a leaf in alcohol inside a test tube standing in a beaker of water. The beaker is supported above a spirit lamp. A second drawing shows iodine solution being added to a leaf on a plate.

See Fig. 10.2 in your NCERT textbook

Why remove the green colour first?

Decolourisation allows the iodine colour change to be observed easily. Without distinguishing the original leaf colour from the test result, it would be harder to judge whether starch is present. The significant observation is the blue-black colour after adding iodine.

The test can be used to compare leaves grown in different conditions or different patches of the same leaf. Such comparisons connect an observed colour change with the availability of light, chlorophyll or carbon dioxide during food preparation.

How do experiments show the roles of sunlight and chlorophyll?

A leaf with green and non-green patches provides a useful comparison. The position of each patch is recorded before testing. After decolourisation, the original green colour is no longer visible, so a tracing helps match the result to the correct region.

What happens in light and darkness?

Keep one of two similar potted plants in sunlight and the other in darkness for 36 hours. Then take a leaf with green and non-green patches from each plant. Sketch the patch positions using tracing paper, then carry out the iodine test.

The green patches from the plant kept in sunlight turn blue-black. The leaf from the plant kept in darkness does not show a blue-black colour, even in its originally green patches. The non-green patches from the sunlit plant also do not turn blue-black.

The comparison shows that chlorophyll is essential for preparing starch in the presence of sunlight. Green colour alone does not mean that starch will be produced under every condition: the green patches on the leaf kept in darkness do not show the positive result.

Note: Non-green patches may not have sufficient chlorophyll to prepare enough starch to be detected using the iodine test. A negative result should not be changed into the stronger claim that these patches contain no chlorophyll.

Can leaves of other colours prepare food?

Some leaves appear red, violet or brown because other coloured pigments are present in greater amounts than green chlorophyll. These pigments hide its green colour. Some of these pigments also help in photosynthesis.

An iodine test can check for starch in such leaves, indicating that photosynthesis has taken place. Visible colour and the presence of chlorophyll are therefore related, but a leaf that does not look green should not automatically be described as unable to photosynthesise.

How can we test whether carbon dioxide is needed?

Carbon dioxide is a gas in the air that plants use in photosynthesis. Its role can be investigated by comparing two parts of a single leaf, one with access to carbon dioxide and the other in air from which it has been absorbed.

First, the potted green plant is kept in darkness for two to three days. This allows it to destarch, meaning to lose previously stored starch. Destarching helps distinguish newly produced starch from starch that was already present.

How is the comparison arranged?

1. Select a leaf on the destarched plant. Put some caustic soda, also called sodium hydroxide, into a wide-mouthed bottle. This substance absorbs carbon dioxide from air.
2. Pass half of the leaf into the bottle through a split cork. Leave the other half outside the bottle, with the leaf still attached to the plant.
3. Keep the arrangement in sunlight for a few hours. Consider the availability of water, sunlight, chlorophyll and carbon dioxide for both parts of the leaf.
4. Remove the leaf and carry out the iodine test for starch. Compare the colour in the part that was inside the bottle with the colour in the part outside it.

Note: Caustic soda is a strong chemical that can burn skin. Only teachers should handle it. The experiment is a teacher demonstration.

What result supports the conclusion?

The part outside the bottle turns blue-black, indicating starch. The part inside does not turn blue-black. Caustic soda has absorbed carbon dioxide from the air inside, so food is not made in that part of the leaf.

The conclusion is that carbon dioxide is essential for preparing starch. The observation is a difference in the iodine-test result; the explanation connects that difference to the removal of carbon dioxide. The bottle is not being used to keep the leaf in darkness.

Together with the light and chlorophyll experiment, this comparison establishes the requirements for food synthesis, meaning food preparation. Water, sunlight, chlorophyll and carbon dioxide are all necessary for photosynthesis.

What does photosynthesis produce, and how is oxygen detected?

Photosynthesis produces glucose and releases oxygen, a gas that plants also use when releasing energy from glucose. A water plant can demonstrate gas release because bubbles can be seen and collected above it.

What does the water-plant experiment show?

Compare two arrangements with a water plant beneath a funnel and an inverted test tube, meaning a tube turned upside down. Keep one arrangement in sunlight and the other in darkness. In the sunlit arrangement, bubbles emerge and gas accumulates in the inverted tube.

When sufficient gas has collected, its effect on a lit matchstick can be demonstrated. The tube is removed with its mouth covered by a thumb, and a lit matchstick is quickly introduced. The matchstick produces an intense flame.

The collected gas is inferred to be rich in oxygen. This is evidence that oxygen is released during photosynthesis and that photosynthesis occurs in the presence of sunlight. Calling the gas oxygen-rich preserves the conclusion without claiming that the collected gas is pure oxygen.

How is the process written as a word equation?

A word equation names the substances used and produced in a process. Here, the plus sign means “and”, while the arrow means “produces”. Sunlight and chlorophyll are necessary conditions for the process.

Carbon dioxide + Water → Glucose + Oxygen

This equation operates in the presence of sunlight and chlorophyll. Glucose can supply energy and later be converted to starch for storage. Oxygen is released rather than being the food prepared by the plant.

The drawing shows a flowering plant rooted in soil. Labels identify light energy, chlorophyll pigment, carbon dioxide, water and minerals, carbohydrates as glucose, and oxygen. Arrows connect these labels with the plant and its surroundings.

See Fig. 10.5 in your NCERT textbook

The equation and diagram bring together the roles of roots, leaves and air. Water reaches the plant from below, while the leaf uses carbon dioxide and captured light in preparing food.

How do leaves exchange gases with their surroundings?

Stomata are tiny pores on the surface of leaves that help gases move between the plant and its surroundings. This movement is called gas exchange. Carbon dioxide is needed during photosynthesis, while oxygen is released.

The pores can be observed in a thin layer peeled from the lower surface of a leaf. A microscope, an instrument used to view very small structures, makes the pores visible. The leaf-peel preparation is a teacher demonstration.

How is a leaf peel prepared for observation?

1. Collect a suitable leaf, such as one from rhoeo, money plant, onion, hibiscus, coleus or grass. Place the leaf in a beaker containing water.
2. Carefully remove a thin layer from its lower surface. Keep this peel in water in a watch glass, a small shallow laboratory dish.
3. Put a drop of water on a microscope slide, the glass support for the specimen. Transfer the peel onto it using forceps, an instrument for picking up small objects.
4. Add a drop of ink. Cover the peel with a coverslip, a thin glass cover, and examine it under the microscope.

The tiny pores seen in the peel are the stomata. Their position on the leaf surface allows gas exchange, connecting the leaf's internal processes with the surrounding air.

How does gas exchange fit into food preparation?

The carbon dioxide needed for photosynthesis comes from air, rather than being taken up as ready-made food from soil. Stomata help the leaf exchange this gas and oxygen. The leaf therefore needs access to gases as well as the water supplied through the plant.

Locating stomata answers where gas exchange can occur. It is a different kind of evidence from the iodine test, which detects starch, or the collection of bubbles, which investigates a gas released during food preparation.

How are water, minerals and food transported through plants?

Roots take up water together with minerals from soil. These materials must reach leaves and other plant parts. Transport is the movement of materials from one part of a plant to another, linking the places of uptake, preparation and use.

What does the coloured-water experiment demonstrate?

1. Label two glass tumblers A and B to identify the plain-water and coloured-water arrangements. Fill one-third of each tumbler with water.
2. Add a few drops of red ink to tumbler B. Use twigs from two similar tender plants, preferably with white flowers, such as white sadabahar or balsam.
3. While keeping the stem bases in water, cut them obliquely, meaning at a slant. Immediately place one plant in each tumbler.
4. Observe the plants the next day. The plant in red-coloured water shows red colour in its stem, leaves and flowers.
5. Cut the upper part of the stem that was not immersed in the red water. Examine the cut using a magnifying glass and look for the red colour inside.

The red ink moves upwards with water through xylem, thin tube-like structures found in stems, branches and leaves. Minerals dissolved in water also move up the stem through xylem. Thus, the coloured water makes the water-transport pathway visible.

How does food reach the rest of the plant?

Phloem is another set of thin tube-like structures. It transports food prepared in leaves to all parts of the plant. The transported food may also be stored in other parts, such as seeds and roots.

Xylem and phloem have different transport roles. Water and minerals travel through xylem; food travels through phloem. Food made in a leaf is therefore available beyond the leaf itself.

The drawing shows roots, a cut stem and leaves. Blue arrows are labelled as water transport through xylem, while orange arrows are labelled as food transport through phloem.

See Fig. 10.8 in your NCERT textbook

These pathways connect the earlier experiments. Roots supply water needed for photosynthesis, and leaves prepare food that can be moved elsewhere. Absorption, food preparation and transport are linked functions rather than separate descriptions of how plants obtain the same material.

How do plants release energy through respiration?

Respiration is the process in which glucose is broken down in the presence of oxygen, releasing carbon dioxide, water and energy. Plants use the released energy for growth and development. All parts of a plant, green or non-green, carry out respiration.

Respiration is therefore different from preparing food. Photosynthesis makes glucose; respiration breaks it down to release energy. A green leaf is not the only part of a plant that needs to release energy.

How can germinating seeds provide evidence?

Germinating seeds are seeds beginning to grow. Soak moong bean seeds overnight and place them on wet cotton in a conical flask, a laboratory vessel with a broad base and narrow neck. Close the flask using a cork with two holes.

Fit two tubes through the holes and leave the flask undisturbed in darkness for 24 hours. Prepare two test tubes of lime water, a test liquid whose milkiness indicates carbon dioxide in this experiment. Connect the flask to one tube using the demonstrated glass-tube and rubber-pipe arrangement.

Compare the lime water in the connected tube with the other tube. The connected tube turns milky because the flask contains more carbon dioxide. Air naturally contains carbon dioxide in very small quantities; the respiring seeds produce additional carbon dioxide.

The drawing shows germinating seeds in a flask closed with a two-holed cork. Glass tubes pass through the cork, and a curved connecting pipe leads to a test tube containing lime water. A second lime-water tube stands beside it.

See Fig. 10.9 in your NCERT textbook

What is the word equation for respiration?

Glucose + Oxygen → Carbon dioxide + Water + Energy

The plus signs join the substances involved, and the arrow indicates the products and released energy. Glucose supplies the food being broken down, while oxygen is used in the process. Carbon dioxide and water are released along with energy.

The seed experiment supports the carbon-dioxide part of this explanation. It does not depend on seeds being green: respiration takes place in non-green plant parts as well.

How do photosynthesis and respiration work together?

Photosynthesis and respiration have different roles, but both help explain how plants live. Preparing glucose provides food; breaking down glucose releases usable energy. Water and food transport connect these processes across the plant.

What are the main differences?

The gases link the two processes. Oxygen generated in photosynthesis can be used in respiration. Carbon dioxide produced in respiration can be used in photosynthesis. Stomata help exchange these gases during both processes.

What can a bottle garden illustrate?

A bottle garden can be made with a growing plant such as spider plant or jade plant in a large transparent bottle. After allowing it to grow properly for some time, seal the bottle and observe its growth.

If the plant is growing well, it indicates that the plant is maintaining gas exchange inside the bottle. Carbon dioxide from respiration is used for photosynthesis, while oxygen generated during photosynthesis is used in respiration. The observation is conditional, rather than a guarantee that every sealed plant will grow well.

These connections also explain the importance of plants to animals. Animals obtain food from plants directly or indirectly. Photosynthesis provides food and releases oxygen, while respiration uses food and oxygen to release energy. Understanding both processes avoids the mistaken idea that plants prepare food but do not use it.

Glossary

• Nutrients — Substances supplied by food that support the growth and development of living organisms.
• Photosynthesis — Food preparation from carbon dioxide and water in the presence of sunlight and chlorophyll.
• Chlorophyll — Green pigment that helps plants capture sunlight for the preparation of food.
• Glucose — A simple carbohydrate produced during photosynthesis, supplying energy and later converted into starch.
• Starch — A carbohydrate in which plants store food, detected by a blue-black colour with iodine.
• Iodine test — A test in which a blue-black colour after adding iodine indicates the presence of starch.
• Decolourisation — Removal of a leaf's colour to make the iodine-test colour change easier to observe.
• Destarching — Loss of previously stored starch when a plant is kept in darkness before an experiment.
• Caustic soda — Sodium hydroxide, a strong chemical used to absorb carbon dioxide in the leaf experiment.
• Stomata — Tiny pores on leaf surfaces that help in exchanging gases with the surroundings.
• Minerals — Important nutrients taken up from soil along with water by the roots of plants.
• Xylem — Thin tube-like structures that transport water and dissolved minerals to different plant parts.
• Phloem — Thin tube-like structures that transport food prepared in leaves to all parts of a plant.
• Respiration — Breakdown of glucose using oxygen, releasing carbon dioxide, water and energy for growth and development.
• Germinating seeds — Seeds beginning to grow, used to demonstrate the release of carbon dioxide during respiration.

Common errors and misconceptions

• Misconception: Plants take ready-made food from soil. Correct: Roots absorb water and minerals; plants prepare glucose through photosynthesis using carbon dioxide and water in the presence of sunlight and chlorophyll.
• Misconception: Starch is the food directly produced during photosynthesis. Correct: Glucose is produced and later converted into starch for storage.
• Misconception: Any leaf that is not visibly green lacks chlorophyll. Correct: Other pigments can hide chlorophyll's green colour in red, violet or brown leaves.
• Misconception: A non-green patch that fails the iodine test definitely contains no chlorophyll. Correct: It may not have sufficient chlorophyll to prepare enough starch for detection.
• Misconception: Xylem transports food while phloem carries water. Correct: Xylem transports water and minerals; phloem transports food.
• Misconception: Only green plant parts respire. Correct: All plant parts, green or non-green, carry out respiration.
• Misconception: A plant receiving sunlight will definitely grow well without water. Correct: Plants need both; the unwatered plant in the growth experiment may have died despite receiving sunlight.
• Misconception: Alcohol can be heated directly during the starch test. Correct: It is highly flammable; the test tube containing it is placed in hot water during a teacher demonstration.

Exam-style questions with model answers

Q1. Name the food actually produced during photosynthesis and explain how it is stored. [2 marks]

1. Photosynthesis produces glucose, a simple carbohydrate that serves as an instant source of energy.
2. Glucose is later converted into starch, the form in which plants store food.

Q2. State the transport functions of xylem and phloem in a plant. [2 marks]

1. Xylem carries water and dissolved minerals from the roots to leaves and other plant parts.
2. Phloem carries food prepared in leaves to all parts of the plant.

Q3. Similar saplings are observed for two weeks. Pot A receives direct sunlight and adequate water, pot B receives direct sunlight without added water, and pot C receives adequate water in darkness. State the likely comparison between A and C, a possible outcome for B, and the conclusion about growth requirements. [3 marks]

1. The plant in pot A is likely to grow better than the plant in pot C because A receives both sunlight and adequate water.
2. The plant in pot B may have died because water was not supplied, even though sunlight was available.
3. The results indicate that both sunlight and water are required for plant growth; supplying one does not replace the other.

Q4. In an iodine test, green patches of a leaf from a sunlit plant turn blue-black, but its non-green patches do not. Green patches from a similar plant kept in darkness for 36 hours also fail to turn blue-black. Explain these three observations without assuming that the non-green patches contain no chlorophyll. [3 marks]

1. The sunlit green patches contain starch, shown by the blue-black colour. Chlorophyll helps prepare starch in the presence of sunlight.
2. The leaf kept in darkness does not show the blue-black colour even in its green patches, indicating that no starch has been produced under those conditions.
3. The non-green patches may not have sufficient chlorophyll to prepare enough starch to be detected by the iodine test.

Q5. A plant is destarched in darkness for two to three days. Half of an attached green leaf is placed in a bottle containing caustic soda, which absorbs carbon dioxide; the other half remains outside. After a few hours in sunlight, only the outside half turns blue-black with iodine. Explain destarching, the inside result, the outside result and the conclusion. [4 marks]

1. Destarching allows the plant to lose previously stored starch, so the later test can investigate starch produced during the experiment.
2. The inside half lacks carbon dioxide because caustic soda absorbs it. This part does not prepare food and fails to turn blue-black.
3. The outside half has access to carbon dioxide and produces starch, indicated by the blue-black colour after iodine is added.
4. The contrasting results show that carbon dioxide from air is essential for the preparation of starch in plants.

Q6. Describe five stages of the teacher-demonstrated iodine test for starch in a leaf, including the alcohol-heating precaution and the positive result. [5 marks]

1. Place the leaf in boiling water for five minutes to soften it before carrying out the remaining steps of preparation.
2. Transfer it into alcohol in a test tube. Put the test tube in a beaker of boiling water; never place alcohol directly near a heat source because it is highly flammable.
3. Wait until the leaf becomes colourless. Removing its original colour makes the later iodine colour change easier to observe clearly.
4. Take out the decolourised leaf and place it on a plate. Add a few drops of diluted iodine solution using a dropper.
5. Wait a few minutes and observe the colour. A blue-black colour indicates that starch is present in the tested leaf.

Q7. Two similar tender plant twigs with white flowers are placed in tumblers filled one-third with water. Only tumbler B receives red ink. Their stem bases are cut obliquely under water before immediate placement. After one day, B's twig has red colour in its stem, leaves and flowers, including an upper stem cut above the water. Explain five features of the evidence and transport pathway. [5 marks]

1. The twig in plain water provides a comparison for the twig in red-coloured water. Similar plants are used, and both tumblers begin one-third filled with water.
2. The red colour observed in the stem shows that coloured water has moved into the plant from tumbler B.
3. Red colour in the leaves and flowers shows that the water has reached these parts, rather than remaining at the immersed stem base.
4. Red colour in the cut upper stem reveals the internal route taken by coloured water, even above the part immersed in the tumbler.
5. The route is xylem, the thin tube-like structures that carry water through the plant. Minerals dissolved in water also move up the stem through this pathway.

Q8. Germinating moong seeds are kept on wet cotton in a flask for 24 hours in darkness. Gas from the flask reaches one of two lime-water tubes. The connected tube turns milky, indicating additional carbon dioxide from the seeds. Explain the observation, write the word equation for respiration, and state which plant parts respire. [3 marks]

1. The seeds release carbon dioxide as they respire. This adds to the small amount naturally present in air and makes the connected lime water turn milky.
2. The word equation is: glucose + oxygen → carbon dioxide + water + energy. Glucose is broken down using oxygen, releasing energy.
3. All parts of a plant, green or non-green, carry out respiration. The released energy supports their growth and development.

Key takeaways

• Plants require both sunlight and water for growth; an unwatered plant may die even when sunlight is available.
• Photosynthesis uses carbon dioxide and water in the presence of sunlight and chlorophyll to produce glucose and oxygen.
• Glucose can provide energy and is later converted into starch, which gives a blue-black colour with iodine.
• Leaves are the primary site of photosynthesis, but other plant parts containing chlorophyll also perform this process.
• Stomata are tiny leaf-surface pores that help exchange carbon dioxide and oxygen during photosynthesis and respiration.
• Xylem transports water and minerals, while phloem distributes food from leaves to all parts of the plant.
• Respiration breaks down glucose using oxygen, releasing carbon dioxide, water and energy for plant growth and development.
• All plant parts respire, including non-green parts; photosynthesis and respiration have different roles in the use of food and gases.

Test yourself

Why is a leaf decolourised before adding iodine?

Removing its original colour makes the iodine colour change easier to observe, allowing starch to be detected clearly.

What does a blue-black colour after the iodine test indicate?

It indicates that starch is present in the tested part of the leaf.

Why is a plant kept in darkness before the carbon-dioxide experiment?

Keeping it in darkness for two to three days allows it to lose previously stored starch before testing new food preparation.

What does caustic soda do in the leaf experiment?

It absorbs carbon dioxide from the air inside the bottle, removing a requirement for photosynthesis.

Why can a red or brown leaf still carry out photosynthesis?

Other pigments can hide the green colour of chlorophyll. Such leaves can still contain chlorophyll and prepare food.

Where can food transported away from leaves also be stored?

Transported food may also be stored in other plant parts, such as seeds and roots.

What does an intense flame in the collected water-plant gas suggest?

It suggests that the collected gas is rich in oxygen, supporting oxygen release during photosynthesis.

How is respiration different from photosynthesis in its use of glucose?

Photosynthesis produces glucose as food, while respiration breaks down glucose using oxygen to release energy.