Photosynthesis ❮ 85There are three types of photosynthesis reactions:
(Noncyclic)light-dependent reactions
- Occur in thylakoid membrane of chloroplast.
- Inputs are light and water.
- Light strikes photosystem II (P680).
- Electrons pass along until they reach primary electron acceptor.
- Photolysis occurs—H 2 O is split to H+and O 2.
- Electrons pass down an ETC to P700 (photosystem I), forming ATP by chemiosmosis.
- Electrons of P700 pass down another ETC to produce NADPH.
- Three products of light reactions are NADPH, ATP, and O 2.
- Oxygen produced comes from H 2 O.
(Cyclic)light-dependent reactions
- Occur in thylakoid membrane.
- Only involves photosystem I; no photosystem II.
- ATP is the only product of these reactions.
- No NADPH or oxygen are produced.
- These reactions exist because the Calvin cycle uses more ATP than NADPH; this is how
the difference is made up.
Light-independent reactions (Calvin cycle)
- Occurs in stroma of chloroplast.
- Inputs are NADPH, ATP, and CO 2.
- First step is carbon fixation, which is catalyzed by an enzyme named rubisco.
- A series of reactions lead to the production of NADP+, ADP, and sugar.
- More ATP is used than NADPH, which creates the need for the cyclic light reactions.
- The carbon of the sugar product comes from CO 2.
Also:
C 4 plants—plants that have adapted their photosynthetic process to more efficiently handle
hot and dry conditions.
C 4 photosynthesis—process that first converts CO 2 into a 4-carbon molecule in the mesophyll
cells, converts thatproduct to malate, and then shuttles it to the bundle sheath cells, where
the malate releases CO 2 and rubisco picks it up as if all were normal.
CAM plants—plants that close their stomata during the day, collect CO 2 at night, and store
the CO 2 in the form of acids until it is needed during the day for photosynthesis.
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