The importance of photosynthesis cannot be overstated.All life on earth depends on it.
Every living organism depends either directly or
indirectly on the products of photosynthesis, a process that takes place only in green
plants (and a few types of microorganisms).
Photosynthesis
is the chemical reaction by which plants manufacture carbohydrates. Plants, like animals,
need "food," or carbohydrates, to provide energy for metabolism. Animals get
their food by consuming it; plants manufacture their own carbohydrates using energy from
light.
The chemical reaction for photosynthesis is:
Carbon dioxide plus water combine to produce carbohydrates
and oxygen.
(CH2 O isn’t the formula for a specific
carbohydrate, but represents the basic ratios; for example, sucrose is C6 H12
O6.)
The chemical reaction looks pretty straightforward. But
think for a moment about what happens if you simply mix carbon dioxide with
water—imagine exhaling under water, for example. You certainly don’t end up with
sugar and oxygen! The process of photosynthesis is complex and involves many intermediate
steps.
First of all, note from the equation that the process of
photosynthesis takes place only in chlorophyll-containing plant cells, and only
in the presence of light. And note that two raw materials must be available:
1. Carbon dioxide, from air that enters the stomata on the
undersides of the leaves.
2. Water, absorbed through the roots and transported to
the leaves through the xylem.
During photosynthesis, the green pigment chlorophyll
(contained in the cell’s chloroplasts) absorbs light energy. The plant uses this
energy to split water into its components (hydrogen and oxygen). Then, through a series of
chemical reactions, carbon (C) and oxygen (O) from the carbon dioxide combine with
hydrogen (H) from the water to form carbohydrates (CH2O).
Green plants have the unique ability to capture
the energy of the sun and transform it into the energy contained in the chemical bonds of
carbohydrates.
And what about the oxygen left over when the water
molecules are split? This is the oxygen that is released into the atmosphere. Now we can
all breathe easier!
Let’s explore what happens during photosynthesis. How
do plants "capture" the energy of the sun? First we need to discuss the nature
of light. Light is a form of radiant energy, and can be described in two ways. Light can
be thought of as consisting of particles called photons, or it can be thought of as
consisting of waves. Sunlight is made up of all the colors of light—shown by the
colors in a rainbow—and each of these colors has a different wavelength.
Now let’s jump to the green pigment
chlorophyll. As you may know, the color of an object indicates what color of light it is
reflecting. If a ball is blue, then it is absorbing all colors (or wavelengths) of light
except blue, which it reflects and which our eyes and brains perceive as the color blue.
Chlorophyll, then, absorbs all colors of light except the color green.
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It's Not Easy Being
Green
My earliest scientific and humanitarian endeavors were
thwarted when my 6th grade teacher rejected my proposal for a science project.
I wanted to invent people with chlorophyll so I could end world hunger. Instead I ended up
doing some forgettable experiment using fruit flies. |
Energy matters
What do we mean when we say plants "convert the energy of sunlight into
chemical bonds"?
You may be familiar with the concept
of conservation of energy, and the different forms energy can take. For example, a rock
positioned at the edge of a cliff has potential energy. If the rock falls down the slope,
that potential energy is converted to kinetic energy, the energy of movement.
Other types of energy include
electricity, heat, and light. And matter contains energy in the form of chemical bonds.
Energy can be converted from one form to another, but it can never be destroyed. If you
burn a log, you convert the energy in the chemical bonds of the wood into heat energy.
During photosynthesis, plants convert the sun’s light energy into chemical bonds,
creating carbohydrates. |
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