Aquatic ecology and Biodiversity

Unit 5- Productivity

5.1.4. Stages of photosynthesis
• The light dependent Process (Light reactions) The Light Reactions occur in the grana of the chloroplasts
• The light independent process (Dark reactions) Dark reactions take place in the stroma of the chloroplasts.

Photosynthesis process. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Light reaction: (Hills reaction)

Light energy in converted into chemical energy. Chemical energy is stored is chemical bonds- ATP, NADPH₂( energy coins). Light reaction takes place in thyllakoids of “grana”.

ADP + Pi --------------> ATP (Cyclic photophosphorylation)
2NADP +2H₂O + ADP + 2Pi --------------> 2NADPH₂ + O₂ +2ATP (Non cyclic photo phosprylation)

Photophosphorylation
Photophosphorylation is the process of converting energy from a light-excited electron into the pyrophosphate bond of an ADP molecule.

Noncyclic photophosphorylation (top) and cyclic photophosphorylation (bottom). These processes are better known as the light reactions. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The above diagrams present the "old" view of photophosphorylation. We now know where the process occurs in the chloroplast, and can link that to chemiosmotic synthesis of ATP.

In the light dependent processes (Light reaction) light strikes chlorophyll a in such a way as to excite electrons to a higher energy state. In a series of reactions the energy is converted into ATP and NADPH. Water is split in the process, releasing oxygen as a by-product of the reaction. The ATP and NADPH are used to make C-C bonds in the light independent process (Dark reaction). In the light independent process, carbon dioxide and water is modified by the addition of hydrogen to form carbohydrates [CH₂O]n). The incorporation of carbon dioxide into organic compounds is known as carbon fixation. It converts it into C-C energy bond that can be released by glycolysis and other metabolic processes.