Enzymes are proteins that speed along the chemical reactions necessary for all life on Earth -- plants as well as animals -- and many of the enzymes are the same. One of the jobs of enzymes is to process food and convert it into a form that can provide energy for the organism. Animals get their food from what they eat, but plants get their food in a process called photosynthesis -- a process in which enzymes play key roles.
Every chemical reaction requires a little kick of energy to get started. That energy is called the activation energy. Enzymes bring together molecules -- called substrates -- and lower the activation energy of chemical reactions, creating chemical products. Enzymes increase the rate of chemical reactions by as much as many millions of times. Enzymes catalyze, or promote, chemical reactions, and they are not used up in the chemical reaction. That is, once the products have been released from the enzyme, it's ready to accept new substrates and start again.
Photosynthesis is a series of chemical and physical interactions that uses energy from light to convert six carbon dioxide molecules and 12 water molecules into one molecule of glucose, six water molecules and six oxygen molecules. The energy harvesting steps are light-dependent, while the steps that build the glucose molecule are light-independent -- that is, they can even happen in the dark. The entire process involves several enzymes, but two are especially important: ATPase and Rubisco.
The first stage of the light-dependent reaction occurs when a photon -- a tiny packet of light energy -- is absorbed by chlorophyll. The energy is transferred along a chain of neighboring molecules until it jumps to another molecule. Along the way, the electron energy moves some protons from one side of a membrane to another.
As the captured light energy is used to transfer protons to one side of a membrane within the plant cell, the extra protons create an electric field across the membrane. The force of the electric field pushes the protons back to the other side of the membrane, but there's only one way across: through an enzyme called ATPase. ATPase is built right into the membrane, like a turnstyle that lets one proton through at a time as it rotates. The rotation adds energy to a molecule of adenosine diphosphate, by converting the adenosine diphosphate, ADP, to adenosine triphosphate, ATP. ATP is like a battery for the cell. It contains energy that the cell can use to do other things.
The "synthesis" of "photosynthesis" is the building of a sugar molecule from carbon dioxide, and that part of the process is done by another enzyme: ribulose bisphosphate carboxylase, often abbreviated as rubisco. Rubisco combines three five-carbon sugar molecules with three carbon dioxide molecules -- which the plant extracts from the air -- to make three six-carbon molecules. Several molecules of ATP give their energy to the next steps, which convert the rubisco products to glucose and other molecules and another set of five-carbon sugars to start all over again. This process is called the Calvin cycle. Rubisco works slowly, as enzymes go, creating only about three glucose molecules a second, so plants need a lot of rubisco. Some scientists believe rubisco is the most abundant protein on Earth.
