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