Episode 145 – Photosynthesis

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We live in a solar-powered world. This episode, we discuss the process that provides the energy that every organism on the planet relies on, as well as the oxygen that characterizes the atmosphere that allows life as we know it to thrive: Photosynthesis.

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Solar-Powered Life

Photosynthesis is a chemical reaction that takes place in the cells of certain organisms. Broadly speaking, it’s pretty straightforward: photosynthesizers take in carbon dioxide and water from the environment and harness the energy of sunlight to convert those substances into sugars (food), releasing oxygen as a waste product.

It’s as simple as that.
Until you look closer, then it becomes very complicated. Image: At09kg, Wattcle, Nefronus, CC BY-SA 4.0

If you happen to be an organism that eats food and breathes oxygen, you can see why photosynthesis is so important. It’s an incredible innovation of evolution that takes in energy from our cosmic companion (the sun) and makes it available to nearly every living thing on Earth.

Photosynthesizers come in many forms and even many colors.
Top left: Cyanobacteria. Image: Argonne National Laboratory, CC BY-NC-SA 2.0
Top right: Green algae. Image: James St. John, CC BY 2.0
Bottom left: Red and brown algae. Image: John Turnbull, CC BY-NC-SA 2.0
Bottom right: Our most familiar photosynthesizing organisms, plants. Image: Public Domain.

There are, in fact, multiple flavors of photosynthesis. Oxygenic photosynthesis is the classic version, but anoxygenic photosynthesis, typically seen in certain bacteria, uses compounds other than water (such as hydrogen sulfide) and doesn’t produce oxygen. The most common version of oxygen photosynthesis is called C3 photosynthesis, named for the three-Carbon compounds important in the process, but there are other strategies: some plants, notably grasses, use C4 photosynthesis, a version that expends more energy for a more efficient metabolism; and others, notably plants in hot, dry habitats, use CAM photosynthesis, a strategy that allows them to take in gases at night and perform sun-powered reactions during the day.

Green Strategy

Within the cells of photosynthesizers, the molecules responsible for harnessing light are pigment molecules, the most famous of which is chlorophyll, the same pigment that gives most photosynthesizers their characteristic green color. But not all photosynthesizers are green! Different colors can come from different pigments used for light-catching, or from other color-producing features in the same organism. In plants, most photosynthesis takes place in the chlorophyll-containing structures (chloroplasts) of the leaves.

Chloroplasts are the centers of photosynthesis in plants and algae. Image: Kelvinsong, CC BY-SA 3.0

The oldest evidence for photosynthesis is about the same age as the oldest evidence of life on Earth, nearly 4 billion years old. The earliest well-known fossils, structures called stromatolites, were formed by mats of photosynthesizing bacteria. This process is about as old as life itself.

Photosynthesis seems to have originated in bacteria, and eukaryotes (including all plants and algae) are thought to have inherited this feature through endosymbiosis, when early eukaryote precursors developed a symbiotic relationship with bacteria inside them. Those internal bacteria gave rise to chloroplasts. Image: Kelvinsong, CC BY-SA 3.0

Absorb More

What is Photosynthesis?

The paleobiological record of photosynthesis (technical, open access)

If you enjoyed this topic and want more like it, check out these related episodes:

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