Light Reactions & Photosynthetic
Pigments
LIGHT!
• Of all the light energy that reaches the earth’s
surface, ~5% is transferred to carbohydrates by a
leaf
• Light = a form electromagnetic radiation that
travels at 3x108 m/s as photons (
– That’s 300,000,000m in ONE SECOND!
LIGHT!
• Light from the sun is a mixture of photons with different
energies
• When light passes through a spectroscope which
contains a prism, photons are separated by energy
• Most of the photons are invisible to humans but we can
see wavelengths of light that range from 380-750nm
– nm = nanometer = 1 x 10-9 m
(it’s really small)
**The Vatican’s chief astronomer invented the spectroscope (1860s)
A Short and Painless History of Photosynthesis
• Like many people, it was initially thought that plants
obtained their ‘food’ from the soil
– They do? Don’t they?
• In the early 1600s, J.B. Van Helmont decided to test this
theory
– Planted a willow tree in soil and
ONLY added water for 5 years
• After the 5 years, the tree’s mass
increased by 75kg
• The soil decreased by only 60g!!
– Thus, the soil was NOT a
contributing factor
– Thought that it must be water!
A Short and Painless History of Photosynthesis
• In 1770, Joseph Priestly discovered, by accident, that
gases in the air played a role in photosynthesis
– Placed a burning candle in a closed container
– When it went out, he placed a plant in the container with
the candle
– After 10 days, he was able to reignite the candle
– Thus, proving that plants
release a gas into the
atmosphere
• This gas was confirmed as
oxygen in 1796
A Short and Painless History of Photosynthesis
• Jan Ingenhousz, 1796, was the first to realize that
sunlight is essential to photosynthesis
– Also noted that CO2 was the source of carbon in plants
– But, mistakenly said that CO2 broke down to produce
oxygen
• “Sunshine splits apart the carbon dioxide molecule
that a plant has absorbed in the air; the plant throws
out at that time the oxygen alone, and keeps the
carbon to itself as nourishment”
• In 1930, C.B. Van Niel proved that oxygen was
produced by water splitting (i.e. Not from CO2)
– This was confirmed using isotopes (18O) in 1938
A Short and Painless History of Photosynthesis
Light and Photosynthesis
• In 1905, F.F. Blackman measured the effect in changes in
light intensity, [CO2], and temperature on photosynthesis
• Two main results:
1. At low light intensities, the rate of photosynthesis increased
by increasing light intensity but not temperature
2. At high light intensities, the rate of photosynthesis increased
by increasing temperature but not light intensity
• From this study, Blackman concluded
that there must be:
– a light-dependent reaction (photochemical)
– a light-independent reaction (biochemical)
A Short and Painless History of Photosynthesis
• Blackman then showed that the rate of photosynthesis is
sensitive to [CO2]
– Controlling for temperature, he subjected plants to different [CO2]
– He found that photosynthesis decreased with a decreased [CO2]
• CONCLUSIONS:
1. Light reactions occur in the presence of
light, independent of temperature
*use light & water to produce NADPH
and ATP
2. Carbon fixation reactions rely on the
energy of light reactions
*dependent on temperature not light intensity
LIGHT and PLANTS
• Plants have photosynthetic pigments called photosystems
– Found in the thylakoid membrane
• Photosystems absorb photons of specific wavelengths
– Then, use light reactions to form ATP and NADPH in the stroma
LIGHT and PLANTS
• Action spectrum
– illustrates the effectiveness with which different
wavelengths of light promote photosynthesis
• Absorption spectrum
– Illustrates how much light, at a specific wavelength, is
absorbed by a specific molecule/substance (ex.
chlorophyll a or b)
Chlorophyll & Accessory Pigments
• Chlorophyll a & b absorb photons with
energies in the blue-violet and red regions
– They reflect energies between 500nm – 600nm
– Hence, why we see photosynthesizing
organisms as green
• Chlorophyll a transfers light energy to
carbon fixation reactions
• Chlorophyll b acts as an accessory pigment
– Absorbs photons that chlorophyll a cannot
Chlorophyll & Accessory Pigments
• Carotenoids = accessory pigments
– Ex. ß-Carotene
– Enzymes split ß-Carotene into vitamin A
– Vitamin A is used to help vertebrae vision in
dim light environments
– Carrots help vision as they have ß-Carotene
• Carotenoids absorb light energy from
400nm - 500nm (blue-violet)
– Reflect yellow-red  appear yellow-orange
Functions:
Why do leaves change colors in the fall?
• Leaves have accessory pigments that reflect multiple
colors of light energy (red, yellow, orange)
• In spring/summer leafs appear green because they
have [chlorophyll] in their chloroplasts of leaf cells
– Remember: chlorophyll reflect green light!
– Most likely temperature dependent
• In the fall, plants stop producing chlorophyll and
dissemble any chlorophyll that is left in the leaves
– Occurs due to the cooler temperatures
• This allos the reflected light of the accessory
pigments to become visible!
PHOTOSYNTHETIC REACTION STAGES
• All photosynthetic reactions can be broken down
into
3 distinct, but connected stages:
1. Capturing light energy
2. Using captured light energy to make ATP and
reduced NADP+
**NADP+ is the photosynthetic equivalent of
NAD+**
3. Using the free energy of ATP and the reducing
power of NADPH to synthesize organic
compounds
(ex. creating glucose from CO2)