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Chapter 7: Photosynthesis
I. Photosynthesis consists of a
complex set of reactions
A. Overall balanced
equation:
1.
chlorophyll
6CO2 + 6H20 + light  C6H12O6 + 6O2
enzymes
2. H2O is oxidized and CO2 is
reduced via H atom
transfer
3. endergonic – requires a net
input of energy from
sunlight (products are at a
higher energy than
reactants)
B. Significance
1. all food directly or
indirectly comes from
plants (primary producers
– first link in food chains)
2. autotrophs (self feeding –
plants make their own
food) vs heterotrophs
(other feeding – animals
and fungi)
3. In the process of
photosynthesis, plants also
manufacture O2
4. Other photosynthetic
organisms include bacteria:
blue-green bacteria,
protists (seaweed and
phytoplankton)
C. Light energy
1. part of the electromagnetic
spectrum – travels in waves
2. visible light (ROYGBIV or
VIBGYOR)
3. 400-700nm = wavelength of
visible light
4. plants (primary producers)
differentially absorb
various wavelengths of light
with different “pigments”
D. Plant pigments – capture
sun light and convert it into
chemical energy to “power”
photosynthesis
1. each pigment has a unique
chemical structure sensitive
to specific wavelengths of
light
2. main light trapping
pigment = chlorophyll a
(also chlorophyll b and c –
in various primary
producers)
3. other accessory pigments
include: carotenoids,
phycobilins, phycocyanins
which capture different
wavelengths of light than
chlorophyll a,b,and c and
“protect” chlorophyll from
bright light
E. Structure of chloroplasts,
leaf, and chlorophyll
1. chloroplasts = stacks of
internal membranes
(thylakoids) and spaces
inbetween them (stroma)
2. chlorophyll molecules
(chlorophyll a) are organized
into two photosystems (I –
P700 and II – P680) within
the thylakoid membranes
3. Each chlorophyll molecule
has an “antennae” porphyrin
ring at one end with Mg
atom in the center. This
“antennae” region is
sensitive to light.
4. Leaf = main photosynthetic
organ of the plant
a. stomata = openings
through which gases
diffuse (CO2, O2, H2O)
b. veins (xylem and phloem)
conduct sugars (phloem)
from the leaves and
minerals (xylem) from the
roots
c. palisade layer and spongy
layer
d. “transpirational pull” –
when stomata are open,
water vapor molecules
leave the stomata creating
a “negative pressure”
which pulls up water and
minerals through the
xylem from the roots.
Capillary action (cohesion
and adhesion) assists in
water motion up from the
roots (root pressure –
positive pushing force).
e. Most leaves are covered
with a waxy layer to
prevent water loss.
II. Chemical Reactions of
Photosynthesis (light vs dark
reactions)
A. Photochemical reactions
(light) include: light
absorption (#1-2), electron
transport (#3-5) and
chemiosmotic ATP synthesis
(#6-7)
1. light is absorbed by
chlorophyll a molecules in
PSII and PSI. Electron is
boosted to a higher energy
level in the reaction center.
2. two water molecules are
“split” releasing 4H+ ions,
O2, and 4 electrons which
replace the electrons lost by
PSII.
3. electrons lost by PSII and
PSI are passed on to protein
carriers that are part of the
electron transport system in
the thylakoid membrane
4. electrons fromPSII P680
reaction center are passed on
to proteins in the electron
transport system embedded
in the thylakoid membrane
(plastoquinone, cytochromes
b-f, and plastocyanin),
eventually reaching PSI
P700.
5. electrons leaving PSI P700
are picked up by ferredoxin
(another electron transport
protein) and passed on to
NADP reductase complex
where NADP+ combines
with H+ to form NADPH –
the final electron pair
acceptor
6. H+ ions are built up inside
the thylakoid membrane
through decomposition of
water and active transport
by the electron transport
system (thylakoid membrane
space inside serves as a
hydrogen ion reservoir)
7. H+ ions are used to make
ATP through chemiosmosis.
a. H+ concentration builds
to 1000x inside the
thylakoid
b. Chemical gates in the
ATP synthase complex
open when this
concentration is reached,
allowing H+ ions to rush
through it
c. Energy transition is used
to bond P to ADP to make
ATP
8. non-cyclic (generates ATP
and NADPH) vs cyclic
(generates ATP only – found
in photosynthetic bacteria)
electron transport
B. Carbon fixation or Calvin
cycle (C3 cycle) – stroma
(dark reactions or light
independent reactions)
1.carbon dioxide is attached to
a 5 carbon compound
(ribulose biphosphate)
2.RuBP is immediately broken
down into 2PGA
(phosphoglycerate)
3.a series of intermediate
molecules are then formed,
ultimately resulting in the
formation of hexose (glucose)
and more complex sugars
4.RuBP is regenerated
5.NADPH and ATP from the
light reactions are used to
change PGA into PGAL
(phosphoglyceraldehyde)
6.3CO2 molecules enter the
cycle to produce one PGAL
that leaves the cycle while
regenerating RuBP
7.2PGAL form glucose,
fructose or can be combined
to form sucrose, starch, and
cellulose
8.PGAL can also be used to
synthesize amino acids and
fatty acids – precursors of
other biomolecules
III. Factors affecting the rate of
photosynthesis
A. Light
1. intensity or brightness – as
intensity increases, the rate
increases up to a point
(limit)
2. wavelength – each pigment
absorbs specific
wavelengths of light
(chlorophyll a – blue and
violet as well as red – 2
peaks)
3. longer periods of light
increase photosynthesis
(summer vs winter) =
photoperiods
B. Temperature
1. warmer temperatures
increase the rate of
photosynthesis up to a
point
C. Concentrations of
reactants and products
1. Increasing the concentration
of reactants (CO2 and H2O)
increases photosynthesis up
to a point
2. Removing the products
(C6H12O6 and O2)
increases photosynthesis up
to a point
IV. Variations in Photosynthesis
A. C4 plants
1. have extra metabolic steps
preceding the C3 pathway
2. have very high rates of
photosynthesis
3. examples: crab grass,
sugar cane, corn
4. a high concentration of
CO2 is built up in
mesophyll cells
surrounding bundle sheath
cells and then changed into
intermediate compounds
(phosphophenolpyruvate,
oxaloacetate, and malate =
C4) before entering the C3
cycle
5. net effect is to increase the
rate of photosynthesis (C3
cycle) while decreasing
photorespiration – thus
leading to faster growth
and reproduction in these
plants
B. CAM (Crassulacean Acid
Metabolism)
1. succulents and cacti
2. stomata must often be closed
during very hot days to
prevent water loss, however,
this prevents entrance of
CO2
3. during cooler nights, stomata
open, carbon dioxide diffuses
in, and is fixed and stored in
organic acids – crassulacean
acid
4. during the day, carbon
dioxide is removed from the
organic acids and enters the
C3 cycle
C. Sun vs Shade Plants
(variations in the C3
pathway)
1. Sun plants – prefer bright
light and achieve fast rates of
photosynthesis (soybeans,
cotton, tomatoes)
2. Shade plants – prefer dim or
“filtered sunlight” to achieve
their maximum rates of
photosynthesis (ferns,
orchids, African violets)
V. Photorespiration = oxidizes
organic compounds using
oxygen and ATP while
releasing CO2
A. Conditions
1. occurs in the presence of
light
2. does not involve electron
transport
3. does not produce ATP
4. occurs because CO2 and
O2 can both bind to RuBP
carboxylase
5. if O2 binds with RuBP
carboxylase along with
RuBP  one molecule of
PGA and a 2 carbon
compound (which later
releases CO2) are produced
and leave the chloroplast
instead of two PGA
(normal C3 cycle)
B. Significance
1. half of the carbon dioxide
fixed in photosynthesis may
be “lost” and not converted
into glucose
2. may have a benefit in
removing high
concentrations of O2 which
can be destructive to living
tissue