Let’s talk about Leaves
Single Most Important chemical reaction in the world: Photosynthesis -Why?
We have all held leaves, driven miles to see their fall colors, eaten them, raked them, sought their shade. Since they are everywhere, it’s easy to take them for granted.
But even when we do, they continue in their one occupation: turning light into life. When rays of sunlight strike green leaves, wavelengths in the green spectrum bounce back toward our eyes. The rest—the reds, blues, indigos, and violets—are trapped. A leaf is filled with chambers illuminated by gathered light. In these glowing rooms photons bump around, and the leaf captures their energy, turning it into the sugar from which plants, animals, and civilizations are built.
Chloroplasts, fed by sun, water, carbon dioxide, and nutrients, do the leaf’s work. They evolved about 1.6 billion years ago when one cell, incapable of using the sun’s energy, engulfed another cell—a cyanobacterium—that could. That cyanobacterium became the ancestor of every living chloroplast. Without their chloroplasts plants would be left like the rest of us, to eat what they find. Instead they hold out their green palms and catch light. Putting the variety of structures aside, most leaves do essentially the same thing: They exist in the main to hold chloroplasts aloft. How can so many different geometries all perfectly capture the sun?
The work of natural selection offers a key to the puzzle. Desert leaves tend to be small, thick-skinned, waxy, or spiny‚ just like leaves in salty regions or other harsh lands—clear examples of the relatively few ways evolution can deal with a lack of water. Rain forest plants often have narrow leaves, with long, thin “drip tips,” to drain away excess water. In cold places one finds leaves with teeth—like birches and cherries—though why this particular pattern exists is the subject of debate.
Some of the most extreme examples of the way natural selection shapes leaves can be found at high elevations in the tropics, where nights are consistently cold and damp and the days hot and dry. Scramble high enough above the tree line in the mountains of Africa, Asia, Hawaii, and the Americas, and you will see thick towers of plants crowned by mops of living and dead leaves.
The battles among plants have changed their stems and their veins. Leaves with more veins can carry more water to the chloroplasts, allowing the chloroplasts to make more sugar and the plants to grow faster. These species in turn can hold their leaves aloft to occupy more space in the sky and consume more sunlight before others get to it. Through time the plants that were able to produce more and more veins in their leaves won many battles and some wars.
Do green leaves also contain other pigments?
*Paper chromatography is a process that uses special filter paper to separate and identify the different substances in a mixture. Chromatography means “to write with color.” The substances in the mixture dissolve in the alcohol and move up the paper. The heavier substances move up the paper more slowly. The lighter substances move up the paper more quickly. So heavy and light substances get separated from one another on the paper.
*Plants contain chlorophyll, a green pigment, as well as carotenoid, a pigment that ranges in color from red to orange to yellow.
Most leaves are green due to chlorophyll. This substance is important in photosynthesis (the process by which plants make their food). In this experiment, the different pigments present in a leaf are separated using paper chromatography. Most leaves are green due to chlorophyll.
1 tall water glass
Colored pencils or crayon
- Cut a 2 ½ cm strip from a coffee filter and Use a ruler to measure and draw a light pencil line 2 cm above the bottom of the coffee filter strip. Tape the top of the paper strip to a pencil so that the end of the strip with the green line hangs down. The pencil should be able to sit across the top of the clear cup with the bottom of the paper strip just touching the bottom of the glass. Cut off any excess paper from the top of the strip if it is too long. DO NOT CUT THE BOTTOM OF THE STRIP WITH THE GREEN LINE. Image A
- Wrap a leaf around a coin with the waxy side of the leaf facing outward. –Image B
- Now rub the leaf along the light pencil line on the coffee filter strip until you make a dark green line. DO NOT RUB THE LEAF ABOVE OR BELOW THE LINE. RUB THE LEAF ON THE LINE ONLY. Image C
- Remove the pencil/paper strip from the clear cup for now.
- Carefully add isopropyl alcohol to the glass until it reaches a depth of 1 cm in the beaker. Lay the pencil across the top of the glass with the paper strip extending into the alcohol. MAKE SURE THAT THE LEVEL OF THE ALCOHOL IS BELOW THE GREEN LINE ON YOUR PAPER STRIP. IF THE ALCOHOL IS GOING TO COVER THE GREEN LINE, POUR OUT SOME ALCOHOL BEFORE YOU GET THE GREEN LINE WET. Image D, E
- Observe as the alcohol gets absorbed and travels up the paper. This may take up to 20 minutes. Do not touch your experiment during this time.
- Using colored pencils or crayons, draw your results. Be sure to dispose the material in an environmentally safe manner.
Results: Include the following in your results section:
|Coffee paper strip||Use colored pencils or crayons to draw your observations|
|Before paper chromatography|
|After paper chromatography|
Why is paper chromatography an appropriate technique to use to determine if different pigments are present in a leaf? How does paper chromatography work? Did the leaf you tested contain different pigments? Use your results to support your answer. Based on what you have learned, explain why leaves tend to change color in the fall. Leaves in New England change color in the fall. However, leaves in Florida do not change color in the fall. Why is this so? (Hint: Think of a difference between the two locations that might act as a trigger for leaves to change color in the fall.) Was your hypothesis correct? If not, rewrite your hypothesis here so that it is correct.
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