Living organisms must concern themselves with two things -- capturing energy and reproducing their kind. On this page we will learn about how photosynthetic organisms capture energy and make food.

     We tend to think that everything green and alive on the earth is a plant.  This is not precisely true.  The first photosynthetic organisms were bacteria, and they are still with us today and still pumping out oxygen.  These tiny organisms are prokaryotes, and they made all higher life on the earth possible by altering the atmosphere so that it contained oxygen.   Great among the photosynthetic bacteria are the cyanobacteria, which are still providing a good part of the earth's oxygen.

     When eukaryotes appeared, making use of the more abundant oxygen available, algae developed.  These green organisms grew in the oceans, and some became multicellular.  They also contributed oxygen to the atmosphere.

     True plants did not appear until after bacteria and algae came out of the ocean and began to grow on land.  It was on land that the true plants evolved, with their stems, roots, leaves, and flowers.  However, we will consider photosynthesis here.  The process of photosynthesis provides both oxygen and edible biomass (leaves, grass, etc) and photosynthetic activity supports life on the earth.

     The earth is an almost closed system. By this we mean that everything that is here is recycled: the same molecules are used over and over again. Atoms do not wear out, but chemical changes require energy. Energy eventually gets transformed into heat and lost. Fortunately for us, the earth does receive energy from the sun, which comes into our closed system and makes life possible.

     Photo synthesizers are the primary producers on earth. They contain chloroplasts, which work as tiny energy capturing factories.  There are reasons to believe that chloroplasts were once independent prokaryote cells that moved into larger eukaryote cells long ago. Chloroplasts are tiny and green, and they contain chlorophyll. They still act like prokaryotes, and reproduce themselves inside cells by simply dividing into two identical parts.

So -- what exactly do the chloroplasts do?

Well, let's think for a minute. What are plants made out of?  Wood. Leaves. Tough fibers. Bark. Blossoms. Fruit. Plants are made mostly out of carbohydrates. Carbon is a very important component of plant tissues. Chloroplasts capture the energy from the sun and also capture the carbon dioxide molecules in the air or dissolved in the water.. They make food for the plant and for whatever animal eats that plant.

What do photosynthesizers  have to work with?

Well, they have water and air. There is carbon in the air in carbon dioxide.  Some carbon dioxide is also dissolved in water.  There is water in lakes and seas as well as in the ground.  The sun provides light and heat energy. The plants use what they have: carbon dioxide, water, and light. In a process called photosynthesis, plants capture energy using only carbon dioxide, water, and light to make a sugar called glucose. They also release oxygen and water molecules into the air.

Here is a picture of what actually happens:

Let's see what is happening here. This plant will make one molecule of glucose.

The plant has 6 carbon dioxide molecules in the air near it. Each carbon dioxide molecule has one atom of carbon and two atoms of oxygen.

6 CO₂

This is where all the carbon in plants comes from -- the air!

Amazing! Trees are made out of the carbon dioxide in the air!

Why do plants put roots into the soil? They need to get minerals and water. Their roots cling to the soil so that the plant can stand up.

This plant has 12 water molecules in the soil near the roots.

Each water molecule has two atoms of hydrogen and one atom of oxygen. Chemists would write down

12 H₂O

The plant gathers up the carbon dioxide from the air. It sucks up water with its roots.

6CO₂ + 12 H₂O -- and the sun is shining!

The plant breaks the water molecules apart. This releases 24 hydrogen atoms and 12 oxygen atoms.

The 12 oxygen atoms pair up to make 6 oxygen molecules. They leave the plant and float out into the air. Later on animals will breathe them in!

Now, what is left?

          6 carbon dioxide molecules (one carbon atom and 2 oxygen atoms each)

24 hydrogen atoms (left over from splitting water molecules)

Here is the glucose molecule, a kind of sugar. The plant makes itself out of glucose. It makes cellulose and starches out of long strings of sugar.

Notice how five of the carbon atoms form a hexagon with just one oxygen atom in it. Notice how each carbon atom has an oxygen atom connected to it. Each carbon atom also has at least one hydrogen atom attached to it as well.

Six oxygen atoms and 12 hydrogen atoms are left over. They combine to make six water molecules and are used by the plant or released into the air.

Glucose feeds animals as well as plants. There are glucose molecules flowing in your blood right now!

This is the process that supports life on earth. It is called photosynthesis. Plants use the sun's energy to transform carbon dioxide and water into a sugar called glucose. Oxygen molecules and water molecules are released when the glucose is made.

Now the next question is -- how do animals use this sugar? Go on to the next page --

Respiration!