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Chapter
5. Photosynthesis
BI 101 Davison Fall 2006
Photosynthesis
is an energy conversion
converting light
energy to chemical
bond energy
Organic molecules are the key building blocks and energy stores for life.
Plants make (synthesize) their own organic molecules from inorganic
atmospheric CO2 . To
do this plants trap light energy to
drive the metabolic pathways we call photosynthesis.
When eaten by animals or decay organisms, plants provide the
organic molecules needed by those forms of life.
You also need plant-made organic molecules to supply the building
blocks and energy needs required for growth and life itself.
You get these organic molecules by eating the plant that created
them or by eating organic molecules originally found in plants but now reside in some
food product you may be about to eat.
Thus plants are the primary source and sole producer of original
organic food.
I. Historical
Perspective
In the early 1600's it was a widely held notion that
plants absorbed food from the soil (Aristotle wrote that leaves in fact
were mere shading devices providing a comfort zone for roots; everyone
new adding manure and humus to soil resulted in plants growing faster
and growing larger.)
1. Mid
1600's van Helmont - a Belgian physician, devised simple experiment in
which he grew a willow tree seedling for five years in a bucket containing
a known quantity of soil. The
tree gained roughly 195 pounds while the soil lost less than one pound.
5 lb. willow
200 lb.
----------->
5 yr
200 lb soil
199.8 lb soil
Van
Helmont concluded wrongly that plants obtain food from water.
If most of
the dry material (dry biomass) didn’t come from soil or rain water, then
where did it come from? It
seems unlikely based on perceptions limited to our bodily senses, but
tremendous tonnage of biomass accumulates each day from thin air.
Photosynthesis is the process by which this happens—that
seemingly weightless CO2 is pulled from the air and accreted
into an organic form, the form of carbohydrates.
Water from the soil plays an important role in supplying atoms of
Hydrogen that are required in building carbohydrates (so Van Helmont
wasn't entirely incorrect). Soil
also provides inorganic nutrients that supply the other elements used in
constructing other organic molecules such as amino acids and nucleotides
(all but C, H, & O are from the soil, recall the mnemonic device for
the chemical elements of life “C HOPKNS CaFe Mg B Mn CuZn Mo Cl Na”).
2. 1700's
Joseph Priestly (England) - put things under bell jars. Mouse kept alive in bell jar with plant.
King of England presented Joseph Priestly with a medal and declared
"for these discoveries we are assured that no vegetable grows in
vain... but cleanses and purifies our atmosphere." Thus before
1800, it was a known fact that plants give off oxygen gas needed in the
respiration of animals. Why the oxygen was given off, or the process
by which plants produce oxygen gas, remained a mystery.
II.
The
Chemistry of Photosynthesis:
1. Light Dependent Reactions
2. Light Independent
Reactions
1.
Light Dependent Reactions - require light
(see section 5.2, p. 76-77)
Within the
range of visible light (recall that white light consist of a spectrum of
colors-energy waves-ROY G BIV) photosynthetic plant pigments capture
(absorb) energy from the blue/violet and red portions of the spectrum
while green and yellow light is reflected.
The “z-scheme” (fig.
5.6a) is the model that best explains these
reactions. These reactions
occur on the inner membranes of the chloroplast; these membranes hold the
pigments and other molecules involved.
Upon the inner chloroplast membranes photosynthetic pigments are arranged into discrete patches
called photosystems (illustrated as
green globs embedded in the phospholipid bilayer shown in figs. 5.7).
It is these pigment molecules (largely chlorophyll)
that absorb light energy. Absorbed
light energy, from the blue and red portions of the spectrum
predominantly, is transferred to electrons causing the electrons to leave the
photosystems. These removed electrons give up their energy absorbed from
the sun to produce ATP during the Electron
Transport Chain (more about
ETC below). To replace electrons removed from
the photosystems, electrons are taken from water molecules.
Removing electrons from two H2O’s results in the water
molecules splitting into four H+ and two O atoms.
The O atoms combine forming O2 as a waste molecule or
byproduct. This process of
splitting water molecules is called photolysis.
Electron transport chain
– a difficult concept, but go with the following analogy:
The electrons that begin the electron transport chain have
potential energy stored from the sun just as water behind a dam stores
potential energy from the sun (Recall the sun is needed to elevate water,
via evaporation and rainfall, from sea level to the level of the dam.
Similarly, sunlight elevates the potential energy of electrons).
The electrons are transported between various molecules in the
membranes of the chloroplast such that they give up their potential energy
in a form that is used to add a phosphate to ADP, thus making ATP.
This is very much like the water behind a dam giving up its
potential energy to generate electricity as it (the water) is transferred
via the pull of gravity through the turbines in the dam.
A more detailed explanation is found in your text (a buildup of
hydrogen ions that form from the ETC and from photolysis is used as a
force that makes ATP just as water behind a dam is used as a force to make
electricity; the concentrated hydrogen ions flow across the membrane
to where they are less concentrated through a transport protein that then
makes ATP--hydrogen ions passing through the special transport protein
that makes ATP is like water passing through a turbine that makes
electricity).
In summary
of the Light Dependent Reactions, O2 has been produced as a
byproduct and light energy has been converted into the chemical bond
energy of ATP. But ATP is not
food, it is incapable of being stored, and thus photosynthesis is not yet
complete.
2.
Light Independent Reactions- do not require light. See fig. 5.8.
=Carbon fixation – the conversion of inorganic CO2 into
carbohydrates.
=Calvin cycle, Calvin-Benson cycle, or C3 cycle.
1940's - Calvin & Benson - used 14C radioactive
isotope and traced the path of carbon
fixation - conversion of inorganic CO2 into organic
carbohydrates. Today this
reaction series bears their name as the Calvin-Benson Cycle or Calvin
Cycle.
The actual synthesis reactions that produce carbohydrates during
photosynthesis do not directly require light.
These so-called light independent reactions do however require the
ATP made during the light reactions.
Without ATP, no energy would be available to drive the carbon
fixation process. Carbon and
Oxygen atoms from atmospheric CO2 and the H+
derived from water pulled from the soil are combined to
make carbohydrates during the Calvin Cycle.
All the chemicals involved are dissolved in the solution that fills
the chloroplast, thus the internal membranes of the chloroplast are not
directly involved in the Light Independent Reactions.
As carbohydrates accumulate they are transported to other plant
organs or stored as starch granules inside the chloroplast.
Some of the products of photosynthesis (i.e. carbohydrates) are
eventually converted to lipids or combined with nitrogen taken from soil
and used to construct proteins and nucleic acids (e.g. DNA).
In summary
of the Light Independent reactions:
·
consumes six molecules of CO2 to produce one molecule of glucose, C6H12O6
·
consumes ATP made during light reactions (ATP is the energy
source that directly drives the carbon fixation reactions)
III. Importance
of Photosynthesis:
Fossil fuels - plants from 350 MYA (Carboniferous Period)
died, were covered with sediments, and became coal.
Food - 250,000 plant species, yet remarkable few used as human
food. Only 150 crop species.
The big four are: rice, wheat, corn, & beans (the first 3 are
grasses).
Removes CO2 - reduces greenhouse gas.
– today only 0.033% CO2 ;
earth billions of years ago had much higher CO2 levels,
much of this early CO2 has been incorporated by plants into
biomass (or into fossil fuels). Our
removal of forests and combustion of fossil fuels returns CO2
to the atmosphere and results in: 1) theoretically with all other factors
removed, faster plant growth & 2) global warming.
Created O2 rich atmosphere (about 21 % O2)
Lead to the formation of the ozone
(O3) layer which filters harmful UV radiation).
3O2 ---> 2O3 in stratosphere
An O2 rich atmosphere and ozone layer are prerequisites
for life on land---a rather recent occurrence.
Only 10% of the time that life has been on earth has there been
substantial terrestrial life. For
the first 3 billion years life on earth was essentially limited to aquatic
environments and only in the
last 400 million years has terrestrial life appeared.
Photosynthesis performed by algae and seaweed changed remarkably
the earth’s atmosphere making the life on land possible.
IV.
Chemosynthesis
- some bacteria use chemical energy to fix CO2
Not all organic molecules are created by plants and photosynthetic
algae, a minor
amount of primary production occurs by chemoautotrophs,
bacteria which use chemical bond energy of iron and sulfur compounds to
fix carbon dioxide. Chemoautotrophs are important in nutrient
cycling.
V.
Additional Vocabulary:
Autotrophs – organisms that create their own food by fixing
CO2 into an organic form: requires outside energy source –
sun in the case of photoautotrophs
(ex. Algae, plants) & inorganic chemical energy in the case of
chemoautotrophs (some
bacteria).
Heterotrophs – organisms that feed on food already formed by
autotrophs. (ex. some
non-green plants, all animals, some bacteria, all fungi)
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