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2. Photosynthesis
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Notes taken from lecture slides and lecturers as well as the textbook. Notes will help with the photosynthesis topic within Biology 1B
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Photosynthesis• ATP – source of energy
• NADPH – source of reduction
• Light reaction – on or in the thylakoid membrane
• Dark reaction – in the stroma
On the outside of the thylakoids, molecules of NADP+ and ADP pick up electrons and phosphates,
respectively, and NADPH and ATP are the released to the stroma, where they play crucial roles in the
Calvin cycle.
Light reaction
The light reactions are the steps of photosynthesis that convert solar energy to chemical. Water is
split, providing a source of electrons and protons (hydrogen ions, H+) and giving off O2 as a by-
product. Light absorbed by chlorophyll drives a transfer of the electrons and hydrogen ions from
water to an accepter called NADP+, where they are temporarily stored. The light reactions use solar
energy to reduce to NADP+ to NADPH by adding a pair of electrons along with an H+. The light
reaction also generates ATP using chemiosmosis to power the addition of a phosphate group to ADP,
a process called photophosphorylation. Thus, light energy in the form of two compounds: NADPH
and ATP. NADPH, a source of electrons, acts as ‘reducing power’ that can be passed along to an
electron acceptor, reducing it, while ATP is the versatile energy currency of cells.
Chlorophyll and Carotenoids
• Chlorophylls - usually most abundant and most important pigments in
light harvesting. Contain tetrapyrrole ring (chlorin) similar to heme but
contains Mg2+. Chlorophylls a (Chl a) and b (Chl b) in plants
• Bacteriochlorophylls a (BChl a) and b (BChl b) are major pigments in
anaerobic photosynthetic bacteria.
• There is a Division of Labor Between the Chlorophyll Molecules - Light
can be captured by antenna pigments (green) and transferred among
themselves until reaching the reaction centers -------→
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