3. introduction
Photosynthesis is the process by which a
plant produces its food by converting light
energy into chemical energy. Plants use
water, carbon dioxide and sunlight in the
presence of chlorophyll to produce their
food or energy in the form of sugar and
release oxygen as the byproduct.
Photosynthesis is influenced by a number
of factors, including its rate and efficiency.
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4. External Factor Affecting
Photosynthesis
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• LIGHT
It is one of the major factors affecting photosynthesis. Photosynthesis cannot occur in the dark and the source of
light for the plants is sunlight. Three attributes of light are important for photosynthesis:
QUALITY OF LIGHT
Light between the wavelength of 400 nm & 700 nm is the most effective for photosynthesis and this light is called
photosynthetically active radiation (PAR). It is because the different rays of light are not absorbed equally by the
chlorophyll.
INTENSITY OF LIGHT
Under low light intensity, the rate of photosynthesis is low.
As the intensity of light increases, the rate of photosynthesis increases. At very high light intensities, the rate of
photosynthesis decreases.
DURATION OF LIGHT
The extent of photosynthesis is more when a plant is
exposed to long periods of light.
Uninterrupted and continuous photosynthesis for
relatively long periods of time may be sustained without
any visible damage to the plant.
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• TEMPERATURE
The rate of photosynthesis increases with a rise in
temperature, over a range from 6°C to about 37°C. Above this
temperature, there is an abrupt fall in the rate and the tissue
dies at 43°C.
High temperatures cause the inactivation of enzymes and
therefore affect the enzymatically controlled ‘dark’
reactions of photosynthesis.
The optimum temperature for the maximum falls
between 20-30°C.
Above 25-30°C the maximum rate is not maintained
as the time factor begins to operate and the
optimum temperature is reduced from 37°C to 30°C.
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.
• WATER
Water is one of the raw materials of photosynthesis. The amount of water used in
photosynthesis is very small.
Less than 1 % water is absorbed by the plant.
But the water content of the leaf often acts as a limiting factor indirectly. It maintains the
turgor of the assimilatory cells.
The rate of photosynthesis decreases in the cells which have lost their turgor, the loss
of turgor of
guard cells closed the stomata. It reduces the rate of photosynthesis.
• OXYGEN
Oxygen is essential for photosynthesis. Photosynthesis does not take place in cells
which lack oxygen.
The energy produced in oxygen respiration is necessary for photosynthesis. Oxygen is
required for the production and maintenance of some substance.
High concentrations of oxygen inhibit the rate of photosynthesis. It promotes
photorespiration.
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•CARBON DIOXIDE
The concentration of CO2 in atmosphere varies between 0.03% and
0.04%.
Increase in the concentration of CO2 up to 0.05% increases CO2
fixation.
The increase in the amount of carbon dioxide increases the
photosynthesis.
Higher concentrations have an inhibitory effect on photosynthesis.
8. Internal Factors Regulating the
Rate of Photosynthesis
STRUCTURE OF LEAVES
PROTOPLASMIC FACTORS
ACCUMULATION OF END
PRODUCTS
CO₂ COMPENSATION
POINT
CHLOROPHYLL CONTENT
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Protoplasmic Factors
There is an unknown factor in protoplasm that influences photosynthesis rate. This factor has an
impact on the dark reactions. The decrease in photosynthesis rate at temperatures above 30°C or at
high light intensities in many plants suggests that this unknown factor is an enzyme.
Chlorophyll Content
Chlorophyll is a critical internal factor in photosynthesis. The amount of CO₂ Fixed by one gram of
chlorophyll in one hour is referred to as the photosynthetic number or assimilation number. It is
usually constant for a plant species, but it varies on occasion. The assimilation number of a species’
variegated variety was discovered to be greater than the variety with green leaves.
Accumulation of End Products
The accumulation of food in the chloroplasts slows photosynthesis.
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Structure of Leaves
The amount of CO₂ that reaches the chloroplasts is determined by structural features of the
leaves such as stomatal size, position, and behaviour, as well as the number of intercellular
spaces. Other characteristics, such as cuticle thickness, epidermis, the presence of epidermal
hairs, the amount of mesophyll tissue, and so on, influence the intensity and quality of light
reaching to the chloroplast.
CO₂ Compensation Point
It is the value or point in light intensity and atmospheric CO₂ concentration at which the rate of
photosynthesis is just equal to the rate of respiration in photosynthetic organs, resulting in no
net gaseous exchange. The light compensation point for shade plants is 2.5-100 ft. candles
and 100-400 ft. candles for sun plants. The CO₂ compensation point in CO₂ plants is very low
(0-5 ppm), whereas it is quite high in C₃ plants (25-100 ppm). A plant cannot survive for long at
the compensation point because there is a net loss of organic matter due to non-green organ
respiration and dark respiration.
11. Blackman’s law of limiting factors
‘When a process is conditioned as to its rapidity by a number of separate factors, the rate of the
process is limited by the pace of the slowest factor’.
This implies that if more than one factor affects a chemical process, then its rate will be determined
by a factor which is nearest to its minimal value. It is the factor which directly affects the rate if its
quantity is changed.
Example: Despite the presence of a green leaf and optimal light and CO2 conditions, the plant may
not photosynthesise if the temperature is very low. This leaf will start photosynthesising if provided
with the optimal temperature.
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