PHOTOSYNTHESIS
CH 10
• Autotrophs are the worlds producers.
• Photoautotrophs produce organic molecules
using solar energy.
• Chemoautotrophs produce organic molecules
using energy from inorganic hydrogen sulfide
or methane
• Plants, algae, cyanobacteria, phytoplankton
are photoautotrophs
• Certain bacteria are chemiautotrophs
I. Overview of Photosynthesis
•
•
•
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Synthesis of glucose using sun’s NRG
Is anabolic (endergonic)
Occurs in photoautotrophs
Occurs in 2 stages:
– The light dependent reaction (LDR) uses the energy
from the sun to make ATP and NADPH
– The Calvin cycle uses the ATP and NADPH from the
LDR to make a precursor to glucose
• The chemical reaction is:
•
• 6CO2 + 6H2O + light → C6H12O6 + 6O2
II. The chloroplast: site of
photosynthesis in eukaryotic cells
• structurally similar to photosynthetic
prokaryotes and most likely evolved from
them
• The light reactions take place in the
membranes of the thylakoids
• The Calvin cycle takes place in the stroma
III. Photosynthesis and Aerobic
Respiration: a series of redox
reactions
• Transfer of electrons from reactant to
product
• in photosynthesis, water becomes oxidized
and CO2 becomes reduced
• in photosynthesis, water is split using its
electrons to reduce CO2
IV. The Two Stages of Photosynthesis:
a preview
• photosynthesis consists of 2 stages:
– Light reactions (LDR)
• Split water
• Generate O2
• Reduce NADP+ to NADPH
• Make ATP from ADP
– Calvin Cycle
• Makes sugar from CO2
• Uses ATP and NADPH
III. The Light Reactions (LDR): convert
solar energy to the chemical energy
of ATP and NADPH
• A. Light
• composed of
different
wavelengths
• also behaves like it
has particles
called photons
B. Photosynthetic Pigments
• pigments absorb wavelengths of visible light
• wavelengths not absorbed are reflected or
transmitted
• why do plants appear green?
• what wavelengths are used for
photosynthesis? What pigments do these
correspond to?
• Chlorophylls absorb the photons of energy
from RO IV
• Chlorophylls found in plants, algae, and
cyanobacteria
• Cyanobacteria are ancestor of chloroplasts
• Other photosynthesizing bacteria have
bacteriorhodopsin which absorbs green
wavelengths
C. The components of the Light
Reactions
• 1. Photosystem I and II They have:
• Light absorbing
complex: contains
chlorophyll and
absorbs photons
• Reaction center:
uses energy from
photons to boost
electrons.
• 2. Electron Transport Chain (ETC)
– series of proteins that transfer electrons by redox
reactions
• 3. Proton pump
– uses the energy in the electron to pump protons (H+)
across the membrane.
• 4. NADP reductase
– uses the energy from the boosted electron to make
NADPH
•
• 5. ATP synthase
– uses energy from the proton gradient to make ATP
•
D. How the light dependent reaction
works
• Linear electron flow:
• 1. photon strikes pigment of PSII and energy is transferred to
P680 of chlorophyll and electron is boosted. P680 becomes
oxidized
• 2. electron is transferred to electron acceptor
• 3. water is split and electron from water reduces oxidized
P680 back to its original state. H+ ions enter thylakoid and
oxygen is made
• 4. excited electron moves thru electron transport chain by
redox reactions pumping H+ ions up the gradient
• 5. proton diffuse thru ATP synthase making ATP
• 6. photon strikes pigments of PS1 and energy is transferred
to P700 and electron is boosted oxidizing P700. Electron
boosted from P680 is used to reduce P700
• 7. excited electron from P700 moves thru electron transport
chain by redox reactions
• 8. NADP reductase uses P700’s electrons to make NADPH
•
http://faculty.nl.edu/jste/noncyclic_photoph
osphorylation.htm
• Chemiosmosis:
http://highered.mcgrawhill.com/sites/0072437316/student_view0/
• Cyclic electron flow:
Difference between cyclic and linear?
IV. The Calvin Cycle uses chemical
energy of ATP and NADPH to reduce
CO2 to sugar
• It uses ATP and NADPH from the LDR as well
as CO2 from the atmosphere to make a
precursor to glucose (PGAL)
• It occurs in the stroma of the chloroplast
• RUBISCO is the enzyme that fixes CO2 to start
the Calvin cycle
• http://glencoe.mcgrawhill.com/sites/9834092339/student_view0/c
hapter39/calvin_cycle.html
V. Alternative Methods of Carbon
Fixation
• Most plants are C3 plants and use RUBISCO
to fix CO2 into a C3 compound
A. Photorespiration
• When Rubisco uses O2 instead of CO2
• Causes calvin cycle to shut
• occurs in C3 plants
http://www.uic.edu/classes/bios/bios100/lec
tures/c4.htm
C4 plants:
• CO2 is fixed by
another enzyme,
that can work with
low CO2 levels
• C4 compound gives
CO2 to RUBISCO
CAM plants
• It is primarily the
C4 pathway, but
plants keep their
stomata open at
night when the
atmospheric O2
levels decrease
V. How the LDR and Calvin cycle
interact
• How would adding an inhibitor to Calvin cycle
affect photosynthesis?
• How would adding an inhibitor to ATP
synthase affect photosynthesis?