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petak, 19.02.2010.
Cells and Energy
Cells and Energy
To stay alive, you need a constant supply of energy. You need energy to move, think,
grow, and even sleep. Where does that energy come from? It all starts with the sun.
Plant cells store energy from the sun in the form of molecules. In this section you’ll
learn about how cells store and release energy.
What is photosynthesis?
Solar cells and
chloroplasts
A solar calculator has solar cells that convert light into electrical
energy. The electrical energy powers the calculator. Some of
it is stored in a battery. A plant cell has chloroplasts that also
convert energy. Chloroplasts are where photosynthesis occurs.
Photosynthesis is a process where plants use the energy of
sunlight to produce energy-rich molecules (carbohydrates).
Photosynthesis takes place in the
chloroplasts.
How does a tiny
seed grow into a
massive tree?
Before our knowledge of photosynthesis, gardeners wondered how
a tiny seed could grow into a massive tree. Where did all of that
mass come from? In the 1600s, a Flemish scientist named Jan Van
Helmont (1580–1644) conducted an important experiment. He
grew a willow tree in a carefully weighed amount of soil. He
noticed that the mass of the soil barely changed while the mass of
the tree greatly increased. He concluded that the extra mass did
not come from the soil.
Photosynthesis
is a chemical
reaction
Later experiments carried out by other scientists showed that
plants use carbon dioxide (from the air) and water to make a
simple carbohydrate (glucose). They also release oxygen. This
chemical reaction (photosynthesis) takes place only in the
presence of light (Figure 8.6).
Light and color
Visible light The Sun provides Earth with a steady source of light. Your eyes
perceive sunlight as white light. However, it is really made up of
different colors of light. The colors that make up sunlight are called
visible light. There are other forms of light we cannot see such as
ultraviolet and infrared light.
Light is a wave Light is a wave, like a ripple on a pond. Waves can be described by
their wavelength (the length from peak to peak), and energy. Light
is part of a continuum of waves known as the electromagnetic
spectrum. Light waves have very short wavelengths. They range
from 800 nm (red light) to 400 nm (violet light). One nanometer
(nm) is equal to one-billionth of a meter!
Color A prism splits white light into all of its colors. Color is how we
perceive the energy of light. All of the colors of visible light have
different energies. Red light has the lowest energy and violet light
has the highest energy. As we move through the rainbow from red
to violet, the energy of the light increases (Figure 8.7).
