Wouldn't that be nice! If all living things were able to photosynthesize, nothing would ever have to eat to survive... every living thing could manufacture its own food within its own body. There would be no need for teeth, no need for stomachs.

The fact of the matter is, plants are incredibly special because only plants can do photosynthesis. For this reason, every organism on earth ultimately relies on plants for its survival (with a few exceptions, which I'll discuss later). Even animals that eat only meat, hunting and catching their food as prey, ultimately rely on plants for survival because the prey are eating plants, or are eating insects that are eating plants, or are eating animals that are eating animals that are eating plants, etc.

Only plants can do photosynthesis. Photosynthesis is the process of harnessing energy from sunlight to generate chemical energy, which can be stored and used later. This stored chemical energy comes from the conversion of inorganic carbon (carbon dioxide) into organic carbon, or food. Every living thing needs a source of stored energy for survival, even plants! Think of the stored energy as a battery, which is providing energy not just for muscle movement (which plants obviously don't worry about), but for all of the essential processes of life (cell growth and repair, for example). Living things that can not harness and store energy themselves through photosynthesis (and this includes humans, animals, insects, bacteria, viruses) HAVE to use the energy harnessed and stored by plants to survive, or their battery runs out. Only plants have the ability to recharge the batteries, so living organisms are dependent on plants. The process that releases the energy stored in these batteries is called the Krebs cycle.

You can thing of the Krebs cycle as the opposite of photosynthesis: a process which releases energy by converting organic matter back into carbon dioxide. Do you think plants use the same process (go through the Krebs cycle) to release the energy from their own "batteries"? I like to think of life as a giant cycle of carbon. The carbon begins and ends the cycle in the form of carbon dioxide but goes through many different forms and transformations along the way: photosynthesis converts the carbon dioxide into plant matter, which is passed along the food chain as organic carbon and eventually the Krebs cycle converts this organic carbon (which is now grass or cow or tiger or grasshopper) back into carbon dioxide and the whole thing begins again.

So I said with a VERY small exception, every living thing relies on plant matter for survival. What's the exception? Some special bacteria that live deep in the ocean, far away from sunlight, have evolved the ability to convert carbon dioxide into organic matter (store energy) without using energy from sunlight. What energy are they storing, then? These bacteria store energy from chemical reactions, a process which is less efficient than storing energy from sunlight but since there's no sunlight, it's the only way of harnessing and storing energy (recharging the batteries). This process is not called PHOTOsynthesis (the synthesis of organic matter from light energy) but CHEMOsynthesis (the synthesis of organic matter from chemical energy). These special bacteria are kept as symbionts inside the bodies of deep-sea animals, so some deep-sea communities do not rely on plant matter. Other deep-sea communities do, however. How do you think the plant matter makes it to the bottom of the ocean?

We don't all do photosynthesis, but without it almost nothing would be alive. Without plants using light energy to make food from carbon dioxide, we animals (and the fungi, and some one-celled creatures) would have nothing to eat. Without plants giving off oxygen as a waste, we couldn't break down food to make the high-energy molecules (ATP) that keep our bodies alive. One good clue about whether something does photosynthesis is color. If a living thing is green or blue-green it probably does photosynthesis.

So if plants can get their energy from light, where does their energy come from when it is dark?

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