Photosynthesis:
The Light Reactions &
The Calvin Cycle
Photosynthesis: Overview
4.3 The Light Reactions
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General Summary Info
Conversion of visible light into chemical
energy needed to produce sugars in the Calvin
cycle
Chlorophyll in the thylakoid absorb the light,
water is split into Hydrogen and Oxygen, &
light energy is converted into chemical energy.
4.3 The Light Reactions
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Light absorbing pigments form 1 of 2 kinds of
clusters: Photosystem (PS) I and II.
The PS absorbs light energy & this energy gets
transferred to different molecules until it ends
up at a reaction center
Reaction center – a specific chlorophyll a
molecule
The reaction center accumulates so much
energy that some of its e- jump to electron
carriers
4.3 The Light Reactions
4.3 The Light Reactions
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These electron carriers form an electron
transport system between the 2 photosystems.
Electrons from PS II replace electrons lost
from PS I.
PS II receives replacements from an enzyme
near its reaction center that splits water into
protons, electrons, & oxygen.
2 H2O  4 H+ + 4 e- + O2
4.3 The Light Reactions
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When the enzyme oxidizes the water, oxygen
is released as a gas & protons build up in the
thylakoid.
The e- replace the lost e- in PS II.
4.3 The Light Reactions
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When e- from H2O reach PS I, they receive an
energy boost from the reaction center & this
energy is used to reduce NADP+ into NADPH.
Both the protons and electrons from water are
used to convert NADP+
The protons & electrons of NADPH is then
used later to reduce CO2 in the Calvin Cycle.
4.3 The Light Reactions
4.3 The Light Reactions
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As electrons are flowing, some of the solar
energy powers the active transport of H+
across the thylakoid membrane.
So a large number of H+ build up inside the
thylakoid causing a difference in charge,
which creates a difference in potential energy
(think of a battery).
Just like a battery, this potential energy can do
work.
4.3 The Light Reactions
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The protons diffuse out of the thylakoid
through an enzyme complex (ATP synthetase)
& as they pass through, they transfer energy to
the ATP synthetase.
ATP synthetase uses the energy to synthesize
ATP from ADP & a phosphate.
4.3 The Light Reactions
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A Summary
Energy from light forces electrons to flow
from water to NADP+.
The electrons retain the energy in NADPH,
which is then used to synthesize ATP.
So the light reactions, convert light energy into
the chemical energy found in ATP and
NADPH, with the overall products being O2,
ATP, & NADPH.
Light Reactions Animation
http://www.stolaf.edu/people/giannini/flashanim
at/metabolism/photosynthesis.swf
4.4 The Calvin Cycle
General Summary Intro
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Saves the chemical energy produced in the
light reactions in the form of sugars
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The Calvin Cycle occurs in the stroma of the
chloroplasts.
4.4 The Calvin Cycle
1. CO2 combines with a 5 carbon phosphatesugar, called ribulose biphosphate (RuBP).
This is called carbon fixation because carbon
dioxide gas is “fixed” into an organic molcule.
This produces an unstable 6 carbon molecule,
which instantly breaks down into two 3 carbon
molecules called phosphoglyceric acid (PGA).
Catalyzed by Rubisco
4.4 The Calvin Cycle
2. Each molecule of PGA is reduced to the 3
carbon sugar-phosphate molecule,
phoshpoglyceraldehyde (PGAL).
This requires 1 ATP and 1 NADPH (from the
light reactions).
4.4 The Calvin Cycle
3. A series of enzymes catalyzes the
combination and rearrangement of the PGAL,
producing a 5 carbon sugar-phosphate,
ribulose 5-phosphate.
4.4 The Calvin Cycle
4. An ATP molecule is used to add a 2nd
phosphate group to the Ribulose 5-phosphate,
producing a molecule of the starting product,
RuBP (completing the cycle).
4.4 The Calvin Cycle
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Three turns of the cycle (which uses 3 CO2
molecules) results in 6 PGAL molecules.
5 of those PGAL are used to regenerate RuBP
& the last one is available to the organism to
use for maintenance & growth.
The PGAL is removed from the Calvin Cycle
to synthesize other compounds like complex
carbohydrates or amino acids.
Animation of The Calvin Cycle
http://www.sinauer.com/cooper/4e/animations03
05.html