Leaf cells have a special feature: pigment-containing chloroplasts in certain cells that enable them to produce energy and their own food through photosynthesis. What does that mean? Well, the chloroplasts within a cell contain different pigments, which are what gives a leaf its color. Green chlorophyll is the most common type of pigment, but there are also xanthophylls (yellow), cartenoids (yellow, orange), and anthocyanins (red). The chlorophylls usually hide the other pigments, except when autumn comes along and chlorophyll begins to break down. This is why leaves turn different colors in the fall.
So then, what is photosynthesis? Simply put, it's the capture of light energy to produce food. Light energy from the sun is transmitted through a leaf's cells to chloroplasts, where chlorophyll and other absorbing pigments serve as receptors to collect the energy. In the process of photosynthesis, carbon dioxide from the air is converted into energy-rich carbon compounds called carbohydrates. As this happens, oxygen is given off into the air, providing the oxygen that we breathe.
You can test the importance of light energy in plant growth by doing a simple experiment. using 2-3 small plants. (Bean plants are a good choice, as they sprout quickly.) You'll need one to be the control, with normal growing conditions, either outside in sunlight or inside by a bright window. See how light effects growth by covering the other test plants with a paper bag or small box during part of the day. Try covering one for four hours during the morning, and another for the whole day. Observe changes to the plants over the course of a week. Which grows the best? What is the result of light-deprivation?
To find out more about leaf pigments, do this next experiment. First, you'll need to extract pigments from leaves. Collect several green leaves from different trees, a few from each one. Maples and others that have dramatic color changes in the fall will work best, but you can use any deciduous leaves (from trees that lose their leaves in the winter). Tear each set of leaves into several pieces and place them in a glass beaker or small drinking glass, then add just enough rubbing alcohol to cover them. (You can cover the containers with foil or plastic wrap to keep the alcohol from evaporating into the air.) Put the containers in a dish of hot tap water for about 30 minutes, until the alcohol turns green as the pigments from the leaves are absorbed into it.
Next, test to find out what colors are really present in a leaf. You'll need coffee filters, filter paper, or chromatography paper for this part of the experiment. Cut a strip out of the middle of a coffee filter, about one inch wide, for each of the leaf sets that you want to test. Tape one end of the paper to a pencil or stick, and suspend it across the container, with the other end just touching the alcohol and pigment mixture. A bit of the mixture will travel slowly up the paper. After about 30-90 minutes you should be able to see the 'green' color break up into several different colors as the different pigments begin to separate. You'll see different shades of green, and perhaps other colors as well. Which leaves had the most colorful pigments? Based on your experiment, which trees' leaves do you think will turn the brightest and least brightest colors this fall? Try the experiment again with evergreen leaves or needles to compare the results.
Taking a Closer Look at Plant Cells
Learn even more about plants by studying different sections of real leaves. You can make your own microscope slide of a leaf section and view it under high power with a compound microscope to see cell detail. All you need is a fresh leaf specimen (use one without many holes or blemishes), a plain glass microscope slide, slide coverslip, sharp knife or razor blade, and water.
Before you begin, make sure the leaf is clean and dry. Lay it out flat on your working surface and slice about a 1'' section crosswise out of the center using the knife. The cells surrounding the central vein of the leaf are what you will want to look at; so make sure you slice across a section of the vein. Then, starting at one of the short ends of the strip (the edges that you did not cut), tightly roll the leaf section. Carefully make several very thin slices off one end of the roll with a razor blade or knife. This is a 'cross section' of the leaf.
Make a wet mount on a plain slide with the inner part of the leaf section facing up (so the inner cells are visible). You can do this by adding a drop or two of water over the leaf section and then covering it with the coverslip. Look at the slide with your microscope's 10x objective to see the general structure, and higher power to see cell detail. Record your observations on a copy of our free Microscope worksheet.