Photosynthesis
- The process of converting sunlight energy to energy in chemical bonds. It
is how photosynthetic autotrophs manufacture food.
light + 6 H2O + 6 CO2  C6H12O6 + 6 O2
Chloroplasts - the site of photosynthesis in plants.
(diagram of chloroplast)
Vocab:
Non-cyclic photophosphorylation
ATP
Electron transport chain
Calvin Benson Cycle
NADH
H2O
PGAL
NADPH
O2
Light
Light Reaction
Dark Reaction
PGA
CO2
Carbohydrate
Photorespiration
C3
C4
CAM
Photophosphorylation - the process of making ATP from ADP and Pi
using energy from light. (diagram of ATP to ADP)
Types of Photosynthesis:
I. Light Reactions:
A. Noncyclic Photophosphorylation (Light Reactions) H2O + ADP + Pi + NADP+ + light  ATP + NADPH + O2 + H+
- Process which takes the energy in light and the electrons in H2O to
make energy rich molecules ATP and NADPH.
- Occurs within the thylakoids of the chloroplasts.
- Require light.
Process: 1. Pigments, such as chlorophylls and carotenoids, within the
thylakoid absorb different wavelengths of light and
incorporate that energy into their electrons. These
energized electrons immediately re-emit the energy to
nearby pigment molecules.
2. Photophosphorolation begins when one of two special
chlorophyll a molecules, P680 or P700, absorbs an energized
electron.(the numbers represent the wavelength at which
they absorb the most amount of light) Chlorophyll P 700 and
the surrounding pigments form a cluster called
photosystem I (PS I) and Chlorophyll P680 form a cluster
called photosystem II (PS II).
3. PS II is the first step. Absorbed energy excites 2 electrons.
These electrons are the passed to the primary electron
acceptor molecule. Thus leaving the PS II with a net +
charge.
4. Electrons from the primary electron acceptor pass through
the electron transport chain (transport proteins) to PS I. As
the electrons move through, they lose energy. This energy
is used to phosphorylate ADP to form ATP molecules. (2
electrons yield about 1.5 ATPs)
5. Once in PS I, the electrons are again energized by light and
passed to a primary electron acceptor (different from the
PS II one).
6. The electrons from the primary electron acceptor are
passed through a short electron transport chain and
combine with NADP+ and H+ to form an energy rich
NADPH molecule.
7. An H2O molecule undergoes photolysis (light
decomposition) and is split into 2 H+ and 1/2 O2. 2
electrons from the H2O replace the lost electrons from
PS II and one of the H+ provides the H for NADPH.
(diagram on Light Reaction)
B. Cyclic Photophosphorylation (Alternative Light Reactions) - Regulated by a build up of NADPH.
- Can occur simultaneously with cyclic phosphorolation.
- Uses PS I but not PS II .
- The electrons cycle back from the primary electron acceptor to PS I
and generate ATP but no NADPH or O2.
- The ATP is then used in the Calvin cycle.
II. Dark Reactions:
A. Calvin-Benson Cycle (C3 Cycle, Carbon Reduction Cycle) 6 CO2 + 18 ATP + 12 NADPH + H+  18 ADP + 18 Pi + 12 NADP+ + 2 C6H12O6
- Process which takes the energy rich molecules (ATP and NADPH)
made in the light reaction and uses it to “fix” CO2 into organic
molecules to produce C6H12O6 (glucose). 1 glucose = 6 times
through the calvin cycle.
- Occurs within the stroma of the chloroplasts.
- Does not require light but does require ATP and NADPH from
light reaction.
Process: 1. Carbon fixation: The enzyme rubisco combines
6CO2 with 6RuBP (ribulose biphosphate) to produce
12PGA (phosphoglycerate, 3 carbons).
2. Reduction: 12 ATP and 12 NADPH are used to convert 12
PGA to the energy rich PGAL or G3P
(phosphoglyceraldehyde or glyceraldehyde 3- phosphate).
ADP, Pi, and NADP+ are released to be re-energized in
noncyclic phosphorylation.
3. Regeneration: 6 ATP is used to convert 10 PGAL
(G3P) to 6 RuBP. This allows for the cycle to repeat.
4. Carbohydrate synthesis: The remaining 2 PGAL (G3P)
molecules are combined to form C6H12O6 (glucose). Other
monosaccharides like fructose and maltose can also be
formed. Monosaccharides can be combined to form
disaccherides like sucrose and polysaccharides like
starch and cellulose.
(diagram on Dark Reaction)
B. Alternative Methods of Carbon fixation (in arid climates):
1. Photorespiration - Rubisco can also fix O2, but it is not as
efficient as the CO2 fixation process and the product formed
when O2 combines with RuBP is not useful or high energy.
Peroxisomes must exert energy to break down the products of
photorespiration.
2. C4 - Some plants have a special “add-on” feature to C3
photosynthesis where PEP carboxylase combines the CO2 with
PEP (phosphoenolpyruvate) to form OAA (oxaloacetic Acid),
which has 4 carbon atoms. OAA is then converted to Malate
which is then shuttled to specialized bundle sheath cells and
converted to pyruvate and CO2. An ATP is broken down into an
AMP to turn pyruvate into PEP where it can begin the process
again. The CO2 is fixed by rubisco and the Calvin-Benson cycle
proceeds. Moving CO2 to the bundle sheath cells increases the
efficiency of photosynthesis since these cells are tightly packed
and very little O2 reaches these cells to compete for fixation by
rubisco. For photosynthesis to occur, the stomata of plants must
be open to allow CO2 to enter. This allows H2O to escape. C4
plants have a higher rate of photosynthesis which reduces the
time that the stoma are open and limits H2O loss.
3. CAM - (Crassulacean Acid Metabolism) Another “add-on “
feature to C3 photosynthesis which is similar to C4. PEP fixes
CO2 to OAA which is converted to Malic acid instead of Malate
(ionized Malic acid). The Malic acid is shuttled to the vacuoles
of the cells instead of the bundle sheath cells. Stomata are open
at night when PEP carboxylase is active and Malic acid
accumulates in the vacuoles. Unlike most plants, the stomata are
closed during the day, greatly reducing H2O loss, while Malic
acid is shuttled out of the vacuole and converted back to OAA,
requiring 1 ATP to ADP and releasing CO2. The CO2 is fixed by
rubisco and the Calvin-Benson cycle proceeds.