Most people know that plants use the process of photosynthesis to take the sun’s energy and convert carbon dioxide and water into sugar (glucose). This is an amazing process which makes life of earth possible. Photosynthesis provides the food we eat, the oxygen we breath and the gasoline in our cars.
The process of photosynthesis involves two parts. The first part is called the light reaction and in this part of the process, light is used to create chemical energy and water molecules are broken down and oxygen gas is released by the plant.
The second step is the dark reaction and here the chemical energy from the light reaction is used along with carbon dioxide to create sugars. The pathway in which these sugars are created is called the Calvin cycle, a series of biochemical steps discovered by Melvin Calvin who won the Nobel prize for his research. I won’t get into all of the details of the Calvin cycle but if you’re interested, you can find it described in any good botany textbook. Wikipedia also have a pretty good description of the Calvin cycle.
In the beginning of the Calvin cycle, the molecule that fixes CO2 splits into two three-carbon molecules. Plants that use this method of fixing carbon are called C3 plants. Most of the plants in the average temperate garden are C3 plants.
But this wouldn’t be botany if there weren’t exceptions! Other plants use a different approach. CO2 if first combined with a three-carbon molecule to produce a four-carbon compound. This compound is moved to special cells in the leaf where the CO2 is removed and starts the Calvin cycle like a C3 plant. Since the first carbon fixation forms a four-carbon compound, these plants are called C4 plants. Some C4 plants are corn, crabgrass and redroot pigweed. (A variant of these plants use the same biochemical system but it occurs in a different location within the plant. These are called CAM (crassulacean acid metabolism) plants and include succulents and cacti.)
It initially seems strange that plants would develop this additional, even cumbersome, step in the process of carbon fixation. But in plants, there’s another process that’s going on simultaneously with photosynthesis and that’s photorespiration. This process produces no energy for the plant and causes over half of the carbon fixed by a plant to be released back into the air as CO2. In C3 plants, the efficiency of photosynthesis is reduced by up to 1/2 because of photorespiration.
C4 plants also have photorespiration but due to their structure and where the Calvin cycle occurs within the plant (or when it occurs for CAM plants), they are much more efficient in fixing carbon than are C3 plants. They are especially efficient in high temperatures and intense light – two conditions that increase photorespiration in C3 plants. C4 plants grow best in the tropics or during the heat of summer – that’s why corn, pigweed and cacti use this system.
This difference in photosynthesis might seem a little esoteric but it helps to explain why some plants do better in the heat of summer. You can see this in the case of weeds. In the spring and fall, C3 weeds like lambsquarter and dandelion grow well. When summer is in full swing, the photosynthetic inefficiency of C3 plants begins to slow their growth. But in these conditions of intense light and heat, C4 weeds like crabgrass, purslane and pigweed grow their best. This also explains why planting corn too early in the season isn’t a good idea – this C4 plant does its best in July and August.
I find the whole process of photosynthesis fascinating and different biochemical pathways that have evolved to help plants thrive in different conditions is amazing. You don’t need to know about the C3 and C4 methods of carbon fixation to grow a garden. But when you do know about them, it makes the plants that are growing and thriving even more amazing – at least for me it does!