Download Photosynthesis Part 1

Photosynthesis Notes Part 1
Photosynthesis consists of both Light Reaction and the Calvin Cycle
Light reaction -- converts light energy to chemical energy (ATP and NADPH).
Calvin cycle -- Takes CO2 from the air and combines it with H from water to
make sugars.
Light Reaction
Light reactions can be divided into two systems: Photosystem I and Photosystem II.
Each system consists of chlorophyll and associated proteins. The systems differ by in the
types of associated proteins.
Photosystem I (PSI)
An electron in chlorophyll molecules is excited to a higher energy state when it absorbs a
photon. This high energy electron is then transferred to an electron acceptor via a redox
reaction. The final electron acceptor in this case is NADP+. When it accepts two
electrons it becomes NADPH (very similar to what we learned for NAD+ and NADH).
It takes two photons to excite two electrons to reduce to NADP+ to NADPH.
Notice that electrons are permanently lost from chlorophyll molecules and transferred to
NADPH.
Photosystem II (PSII)
Just like PSI, electrons in chlorophyll molecules are excited to a higher energy state when
it absorbs a photon. This high energy electron is passed between several electron carriers
in a similar manner as the electron transport system in mitochondrial membranes. The
electron is finally passed to the chlorophyll molecule that lost its electron in PSI. In the
process of electrons passing down the electron carriers on their way to the chlorophyll
molecules in PSI, the energy, released by these electrons dropping in free energy as they
move down the electron train, is used to pump H+ ions into the inner thylakoid space.
H+ ions build up in this space and then can only get out of the space through ATP
synthase (sound familiar). The energy from PSII photons is used to make ATP. The
energy from PSI is used to reduce NADPH.
Notice the electrons lost from the chlorophyll molecules in PSI are being replaced by
those electrons traveling down the electron chain from PSII. However, the electrons lost
from the chlorophyll molecules in PSI are not being replaced. This loss can not continue
very long, so plants have a mechanism to replace the electrons lost in PSII. Plants can
break water up into 2H2O  4H+ + 4e- + O2 . The electrons replace those lost in PSII,
the H+ (or their equivalent) become part of NADPH as NADP NADPH. The O2 is a
waste product.
Calvin Cycle
Takes CO2 from the air and combines it with H from water to
make sugars.
Calvin cycle is divided into three phases:
1. Carbon fixation phase
2. Energy input (book calls it reduction phase)
3. Regeneration phase
Carbon fixation phase
3 CO2 combines with 3 RuBP (5C)  3 short-lived (6C) molecule 6 PGA (3C)
Enzyme that combines CO2 with RuBP is Rubisco (arguably the most abundant
enzyme in the word.
Energy input phase
6 PGA (3C) + 6 ATP + 6NADPH 6 G3P + 6 ADP + 6 NADP+
The free energy of G3P is much higher than PGA
1 of the six G3P is exported from the chloroplast
Regeneration phase
5 out of the 6 G3P (15Cs in all) are used to make 3 RuPBs (5C) which replenish the 3
RuBPs used in the carbon fixation phase.
The regeneration phase costs 3 ATP.
Summary of Calvin Cycle
Each turn of the cycle uses 3 CO2 9ATP and 6 NADPH.
It takes 2 turns of the cycle to make 2 G3Ps which can be combined to make 1 glucose.
It costs a total of 18 ATP and 12 NADPH to make 1 glucose.