Photosynthesis Recap

Photosynthesis is basically the opposite of cell respiration. Instead of taking in oxygen and creating carbon dioxide, along with ATP, photosynthesis takes in carbon dioxide and releases oxygen.

Plant photosynthesis has two parts to it, light reaction and dark reaction. 

Light reaction takes place in the grana on the chloroplast. The first part of the light reaction is Photosystem II (PS2). In this part, water molecules get split into oxygen and hydrogen atoms. The electrons from the hydrogens are carried to the electron transport chain to provide energy to produce ATP by chemiosmosis. This step is called photophosphorylation because it uses light to provide energy.

The next step is Photosystem I (PS1). Light energy is absorbed once again. PS 1 is smilier to PS 2 except the end product is not ATP but NADPH.

So basically for light reactions,
PS2

1. Light is absorbed (680)
2. oxygen released; hydrogen ions move to ATP synthase
3. ATP produced

PS1

1. Light absorbed (700)
2. NADPH produced (carries H+ to Calvin cycle)

Dark Reactions are called dark reactions because there is no need for light absorption during this phase. It's also known as the Calvin Cycle.

This process is called carbon fixation. Basically, CO2 enters the Calvin cycle and becomes attached to a 5-C sugar to form a 6-C molecule. This molecule is unstable and immediately breaks down into 2 3-C molecules by the enzyme rubisco. The end product made is ADP and NADP, which go back into the cycle, and G3P, a 3-C sugar.


And that, in a large nutshell, is how we are able to live and breathe. Thank you plants!!