TSR Wiki > Study Help > Subjects and Revision > Revision Notes > Biology > Photosynthesis - Light Independent/Dependent Stages
Light-Dependent reactions
THYLAKOID MEMBRANES in CHLOROPLAST
Photophosphorylation
Creating ATP
- Chlorophyll molecules absorb light energy
- Two electrons (2e-) are excited (higher energy levels)
- Electrons leave chlorophyll = positively charged
- Electrons pass thru electron carrier molecules - electron transport chain
- Energy is lost from excited electrons
- Lost energy = synthesis of ATP from ADP
- ATP used in light-independent reactions
Electrons driven from chlorophyll by light energy need to be replaced…
Photolysis
Splitting of water, production of electrons
- Splitting of water = hydrogen ions (H+), oxygen and electrons (2e-)
- Electrons from photolysis replace those emitted by chlorophyll, so it is no longer positively charged
- Hydrogen ions and electrons (that passed thru transport chain) combine with NADP = Reduced NADP
- Reduced NADP used in light-independent reactions
Light-Independent reactions (Calvin Cycle)
STROMA of CHLOROPLAST
- ATP & Reduced NADP required here
- CO2 from air + 5-carbon compound Ribulose Biphosphate (RuBP) = 6-carbon compound › CO2 + RuBP = 6-carbon compound
- 6-carbon compound unstable, therefore breaks down = 2 x 3-carbon compounds (PGA)
- 2 x PGA is reduced = 3-carbon sugar phosphoglyceraldehyde (PGAL) › ATP provides energy, Reduced NADP provides hydrogen and electrons for the reduction
- 2 of the PGALs = creation of carbohydrates, e.g. glucose and starch
- 10 PGALs = regenerate RuBP › PGAL molecules rearrange to form 5-carbon compounds, then ATP forms RuBP
- Continuous cycle
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