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Photosynthesis
Using sunlight to make food
Chapter 6
Photosynthesis
Some interesting facts
Life on earth is solar powered – It means life can continue to exist on earth only with a continued
supply of sunlight.
The favorable temperature on earth is maintained by greenhouse effect of gases like CO2 (slow
down heat loss from earth into space). Earth would be much colder without this effect.
In one year Photosynthesis produces 160 billion tons of carbohydrates.
There are about 500,000 photosynthetic factories/square mm.
Trees like willow and Eucalyptus are grown as energy plantations to supply wood to burn in power
plants. Wood burning does not cause acid rain like burning of coal or diesel (fossil fuels). These trees
grow fast and unlike fossil fuels are renewable.
Visible light has 7 colors and 3 main regions Blue, Green and Red. Plants absorb and utilize blue and
red regions of visible light and emit green region. That is why plants look green.
The Basics of Photosynthesis - Chloroplasts
Chloroplasts – The Site of Photosynthesis
Chloroplasts are organelles bound with double membranes and found in most plant cells exposed to
sunlight.
Stroma: The inner membrane encloses a thick fluid called Stroma. Interconnected Membranous sacs
called Thylakoids are present inside stroma.
Grana: The thylakoids arrange at many places into stacks called Grana (sing. Granum).
Photosynthesis – Light Reactions
During light reactions phosphate is added to ADP to produce ATP. The process is called
Photophosphorylation.
Photosynthesis – The Calvin Cycle
Dark phase = The Calvin Cycle – is the making of Glucose from CO2. It can operate in darkness but
utilizes the products of light phase, NADPH and ATP to reduce CO2 into Glucose. It occurs in Stroma.
Calvin Cycle has 3 main parts.
1. CO2 Fixation: A 5C molecule RuBP (P - 5C – P)accepts CO2 in the presence of enzyme Rubisco and
splits into 2 molecules of 3C – P, Phosphoglyeraldehyde.
2. Glycolytic Reversal: ATP and NADPH produced during light reactions help to make Glucose and
other sugars from P – 3C.
3. Regeneration of RuBP (Ribulose Bi Phosphate): RuBP is regenerated through interconversions of
sugars. The dark phase is a Cycle because the starting chemical RuBP is regenerated.
Calvin Cycle 2
6 cycles of Calvin Cycle use 6 CO2 to produce 1 molecule of Glucose and uses 12 NADPH + 18 ATP
produced in the light phase.
Overall Reaction:
6 CO2 + 12 NADPH + 18 ATP  C6H12O6 + 12 NADP + 18 ADP
Photorespriation
In tropics during noon C3 plants close their stomata
It traps O2 inside the leaves
Rubisco enzyme start fixing O2 instead of CO2
Photorespiration: Like respiration it uses O2 and releases CO2 but needs high intensity of light.
It reduces the efficiency of photosynthesis
C4 and CAM Plants
C3 Plants plants fix CO2 directly and form 1st stable 3-C molecule PGA in Calvin Cycle = C3 Cycle. Only
C3 Cycle.
C4 Plants plants store CO2 into acids with 4-C. Later they obtain CO2 from acids and fix it in Calvin
Cycle. C4 cycle operates in mesophyll chloroplasts and C3 cycle takes place in bundle sheath
chloroplasts. In C4 plants C4 and C3 Cycles are separated by space.
CAM Plants store CO2 in acids in C4 cycle during night but operates C3 cycle during day when light
energy is available. These plants are adapted to very dry conditions. In CAM plants C4 and C3 Cycles
are separated by time.