Download 3.2

PHOTOSYNTHESIS 2
3.2
Photosynthesis
Can be broken down into
three stages.
LIGHT REACTIONS (in
thylakoids)
1)
Capturing light energy.
2)
Using captured light energy
to make ATP and reduce
NADP+ (nicotinamide
adenine dinucleotide
phosphate) to NADPH
CARBON FIXATION (in stroma)
3) Uses the energy of ATP and the
reducing power of NADPH
to drive Calvin Cycle, which
incorporates CO2 into
carbon compounds such as
glucose.

Properties of Light
 Of the total solar energy that reaches the earth, 60%
is lost to the atmosphere. Of the 40% which reaches
plants, only 5% is used in photosynthesis.
 Light is also known as electromagnetic (EM)
radiation that travels at 300,000,000m/s
Properties of Light
 That light comes in packets known as photons (or
quanta). Photons of light energy come in different
wavelengths of light energy.



The longer the wavelength, the lower the energy.
The shorter the wavelength, the higher its energy.
Visible light is between 380 nm (violet) and 750nm (red).
Photosystems
 Are clusters of photosynthetic pigments embedded
in the thylakoid membranes which absorb photons
of particular wavelengths.
 transfers energy to ADP, P, and NADP+, forming
ATP and NADPH.
 The electrons that reduce NADP+ to NADPH are
supplied by water molecules that enter the
thylakoids through the stroma.
 ATP and NADPH are synthesized in the stroma,
where carbon fixation reactions occur.
History of Photosynthesis
 In ancient times, people
thought all food for the plant
was from the soil.
 In early 1600’s, J.B. Van
Helmont planted s a willow tree
after predetermining the weight
of the soil and the willow tree.
 After five years, the tree’s mass
had increased by 74.4 kg, but
the soil’s mass only decreased
by 60 g. Therefore, most of the
added weight of the plant could
be attributed to water
absorption (incorrect)
History of Photosynthesis
 1771, Joseph Priestley discovers by accident that gases play a
role in photosynthesis.
 Put a candle in a glass container, and it eventually burns out.
 Put a candle in a glass jar with a plant, and in ten days the
candle was able to combust again.
 There must be a gas released by the plant that supports
combustion.
History of Photosynthesis
 In 1776, Dutch medical doctor Jan Ingenhousz
identified the gas as oxygen, and was the first to
realize light was necessary for photosynthesis.
 He was wrong in thinking that O2 was produced by
the sun splitting CO2 and leaving the carbon atom for
the plant to use.
History of Photosynthesis
 In the 1930’s, C.B. Van Niel was able to show that
water provided the oxygen gas by way of purple
sulfur bacteria.
 CO2 + 2H2S + light  [CH2O](aq) +H2O(l) )+ 2S(s)
(hydrogen sulfide)
(carbohydrate)
History of Photosynthesis
 In 1938, S.M. Ruben and M. Kamen confirm Niel’s
findings.
 Use Chlorella algae - grow it in heavy water (uses an
oxygen-18 isotope)
 Used mass spectrometer to track the isotope. The
spectrometer separates and detects molecules by
mass.
 The isotope was found in the oxygen gas.
Rate of Photosynthesis
 In 1905, Blackman
measured the effect in
changes in light intensity,
CO2 and temperature and
temperature. Concluded
that,
 1) At low light intensities the
rate of photosynthesis is
increased by increasing the
light intensity, but not the
temperature.
 2) At high light intensities,
the rate of photosynthesis is
increased by increasing
temperature, not light
intensity.
Rate of Photosynthesis
 Concluded that photosynthesis
takes place in two stages, an
initial light-dependent stage,
and a second, lightindependent stage that is
affected by heat, not light.
 He later showed that the level
of carbon dioxide affects the
rate of photosynthesis.
 The light reactions use light
and water to produce NADPH
and ATP.
 The carbon fixation reactions
then use that NADPH and ATP.
Colours and Photosynthesis
 In 1882 T.W. Engelmann placed a triangular glass
prism between a light source and a microscope stage.
On the stage was the algae Spirogyra, long and
filamentous.
 He placed aerobic bacteria along the length of the
alga, to see if photosynthesis was equal across all
colour wavelengths.

Aerobic bacteria would grow where oxygen is produced.
Colours and Photosynthesis
 Very little aerobic
bacteria grew near the
green spectrum.
 Why is chlorophyll
green?
 Colours are determined
by the wavelengths of
light that reflect back to
us (not absorbed)
Absorption Spectrum
 The wavelengths of light
absorbed by a pigment.
 Chlorophylls a and b
absorb blue-violet and red
and transmit green 500600 nm.
 Chlorophyll a is the
pigment that transfers
energy from light to the
carbon fixation reactions.
Action Spectrum
 Graph illustrating the
effectiveness with
which different
wavelengths of light
produce
photosynthesis.
Accessory Pigments
 Carotenoids: Absorb in the blue-violet range (400-500nm).
Contain two hydrocarbon rings connected by an alternating single
and double-bond hydrocarbon chain.
 Reflect red and yellow. Dispersed in the thylakoid membranes.
 Some don’t participate in photosynthesis, but rather absorb energy
that can damage chlorophyll release that energy as heat.
Accessory Pigments
Carotenoids: (yellow-red)
Xanthophylls – pigments in chloroplast (thylakoid)
membranes that give rise to yellow colour in leaves
Anthocyanins – pigments in vacuoles that give rise to
the red colour in autumn leaves.
So, why do plants appear to be green in the
spring/summer?
Why do leaves turn colour in the fall?
Photosynthetically Active Radiation (PAR)
 The wavelengths from 400 nm to 700 nm that
support photosynthesis.
 Chlorophylls a and b combined with other pigments
pretty much absorb the entire visible spectrum.
Summary
Seatwork/Homework
 Reread section 3.2
 Answer PPs on page 154:
 #1,2,3, 5, 7 (read text), 8.