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PHOTOSYNTHESIS
I. Virtually all energy on earth comes from sunlight
A. Plants use energy from the sun to make the bonds which hold organic
molecules together
B. When these bonds are broken the energy is ultimately transferred to ATP,
which is then moved about cells and organisms to power their needs
C. Since these molecules are synthesized from the energy in sunlight the
process is called photosynthesis
1. Autotrophs = organisms that can make their own
food
a. Plants are photoautotrophs because they use light
to make their food
2. Heterotrophs = organisms that can not make their
own food and therefore must get it from their environment
II. Photosynthesis
A. Electricity, one of our most common energy sources, consists of a flow
of electrons
B. During photosynthesis the sun's energy is used to split water
molecules, starting a flow of electrons
C. The energy from this flow of electrons is harnessed and used to make the
bonds in organic molecules
D. 3CO2 + 6H20----- light -----> C3H6O3
+ 3O2+ 3H20
1. Since organic molecules contain carbon, a supply of carbon is needed
for this process, it comes from carbon dioxide in the
atmosphere
2. Note the starting products are water, which supplies the electrons,
and carbon dioxide, which provides the carbon
3. The end products are oxygen, a triose (two combined make glucose) and
water
III. Importance of photosynthesis
A. Note that photosynthesis produces:
1. Glucose - which provides us with both food (alpha glucose
polymers) and structural materials (beta glucose polymers)
2. Oxygen
a. The early atmosphere of the earth lacked oxygen
and it took about 3 billion years of photosynthesis to produce the current
21% oxygen atmosphere we now enjoy
b. Some of this oxygen reacted with sunlight in the upper atmosphere to
produce ozone (O3), which protects us from
harmful solar radiation
IV. Energy transformations
A. First law of thermodynamics - energy can not be created nor destroyed,
only changed from one form to another
B. Plants transform the energy from the sun into a more useable form,
carbohydrates
C. Energy transformations in living systems involve the transfer of
electrons and protons [H+] from one energy level to another,
or from one atom or molecule to another
1. Reduction/ oxidation (REDOX) reactions:
a. Oxidation = loss of electrons, e.g. Na to Na+
b. Reduction = gain of electrons, e.g. Cl to Cl-
V. Overview of photosynthesis
A. Photosynthesis is actually a two step process:
1. Energy transduction reactions - energy from the sun
is captured in energy carrying molecules. Since it must take place in
sunlight it is often called the light-dependent reaction(s)
2. Carbon-fixation reactions - The energy from the
transduction reactions is used to fix the carbon from atmospheric carbon
dioxide into organic compounds. It does not have to take place in the light
so it is often called light-independent reaction(s)
VI. Energy transduction reactions
A. The sun's energy is used to split water molecules which starts a
flow of electrons
B. The energy from this flow of electrons is used to convert ADP to
ATP and to form an additional electron carrier molecule called
NADPH
C. The first step in the conversion of energy is the absorption of light by
pigments
1. Pigment = a substance that absorbs light
2. Major photosynthetic pigments:
a. Chlorophylls - absorb violets, blues and reds, reflect green
b. Carotenoids - red, yellow and orange pigments. E.g.
betacarotene is principle source of vitamin A required by humans for
proper vision
c. Phycobilins - pigments found in cyanobacteria and some red
algae
3. When chlorophyll pigments absorb light, electrons are boosted to a
higher energy level and the energy is captured in a chemical bond
4. Photosynthetic pigments are embedded in the thylakoids, membrane
bound sacs within the chloroplasts
a. 250 - 400 pigment molecules are organized into a photosystem
5. Photosystems have two components:
a. Antenna complex - pigment molecules which gather light
energy and funnel it to the reaction center
b. Reaction center - a complex of proteins and pigment
molecules that enable light energy to be converted to chemical energy
6. All the pigments within a photosystem can absorb photons
(particles of light energy)
7. However, only one pair of chlorophyll molecules per
photosystem can actually use the energy. These special chlorophyll molecules
are at the core of the reaction center
8. Light energy gathered by the antenna pigments is transferred from one
molecule to the next until it reaches the reaction center
9. When the reaction center absorbs the energy, electrons from one of the
core molecules is boosted to a higher energy level and transferred to an
acceptor molecule which initiates electron flow along an electron
transport chain
a. There may be one or two electron transport systems and one or two
photosystems working together
10. The molecules of the electron transport chain harness the energy from
the flow of electrons and use it to make ATP and another
energy carrier called NADPH
a. A molecule of NADPH carries seven times more energy than an
ATP molecule
11. Since a phosphate is added to ADP (converting it to ATP) using
sunlight the process is called photophosphorylation
12. The energy captured in ATP and NADPH will be used to power
the second phase of photosynthesis, carbon fixation
VII. Carbon fixation (light-independent reactions)
A. During carbon fixation reaction the energy in the ATP and NADPH produced
in the transduction reactions is used to extract carbon from atmospheric (or
dissolved in water) carbon dioxide to make organic compounds, chiefly glucose
B. The reactions are sometimes called the Calvin cycle for
its discoverer. Since the initial product is a three carbon compound it is
also often called the C3 or three carbon pathway
C. Carbon fixation occurs in the stroma or ground substance of the
chloroplast
D. The starting compound is a 5-carbon sugar with 2 phosphates attached
called RuBP (Ribulose 1,5 bisphosphate)
E. Carbon fixation:
1. Carbon dioxide enters through small pores in the leaves called stomates
and enzymes extract and bond the carbon to RuBP forming a 6 carbon compound
2. The 6 carbon compound is quite unstable and immediately hydrolyzed to
two 3 carbon compounds called PGA. This is why it is often
called C3 photosynthesis
3. The PGA is reduced to PGAL
4. Once 6 molecules of PGAL accumulate 5 are used to regenerate RuBP
5. The sixth one is used to form organic compounds
6. Note that each process is powered by energy from the transduction
reactions
F. Although glucose is often represented as the byproduct of
photosynthesis, very little glucose is actually generated. Most of the fixed
carbon is converted either to sucrose (a disaccharide), the
major transport sugar in plants, or starch, the major storage
carbohydrate in plants
TWO VARIATIONS ON PHOTOSYNTHESIS
I. C4 photosynthesis
A. Plants that use only the Calvin cycle are called C3
and the process takes place in the mesophyll of the leaves
B. However is some plants the carbon dioxide is initially fixed into a 4-carbon
compound called oxaloacetate
1. Therefore these plants are called C4
2. The oxaloacetate is then converted to either malate or aspartate,
which is then transported to the bundle sheath cells
3. The malate or aspartate is decarboxylated and
the resulting carbon dioxide enters the Calvin (C3) cycle
C. Why such a seemingly complex system?
1. RUBP
carboxylase/oxygenase works differently depending on the relative
concentrations of carbon dioxide and oxygen:
a. At high carbon dioxide levels it fixes carbon (carboxylase)
b. At low concentrations of carbon dioxide it acts as an oxygenase,
consuming oxygen and releasing carbon dioxide (rather than fixing it)
c. This is called photorespiration
2. Photorespiration is a common phenomenon in C3 plants
a. As much as 50% of the fixed carbon may be reoxidized to
oxygen, making C3 photosynthesis inefficient
b. When it is hot and/or dry they must close their stomates
c. This cuts off the supply of carbon dioxide so photorespiration
increases
3. C4 plants are well adapted to high light intensities, high
temperatures and dry conditions - photorespiration is nearly absent in them
a. This is because the enzyme PEP carboxylase takes the place of the
RUBP carboxylase/oxygenase found in C3 plants
b. C4 grasses have net photosynthetic rates 2-3 greater than
C3 plants.
c. Note the distribution of C4 grasses in North America on
p. 147. There are many more C4 species in the hotter and drier
areas
d. Many of our most important crops are C4 plants, e.g. corn
and sugarcane
e. Crabgrass is also a C4 plant and it tends to overtake
traditional C3 lawn grasses as the summer gets hotter and drier
II. CAM photosynthesis
A. Many succulent plants, e.g. cacti, have a system very similar to C4
plants
B. It is called CAM (crassulacean acid metabolism) because it was
discovered in members of the stone crop family, the Crassulaceae
C. To prevent water loss they open their stomates at night and accumulate
carbon as malate like C4 plants
D. The malate is converted to malic acid and stored until the following day
E. It is then decarboxylated and the carbon enter the Calvin cycle.
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