Life Processes in Plants

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CLASS VII Science ~6 marks/year Ch 10 of 12
Life Processes in Plants

Class 7 · Science · NCERT chapter notes · Akanksha Classes

Snapshot
  • Like animals, plants also carry out life processes — nutrition, transport, respiration and excretion — but in their own special ways.
  • Photosynthesis is the process by which green plants make their own food (glucose) using sunlight, chlorophyll, carbon dioxide and water. Plants are therefore called autotrophs.
  • Raw materials: CO2 from air (through tiny pores called stomata) and water + minerals from the soil (through roots). The green pigment chlorophyll traps sunlight.
  • Not all plants are green-feeders: there are parasitic (Cuscuta), insectivorous (pitcher plant, Venus flytrap), saprotrophic (fungi/mushrooms) and symbiotic (lichens) modes of nutrition.
  • Transport in plants: water and minerals rise up through xylem; prepared food travels through phloem to all parts of the plant.
  • Transpiration is the loss of water as vapour from the leaves; it helps suck water up and cools the plant.
  • Plants respire day and night, taking in O2 and giving out CO2; they remove some wastes through stomata and shedding of leaves.
  • Board weightage: ~6 marks/year — 1-mark definitions, 2-mark raw-material/role questions, 3-mark process or comparison answers.
Detailed Notes

1. How Do Plants Get Their Food?

Animals depend on plants or other animals for food, but green plants are different — they make their own food. Because they prepare their own food, plants are called autotrophs (auto = self, troph = nourishment). The way most green plants make food is called photosynthesis.

Autotrophs are organisms (mainly green plants) that make their own food from simple raw materials. Heterotrophs (most animals) depend on others for food.

Plants are the only living things that can capture the energy of sunlight and store it in food. This makes them the producers of every food chain — directly or indirectly, all animals depend on plants for food.

2. Photosynthesis — The Food Factory of Plants

The word photosynthesis means "building with light" (photo = light, synthesis = to make). It mainly takes place in the green leaves, which act like tiny food factories.

Photosynthesis is the process by which green plants make their food (glucose) from carbon dioxide and water, in the presence of sunlight and chlorophyll, releasing oxygen.

Carbon dioxide + Water → (sunlight, chlorophyll) → Glucose + Oxygen

2.1 The Four Requirements for Photosynthesis

  • Sunlight — provides the energy needed for the process.
  • Chlorophyll — the green pigment in leaves that traps sunlight.
  • Carbon dioxide — taken from the air through tiny pores called stomata.
  • Water — absorbed from the soil by the roots and carried up to the leaves.

Using these, the leaf makes a simple sugar called glucose. The glucose may be used at once for energy, or stored as starch. Oxygen is given out as a by-product — this is the oxygen we breathe.

2.2 The Leaf — Where Food is Made

The leaf is beautifully designed for photosynthesis. It is broad and flat to catch maximum sunlight, contains plenty of chlorophyll, and has thousands of tiny pores called stomata (singular: stoma) on its surface for the exchange of gases. Each stoma is guarded by two guard cells that open and close the pore.

Stomata are tiny pores, mostly on the underside of leaves, through which gases (CO2 and O2) and water vapour move in and out. They are opened and closed by guard cells.
NCERT Activity — Testing a leaf for starch (iodine test)

Take a leaf from a plant kept in sunlight. Boil it in water, then in alcohol to remove its green colour (chlorophyll). Wash the colourless leaf and add a few drops of iodine solution. The leaf turns blue-black, showing the presence of starch. This proves that the leaf has made food (starch) by photosynthesis. If you test a leaf from a plant kept in the dark, it does not turn blue-black, proving that sunlight is necessary for photosynthesis.

NCERT Activity — Is chlorophyll necessary? (variegated leaf)

Take a variegated leaf (one with green and white/pale patches, like a money plant or croton). Test it for starch with iodine. Only the green parts turn blue-black; the white parts do not change colour. This shows that food (starch) is made only in the parts that have chlorophyll — chlorophyll is essential for photosynthesis.

2.3 How Plants Get Other Nutrients

Besides carbohydrates made in photosynthesis, plants also need minerals like nitrogen to make proteins and other substances. Plants take these minerals dissolved in water from the soil. Soil nitrogen is enriched by bacteria such as Rhizobium, which live in the root nodules of leguminous plants (like peas and beans) and fix nitrogen from the air.

3. Other Modes of Nutrition in Plants

Not all plants make their food by photosynthesis. Some have special ways of getting food.

3.1 Parasitic Plants

A parasite takes its food from another living plant (the host) and gives nothing in return. Cuscuta (Amarbel) is a yellow, leafless creeper with no chlorophyll. It twines around a host plant and sends suckers into it to take ready-made food, harming the host.

3.2 Insectivorous Plants

Some plants grow in soil that is poor in nitrogen, so they trap and digest insects to get this nutrient. They are called insectivorous (insect-eating) plants. The pitcher plant has a pitcher-shaped leaf with a lid; an insect that lands inside slips down and is digested by juices at the bottom. The Venus flytrap snaps its leaf shut when an insect touches its trigger hairs. These plants still photosynthesise, but trap insects only for extra nitrogen.

Insectivorous plants are green plants that trap and digest insects to obtain nitrogen, because they grow in nitrogen-poor soil. Examples: pitcher plant, Venus flytrap, sundew.

3.3 Saprotrophic Nutrition

Saprotrophs get their food from dead and decaying matter. Fungi such as mushrooms and the green mould on bread feed in this way. They release digestive juices on the dead matter, which breaks it down, and then absorb the dissolved food. Saprotrophs are important because they decompose dead plants and animals and return nutrients to the soil.

3.4 Symbiotic Relationships

In symbiosis, two different organisms live together and both benefit. Lichens are a partnership between a fungus and an alga: the alga makes food by photosynthesis and shares it, while the fungus provides shelter, water and minerals. Another example is Rhizobium bacteria living in the root nodules of legumes — the plant gives shelter and food, and the bacteria fix nitrogen for the plant.

NCERT Activity — Observing bread mould (saprotroph)

Take a moist piece of bread and keep it in a warm, dark place for a few days. A fuzzy growth, often greenish or black, appears on it — this is bread mould, a fungus. It feeds on the bread (dead organic matter) by saprotrophic nutrition. This shows how saprotrophs grow on and decompose dead organic material. (Do not smell or eat the mouldy bread.)

4. Transport of Water and Minerals in Plants

A plant must move water, minerals and food from one part to another, sometimes over several metres in a tall tree. This is done by two kinds of pipe-like tissues running through the plant: xylem and phloem.

4.1 Xylem — Carrying Water Upward

Water and dissolved minerals are absorbed from the soil by the root hairs on the roots. From there they travel upward through the xylem tubes to the stem and finally to every leaf. Xylem carries water and minerals in only one direction — upward.

Xylem carries water and dissolved minerals from the roots upward to the leaves. Root hairs greatly increase the surface area of the root for absorbing water.

4.2 Phloem — Carrying Food

The food (glucose) made in the leaves must reach all parts of the plant, including the roots, stem, flowers and fruits. This food is carried by the phloem. Unlike xylem, phloem can transport food in both directions — upward and downward — to wherever it is needed or stored.

NCERT Activity — Coloured water rising in a stalk

Take a glass of water and add a few drops of red or blue ink. Place the freshly cut stalk of a white flower (such as a balsam or a stick of celery) in the coloured water and leave it for a few hours. The flower petals (or the veins of the leaf) slowly turn the colour of the ink. If you cut the stalk across, you see coloured spots — these are the xylem tubes. This shows that water rises up the plant through the xylem.

Feature Xylem Phloem
CarriesWater and mineralsPrepared food (glucose)
DirectionUpward only (roots → leaves)Both directions
From / ToRoots to all partsLeaves to all parts

5. Transpiration

Plants absorb far more water than they need. Most of this extra water is given out as water vapour from the leaves through the stomata. This loss of water as vapour from the aerial parts of a plant is called transpiration.

Transpiration is the loss of water in the form of vapour from the leaves and other aerial parts of a plant, mainly through the stomata.

5.1 Why Transpiration is Useful

  • It helps in sucking water and minerals up from the roots to the leaves — the pull created by transpiration draws water up the xylem.
  • It cools the plant on a hot day, just as sweating cools our body.
  • It helps in the upward movement of dissolved minerals.
NCERT Activity — Showing transpiration with a polythene bag

On a sunny day, tie a clear polythene bag tightly around the leafy branch of a potted plant. After a few hours, tiny droplets of water appear on the inside of the bag. These droplets are water vapour given out by the leaves through transpiration, which has condensed on the cooler plastic. This proves that plants lose water through their leaves.

6. Respiration in Plants

Like all living things, plants also respire to release energy from food. Plant cells take in oxygen and break down glucose to release energy, giving out carbon dioxide and water. Plants respire all the time — day and night.

Glucose + Oxygen → Carbon dioxide + Water + Energy. Plants respire through stomata in leaves, lenticels in stems, and through the surface of roots.

6.1 Photosynthesis and Respiration Together

It is easy to confuse the two. During the day, a plant carries out both photosynthesis and respiration. Photosynthesis happens only in light and gives out oxygen, while respiration happens all the time and gives out carbon dioxide. In bright sunlight, photosynthesis is much faster than respiration, so overall the plant gives out oxygen. At night, only respiration occurs, so the plant gives out carbon dioxide.

Feature Photosynthesis Respiration
WhenOnly in sunlight (daytime)All the time (day and night)
Gas taken inCarbon dioxideOxygen
Gas given outOxygenCarbon dioxide
EnergyStored in foodReleased from food

7. Excretion in Plants

Plants produce far fewer wastes than animals, and they get rid of them in simple ways. The gases CO2 (from respiration) and O2 (from photosynthesis) are released through the stomata. Extra water leaves through transpiration. Some solid and liquid wastes are stored harmlessly inside the plant or removed when old leaves, flowers and bark are shed. A few useful plant products that are really stored wastes include gums, resins and latex.

Plants excrete gaseous wastes through stomata, extra water by transpiration, and store or shed other wastes (as falling leaves, bark, gums, resins and latex).
Practice MCQs
1. Plants that make their own food are called:
  1. Heterotrophs
  2. Autotrophs
  3. Parasites
  4. Saprotrophs
Answer: (B) Green plants make their own food by photosynthesis, so they are called autotrophs.
2. The green pigment that traps sunlight for photosynthesis is:
  1. Glucose
  2. Starch
  3. Chlorophyll
  4. Carotene
Answer: (C) Chlorophyll is the green pigment in leaves that absorbs sunlight, which provides the energy for photosynthesis.
3. The gas taken in by leaves during photosynthesis is:
  1. Oxygen
  2. Nitrogen
  3. Carbon dioxide
  4. Hydrogen
Answer: (C) Plants take in carbon dioxide from the air through stomata and use it to make glucose, releasing oxygen.
4. The tiny pores on the leaf surface through which gases are exchanged are called:
  1. Stomata
  2. Root hairs
  3. Lenticels only
  4. Villi
Answer: (A) Stomata are tiny pores, mostly on the underside of leaves, that allow CO2, O2 and water vapour to move in and out. Each is controlled by guard cells.
5. In the iodine test, the part of a variegated leaf that turns blue-black is the:
  1. White part
  2. Green part
  3. Veins only
  4. Stalk
Answer: (B) Only the green parts have chlorophyll and so make starch; they turn blue-black with iodine. The white parts do not, proving chlorophyll is needed for photosynthesis.
6. Cuscuta (Amarbel) is an example of a:
  1. Saprotroph
  2. Insectivorous plant
  3. Parasitic plant
  4. Autotroph
Answer: (C) Cuscuta is a leafless, yellow creeper with no chlorophyll that takes ready-made food from a host plant, so it is a parasite.
7. Insectivorous plants such as the pitcher plant trap insects mainly to obtain:
  1. Carbon dioxide
  2. Nitrogen
  3. Oxygen
  4. Water
Answer: (B) Insectivorous plants grow in nitrogen-poor soil, so they trap and digest insects to get the nitrogen they need.
8. Fungi that feed on dead and decaying matter show which mode of nutrition?
  1. Autotrophic
  2. Parasitic
  3. Saprotrophic
  4. Symbiotic
Answer: (C) Saprotrophs like mushrooms and bread mould release digestive juices onto dead matter and absorb the dissolved food.
9. Water and minerals are transported in a plant through the:
  1. Phloem
  2. Xylem
  3. Stomata
  4. Guard cells
Answer: (B) Xylem carries water and dissolved minerals upward from the roots to the leaves.
10. The food prepared in the leaves is carried to other parts of the plant by the:
  1. Xylem
  2. Phloem
  3. Root hairs
  4. Stomata
Answer: (B) Phloem transports the glucose made in the leaves to all parts of the plant, in both directions.
11. The loss of water as vapour from the leaves of a plant is called:
  1. Respiration
  2. Transpiration
  3. Photosynthesis
  4. Condensation
Answer: (B) Transpiration is the loss of water as vapour through the stomata. It helps pull water up and cools the plant.
12. During the night, a plant gives out:
  1. Only oxygen
  2. Only carbon dioxide
  3. Both oxygen and carbon dioxide equally
  4. No gas at all
Answer: (B) At night there is no light, so photosynthesis stops, but respiration continues. Hence the plant takes in oxygen and gives out carbon dioxide.
13. The bacteria living in the root nodules of leguminous plants that fix nitrogen are:
  1. Rhizobium
  2. Amoeba
  3. Lactobacillus
  4. Cuscuta
Answer: (A) Rhizobium bacteria live symbiotically in the root nodules of legumes and fix atmospheric nitrogen into a form the plant can use.
14. The by-product released during photosynthesis is:
  1. Carbon dioxide
  2. Nitrogen
  3. Oxygen
  4. Water vapour
Answer: (C) Photosynthesis releases oxygen as a by-product — this is the oxygen used by animals for breathing.
15. A partnership between a fungus and an alga, in which both benefit, is seen in:
  1. Cuscuta
  2. Lichens
  3. Pitcher plant
  4. Mushroom
Answer: (B) Lichens are a symbiotic association: the alga makes food and the fungus provides shelter, water and minerals.
Important Questions
Q1. Define photosynthesis and write its word equation. (3 marks)
Answer: Photosynthesis is the process by which green plants make their own food (glucose) from carbon dioxide and water, in the presence of sunlight and chlorophyll, releasing oxygen. Word equation: Carbon dioxide + Water → (sunlight, chlorophyll) → Glucose + Oxygen.
Q2. Why do insectivorous plants trap and eat insects even though they are green? (2 marks)
Answer: Insectivorous plants such as the pitcher plant are green and can make carbohydrates by photosynthesis. However, they grow in soil that is poor in nitrogen. They trap and digest insects to obtain the nitrogen needed to make proteins, which they cannot get enough of from the soil.
Q3. Differentiate between xylem and phloem (any two points). (2 marks)
Answer: (1) Xylem carries water and dissolved minerals, while phloem carries the food (glucose) prepared in the leaves. (2) Xylem transports materials upward only (from roots to leaves), whereas phloem can transport food in both directions, to wherever it is needed or stored.
Q4. State three ways in which transpiration is useful to a plant. (3 marks)
Answer: (1) Transpiration creates a pull that helps suck water and minerals up from the roots to the leaves through the xylem. (2) It cools the plant on a hot day, like sweating cools our body. (3) It helps in the upward movement of dissolved minerals throughout the plant.
Q5. How can you show that starch is produced in a green leaf kept in sunlight? (3 marks)
Answer: Take a leaf from a plant kept in sunlight. Boil it in water and then in alcohol to remove the green chlorophyll. Wash the colourless leaf and put a few drops of iodine solution on it. The leaf turns blue-black, which shows the presence of starch. This proves the leaf made food by photosynthesis. A leaf from a plant kept in the dark does not turn blue-black, confirming that sunlight is needed.
Q6. Why does a plant give out oxygen during the day but carbon dioxide at night? (2 marks)
Answer: During the day the plant carries out both photosynthesis and respiration, but photosynthesis is much faster, so overall it takes in CO2 and gives out oxygen. At night there is no light, so photosynthesis stops and only respiration continues; the plant then takes in oxygen and gives out carbon dioxide.
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