How to Understand Photosynthesis

Billions of years ago, single-celled algae and bacteria figured out a way to harvest energy from sunlight, a key innovation in the eventual propagation of life around the globe. Here’s a quick overview of how photosynthesis evolved, how it works, and how it has changed the face of our planet.

Instructions

Difficulty: Easy

1. Before photosynthesis, single-celled organisms depended on organic molecules. Three and one-half billion years ago (give or take a few hundred million years), primitive bacteria thrived by soaking up the organic residues floating in the Earth’s oceans. Like oil today, once these nutrients were gone, they were gone for good—creating evolutionary pressure for new means of finding sustenance.
2. Primitive bacteria evolved photosynthetic mechanisms. One thing the ancient Earth had in abundance was sunlight. Scientists think the earliest photosynthetic organisms used the elements at hand (such as sulfur) as the motors of primitive photosynthetic assembly lines, in which the energy of an incoming ray of sunlight is stored, processed and used to convert organic molecules into food.
3. Plants rely on organelles called “chloroplasts.” In the evolutionary sweepstakes, it turned out that the complex molecule chlorophyll was best suited to harvesting sunlight (and certainly better suited than sulfur). Billions of years ago, primitive cells that used chlorophyll were co-opted by plant cells (in much the same way that mitochondria, the engines of oxygen respiration, were co-opted by animal cells). Today, all the cells of photosynthesizing plants contain chloroplasts.
4. Photosynthesis consumes carbon dioxide and produces oxygen. One of the reasons deforestation is such an important issue is that plants and trees remove carbon dioxide from the atmosphere—thus slowing global warming. But photosynthesis has another profound effect: billions of years ago, the worldwide spread of photosynthetic organisms created huge levels of oxygen in the atmosphere, paving the way for the evolution of oxygen-breathing creatures (like us).
5. Photosynthesis drives the process of carbon and nitrogen fixation. When a photosynthesizing plant removes a molecule of carbon dioxide from the air, it breaks off the carbon atom and “fixes” it in an organic molecule (which it stores as food, usually in the form of starch). Although not only photosynthetic organisms fix nitrogen, and not all plants do, this process is crucial for maintaining the fertility of soil. Because nitrogen molecules are joined by triple electron bonds, they’re especially difficult to break, and this is another benefit of the endless power of sunlight.