Dark Reactions Of Photosynthesis:
The alkaloid in tobacco responsible for its highly addictive properties is nicotine, a mild stimulant of the central nervous system. In its pure form, nicotine is highly poisonous and is used as an insecticide. Nicotine is derived from nicotinic acid, a B-vitamin also known as niacin. Niacin prevents pellagra, a disease characterized by severe damage to the tongue, skin and digestive tract. Nicotinic acid is also converted into nicotinamide, a precursor of nicotinamide adenine dinucleotide phosphate (NADP), a vital coenzyme required for photosynthesis. [NAD is another vital coenzyme that carries electrons to the electron transport system in mitochondria.] During the light reactions of photosynthesis, NADP picks up two hydrogen atoms from water molecules forming NADPH2, a powerful reducing agent that is used to convert carbon dioxide into glucose during the dark reactions of photosynthesis (also called the Calvin Cycle). When the two atoms of hydrogen join with NADP, oxygen is liberated, and this is the source of oxygen gas in our atmosphere. The following equation shows the overall reactants and products of photosynthesis:
6 CO2 + 6 H2O (ATP & NADPH2 from Light Reactions) = C6H12O6 + 6 O2
Note: The oxygen liberated during the light reactions comes from water.
Some plants adapted to hot, arid regions have a different
photosynthetic mechanism called CAM photosynthesis. CAM
(Crassulacean Acid Metabolism) photosynthesis is also found in
cacti and succulents, including the crassula family (Crassulaceae).
During the hot daylight hours their stomata are tightly closed;
however they still carry on vital photosynthesis as carbon dioxide
gas is converted into simple sugars. During the cooler hours of
darkness their stomata are open and CO2 enters the leaf cells where
it combines with PEP (phosphoenolpyruvate) to form 4-carbon organic
acids (malic and isocitric acids). The 4-carbon acids are stored in
the vacuoles of photosynthetic cells in the leaf. During the
daylight hours the 4-carbon acids break down releasing CO2 for the
dark reactions (Calvin cycle) of photosynthesis inside the stroma
of chloroplasts. The CO2 is converted into glucose through a series
of complicated reactions involving ATP (adenosine triphosphate) and
NADPH2 (nicotinamide adenine dinucleotide phosphate), the latter
two compounds which were synthesized during the light reactions of
daylight in the grana of chloroplasts. The adaptive advantage of
CAM photosynthesis is that plants in arid regions can keep their
stomata closed during the daytime, thereby reducing water loss from
the leaves through transpiration; however, they can still carry on
photosynthesis with a reserve supply of CO2 that was trapped during
the hours of darkness when the stomata were open.
Another interesting modification of the photosynthetic pathway is called C-4 Photosynthesis. During C-4 photosynthesis, CO2 combines with phosphoenolpyruvate (PEP) to form a 4-carbon organic acid (oxaloacetic acid) which migrates (diffuses) to the photosynthetic bundle sheath cells surrounding the vascular bundles (veins) of the leaf. PEP essentially shuttles the CO2 to the bundle sheath cells where it is released for the dark reactions (Calvin cycle) of photosynthesis. During hot weather the CO2 level inside leaves is greatly reduced because the leaf stomata are closed. In ordinary C-3 plants which form a 3-carbon compound (PGA) during the initial steps of the dark reactions, photosynthesis in the leaf shuts down without a sufficient supply of CO2. C-4 plants have a competitive advantage during hot summer days because they are able to carry on photosynthesis in the bundle sheaths where CO2 levels are concentrated. C-4 plants such as Bermuda grass and purslane grow rapidly during hot summer days, while photosynthesis and growth in C-3 plants shuts down.
Early explorers in the New World encountered native people in the Caribbean region smoking tobacco. Tobacco seeds were brought back to Europe by 1558. Sir Walter Raleigh planted tobacco plants and potatoes on his estate in Ireland in 1586. The famous taxonomist Carolus Linnaeus named the tobacco plant Nicotiana, in honor of the French ambassador Jean Nicot who distributed the seeds. The most carcinogenic (cancer causing) effect of habitual tobacco smoking is from the inhalation of tars produced from the burning leaves. This applies to the tobacco smoker and the other unfortunate souls breathing second hand smoke. Curing of tobacco leaves involves a series of complex biochemical events, including fermentation, browning, and the conversion of starches into sugars. During the curing process the water content is reduced from 80 percent to 20 percent. Dried tobacco leaves are used for cigarettes and cigars, and for pipe and chewing tobaccos. Tobaccos often contain other ingredients which enhance the flavor and aroma, including sugars, licorice, rum and menthol. Some of these compounds, such as coumarin, have been shown to be carcinogenic when burned. Most cultivated tobacco plants are tetraploids with four haploid sets of chromosomes. Polyploid plants (with extra sets of chromosomes) often have larger leaves, flowers and fruits, so the advantage of polyploid tobacco plants is obvious.