Photosynthesis
Thomas G Chastain
CROP 200 Crop Ecology and Morphology
Photosynthesis
• Photosynthesis and Respiration are
oxidation and reduction Processes.
• Oxidation - A compound is oxidized
when it loses electrons by the
addition of oxygen or loss of
hydrogen.
• Reduction - A compound is reduced
when it gains electrons by the
addition of hydrogen or loss of
oxygen.
• Photosynthesis is a reduction
process: light energy is stored as a
reduced carbon compound, a
carbohydrate. Photosynthesis consists
of two basic reaction processes: the
light reaction and the carbon
reaction.
Meadowfoam crop
(TG Chastain photo)
Photosynthesis
• Green plants capture solar energy
and CO2 from the atmosphere and
convert this energy into a stable
chemical form.
• Instead of solar panels to capture
solar energy, green plants employ a
canopy of leaves and stems to collect
the sun’s energy. Conversion of solar
energy to chemical energy by plants is
known as photosynthesis.
• Carbon from atmospheric CO2 is
reduced in several steps to a reduced
and stable form, a carbohydrate.
CO2 + H2O
Oxidized
Carbon
Reductant
CH2O
Reduced
Carbon
Photosynthesis
• Photosynthesis takes place in the
chloroplast, an organelle found in the
green parts of plants. The chloroplast
contains chlorophyll, a green-colored
pigment.
• Light energy is captured by
chlorophyll which forms a light
harvesting complex with other
pigments. The light reactions take
place in the thylakoid membranes. A
stack of thylakoids are known as
grana.
Chloroplast digram (top left), cells with
chloroplasts (bottom left – Thomas Dreps photo)
Photosynthesis
• Light is radiant energy that is
a portion of the
electromagnetic spectrum
• Photosynthetically active
radiation (PAR) is radiant
energy between 400 nm and
700 nm - 45% of the
electromagnetic spectrum’s
energy is PAR. Peak
absorption of light by
chlorophyll is in the red and
blue wavelengths.
100
Chlorophyll a
Absorption
75
Chlorophyll b
50
25
0
400
500
600
Wavelength (nm)
700
Electromagnetic spectrum
(top), chlorophyll absorption
spectra (bottom)
Time course of photosynthesis response to solar
radiation in maize (Moss, 1961)
Photosynthesis
• The quantity of solar radiation
has a direct effect on the
amount of photosynthesis taking
place in a crop field.
• Over the course of a day from
dawn to mid-day, solar radiation
increases, causing a concomitant
increase in the amount of CO2
fixed in the photosynthetic
process. From mid-day to dusk,
solar radiation and
photosynthesis decline.
• More photosynthesis takes place
on a clear day than on a cloudy
day.
Photosynthesis
Photosynthesis
• Light Reaction of Photosynthesis –
essential steps:
1. Capture of light energy
2. Photophosphorylation – the
production of ATP by light
3. Water serves as the reductant
in the reaction and is oxidized to 2
H+ and O. Electrons are
transferred to CO2.
4. NADP is reduced to NADPH
(nicotinamide adeninedinucleotide
phosphate), a reductant
Radish seedling (TG
Chastain photo)
Photosynthesis
• Two Important Carboxylase Enzymes in
Crop Plants:
1. Ribulose bisphosphate carboxylase
(Rubisco) – found in C3 plants such as
wheat, potato, alfalfa, canola, tall
fescue, and mint.
2. Phosphoenolpyruvate carboxylase
(PEP carboxylase) – found in C4 plants
such as corn, sugar cane and
bermudagrass.
Corn – a C4 plant
C3 carbon reaction
Photosynthesis
• The C3 carbon reaction,
formerly known as the dark
reaction and also known as
the Calvin-Benson Cycle, is a
2-step process:
1. Fixation of CO2 by
carboxylase and reduction to
a carbohydrate, 3phosphoglycerate (3-PGA).
2. Reduction of 3-PGA to
glyceraldehyde 3-phosphate
(GAP). NADPH, a reductant
from the light reaction, is
involved.
Mesophyll
cell
Bundle
sheath cell
Photosynthesis
• The C4 Carbon Reaction is
compartmentalized so the the
carboxylation step takes place in one
cell and the reduction step takes
place in another cell.
• Phosphoenolpyruvate is carboxylated
to a C4 acid by PEP carboxylase in the
mesophyll cell. The C3 carbon
reaction takes place in the bundle
sheath cell.
• The advantages of C4 photosynthesis
include the adaptation of crops to hot
and dry climates. C4 plants can
continue photosynthesis under these
adverse conditions when C3 plants
cannot.
Photosynthesis
• The net photosynthetic rate for
each soybean leaf rises as leaf
area expands to a maximum
and then declines as the leaf is
progressively shaded by the
next developing leaf.
Leaf development and
photosynthesis in
soybean (left), soybean
plant (top).
Rust (Cynthia Ocamb photo -OSU)
Photosynthesis
• Many plant diseases can reduce
crop productivity directly by
reducing the plant’s
photosynthetic capacity.
• Reduction in grass seed crop
seed number and seed weight by
fungal diseases such as rust.
These yield components are
affected by reductions in
photosynthetic leaf area.
Biological Solar Energy Collectors
• Crop plants are biological solar
energy collectors. How big is this
collector? For every acre of wheat
field, there are 3 acres of leaves
deployed to capture solar energy.
• The number of plants in the stand
(plant population) as well as
number of tillers and leaves must
be managed so as to optimize the
size and efficiency of this
biological solar energy collector.