An ecosystem needs energy to sustain itself; energy is continually flowing within these systems (Figure 2.1). As with all flows of energy, ecosystems are ruled by the basic laws of thermodynamics.
The first law of thermodynamics states, quite simply: Energy can be changed from one form to another but cannot be created or destroyed. An example of this law is the flow of biological and cellular energy (Figure 2.2). The radiant energy of sunlight is produced by the fusion reactions taking place in the sun. Chloroplasts, present in all photosynthetic eukaryote cells, capture this energy and use it to convert water and carbon dioxide into carbohydrates, such as sucrose, starch and other nutritive molecules. Oxygen is released into the air as a product of these photosynthetic reactions. Mitochondria organelles present in all eukaryotic cells, break down these carbohydrates and transform their stored energy into ATP molecules. This process, called cellular respiration, consumes oxygen and produces carbon dioxide and water, completing the cycle. 
In a biological context, the second law of thermodynamics becomes more tangible. The second law of thermodynamics states: In all energy changes and conversions, if no energy leaves or enters the system under study, the potential energy of the final state will always be less than the potential energy of the initial state (Figure 2.3). This energy, called entropy, gets dissipated as it moves up through a natural system. The energy that is no longer in the final product has been used by the reaction, most commonly being converted to and released as heat. Temperature changes are a common by-product of chemical reactions, this places the energy in an inaccessible form for plants. All of this is important when dealing with biological processes such as oxidation and reduction, and metabolic and enzymatic processes. 
There are two types of photosynthesis which are important to understanding how grasses grow. Photosynthesis is a common process among green plants of processing carbon from carbon dioxide in the presence of chlorophyll (a green light energy receptor), using the energy (radiation) of sunlight. This is the process by which the sugars and starches necessary for respiration and plant processes are produced within the plant.
One process is called C3 (Carbon Three) photosynthesis because the first detectable molecule produced from the processing of the sun's radiation is phosphoglycerate, a compound with three carbon atoms. There are also 15 families of plants which use a more complex system of photosynthesis; a process called C4 (Carbon Four) photosynthesis (Figure 2.5). C4 photosynthesis is named so because the first detectable molecule has four carbon atoms. C4 grasses perform better during the warmer seasons, need less water, less nitrogen, and perform poorly in soils polluted by high levels of nitrogen.
A third method of photosynthesis is CAM (Crassulacean acid metabolism) which is common in desert plants (xerophytes). However, this type of photosynthesis rarely occurs outside of desert plants and is not described further here. Overall, different leaf anatomies, biochemical pathways and ecological distributions fall collectively into well defined groups defined by the type of photosynthesis that they use.
Grasses that have evolved in warmer climates have tended to be C4 plants. Given this, it is reasonable to expect that warm season grasses are all C4 grasses, where as cool season grasses are all C3 grasses (Chapman, 1996). Of the grasses grown for this project the only exception is Little Bluestem, a C3 warm season grass. C3 photosynthesis is thought to be the earlier derivation of the evolution of the photosynthetic process, so C3 grasses are considered to be older grasses, which suggests that grasslands first evolved in the cool climates and then spread to the warmer zones.
