Craig and I have arrived in Tucson, where we'll spend the month of February. In an attempt to feel more productive, Craig's reading an Organic Chem textbook, and I am learning stuff too. When we're not reduced to making funny noises and chasing Fatty around in circle, we like to talk about science. Wandering around Joshua Tree a few days ago, we started talking about photosynthesis. Desert plants practice a special kind of photosynthesis, called CAM, that allows them to use lose less water (a precious resource) at the expense of slower growth.
I had to review this, biochemistry not being my natural forte. All photosynthetic plants use carbon dioxide and water to produce sugar, with oxygen as a byproduct. Most plants practice ordinary C3 photosynthesis, which is most efficient when water and carbon dioxide are in high supply. C3 photosynthesis involves the carbon dioxide being converted into a 3-carbon chain, with the aid of the enzyme rubisco. Those 3-carbon chains will in turn be converted into glucose, the main sugar product of photosynthesis. Side note: rubisco is arguably the most abundant enzyme (and protein) in the world. Wooo...
Rubisco, as ubiquitous as it is, is also the enzyme catalyst for photorespiration, which grabs oxygen, instead of carbon dioxide, and releases carbon dioxide as a byproduct. Photorespiration is considered "wasteful" in a plant, because it uses more "energy" - or more specifically, ATP and NADPH - the energetic molecules of the cell. Ordinarily, photorespiration is not a problem for plants. The carbon dioxide concentration of mild climates is high enough that rubisco acts primarily as the photosynthetic catalyst.
In hot climates, the ratio of oxygen to carbon dioxide is higher, because oxygen is more soluble at higher temperatures. That makes plants growing in hot climates more susceptible to wasteful photorespiration. In order to protect themselves from photorespiration, plants that thrive in hot climates - like the tropics and the desert - have developed C4 photosynthesis. C4 photosynthesis has developed a way to increase the concentration of carbon dioxide inside the plant tissue, in order to prevent oxygen from binding to rubisco.
Instead of converting carbon dioxide into a 3-carbon chain intermediate, like in C3 photosynthesis, corn and sugarcane (and other tropical plants) convert carbon dioxide into a 4-carbon chain intermediate by a different enzyme - PEP carboxylase. This 4-carbon chain is used to transport the carbon into more-interior cells, farther away from the oxygen supply, where it can be broken down into a 3-carbon chain and safely converted into glucose, using rubisco. The downside of C4 photosynthesis is that it uses more energy (ATP) than C3 photosynthesis.
Desert plants, including most succulents and cacti, use a third type of photosynthesis to create their food - the CAM pathway. The CAM pathway is really a specialization of C4 photosynthesis, allowing plants in hot and extremely dry climates to create glucose without photorespiring (when rubisco grabs oxygen instead of carbon dioxide) or losing water. Instead of separating the conversion of carbon dioxide into a C4 chain and the conversion of the C4 chain into glucose into two different CELLS, these processes are separated into night and daytime reactions. At night, when temperatures and evaporation (water loss) are low, cacti open their stomata (openings on plant leaves that are closed by kidney-shaped "guard cells" bloating to close the space between them). Carbon dioxide is let into the plant cells through these stomata, where it is converted into a C4 chain, using PEP carboxylase. Throughout the night, the C4 chains are stored in vacuoles (cell storage units).
During the day, the stomata are closed, preventing carbon dioxide, oxygen, or water from entering or leaving the plant. The carbon from the 4-carbon chains can only be converted into glucose when it is light outside. Cacti prevent photorespiration by not allowing oxygen into the cells while rubisco is helping to produce glucose. Additionally, cacti don't lose precious water during the extremely hot days in the desert, when evaporation can be very high. As a tradeoff, plants using CAM photosynthesis tend to grow more slowly than C3 plants in milder climates.
