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Photosynthesis
How do plants grow?
Van Helmont - 1648
Joseph
Priestley
Priestley – 1771 – plants restore “good quality” to air
Jan Ingenhousz – 1796 – plants only restore good
quality to air in the presence of light
Water is source of oxygen released
during photosynthesis
C.B. van Niel
1930’s
Van Niel was studying the activities of
photosynthetic bacteria - he found that purple
sulfur bacteria reduce carbon to carbohydrates
but do not release oxygen; instead the purple
sulfur bacteria use hydrogen sulfide in their
photosynthesis - so for them the reaction is as
follows:
CO2 + 2H2S + light energy =>(CH2O) + H2O + 2S
Van Niel then generalized this to the following
reaction for all photosynthetic activity
CO2 + 2H2A + light energy=>(CH2O) + H2O +2A
Photosynthesis has two separate reactions
• Experiments by F.F. Blackman in 1905
demonstrated that photosynthesis has two
stages or steps - one is a light-dependent stage
and the other is a light-independent stage - due
to changes in the effectiveness of the lightindependent stage with increases in
temperature, Blackman concluded that this
stage was controlled by enzymes
The role of pigments
• A pigment is any substance that absorbs visible light most absorb only certain wavelengths and reflect or
transmit the wavelengths they don't absorb
• Chlorophyll absorbs light primarily in the violet, blue
and red wavelengths and reflects green wavelengths,
and thus appears green
• Absorption spectrum - the light absorption pattern of
a pigment
• Action spectrum - the relative effectiveness of
different wavelengths for a specific light-requiring
process
• Chlorophyll is implicated as the principle pigment in
photosynthesis because its absorption spectrum is the
same as the action spectrum for photosynthesis
The Photosynthetic Pigments
• Chlorophyll a - found in all photosynthetic eukaryotes and
cyanobacteria - essential for photosynthesis in these organisms
• chlorophyll b - found in vascular plants, bryophytes, green
algae and euglenoid algae - it is an accessory pigment - a
pigment that serves to broaden the range of light that can be
used in photosynthesis - the energy the accessory pigment
absorbs is transmitted to chlorophyll a
• carotenoids - red, orange or yellow fat-soluble accessory
pigments found in all chloroplasts and cyanobacteria caroteniods are embedded in thylakoids as are chlorophylls two types - carotenes and xanthophylls (xanthophylls have
oxygen in their structure, carotenes don't)
When pigments absorb light, electrons are
temporarily boosted to a higher energy level
One of three things may happen to that energy:
1. the energy may be dissipated as heat
2. the energy may be re-emitted almost instantly
as light of a longer wavelength - this is called
fluorescence
3. the energy may be captured by the formation
of a chemical bond - as in photosynthesis
Overview of Photosynthesis
The Photosystems
• The chlorophylls and other pigments are embedded in
thylakoids in discrete units called photosystems
• Each photosystem has 250 to 400 pigment molecules in two
closely linked components - the reaction center-protein
complex and the antenna protein complex
• All pigments in the photosystem are capable of absorbing
photons of light, but only one pair of those in the reaction
center-protein complex can actually use the energy in a
photochemical reaction
• The other pigments in the antenna protein complex act like
antenna to gather light and transfer that energy to the
photochemically active pigments
The Photosystems
• There are two different kinds of photosystems –
• Photosystem I - has chlorophyll a - has an optimum
absorption peak of 700 nanometers of light - the
chlorophyll a is called P700 because of this activity
• Photosystem II - has special chlorophyll a active at
680 nanometers - the P680 chlorophyll a
• In general the two photosystems work together
simultaneously and continuously - however,
photosystem I can work independently
Overview of Photosynthesis
Calvin Cycle - details
• The Calvin cycle begins when CO2 enters the cycle and is
joined to RuBP this forms a 6 carbon compound which
immediately splits into two 3 carbon compounds (the 6 carbon
intermediate has never been isolated) - the 3 carbon compound
is 3-phosphoglycerate (PGA)
• Because each PGA has three carbons, this is sometimes also
called the C3 pathway
• Each full turn of the Calvin cycle begins with entry of a CO2
molecule and ends when RuBP is regenerated - it takes 6 full
turns of the Calvin cycle to generate a 6 carbon sugar such as
glucose
• Although we usually report glucose as the product of
photosynthesis, the cell usually produces either sucrose or
starch as its storage products
• At night, sucrose is produced from the starch and it is
transported from the chloroplast to the rest of the cell
The full Calvin Cycle equation
6CO2 + 12NADPH + 12H+ + 18ATP =>
C6H12O6 (GLUCOSE) + 12NADP+ + 18ADP +
18 Pi + 6H2O
The C4 Pathway
• In some plants the first carbon compound
produced through the light-independent
reactions is not the 3 carbon PGA, but rather is
a 4 carbon molecule oxaloacetate - plants that
use this pathway are called C4 plants
• Leaves of C4 plants typically have very
orderly arrangement of mesophyll around a
layer of bundle sheath cells
Electron
micrograph
with C4
pathway
shown
Why use C4 pathway?
• A problem with C3 is that for all C3 plants, photosynthesis is
always accompanied by photorespiration which consumes and
releases CO2 in the presence of light - it wastes carbon fixed
by photosynthesis - up to 50% of carbon fixed in
photosynthesis may be used in photorespiration in C3 plants as
fixed carbon is reoxidized to CO2
• Photorespiration is nearly absent in C4 plants - so greatly
increases their efficiency - this is because a high CO2: low O2
concentration limits photorespiration - C4 plants essentially
pump CO2 into bundle sheath cells (or the products of its
reduction) thus maintaining high CO2 concentration in cells
where Calvin cycle will occur
• Thus net photosynthetic rates are higher for C4 plants (corn,
sorgham, sugarcane) than in C3 relatives (wheat, rice, rye,
oats)
Why use C4 pathway?
• C4 plants evolved in tropics and are well adapted to
life at high temperature, high light intensity and dry
conditions - optimal temperature for C4
photosynthesis is much higher than for C3 - efficient
use of CO2 allows C4 plants to keep stomata closed
longer and thus they lose less water during
photosynthesis than do C3 plants
• C4 monocots do especially well at high temperature
• C4 dicots do especially well in dry conditions
Crassulacean Acid Metabolism
• Crassulacean Acid Metabolism (CAM) has evolved
independently in many plant families including the
stoneworts (Crassulaceae) and cacti (Cactaceae)
• Plants which carry out CAM have ability to fix CO2
in the dark (night) via the activity of PEP carboxylase
- malic acid (malate) so formed is stored in the cell's
vacuole - during the light (day) the malic acid is
decarboxylated and CO2 is transferred to RuBP in
Calvin cycle within the same cell
• so CAM plants, like C4 plants, use both C4 and C3
pathways, but CAM plants separate the cycles
temporally and C4 plants separate them spatially
Comparison of C4 and CAM pathways