Read Aloud: Photosynthesis

By Charles Wohlforth

Excepted from Frommer’s Family Vacations in the National Parks (Wiley)

All the world’s plants, trees and seaweed, in the forests, in deserts, or floating in the ocean, from the tallest redwood to the tiniest spec of pond scum that you can see only with a microscope--all work the same way. They all use photosynthesis to catch the energy in sunlight and turn it into a solid form. Only plants and algae (which are like plants of the sea) can make light into food, but all animals need that food to live. Plants feed us, and they make the oxygen we breathe. Without photosynthesis, there would be no life on Earth.

Almost everything around us is made of atoms of a few basic elements, like oxygen, carbon, and hydrogen. They stick together to make molecules, like water or sugar. To glue atoms together into some kinds of molecules, you have to add energy, such as heat from the sun. When those molecules come apart again, dividing into atoms or smaller molecules, the energy that went into sticking them together comes out again. A corn plant, for example, takes the energy of the sun to make the sugar molecules in an ear of sweet corn. When you eat the ear of corn, your body breaks those molecules apart to get the energy back out[md]that’s the energy that lets you live and grow. The same thing happens when a tree stores the sun’s energy in its wood, which is made of sugar and other molecules that the tree makes from sugar. A fire burns the wood the way your body burns the corn, breaking up the molecules and releasing the energy the sunlight put in to make them. In a wood fire, it comes out as heat and light.

For photosynthesis to happen, plants need light, water, and a gas in the air called carbon dioxide. Carbon dioxide molecules are made of carbon and oxygen. Water is a molecule made of hydrogen and oxygen. Chemists use letters and numbers to stand for these atomic elements because they make it easier to understand how the molecules break apart or add together. (This may be easier to understand if you write it out on a piece of paper.) Hydrogen is H, oxygen is O, and carbon is C. Water is H₂O, or two atoms of hydrogen and one of oxygen. Carbon dioxide is CO₂, or one carbon and two oxygen. Adding up the atoms, water and carbon dioxide have one C, two H, and three O atoms between them. That’s almost the same as sugar, which is CH₂O, or one C, two H and one O atom. Two O atoms are left over. With the energy of the sun, the water and carbon dioxide do add together, and make sugar with a two-atom oxygen molecule left over. That oxygen molecule, which is the same as the oxygen we breathe, floats off from the plant back into the air.

To get the energy back out of the sugar, you need to turn it into water and carbon dioxide again. To do that, you have to add an oxygen molecule to replace the one that floated off from the plant. That’s why you breathe. Your lungs draw in oxygen (O₂), and your body puts it together with sugar (CH₂0), to release the energy in the sugar. That makes water (H₂O) and carbon dioxide (CO₂), which you breathe out again. Fire works the same way: it takes in oxygen, and it puts out carbon dioxide. The carbon dioxide from your breath or your campfire floats around until it runs into a plant that makes it into oxygen and sugar again.

All the plants and animals on Earth are passing oxygen, carbon dioxide, and water back and forth. At the rate that living things on Earth use oxygen, it would run out in a few thousand years if plants didn’t keep making it. The carbon dioxide would run out even quicker--in a few hundred years--if animals, plants, and fires weren’t burning sugar and breaking it up into carbon dioxide and water. Through this carbon cycle, all living things on earth are related and working together. Animals make carbon dioxide for plants, and plants make oxygen for animals.

Look around you and see if you can find things made by photosynthesis. Sugar is only the first step in plants. They turn sugar and soil into many other materials, including starch and fiber. Wood and all the things we make it into--paper, cardboard, fabric--come from photosynthesis, and so does the cotton in your clothes. Photosynthesis made the plants that sank to the bottom of the ocean and got pressed together into coal and oil. We use coal and oil for gasoline and to make plastic and the nylon in your clothing and carpets. Your food comes from photosynthesis, even the meat--it has the energy an animal got from eating a plant, and the plant got that energy from photosynthesis. And you--you’re made of the food you eat and the air your breathe. Plants used photosynthesis to put together the water, carbon dioxide, and sunlight to make you.