Science 7

237 Pages·2014·8.343 MB·English

by  Various

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In Class VI you learnt that food is utilisation by the body. The mode of
essential for all living organisms. nutrition in which organisms make food
You also learnt that carbohydrates, themselves from simple substances is
proteins, fats, vitamins and minerals are called aaaaauuuuutttttoooootttttrrrrroooooppppphhhhhiiiiiccccc (auto = self; trophos
components of food. These components = nourishment) nutrition. Therefore,
of food are necessary for our body and plants are called aaaaauuuuutttttoooootttttrrrrroooooppppphhhhhsssss. Animals
are called nnnnnuuuuutttttrrrrriiiiieeeeennnnntttttsssss. and most other organisms take in ready
All living organisms require food. made food prepared by the plants. They
Plants can make their food themselves are called hhhhheeeeettttteeeeerrrrroooootttttrrrrroooooppppphhhhhsssss (heteros =
but animals including humans cannot. other).
They get it from plants or animals that
eat plants. Thus, humans and animals
are directly or indirectly dependent on Paheli wants to know why
plants. our body cannot make food
from carbon dioxide, water
and minerals like plants do.
Boojho wants to know
how plants prepare Now we may ask where the food
their own food. factories of plants are located: whether
food is made in all parts of a plant or
only in certain parts? How do plants
11111.....11111 MMMMMOOOOODDDDDEEEEE OOOOOFFFFF NNNNNUUUUUTTTTTRRRRRIIIIITTTTTIIIIIOOOOONNNNN IIIIINNNNN PPPPPLLLLLAAAAANNNNNTTTTTSSSSS
obtain the raw materials from the
Plants are the only organisms that can
surroundings? How do they transport
prepare food for themselves by using
them to the food factories of the plants?
water, carbon dioxide and minerals. The
raw materials are present in their 11111.....22222 PPPPPHHHHHOOOOOTTTTTOOOOOSSSSSYYYYYNNNNNTTTTTHHHHHEEEEESSSSSIIIIISSSSS ————— FFFFFOOOOOOOOOODDDDD
surroundings. MMMMMAAAAAKKKKKIIIIINNNNNGGGGG PPPPPRRRRROOOOOCCCCCEEEEESSSSSSSSSS IIIIINNNNN PPPPPLLLLLAAAAANNNNNTTTTTSSSSS
The nutrients enable living Leaves are the food factories of plants.
organisms to build their bodies, to grow, The synthesis of food in plants occurs
to repair damaged parts of their bodies in leaves. Therefore, all the raw
and provide the energy to carry out life materials must reach there. Water and
processes. NNNNNuuuuutttttrrrrriiiiitttttiiiiiooooonnnnn is the mode of minerals present in the soil are absorbed
taking food by an organism and its by the roots and transported to the
CCCCCeeeeellllllllllsssss
You have seen that buildings are made of bricks.
Nucleus
Similarly, the bodies of living organisms are
made of tiny units called ccccceeeeellllllllllsssss. Cells can be seen
only under the microscope. Some organisms
are made of only one cell. The cell is enclosed by
Cytoplasm
a thin outer boundary, called the ccccceeeeellllllllll mmmmmeeeeemmmmmbbbbbrrrrraaaaannnnneeeee.
Most cells have a distinct, centrally located
Cell membrane
spherical structure called the nnnnnuuuuucccccllllleeeeeuuuuusssss (Fig. 1.1).
The nucleus is surrounded by a jelly-like
substance called cccccyyyyytttttoooooppppplllllaaaaasssssmmmmm.
FFFFFiiiiiggggg..... 11111.....11111 Cell
leaves. Carbon dioxide from air is taken The leaves have a gggggrrrrreeeeeeeeeennnnn pppppiiiiigggggmmmmmeeeeennnnnttttt
in through the tiny pores present on the called ccccchhhhhlllllooooorrrrroooooppppphhhhhyyyyyllllllllll. It helps leaves to
surface of the leaves. These pores are capture the energy of the sunlight. This
surrounded by ‘guard cells’. Such pores energy is used to synthesise (prepare)
are called ssssstttttooooommmmmaaaaatttttaaaaa [Fig. 1.2 (c)]. food from carbon dioxide and water.
Since the synthesis of food occurs in the
presence of sunlight, it is called
Boojho wants to know how water
ppppphhhhhoooootttttooooosssssyyyyynnnnnttttthhhhheeeeesssssiiiiisssss (Photo: light; synthesis :
and minerals absorbed by roots
to combine). So we find that chlorophyll,
reach the leaves.
sunlight, carbon dioxide and water are
necessary to carry out the process of
Water and minerals are transported
photosynthesis. It is a unique process
to the leaves by the vessels which run
on the earth. The solar energy is
like pipes throughout the root, the stem,
captured by the leaves and stored in the
the branches and the leaves. They form
plant in the form of food. TTTTThhhhhuuuuusssss,,,,, sssssuuuuunnnnn iiiiisssss
a continuous path or passage for the
ttttthhhhheeeee uuuuullllltttttiiiiimmmmmaaaaattttteeeee sssssooooouuuuurrrrrccccceeeee ooooofffff eeeeennnnneeeeerrrrrgggggyyyyy fffffooooorrrrr aaaaallllllllll
nutrients to reach the leaf. You will learn
llllliiiiivvvvviiiiinnnnnggggg ooooorrrrrgggggaaaaannnnniiiiisssssmmmmmsssss.
about transport of materials in plants
Can you imagine the earth in the
in Chapter 11.
absence of photosynthesis!
In the absence of photosynthesis
there would not be any plants. The
Paheli wants to know what is so
survival of almost all living organisms
special about the leaves that they
directly or indirectly depends upon the
can synthesise food but other
food made by the plants. Besides,
parts of the plant cannot.
oxygen which is essential for the survival
22222 SSSSSCCCCCIIIIIEEEEENNNNNCCCCCEEEEE
Besides leaves, photosynthesis also takes place in other green parts of the
plant — in green stems and green branches. The desert plants have scale- or
spine-like leaves to reduce loss of water by transpiration. These plants have
green stems which carry out photosynthesis.
of all living organisms is produced Light energy
during photosynthesis. In the absence
of photosynthesis, life would be
impossible on the earth.
During photosynthesis, chlorophyll
Carbon
containing cells of leaves (Fig. 1.2), in
dioxide
the presence of sunlight, use carbon Chlorophyll
dioxide and water to synthesise Oxygen in leaf
carbohydrates (Fig. 1.3). The process
can be represented as an equation:
Water and
Carbon dioxide water su nl ight minerals
chlorophyll
Carbohydrate oxygen FFFFFiiiiiggggg..... 11111.....33333 Schematic diagram showing
photosynthesis
Chlorophyll
During the process oxygen is
released. The carbohydrates ultimately
get converted into starch. The presence
of starch in leaves indicates the
occurrence of photosynthesis. The
starch is also a carbohydrate.
Guard cell Stoma
(a) Leaf Boojho has observed some
(b) A section through a leaf
plants with deep red, violet or
Guard cells brown leaves. He wants to
know whether these leaves
also carry out photosynthesis.
AAAAAccccctttttiiiiivvvvviiiiitttttyyyyy 11111.....11111
Stomatal opening
Take two potted plants of the same kind.
(c) Stomata Keep one in the dark (or in a black box)
FFFFFiiiiiggggg..... 11111.....22222 for 72 hours and the other in the
NNNNNUUUUUTTTTTRRRRRIIIIITTTTTIIIIIOOOOONNNNN IIIIINNNNN PPPPPLLLLLAAAAANNNNNTTTTTSSSSS 33333
sunlight. Perform iodine test with the process of photosynthesis. The
leaves of both the plants as you did in carbohydrates are made of carbon,
Class VI. Record your results. Now leave hydrogen and oxygen. These are used
the pot which was earlier kept in the to synthesise other components of food.
dark, in the sunlight for 3 – 4 days and But proteins are nitrogenous substances
perform the iodine test again on its which contain nitrogen. From where do
leaves. Record your observations in your the plants obtain nitrogen?
notebook. Recall that nitrogen is present in
The leaves other than green also have abundance in gaseous form in the air.
chlorophyll. The large amount of red, However, plants cannot absorb nitrogen
brown and other pigments mask the in this form. Soil has certain bacteria
green colour (Fig. 1.4). Photosynthesis that convert gaseous nitrogen into a
takes place in these leaves also. usable form and release it into the soil.
These soluble forms are absorbed by the
plants along with water. Also, you might
have seen farmers adding fertilisers rich
in nitrogen to the soil. In this way the
plants fulfil their requirements of
nitrogen along with the other
constituents. Plants can then
synthesise components of food other
than carbohydrates such as proteins
and fats.
FFFFFiiiiiggggg..... 11111.....44444 Leaves of various colours
11111.....33333 OOOOOTTTTTHHHHHEEEEERRRRR MMMMMOOOOODDDDDEEEEESSSSS OOOOOFFFFF NNNNNUUUUUTTTTTRRRRRIIIIITTTTTIIIIIOOOOONNNNN IIIIINNNNN
You often see slimy, green patches PPPPPLLLLLAAAAANNNNNTTTTTSSSSS
in ponds or in other stagnant water There are some plants which do not have
bodies. These are generally formed by chlorophyll. They cannot synthesise
the growth of organisms called aaaaalllllgggggaaaaaeeeee. their food. How do they survive and from
Can you guess why algae are green in where do they derive nutrition? Like
colour? They contain chlorophyll which humans and animals such plants
gives them the green colour. Algae can depend on the food produced by other
also prepare their own food by plants. They use the hhhhheeeeettttteeeeerrrrroooootttttrrrrroooooppppphhhhhiiiiiccccc
photosynthesis. mmmmmooooodddddeeeee of nutrition. Look at Fig. 1.5. Do
you see yellow tubular structures
SSSSSyyyyynnnnnttttthhhhheeeeesssssiiiiisssss ooooofffff ppppplllllaaaaannnnnttttt fffffooooooooooddddd ooooottttthhhhheeeeerrrrr
twining around the stem and branches
ttttthhhhhaaaaannnnn cccccaaaaarrrrrbbbbbooooohhhhhyyyyydddddrrrrraaaaattttteeeeesssss
of a tree? This is a plant called Cuscuta
You have just learnt that plants
(Amarbel). It does not have chlorophyll.
synthesise carbohydrates through the
It takes readymade food from the plant
44444 SSSSSCCCCCIIIIIEEEEENNNNNCCCCCEEEEE
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 hair
which are directed downwards. When
an insect lands in the pitcher, the lid
closes and the trapped insect gets
entangled into the hair. The insect is
digested by the digestive juices secreted
in the pitcher. Such insect-eating plants
are called iiiiinnnnnssssseeeeeccccctttttiiiiivvvvvooooorrrrrooooouuuuusssss ppppplllllaaaaannnnntttttsssss.
Is it possible that such plants do not
get all the required nutrients from the
soil in which they grow?
FFFFFiiiiiggggg..... 11111.....55555 Cuscuta (Amarbel) on host plant
on which it is climbing. The plant on
which it climbs is called a hhhhhooooosssssttttt. Since it Boojho is confused. If the
deprives the host of valuable nutrients,
pitcher plant is green and
it is called a pppppaaaaarrrrraaaaasssssiiiiittttteeeee. Are we and the carries out photosynthesis, then
other animals also parasites for the
why does it feed on insects?
plants? You should think about it and
discuss with your teacher.
Lid
Paheli wants to know whether
mosquitoes, bed bugs, lice and
leeches that suck our blood are
also parasites.
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 Leaf modified
into pitcher
green or of some other colour. Look at
the plant in Fig. 1.6. The pitcher-like FFFFFiiiiiggggg..... 11111.....66666 Pitcher plant showing lid and pitcher
NNNNNUUUUUTTTTTRRRRRIIIIITTTTTIIIIIOOOOONNNNN IIIIINNNNN PPPPPLLLLLAAAAANNNNNTTTTTSSSSS 55555
11111.....44444 SSSSSAAAAAPPPPPRRRRROOOOOTTTTTRRRRROOOOOPPPPPHHHHHSSSSS
You might have seen packets of
mushrooms sold in the vegetable
market. You may have also seen fluffy
umbrella-like patches growing on rotting
wood during the rainy season (Fig. 1.7).
Let us find out what type of nutrients
FFFFFiiiiiggggg..... 11111.....88888 Fungi
they need to survive and from where
growing on bread
they get them.
Write down your observations in your
notebook. Most probably you will see
cotton-like threads spread on the piece
of bread.
These organisms are called fungi.
They have a different mode of nutrition.
They secrete digestive juices on the dead
FFFFFiiiiiggggg..... 11111.....77777 Packet of mushrooms, a mushroom
and decaying matter and convert it into
growing on decayed material
a solution. Then they absorb the
nutrients from it. This mode of nutrition
in which organisms take in nutrients in
solution form from dead and decaying
Boojho wants to know how
matter is called sssssaaaaappppprrrrroooootttttrrrrroooooppppphhhhhiiiiiccccc nnnnnuuuuutttttrrrrriiiiitttttiiiiiooooonnnnn.
these organisms acquire
Plants which use saprotrophic mode of
nutrients. They do not have
nutrition are called sssssaaaaappppprrrrroooootttttrrrrroooooppppphhhhhsssss.
mouths like animals do. They are
Fungi also grow on pickles, leather,
not like green plants as they lack
clothes and other articles that are left
chlorophyll and cannot make food
in hot and humid weather for long time.
by photosynthesis.
Paheli is keen to know
AAAAAccccctttttiiiiivvvvviiiiitttttyyyyy 11111.....22222
whether her beautiful shoes,
Take a piece of bread and moisten it with
which she wore on special
water. Leave it in a moist warm place
occasions, were spoiled by fungi
for 2–3 days or until fluffy patches
during the rainy season. She
appear on them (Fig. 1.8). These patches
wants to know how fungi appear
may be white, green, brown or of any
suddenly during the rainy
other colour. Observe the patches under
season.
a microscope or a magnifying glass.
66666 SSSSSCCCCCIIIIIEEEEENNNNNCCCCCEEEEE
fungus provides shelter, water and
minerals to the alga and, in return, the
alga provides food which it prepares by
Boojho says once his
photosynthesis.
grandfather told him that his
11111.....55555 HHHHHOOOOOWWWWW NNNNNUUUUUTTTTTRRRRRIIIIIEEEEENNNNNTTTTTSSSSS AAAAARRRRREEEEE
wheat fields were spoiled by a
fungus. He wants to know if RRRRREEEEEPPPPPLLLLLEEEEENNNNNIIIIISSSSSHHHHHEEEEEDDDDD IIIIINNNNN TTTTTHHHHHEEEEE SSSSSOOOOOIIIIILLLLL
fungi cause diseases also. Have you seen farmers spreading
manure or fertilisers in the fields, or
Paheli told him that many fungi
gardeners using them in lawns or in
like yeast and mushrooms are
pots? Do you know why they are added
useful, but some fungi
to the soil?
cause diseases in plants,
You learnt that plants absorb
animals and humans.
mineral nutrients from the soil. So, their
Some fungi are also used
amounts in the soil keep on declining.
in medicines.
Fertilisers and manures contain plant
nutrients such as nitrogen, potassium,
phosphorous, etc. These nutrients need
During the rainy season they spoil many to be added from time to time to enrich
things. Ask your parents about the the soil. We can grow plants and keep
menace of fungi in your house. them healthy if we can find out the
The fungal spores are generally nutrient requirement of plants.
present in the air. When they land on Usually crops require a lot of nitrogen
wet and warm things they germinate to make proteins. After the harvest, the
and grow. Now, can you figure out how soil becomes deficient in nitrogen.
we can protect our things from getting Though nitrogen gas is available in
spoiled? plenty in the air, plants cannot use it in
Some organisms live together and the manner they can use carbon dioxide.
share shelter and nutrients. This is They need nitrogen in a soluble form.
called sssssyyyyymmmmmbbbbbiiiiioooootttttiiiiiccccc rrrrreeeeelllllaaaaatttttiiiiiooooonnnnnssssshhhhhiiiiippppp. For The bacterium called Rhizobium can
example, certain fungi live in the roots take atmospheric nitrogen and convert
of trees. The tree provides nutrients to it into a soluble form. But Rhizobium
the fungus and, in return, receives help cannot make its own food. So it lives in
from it to take up water and nutrients the roots of gram, peas, moong beans
from the soil. This association is very and other legumes and provides them
important for the tree. with nitrogen. Most of the pulses (dals)
In organisms called llllliiiiiccccchhhhheeeeennnnnsssss, a are obtained from leguminous plants.
chlorophyll-containing partner, which is In return, the plants provide food and
an alga, and a fungus live together. The shelter to the bacteria. They have a
NNNNNUUUUUTTTTTRRRRRIIIIITTTTTIIIIIOOOOONNNNN IIIIINNNNN PPPPPLLLLLAAAAANNNNNTTTTTSSSSS 77777