PHOTOSYNTHESIS
GENERAL INFO
What is PS?
Photosynthesis produces glucose and oxygen from carbon dioxide and water
Solar Energy + 6 CO2 + 6 H2O ! C6H12O6 + 6O2
(there is an inverse relationship of photosynthesis to cellular respiration, where C6H12O6 + 6O2!6 CO2 + 6 H2O+ E)
How energy from the sun participates in photosynthesis:
Sun's hydrogen is converted to helium, energy in the form of photons is produced. Those within visible light
portion of electromagnetic spectrum are absorbed by pigments in the chloroplasts of plants, initiating the
process of photosynthesis.
Glucose produced in photosynthesis is turned into starch for storage or sucrose for transport
Where does PS occur?
Chloroplasts – by plants, some protists
Across membranes – by some bacteria
Who undergoes PS?
Plants, some protists, some bacteria (NOTE: lichens have a photosynthetic component—keep an eye out for them]
NOT animals or fungi
How do we check experimentally for photosynthetic activity?
(1) presence of starch or
[we did this in experiment]
(2) presence of oxygen (if experimental system is a closed container)
[we could have done this instead]
COLOR/LIGHT THEORY
Light is part of the electromagnetic spectrum.
Visible light is between about 400 nm and 700nm.
Wavelength is inversely proportional to energy.
Pigments absorb different wavelengths (colors).
Colors that are absorbed disappear (e.g. a red
firetruck absorbs all non-red colors).
Colors that are transmitted/reflected hit your
retina and are perceived (e.g. a red firetruck reflects
red; a red umbrella/filter reflects red, but also light
passing through it transmits red; anything under the
umbrella/filter is only receiving red, therefore it (e.g. a
plant) cannot absorb anything but red).
White reflects all wavelengths (why a white rooftop keeps a house cool in the summer).
Black absorbs all wavelengths (why wearing black in the summer is a bad idea).
How this relates to Plant Pigments:
Green pigments only absorb non-green light (since they reflect green). Xanthophylls (yellow) only absorb non-yellow light
(since they reflect yellow). Carotenoids (orange) only absorb non-orange light (since they reflect orange). And so on... If a
leaf is only exposed through e.g. green light (under a green filter), then it will only receive green light, and will be unable to
photosynthesize...
EXPERIMENTS
Experiment 1: What wavelengths of light are useful for photosynthesis?
Question: Does the wavelength of light (which part of the electromagnetic spectrum ) that reaches the photosynthetic part
of a plant cell affect its photosynthetic ability?
Hypothesis: The wavelength of light that reaches the photosynthetic part of a plant will affect its photosynthetic ability, a
fact which can be measured by its capacity to produce storage starch.
Prediction: Green filters will allow only green light to be transmitted to the leaf under it. Green has a wavelength
photosynthetic pigments have no use for and hence very little if any photosynthesis will occur in leaves under a green filter.
Blue filters are very close to the green part of the spectrum, and thus photosynthesis may not be as great as under
another filter.
Red filters will transmit the red spectrum and will have the maximum positive effect on photosynthetic ability of any of the
other filters used (perhaps not as much as white light).
The black construction paper or foil will absorb all wavelengths and prevent any from getting to the leaf, and will thus
have the most hindering effect on photosynthetic ability.
Experiment 2: What parts of a multicolored leaf carry out photosynthesis?
Coleus has multicolored leaves.
Green parts of leaf: most photosynthetic
Pink/White parts: least photosynthetic
Purple: in some cases, photosynthesis occurred. Explanation: some colors on a leaf
are the result of pigment mixtures, i.e. anthocyanins plus chlorophylls.
Carotenoids are revealed as chlorophyll breaks down (e.g. in the fall). Carotenoids may
absorb light as accessory pigments, and are therefore useful in photosynthesis.
Anthocyanins are non-photosynthetic pigments. They may act as “sunscreen” in young and
old foliage.
Question: Are only the green parts of a leaf photosynthetic?
Hypothesis: Only green parts of a leaf are photosynthetic
Prediction: Green parts will turn black after leaves have been boiled and Lugol's has been
applied to test for presence of starch (result of photosynthesis)
Actual Result: Some originally purple parts of the leaf turned black as well – leading us to
believe that they contained mixtures of chlorophylls and anthocyanins that we were not
capable of seeing with the naked eye.
Experiment 3: Separation and Identification of Plant Pigments by Paper
Chromatography
Substances and their respective polarities:
Solvents: “like dissolves like”.
Petroleum ether (= benzine) is a non-polar solvent, it will dissolve the nonpolar molecules beta carotene and xanthophyll.
Acetone is a polar solvent, it will dissolve the polar molecules chlorophyll
a and b.
Chromatography Paper: is a polar substance (cellulose) and as such will “grab onto” the
other polar substances (xanthophyll, chlorophylls a and b) to varying
degrees) and “slow down” their ascent to the top of the chromatography paper.
Pigments:
Beta carotene = least polar molecules (no polar oxygens) ! first to top
Xanthophyll = second least polar molecules (hydroxyl group) ! second to top
Chlorophyll a = some polar molecules (polar oxygen) ! third to top
Chlorophyll b= most polar molecules (two polar oxygens) ! fourth to top
Experiment 4: Determining Absorption Spectrum for Leaf Pigments:
We separated out each pigment (through paper chromatography), and different groups tested the absorbtion of the
individual pigments at various wavelengths of light. Here is the graph from our class data (no data for chlorophyll a):
Absorbance (%)
SPECTROPHOTOMETRY
Absorption spectrum can be determined using
Absorbance of Photosynthetic Pigments Extracted from Fresh Spinach
a spectrophotometer, which measures the proportions of
0.55
light of different wavelengths (colors) absorbed and
0.5
transmitted by a pigment solution. It passes a beam of light
0.45
of a particular wavelength (designated by the operator)
0.4
through the pigment solution being tested. The
0.35
spectrophotopmeter then measures the proportion of light
Chlorophyll a
0.3
Chlorophyll b
transmitted or, conversely, being absorbed by that particular
Xanthophyll
0.25
pigment and shows the reading on the calibrated scale.
Carotene
0.2
Total Pigment
0.15
0.1
0.05
0
400
450
500
550
600
650
Wavelength (nm)
700
750
Graph Types:
An absorption spectrum graph shows the relative absorption of wavelengths by individual pigments in a sample
An action spectrum graph shows the relative effectiveness of the total extract in photosynthesis
x-axis=Independent variable=wavelength
y-axis=dependent variable=absorbtion reading