Everything that lives needs energy.  And since everything that lives is made up of cells, cells need energy too. But where do they get it?

They get it like we do -- from food. Like us, cells eat and digest food.

More specifically, cells break down sugars and turn them into energy. That process is called metabolism. Metabolism is a big word. It is the sum total of all the chemical reactions an organism (like us) goes through to survive.

There are two processes involved in metabolism -- and they make our world go around. Those processes are photosynthesis and cellular respiration.

Photosynthesis is the process by which plants make carbohydrates (sugars).

cellular respiration is the process by which cells convert these sugars into ATP, the basic “fuel” for all living things.  You’ll learn more about it in our next Instruction.

But now we’d like to talk about photosynthesis.

Photosynthesis
First of all, only plants do it – and it’s a pretty amazing process. Because plants actually take one kind of energy (light from the Sun) and turn it into another kind of energy, sugar (carbohydrate is just a fancy name for sugar). All life depends on photosynthesis because animals (like us) either eat plants (fruits, vegetables, grains, etc.) or eat the animals that eat plants. We also breathe the oxygen that’s released at the end of photosynthesis, but we’ll tell you more about that later.

Here’s how photosynthesis works:

• The light from the Sun shines down on a green plant.
   
• Plants, like all living things, are made up of cells.
   
• The cytoplasm (cell fluid) in plant cells contains special organelles (little organs) called chloroplasts (animal cells don’t have them).

Let’s look at these diagrams of plant cells and chloroplasts:

The chloroplasts are where photosynthesis actually takes place. And it happens because of a chemical called chlorophyll.

Besides sunlight, the raw materials for photosynthesis are carbon dioxide (CO₂) and water (H₂O). (A plant gets water through its roots or leaves and gets carbon dioxide from the atmosphere).

The chlorophyll in the chloroplast traps the light (solar energy) from the Sun. Photosynthesis then takes part in two stages:

During the first (“light reaction”) stage, the chlorophyll uses this solar energy to split water (H₂O) into Oxygen (O₂), protons (hydrogen ions, H⁺) and electrons.

During the second (“dark reaction”) stage, no light is needed. The chloroplasts can make carbohydrate in the dark if they’re provided with ATP (fuel), NADH (an enzyme catalyst) and CO₂. This is when the protons and electrons actually convert the carbon dioxide into carbohydrates (sugar) and oxygen.

The sugar is then stored in the plant (for its own use or to be consumed by others) and the oxygen is released into the air for animals to breathe. As you learned in our instruction on Early Earth, once the very first prokaryotic cells began to exude oxygen, an explosion of life began.

During photosynthesis the CO₂ is reduced and the water oxidized. In green plants most of the ATP for protein synthesis comes from oxidative phosphorylation.

Here is the chemical formula for photosynthesis:

Energy from SUNLIGHT + 6H₂O+6CO₂ produce C₆H₁₂O₆+60₂

If you don’t speak “chemistry,” it goes like this:

Light plus six molecules of water plus six molecules of carbon dioxide produce one molecule of sugar plus six molecules of oxygen.

Here’s another brief description of how photosynthesis works:

• Energy from the Sun reaches a plant.
• Water and carbon dioxide enter the plant through its roots or its leaves.
• The plant uses the energy of the Sun to turn the water and carbon dioxide  into carbohydrates (sugar) and oxygen.
• The sugar stays in the plant.
• Oxygen leaves the plant through its leaves.
  
   

Something else you should know is that it’s the chlorophyll in the chloroplast of plant cells that makes plants green. Of course not all plants are green, but most are – and here’s why.

Why Plants Look Green
http://webexhibits.org/causesofcolor/7.html

As you know, light is transmitted at different wavelengths. And it’s those wavelengths that determine the colors we see. Any substance that absorbs light is called a pigment. Chlorophyll absorbs light, so it’s a pigment.

Different pigments absorb different wavelengths of light. What they don’t absorb, they reflect. And it’s the wavelengths they reflect that determine what colors we see. Auxiliary pigments in the chloroplasts of plant cells are responsible for absorbing protons of different wavelengths of light. 

Chlorophyll absorbs red and blue wavelengths -- but it doesn’t absorb green. It reflects green. So when we look at most plants, we see green.

Experiments for Home and Classroom

Why do leaves change color in the fall? -- photosynthesis. This web site explains the process very carefully (ignore the overly simple "I Can Read" section) and also has three interesting experiments to help students understand how the process works. Don't be put off by the word chromatography -- it simply means "separation of colors." Scroll down to "Projects to Do Together." Click:
http://photoscience.la.asu.edu/photosyn/education/colorchange.html 

In order to carry out photosynthesis, plants must pull water from the soil. This entertaining experiment, called "Celery Stalks at Midnight," shows how this happens (the process is called capillary action). Materials required include 4 stalks of celery, 4 cups or glasses, food coloring, a measuring cup, paper towels, a vegetable peeler, a ruler and some old newspapers. Click:
http://www.ed.gov/pubs/parents/Science/celery.html 

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