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Revision Notes on Photosynthesis in Higher Plants

Chloroplast-The site of photosynthesis:

The most active photosynthetic tissue in higher plants is the mesophyll of leaves. Mesophyll cells have many chloroplasts. Chloroplast is present in all the green parts of plants and leaves. There may be over half a million chloroplasts per square millimetre of leaf surface. In higher plants, the chloroplasts are discoid or lens-shaped. They are usually 4-10mm in diameter and 1-3mm in thickness.


Chloroplast pigments:

Pigments are the organic molecules that absorb light of specific wavelengths in the visible region due to presence of conjugated double bonds in their structures. The chloroplast pigments are fat soluble and are located in the lipid part of the thylakoid membranes.

(i) Chlorophylls: The chlorophylls, the green pigments in chloroplast are of seven types i.e., chlorophyll a, b, c, d, e, bacteriochlorophyll and bacterioviridin.


Chemical Formula


Chlorophyll a


All photosynthetic organisms except photosynthetic bacteria.

Chlorophyll b


Chlorophyta, Euglenophyta and in all higher plants.

Chlorophyll c


Brown algae (Phaeophyta), Diatoms and Pyrrophyta.

Chlorophyll d


Red algae (Rhodophyta).

Chlorophyll e

Not fully known




Purple photosynthetic bacteria.

Chlorobiumchlorophyll (Bacterioviridin)


Green sulphur bacteria.

(ii) Carotenoids : The carotenoids are unsaturated polyhydrocarbons being composed of eight isoprene (C5H8) units. They are made up of two six-membered rings having a hydrocarbon chain in between. They are sometimes called lipochromes due to their fat soluble nature.

Difference between Photosystem I and Photosystem II


Photosystem I

Photosystem II


PS I lies on the outer surface of the thylakoids

PS II lies on the inner surface of the thylakoid.


In this system molecular oxygen is not evolved.

As the result of photolysis of water molecular oxygen is evolved.


Its reaction center is P700.

Its reaction center is P680.


NADPH is formed in this reaction.

NADPH is not formed in this reaction.


It participate both in cyclic and noncyclic photophosphorylation.

It participates only in noncyclic photophosphorylation.


It receives electrons from photosystem II.

It receives electrons from photolytic dissociation of water.


It is not related with photolysis of water.

It is related with photolysis of water.

Difference between C3 Plants and C4 Plants



C3 plants

C4 plants


CO2 acceptor

The CO2 acceptor is Ribulose 1, 5 diphosphate.

The CO2 acceptor is phosphoenol-pyruvate.


First stable product

The first stable product is phosphoglyceric acid.

Oxaloacetate is the first stable product.


Type of chloroplast

All cells participating in photosynthesis have one type of chloroplast.

The chloroplast of parenchymatous bundle sheath is different from that of mesophyll cells. Leaves have 'Kranz' type of anatomy. The bundle sheath chloroplasts lack grana. Mesophyll cells have normal chloroplasts. 



Only reductive pentose phosphate cycle is found.

Both C4-dicarboxylic acid and reductive pentose phosphate cycles are found.


Optimum temperature

The optimum temperature for the process is 10-25°C.

In C4 plants, it is 30-45°C.


Oxygen inhibition

Oxygen present in air (=21% O2) markedly inhibit the photosynthetic process as compared to an external atmosphere containing no oxygen.

The process of photosynthesis is not inhibited in air as compared to an external atmosphere containing no oxygen.


PS I and PS II

In each chloroplast, photosystems I and II are present. Thus, the Calvin cycle occurs.

In the chloroplasts of bundle sheath cells, the photosystem II is absent. Therefore, these are dependent to mesophyll chloroplast for the supply of NADPH + H+



The Calvin cycle enzymes are present in mesophyll chloroplast.

Calvin cycle enzymes are absent in mesophyll chloroplasts. The cycle occurs only in the chloroplasts of sheath cells.


Compensation point

The CO2 compensation point is 50-150ppm.

CO2 compensation point is 0-10ppm.



Photorespiration is present and easily detectable.

Photorespiration is present only to a slight degree and difficult to detect.


Net rate

Net rate of photosynthesis in full sunlight (10,000-12,000 ft.c) is 15-35mg. of CO2 per dm2 of leaf area per h.

It is 40-80mg. of CO2 per dm2 of leaf area per h. That is photosynthetic rate is quite high. The plants are efficient.


Saturation intensity 

The saturation intensity reached in the range of 100-4000 ft.c.

It is difficult to reach saturation even in full sunlight.

Difference between Photorespiration, Photosynthesis and True Respiration




True Respiration


Occurs in green plants in light.

Occurs in green plants in light.

Occurs in all living organisms in light and dark.


The primary substrate is glycolate formed from RuBP.

Substrate is CO2 and H2O.

Substrates are carbohydrates, fat and proteins.


Occurs in most of the C3 plants.

Occurs in all green plants.

Occurs in all living organisms.


Intracellularly, the process occurs in peroxisomes in association with chloroplasts and...

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