Biology (Holt)

Stage One:

Absorption of Light Energy

The chemical reactions that occur in the first and second stages of

photosynthesis are sometimes called “light reactions,” or light-

dependent reactions. Without the absorption of light, these

reactions could not occur. Light energy is used to make energy-

storing compounds. Light is a form of radiation—energy in the form

of waves that travel through space. Different types of radiation, such

as light and heat, have different wavelengths (the distance between

two consecutive waves). When the sun shines on you, your body is

bombarded by many kinds of radiation from the sun. However, you

can see only radiation known as visible light. You see wavelengths of

visible light as different colors. As shown in Figure 5,sunlight con-

tains all the wavelengths of visible light, red through violet.

Pigments

How does a human eye or a leaf absorb light? These structures con-

tain light-absorbing substances called .Pigments absorb

only certain wavelengths and reflect all the others.

(KLOR uh fihl),the primary pigment involved in photosynthesis,

absorbs mostly blue and red light and reflects green and yellow

light. This reflection of green and yellow light makes many plants,

especially their leaves, look green. Plants contain two types of

chlorophyll, chlorophyll aand chlorophyll b.Both types of chloro-

phyll play an important role in plant photosynthesis.

The pigments that produce yellow and orange fall leaf colors, as

well as the colors of many fruits, vegetables, and flowers, are called

(kuh RAH tuh noydz). Carotenoids absorb wavelengths

of light different from those absorbed by chlorophyll, so having both

pigments enables plants to absorb more light energy during photo-

synthesis. The graph in Figure 6 shows the wavelengths of light

absorbed by chlorophyll a,chlorophyll b,and carotenoids.

carotenoids

Chlorophyll

pigments

98 CHAPTER 5Photosynthesis and Cellular Respiration

Sun

Sunlight

Visible spectrum

Increasing

wavelength

400 nm 700 nm

Prism

Wavelength (nm)

Carotenoids

Chlorophyll b

Chlorophyll a

400 500 600 700

Percentage oflight absorbed

Absorption Spectra of Photosynthetic Pigments

Figure 5 Visible spectrum.

Sunlight contains a mixture of

all the wavelengths (colors) of

visible light. When sunlight

passes through a prism, the

prism separates the light into

different colors.

Figure 6 Light absorption

during photosynthesis.

Chlorophylls absorb mostly

violet, blue, and red light, while

carotenoids absorb mostly

blue and green light.