MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
C
Chhaapptteerr 77:: TThhee D
Daarrkk R
Reeaaccttiioonn ooff P
Phhoottoossyynntthheessiiss
- Part II The C
Caallvviinn--B
Beennssoonn ccyyccllee

Plants are the primary producers of food materials of all major food chains on planet
Earth

Due to the importance of plants as the primary suppliers of raw materials, most
importantly in form of glucose, fructose and starch, to most forms of life, the chemical
process called Calvin-Benson cycle (or short: Calvin cycle) has to be considered as
the single most important biological process on planet Earth

the Calvin cycle is a cyclic series of chemical reactions happening in the stroma of the
chloroplast that leads to the build-up (= synthesis) of the mono sugar molecules
glucose and fructose and the polysaccharide starch

the Calvin cycle builds-up (= synthesizes) the energy-rich 3 carbon molecule
glyceraldehyde-3-phosphate (= G3P) from CO2, ATP and NADPH + H+
 CO2 is extracted from air, which diffuses freely into the
chloroplast via special leaf openings, called stomata
 ATP and NADPH + H+ is supplied via the two light reactions in PS I and PS II
(see part I)

G3P the end product of the Calvin cycle is the key precursor molecule of all important 6
carbon sugar molecules, most importantly of glucose and fructose

the chemical reactions of the Calvin cycle occur in the stroma of the chloroplast
TThhee tthhrreeee sstteeppss ooff tthhee C
Caallvviinn ccyyccllee
11.. C
Caarrbboonn ffiixxaattiioonn

3C
CO
O222 molecules are combined with 3 molecules of the 5 carbon- molecule R
Riibbuulloossee-bbiisspphhoosspphhaattee ((R
i
b
P
2
)
Rib-P2) to receive 6 molecules of the 3 carbon compound
33--P
Phhoosspphhooggllyycceerriicc aacciidd ((== 33--P
PG
G))
 this reaction is catalyzed by Rubisco, the key enzyme of
the Calvin cycle
 of C:
3 xC
C +
3
3 xC
C55 
6 xC
C33
+
15 
18
1
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
C
Caarrbboonn ffiixxaattiioonn rreeaaccttiioonn ooff tthhee R
Ruubbiissccoo eennzzyym
mee

despite its central role in PS, the Rubisco
enzyme is remarkably inefficient and
slow
 typical enzymes can process
more than 1000 molecules per
second
 Rubisco fixes only about three
CO2 molecules per second

plant cells compensate for this slow rate by
building lots of the enzyme

since chloroplasts are literally packed with
Rubisco, it makes it
the most plentiful single enzyme on
planet Earth!
M
Mooddeell ooff tthhee ccaarrbboonn--ffiixxiinngg
eennzzyym
mee R
Ruubbiissccoo
 based on X-ray crystallography
Plants & Algae:
 8 copies of a large protein chain
 8 copies of a smaller protein chain
 contains Mg2+ as co-factor
22.. E
Enneerrggyy ccoonnssuum
mppttiioonn aanndd rreeddooxx rreeaaccttiioonnss

2 chemical reactions of the Calvin cycle consume energy from 6 ATP and oxidize 6
NADPH + H+ molecules to reduce 6 molecules of 3-PG to the energy-rich G3P molecule
2
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
33.. R
Reelleeaassee ooff oonnee m
moolleeccuullee ooff G
G33P
P ppeerr ccoom
mpplleettee ccyyccllee

5 of the synthesized 6 G3P (C3) molecules remain in the Calvin cycle to recover 3
molecules of Rib-P2 (C5), which can re-enter the cycle

only one molecule G3P (C3) leaves the Calvin cycle and is available for the
subsequent glucose (C6) synthesis

since G3P is only a 3 carbon compound (C3) it takes two rounds of the Calvin cycle to
finally make one glucose molecule (C6)

since plants usually produce more sugar than they actually need for maintaining their vital
biological functions, many sugars are stock-piled as storage sugars in form of starch in
roots, tubers and fruits

the photosynthesis reaction of plants produces billions of tons of organic matter or biomass each year; this bio-production is unmatched by any other chemical process on
Earth! (see Graphic below)
most of the organic matter produced by plants via photosynthesis is the ultimate source of
food for virtually all organisms on our planet


plants are the foundation of our Earth’s diverse food chains
 life on Earth as we know it is not possible without this unique
photochemical process called photosynthesis!
C
Crruucciiaall cchheem
miiccaall sstteeppss ooff tthhee C
Caallvviinn ccyyccllee
(molecule numbers shown for two complete rounds within the cycle
to make one molecule of glucose )
3
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

the Calvin cycle is the plants core chemical reaction and is the ‘turbine’ of its sugar
manufactory process

the sugar produced during the Calvin cycle is used by the plants as:
1.
fuel molecule for cellular respiration
2.
as nectar for insect attraction or
3.
as starting material for the biosynthesis of structural molecules, e.g. cellulose or
storage molecules, e.g. starch
Carbon fixation and release in numbers
• Carbon fixation rate:
5g CO2 / m2 x day
(temperate forest)
• Global carbon fixation
100 Giga t C / y
(due to photosynthesis)
• Global carbon fixation
92 Giga t C / y
(due to Ocean uptake)
• Global atmospheric carbon
• Carbon (CO2) release
750 Giga t C
5.4 Giga t C / y
(anthropogenic, fossil fuels)
• Plant and soil respiration
50 + 50 Giga t C / y
Global atmospheric CO2 concentration
4
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
C
Caarrbboonn ffiixxaattiioonn ssttrraatteeggiieess ooff ppllaannttss

One of the reasons for the great evolutionary success of plants on Earth lays in their
adaptive flexibility and in their different ways of taking up and fixing atmospheric CO2
as well as saving water during photosynthesis

Depending on their way of fixing the air’s trace gas CO2 (only 0.03% of air is CO2!), plants
are classified into three groups:
11..
C
C33--ppllaannttss

the Calvin cycle in these plants uses CO2 directly from the entered air

the first carbon compound synthesized is the 3 carbon molecule (= C3) 3-PGA

C3-plants are common and widely distributed; amongst it are many agriculturally
important plants, e.g. soybeans, oats, wheat, rice

C3 plants are draught-sensitive and close their stomata (= leaf openings) on hot, dry days
to prevent loss of water

as an unavoidable consequence of the stomata closure on hot, dry days, the C
CO
O222 supply
s
l
o
w
s
d
o
w
n
also slows down, while the O2 released from the PS reaction builds up in the plant cell

as a result of this, the Rubisco enzyme of the Calvin cycle incorporates O2 into
Ribulose-Bisphosphate (RibP2) instead of CO2; as a consequence Rubisco enzyme
cleaves and retrieves only one 3-PG (= C3) molecule (instead of two) and one C2
compound (= Phospho-glycolate) and triggers a process called photorespiration
 since Rubisco is an extraordinarily ancient enzyme that evolved when the
planet's atmosphere lacked oxygen, it “never learned” how to distinguish
CO2 from oxygen

in photorespiration the uptake of O2 instead of carbon dioxide by Rubisco leads to one
C3 molecule and one C2 molecule:
1. 3-phosphoglyceric acid (3-PG) (= C3)
 just as in the Calvin cycle under “normal” temperature conditions
22.. P
Phhoosspphhoo--ggllyyccoollaattee ((== C
C22))
 this molecule enters the peroxisome where it forms an intermediate molecule
(= glycoxylate) under consumption of oxygen (see Graphic below)
 a further derivative of the intermediate C2 molecule (= glycine) enters the
mitochondrion where it is cleaved to CO2 and water
5
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
The major chemical events during photorespiration in C3-Plants
- Happens at high light intensities
- Happens at high daylight temperatures
Plant Cell
2-Oxoglutarate
P O – CH2 – CH – COOH
3-PG
Calvin
Cycle
P O – CH2 – COOH
L- Glycine
Phosphoglycolate
O2
NH4+
H2O2
Glu
Pi
Rubisco
HOCH2 – COOH
Glycolate
Rib-P2
Peroxisome
CO2 Mitochondrion
NADH + H+
O2
CO2
Glycolate
Chloroplast
Oxygenase
act.
NAD+
H2N – CH2 – COOH
H2N – CH2 – COOH
Serine
L- Glycine (2x)
Hydroxymethyl
H2N – CH2 – COOH
L- Serine
NH4+
Stomata
closed
Glycoxylate
Transferase
Graphics©E.Schmid/SWC2003

photorespiration, similar to cellular respiration, consumes oxygen and releases carbon
dioxide, but unlike photosynthesis yields no sugar and does nnoott produce ATP
molecules
 this outcome is very disadvantageous for the many C3 agricultural plants!!

since in agricultural plants like soybean, Rubisco captures oxygen instead of CO 2 about
20 percent of it’s active time, today scientists try to genetically replace the inefficient plant
Rubisco enzyme with an enzyme from green algae that captures CO 2 more quickly
22..
C
C44--ppllaannttss

they evolved special adaptations to save water and also to prevent photorespiration

C4-plants also close their stomata on dry, hot days, but they have a special enzyme
which fixes CO2 into a four carbon (= C4) compound instead of incorporating it into 3PGA!

more importantly: this enzyme does not switch to O2 incorporation under dry conditions
6
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

therefore, on hot, dry days C4 plants can continue to fix carbon in the Calvin cycle
even though the CO2 concentration within the leaf is low due to the stomata closure

important agricultural plants e.g. corn and sugar cane are C4 plants
 the C4 plants evolved in the tropics as a adaptive response to the
permanently hot-dry climate in these regions
33..
C
CA
AM
M ((== ccrraassssuullaacceeaann aacciidd m
meettaabboolliissm
m)) ppllaannttss

have evolved in form of e.g. cacti, pineapple and succulents (ice plant!) as a response to
persistently dry and hot climates

these plants conserve water by allowing opening of its stomata and influx of CO2 only
at night (when the temperatures usually are lower!)

during night, CO2 is fixed into a four carbon (C4) molecule which is later used during
the day to retrieve CO2 for the ATP synthesis
7
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.
P
Phhoottoossyynntthheessiiss,, TTooxxiinnss &
&E
Ennvviirroonnm
meennttaall P
Poolllluuttaannttss

photosynthesis is a highly complex biological process, enabled and driven by many
enzymes and enzyme systems, which work in well-defined structures in well ordered
and integrated sequences (= enzyme cascades)

these enzymes and structures, playing a crucial role in photosynthesis, are the point of
attack of many molecules released by bacteria, fungi or herbivorous insects
 e.g. Fusarium solani, a fungal pathogen of many plants, produces
Naphthazarian toxins which destroy the chloroplast membranes
 as a consequence the chlorophyll of the affected leaves bleaches and the
plant starts to wilt

the “photosynthesis bioapparatus” is also highly vulnerable to many environmental
pollutants released by human activity and is purposely attacked by commonly used
herbicidal agents

critical plant-harming environmental pollutants are sulfur dioxide (= SO2) and ozone (=
O3), which are common air pollutants in today’s urban and inner-city areas
 Sulfur dioxide results from the burning of fossil fuels such as oil, gasoline
and coal (especially brown coal combustion)
 Ozone is a highly aggressive by-product of automobile exhaust and may
accumulate in urban areas on sunny days
 it is estimated that air pollution may reduce yields of some farm crops by
as much as 20 percent!

herbicides which are commonly used in agricultural weed control and known to target
the enzymatic reactions, structures or processes in the chloroplasts are:
1. Triazines (e.g. Evik, Bladec) and Phenylureas (e.g. Lorox, Spike)
 site of action in the chloroplast is the D-1 quinone-binding protein of the
photosynthetic electron transport chain
2. Diphenylether and Bipyridylium herbicides
 these contact herbicides destroy plant cell membranes after activation by
exposure to sunlight and formation of aggressive oxygen compounds such as
hydrogen peroxide
3. Bipyridiliums (e.g. Paraquat)
 the non-selective weed controlling herbicide Paraquat (Gramoxone Extra) is
activated by the photosystem I (PSI) and destroys the chloroplast by generating
free radicals
4. Diphenylethers (e.g. Blazer, Cobra)
 these herbicides inhibit the protoporphyrinogen oxidase enzyme of the electron
transport chain
8
MESA COLLEGE, SAN DIEGO
SCHOOL OF MATHEMATICS & NATURAL SCIENCE
General Biology (BIO107); Instructor: Elmar Schmid, Ph.D.

Certain herbicides work as so-called un-couplers, e.g. DNP or CCCP and prevent the
synthesis of ATP in chloroplasts by shuttling protons across the thylacoid membrane
 they do not interfere with the passage of electrons down the electron
transport chain to NADP, but destroy the proton gradient essential for ATP
synthesis
9