Photosynthesis: The process
by which plants prepare their food
Photosynthesis is a process in which green plant prepare
carbohydrate food (Glucose) with help of chlorophyll pigments
by using carbon dioxide, water and sunlight
Equations: 6CO2+12H2O+ sunlight
= C6H12O6 +6H2O
+6O2
Raw material and Products of photosynthesis: The
raw materials of photosynthesis are water and carbon dioxide,
enter the cells of the leaf, and the products are sugar
and oxygen. Water enters the root and is transported up
to the leaves through specialized plant cells known as xylem
Carbon dioxide cannot pass through the protective
waxy layer covering the leaf (cuticle), it enters
the leaf through an opening (the stoma; plural =
stomata)
Pigments: Chlorophyll pigments play important
role in photosynthesis. They are situated in chloroplast
of plant cell
Plants are green due to the photosynthetic pigment chlorophyll.
Chlorophyll serves to reflect green and yellow wavelengths
of light and absorb all the others.
Chlorophyll exists in two forms, chlorophyll a (the
primary photosynthetic pigment) and chlorophyll b
Accessory Pigments: This is due to the presence of accessory
pigments such as chlorophyll b, the Carotenoids,
Phycoerythrin and Phycocyanin. Accessory pigments
serve to capture light energy and transfer this energy to
chlorophyll. Once absorbed the energy associated with the
photon is used to begin the process of photosynthesis
These pigments are arranged in two types of unit photosystem
I (P700) and photosystem II (P680).
Photosystems I and II (light dependent Reactions): Photosynthesis
is a two-stage process. The first process is the Light
Dependent Process (Light Reactions) and the other
light independent one (dark reactions). The first one
requires the direct energy of light to make energy carrier
molecules that are used in the second process. The Light
Independent Processes (or Dark Reactions) occur when the
products of the Light Reaction are used to form glucose.
In the Light Dependent Processes, light strikes Photosystem
I to excite electrons to a higher energy state. At the same
time photolysis of water molecule take place and
forms free oxygen, hydrogen and electrons. These electrons
are transferred to Photosystem II to compensate its loss
of electrons. Excited electron travel through electron transport
chain, energy of flow of electron is use to convert ADP
to ATP and then is passed to photosystem II. Again light
energy excites it's electron and then it travel through
ETS of photosystem II, at the end of chain NADP is
converted into NADPH
Light independent process: In the Light Independent Process,
carbon dioxide from the atmosphere is captured and combines
with enzyme ribulose biphosphate form glucose using ATP
and NADPH. A basic version of calvin cycle is shown.
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