Let’s assume that you are conducting a popular
experiment to test whether plants need light to
make starch, a carbohydrate that they use to store
energy: Cover part of a leaf on a living plant
(geraniums are popular) with a material that will
block light – black paper or tin foil work well.
Expose the leaf to sunlight for several days and
then remove the leaf from the plant. After
removing the paper/foil, you need to break down
the colored pigments in the leaf by placing it in
boiling water for several minutes, until pale and
soft. After boiling, cool it under tap water and
pat dry with paper towel. Place the leaf in a dish
and add some dilute potassium iodide solution. The
iodide (which is brown) will react with starch and
turn purple/blue. The places that were exposed to
light (which enabled the plant to make starch)
should turn blue, while the parts covered by paper
will turn brown.
This experiment is described here:
Let’s examine why it works. While light,
heat and starch are all forms of energy, they
can’t be used in the same way. The plant needs
visible light in a particular energy band to drive
the photosynthetic chemical reaction. Each
packet of infrared light (heat) is too low in
energy to do that work, so it gets distributed
into making molecules vibrate. Even if there is
more total energy in heat, the molecular vibration
is too difficult to concentrate in one place to
drive the photosynthetic reaction. Thus, only
the parts of the leaf that have access to sunlight
can easily build starch. Starch is a way of
storing the energy so that it can be moved around
the plant and broken down to power other chemical
reactions that allow the plant to live and grow.
Where you block access to sunlight, the leaf
can’t make more starch but that doesn’t mean the
cells there stop working. Instead, they keep
carrying on chemical reactions using the starch
they made and stored previously, eventually
consuming it. That’s why these areas stay
brown after you dye the leaf with iodine – there
isn’t enough starch for the iodine to react with.
Now, do you have to use black paper or tin
foil? No, anything that significantly blocks
the useful wavelengths of light will give you
similar results. Try using white paper – it will
reflect most of the light away from the leaf too.
Or, try colored transparent plastic. The color the
plastic appears is the opposite color that it
absorbs, e.g. you can block red light by using
green plastic. How could you determine which
wavelengths are the most useful for
photosynthesis? Stay curious!
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