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Photosynthesis
2 Parts of Photosynthesis
Light Dependent Reaction
Calvin Cycle or Light Independent Rxn
(aka Dark Reaction)
Structure of a Chloroplast
The light
reactions take
place in the
thylakoid
membrane
The dark
reactions take
place in the
stroma
Light energy is packaged into photons.
Light is a form of Electromagnetic
energy move in waves.
Each level of wavelength emits various
amounts of energy.
Molecules of plants that absorb light are
called Pigments.
Chlorophyll is the major absorbing
pigment in green plants and
photosynthesis.
Electromagnetic Spectrum
Why are
plants
green?
Absorption Spectrum of green
plant.
Pigments
Pigments are light-absorbing
compounds.
Pigments appear colored because they
absorb light of certain wavelengths and
reflect that of others.
Chlorophyll a is the primary pigment in
green plants that absorbs red and
blue/violet light and reflects green light.
Stomata
GAS EXCHANGE
Stomata Anatomy
1. Stomata are microscopic openings found on the
bottom sides of the leaves.
2. The stomata is bounded by two half moon
shaped guard cells.
3. Found on the bottom surface of the leaf because
the guard cells are partially light activated, plants
under direct light would constantly have their
stomata open and would thus lose much water
and the plant would die.
4. Guard cells
Each guard cell contains water and ions.
Stomata Physiology
1. LIGHT : Guard cells Swell; stomata
open.
A. Carbon Dioxide diffuses into the cell to
be used for photosynthesis.
B. Water and Oxygen diffuse out of the
leaf.
2. DARK: Guard cells Shrink; stomata
close
No photosynthesis takes place.
Light Reaction
Light Reaction
Also called Light Dependent Reaction
Pigments that are in the chloroplasts
intercept light and begin the light reactions of
photosynthesis.
The light reactions occur in two
photosystems (located in the thylakoid
membrane): -photosystem I (PSI)
-photosystem II (PSII)
Photosystem: a unit of several hundred
chlorophyll a molecules and associated
acceptor molecules
Photosystems
Excited Electrons
NADPH
NADP+ is an electron acceptor
NADP+ + 2 e- + 2 H+
NADPH + H+
Steps of LIGHT Reaction
1. Light Energy is absorbed by the
grana.
2. Water is split into H+ , and O2 The
oxygen is then released.
3. The Electrons from water are
excited and passed down from
PhotosystemII to PhotosystemI. The
free Hydrogen ions are transported
across the thylakoid membrane and
are used to produce ATP and
NADPH.
Photosystem II
& Photosystem I
STEPS of LIGHT REACTION
4. The electrons arrive at Photosystem I and
are excited again by the sunlight .
5. They are then passed to NADP and form
NADPH.
6. The build up of H+ inside the Thylakoid
causes the H+ to move through ATP
Synthetase.
7. This energy forces a phosphate onto ADP
to generate ATP. This movement is due to a
difference in concentration.
ETC of Photosynthesis
to Calvin Cy
From Light reactions to Calvin cycle
 Calvin cycle

chloroplast stroma
 Need products of light reactions to
drive synthesis reactions
stroma
ATP
 NADPH

ATP
thylakoid
AP Biology
Dark Reaction
Dark Reaction
The dark reaction is also
known as the Calvin Cycle,
after an American scientist
(Melvin Calvin) who figured
out the pathway.
The dark reaction is also
known as the C3 Cycle
because the first stable
products of this pathway are
molecules that contain three
carbon atoms.
The Calvin Cycle occurs in
the stroma.
6 CO2
C
6 RuBP
C C C C C
6 ATP
C C C
~
12 PGA
e
6 ADP + P
12 ATP
~
e
12 ADP + P
C C C C C C
Glucose
C C C
12 PGAL
C C C
e-
Calvin Cycle
1. Takes place in the stroma
2. Carbon dioxide is fixed to RuBP by
the enzyme Rubisco.
3. One - 3 Carbon molecule known as G3P
is formed for each Carbon dioxide(3) that
gets fixed. It takes two turns of the cycle to
produce ONE 6 carbon molecule of sugar.
4. ATP and NADPH are necessary to run this
reaction and generate ADP, and NADP+ for
the light reactions.
Adenosine triphosphate
ATP Synthesis in the Cell
ENERGY
from food
1.Compounds that store energy are ATP, NADPH, FADH, &
NADH
2. The most important energy storing compound is ATP
(Adenosine triphosphate.)
3. ATP releases energy when a bond holding a phosphate
group is broken. This creates a new molecule called ADP
(adenosine di-phosphate)
4. What is the difference between ATP and ADP? # of
Phosphates
5. Cells generate 10 million new ATP molecules per second.
ATP – P = ADP
ADP + P = ATP
Adenosine
Monophosphate
Adenosine
Diphosphate
7.3 kcal/mole energy released
Adenosine
Triphosphate
T/F Questions
ATP has
more energy
than ADP.
T/F
There are three
phosphate atoms in
ATP?
T/F
You are storing
energy right now in
your body in the
phosphate bonds of
ATP.
T/F
ATP is made of a
sugar, the base
adenine and a
phosphate.