Photosynthesis—Energy Acquiring Pathway
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Photosynthesis is the process of taking sunlight, water, and carbon dioxide and
converting them into oxygen, sugar, and glucose.
Photosynthesis is consist of 2 stages:
1. Light-dependent reaction (Light reaction)—sunlight energy is absorbed and
converted to ATP/NADPH energy.
2. Light-independent reaction (Dark reaction)—ATP/NADPH energy drives the
assembly of carbon, hydrogen, and oxygen into molecules of glucose (C6H12O6).
WHERE THE REACTIONS OCCUR
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The 2 stages of photosynthesis proceed at different sites inside the chloroplast.
Inside the Chloroplast:
1. stroma—semifluid interior
2. grana/granum—stack of disks or pancakes
3. thylakoid membrane system—all the stack of disks plus channels between them
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The spaces inside the disks and channels are used for ATP production.
The Light-dependent reaction occurs inside the thylakoid membrane system.
The Light-independent reaction occurs in the stroma.
LIGHT-DEPENDENT REACTION
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3 events occur during this reaction:
1. Pigments absorb sunlight energy and give up electrons
2. Electron and hydrogen transfers lead to ATP and NADPH formation.
3. The pigments that gave up electrons in the first place get electron replacements
Absorbing Sunlight Energy:
 In thylakoid membranes, clusters of pigments (light-absorbing molecules) trap
photons (packets of energy that travel through space) from the sun.
 Examples of some pigments are chlorophyll a, chlorophyll b, carotenoid, and
phycobilin.
 In all plants, chlorophyll a is the main pigment of photosynthesis.
 These pigments that are organized as light-trapping clusters are called photosystems.
 When they absorb a photon, one of their electrons gets boosted to a higher energy
level.
 When the electron returns to a lower level, it quickly gives up the added energy.
 The energy bounces among pigments, and a bit is lost at each bounce as heat.
ATP and NADPH Formation:
 ATP are made by 2 different pathways—cyclic and noncyclic
1. Cyclic Pathway—in this pathway, electrons “cycle” from P700 through a transport
system then back to P700 (also known as photosystem I or light-trapping pigment)
 Energy released during the electron flow is used in the formation of ATP from
ADP and unbound phosphate.
2. Noncyclic Pathway—in the pathway, it goes into operation when the sun’s rays
bombard photosystem II; thus, P680 give up electrons.
 This also trigger photolysis—a reaction sequence that splits apart water
molecules into oxygen, hydrogen ions, and electrons.
 P680 attracts the electrons released from water as replacements for the excited
ones that got away.
 The excited electrons are transferred through a transport system and then to
chlorophyll P700 of photosystem I.
 Because electrons arriving at P700 have not lost all of their extra energy and
because photons are also bombarding P700, the incoming energy boosts the
electrons to a higher energy level that allow them to enter a second transport
system.
 Since one of the enzymes of this system has a helper called NADP+, when the
enzyme takes part in the transfers, it picks up 2 electrons and a hydrogen ion, and
so becomes NADPH
 In the light-dependent reaction, energy from the sun drives the formation of
1. ATP (which carries energy)
2. NADPH (which carries hydrogen and electrons.)
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LIGHT-INDEPENDENT REACTION
This pathway is the synthesis part of photosynthesis and it does not require sunlight.
For this reaction to occur, it needs
1. ATP to deliver the required energy
2. NADPH to deliver the required hydrogen and electrons
3. Carbon dioxide in the air to provide the carbon and oxygen.
Calvin –Benson Cycle:
 This reaction starts when a carbon atom from carbon dioxide becomes attached to
RuBP (ribulose bisphosphate).
 This attachment produces an unstable six-carbon intermediate; thus, it splits into 2
molecules of PGA (phosphoglycerate)
 ATP donates a phosphate group to each PGA, and NADPH donates hydrogen and
electrons to the resulting intermediate, forming PGAL (phosphoglyceraldehyde)
 Most of the PGAL are used to replace RuBP used in carbon dioxide fixation.
 Some PGAL are combined to form a 6 carbons sugar phosphate.
 The sugar phosphate formed in the cycle serves as a building block for sucrose,
starch, or cellulose
 ADP, NADP, and phosphate leftovers diffuse back to sites of the light-dependent
reaction or the thylakoid system to be converted back to NADPH and ATP.