Chapter Notes: Nutrition in Plants

Table of Contents
1. Nutrition
2. Modes of Nutrition
3. Autotrophic Nutrition
4. Photosynthesis
5. Heterotrophic Nutrition
6. Replenishment of Nutrients in the Soil
7. Activity 1
8. Activity 2
9. Activity 3
10. Activity 4
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Nutrition

Nutrition is the process by which living organisms obtain and use food to gain energy and materials for growth, repair and maintenance. Food is a substance that is chemically broken down in an organism's body to release energy and provide raw materials for body processes.

Modes of Nutrition

• There are two main modes of nutrition: autotrophic and heterotrophic.
• Autotrophic nutrition and heterotrophic nutrition differ in how organisms obtain and prepare their food.

Autotrophic Nutrition

Autotrophic nutrition is the mode of nutrition in which an organism makes its own food from simple inorganic substances. Green plants are the common example of autotrophs; they make food by the process of photosynthesis.

Autotrophs

• Organisms that follow the autotrophic mode of nutrition are called autotrophs.
• The word autotroph comes from two Greek words: auto (self) and trophe (nutrition).
• Autotrophs are self-sufficient in nutrition and do not depend on other organisms for food.

Photosynthesis

Photosynthesis is the biochemical process by which green plants, some algae and certain bacteria use sunlight energy to synthesise organic food (mainly glucose) from carbon dioxide and water. The process produces oxygen as a by-product.

This process occurs mainly in the green parts of plants, especially in the leaves, and inside the leaf cells in organelles called chloroplasts. The overall simplified chemical equation of photosynthesis is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Parts of a Plant Involved in Photosynthesis

• Leaves are the principal organs of photosynthesis because they contain many chloroplasts and have large surface area to absorb sunlight.
• Chloroplasts are the cell organelles that contain the green pigment chlorophyll, which absorbs light energy required for photosynthesis.
• Stomata (plural of stoma) are small pores on the leaf surface that allow exchange of gases - carbon dioxide enters and oxygen exits through stomata.

Chloroplasts and Chlorophyll

• Chloroplasts contain chlorophyll and the internal membrane system (thylakoids) where light-dependent reactions take place.
• Chlorophyll absorbs light (mainly blue and red wavelengths) and converts light energy into chemical energy, driving the synthesis of organic molecules.

Process of Photosynthesis

• Photosynthesis can be divided into two linked stages: the light-dependent reactions (light reactions) and the light-independent reactions (Calvin cycle or dark reactions).
• The light-dependent reactions occur on the thylakoid membranes of chloroplasts and convert light energy into chemical energy in the form of ATP and NADPH, producing oxygen from water.
• The light-independent reactions take place in the stroma of the chloroplast and use ATP and NADPH to fix carbon dioxide into glucose via the Calvin cycle.

The light-independent reactions occur in the stroma of the chloroplasts and involve the conversion of carbon dioxide and water into glucose (a simple sugar) using the ATP and NADPH produced during the light-dependent reactions.

Importance of Photosynthesis

• Photosynthesis is the primary source of organic matter for almost all organisms, forming the base of food chains.
• It produces oxygen required by most living organisms for respiration.
• Photosynthesis helps maintain the balance of gases in the atmosphere by removing carbon dioxide and releasing oxygen.

Necessary Conditions for Photosynthesis

Here are the necessary conditions for photosynthesis:

1: Sunlight
Sunlight is the primary source of energy for photosynthesis. Plants absorb sunlight through chlorophyll in their leaves. Without sufficient light, the rate of photosynthesis decreases.

2: Chlorophyll
Chlorophyll is the green pigment required to absorb light energy. Plants without chlorophyll cannot carry out photosynthesis (although some plants may obtain food in other ways).

3: Carbon Dioxide
Carbon dioxide from the air is one of the raw materials for photosynthesis. It enters the leaf through open stomata.

Carbon dioxide is a gas present in the air. It is an essential component of photosynthesis as it provides the carbon atoms needed to form glucose. Plants take in carbon dioxide from the air through tiny pores called stomata.

 If there is sufficient light and water, the guard cells expand and bend apart, which results in the stoma opening up. This enables carbon dioxide to penetrate the leaf cells.

A stoma is surrounded by two guard cells, which are shaped kidney. These cells control the opening and closing of the stoma.

4: Water

• Water is essential for photosynthesis and is absorbed from the soil by the roots.
• Xylem transports water and dissolved minerals from roots to leaves.
• Phloem transports the manufactured food (dissolved sugars) from the leaves to other parts of the plant.

Phloem, on the other hand, is responsible for transporting sugars and other nutrients from the leaves to the rest of the plant. It is a system of tubes that run from the leaves to the roots and other parts of the plant. The movement of nutrients through the phloem is driven by a process called translocation, which is the movement of nutrients from areas of high concentration to areas of low concentration.

5: Temperature

Temperature affects the rate of the chemical reactions of photosynthesis. Photosynthesis generally proceeds best between about 20-25 °C for many temperate plants; too low or too high temperatures reduce the rate.

Heterotrophic Nutrition

Heterotrophic nutrition is the mode of nutrition in which organisms obtain prepared organic food from other organisms (plants or animals). Animals, fungi and many bacteria are examples of heterotrophs. Heterotrophs depend directly or indirectly on autotrophs for their food.

Heterotrophic Organisms

Organisms that rely on heterotrophic nutrition are called heterotrophs. They cannot produce their own food and must obtain it from other organisms.

Types of Heterotrophic Plants

1. Parasitic plants
2. Saprophytic plants
3. Insectivorous (carnivorous) plants
4. Symbiotic plants

1: Parasitic Plants

Parasitic plants depend on another living plant (the host) for water, minerals and sometimes food. They may attach to the host's stem, roots or leaves.

Examples: Cuscuta (dodder), mistletoe, and Rafflesia.

Dodder

Parasitic plants can harm their hosts by reducing the host's growth and vigour; in severe infestations they may weaken or kill the host.

Types of parasitic plants:

• Root parasites: Attach to host roots and extract water and nutrients. Example: some species of the genus Orobanche.
• Stem parasites: Twine around or penetrate the host stem. Example: Cuscuta (dodder).
• Leaf parasites: Attach to leaves of the host to draw nutrients. Example: some species of mistletoe.

Characteristics of parasitic plants:

• Often have reduced or absent leaves and little or no chlorophyll.
• Obtain nutrients by special structures (haustoria) that penetrate the host.
• Some are partially parasitic and can perform some photosynthesis but still depend on the host for water and minerals.

Impact and control:

• Parasitic plants can reduce crop yield and damage ornamental plants.
• Control measures include physical removal, chemical control with suitable herbicides and biological control where applicable.

Fun fact: Rafflesia produces one of the world's largest flowers can reach up to about 106 cm in diameter and weigh up to 10 kg. Rafflesia is a specialised obligate parasite.

2: Saprophytic Plants

Saprophytic plants obtain their nutrients from dead and decaying organic matter. Many such plants are non-green and obtain nutrients via a close association with fungi (mycorrhizal or parasitic fungi).

• Saprophytes are often whitish or pale because they lack chlorophyll.
• They commonly grow in deep shade in forests where dead organic matter is abundant.
• They make use of fungi (saprotrophs) that break down dead matter into soluble nutrients which the plant can absorb.

Examples include Indian pipe and Coral root.

Role of fungi:

• Fungi secrete enzymes that decompose dead organic material into simpler substances which saprophytic plants and other organisms can use.
• Fungi that feed on dead matter are called saprotrophs.

Importance: Saprophytic organisms help in nutrient recycling and maintain soil fertility by breaking down dead matter. No control is generally required for saprophytes because they do not harm living plants.

3: Insectivorous (Carnivorous) Plants

Insectivorous plants trap and digest small animals, mainly insects, to obtain nitrogen and other nutrients that are scarce in the soil where they grow.

They are common in nutrient-poor habitats such as bogs and water-logged soils.

Examples of insectivorous plants:

Venus flytrap - leaves are modified into snap traps with stiff hairs on the inner surface; when an insect touches the hairs the trap snaps shut and the insect is digested.

Pitcher plant - a leaf is modified into a pitcher-shaped structure with slippery walls and downward-pointing hairs; insects fall in and are digested by enzymes in the fluid at the bottom.

Sundew - leaves bear glandular tentacles that secrete sticky mucilage; insects get stuck and are gradually digested.

Sundew

Bladderwort - aquatic or wet-soil plants with small bladder-like traps that suck in tiny organisms when triggered.

4: Symbiotic Plants

Symbiosis is a close and long-term biological interaction between two different organisms, often mutually beneficial.

Some plants form symbiotic relationships with fungi, bacteria or other plants to obtain nutrients.

Lichens are a symbiotic association between a fungus and a photosynthetic partner (green algae or cyanobacteria). The algal partner produces food by photosynthesis, while the fungus provides shelter and retains water and minerals. Lichens can grow in harsh environments such as bare rocks and deserts.

Lichens on rock

Legume-Rhizobium symbiosis: Many leguminous plants (e.g., peas, beans) form a symbiotic association with Rhizobium bacteria that live in root nodules. Rhizobium bacteria fix atmospheric nitrogen into forms usable by plants; in return the plant supplies the bacteria with carbohydrates. This process enriches soil nitrogen.

Roots of some plants, such as peas, contain bacteria called Rhizobium. These bacteria convert atmospheric nitrogen into usable forms like ammonia, and the plant provides nutrients for the bacteria's growth. This symbiotic association of Rhizobium and leguminous plants, such as peas and beans, is a natural way of replenishing the soil with nitrogen. Farmers sometimes grow these plants alternately with other crops to restore the nitrogen content of the soil.

Replenishment of Nutrients in the Soil

Plants require mineral nutrients such as nitrogen, phosphorus and potassium for healthy growth. Continuous cropping and harvesting can deplete these nutrients.

• Manures and fertilisers are added to soil to replenish nitrogen (N), phosphorus (P) and potassium (K) and other micronutrients.
• Use of legume crops and their symbiosis with Rhizobium bacteria is an eco-friendly method to increase soil nitrogen through biological nitrogen fixation.

Activity 1

Aim: To prove that the green parts of a plant, which have chlorophyll, can do photosynthesis.

Materials: A variegated (two-coloured) leaf, a beaker, a stand, a heat source (Bunsen burner or spirit lamp), a test tube, water, alcohol (ethanol), and iodine solution.

Method:

1. Half fill a beaker with water and place it on a stand; heat the water to boiling.
2. Soak the leaf in the boiling water for about two minutes to soften and kill the cells; this stops further chemical changes.
3. Place the leaf in a test tube containing alcohol that is about three-fourths full.
4. Place the test tube in the hot water for about ten minutes; the alcohol will decolourise the leaf by removing chlorophyll, making it nearly colourless.
5. Remove the leaf from the alcohol, rinse in warm water to remove alcohol, spread it on a white tile and add four drops of iodine solution on different parts.

Observation: The formerly green areas of the leaf turn blue-black on adding iodine (indicating starch), while the non-green (colourless) areas do not change colour. This shows that only green parts with chlorophyll formed starch by photosynthesis.

Activity 2

Aim: To investigate whether light is required for photosynthesis.

Materials: A green potted plant, black paper, scissors and a clip.

Method:

1. Keep the plant in a dark place for three days so that existing starch in leaves is exhausted.
2. Pluck a leaf and test it for starch to ensure it is starch-free (the whole leaf shows no blue-black colour with iodine).
3. Cover a part of a fresh leaf with a strip of black paper and fix it with a clip so that the covered part gets no light.
4. Expose the plant to sunlight for at least six hours.
5. Remove the leaf, remove the paper strip and test the leaf for starch using iodine solution.

Observation: The part of the leaf that was covered and had no light does not turn blue-black; the exposed part turns blue-black on adding iodine. This demonstrates that light is required for photosynthesis and starch formation.

Activity 3

Aim: To show that carbon dioxide is required for photosynthesis.

Materials: A potted plant, dilute potassium hydroxide (KOH) solution (which absorbs CO₂), a conical flask with a hole in the cork for the leaf, iodine solution and a dropper. (Handle KOH with care; it is corrosive.)

Method:

1. Keep the potted plant in a dark room for a few hours and water it.
2. Pour dilute KOH solution into a conical flask (KOH absorbs CO₂ from air).
3. Insert one leaf (still attached to the plant) into the flask without touching the solution and seal the flask with the cork.
4. Place the setup in sunlight alongside the plant itself.
5. After a few hours, remove the leaf from inside the flask and another leaf from the same plant and test both for starch using iodine solution.

Observation: The leaf kept in the flask (with CO₂ removed) shows no blue-black colour with iodine, while the other leaf turns blue-black. This indicates carbon dioxide is necessary for photosynthesis.

Activity 4

Aim: To grow fungi and observe saprophytic growth.

Materials: A piece of bread, water and a closed box (or petri dish).

Method: Moisten the bread with a little water and keep it in the closed box at room temperature for a few days.

Observation: After a few days grey or white patches (fungal growth / mould) appear on the bread. This demonstrates saprophytic organisms (fungi) growing on dead organic matter.