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An over view of Photosynthesis
• Photosynthesis is a series of reactions in which energy transferred
as light is transformed to chemical energy. Energy from light is
trapped by chlorophyll, and this energy is then used to
i. Split apart the strong bonds in water molecules to release
ii. Produce ATP
iii. Reduce a substance called NADP
NADP stands for nicotinamide adenine dinucleotide phosphate and is
a coenzyme like NAD.
Overall equation of Photosynthesis is 6CO2+6H2O=C6H12O6+ O2
There are many different steps in photosynthesis, which can be
divided into two main stages – Light-dependent reactions and Light
• Photosynthesis takes place inside chloroplasts. These
organelles surrounded by two membranes, called an
Ribosomes in stroma
Synthesising enzymes – Ribulose
bisphosphate carboxylase, for
The soluble sugars made in
cannot be stored. They can only be
they are converted into an insoluble
Creates large surface area for
so lots of light can be absorbed.
Driven reactions to occur within
the chloroplasts at the same tiime.
Codes for some of the chloroplast
Light dependent reaction
• The first reactions of photosynthesis require light energy, and are
called light dependent reactions.
• When light is absorbed by PS I and PS II, electrons in the chlorophyll
molecule are boosted to higher energy levels. They are emitted and
passed on to electron carriers.
• The loss of electrons from PS II causes the splitting of water. The
water gives up its electrons to PS II to fill the gap.
• This leaves oxygen, which is given off as waste, and hydrogen ions.
• The electrons are passed through a series of electron carriers at
successively lower energy levels. This means that energy is released
and used to form ATP out of ADP and P.
Light dependant reactions
• The electrons from PSI are passed along a different chain of carriers
to NADP. The NADP also picks up the hydrogen ions from the split
water molecules. The NADP becomes reduced NADP.
• We can show all of this in a diagram called the Z-scheme. The higher
up the energy level.
Light Independent reactions
• This reaction takes place in the stroma of the chloroplast, where
the enzyme ribulose bisphosphate carboxylase, usually known as
rubisco, is found.
• Carbon dioxide diffuses into stroma from the air spaces within the
leaf. It enters the active site of rubisco, which combines it with a 5carbon compound called ribulose biphosphate,RuBP. The products
of this reaction are two 3-carbon molecules, glycerate 3-phosphate,
GP. The combination of CO2 with RuBP is called carbon fixation.
• The energy from ATP and hydrogen from reduced NADP are then
used to convert the GP into triose phosphate, TP. Triose phosphate
is the first carbohydrate produced in photosynthesis.
• Most of the TP is used to produce RuBP, so that more CO2 can be
Light independent reactions
• This series of reactions is known as the Calvin cycle.