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”

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

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

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

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

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+

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

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

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

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

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

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.

◦ 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

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

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

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

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

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

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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