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B
CLASS-X
EPISODE-2
STATE SYLLABUS
LIFE PROCESSES
PHOTOSYNTHESIS – FACTORS
In the last episode we have studied the importance of Photosynthesis.
In this episode we shall examine the factors of Photosynthesis.
The factors of Photosynthesis are
a) Carbon dioxide.
b) Water
c) Sun light
d) Chloroplasts for synthesis of Carbohydrates.
The factors “a” and “c” i.e., CO2 and Sun light are external
factors while the factors “b” and “d” are internal factors.
Let us now examine the importance of external factors.
1. Light:- Light is a form of energy.
What do the light rays contain?
The light rays have tiny particles known as “Photons”. The energy is
carried by a photon is called ”Quantum”.
Light is an electromagnetic radiation. It has many components with
different wave lengths. Comic rays, gamma rays, X rays, ultra
violet rays, (UV) infrared rays (IR) and radio waves are some of
the components of light.
The wave length of visible light lies between 400-700mm ie., lies
between UV and IR. As the wave length increases there is decrease in
the energy quantum.
See the image
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From this image we can understand that the wave length of visible
light is between 100mm to 1000mm. Violet has a minimum wave
length of 100mm while red has 700mm.
Properties of Light:Light has seven different colours. The colours as we see in the rainbow
are Violet, Indigo, Blue, Green, Yellow, Orange, Red, or
VIBGYOR, in short.
Show rainbow
When light strikes any object, the light rays undergo reflection,
refraction, transmission or absorption. This depends on the nature of
object.
When the light is absorbed the energy present in photons is
transferred to the object. For photosynthesis maximum energy is
required. Light rays with small wave lengths would not be useful. Only
the light rays with maximum wave lengths are useful. Therefore the
length of light becomes a critical factor.
In a spectrum photosynthesis will be more in the regions of red and
blue rays. Chlorophy reflects green light. Hence no photosynthesis is
possible in green light. The chlorophyll molecules, located on the
thylakoid membranes of the chloroplast, absorb light energy and
initiate photosynthesis.
The importance of light can be understood by the experiment that
follows.
1.Keep a potted plant in dark for two three days. After two or three
days the leaves become free of starch.
2.Cut a design in the center of light screen. Place the light screen on
one of the leaves. The center of the screen.
3.Place the pot in sun light for about 5 hours.
4. Pluck the leaf with light screen and remove the light screen.
5. Dip the plucked leaf in iodine solution to test for the presence of
starch.
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6. The areas where starch is present turns blue. You will observe that
the area other than the area where leaf was covered by the light
screen turn blue.
Animate
Story Board
1. (image) Potted plant kept in dark.
2. Light screen(image) place light screen on one leaf with a design
in center.
3. (image) keep the pot in sun light.
4. (image).
5. (image).
6. (image).
The experiment proves that photosynthesis can take place only in the
presence of light. As certain part of the leave was covered and did not
receive light, photo synthesis did not take place and no starch has
formed.
The other important factor for photosynthesis is carbon dioxide. Let us
conduct an experiment to test the importance of CO2 for
photosynthesis.
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Animate
Story Board
1.Pot image
1.Take a potted plant with long
leaves
2.Take a bottle with a cork.
3.Split the cork vertically
4.Keep the pot in dark for two to
three days. The leaves will be free
from starch.
2.Bottle Cork.(image)
3.Cork up down
4.Potted Plant (image)
5.Take 5 – 10 ml of potassium
hydroxide in the bottle. This will
dissolve the CO2 present in the
bottle.
5. Potassium Hydroxide taken in
bottle (image).
6. After three days insert a leaf
into the bottle through the slit in
the cork without detaching it from
the plant. Keep the bottle in sun
light for 4 to 5 hours.
The mouth of the bottle should be
made air tight by applying
Vaseline or grease. This prevents
any air going into the bottle.
7. After 4-5 hours detach the leaf
from the plant and dip it in iodine
solution to test for the presence of
starch.
8. You will observe that the part
of the leaf outside the bottle
exposed to environment turns
blue. The part of leaf does not
turn blue.
6. Potted Plant and bottle kept in
sun light (image)
7. leaf becomes blue in colour.
(image)
8. leaf testing (image)
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this proves that the plant of leaf in the bottle did not receive CO2 while
the one outside the bottle received CO2 . Hence starch was synthesized
by Photosynthesis.
Thus we know that CO2 and light are both essential external factors for
Photosynthesis.
How does the leaf absorb CO2? Are there any special structures?
Leaves have natural openings in their epidermic. These are called
STOMATA. It is through this stomata the leaves exchange gases i.e.,
absorb CO2 and release O2. The efficiency of photosynthesis depends
on the number of stomata on leaves. More the number of stomata
more photosynthesis and lesser number of stomata reduced
photosynthesis occurs in plants.
Epidermal peels of a leaf showing stomata and guard cells
Let us now examine the internal factors of photosynthesis.
All green plants have in their leaves special membrane bound
organells called chloroplasts. The chloroplasts are disc or spindle
shaped. They are filled with colourless area called STROMA. In the
stroma are a number of stacks of structures known as THALAKOIDS.
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The thalakoids are connected to one another by structures called
LAMELLAE.
Chloroplast
In the lipid part of thalakoid a green pigment CHLOROPHYL is present.
Due to this pigment the chloroplasts are green in colour. The
molecules in the pigment absorb light rays of a specific wave length
i.e., Red and blue rays.
Chlorophyll and other accessory pigments in the thalakoids
membranes organize to form REACTION CENTERS called photo
system-I and Photo system-II (PSI and PSII).
The pigments harvest solar energy and pass it into the reaction
centers. They absorb photons and convert CO2 in the air to sugar and
release O2. some enzymes present in stroma are utilized during this
process.
If you dip a leaf in iodine solution in the evening you will observe that
some areas of the leaf turn blue.
If you observe a leaf you will find that some areas are green while
some are not. Dip this leaf in iodine solution in the evening. You will
observe that the green areas turn blue indicating presence of starch
and hence photosynthesis. The green colour in these areas is due to
presence of chlorophyll.
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The area of leaf which does not turn blue indicates absence of
Photosynthesis due to absence of chlorophyll. This proves that
chlorophyll is essential for Photosynthesis.
Story
1.
2.
3.
Animate
Board
Take a Green leaf. Show patches of non green or pale areas.
Dip the leaf in iodine solution.
Take out the leaf. Show the leaf in blue colour except the pale
areas which remain pale green.
Fig 1.2.5 state syllabus X page no.8
In the next episode we will examine the mechanism of Photosynthesis.
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