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Photosynthesis is the process plants use to manufacture glucose. It uses sunlight to make their own food from water and carbon dioxide.

Photosynthesis occurs in plant cells where chlorophyll is present. Chlorophyll is a green pigment that gives plant its color and is found in chloroplast cells. 

Chlorophyll is a very important component for photosynthesis because it can absorb sunlight of different wavelengths. Without the chlorophyll pigments, plants cannot carry out photosynthesis.

There are two main stages of photosynthesis. The first stage is carried out in the thylakoids, the second stage is carried out in the
stroma.

Stage 1 - Light-dependent reactions

Photosystem II & Photosystem I

Chlorophyll pigments are clustered together in the thylakoid membranes.

There are two photosystems (PSII and PSI) where sunlight is absorbed. The photosystems reside in the thylakoid membranes where the chlorophyll pigments are concentrated.

When sunlight hits the pigment in PSII, electrons in the pigment molecules are energized. The extra energy makes the molecules unstable and need to be redistributed. This is carried out by a series of redox reactions in the electrons transport chain (ETC). Thus the energized electrons are transferred from PSII to PSI along the ETC. The ETC consists of a series of molecules known as electron receptors. They are capable of accepting electrons.

Photosynthesis - Stage 1 Light dependent - Photosystem I & II - Light-dependent reactions

Water splitting in PSII

  • Water donates electrons

Meantime, water in the PSII system goes through a light-induced catalytic water splitting process. This releases electrons in the water and water becomes an electron donor. Electrons in PSII are replenished.

  • Water releases oxygen

Oxygen is released as a waste product into the air for us to breath. Basically, we are breathing plant's waste product.

  • Water releases hydrogen ions

When hydrogen ions are released in the process of water splitting, they go into the the lumen (inside) of the thylakoid. They are later used in making energy storing molecules (ATP & NADPH) which are instrumental in the formation of glucose.

In addition, hydrogen ions are also pumped across the thylakoid membrane from the stroma into the lumen. This is caused by the increase in electron charge along the membrane when sunlight is absorbed.

When hydrogen ions increase in the lumen, a concentration gradient across the thylakoid membrane is formed. This simply means the concentration of (H+) in the lumen is higher than that in the stroma. Thus, an active transport situation is created and this powers the movement of hydrogen ions. It's called active transport because the hydrogen ions will automatically move by themselves through the ATP synthase to create ATP. How amazing that plants can create their own pump systems to move molecules around where needed!

photosynthesis ATP Synthase

  • Movement of hydrogen ions

In the ATP synthase, ADP is phosphorylated, meaning a phosphate group is added to it. ADP is converted to ATP by the enzymes in the synthase.

  • ATP & NADPH

In this area, ATP & NADP special energy molecules are created using hydrogen ions.

Stage 2 - Calvin cycle

  • The dark reactions

The Calvin cycle is also referred as the "dark reactions". Here, the ATP & NADP high energy molecules are used for maufacturing glucose.

Photosynthesis - The Calvin Cycle - Light-independent
The Calvin cycle takes place in the stroma and sunlight is not needed here.

In the Calvin cycle, glucose is made with the help of carbon dioxide and water.

Oxygen is released as a waste product and travels into the air.

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