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PHOTOSYNTHESIS. Photosynthetic Carbon Fixation removes tons of CO 2 from the atmosphere every year. The greenhouse effect , contributed to by CO 2 , has some beneficial effects, the problem lies in the recent rate of increase in CO 2 emissions from fossil fuel consumption.
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Photosynthetic Carbon Fixation removes tons of CO2 from the atmosphere every year.
The greenhouse effect, contributed to by CO2, has some beneficial effects, the problem lies in the recent rate of increase in CO2 emissions from fossil fuel consumption.
The formula for photosynthesis is, in a sense, the opposite of that for aerobic respiration.
A Brief overview of the structures involved in Photosynthesis : where the chloroplasts live.
An actual dicot leaf cross section micrograph. The upper and lower-most epidermal layers are essentially composed of transparent “window” cells with several layers of green photosynthetic cells in between.Cells in the lower layers are loosely connected with the air spaces connected through stomatal openings (bottom center) to the outside.The circular groups of cells in the center are cross sections of small vascular bundles (leaf “veins”) that contain xylem (deliver water) and phloem (take sugar away) cells.
Stomatal opening is controlled by two guard cells that swell when illuminated and fully hydrated (left) causing the stomate to open. Darkness or low water content cause the guard cells to relax (right) closing the opening and limiting CO2 - O2 gas exchange and water flow (transpiration) powered by evaporation.
Each vascular bundle (see detail) has an outer grouping of phloem cells that carry sugar away from the leaves and an inner grouping of xylem cells that carry water up to the leaves. The branching“veins” of a leaf are the continuation of the vasculature in the leaf. phloem The plant’s vascular system conducts water from the roots to the leaves. xylem
In the photosystems ( II & I) energy is funneled to a primary reaction center where a high energy electron is created and captured in a high energy state before it can “decay” by releasing its energy as longer wavelength (red) light.
Slight structural differences distinguish chlorophyll a & b which are slightly different shades of green. The large porphyrin ring structure holds a magnesium atom (green) in the center complexed to four nitrogen atoms (blue). Pigment molecules. Carotenoids, such as the b-carotene shown above, are orange to yellow pigments that capture light in the blue -to- near uv end of the spectrum.
Chlorophyll released from the other components of a photosystem can generate high energy electons when illuminated with white light, but they quickly decay re-emitting red light similar to the yellow chemical fluorescence emitted by a glow stick.
An elegant experiment showing that oxygen is evolved from water only by light absorption in the red and blue ends of the visible spectrum. An algal filament illuminated by white light separated by a prism attracts oxygen-seeking bacteria only to those cells receiving red or blue light.
Jack Frost at work. As the days shorten, leaves are “programmed” to slowly recycle their contents back to the tree and then drop off. As the chlorophyll fades the remaining yellow-orange carotenoid pigments show through and may be joined by new red anthocyanin pigments synthesized in response to cold.
The guys in yellow represent light energy input. The “water wheel” shows how electron energy is used to pump hydrogen ions across the thylakoid membrane, creating a H+ gradient that then is used to make ATP.Finally the electron is re-energized and used to load hydrogen onto an NADP “pickup truck” for delivery to the Calvin cycle. A light reaction analogy.
A slightly more realistic representation of the Light Reaction with all of the components (compare to the previous slide).
3-PGA is 3-phosphoglyceric acid; it becomes G3P, glyceraldehyde-3-phosphate, when H is added from NADPH. One of these G3Ps (or PGAL) is then removed and the remaining 5 G3Ps (= 15 carbons) are re-arranged back into the three 5C pick-up molecules, RuBP, ribulose-bis-phosphate (a 5C sugar). The re-arrangement steps (step 4 on the left) are amazingly complicated and are (thankfully) not shown in detail here. The RuBP + CO2pick-up step is catalyzed by the enzyme RUBISCO = ribulose-bis-phosphate carboxylase. Melvin (no lie, that’s his name) Calvin won the Nobel Prize years back for figuring this out, although his post-doc Benson is rumored to have done much of the work (such is life).
One last point: remember how stomata control both transpiration and gas exchange? So, in hot dry weather, many plants have a problem
Low water content causes the guard cells to relax (right) closing the opening and limiting CO2 - O2 gas exchange and water evaporation. Unfortunately the Rubisco enzyme has difficulty distinguishing between CO2 and O2 when the former is low and the later is high, so it begins to join O2 onto RubP, leading to photorespiration and a loss of ATP. As a result the plant must go dormant, or die.
The standard photosynthesis process is termed C3 photosynthesis because 3-PGA, the first stable Calvin cycle intermediate, is a 3C compound. The C3 process RUBISCO pick-up enzyme, however, gets “confused” when O2 inside the leaf is high and CO2 is low causing a wasteful process called photorespiration to occur instead of the standard Calvin cycle events. This happens when leaf stomates are closed under hot dry conditions. The result is that C3 plants go dormant (think the grass in your lawn) during a summer drought. C4 and CAM plants have a different enzyme (not RUBISCO) that loads CO2 onto a 3C molecule to make a 4C product. These enzymes work well even when O2 is high in the leaf and CO2 is low. C4 plants, like crabgrass and corn, can grow well even when it’s hot and dry, as the C4 pick up enzyme creates a 4C acid in some cells that is then transported to other cells where C is transferred to the normal Calvin cycle. CAM plants, like cacti, keep their stomates open only at night and pick up CO2 to create 4C acids (using ATP made the previous day) that are stored until the next day when they are then fed into the Calvin cycle, even though the cactus stomates are now closed.
Plant Genetic Engineering - Is misunderstood by many which has lead to much misinformation being circulated about the risks and benefits involved.
Plants have been selectively modified using artificial selection (cross breeding, hybridizing, and selection) since agriculture began. Genetic engineering (GE) expands this process to include gene transfer from other species, even across Kingdoms. Plants are modified to resist pests, grow better, or produce crops that are of higher quality than before. Shown below “Golden rice”plants produce β-carotene (provitamin A) not only in green tissues (the leaves) but also in the seed endosperm. Vitamin A deficiency causes childhood blindness in societies that consume high quantities of normal rice.
GE plants have been created to produce crops with a greater shelf life (tomatoes), be resistant to herbicides (cotton), and to create their own insecticides. This last objective has been achieved in several plant species, field corn and cotton to name two. Corn earworm (a moth larva) is one insect controlled in this manner using a protein toxin gene from Bacillus thuringiensis (Bt toxin).
GE plants are possible because scientists were able to combine discoveries from different areas of biology, some of which originally seemed to have no obvious practical application. Bacterial plasmids, small circles of extra-chromosomal DNA, were discovered in experiments exploring bacterial drug resistance. These DNAs contain genes, that can produce proteins, just like those in chromosomes. They are also easy to remove, manipulate, and then re-insert into their bacterial hosts. Someone realized they could be used as”vectors” to move foreign genes into bacteria which would then be able to replicate them and produce the foreign gene products as if they were their own.
Bacterial “restriction” enzymes were discovered that cut DNA internally, but only at particular recognition sites. Some create a staggered cut that always leaves the same “sticky ends” making it easy to join different DNAs cut with the same enzyme together. These became a tool that “gene jockeys” use to isolate genes and place them into plasmids for cloning.
The DNA donor cell can be animal, plant, or whatever. The process shown here allows a desired human protein to be produced by engineered bacteria so that it can be purified and used as a medicine.
Plant tumors, or crown galls, were found to be produced by a bacterium that transfers a plasmid (the Ti plasmid) to the host plant cells. Plasmid genes cause the cells so transformed to grow out of control; as they do so, they produce several odd amino acids that only Agrobacterium can use as food. Modified Ti plasmids with the disease-producing genes removed can be used as vectors to move desirable foreign genes into plants. The last piece of the plant GE puzzle is provided by Agrobacterium tumefaciens, the Crown Gall bacterium.
If the foreign gene codes for the insect toxin from Bacillus thuringiensis, the plants would now be able to make their own protein insecticide (Bt toxin).
