Page 645 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

From the equation, you can see that photosynthe- sis removes carbon (in the form of CO2) from Earth’s atmosphere. The quantity is enormous: approximately 91 billion tonnes of CO2 per year. Carbohydrates, the or- ganic result of the photosynthetic process, are combina- tions of carbon, hydrogen, and oxygen. The simple sugar glucose (C6H12O6) is an example. Plants use glucose to build starches, which are more-complex carbohydrates and the principal food stored in plants.
Plants store energy (in the bonds within carbohy- drates) for later use. They consume this energy as needed through respiration, which converts the carbohydrates to energy for their other operations. Thus, respiration is es- sentially a reverse of the photosynthetic process:
C6H12O6 + 6O2 S 6CO2 + 6H2O + energy
Chapter 19 ecosystem essentials 609 are under study at a number of Free-Air CO2 Enrichment
(FACE) facilities in the United States. This research shows that fumigating natural and agricultural ecosystems with elevated concentrations of CO2 stimulates the photosyn- thetic processes, increasing plant growth, but also ap- pears to have some complex and varied side effects. At the same time, the presence of ground-level ozone (O3), a toxic gas (discussed in Chapter 3) that is increasing in the lower atmosphere, decreases plant growth, offsetting the effects of increased CO2 when the two occur together in the atmosphere. (For more information, see aspenface .mtu.edu/ or climatechangescience.ornl.gov/content/free-air-co2- enrichment-face-experiment.)
Net Primary Productivity The net photosynthesis for an entire ecosystem is its net primary productivity. This is the amount of stored chemical energy that the ecosystem generates. The total organic matter (living and recently living, both animal and plant) in an ecosystem, with its associated chemical energy, is the ecosystem’s biomass and is often measured as the net dry weight of all organic material. Net primary productivity is an important as- pect of any type of ecosystem because it determines the biomass available for consumption by heterotrophs, or consumers—organisms that feed on others. The distribu- tion of productivity over Earth’s surface is an important aspect of biogeography.
Net primary productivity is measured as fixed car- bon per square metre per year. The map in Figure 19.4 shows that on land, net primary production tends to be highest between the Tropics of Cancer and Capricorn at sea level and decreases toward higher latitudes and elevations. Productivity levels are tied to both sunlight and precipitation, as evidenced by the correlations of
(glucose, (oxygen) (carbon (water) carbohydrate) dioxide)
(heat energy)
In respiration, plants oxidize carbohydrates (break them down through reaction with oxygen), releasing CO2, water, and energy as heat. The difference between pho- tosynthetic production of carbohydrates and respiration loss of carbohydrates is net photosynthesis. The overall growth of a plant depends on the amount of net photo- synthesis, a surplus of carbohydrates beyond those lost through plant respiration.
The compensation point is the break-even point be- tween the production and consumption of organic mate- rial. Each leaf must operate on the production side of the compensation point, or else the plant eliminates it—you may have observed a plant shedding leaves if it receives inadequate water or light.
The effects of today’s rapidly increasing atmospheric CO2 concentrations on plant photosynthesis and growth
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▲Figure 19.4 Net primary productivity. Worldwide net primary productivity in grams of carbon per square metre per year (approximate values). [adapted from D. e. reichle, Analysis of Temperate Forest Ecosystems (Heidelberg, germany: Springer, 1970).]
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