Process of photosynthesis
The City School
Gulshan campus A
Photosynthesis is the process of converting light energy to chemical
energy and storing it in the bonds of sugar. This process occurs in plants
and some algae (Kingdom Protista). Plants need only light energy, CO2,
and H2O to make sugar. The process of photosynthesis takes place in
the chloroplasts, specifically using chlorophyll, the green pigment
involved in photosynthesis.
The overall chemical reaction involved in photosynthesis is:
6CO2 + 6H2O (+ light energy) → C6H12O6 + 6O2.
This is the source of the O2 we breathe, and thus, a significant factor in
the concerns about deforestation.
There are two parts to photosynthesis. They are called the light
reaction and the dark reaction
Photosynthesis is a very important process because it harnesses the sun's
energy into forms of energy on earth. Photosynthesis converts light energy
into chemical energy in the form of glucose and then the process of cellular
respiration converts the energy in glucose to energy. It also states that the
energy produced by photosynthesis forms the basis of virtually all
terrestrial and aquatic food chains. It is the ultimate source of carbon in the
organic molecules that is found in most organisms.
The primary function of photosynthesis is to convert solar energy into
chemical energy and then store that chemical energy for future use. For the
most part, the planet’s living systems are powered by this process. It’s not
particularly efficient by human engineering standards, but it does the job.
Photosynthesis happens in regions of a cell called chloroplasts. The
chemistry and physics are complex.
1. We take a potted plant having long and narrow leaves and place it in a completely dark place for; about three
days to destarch its leaves.
2. Take a glass bottle having a wide mouth and put some potassium hydroxide solution (KOH solution! in it.
(This potassium hydroxide solution is to absorb the carbon dioxide gas from the air present in the glass bottle
so that no carbon dioxide is left in the air inside the glass bottle).
3. Take a rubber cork which fits tightly into the mouth of the glass bottle and cut it into two halves.
4. Put a destarched leaf of the potted plant (while it is still attached to the plant), in-between the two halves of
the cut cork and then fit the cork in the mouth of the glass bottle. The; upper half of the leaf should remain
outside the glass bottle and only the lower half of the leaf should: be inside the glass bottle.
5. The potted plant (with its one destarched leaf half inside the glass bottle containing potassium hydroxide
solution) is kept in sunlight for 3 to 4 days. During this period, the upper half of the leaf (which is outside the
glass bottle) gets carbon dioxide from the air but the lower half of the leaf (which is inside the glass bottle)
does not get any carbon dioxide. This is because all the carbon dioxide of the air present in the glass bottle has
been absorbed by potassium hydroxide solution. And no fresh air can come into the closed glass bottle.
6. Pluck the leaf from the plant and take it out from the glass bottle. Remove the green coloured chlorophyll
from the leaf by boiling it in alcohol. In this way, we get a decolourised leaf.
7. Wash the decolourised leaf with water to remove any chlorophyll which may be sticking to it.
8. Pour iodine solution over the colourless leaf and observe the change in colour of the leaf.
9. We will find that the lower half part of the leaf (which was inside the glass bottle having no carbon dioxide
around it), does not turn blue-black on adding iodine solution showing that no starch is present in this lower
half of the leaf. From this observation we conclude that the photosynthesis to make starch in the leaf does not
take place without carbon dioxide.
10. The upper half part of the leaf (which was outside the glass bottle, having carbon dioxide around it) turns
blue-black on adding iodine solution showing that starch is present in this upper half of the leaf. From this
observation we conclude that photosynthesis (to make starch) takes place in the presence of carbon dioxide.
In other words, carbon dioxide is necessary for the process of photosynthesis to take place.
Raw Materials for Photosynthesis :
The preparation of carbohydrates (food) by plants by the process of photosynthesis requires two
materials (or substances): carbon dioxide, and water. Thus, the raw materials for photosynthesis are:
(i) Carbon dioxide, and
We will now describe how these two raw materials become available to plants for photosynthesis.
1. How the Plants Obtain Carbon Dioxide :
There are a large number of tiny pores called stomata on the surface of the leaves of plants (The
singular of stomata is stoma). The green plants take carbon dioxide from air for photosynthesis. The
carbon dioxide gas enters the leaves of the plant through the stomata present on their surface.
Each stomata pore (or stoma) is surrounded by a pair of guard cells. The opening and closing of
stomata pores is controlled by the guard cells. When water flows into the guard cells, they swell,
become curved and cause the pore to open. On the other hand, when the guard cells lose water, they
shrink, become straight and close the stomata pore.
A large amount of water is also lost from the cells of the plant leaves through open stomatal pores. So,
when the plant does not need carbon dioxide and wants to conserve water, the stomatal pores are
closed. The oxygen gas produced during photosynthesis also goes out through the stomatal pores of
the leaves. Please note that in addition to leaves, the stomata are also present in the green stems (or
shoots) of a plant.
So, the green stems (or shoots) of a plant also carry out photosynthesis. It is clear from the above
discussion that stomata allow the movement of gases in and out of plant cells. In other words, the
gaseous exchange in plants takes place through the stomata in leaves (and other green parts).
Please note that in most broad-leaved plants, the stomata occur only in the lower surface of the leaf
but in narrow-leaved plants, the stomata are equally distributed on both the sides of the leaf. Another
point to be noted is that the aquatic plants (or water plants) use the carbon dioxide gas dissolved in
water for carrying out photosynthesis.
2. How the Plants Obtain Water for Photosynthesis:
The water required by the plants for photosynthesis is absorbed by the roots of the plants from the soil
through the process of osmosis. The water absorbed by the roots of the plants is transported upward through
the xylem vessels to the leaves where it reaches the photosynthetic cells and utilized in photosynthesis.
The two raw materials, carbon dioxide and water, are required by the plants to prepare energy foods called
carbohydrates (such as glucose and starch). But the plants also need other raw materials such as nitrogen,
phosphorus, iron and magnesium, etc., for building their body.
The plants take materials like nitrogen, phosphorus, iron and magnesium, etc., from the soil. For example,
nitrogen is an essential element used by the plants to make proteins and other compounds.
The plants take up nitrogen from the soil in the form of inorganic salts called nitrates (or nitrites), or in the
form of organic compounds which are produced by bacteria from the atmospheric nitrogen.
Site of Photosynthesis: Chloroplasts
Chloroplasts are the organelles in the cells of green plants which contain chlorophyll and where photosynthesis
takes place. Thus, photosynthesis occurs in the organelles called chloroplasts present in the photosynthetic cells
(or mesophyll cells) of green plants. In other words, the site of photosynthesis in a: cell of the leaf are
chloroplasts. Chloroplasts can be seen easily by using a light microscope. In a cross-section of a leaf, chloroplasts
can be seen as numerous disc-like organelles in the photosynthetic cells (or mesophyll cells) of the palisade
tissue just below the upper epidermis.
In the structure of a leaf we can see that the middle layers in the leaf (palisade layer and spongy layer) contain
photosynthetic cells called mesophyll cells. These cells contain more chlorophyll than other plant cells. A typical
photosynthetic cell (or mesophyll cell) of a green leaf may contain 100 or more tiny chloroplasts in it, and a
whole leaf may contain many thousands of photosynthetic cells.
Carbon dioxide needed for photosynthesis enters from the air into the leaf through the stomata in its surface,
and then diffuses into the mesophyll cells and reaches the chloroplasts.Water is carried to the leaf by xylem
vessels and passes into the mesophyll cells by diffusion and reaches the chloroplasts.There is a thin, waxy
protective layer called cuticle above and below a leaf which helps to reduce the loss of water from the leaf.
1. We take a potted plant having green leaves and place it in a completely dark place for
about three days to destarch its leaves. So, in the beginning of the experiment, the leaves
do not have any starch in them.
2. Take a thin strip of aluminium foil (or black paper) and wrap it in the centre of one leaf
on both the sides (while the leaf is still attached to the plant). The aluminium foil should
be fixed tightly to the leaf by using paper clips so that sunlight may not enter it from the
sides. The aluminium foil should cover only a small part of the leaf so that the remaining
part of the leaf remains uncovered and exposed to sunlight. We have covered the centre
part of the leaf with aluminium foil so that sunlight may not fall on this covered part of
3. Keep this potted plant (with partially covered leaf) in bright sunshine for three to four
4. Pluck the partially covered leaf from the plant and remove its aluminium foil. Immerse
this leaf in boiling water for a few minutes. This will break down the cell membranes of
leaf cells and make the leaf more permeable to iodine solution (so that it may reach the
starch present inside the leaf cells).This leaf is now to be tested for the presence of
starch. But before testing for starch, chlorophyll has to be removed from the leaf. This is
because chlorophyll interferes in the test for starch due to its green colour.
5. Put the plucked leaf in a beaker containing some alcohol. Place the beaker containing
alcohol and leaf in a water bath (A water bath can be a bigger beaker containing water). A
water bath is being used here for heating alcohol because alcohol is a highly inflammable
liquid. So, if alcohol is heated directly over a flame, then it will catch fire at once.
1. We take a potted plant like croton whose leaves are partly green and partly white.
The green part of the leaf has chlorophyll but the white part of the leaf does not
2. Place this plant in a completely dark place for about three days to destarch its
3. Take out the potted plant from the dark place and keep it in bright sunshine for
three to four days.
4. Pluck the variegated leaf from the plant, boil it in water for a few minutes and then
remove its green colour 'chlorophyll' by boiling it in alcohol. The green parts of the
leaf get decolourised. So, we get decolourised leaf.
5. Wash the decolourised leaf with hot water to soften it and remove any chlorophyll
which may be sticking to it.
6. Pour iodine solution over the colourless leaf and observe the change in colour of
7. We will find that the outer part of leaf that was originally white
(without chlorophyll) does not turn blue-black on adding iodine solution
showing that no starch is present in this outer part of the leaf. From this
observation we conclude that the photosynthesis to make starch does
not take place without chlorophyll.
8. The inner part of leaf which was originally green (contained
chlorophyll) turns blue-black on adding iodine solution showing that
starch is present in this inner part of the leaf. From this observation we
conclude that the photosynthesis to make starch takes place in the
presence of chlorophyll. In other words, chlorophyll is necessary for the
process of photosynthesis to take place.
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