Download Unit 12 - Plant and Animal Nutrition (Photosynthesis).

UNIT 12 : PLANT AND ANIMAL NUTRITION
YEAR 8, 2013
12.1 – PHOTOSYNTHESIS
At the end of the lesson, you should be able to:
 define photosynthesis as the process by which plants manufacture food using carbon dioxide and water in the
presence of sunlight and chlorophyll.
 state the word equation for photosynthesis.
All organisms require the energy from the Sun.
I use the energy from the Sun to Sunbath my body to give me a
nice brown colour and my mother dries the clothes under the
sun everyday.
However, there are some living organisms such as green plants
that make use of this solar energy to make their own food.
Green plants are at the beginning of all food chains. Plants are known as 'producers' because
only plants make their own food by a process called “photosynthesis”
[Photo means light and synthesis means to combine]
Animals depend directly or indirectly on green plants for food. Green plants need sunlight to
produce food. They 'capture' the sun's light energy using the chlorophyll in their leaves and use it to
make a sugar called glucose. Glucose is a simple sugar and is needed to provide energy for the plant.
PHOTOSYNTHESIS is the process in which green
plants make food (glucose) using raw materials
such carbon dioxide and water with the help of
sunlight and chlorophyll.
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UNIT 12 : PLANT AND ANIMAL NUTRITION
YEAR 8, 2013
How photosynthesis works
Plants can photosynthesize to make their own food
because they have chloroplasts (a disc-like structure) in
the green parts of their leaves. Inside these chloroplasts
are green pigments called the chlorophyll.
These chlorophylls are the ones which absorb the light
energy from the sun and use it to change carbon dioxide
and water into glucose and releases oxygen. The glucose
produced in the photosynthesis reaction is carried to other parts of the plant through the phloem
vessels. Glucose cannot be stored as it is very soluble, therefore it is converted to starch and stored in
the leaves.
Oxygen is a 'waste' product of photosynthesis which is released into the air through tiny pores under
the leaves called stomata.
Exchange of gases such as carbon dioxide,
water vapour and oxygen takes place
through the stomata of the leaves.
Photosynthesis is a complex process (chemical reaction) which
occurs in a series of steps. However, it can be summarized into a
simple word equation like this:
Light energy from the sun
Carbon dioxide + water ----------------------------------------> glucose + oxygen
Absorbed by chlorophyll
Conditions needed for photosynthesis
Plants need 4 important factors in order
to photosynthesize:
1.
2.
light energy (from the sun)
chlorophyll (the green
3.
4.
substances in plant)
carbon dioxide (from the air)
water (from the soil)
Photosynthesis produces:
1.
2.
glucose (converted to starch)
oxygen (a waste product or byproduct of photosynthesis)
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UNIT 12 : PLANT AND ANIMAL NUTRITION
YEAR 8, 2013
The following practical activities will investigate some of these factors listed above.
(1) LIGHT & PHOTOSYNTHESIS
Aim: To show that light is necessary for photosynthesis.
Conclusion:
Starch is produced only in areas that received light, showing that light is necessary for photosynthesis.
(2) CHLOROPHYLL & PHOTOSYNTHESIS
Aim: To show that chlorophyll is necessary for photosynthesis.
A variegated leaf (a leaf with green and non-green parts)
Conclusion:
Starch is made only in areas of the leaf with chlorophyll, showing that chlorophyll is necessary for
photosynthesis.
Only the green parts of the leaf make starch because the non-green parts have no chloroplast hence
no chlorophyll.
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UNIT 12 : PLANT AND ANIMAL NUTRITION
YEAR 8, 2013
3) CARBON DIOXIDE (GASES) & PHOTOSYNTHESIS
Both soda lime and
sodium hydroxide
solution absorbs
carbon dioxide from
the air.
The presence of starch can be tested using ‘iodine solution’ which will turn from yellow to blue-black
in colour when starch is present.
When testing the leaves from plant A & B, it is found that:
In plant A
The leaves do not turn blue-black upon
testing with iodine solution (the iodine
solution remains yellow).
It is concluded that STARCH IS ABSENT here.
In plant B
The leaves turn blue-black upon testing with
iodine solution.
It is concluded that STARCH IS PRESENT.
Therefore CARBON DIOXIDE must be present for photosynthesis to take place.
STARCH TEST IN A GREEN LEAF
To find out that food (starch) is really synthesized by green plants during photosynthesis, we can do a
test on a green leaf for starch.
Here are the steps necessary to carry out when testing a green leaf for starch:
(1)
Pick a small leaf.
Boil the leaf in a beaker
of water to kill and
soften it.
(2)
Heat the leaf again but
in a test tube of alcohol.
The alcohol
decolourizes the leaf.
Caution:
Turn off the flame as
alcohol burns easily.
(3)
Remove the
decolourised leaf and
put it in a beaker of
hot water to soften it.
(4)
Place the soften
leaf on a white tile
and add a few
drops of iodine
solution to it.
Conclusion: Starch is present if the leaf turns from
yellow to dark blue-black.
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UNIT 12 : PLANT AND ANIMAL NUTRITION
YEAR 8, 2013
IMPORTANCE OF PHOTOSYNTHESIS
Photosynthesis is the only natural process
that:
 can convert light energy into chemical
energy.
 enables green plants to make food.
 releases oxygen into the atmosphere
and removes carbon dioxide from the
atmosphere. In this way,
photosynthesis keeps the proportion
of oxygen and carbon dioxide in the
atmosphere balanced.
Glucose to all
parts of the
plant
Mineral requirements of plants
Green plants make food by photosynthesis. However, for healthy growth, plants also need minerals
such as nitrates, calcium, phosphates and potassium. These nutrients (absorbed as minerals, dissolved
in water) which plants take through their roots in the soil.
The experiment below shows the effect of minerals on plant growth.
[Yellow leaves]
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UNIT 12 : PLANT AND ANIMAL NUTRITION
YEAR 8, 2013
It is found that:
1. Seedlings are grown in the test tube containing different minerals solutions.
2. After a few weeks, the seedlings look quite different. Only in the first tube, with all minerals
present, the seedling grows well.
3. This experiment demonstrates the importance of certain elements in healthy plant growth.
4. Plants grow well in soil which contains minerals. Only small quantities of the mineral are needed
for healthy growth.
A healthy tomato plant.
Tomato plant suffering from
potassium deficiency.
In smaller quantities, plants also need minerals such as:




Magnesium
Iron
Calcium
Sulphur
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