The Job of Producing
There’s a law of physics that is deceptively simple. The Second Law of Thermodynamics says: “Energy spontaneously disperses from being localized to becoming spread out if it is not hindered from doing so.” Every system degenerates unless energy and effort are put into it. Your house doesn’t get neater unless you clean it. Your workplace doesn’t become more organized unless you make an effort. Living systems must obey the laws of physics too. It takes a constant flow of new energy to keep systems going. From producers to consumers to the decomposers that ultimately break the last bits of organic molecules down to get the last calories from them, every ecosystem is based on an exchange of energy. It doesn’t cycle; it dissipates so we always need more.
Visible light from the sun becomes chemical energy, stored chiefly in the bonds between the carbons. So the plant or single celled organism (which we call a producer) can break the organic molecule apart again to use that same energy when it’s needed, or pass it on to a consumer or a decomposer.
Still on your imaginary trip? Think about the “rules of the game” for the organisms there. Is there a lot of light available, or a little? Is it normally warm or cool? Is the wind a limiting factor? Is water ample or in short supply? Without significant interference from human activity, the organisms in your ecosystem have achieved a “negotiated agreement” on how to share limited resources--especially the energy available to this area.
Photosynthesis isn’t a single process but a complex series of reactions. Many books summarize that series this way:
6H2O + 6CO2 ----------> C6H12O6+ 6O2
But of course, that summary is really “truthiness.” The process begins in a single celled organism or a plant, when visible light is absorbed by a pigment—usually a form of chlorophyll. Chlorophyll is a molecule made of carbons, hydrogens and oxygens arranged around a single, electronegative atom of magnesium. In sunlight, chlorophyll is charged—much like a solar cell—and the energy is used to power a series of reactions that forms chains of carbons (organic molecules) that we often just call “sugars.”
In most land systems, we can find one of three types of photosynthesis:
v C3 photosynthesis builds a 3-carbon sugar first. It’s most efficient when it’s cool and there is plenty of water. That’s what most plant do.
v C4 photosynthesis builds a 4-carbon sugar first. It requires more enzyme steps, but works well at high temperatures and light. (Example: corn, crabgrass, sugar cane)
v CAM photosynthesis is an adaptation for very low water conditions. The first product is an acid. (Example: bromeliads)
Photosynthesis is far more ancient than plants. Life probably began on Earth 3.8 billion years ago, and by 2.5 billion years ago (what we call the end of the Archaean era) there were very sophisticated metabolic pathways in the one-celled organisms on Earth. We know that at least some of those organisms could make organic molecules using light energy in a simple process of photosynthesis. How do we know? Because the rocks from that time contain “biomarkers” that are relatively simple sugar-like organic molecules called 2-methylhopanes, oil, and other organics. Some time later, there is evidence that free oxygen was being produced. (It was millions of years before that oxygen could actually enter the atmosphere, because it was captured by atoms like iron.) Once the atmosphere began to change, and the carbon dioxide began to be quickly removed, the climate changed and the Earth became much cooler in a reverse of what we call global warming. In plants, photosynthesis consists of a complex series of reactions divided into:
Light-dependent reactions, where chlorophyll absorbs energy from sunlight and the energy is transferred along the outer membrane of an organelle called a chloroplast. In this phase water molecules are broken down and oxygen molecules are released.
In the light-independent reactions (which occur in the internal stroma of chloroplasts) carbon dioxide is added to build larger organic molecules
Many tropical plants use the C4 pathway for photosynthesis. (It’s named because the first carbohydrate formed is a 4-carbon compound rather than a 3-carbon compound produced by the Calvin cycle used by most temperate plants.) This pathway requires more energy (in abundant supply in the tropics), high concentrations of water and carbon dioxide, and produces more stored energy in the end. C4 plants often have a characteristic anatomy; there are two rings of cells around the veins called “bundle sheath cells” which contain starch-rich chloroplasts. This so-called “Kranz anatomy” which is common in the leaves of the tropical rainforest can sometimes be spotted in the dark green veins of tropical plants. You can tell by looking at the rich chloroplasts around each vein that they are metabolically suited for a high energy environment.How much product can a producer produce?
Image source for the Absorption, Reflectance in Plants: USGS http://landsat.usgs.gov/resources/remote_sensing/radiation.php