Welcome dear students! Today we are going to learn about Nutrition in Plants from Class 7 Science.
In Class 6, you learnt that food is essential for all living organisms. You also learnt that carbohydrates, proteins, fats, vitamins, and minerals are components of food. These components are called nutrients and are necessary for our body. All living organisms require food. Plants can synthesise food for themselves, but animals, including humans, cannot. They get it from plants or from animals that eat plants. Thus, humans and animals are directly or indirectly dependent on plants.
[CHECKPOINT]
Let us explore section one point one, Mode of Nutrition in Plants. Plants are the only organisms that can prepare food for themselves by using water, carbon dioxide, and minerals. The raw materials are present in their surroundings. Nutrients enable living organisms to build their bodies, to grow, to repair damaged parts, and provide the energy to carry out life processes. Nutrition is the mode of taking food by an organism and its utilisation by the body. The mode of nutrition in which organisms make food themselves from simple substances is called autotrophic nutrition. Auto means self, and trophos means nourishment. Therefore, plants are called autotrophs. Animals and most other organisms take in food prepared by plants. They are called heterotrophs. Heteros means other. Boojho wants to know how plants prepare their own food.
[CHECKPOINT]
We may ask where the food factories of plants are located. Are they in all parts of a plant or only in certain parts? How do plants obtain the raw materials from the surroundings, and how do they transport them to the food factories? Paheli wants to know why our body cannot make food from carbon dioxide, water and minerals like plants do. You have seen that buildings are made of bricks. Similarly, the bodies of living organisms are made of tiny units called cells. Cells can be seen only under a microscope. Some organisms are made of only one cell. The cell is enclosed by a thin outer boundary called the cell membrane. Most cells have a distinct, centrally located spherical structure called the nucleus. The nucleus is surrounded by a jelly-like substance called cytoplasm.
[CHECKPOINT]
Moving to section one point two, Photosynthesis, the food making process in plants. Leaves are the food factories of plants. Therefore, all the raw materials must reach the leaf. Water and minerals present in the soil are absorbed by the roots and transported to the leaves. Carbon dioxide from the air is taken in through the tiny pores present on the surface of leaves. These pores are surrounded by guard cells. Such pores are called stomata. In Figure one point two, we see a leaf, a cross section of a leaf, and a close up of a stoma showing the stomatal opening and the surrounding guard cells. Boojho wants to know how water and minerals absorbed by roots reach the leaves. Water and minerals are transported to the leaves by vessels which run like pipes throughout the root, the stem, the branches, and the leaves. They form a continuous path for nutrients to reach the leaf.
[CHECKPOINT]
Paheli wants to know what is so special about the leaves that they can synthesise food but other parts of the plant cannot. The leaves have a green pigment called chlorophyll. It helps leaves to capture the energy of sunlight. This energy is used to synthesise, or prepare, food from carbon dioxide and water. Since the synthesis of food occurs in the presence of sunlight, it is called photosynthesis. Photo means light, and synthesis means to combine. So, chlorophyll, sunlight, carbon dioxide, and water are necessary to carry out photosynthesis. It is a unique process on Earth. The solar energy is captured by the leaves and stored in the plant in the form of food. Thus, the sun is the ultimate source of energy for all living organisms. Can you imagine life on Earth without photosynthesis? In its absence, there would be no food. The survival of almost all living organisms depends directly or indirectly on the food made by plants. Besides, oxygen, which is essential for survival, is produced during photosynthesis.
[CHECKPOINT]
During photosynthesis, chlorophyll containing cells of leaves, in the presence of sunlight, use carbon dioxide and water to synthesise carbohydrates. The process can be represented in an equation: Carbon dioxide plus H₂O, in the presence of sunlight and chlorophyll, yields Carbohydrate plus oxygen. During this process, oxygen is released. The presence of starch in leaves indicates the occurrence of photosynthesis, and starch is also a carbohydrate. In Figure one point three, we see a diagram showing light energy entering a leaf, carbon dioxide entering through the stoma, water and minerals coming up through the stem, chlorophyll inside the leaf, and oxygen being released.
[CHECKPOINT]
Let us perform Activity one point one to understand this better. Take two potted plants of the same kind. Keep one in the dark, or in a black box, for seventy two hours, and keep the other in sunlight. Perform the iodine test with the leaves of both plants as you did in Class 6. Record your results. Now, leave the pot which was earlier kept in the dark, in the sunlight for three to four days and perform the iodine test again on its leaves. Record your observations in your notebook.
[CHECKPOINT]
Boojho has observed some plants with deep red, violet, or brown leaves and wants to know whether these leaves also carry out photosynthesis. Leaves other than green also have chlorophyll. The large amount of red, brown, and other pigments simply mask the green colour. In Figure one point four, we see leaves of various colours. Photosynthesis takes place in these leaves also. Besides leaves, photosynthesis also takes place in other green parts of the plant, such as green stems and green branches. Desert plants have scale or spine like leaves to reduce water loss by transpiration. These plants have green stems which carry out photosynthesis.
[CHECKPOINT]
You often see slimy, green patches in ponds or stagnant water bodies. These are generally formed by the growth of organisms called algae. Can you guess why algae are green in colour? They contain chlorophyll, which gives them their green colour, and algae can also prepare their own food by photosynthesis. Now, let us look at the synthesis of plant food other than carbohydrates. You have just learnt that plants synthesise carbohydrates through photosynthesis. Carbohydrates are made of carbon, hydrogen, and oxygen. These are used to synthesise other components of food such as proteins and fats. But proteins are nitrogenous substances which contain nitrogen. From where do plants obtain nitrogen? Recall that nitrogen is present in abundance in gaseous form in the air. However, plants cannot absorb nitrogen in this form. Soil has certain bacteria that convert gaseous nitrogen into a usable form and release it into the soil. These are absorbed by plants along with water. Also, farmers add fertilisers rich in nitrogen to the soil. In this way, plants fulfil their nitrogen requirements and can then synthesise proteins and vitamins.
[CHECKPOINT]
Let us move to section one point three, Other Modes of Nutrition in Plants. There are some plants which do not have chlorophyll. They cannot synthesise food. How do they survive and from where do they derive nutrition? Like humans and animals, such plants depend on the food produced by other plants. They use the heterotrophic mode of nutrition. Look at Figure one point five. Do you see a yellow, wiry, branched structure twining around the stem and branches of a tree? This is a plant called Cuscuta, also known as Amarbel. It does not have chlorophyll. It takes readymade food from the plant on which it is climbing. The plant on which it climbs is called the host. Since it deprives the host of valuable nutrients, Cuscuta is called a parasite. Paheli wants to know whether mosquitoes, bed bugs, lice and leeches that suck our blood are also parasites.
[CHECKPOINT]
Have you seen or heard of plants that can eat animals? There are a few plants which can trap insects and digest them. Is it not amazing? Such plants may be green or of some other colour. Look at the plant in Figure one point six, the pitcher plant. The pitcher like or jug like structure is the modified part of the leaf. The apex of the leaf forms a lid which can open and close the mouth of the pitcher. Inside the pitcher, there are hairs which are directed downwards. When an insect lands in the pitcher, the lid closes and the trapped insect gets entangled in the hair. The insect is digested by the digestive juices secreted in the pitcher, and its nutrients are absorbed. Such insect eating plants are called insectivorous plants. Boojho is confused. If the pitcher plant is green and carries out photosynthesis, then why does it feed on insects? Is it possible that such plants do not get all the required nutrients from the soil in which they grow? Can we say that the insectivorous plants are partial heterotrophs?
[CHECKPOINT]
Now let us explore section one point four, Saprotrophs. You might have seen packets of mushrooms sold in the vegetable market. You may have also seen fluffy, umbrella like patches growing in moist soils or on rotting wood during the rainy season, as shown in Figure one point seven. Let us find out what type of nutrients they need to survive and from where they get them. Boojho wants to know how these organisms acquire nutrients. They do not have mouths like animals do. They are not like green plants as they lack chlorophyll and cannot make food by photosynthesis. Let us do Activity one point two. Take a piece of bread and moisten it with water. Leave it in a moist, warm place for two to three days or until fluffy patches appear on it, as shown in Figure one point eight. What is the colour of these patches? Observe the patches under a microscope or a magnifying glass and write down your observations. You will see cotton like threads spread on the piece of bread.
[CHECKPOINT]
These organisms are called fungi. They have a different mode of nutrition. They absorb nutrients from the bread. This mode of nutrition, in which organisms take in nutrients from dead and decaying matter, is called saprotrophic nutrition. Such organisms with a saprotrophic mode of nutrition are called saprotrophs. Fungi also grow on pickles, leather, clothes, and other articles left in hot and humid weather for a long time. During the rainy season, they spoil many things. Ask your parents about the menace of fungi in your house. The fungal spores are generally present in the air. When they land on wet and warm things, they germinate and grow. Now, can you figure out how we can protect our things from getting spoiled? Paheli is keen to know whether her beautiful shoes, which she wore on special occasions, were spoiled by fungi during the rainy season. She wants to know how fungi appear suddenly during the rainy season.
[CHECKPOINT]
Some organisms live together and share both shelter and nutrients. This relationship is called symbiosis. For example, certain fungi live inside the roots of plants. The plants provide nutrients to the fungus, and in return, the fungus provides water and certain nutrients. In organisms called lichens, a chlorophyll containing partner, which is an alga, and a fungus live together. The fungus provides shelter, water, and minerals to the alga, and in return, the alga prepares and provides food to the fungus.
[CHECKPOINT]
Moving to section one point five, How Nutrients are Replenished in the Soil. Have you seen farmers spreading manure or fertilisers in the fields, or gardeners using them in lawns or pots? Do you know why this is done? You learnt that plants absorb minerals and nutrients from the soil, so their amounts in the soil keep on declining. Fertilisers and manures contain nutrients such as nitrogen, potassium, phosphorus, and so on. These nutrients need to be added from time to time to enrich the soil. We can grow healthy plants if we fulfil their nutrient requirements. Usually, crop plants absorb a lot of nitrogen, and the soil becomes deficient in it. Though nitrogen gas is available in plenty in the air, plants cannot use it directly. They need nitrogen in a soluble form. The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a usable form. But Rhizobium cannot make its own food. So, it often lives in the roots of gram, peas, moong, beans, and other legumes, providing them with nitrogen. In return, the plants provide food and shelter to the bacteria. They thus have a symbiotic relationship. This association is of great significance for farmers, as they can reduce the use of nitrogenous fertiliser where leguminous plants are grown. Most of the pulses, or dals, are obtained from leguminous plants.
[CHECKPOINT]
Boojho says once his grandfather told him that his wheat fields were spoiled by a fungus. He wants to know if fungi cause diseases also. Paheli told him that many fungi like yeast and mushrooms are useful, but some fungi cause diseases in plants, animals including humans. Some fungi are also used as medicines. In this chapter, you learnt that most plants are autotrophs. Only a few plants are parasitic or saprotrophic. They derive nutrition from other organisms. All animals are categorised as heterotrophs since they depend on plants and other animals for food. Can we say that insectivorous plants are partial heterotrophs? Let us quickly review what you have learnt. All organisms need food and utilise it to get energy for growth and maintenance of their body. Green plants synthesise food for themselves by the process of photosynthesis and are called autotrophs. Plants like Cuscuta are parasites that take food from the host plant. Plants use simple chemical substances like carbon dioxide, water, and minerals for food synthesis. Chlorophyll, water, carbon dioxide, and sunlight are the essential requirements for photosynthesis. Complex chemical substances such as carbohydrates are the products of photosynthesis. Solar energy is absorbed by the chlorophylls present in leaves. Oxygen is produced during photosynthesis and is utilised by living organisms for survival. Many fungi derive nutrition from dead and decaying matter and are called saprotrophs. A few plants and all animals are dependent on others for their nutrition and are called heterotrophs. The keywords for this chapter are Autotrophic, Chlorophyll, Heterotrophs, Host, Insectivorous, Nutrient, Nutrition, Parasite, Photosynthesis, Saprotrophs, Saprotrophic, and Stomata.
[CHECKPOINT]
Now let us move to the exercises. I will guide you through each question. Question one: Why do organisms take food? Organisms take food because nutrients enable them to build their bodies, grow, repair damaged parts, and provide energy to carry out life processes. Question two: Distinguish between a parasite and a saprotroph. A parasite is an organism that lives on or inside another living organism, called the host, and takes readymade food from it, like Cuscuta. A saprotroph is an organism that takes in nutrients from dead and decaying matter, like fungi. Question three: How would you test the presence of starch in leaves? Perform the iodine test on the leaves as you did in Class 6. If starch is present, the leaf will turn blue black. Question four: Give a brief description of the process of synthesis of food in green plants. The process is photosynthesis. Leaves containing chlorophyll capture sunlight energy. Using this energy, they combine carbon dioxide and water to synthesise carbohydrates, releasing oxygen in the process.
[CHECKPOINT]
Question five: Show with the help of a sketch that plants are the ultimate source of food. Since this is an audio lesson, I will describe the sketch. Draw a sun at the top and a green plant below it. Draw an arrow from the sun to the plant. Draw an arrow from the plant to a herbivore, and from the herbivore to a carnivore. Also draw an arrow from the plant directly to a human. This shows all animals depend directly or indirectly on plants. Question six: Fill in the blanks. (a) Green plants are called autotrophs since they synthesise their own food. (b) The food synthesised by plants is stored as starch. (c) In photosynthesis, solar energy is absorbed by the pigment called chlorophyll. (d) During photosynthesis, plants take in carbon dioxide and release oxygen gas. Question seven: Name the following. (i) A parasitic plant with yellow, slender, and branched stem is Cuscuta. (ii) A plant that is partially autotrophic is the pitcher plant. (iii) The pores through which leaves exchange gases are stomata.
[CHECKPOINT]
Question eight: Tick the correct answer. (a) Cuscuta is an example of a parasite. (b) The plant which traps and feeds on insects is the pitcher plant. Question nine: Match the items given in Column I with those in Column II. Chlorophyll matches with Leaf. Rhizobium matches with Nitrogen. Heterotrophs matches with Animals. Cuscuta matches with Parasite. Pitcher plant matches with Insects. Question ten: Mark T if true and F if false. (i) Carbon dioxide is released during photosynthesis. False. (ii) Plants which synthesise their food are called saprotrophs. False. (iii) The product of photosynthesis is not a protein. True. (iv) Solar energy is converted into chemical energy during photosynthesis. True. Question eleven: Which part of the plant takes in carbon dioxide from the air for photosynthesis? The correct option is Stomata. Question twelve: Plants take carbon dioxide from the atmosphere mainly through their leaves. Question thirteen: Why do farmers grow crops inside large green houses? What are the advantages? Farmers grow crops in green houses to protect them from diseases and adverse climatic conditions. The advantages are controlled temperature, light, and water, leading to higher yield, better quality, and year round production.
[CHECKPOINT]
Now let us look at the extended learning activities and projects. Project one: Take a potted plant with broad leaves. Cut a small square in the centre of two black paper strips. Cover a part of two leaves with these papers and secure them with paper clips. Keep the plant in sunlight for two to five days. Observe the difference in colour between the covered and uncovered portions. Perform the iodine test on this leaf. The uncovered part will turn blue black showing starch, while the covered part will not. Now take the second leaf, remove the strip, expose the covered part to sunlight for two to three days, and do the iodine test again. It will now turn blue black, proving light is necessary for starch formation. Project two: Visit a green house if there is one near you. Observe how they grow plants and find out how they regulate light, water, and carbon dioxide. Project three: Try growing a sweet potato just in water. Describe your experiment and observations. You will observe that the sweet potato will sprout and grow roots and shoots in water. You can read more on the following website: www dot pschool dot com slash science slash biology underscore place slash biocoach slash photosynth slash overview dot htm.
[CHECKPOINT]
Finally, a quick fact for you. Did you know? Light is so important to plants that their leaves grow in many patterns so as to absorb maximum sunlight. Isn't that fascinating?
Thank you for listening! Keep revising and practicing. Goodbye! [CHAPTER_COMPLETE]