PHOTOSYNTHESIS
Class Notes
PHOTOSYNTHESIS
Photosynthesis is ability of a plant to turn
sunlight, air, and water into sugar
(energy).
 The overall reaction is:

light
chlorophyll
H2O -------> CH2O
CO2 +
+ O2
 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 Chloroplast
-double membrane, stroma,
thylakoid, grana
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?

.
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
Carotenoids are 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. 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
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 use more ATP
then NADPH.
 Each turn of the cycle uses 9 ATP
and only 6 NADPH.
 How are more ATP acquired than
NAPH since one ATP is produced in
photosystem II and one NADPH is
produced in photosystem I?
 Cyclic and noncyclic electron flow
Noncyclic Electron Flow
Cyclic Electron Flow
The Dark Reactions

The dark reactions take place outside
the thylakoid membrane in the stroma.
The Calvin Cycle converts CO2 to
sugar in a three phase process.
1. Carbon fixation - The carbon dioxide is
incorporated into an organic molecule (1C
+ 5C = 6C) called RuBP by the enzyme
Rubisco. This 6 carbon structure is
unstable and immediately splits into 2, 3
carbon molecules. This is the same
molecule that glucose is split into.
2.
3.
Reduction – Each 3 carbon molecule is
phosphorylated by ATP and NAPH to
create G3P. The cycle must turn 3 times for
a net gain of one molecule of G3P.
Regeneration of CO2 acceptor – The
rearrangement of 5 molecules of G3P into
3 molecules of RuBP required 3 more ATP.
( 9 ATP and 6 NADPH yields 1 G3P)
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!
Vocabulary
 Glossary
– blade
– stipules
– petiole
– vein
– midrib
of terms
Vocabulary
carotenoids
chlorophyll
electromagnetic
spectrum
granum
NADP+
NADPH
photon
photosystem
photosystem I
photosystem II
pigment
stroma
thylakoid