Biology (Holt)

Using the Energy in Sunlight

When you eat a hamburger, you get energy from the sun indirectly.
Plants, such as grass, capture the energy in sunlight. The beef in a
hamburger comes from a cow that ate grass. The bun, lettuce, and
tomato come from plants. With few exceptions, you end up with
plants whenever you trace your food back to its origin. Plants, algae,
and some bacteria capture about 1 percent of the energy in the sun-
light that reaches Earth and convert it to chemical energy through
the process of photosynthesis.

The Stages of Photosynthesis

Photosynthesis is the process that provides energy for almost all life.
As Figure 4 shows, photosynthesis has three stages:

Stage 1 Energy is captured from sunlight.

Stage 2 Light energy is converted to chemical energy, which is tem-

porarily stored in ATP and the energy carrier molecule

NADPH.

Stage 3 The chemical energy stored in ATP and NADPH powers the

formation of organic compounds, using carbon dioxide,
CO 2.
Photosynthesis occurs in the chloroplasts of plant cells and algae
and in the cell membrane of certain prokaryotes. Photosynthesis
can be summarized by the following equation:

3CO 2  3H 2 O → C 3 H 6 O 3  3O 2

This equation, however, does not show
how photosynthesis occurs. It merely says
that three carbon dioxide molecules, three
water molecules, and light are needed to
form one three-carbon organic compound
and three molecules of oxygen. Plants use
the organic compounds they make during
photosynthesis to carry out their life
processes. For example, some of these sug-
ars are used to form starch, which can be
stored in stems or roots. The plant may
later break down the starch to make ATP
used to power metabolism. All of the pro-
teins, nucleic acids, and other molecules
of the cell are assembled from fragments
of these sugars.

Photosynthesis Section 2

Objectives

Summarizehow energy is

captured from sunlight in the

first stage of photosynthesis.

Analyzethe function of

electron transport chains in

the second stage of photo-

synthesis.

Relatethe Calvin cycle to

carbon dioxide fixation in

the third stage of photo-

synthesis.

Identifythree environmental

factors that affect the rate of

photosynthesis.

Key Terms

pigment

chlorophyll

carotenoid

thylakoid

electron transport

chain

NADPH

carbon dioxide

fixation

Calvin cycle

carbon

dioxide

water oxygen

gas

3-carbon

sugar

light

NADP+

Light

NADPH

ADP

H 2 O

O 2 CO^2

Organic

compounds

Stage 1 Stage 2 Stage 3

AT P

SECTION 2 Photosynthesis 97

The process of photosynthesis occurs in three stages.

Figure 4 Photosynthesis

9D

4B