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PHOTOSYNTHESIS
PHOTOSYNTHESIS
Photosynthesis is ability of a plant to turn
sunlight, air, and water into sugar
(energy).
 The overall reaction is:

6CO2 +
light
chlorophyll
6H2O -------> C6H12O6
+ 6O2
Photosynthesis is vital to life
for two reasons
–1. The oxygen in the air comes from
photosynthesis. The plants continue to
replenish the oxygen in the air.
–2. All of our food comes directly or
indirectly from photosynthesis.
Parts of a leaf
Stomata
palisade layer
spongy layer
veins
chloroplast
thylakoids
grana
stroma
The Reactions of Photosynthesis
 Remember
it all occurs in the
chloroplast.
– Thylakoids – sac-like photosynthetic
membranes inside chloroplasts, arranged
in stacks called grana
• Photosystems – clusters of pigments in
thylakoid
– Stroma – region outside of the thylakoid
membrane (Calvin Cycle takes place)
Light
 Photosynthesis
uses visible light.
– Only visible light with intermediate
wavelengths has enough energy to cause
chemical change without destroying
biological molecules.
 What
are the colors of the visible
spectrum?
 Why does a leaf look green?
Pigments
 Light
is captured by pigments.
 The main photosynthetic pigment is
chlorophyll.
 There is chlorophyll a, b and c.
 Chlorophyll a is the major
photosynthetic pigment and is found in
all photosynthetic plants, protist, and
cyanobacteria.
Carotenoids
 Accessory
pigments found in all green
plants.
 They absorb blue and green
wavelengths and give a plant a yellow
or orange color.
 In the fall when chlorophyll breaks
down, it is the accessory pigments
which give colors of fall.
Accessory Pigments
 The
red color of some autumn leaves
is due to the anthocyanin pigments.
These are not photosynthetic.
Stages of Photosynthesis
LIGHT REACTIONS
DARK REACTIONS
Light absorption
Carbon Fixation
Electron transport
Oxygen production
Chemiosmotic ATP synthesis
Putting it all together
light
CO2 + H2O + energy  C6H12O6 + O2
H2O
CO2
sunlight
ADP
NADP
Energy
Building
Reactions
Sugar
Building
Reactions
NADPH
ATP
O2
sugars
Plants make both:
 energy
 ATP & NADPH
 sugars
The Energy-Capturing Reactions
 Light
absorption
– Chlorophyll a and b are antennae pigments
 Electron
transport
– Electrons lost by chlorophyll a end up at
NADP
– Produce oxygen
 Chemiosmotic
ATP synthesis
– Powered by H ion reservoir
The Dark Reactions
 The
dark reactions take place outside
the thylakoid membrane.
 Carbon fixation - The carbon dioxide is
incorporated into an organic molecule
(1C + 5C = 6C). This 6 carbon structure
is unstable and immediately splits into
2, 3 carbon molecules. These molecules
go through a series of reactions known
as the CALVIN CYCLE.
What controls photosynthesis?
Intensity
of light
Scarcity of water
Ecological Aspects

C4 PLANTS - Use more ATP but at high
temperatures they can photosynthesize
faster -allowing plants to grow and
reproduce faster. These include many
weeds and important crops such as corn.

A special variation of C4 plants are the
CAM plants. These are mostly desert
plants. These plants keep the stomata
closed during the day and open at
night. They must conserve water!
even though
this equation
is a bit of a lie…
it makes a
better story
sun
Energy
cycle
Photosynthesis
light
CO2 + H2O + energy C6H12O6 + O2
plants
CO2
H2O
animals, plants
glucose O2
ATP
C6H12O6 + O2  energy+ CO2 + H2O
Cellular Respiration
The Great Circle
of Life,Mufasa!
ATP