Photosynthesis
6H20 +
6CO2

C6H12O6 + 6O2
Photosynthesis Occurs in
Chloroplasts of Plant Cells
Plants Produce O2 Gas By Splitting Water
Fates of all the atoms in photosynthesis
Photosynthesis is a Redox Process
Process Overview
Light
H2O
CO2
NADP+
ADP + P
Lightdependent
reactions
O2
Calvin
cycle
Sugars
Visible Light Provides the Energy
that Drives the Light Reactions
• Which wavelengths/
color(s) are absorbed
best?
• Which wavelengths/
color(s) are reflected?
• What is the relationship
between reflection of
light and the perception
of color?
How do Photosystems Capture Solar Power?
• Plants contain the green
pigment chlorophyll, clumped
in photosystems, which takes in
sunlight and captures it’s
energy, in the chloroplasts.
– There are two main types of
chlorophyll, “chlorophyll a”
and “chlorophyll b”
• When chlorophyll absorbs light,
energy is transferred to
electrons in the chlorophyll
molecule, raising their energy
level.
• These high-energy electrons
make photosynthesis work.
Light-Dependent Reactions
Chemiosmosis
Light-Dependent Reactions
1. Light is absorbed by chlorophyll in PS II (P680)
2. An electron is excited and captured by the primary electron
acceptor
3. Water is split to supply electrons to PSII
4. The leftover oxygen atoms pair up to form a molecule of O2
5. The electrons released from PSII are passed to PS I by an
electron transport chain
6. This “fall” of electrons down the chain provides energy to pump
H+ ions across the membrane which will then power the
production of ATP via chemiosmosis. (photophosphorylation_
7. Meanwhile, light excites an electron of chlorophyll at PS I/ The
primary electron acceptor captures this and an electron from the
bottom of the ETC replaces the lost electron.
8. The excited electron of PS I is passed to NADP+, reducing it to
NADPH
The Calvin Cycle
Calvin Cycle
1. Carbon Fixation: The enzyme rubisco combines CO2 with a
5-C sugar called RuBP. The product is unstable and splits into
two molecules of a 3-C organic acid (3-PGA).
2. Reduction: Chemical reactions consume the energy from ATP
and oxidize NADPH to reduce the 3-PGA into an energy-rich
3-C sugar (G3P)
3. Release of one G3P molecule: For every 6 G3P formed, 5
must remain in the cycle. One G3P is available to be used by
the cell to make glucose and other organic compounds.
1. Regeneration of RuBP: A series of chemical reactions used
energy from ATP to rearrange the remaining five G3P
molecules and form three RuBP molecules so that another
turn of the cycle can start.
Water Conserving Adaptations
• C3 plants: (majority of plants)
– initially fix carbon dioxide when the enzyme rubisco adds CO2
to RuBP. (1st organic compound is 3-C acid)
– If stomata close to conserve water, O2 builds up
– Photorespiration occurs instead of photosynthesis
• C4 plants: (ex. Corn & sugarcane)
– Precede Calvin Cycle by first fixing carbon dioxide into a 4-C
compound
– Mesophyll cells have a high affinity for CO2 and will continue to
fix CO2 even at low concentrations to allow photosynthesis to
continue when stomata are closed
• CAM plants: (aloe, jade & cacti)
– Open stomata and intake CO2 only at night
– CO2 fixed into a 4-C compound and released to Calvin Cycle
during the day
Global Impact of Photosynthesis
•Carbon cycle
•Greenhouse effect
•Greenhouses gases in
atm. prevent some heat
from the sun from
escaping
•CO2, methane, H2O vapor
are the main GH gases
•Climate change
•Excessive levels of GH
gases result in more heat
being trapped
•This is not the same
thing as ozone depletion
The Ozone Layer
• The ozone layer protects organisms from harmful UV
radiation which is damaging to DNA (mutagen)
• Ozone layer was depeleted from CFC use
• Use of these chemicals is banned and the ozone layer is
repairing.