Photosynthesis ( photo =light, synthesis-= putting together ) is the process of formation of simple sugars by green plants, some bacteria, and some protistans from water from soil and carbon dioxide from air in presence of sunlight and chlorophyll.

Photosynthesis converts light/solar energy into chemical energy, and thus is very important to life. Chemical equation for photosynthesis (reactants on the left, products on the right):

energy from the Sun + 6CO2 + 6H2O → C6H12O6 + 6O2

In this process sunlight (photons), carbon dioxide (CO2) and water (H2O) is converted into glucose (C6H12O6) and the byproduct oxygen (O2).

Photosynthesis occurs in green parts of the plant, mostly the leaves, sometimes the green stems and floral buds. The leaves contain specialised cells called mesophyll cells which contain the chloroplast the pigment containing organelle. These are the actual sites for photosynthesis.

Photosynthetic Pigment:

The thylakoids of the chloroplast contain the pigments which absorb light of different wave length and carry out the photo chemical reaction of photosynthesis.The role of the pigments is to absorb light energy, thereby converting it to chemical energy.

The photosynthetic pigment chlorophyll is the principle pigment involved in photosynthesis. It is a large molecule and absorbs light in the violet blue and in the red region of the visible spectrum and reflects green light and thus leaves appear green in colour. Carotenoids (carotene and xanthophyll ) absorb light in the regions of the spectrum not absorbed by the chlorophyll.

The 2-step process:

Photosynthesis occurs in two steps, the Light Reactions (also: light-dependent reaction) and the Calvin Cycle (also: dark-reactions, light-independent reactions, carbon fixation).

The light-dependent reactions (which take place in the thylakoid membranes, or grana, of the chloroplasts), in which light energy, trapped by chlorophyll, is used:

  • to split water into O2 and H+ ions, stored as reduced NADPH and
  • to be stored as chemical energy by converting ADP + Pi to ATP.

The inside of the thylakoid membrane is called the lumen, and outside the thylakoid membrane is the stroma, where the light-independent reactions take place.

The light-independent reactions (Calvin Cycle), (take place in the stroma of the chloroplasts) in which:

  • the ATP and reduced NADP (from the light-dependent reactions) are used to reduce CO2 to glucose.

The Calvin cycle takes place within the stroma of the chloroplast. This is where plants make sugar molecules that they can use to make other essential components, and that all other organisms can use for energy.

The cycle uses ATP and NADPH synthesized in the light-dependent reactions to drive its reactions forward. So although the cycle itself doesn’t use light energy, it does depend on the light reactions to provide it with the ATP and NADPH.

The Calvin cycle has four main steps: carbon fixation, reduction phase, carbohydrate formation, and regeneration phase.

Photorespiration:

  • Photorespiration is a wasteful pathway that competes with the Calvin cycle. It begins when rubisco acts on oxygen instead of carbon dioxide. This wasteful metabolic pathway begins when rubisco, the carbon-fixing enzyme of the Calvin cycle, grabs O2 rather than CO2. It uses up fixed carbon, wastes energy, and tends to happens when plants close their stomata (leaf pores) to reduce water loss. High temperatures make it even worse.
  • The majority of plants are C3 plants, which have no special features to combat photorespiration
  • C4​ plants minimize photorespiration by separating initial CO2 fixation and the Calvin cycle in space, performing these steps in different cell types.
  • Crassulacean acid metabolism (CAM) plants minimize photorespiration and save water by separating these steps in time, between night and day.

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