Photosynthesis – class 11 revision notes
Last updated on February 5th, 2022 at 04:48 am
Photosynthesis process – Facts
The photosynthesis process actually consists of two groups of reactions: light reactions and carbon-fixation reactions.
light reactions:
The light reactions capture solar energy. The light reactions occur in the chloroplast thylakoids. When chlorophyll a and accessory pigments absorb visible-light energy, electrons in these molecules become excited or energized.
That energy in excited electrons is used in one of three processes.
(1) Some of the energy in excited electrons is used to split water molecules. The oxygen from those split water is released as molecular oxygen (O2) most of which enters the atmosphere.
(2) Some of that energy is captured in the energy carrier adenosine triphosphate (ATP).
(3) The electrons(e–) and protons(H+) from water combine with the electron acceptor nicotinamide adenine dinucleotide phosphate (NADP+) to form NADPH, which is an energy carrier for those excited electrons.
In summary, the light reactions use solar energy to split water to release electrons (e–), protons (H+), and oxygen. The electrons and protons are used to make ATP and NADPH.
How do the light reactions actually carry out these three processes?
Two discrete groupings of proteins and pigment molecules are photosystem I and photosystem II (named I and II in the order of their discovery).
Both photosystem types are embedded in the thylakoid membranes.
Each photosystem contains two components: an antenna complex consisting of accessory pigment molecules, which collect light energy and pass it to the second component, the reaction center.
Each reaction center contains special molecules of chlorophyll a, which are the only ones that can pass an electron to an electron acceptor molecule and therefore have electrons removed.
An electron transport chain links together the two photosystems. The system ends with a final electron transport chain, which reduces NADP+ to NADPH.