The Process of Photosynthesis Chapter 8.3

Photosynthesis Summary

Light and Energy Because light is a form of energy any compound that absorbs it also absorbs energy. When chlorophyll absorbs energy from light much of that energy is transferred directly to electrons, raising their energy levels. These high energy electrons are used to make ATP (and NADPH – a high energy electron carrier). ATP and NADPH are used to power the Calvin cycle that produces sugar.

Light Dependant Reactions Photosystem I

Light Dependant Reactions The light dependant reactions involve electron transport between an electron donor (NADH) and an electron acceptor (oxygen). This creates a proton gradient which is used to activate an enzyme known as ATP synthase which re-phosphorylates ADP + P into ATP. The light dependant phase is split into three parts; photosystem II, electron transport chain, and photosystem I.

Photosystem II Photosystem II Light absorption – light enters the reaction center and excites chlorophyll Electron capture – the electron is captured by the electron acceptor Splitting water and releasing oxygen – water is split and its electrons replace those lost in the chlorophyll and H+ ions are released into the thylakoid space. Oxygen is released as a waste product.

Electron Transport Chain Electron transport chain (ETC) - Electrons are pass through the chain to Photosystem I. Energy generated is used to pump more H+ ions into the thylakoid space or lumen from the stroma. This creates a gradient that will force H+ ions back across the thylakoid membrane back into the stroma. The movement of ions will provide the power for ATP synthase.

Photosystem I Photosystem I Photosystem I – electrons transferred from ETC and chlorophyll is excited by sunlight. NADP+ >>> NADPH – the excited electrons are transferred in a second ETC to NADP+ to produce NADPH

Hydrogen Movement and ATP Formation In chloroplasts light energy drives the conversion of H2O to O2 and NADP+ to NADPH. NADPH will be used later in the Calvin cycle. The Thylakoid space fills up with H+ ions and becomes positively charged (relative to the outside). H+ ions pass back across the thylakoid membrane, through ATP synthase. As the ions pass through ATP synthase, it rotates and the energy produced converts ADP + P to ATP.

NADP+ and NADPH Nicotinamide adenine dinucleotide phosphate or NADP+ is an electron carrier used in anabolic reactions. NADP+ is reduced to NADPH during the light dependant reactions. When NADPH is sent to the Calvin Cycle it supplies energy and a source of Hydrogen in the production of carbohydrates.

Photosynthesis

The Calvin Cycle The Calvin cycle, also known as the light independent reactions or the CO2 cycle, involves the combining of existing compounds and CO2 to make sugars. These reaction occur in the stroma of the chloroplasts.

The Calvin Cycle

Photosynthesis and Cellular Respiration Photosynthesis and cellular respiration are closely related chains of reactions. We can see that both these reactions are basically the reverse of each other. Photosynthesis involves using ATP to build sugar. Cellular respiration involves breaking down sugar to produce ATP.

Photosynthesis and Cellular Respiration