Photosynthesis (PS)
Chapter 10
“Self” Feeders
Autotrophs (producers)
– Self-produce organic materials such as glucose using
inorganic compounds like CO2 , H2O
– Ultimate source of organic compounds for
heterotrophs
– Classes
• Photoautotrophs
• chemoautotrophs
Autotroph Classification
Photoautotrophs
– use light as the energy
source
(photosynthesis)
– in plants, algae, some
protists, and some
prokaryotes
Chemoautotrophs
- energy source from
oxidizing inorganic
substances (sulfur and
ammonia)
– unique to bacteria
“Other” Feeders
Heterotrophs (Consumers)
– Uses organic compounds created by others
• 1° Consumers (vegetarians)
• 2° Consumers (carnivores and omnivores)
• Decomposers (detritivores)
Chloroplast Structure
Where within this organelle is
the chlorophyll pigment located?
Chlorophyll pigment is embedded in the
thylakoid membrane
Chloroplasts = Sites for PS
• Found mainly in mesophyll cells
(interior leaf tissue)
• O2 exits and CO2 enters the leaf
through stomata underneath the
leaf.
• Veins deliver water
from the roots and
carry off sugar from
mesophyll cells to
other plant areas.
PS Equation
Net equation of photosynthesis:
6CO2 + 6H2O + light energy -> C6H12O6 + 6O2
In reality, photosynthesis adds one CO2 at
a time:
CO2 + H2O + light energy -> CH2O + O2
(CH2O)n = generic formula for a sugar
2 Stages of PS
• Light Reactions
• Calvin Cycle
– In Thylakoid
Membrane
– In Stroma
• Requires Light
– chlorophyll
pigment absorbs
light energy
• AKA Light
Independent or Dark
Reactions
• ** Be careful** occurs
only during the day
because NADPH and
ATP made in the light
reactions drive the
Calvin Cycle
Absorption Spectra
• In the thylakoid, several pigments differ in
their absorption spectrum.
• Chlorophyll a (dominant pigment) absorbs
best in the red and blue wavelengths, and
least in the green.
• Other pigments
(chll b, carotenoids)
have different
absorption spectra and
can transfer energy to chll a
Actions Spectrum
Collectively, these pigments determine an
overall action spectrum for photosynthesis.
Photosystems
• Photosystem = light
gathering complex
• A few hundred chll a, chll
b, and carotenoids
combined =
photosystem
• Only chll a participates
directly in the light
reactions
Thylakoid Membrane Populated
by Two Photosystems
• P700 (Photosystem I) has a reaction
center with an absorption peak at 700nm.
• P 680 (Photosystem II) has a reaction
center with a peak at 680nm.
Photosystem Function
1) Sun E (photons) hit
Photosystems
2) 2 e- excited from chll a
(stores PE)
3) Energy transferred from
one pigment to another
until E tranferred to chll a
4) Position of chll a is near the
“reaction center” where 1st
light driven chemical rxn of
PS occurs
Light Reactions
• Non-cyclic Electron Flow
• Cyclic Electron Flow
Steps to Non-cyclic Electron Flow
1. P680 absorbs light
2. Light excites 2e- and
passes e- to primary
electron acceptor
3. Water is split into
½ O2  oxygen gas
H+  to thylakoid
space
2e-  resupplies
P680
Steps to Non-cyclic Electron Flow
4. e- pass through
ETC, where they
“fall” in E
5. E from e- is
captured to
produce ~ 1.5
ATP’s through
chemiosmotic
phosphorylation
Steps to Non-cyclic Electron Flow
6. e- from P680
replenish e- in
P700
7. e- in P700 are
excited again by
light and passed
to a primary
electron acceptor
Steps to Non-cyclic Electron Flow
8. e- cascades
down short
ETC, where ecombine with
and reduce
NADP+ and H+
to form
NADPH (E rich
molecule)
Non-cyclic Electron Flow
Non-Cyclic Electron Flow
ATP and
NADPH
created in
the Light
Reaction
s are
used in
the
Calvin
Cycle
Cyclic Electron Flow
• Calvin Cycle – uses up more ATP than
NADPH so cyclic electron flow helps to
generate more ATP’s
• Short circuit – e- fall back from P700
primary electron acceptor to the 1st ETC to
generate more ATP’s via chemiosmotic
phosphorylation
Cyclic Electron Flow
Calvin Cycle (in stroma)
•
•
•
ATP drives the cycle
NADPH = reducing power for adding
high energy e- to make sugar
3 Basic parts
1. CO2 fixation = Carboxylation
2. Reduction
3. Regeneration of CO2 acceptor (RuBP) –
Ribulose bi-phosphate
CO2 Fixation
Rubisco catalyzes
the fixation of CO2 to
a C5 compound,
RuBP (Ribulose
biphosphate)
Initial C6 compound=
unstable
Splits into PGA (3phosphoglycerate)
Reduction
Hydrolysis of 6 ATP
NADPH is oxidized to
NADP+
PGAL sugar created =
G3P (glyceraldehyde-3phosphate)
1/6 PGAL made into
glucose and other
organic compounds
Regeneration of RuBP
Hydrolysis of 3
ATP’s to
regenerate
RuBP
Evolution of Photorespiration
• C3 plants = plants that make a C3 compound
like PGA as the 1st product in the Calvin
Cycle
• In arid/dry climates, plants close stomata to
prevent dehydration limits CO2 intake
• Result  Rubisco accepts O2 in place of CO2
• C5 intermediate results which splits into a C3
and C2 compound. C2 compound is exported
as waste to peroxisomes and mitochondria to
regenerate CO2 = photorespiration
Photorespiration – Detriment or
not?
• C2 waste and C3 compound created
means no C5 RuBP regeneration for
Calvin Cycle. This means reduced PS
rate!!
• no sugar made
• C4 Photosynthesis and CAM
Photosynthesis evolved to minimize
photorespiration
C4 Photosynthesis
Leaf Anatomy Differs b/t C3 and C4 plants
• C4 plants = plants that have alternate CO2
•Bundle Sheath Cells @ veins of leaf
fixation route that makes a C4 compound
•Mesophyllst Cells @ outside of bundle
as the 1 compound (corn, grasses, sugar
sheaths loosely arranged
cane)
C3
C4 Photosynthesis
• CO2 fixed by PEP carboxylase
(not Rubisco) in mesophyll
cells
• CO2 and combines with C3
PEP (phosphoenol pyruvate)
to form OAA (oxaloacetate)
• OAA converts to C4 malate
• Malate enters bundle sheath
cells via plasmodesmata
• CO2 released from malate and
captured by Rubisco to enter
Calvin Cycle
CAM Photosynthesis
• CAM = Crassulacean Acid Metabolism
• In succulent plants like cactus
• Succulents open stomata at night and
close them during the day, thus needing a
way to fix CO2 at night
CAM Photosynthesis
• At night, CO2 fixed by
PEP carboxylase (not
Rubisco)
• Same chemical
pathway as C4 PS to
create malate except…
• Malic Acid (malate)
temporarily stored in
Vacuoles at night
• During the day, CO2 is
released from the C4
malate and fixed by
Rubisco to be used in
the Calvin Cycle