Atoms Alive: Cells and energy

REPORTER:

A cell is a self-sustaining chemical factory. It needs energy, and, to get it, it needs fuel. Stentor's energy comes from the organic molecules it digests from its prey, reacted with oxygen, diffused in from the surrounding water. Another approach is photosynthesis. This diatom has green chloroplasts, which are capable of using the energy of light to build organic molecules directly from carbon dioxide and water. Photosynthesis also releases oxygen, bubbles of which can be seen coming off this algae.

The sum of all cell processes is called 'metabolism'. Metabolism drives the cell and the cell drives the organism, no matter how small or big. Metabolic activity divides into two categories — catabolism and anabolism. Catabolism is the process of breaking large molecules down into smaller ones, which releases energy for activity.

In eukaryotes, this means the controlled burning of carbon compounds with oxygen. Anabolism is the opposite process of building more complex molecules from simpler ones, and this uses up energy. For example, the activity of growing — of creating new cells to become more bone, muscles, and all the other parts of a multicellular body — requires energy to drive this anabolic process. Of course, the most important anabolic process of all is considered to be photosynthesis. Since the energy needed to make the complex molecules produced by photosynthesis comes free from space, this process is regarded as the ultimate source of energy for almost all life on Earth. In this sense, those little green chloroplasts are the most important organelles of all.