Photosynthesis
Photosynthesis is the process by which light energy is converted into chemical energy in the presence of carbon dioxide and water and stored in the bonds of stable carbohydrates releasing oxygen as a by product. Most photosynthesis takes place in the green parts of plants, which contain chloroplasts, in particular the leaves.
Photosynthesis is a very important process for life on earth because it recycles carbon dioxide and water, and provides food for many organisms.
Photosynthesis at work
The organelle that is responsible of photosynthesis is the chloroplast.
It is made of a double membrane enclosing a fluid called stroma.
Embedded in the stroma, there are columns of disk shaped structures or thylakoids called grana.
Photosynthesis is divided into two processes. The light dependent and light independent reactions
Photosynthesis in the chloroplast takes place in two different places: The thylacoid membrane and the stroma.
It is divided into the Photosystems I and II or Light Reactions, and the Calvin cycle.
The Light dependent reactions begins as sunlight is absorbed by the chloroplasts. Light activates a carrier protein or Photosystem II in the thylakoid membrane that allows the pass of water and hydrolyzes it into hydrogen ions and oxygen. Then, because of the concentration gradient, an electron transport chain is fromed and charges the energy molecule nicotinamide adenine dinucleotide phosphate (NADP) with two high-energy electrons and a hydrogen ion into NADPH. Some H ions that cannot cross the membrane generate a difference in the membrane potential and activates the Photosystem I which consists of an enzyme called ATP synthase that charges adenosine diphosphate into adenosine triphosphate by adding a phosphate molecule to it.
During the Light Independent Reactions, The energy molecules enter the Calvin Cycle in which carbon dioxide is used to produce glucose or other simple sugars.
ATP molecules in the presence of an enzyme called rubisco trigger the reaction between carbon dioxide and a sugar called ribulose 3-phosphate to produce 3-phosphoglycerate (3-PGA) that in the presence of ATP and NADPH, is decomposed into glyceraldehyde 3-phosphate that is used to build up molecules of sucrose or starch, and to regenerate ribulose diphosphate so the cycle can start again.
