Photosynthesis is the process by which plants, algae, and some bacteria use sunlight, water and carbon dioxide to produce oxygen and energy in the form of glucose. It takes place in two stages - the light dependent reaction which uses energy from sunlight to convert water to oxygen and produces ATP and NADPH, and the light independent reaction (Calvin cycle) which uses ATP and NADPH to incorporate carbon from carbon dioxide into organic compounds. There are several factors that affect the rate of photosynthesis including light intensity, temperature, carbon dioxide concentration, and water availability.

1. PHOTOSYNTHESIS

2. PHOTOSYNTHESIS
 *photosynthesis is a process used by plants and other organism to convert light energy
into chemical energy.
 *this is an energy requiring reaction –the energy source is sunlight.The overall
equation for the type of photosynthesis occur in plants
 .
* the chemical energy is stored in carbohydrate molecules such as sugars, which are
synthesized from Co2 and H2o.
 *Photosynthesis maintains atmospheric oxygen levels and supplies all of the organic
compounds and most of the energy necessary for life on earth.
 *Photosynthetic organisms are photoautotrophs, which means that they are able to
synthesize food directly from Co2 and H2o using energy from light..

3. PHOTOSYNTHESIS
 Photosynthetic organisms are photoautotrophs, which means that they are
able to synthesize food directly from Co2 and H2o using energy from light.
 *There are two types of photosynthetic processes: oxygenic photosynthesis
and anoxygenic photosynthesis.
 *In plants, algae and cyanobacteria, photosynthesis releases oxygen. This is
called oxygenic photosynthesis.
 *In anoxygenic photosynthesis they consumes carbon dioxide but do not
release oxygen.

4. Photosynthesis:Discovery
*The Joseph Priestley discovered oxygen in 1774.
*He conducted a series of experiments that
revealed the essential role of air in the growth of green plants.
*He observed that a candle is burning in a bell jar with the death mouse
immediately went out.
*After he put a plant in the bell jar and found that the mouse still lived.
*Priestly hypothesized as follows: plants restore the air whatever
breathing animals and burning candles remove.

6. WHERE DOES PHOTOSYNTHESIS TAKE PLACE?
*The photosynthesis takes place in chloroplast.
*The innermost membrane of the chloroplast is called
thylakoids.
*Chlorophyll pigments are present in thylakoids.
*The thylakoids are arranged in stacks like the piles of coins
called grana.
*The cytoplasm of the chloroplast is called stroma.
*The stroma of the chloroplast contains enzymes required for
the synthesis of carbohydrates and proteins.

7. HOW MANY PIGMENTS ARE INVOLVED IN
PHOTOSYNTHESIS?
 *There are four pigments seen in leaves through paper chromatography.
 *They are chlorophyll a, chlorophyll b, Xanthophylls and carotenoids. The graph showing
that the maximum absorption by chlorophyll a.
 *The pigment molecules are associated with proteins, which allow them flexibility. A large
collection of 100 to 5,000 pigment molecules constitute “antennae”. It capture light energy
from the sun in the form of a photons.

8. Reactions of plant photosynthesis
• *The reactions of plant photosynthesis are divided into two. They are
* 1) Light dependent reaction (Light reaction)
• 2) Light-independent reaction (Dark reaction)

10. 1) Light dependent reaction (Light
reaction)
 *It take place in thylakoid.
 *The light reaction include light absorption, water splitting, oxygen
release and the formation of ATP and NADPH.
 *The pigments are organized into two light harvesting complexes (LHC)
within the Photosystem I (PS I) and Photosystem II (PS II).
 *These pigments help to make photosynthesis more efficient by
absorbing different wavelengths of light.
 *The single chlorophyll a molecule forms the reaction centre.
 *The reaction centre is different in both PS I and PS II.

11. 1) Light dependent reaction (Light
reaction)
 *In PS I the reaction centre chlorophyll a has an absorption peak at 700nm, hence is
called P700.
 *In PS II it has absorption maxima at
 680nm, hence is called p680.

13. THE ELECTRON TRANSPORT
 *In photosystem II the reaction centre chlorophyll a absorbs 680nm wavelength of red light causing electrons to
become excited and jump into an orbit farther from the atomic nucleus. These electrons are picked by an
electron acceptor which passes them to an electron transport system consisting of cytochromes.
 *The movement of electrons is downhill, in terms of
 an oxidation reduction. The electrons are pass through the
 the electron transport chain. But they are passed through the
pigments of PS I.
*Electrons in the reaction Centre of PS I are also excited when
they receive red light of wavelength 700nm and transferred
into the acceptor molecule.
*The addition of these electrons reduces NADP+ to NADPH +
H+. The whole scheme of transfer of electrons, starting from
the PS II acceptor down to the electron transport chain PS I.

14. ELECTRON
TRANSPORT
 *Finally the NADP+ is reduced into NADPH+ and H+. This is called Z scheme.
 *PS II supply electrons continuously by the splitting of water.
 *The splitting of water is associated with the PS II. Water is split into H+ and
[o] electrons.

15. Cyclic and Non-cyclic photophosphorylation
• *The synthesis of ATP from ADP and inorganic phosphate in presence
of light is known as phosphorylation.
• *Non-cyclic photophosphorylation occurs by two photosystems are
work in a series, first PS II and then the PS I. The two photosystems are
connected through an electron transport chain, as seen in earlier- in
the Z scheme.
• *Cyclic phosphorylation occurs by one photosystem and cyclic flow of
electrons.

16. Cyclic and Non-cyclic photophosphorylation
• Cyclic phosphorylation occurs by one photosystem and cyclic flow of
electrons.
• *It take place in stroma lamellae.
• *There is a synthesis of only ATP, but not of NADP + H+
• *Cyclic phosphorylation also occurs when the light of wavelength
beyond 680nm.

17. Cyclic
phosphorylation

19. CALVIN CYCLE (C3 cycle)
• *The path of carbon in the dark reaction was traced by Melvin
• Calvin using radioactive carbon(C14). The Calvin pathway occurs in all
photosynthetic plants, irrespective of whether they have C3 or C4 pathway
• *The plants in which the first product of Co2 fixation is a PGA (Phospho
glyceric acid) is called C3 pathway. SO Calvin cycle is called C3 cycle.
• *The plants in which the first product of Co2 fixation is an OAA (oxalo
acetic acid) is called C4 pathway.
• *The primary acceptor of Co2 is Ribulose biphosphate. It is 5-carbon ketose
sugar.

20. CALVIN
CYCLE
• *Calvin cycle can be described under three stages:

21. Carboxylation, reduction and regeneration
• Carboxylation
• *Ribulose 1, 5-biphosphate combines with Co2 and fixes a stable
organic intermediate (3-phosphoglycerate).
• *The reaction is catalyzed by enzyme RuBisco (RuBP carboxylase).
• *3 PGA is the first stable product of this cycle
• Reduction
• *Here the two molecules each of ATP and NADPH are required for
fixing 1 molecule of Co2.
• *Glucose is the product of this cycle.

22. Regeneration
*RuBP is crucial if the
cycle to continue
uninterrupted.
*1 ATP molecule is
required.

24. C4 pathway
 *It is otherwise known as Hatch and Slack pathway.
 *It seen in plants like maize, sugar cane etc.
 *The leaves of C4 plants have Kranz anatomy. They have a special large cells around their
vascular bundles are called bundle sheath cells. It forms a several layers around the vascular
bundle; they are characterized by having large number of chloroplast and no intercellular
spaces.
 *Here the C3 cycle occur in Bundle sheath cells and C4 cycle occur in Mesophyll cells.
 *The primary Co2 acceptor is a 3-carbon molecule phosphoenol pyruvate.
 *The enzyme PEP Carboxylase fixes Co2 in the mesophyll cells. It forms 4-carbon compounds
like malic acid and aspartic acid.
 *These compounds are transported into the bundle sheath cells and C4 acid breaks down to
form C3 acid and Co2.
 *C3 acid is transported to the mesophyll cells and regenerated back into PEP.

26. FACTORS AFFECTING PHOTOSYNTHESIS

27. FACTORS AFFECTING PHOTOSYNTHESIS
 *The photosynthesis is under the influence of several factors, both internal and external.
 *The internal include the number, size and age, orientation of leaves, Co2 concentration and
amount of chlorophyll.
 *The external include the availability of sunlight, temperature, Co2 concentration and water.
 *The factors affecting photosynthesis can be explained by Blackman’s (1905) Law of
Limiting Factors. It states that if a chemical process is affected by more than one
factor, its rate will be determined by the factor which is nearest to its minimal value.
 Light
 *There is linear relationship between incident light and Co2 fixation rates at low light
intensity.
 *The figure shows at higher light intensities the rate does not show further increase.
 *Increase in incident light beyond a point causes the breakdown of chlorophyll and
decrease in photosynthesis.

28. FACTORS AFFECTING PHOTOSYNTHESIS
 Carbon dioxide concentration
 *The C3 plants are respond to higher Co2 concentration by showing increased rates of
photosynthesis leading to higher
 Productivity has been used for some greenhouse crops such as tomatoes and pepper.
 Temperature
 *The C4 plants are respond to higher temperature and C3 plants are respond to lower
temperature.
 Water
 *Water is one of the reactant in light reaction.
 *Water stress causes the stomata to close hence reducing Co2 availability.
