This document provides an overview of photosynthesis, including:
- The two pathways of photosynthesis (light reactions and Calvin cycle) that convert solar energy to chemical energy.
- The structure and function of chloroplasts, where the light reactions take place in the thylakoid membranes and the Calvin cycle occurs in the stroma.
- The light-dependent reactions that use photon energy to produce ATP and NADPH via photosystems and electron transport chains, and the light-independent Calvin cycle that fixes carbon into glucose using ATP and NADPH.
2. Objectives
• Understand the difference between autotroph and
heterotroph
• Describe the location and structure of a chloroplast.
Explain how chloroplast structure is related to its function
• Recognize and explain the summary equation for
photosynthesis
• Understand the role of REDOX reactions in
photosynthesis
• Understand the properties of light discussed in class
• Describe the relationship between action and absorption
spectrum
• Explain what happens when chlorophyll or accessory
pigments absorb photons
3. Objectives continued
• List the function and components of a
photosystem
• Compare cyclic and noncyclic electron flow and
explain the relationship between these components
of the light reactions
• Summarize the light reactions of photosynthesis
• Summarize the carbon fixing reactions of the
Calvin cycle
• Describe the role of NADPH and ATP in the
Calvin cycle
• Understand why variations of photosynthesis
evolved
4. Overview of Photosynthesis
• Process by which
chloroplast bearing
organisms transform
solar light energy into
chemical bond energy
• 2 metabolic pathways
involved
• Light reactions:
convert solar energy
into cellular energy
• Calvin Cycle: reduce
CO2 to CH2O
•Organisms that can perform
photosynthesis are called
autotrophs whereas those that
cannot are called heterotrophs
5. Photosynthesis Equation
• Reduction of carbon dioxide
into carbohydrate via the
oxidation of energy carriers
(ATP, NADPH)
• Light reactions energize the
carriers
• Dark reactions (Calvin Cycle)
produce PGAL
(phosphoglyceraldehyde)
Photosynthesis
6CO2 +6H20 + light → C6H1206 + 6O2
7. Structure of the Chloroplast
• Thylakoid: membranous system
within the chloroplast (site of light
reactions). Segregates the chloroplast
into thylakoid space and stroma.
• Grana stacks of thylakoids in a
chloroplast
• Stroma: region of fluid between the
thylakoids and inner membrane where
Calvin Cycle occurs
8. Light
• Electromagnetic energy travelling in waves
• Wavelength (λ): distance from peak of one wave to
the peak of a second wave
• inverse relationship between wavelength and energy
↑ λ ↓energy
9. Visible Spectrum
• The portion of the electromagnetic spectrum that our eyes
can see
• White light contains all λ of the visible spectrum
• Colors are the reflection of specific λ within the visible
spectrum
∀ λ not reflected are absorbed
• Composition of pigments affects their absorption
spectrum
10. Absorption vs. Action
• Absorption spectrum
is the range of
wavelengths that can
be absorbed by a
pigment
• Action spectrum
means the
wavelengths of light
that trigger
photosynthesis
11. Why are plants green?
• Pigments contained
within the chloroplast
absorb most λ of light
but absorb the green λ
the least
• Pigments include
– Chlorophyll a
– Chlorophyll b
– Carotenoids
• Carotenes
• Xanthophylls
12. Chlorophyll a
• Is only pigment that directly
participates in the light
reactions
• Other pigments add energy to
chlorophyll a or dissipate
excessive light energy
• Absorption of light elevates an
electron to a higher energy
orbital (increased potential
energy)
13. Photosystems
• Collection of pigments
and proteins found
associated with the
thylakoid membrane that
harness the energy of an
excited electron to do
work
• Captured energy is
transferred between
photosystem molecules
until it reaches the
chlorophyll molecule at
the reaction center
14. What Next?
• At the reaction center are
2 molecules
– Chlorophyll a
– Primary electron acceptor
• The reaction-center
chlorophyll is oxidized
as the excited electron is
removed through the
reduction of the primary
electron acceptor
• Photosystem I and II
15. Electron Flow
• Two routes for the path of electrons stored in the primary
electron acceptors
• Both pathways
– begin with the capturing of photon energy
– utilize an electron transport chain with cytochromes for chemiosmosis
• Noncyclic electron flow
– uses both photosystem II and I
– electrons from photosystem II are removed and replaced by electrons
donated from water
– synthesizes ATP and NADPH
– electron donation converts water into ½ O2 and 2H+
• Cyclic electron flow
– Uses photosystem I only
– electrons from photosystem I are recycled
– synthesizes ATP only
16. Noncyclic Electron Flow
1 Electrons at reaction-
center are energized
2 H2O split via enzyme
catalysed reaction forming
2H+
, 2e-
, and 1/2 O2.
Electrons move to fill
orbital vacated by
removed electrons
3,4 Each excited electron is
passed along an electron
transport chain fueling the
chemiosmotic synthesis of
ATP
17. 5 The electrons are now
lower in energy and
enters photosystem I via
plastocyanin (PC) where
they are re-energized
6 The electrons are then
passed to a different
electron transport system
that includes the iron
containing protein
ferridoxin. The enzyme
NADP+
reductase assists
in the oxidation of
ferridoxin and
subsequent reduction of
NADP+
to NADPH
Noncyclic Electron Flow
19. Cyclic Electron Flow
• Electrons in Photosystem I is excited and
transferred to ferredoxin that shuttles the electron to
the cytochrome complex.
• The electron then travels down the electron chain
and re-enters photosystem I
20. Where are the photosystems found
on the thylakoid membrane?
21. Chemiosmosis in 2 Organelles
• Both the Mitochondria and Chloroplast
generate ATP via a proton motive force
resulting from an electrochemical
inbalance across a membrane
• Both utilize an electron transport chain
primarily composed of cytochromes to
pump H+
across a membrane.
• Both use a similar ATP synthase
complex
• Source of “fuel” for the process differs
• Location of the H+
“reservoir” differs
22. Calvin Cycle
• Starts with CO2 and
produces
Glyceraldehyde 3-
phosphate
• Three turns of Calvin
cycle generates one
molecule of product
• Three phases to the
process
– Carbon Fixation
– Reduction of CO2
– Regeneration of RuBP
23. 1 A molecule of CO2 is
converted from its
inorganic form to an
organic molecule
(fixation) through
the attachment to a
5C sugar (ribulose
bisphosphate or
RuBP).
– Catalysed by the
enzyme RuBP
carboxylase
(Rubisco).
• The formed 6C sugar
immediately cleaves
into 3-
phosphoglycerate
24. 2 Each 3-
phosphoglycerate
molecule receives an
additional phosphate
group forming 1,3-
Bisphosphoglycerate
(ATP phosphorylation)
• NADPH is oxidized
and the electrons
transferred to 1,3-
Bisphosphoglycerate
cleaving the
molecule as it is
reduced forming
Glyceraldehyde 3-
phosphate
25. 3 The final phase
of the cycle is to
regenerate RuBP
• Glyceraldehyde
3-phosphate is
converted to
RuBP through a
series of
reactions that
involve the
phosphorylation
of the molecule
by ATP
26. Variations Anyone?
• In hot/arid regions plants may run
short of CO2 as a result of water
conservation mechanisms
• C4 Photosynthesis
CO2 may be captured by
conversion of PEP
(Phosphoenolpyruvate) into
oxaloacetate and ultimately malate
that is exported to cells where the
Calvin cycle is active
• CAM Photosynthesis
CO2 may be captured as inorganic
acids that my liberate CO2 during
times of reduced availability
