1 Nutrition
Before we learn about the various life processes, we should know the defining characteristics of
life. How do we understand what is alive and what is not? We see a variety of things such as
mountains, land, buildings, plants, insects, birds, animals, etc., around us. How do we
differentiate these things? Some of these, like mountains, land and buildings, are nonliving
things, while others like plants, insects, birds and animals are living. What are the basic
differences between these? All living things carry out various life processes like taking in food,
obtaining energy from food, throwing out wastes, and so on. Living things also move, grow,
respond to changes in their external and internal conditions, and produce young ones.
All living things (organisms) also have an organized (cellular) structure with different levels
of organization. An organized and ordered structure with cells, tissues, organs, organ systems,
etc., is an important feature that distinguishes the living from the nonliving. If this organization
breaks down, an organism is no longer alive as organization is not only confined to the external
appearance, but is present in the internal structure as well. Therefore, living things have to repair
and maintain their structures through various processes. Let us learn about some of these
processes which help maintain life.
LIVING THINGS AND LIFE PROCESSES
You have learnt that living organisms have tissues, which comprise groups of cells with similar
structures and functions. A cell is the basic structural and functional unit of any living thing. Each
cell is made up of molecules. The molecules exhibit movement during cellular reactions. Such
reactions lead to the cellular life activities. There is an absence of such molecular movements
viruses, which remain nonliving until they infect a living organism. Their molecular movement
begins when they use the other organism's cell molecules and organelles for producing their own
proteins and replicating themselves.
Let us now discuss how living beings grow and how they maintain and repair their
structures. The growth of a living organism starts with the division of its cells. When a cell
divides, it forms two daughter cells from a single mother cell. The daughter cells divide and
redivide to give rise to tissues and organs. Different life processes of an organism, like growth
and maintenance, require energy which is obtained from food by a process called nutrition.
Different organisms have varied nutritional processes depending on their environment and
specific food requirements. Food is broken down into simpler forms by a stepwise
oxidizing--reducing process known as respiration. During this process, oxygen is commonly
required by organisms to release energy from food for carrying out various life processes.
Generally, in multicellular organisms all the cells are not in contact with the environment.
The exchange of gases and the uptake of food occur in specialized tissues. So, food and oxygen
have to be transported to all parts of the body. For the movement of food and oxygen from one
part to another, there is a transportation system. The carrying out of different life processes
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involves metabolism (chemical reactions in organisms), which produces harmful waste products
that have to be removed from the bodies of living beings. The process of elimination of these
waste products of metabolic activity from the body is excretion. In complex organisms a
specialized tissue system carries out excretion and a specialized transportation system carries the
metabolic waste products to the excretory tissues.
The exchange of materials with the environment is accomplished by diffusion in unicellular
organisms as the entire surface of the organism remains in contact with the environment.
Living things also respond to changes in their environment in a particular manner. They
produce new individuals similar to themselves by a process called reproduction. They have a
definite life cycle of birth, growth, reproduction and death.
From the above discussion it is clear that living things can be easily distinguished from
nonliving things as they carry out various life processes such as nutrition, respiration,
transportation,excretion, etc. In this chapter we shall discuss the basic concepts of nutrition.
Nutrition provides nutrients to the body so that it can obtain energy to carry out the activities
required to stay alive. Nutrients are substances that give nourishment, which provides energy to
an organism. Cells obtain nutrients from the food taken by the organism. The food taken by the
organism is complex, but nutrients are much simpler molecules. The digestive system of an
organism breaks down complex food into simpler molecules, so that the cells can take them
and use them for survival, growth and reproduction.
Nutrition promotes growth of the body, which involves the formation of new protoplasm.
Nutrition meets the energy requirement of the body. Nutrition helps synthesize a variety of
substances, like proteins, carbohydrates, lipids, etc., which in turn perform a variety of functions.
MODES OF NUTRITION
Plants and animals do not obtain food by the same processes. Plants and some bacteria have the
green pigment chlorophyll to help synthesize food, while animals, fungi and other bacteria
depend on other organisms for food. Based on this, there are two main modes of nutrition:
autotrophic and heterotrophic.
AUTOTROPHIC NUTRITION
The term 'autotroph' is derived from two Greek words autos (self) and trophe (nutrition). In
autotrophic nutrition, an organism makes its own food from simple raw materials.
Sun CO,from air
Light energy
Chlorophyl
+
Water
Glucose
Oxygen Released into the air
+To different parts of the plant
From soil through absorption
Fig.1.1 A summary of nutrition in green plants
Photosynthesis
Green plants, which are autotrophic, synthesize food through the process of photosynthesis.
Photosynthesis is a process by which green plants, having chlorophyll, synthesize the simple
Nutrition 3
sugar (glucose) from the simple raw materials water and carbon dioxide using the energy of
sunlight. Oxygen is released in this process. The overall equation of photosynthesis is
Sunlight
6CO, +12H,0 ’C,H,0, +6H,0+6o,
Chlorophyll
The sugar produced is stored in the form of starch in plants. (In animals food is stored in the
form of glycogen.) These food reserves provide energy as and when required by the organism.
Since autotrophic plants are able to produce food, they are also known as producers.
Site of Photosynthesis
Though all green parts of a plant are capable of performing photosynthesis, the leaves are the
most suitable organs for this process. The cells of the leaves contain special organelles called
chloroplasts, which are the main sites of photosynthesis. These are plastids which contain the
light-absorbing green pigment chlorophyll.
Requirements for Photosynthesis
Photosynthesis requires chlorophyll, carbon dioxide, water and sunlight.
1. Chlorophyll Chlorophylls are green pigments found in all photosynthetic organisms and are
responsible for their green colour. In plants, chlorophyll is mainly found in the leaves. Young
stems and fruits may also have chlorophyll. In lower plants like algae, the whole plant is green
and takes part in photosynthesis.
2. Carbon dioxide Air contains about 0.03% of carbon dioxide. Terrestrial plants use atmospheric
carbon dioxide in photosynthesis. Aquatic plants use the carbon dioxide dissolved in water.
Plants obtain carbon dioxide through pores called stomata present on the surfaces of leaves. The
opening and closing of these pores are regulated by guard cells, which surround them.
Cuticle
Upper
epidermis
Chloroplast
Xylem
Phloem
Air spaces
Lower
epidermis
Guard cel
CO Stoma
Fig. 1.2 Anatomy of a leaf. Note how plants obtain CO, through stomata.
3. Water Water is an important raw material for photosynthesis. Plants absorb water from the
soil through their root hairs. The water is then transported up to the leaves through the stem.
4. Sunlight Light energy is used in splitting water molecules into hydrogen and oxygen. The
splitting of water in the presence of light is called photolysis.
Mechanism of Photosynthesis
There are two main stages in the entire process of photosynthesis. The first stage is dependent on
light (light reactions). The other stage does not require light (dark reactions). During these two
stages, the following events occur.
1. Light energy is first absorbed by chlorophyll molecules found inside the chloroplasts.
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2. The absorbed energy causes splitting of water molecules into hydrogen and oxygen.
During this process the light energy gets converted into chemical energy.
3. Finally, carbon dioxide is reduced to carbohydrate (the end product of photosynthesis).
Factors Affecting Photosynthesis
Intensity of light, carbon dioxide concentration in the air, temperature and water are the
important external factors that influence photosynthesis. Internal factors include chlorophyll
content and the accumulation of the products of photosynthesis.
1. Experiment to demonstrate that starch is formed during photosynthesis
Pluck a healthy green leaf of a plant which was in the sunlight. Place it in a beaker containing boiling
water for about two minutes. Now transfer the leaf to a beaker containing alcohol. Warm it over a water
bath for a few minutes. You will observe that the leaf turns white, indicating that the chlorophyll has been
removed. Now wash the leaf carefully in water without damaging it. Place the leaf in a dilute solution of
iodine. This will turn the leaf bluish black. The changing of the leaf's colour to bluish black after it has
been treated with iodine solution shows that the leaf contains starch.
2. Experinment to demonstrate that carbon dioxide is essential for photosynthesis
Get two healthy potted plants of almost the same size and place them in the dark for 24 hours to
destarch the leaves. Now place them on glass plates. Cover the plants with separate bell jars. Keep some
crystals of potassium hydroxide (KOH) in a Petri dish and place it under one of the jars. Make the set-up
airtight by applying Vaseline at the bottom of the bell jars.
Bell jar
Potted plant
KOH
crystals in
Petri dish
Bell jar with KOH Bell jar without KOH
Fig. 1.3 Experiment to show that CO, is essential for photosynthesis
Keep the plants in sunlight for photosynthesis to take place. After 3 to 4 hours pluck a leaf from each
plant. Boil the leaves in water and subsequently in alcohol, using a water bath, to remove chlorophyl.
Now use afew drops of iodine to test for starch in each leaf. Only one leaf turns blue-black showing the
presence of starch. This happens because KOH absorbs the CO2 present inside one bell jar. As a result,
the leaves do not get CO; for photosynthesis. Thus the process of photosynthesis is inhibited and starch
is not synthesized.
3. Experiment to show that sunlight is essential for photosynthesis
Keep a potted plant in the dark for 24 hours. On one of the leaves, stick black paper strips (one below
and one above the leaf) with the help of Sellotape. Now, place this plant in sunlight for a few hours. Pluck
the leaf and remove the black strips. Boil this leaf, first in water and then in alcohol, to remove
chlorophyll. After washing the leaf with water, keep it in a Petri dish. Add a few drops of iodine solution.
The leaf turns blue-black except in the region that had been covered. This region did not receive light and
Nutrition 5
hence no starch was formed. The uncovered region received light and starch was formed due to
photosynthesis.
Black paper
Does not turn
blue-black
Blue-black
Fig. 1.4 Experiment to show that sunlight is essential for photosynthesis
Plants take up different nutrients like nitrogen, phosphorus, iron, magnesium, etc., along
with water through the root. These nutrients contribute not only to the process of photosynthesis
but also to the general development of the plants. For example, nitrogen is used in the synthesis
of proteins and other compounds.
HETEROTROPHIC NUTRITION
The word 'heterotroph' is derived from two Greek words-eteros (other) and trophe (nutrition).
Unlike autotrophs, which manufacture their own food, heterotrophic organisms obtain food
from other organisms. As heterotrophs depend on other organisms for their food, they are called
consumers. All animals and nongreen plants like fungi come under this category. Consumers
which consume herbs and other plants are called herbivores, and those which consume animals
are called carnivores. After taking complex organic materials as food, heterotrophs break them
into simpler molecules with the help of biological catalysts, or enzymes, and utilize them for their
own metabolism.
Depending upon the mode of living and the mode of intake of food, heterotrophs may be
parasitic. saprophytic or holozoic.
Parasitic
Parasitic organisms, or parasites, live on or inside other living organisms, called hosts, and obtain
their food from them. The host does not get any benefit from the parasite. Different parasites, like
Cuscuta (akast-bel), Cassytha (amar-bel), hookworms, tapeworms, leeches, etc., have different
modes of feeding, depending upon habit, habitat and modifications.
Saprophytic
Saprophytic organisms, or saprophytes, derive their food from dead organisms. They secrete
enzymes that are released on food material outside their body. These enzymes break down
complex food into simple forms. Common examples of saprophytes are fungi (moulds,
mushrooms, yeasts) and many bacteria.
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Holozoic
In holozoic nutrition complex organic substances are ingested (taken in) without their being
degraded or decomposed. After intake, such food is digested by enzymes produced within the
organism. Digested food is absorbed into the body and the undigested product is egested
(expelled) from the body. This kind of nutrition is found mainly in nonparasitic animalssimple
ones like Amoeba and complex ones like human beings.
How Organisms Obtain Nutrition
Different organisms obtain food in different ways. Nutrition in unicellular organisms, like
Amoeba, involves ingestion by the cell surface, digestion and egestion.
Amoeba takes in complex organic matter as food. Amoeba first identifies its food. It then
throws out a number of small pseudopodia (projections of cytoplasm, also called false feet).
These pseudopodia enclose the food particle and prevent it from escaping. The food enclosed in
the cell membrane forms a food vacuole. The complex food is broken down into simpler
molecules with the help of digestive enzymes of the organelle called lysosome. The digested food
is distributed in the cytoplasm and the undigested food is egested through the cell membrane.
Food Food vacuole
Pseudopodia
a
a)
Cilia
Food particles
Cytostome
Food vacuole
Formation of food vacuole
(b)
Fig. 1.5 (a) Amoeba sends out pseudopodia to engulf food. (b) Feeding in Paramoecium
In Paramoecium, a unicellular organism with a specific shape, food is ingested through a
special opening, the cytostome (cell mouth). Food is brought to this opening by the lashing
movement of cilia that cover the entire surface of the cell.
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