CHAPTER 13 PHOTOSYNTHESIS IN HIGHER PLANTS
Channel: ISHTA’s BIO ACADEMY
Date: 02/04/2023

Green plants make their own food by photosynthesis. During this process carbon
dioxide from the atmosphere is taken in by leaves through stomata and used for
making carbohydrates, principally glucose and starch. Photosynthesis takes place
only in the green parts of the plants, mainly the leaves. Within the leaves, the
mesophyll cells have a large number of chloroplasts that are responsible for CO2
fixation. Within the chloroplasts, the membranes are sites for the light reaction,
while the chemosynthetic pathway occurs in the stroma. Photosynthesis has two
stages: the light reaction and the carbon fixing reactions. In the light reaction the
light energy is absorbed by the pigments present in the antenna, and funnelled to
special chlorophyll a molecules called reaction centre chlorophylls. There are two
photosystems, PS I and PS II. PS I has a 700 nm absorbing chlorophyll a P700
molecule at its reaction centre, while PS II has a P680 reaction centre that absorbs
red light at 680 nm. After absorbing light, electrons are excited and transferred
through PS II and PS I and finally to NAD forming NADH. During this process a
proton gradient is created across the membrane of the thylakoid. The breakdown
of the protons gradient due to movement through the F0
 part of the ATPase enzyme
releases enough energy for synthesis of ATP. Splitting of water molecules is
associated with PS II resulting in the release of O2
, protons and transfer of electrons
to PS II.
In the carbon fixation cycle, CO2
 is added by the enzyme, RuBisCO, to a 5-
carbon compound RuBP that is converted to 2 molecules of 3-carbon PGA. This
is then converted to sugar by the Calvin cycle, and the RuBP is regenerated. During
this process ATP and NADPH synthesised in the light reaction are utilised. RuBisCO
also catalyses a wasteful oxygenation reaction in C3
 plants: photorespiration.
Some tropical plants show a special type of photosynthesis called C4
 pathway.
In these plants the first product of CO2
 fixation that takes place in the mesophyll,
is a 4-carbon compound. In the bundle sheath cells the Calvin pathway is carried
out for the synthesis of carbohydrates.