book:a 4.2
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Book:A4.2 Photosynthesis:
Book A:4-2
Dr. Engelmann
• In 1883, T. W. Engelmann, a German biologist, was at work in his laboratory.• He peered into the microscope at some
strands of algae on a slide. • As Engelmann watched the algae, he
saw gas bubbles forming in the water around some of the cells. • Although Engelmann did not know it
at the time, his experiment provided a clue about how light is involved in photosynthesis.
Book:A4.2
White LightBook:A4.2
• The sun is the source of energy on Earth. • You see the energy in the form of light on objects
around you--The light that you see is called white light.• But when white light passes through a prism, you
can see that it is made up of the colors of the rainbow—red, orange, yellow, green, blue, and violet.• Scientists refer to these colors as the visible
spectrum.
The Nature of LightBook:A4.2
• Some objects such as glass and other transparent materials transmit light, or allow the light to pass through. • Shiny surfaces such as mirrors reflect, or bounce back, light. • Dark objects such as street pavements absorb, or take in and hold, light.
The Nature of LightBook:A4.2
• Most objects, however, reflect some colors of the visible spectrum while they absorb other colors. • For example, when white light strikes a red shirt, the
shirt absorbs most of the light’s colors.• However, the shirt reflects red light. • The shirt looks red because your eyes
see the reflected color.
Plants and LightBook:A4.2
• When light strikes a leaf, it is absorbed by pigments found in the chloroplasts of the cells. • Chlorophyll, the most abundant pigment in plants,
absorbs most of the blue and red light. • Green light, on the other hand, is reflected rather
than absorbed. • This explains why chlorophyll appears
green in color, and why plants appear green.
Plants and LightBook:A4.2
• Other pigments, called accessory pigments, include yellow, orange, and red pigments.
• These pigments absorb colors of light that chlorophyll does not.
• Most accessory pigments are not visible in plants for most of the year because they are masked by the chlorophyll.
• However, in some areas during the fall season, cool temperatures break down the chlorophyll in many plants.
• The colors of the accessory pigments become visible and produce the beautiful orange and yellow colors of fall leaves.
Capturing EnergyBook:A4.2
• Because light is a form of energy, a substance that absorbs light absorbs energy. • Just as a car requires the energy
in gasoline to move, the process of photosynthesis in plants requires energy in the form of light. • Photosynthesis begins when light
strikes the chlorophyll in the chloroplasts of the plant’s cells. • The light energy that is absorbed
powers the process of photosynthesis.
Book:A4.2
1643Jean-BaptisteVan HelmontA Dutch scientist, Jean-Baptiste Van Helmont,planted a willow tree in a tubof soil. After five years ofadding only water, the treegained 74 kilograms. Van Helmontconcluded that trees need only water togrow. Today it is known that water is one ofthe raw materials of photosynthesis
Unraveling the Mysteries of Photosynthesis
Science & History
Book:A4.2
1771Joseph Priestley
When Joseph Priestley, an English
scientist, placed a burningcandle in a covered jar, the
flame went out. When heplaced both a plant and a
candle in a covered jar, thecandle kept burning.
Priestley concluded thatthe plant released
something into the air thatkept the candle burning.
Today we know that plantsproduce oxygen, a product
of photosynthesis.Unraveling the Mysteries of Photosynthesis
Science & History
Book:A4.2
1779Jan Ingenhousz
Jan Ingenhousz, a Dutch scientist,placed branches with leaves in
water. In sunlight, the leavesproduced oxygen bubbles. In the
dark, the leaves produced nooxygen. Ingenhousz concluded that
plants need sunlight to produceoxygen, a product of
photosynthesis.
Unraveling the Mysteries of Photosynthesis
Science & History
Book:A4.2
Science & History
1864Julius Sachs
A German biologist, JuliusSachs, observed living leaf
cells under a microscope. Ashe watched, he tested the
cells for the presence ofcarbohydrates. Sachs
discovered that plantsproduce carbohydratesduring photosynthesis.
Unraveling the Mysteries of Photosynthesis
Book:A4.2
Science & History
Unraveling the Mysteries of Photosynthesis
1883T. W. Engelmann
T. W. Engelmann studied howdifferent colors of light affect
photosynthesis in green algae. Hefound that cells bathed in blue and
red light had the fastest rates ofphotosynthesis. Today scientists
know that the chlorophyll in bothgreen algae and plants absorbs
mostly blue and red light.
Book:A4.2
Science & History
Unraveling the Mysteries of Photosynthesis
1948Melvin Calvin
The American scientistMelvin Calvin traced
thechemical path
that the carbon fromcarbon dioxide followsduring photosynthesis.
Bydoing this, Calvin
learnedabout the complex
chemicalreactions of
photosynthesis.
Book:A4.2
The Chemistry of Photosynthesis
• Light energy is just one of the things that plants need to carry out photosynthesis.
• Just as you need flour and eggs to make cookies, a plant also needs raw materials to make its own food.
• Plants use carbon dioxide gas and water as raw materials for photosynthesis.
Book:A4.2
The Chemistry of Photosynthesis
• During photosynthesis, plants use the energy absorbed by the chlorophyll to power a series of complex chemical reactions.
• In these reactions, carbon dioxide from the air and water from the soil combine to produce sugar, a type of carbohydrate.
• Another product, oxygen gas, is also produced.
Book:A4.2
The Chemistry of PhotosynthesisThe many chemical reactions of photosynthesis can be summarized by the following equation.
• Like all organisms, plants need a steady supply of energy to grow and develop. • Some of the food made by plants supplies that
energy. The excess food is stored by the plants in their roots, stems, or leaves. • Carrot plants, for example, store excess food in their
roots. • When you eat a carrot, you are eating the plant’s
stored food.
Book:A4.2
The Chemistry of Photosynthesis
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