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 occurs in two stages - the light-dependent reactions where sunlight is absorbed to make ATP and NADPH using chlorophyll, and the light-independent Calvin cycle where carbon is fixed into sugars like glucose using ATP and NADPH. Photosynthesis provides food for plants and oxygen for other organisms while consuming carbon dioxide.

2. All life needs a constant input of energy
Autotrophs: Get their energy from “sunlight”
Build organic molecules (food) from CO2
Make energy & synthesize sugars through photosynthesis
Heterotrophs
Get their energy from “eating others”

3. Process that converts solar energy into chemical energy
Occure in plants, algae, certain other protists and some prokaryotes
They use the energy of sunlight to make organic molecules from water and carbon dioxide
The chloroplasts are the major site of photosynthesis located mainly in the leaves

4. It is Double membrane contains thylakoid sacs
Chlorophyll, the green pigment within chloroplasts gives green color of leaf
Chloroplasts also contain stroma,
a dense interior fluid

5. Photosynthesis consists of two processes
The light reactions
Occur in the grana
During the light reaction, there are two possible routes for electron flow
Cyclic Electron Flow
Noncyclic Electron Flow
The Calvin cycle
Occurs in the stroma
Forms sugar from carbon dioxide, using ATP for energy and NADPH for reducing power

6. The overall reactions of photosynthesis that can be summarized as the following equation:
6 CO2 + 12 H2O + Light energy  C6H12O6 + 6 O2 + 6 H2O
sugars C6H12O6
Sugar Building
Reactions
ADP
NADP
NADPH
ATP
CO2

7. Occurs in the Thylakoid membranes
During the light reaction, there are two possible routes for electron flow
Cyclic Electron Flow
Uses Photosystem I only
P700 reaction center- chlorophyll a
Uses Electron Transport Chain (ETC)
Generates ATP only
ADP + Pi ATP
Noncyclic Electron Flow
ADP + Pi  ATP
NADP+ + H  NADPH
Oxygen comes from the splitting of H2O, not CO2
H2O  1/2 O2 + 2H+

8. Powers ATP synthesis
Takes place across the thylakoid membrane
Uses ETC and ATP synthase (enzyme)
H+ move down their concentration gradient through channels of ATP synthase forming ATP from ADP

9. P700
Primary
Electron
Acceptor
e-
e-
e-
e- ATP produced by ETC
Photosystem I
Accessory
Pigments
SUN
Photons
Pigments absorb light energy & excite e- of Chlorophylla to produce ATP

10. P700
Photosystem I
P680
Photosystem II
Primary
Electron
Acceptor
Primary
Electron
Acceptor
ETC
Enzyme
Reaction
H2O
1/2O2 + 2H+ ATP NADPH
Photon
2e-
2e-
2e-
2e-
2e-
SUN
Photon
H2O is split in PSII & ATP is made, while the energy carrier NADPH is made in PSI

11. The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle
The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH
In most plants (C3 plants), initial fixation of CO2, via rubisco, forms a three-carbon compound (3-phosphoglycerate)
In photorespiration, rubisco adds O2 instead of CO2 in the Calvin cycle, producing a two- carbon compound
Photorespiration consumes O2 and organic fuel and releases CO2 without producing ATP or sugar

13. C4 pathway – evolved in hot, dry climate
Form 4-carbon compounds
Partially close stomata
E.g. Corn, sugar cane, crabgrass
CAM – open stomata at night, close in day
Grow slow, lose less water
E.g. cactus, pineapple

14. Dark reactions make use of these organic energy molecules (ATP and NADPH). This reaction cycle is also called Calvin Benison Cycle, and it occurs in the stroma. ATP provides the energy, while NADPH provides the electrons required to fix the CO2(carbon dioxide) into carbohydrates.

15. ◦ Light Intensity
High Intensity Light causes the rate of photosynthesis to increase
The rate will increase until it reaches its saturation point
At the saturation point, the rate of photosynthesis remains constant

16.  Temperature
As the temperature increases, so does the rate of photosynthesis
Enzymes function at an optimal temperature
If the temperature is too high or too low, then the enzyme will not function properly
The rate of photosynthesis will slow down or stop, entirely

17.  Water
Water is one of the raw materials of photosynthesis
A shortage of water can slow or even stop photosynthesis
Water stress causes stomata to close, preventing CO2 from entering the leaf

18.  Carbon Dioxide (CO2) Concentration
An increase in CO2 concentration causes the rate of photosynthesis to increase
More CO2 available means more sugar being made in the light independent reaction

19. i. Helps in conversion of solar energy into organic matter.
ii. Consumes atmospheric carbon dioxide and yields carbohydrates and molecular oxygen.
iii. Evolves molecular oxygen for use by other living organisms and maintains the level of atmospheric oxygen which is continuously consumed by plants and animals during respiration
iv. Produces carbohydrates and used by plants and animals to synthesize organic acids, proteins, fats, nucleic acids, pigments, hormones, vitamins, alkaloids and other metabolites

20. Plants are the only things on earth that turn sunlight into food. They do it through a process called photosynthesis
It is the only known method of manufacture of organic food from inorganic raw materials.
Production of oxygen which is the sole of man’s livelihood is also by this photosynthesis

21.  Dr. B.B Arora and A.K Sabharwal(2010). A Text Book of Biology STD XII Modern publications
Dr.E .Valsala kumar(2004).A Text Book of Botany,Trivandrum Publications
Kumar Pushkar and A.P Singh(2011).A Text book of CSIR-UGC Life Sciences,Upkar publications

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