Photosynthesis
An outline of the process
Photosynthesis is the trapping (fixing)
of carbon dioxide and its subsequent
reduction to carbohydrate, using
hydrogen from water. It takes place
inside chloroplasts.
An overall equation for photosynthesis in green plants is:
Two sets of reactions are involved. These are the light dependent reactions, for
which light energy is necessary, and the light independent reactions, for which
light energy is not needed. The light dependent reactions only take place in the
presence of suitable pigments that absorb certain wavelengths of light.
(a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found
in the thylakoid membrane. Chlorophyll a and b are responsible for the green color of leaves. βcarotene is responsible for the orange color in carrots. Each pigment has (d) a unique
absorbance spectrum.
❖ Light energy is necessary for the splitting (photolysis) of water into hydrogen and
oxygen; oxygen is a waste product. Light energy is also needed to provide chemical
energy, in the form of ATP, for the reduction of carbon dioxide to the carbohydrate in the
light independent reactions.
❖ The photosynthetic pigments involved fall into two categories; primary pigments and
accessory pigments. The pigments are arranged in light harvesting clusters called
photosystems of which there are two types, I and II.
❖ In a photosystem, several hundred accessory pigment molecules surround a primary
pigment molecule, and the energy of the light absorbed by the different pigments is
passed to the primary pigment. The primary pigments are two forms of chlorophyll.
These pigments are said to act as reaction centres.
RuBP
GP
The light dependent
reactions
➔ The light dependent reactions include the
splitting of water by photolysis to give
hydrogen ions (protons) and the synthesis
of ATP in photophosphorylation.
➔ The hydrogen ions combine with a carrier
molecule NADP, to make reduced NADP.
ATP and reduced NADP are passed from
the light dependent to the light
independent reactions.
➔ Photophosphorylation of ADP to ATP can
be cyclic or noncyclic, depending on the
pattern of electron flow in one or both
types of photosystem.
Cyclic photophosphorylation
1. Cyclic photophosphorylation involves only
photosystem I.
2. Light is absorbed by photosystem I and is passed
to the primary pigment.
3. An electron in the chlorophyll molecule is excited
to a higher energy level and is emitted from the
chlorophyll molecule. This is called
photoactivation. Instead of falling back into the
photosystem and losing its energy as thermal
energy or as fluorescence, the excited electron is
captured by an electron acceptor and passed
back to a chlorophyll molecule via a chain of
electron carriers.
4. During this process, enough energy is released to
synthesise ATP from ADP and an inorganic
phosphate group (Pi) by the process of
chemiosmosis.
5. The ATP then passes to the light independent
reactions.
Cyclic phosphorylation produces only ATP in
small amounts and not reduced NADP or
oxygen.
Parts of the cycle are highlighted.
Non-cyclic photophosphorylation
1. Non-cyclic photophosphorylation involves
both photosystems.
2. Light is absorbed by both photosystems and
excited electrons are emitted from the
primary pigments of both reaction centres.
3. These electrons are absorbed by electron
acceptors and pass along chains of electron
carriers, leaving the photosystems positively
charged.
4. The primary pigment of photosystem I
absorbs electrons from photosystem II.
5. Its primary pigment receives replacement
electrons from the splitting (photolysis) of
water. As in cyclic photophosphorylation, ATP
is synthesised as the electrons lose energy
while passing along the carrier chains.
Photolysis of water
Photosystem II includes a water splitting enzyme that catalyses the breakdown of water:
H2O ------> 2H+ + 2e- + ½ O2
Oxygen is a waste product of this process. The hydrogen ions combine with electrons from
photosystem I and the carrier molecule NADP to give reduced NADP.
2H+ + 2e- + NADP --------> reduced NADP
Reduced NADP passes to the light independent reactions and is used in the synthesis of
carbohydrate.
The light independent
reactions - (Calvin cycle)
1. The fixation of carbon dioxide is a light
independent process in which carbon dioxide
combines with a five-carbon sugar, ribulose
bisphosphate (RuBP), to give two molecules of
a three-carbon compound, glycerate 3phosphate (GP). (This compound is also
sometimes known as PGA.)
2. GP, in the presence of ATP and reduced NADP
from the light dependent stages, is reduced to
triose phosphate (TP) (three-carbon sugar).
This is the point at which carbohydrate is
produced in photosynthesis.
3. Most (five-sixth) of the triose phosphates are
used to regenerate RuBP, but the remainder
(one-sixth) are used to produce other
molecules needed by the plant.
4.
Some of these triose phosphates
condense to become hexose phosphates which, in
turn, are used to produce starch for storage, sucrose
for translocation around the plant, or cellulose for
making cell walls.
5.
Others are converted to glycerol and fatty
acids to produce lipids for cellular membranes or to
acetyl coenzyme A for use in respiration or in the
production of amino acids for protein synthesis.
6.
The cycle of events was worked out by
Calvin, Benson and Bassham between 1946 and
1953, and is usually called the Calvin cycle.
7.
The enzyme ribulose bisphosphate
carboxylase (rubisco), which catalyses the
combination of carbon dioxide and RuBP, is the most
common enzyme in the world.
Videos
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https://www.youtube.com/watch?v=KfvYQgT2M-k
https://www.youtube.com/watch?v=CMiPYHNNg28
https://www.youtube.com/watch?v=0UzMaoaXKaM
https://www.youtube.com/watch?v=pwymX2LxnQs
https://www.youtube.com/watch?v=-rsYk4eCKnA
Limiting factors of photosynthesis
Light intensity:
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Light is essential for providing energy to
the light dependent reaction.
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If the light intensity is high, more energy is
provided.
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Light intensity is a limiting factor at night.
Temperature:
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Enzymes are involved in
photosynthesis.
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If the temperature is too low, enzymes
turn inactive and if the temperature is
very high (greater than 45 degrees)
denaturation may occur.
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Stomata closes when the temperature is
high in order to prevent the water loss in
plants.
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This closure will limit the amount of
carbon dioxide entering the leaf and
thus reduces the rate of photosynthesis.
Carbon dioxide:
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The rate of photosynthesis is
increased when there’s a higher rate
of carbon dioxide.
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However, if it becomes too high,
stomata will begin to close.
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Usually on a warm and sunny,
windless day, carbon dioxide acts as
the limiting factor.
Photosynthesis can occur fast when all the
above mentioned factors are at the right
levels. If two factors are at the perfect level
while the other is not, it will still not make a
difference and will slow down the process of
photosynthesis.
Agricultural practices
and photosynthesis
❖ Farmers are well aware of the limiting
factors of photosynthesis and use this
knowledge to control plant growth.
❖ They try to create a suitable environment
with the right conditions at the right amount
to increase the growth and yield of their
crops.
❖ They make use of glasshouses to provide
optimum conditions to the plants.
If the limiting factor is:
Carbon dioxide concentration: addition of
carbon dioxide to the air by means of
burning propane in small amounts in a
carbon dioxide generator.
Light: light can reach the plants through the
glass. Lamps are used at night time.
Temperature: heat energy from sunlight is
trapped in the glass houses and warms the
air. Cooling systems and heaters could be
used to maintain a constant level of
optimum temperature.