Photosynthesis and Light

Ms. Mudd 7th Grade Biology

The Nature of Light

• The sun is a source of energy.• You can feel the sun’s energy as it warms your skin.• You see the energy in the form of light on objects

around you.• The light you see is white light.• When white light passes through a prism you can see

it is made up of the colors of the rainbow.• Scientists refer to the colors-red, orange, yellow,

green, blue, indigo, and violet-as the visible light spectrum.

When Light Strikes An Object

• White light strikes many objects.• Some objects like glass allow light to pass

through them.• This process is called transmission.• When light hits a shiny surface such as a mirror,

the light bounces back.• This process is called reflection.• When dark objects, such as street pavements,

take in light, it is called absorption.

Light Strikes Objects• Most objects reflect some colors while they

absorb others.• When white light strikes a lemon, the lemon

absorbs most of the light’s colors. • However, the lemon reflects the yellow light.• The lemon looks yellow because your eyes see

the reflected color.

Plants and Light

• Like yellow lemons and most other objects plants absorb some colors of the visible spectrum and reflect others.

• When light strikes the green leaves of a plant, most of the green part of the spectrum is reflected.

• Most of the other colors of light are absorbed.

Plant Pigments

• When light strikes a leaf, it is absorbed by pigments found in the leaf’s cells.

• Chlorophyll, the most abundant pigment in leaves, absorbs most of the blue and red light.

• Most of the green light is reflected rather than absorbed.

• This explains why chlorophyll appears green in color, and why leaves usually appear green.

Accessory Pigments

• Other pigments called accessory pigments are also found in leaves.

• These pigments include orange and yellow pigments, and they absorb different colors of light than chlorophyll.

• Most accessory pigments are not visible in plants because they are masked by chlorophyll.

Photosynthesis is how plants use light and water to make sugar. Sugar is created in the green parts of a plant and every animal on earth depends on it. Without plants we would have no food to eat or oxygen to breath. Here is a picture to show how it happens.

Photo means light and synthesis means put together.

   

Plants absorb a common gas called carbon dioxide, pull water up through their roots and use light to make sugar. Plants use the sugar to grow. Plants give off oxygen as a by-product. The green parts of the plant makes the sugar and oxygen.

Why Do Leaves Change Color In Fall?

Leaves are nature's food factories. Plants take water from the ground through their roots. They take a gas called carbon dioxide from the air. Plants use sunlight to turn water and carbon dioxide into glucose. Glucose is a kind of sugar. Plants use glucose as food for energy and as a building block for growing. The way plants turn water and carbon dioxide into sugar is called photosynthesis. That means "putting together with light." A chemical called chlorophyll helps make photosynthesis happen. Chlorophyll is what gives plants their green color.

As summer ends and autumn comes, the days get shorter and shorter. This is how the trees "know" to begin getting ready for winter. During winter, there is not enough light or water for photosynthesis. The trees will rest, and live off the food they stored during the summer. They begin to shut down their food-making factories. The green chlorophyll disappears from the leaves. As the bright green fades away, we begin to see yellow and orange colors. Small amounts of these colors have been in the leaves all along. We just can't see them in the summer, because they are covered up by the green chlorophyll.

The bright reds and purples we see in leaves are made mostly in the fall. In some trees, like maples, glucose is trapped in the leaves after photosynthesis stops. Sunlight and the cool nights of autumn cause the leaves to turn this glucose, into a red color. The brown color of trees, like oaks, are made from wastes left in the leaves.