Nutrition in Plants1In Class VI you learnt that food is

essential for all living organisms.You also learnt that carbohydrates,

proteins, fats, vitamins and minerals arecomponents of food. These componentsof food are necessary for our body andare called nutrients.

All living organisms require food.Plants can make their food themselvesbut animals including humans cannot.They get it from plants or animals thateat plants. Thus, humans and animalsare directly or indirectly dependent onplants.

1.1 MODE OF NUTRITION IN PLANTS

Plants are the only organisms that canprepare food for themselves by usingwater, carbon dioxide and minerals. Theraw materials are present in theirsurroundings.

The nutrients enable livingorganisms to build their bodies, to grow,to repair damaged parts of their bodiesand provide the energy to carry out lifeprocesses. Nutrition is the mode oftaking food by an organism and its

utilisation by the body. The mode ofnutrition in which organisms make foodthemselves from simple substances iscalled autotrophic (auto = self; trophos= nourishment) nutrition. Therefore,plants are called autotrophs. Animalsand most other organisms take in readymade food prepared by the plants. Theyare called heterotrophs (heteros =other).

Boojho wants to knowhow plants prepare

their own food.Now we may ask where the food

factories of plants are located: whetherfood is made in all parts of a plant oronly in certain parts? How do plantsobtain the raw materials from thesurroundings? How do they transportthem to the food factories of the plants?

1.2 PHOTOSYNTHESIS — FOOD

MAKING PROCESS IN PLANTS

Leaves are the food factories of plants.The synthesis of food in plants occursin leaves. Therefore, all the rawmaterials must reach there. Water andminerals present in the soil are absorbedby the roots and transported to the

Paheli wants to know whyour body cannot make foodfrom carbon dioxide, waterand minerals like plants do.

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Paheli wants to know what is sospecial about the leaves that they

can synthesise food but otherparts of the plant cannot.

Boojho wants to know how waterand minerals absorbed by roots

reach the leaves.

Cells

leaves. Carbon dioxide from air is takenin through the tiny pores present on thesurface of the leaves. These pores aresurrounded by ‘guard cells’. Such poresare called stomata [Fig. 1.2 (c)].

Water and minerals are transportedto the leaves by the vessels which runlike pipes throughout the root, the stem,the branches and the leaves. They forma continuous path or passage for thenutrients to reach the leaf. You will learnabout transport of materials in plantsin Chapter 11.

The leaves have a green pigmentcalled chlorophyll. It helps leaves tocapture the energy of the sunlight. Thisenergy is used to synthesise (prepare)food from carbon dioxide and water.Since the synthesis of food occurs in thepresence of sunlight, it is calledphotosynthesis (Photo: light; synthesis :to combine). So we find that chlorophyll,sunlight, carbon dioxide and water arenecessary to carry out the process ofphotosynthesis. It is a unique processon the earth. The solar energy iscaptured by the leaves and stored in theplant in the form of food. Thus, sun isthe ultimate source of energy for allliving organisms.

Can you imagine life on earth in theabsence of photosynthesis!

In the absence of photosynthesisthere would not be any food. Thesurvival of almost all living organismsdirectly or indirectly depends upon thefood made by the plants. Besides,oxygen which is essential for the survival

You have seen that buildings are made of bricks.Similarly, the bodies of living organisms aremade of tiny units called cells. Cells can be seenonly under the microscope. Some organismsare made of only one cell. The cell is enclosed bya thin outer boundary, called the cell membrane.Most cells have a distinct, centrally locatedspherical structure called the nucleus (Fig. 1.1).The nucleus is surrounded by a jelly-likesubstance called cytoplasm.

Cell membrane

Cytoplasm

Nucleus

Fig. 1.1 Cell

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Besides leaves, photosynthesis also takes place in other green parts of theplant — in green stems and green branches. The desert plants have scale- orspine-like leaves to reduce loss of water by transpiration. These plants havegreen stems which carry out photosynthesis.

of all living organisms is producedduring photosynthesis. In the absenceof photosynthesis, life would beimpossible on the earth.

During photosynthesis, chlorophyllcontaining cells of leaves (Fig. 1.2), inthe presence of sunlight, use carbondioxide and water to synthesisecarbohydrates (Fig. 1.3). The processcan be represented as an equation:

During the process oxygen isreleased. The carbohydrates ultimatelyget converted into starch. The presenceof starch in leaves indicates theoccurrence of photosynthesis. Thestarch is also a carbohydrate.

+ ⎯⎯⎯⎯⎯⎯→

+

sunlightchlorophyll

Carbon dioxide water

Carbohydrate oxygen

Boojho has observed someplants with deep red, violet or

brown leaves. He wants toknow whether these leaves

also carry out photosynthesis.

Activity 1.1

Take two potted plants of the same kind.Keep one in the dark (or in a black box)for 72 hours and the other in theFig. 1.2

(a) Leaf

Guard cells

(c) StomataStomatal opening

(b) A section through a leaf

Guard cells Stoma

Chlorophyll

Fig. 1.3 Diagram showingphotosynthesis

Light energy

Carbondioxide

OxygenChlorophyll

in leaf

Water andminerals

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Fig. 1.4 Leaves of various colours

process of photosynthesis. Thecarbohydrates are made of carbon,hydrogen and oxygen. These are usedto synthesise other components of foodsuch as proteins and fats. But proteinsare nitrogenous substances whichcontain nitrogen. From where do theplants obtain nitrogen?

Recall that nitrogen is present inabundance in gaseous form in the air.However, plants cannot absorb nitrogenin this form. Soil has certain bacteria thatconvert gaseous nitrogen into a usableform and release it into the soil. Thesesoluble forms are absorbed by the plantsalong with water. Also, you might haveseen farmers adding fertilisers rich innitrogen to the soil. In this way theplants fulfil their requirements ofnitrogen along with the otherconstituents. Plants can then synthesisecomponents of food other thancarbohydrates such as proteins and fats.

1.3 OTHER MODES OF NUTRITION INPLANTS

There are some plants which do not havechlorophyll. They cannot synthesisetheir food. How do they survive and fromwhere do they derive nutrition? Likehumans and animals such plantsdepend on the food produced by otherplants. They use the heterotrophicmode of nutrition. Look at Fig. 1.5. Doyou see yellow tubular structurestwining around the stem and branchesof a tree? This is a plant called Cuscuta(Amarbel). It does not have chlorophyll.It takes readymade food from the plant

You often see slimy, green patchesin ponds or in other stagnant waterbodies. These are generally formed bythe growth of organisms called algae.Can you guess why algae are green incolour? They contain chlorophyll whichgives them the green colour. Algae canalso prepare their own food byphotosynthesis.

Synthesis of plant food otherthan carbohydratesYou have just learnt that plantssynthesise carbohydrates through the

sunlight. Perform iodine test with theleaves of both the plants as you did inClass VI. Record your results. Now leavethe pot which was earlier kept in thedark, in the sunlight for 3 – 4 days andperform the iodine test again on itsleaves. Record your observations in yournotebook.

The leaves other than green also havechlorophyll. The large amount of red,brown and other pigments mask thegreen colour (Fig. 1.4). Photosynthesistakes place in these leaves also.

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on which it is climbing. The plant onwhich it climbs is called a host. Since itdeprives the host of valuable nutrients,it is called a parasite. Are we and theother animals also parasites on theplants? You should think about it anddiscuss with your teacher.

structure is the modified part of the leaf.The apex of the leaf forms a lid whichcan open and close the mouth of thepitcher. Inside the pitcher there are hairwhich are directed downwards. Whenan insect lands in the pitcher, the lidcloses and the trapped insect getsentangled into the hair. The insect isdigested by the digestive juices secretedin the pitcher. Such insect-eating plantsare called insectivorous plants.

Is it possible that such plants do notget all the required nutrients from thesoil in which they grow?

Fig. 1.5 Cuscuta (Amarbel) on host plant

Paheli wants to know whethermosquitoes, bed bugs, lice andleeches that suck our blood are

also parasites.

Fig. 1.6 Pitcher plant showing lid and pitcher

Leaf modifiedinto pitcher

Lid

Boojho is confused. If thepitcher plant is green and

carries out photosynthesis, thenwhy does it feed on insects?

Have you seen or heard of plants thatcan eat animals? There are a few plantswhich can trap insects and digest them.Is it not amazing? Such plants may begreen or of some other colour. Look atthe plant in Fig. 1.6. The pitcher-like

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1.4 SAPROTROPHS

You might have seen packets ofmushrooms sold in the vegetablemarket. You may have also seen fluffyumbrella-like patches growing on rottingwood during the rainy season (Fig. 1.7).Let us find out what type of nutrientsthey need to survive and from wherethey get them.

Boojho wants to know howthese organisms acquire

nutrients. They do not havemouths like animals do. They arenot like green plants as they lackchlorophyll and cannot make food

by photosynthesis.

Activity 1.2

Take a piece of bread and moisten it withwater. Leave it in a moist warm placefor 2–3 days or until fluffy patchesappear on them (Fig. 1.8). These patchesmay be white, green, brown or of anyother colour. Observe the patches undera microscope or a magnifying glass.

Paheli is keen to knowwhether her beautiful shoes,

which she wore on specialoccasions, were spoiled by fungi

during the rainy season. Shewants to know how fungi appear

suddenly during the rainyseason.

Write down your observations in yournotebook. Most probably you will seecotton-like threads spread on the pieceof bread.

These organisms are called fungi.They have a different mode of nutrition.They secrete digestive juices on the deadand decaying matter and convert it intoa solution. Then they absorb thenutrients from it. This mode of nutritionin which organisms take in nutrients insolution form from dead and decayingmatter is called saprotrophic nutrition.Plants which use saprotrophic mode ofnutrition are called saprotrophs.

Fungi also grow on pickles, leather,clothes and other articles that are leftin hot and humid weather for long time.

Fig. 1.8 Fungigrowing on bread

Fig. 1.7 Packet of mushrooms, a mushroomgrowing on decayed material

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fungus provides shelter, water andminerals to the alga and, in return, thealga provides food which it prepares byphotosynthesis.

1.5 HOW NUTRIENTS ARE

REPLENISHED IN THE SOIL

Have you seen farmers spreadingmanure or fertilisers in the fields, orgardeners using them in lawns or inpots? Do you know why they are addedto the soil?

You learnt that plants absorbmineral nutrients from the soil. So, theiramounts in the soil keep on declining.Fertilisers and manures contain plantnutrients such as nitrogen, potassium,phosphorous, etc. These nutrients needto be added from time to time to enrichthe soil. We can grow plants and keepthem healthy if we can fulfil the nutrientrequirement of plants.

Usually crops require a lot of nitrogento make proteins. After the harvest, thesoil becomes deficient in nitrogen. Youlearnt that though nitrogen gas is availablein plenty in the air, plants cannot use itin the manner they can use carbondioxide. They need nitrogen in a solubleform. The bacterium called Rhizobium cantake atmospheric nitrogen and convert itinto a soluble form. But Rhizobium cannotmake its own food. So it lives in the rootsof gram, peas, moong, beans and otherlegumes and provides them withnitrogen. Most of the pulses (dals) areobtained from leguminous plants. Inreturn, the plants provide food andshelter to the bacteria. They, thus, have

Boojho says once hisgrandfather told him that his

wheat fields were spoiled by afungus. He wants to know iffungi cause diseases also.

Paheli told him that many fungilike yeast and mushrooms areuseful, but some fungi

cause diseases in plants,animals and humans.

Some fungi are also usedin medicines.

During the rainy season they spoil manythings. Ask your parents about themenace of fungi in your house.

The fungal spores are generallypresent in the air. When they land onwet and warm things they germinateand grow. Now, can you figure out howwe can protect our things from gettingspoiled?

Some organisms live together andshare shelter and nutrients. This iscalled symbiotic relationship. Forexample, certain fungi live in the rootsof trees. The tree provides nutrients tothe fungus and, in return, receives helpfrom it to take up water and nutrientsfrom the soil. This association is veryimportant for the tree.

In organisms called lichens, achlorophyll-containing partner, which isan alga, and a fungus live together. The

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What you have learnt

All organisms take food and utilise it to get energy for the growth andmaintenance of their bodies.

Green plants synthesise their food themselves by the process ofphotosynthesis. They are autotrophs.

Plants use simple chemical substances like carbon dioxide, water andminerals for the synthesis of food.

Chlorophyll and sunlight are the essential requirements forphotosynthesis.

Complex chemical substances such as carbohydrates are the productsof photosynthesis.

Solar energy is stored in the form of food in the leaves with the help ofchlorophyll.

Oxygen is produced during photosynthesis.

Oxygen released in photosynthesis is utilised by living organisms fortheir survival.

Fungi derive nutrition from dead, decaying matter. They are saprotrophs.Plants like Cuscuta are parasites. They take food from the host plant.

A few plants and all animals are dependent on others for their nutritionand are called heterotrophs.

a symbiotic relationship. Thisassociation is of great significance forthe farmers. They do not need to addnitrogen fertiliser to the soil in whichleguminous plants are grown.

In this chapter you learnt thatmost of the plants are autotrophs.Only a few plants adopt other modes

of nutrition like parasitic andsaprotrophic. They derive nutritionfrom other organisms. All animalsare categorised as heterotrophs sincethey depend on plants and otheranimals for food. Can we say thatthe insectivorous plants are partialheterotrophs?

Keywords

AutotrophicChlorophyllHeterotrophsHost

InsectivorousNutrientNutritionParasite

PhotosynthesisSaprotrophsSaprotrophicStomata

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Exercise

1. Why do organisms need to take food?

2. Distinguish between a parasite and a saprotroph.

3. How would you test the presence of starch in leaves?

4. Give a brief description of the process of synthesis of food in green plants.

5. Show with the help of a sketch that the plants are the ultimate sourceof food.

6. Fill in the blanks:

(a) Green plants are called _________________ since they synthesisetheir own food.

(b) The food synthesised by the plants is stored as _________________.

(c) In photosynthesis solar energy is captured by the pigment called___________.

(d) During photosynthesis plants take in ______________________ andrelease __________________.

7. Name the following:

(i) A parasitic plant with yellow, slender and tubular stem.

(ii) A plant that has both autotrophic and heterotrophic mode of nutrition.

(iii) The pores through which leaves exchange gases.

8. Tick the correct answer:

(a) Amarbel is an example of:

(i) autotroph (ii) parasite (iii) saprotroph (iv) host

(b) The plant which traps and feeds on insects is:

(i) Cuscuta (ii) china rose (iv) pitcher plant (iv) rose

9. Match the items given in Column I with those in Column II:

Column I Column II

Chlorophyll Bacteria

Nitrogen Heterotrophs

Amarbel Pitcher plant

Animals Leaf

Insects Parasite

10. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(i) Carbon dioxide is released during photosynthesis. (T/F)

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(ii) Plants which synthesise their food themselves are calledsaprotrophs. (T/F)

(iii) The product of photosynthesis is not a protein. (T/F)

(iv) Solar energy is converted into chemical energy duringphotosynthesis. (T/F)

11. Choose the correct option from the following:

Which part of the plant takes in carbon dioxide from the air for photosynthesis?

(i) Root hair (ii) Stomata (iii) Leaf veins (iv) Sepals

12. Choose the correct option from the following:

Plants take carbon dioxide from the atmosphere mainly through their:

(i) roots (ii) stem (iii) flowers (iv) leaves

Extended Learning — Activities and Projects

1. Project

Take a potted plant with broadleaves. Take two strips of blackpaper and cut out a small squarein their centres. Cover a part of twoleaves with these papers and securethem with paper clips (Fig. 1.9).Keep the plant in the sunlight for2–5 days. Observe the difference inthe colour of the covered and theuncovered portions on the one leaf.Perform iodine test on this leaf. Didthe two parts show any differencein results? Now take second leaf.Remove the strip and expose thecovered part to the sunlight for 2–3 days and do the iodine test again.Describe your observations.

2. Visit a green house if there is one near your place. Observe how theyraise plants. Find out how they regulate the amount of light, water andcarbon dioxide to grow the plants.

3. Try growing a sweet potato just in water. Describe your experiment andobservations.You can read more on the following website:www.phschool.com/science/biology_place/biocoach/photosynth/overview.htm

Fig. 1.9 Experiment to test theoccurrence of photosynthesis

Did you know?

Light is so important to plants that their leaves grow in many patterns soas to catch the most sunlight.

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