Essentials of Ecology

S44 SUPPLEMENT 6

Certain Molecules Store and

Release Energy in Cells

Chemical reactions occurring in photosynthesis

(pp. 58–59) release energy that is absorbed by

adenosine diphosphate (ADP) molecules and

stored as chemical energy in adenosine triphos-

phate (ATP) molecules (Figure 13, left). When

cellular processes require energy, ATP molecules

release it to form ADP molecules (Figure 13,

right).

A Closer Look at Photosynthesis

In photosynthesis, sunlight powers a complex

series of chemical reactions that combine water

taken up by plant roots and carbon dioxide

from the air to produce sugars such as glucose.

This process converts solar energy into chemi-

cal energy in sugars for use by plant cells, with

the solar energy captured, stored, and released

as chemical energy in ATP and ADP molecules

(Figure 13). Figure 14 is a greatly simplifi ed

summary of the photosynthesis process.

Photosynthesis takes place within tiny en-

closed structures called chloroplasts found within

plant cells. Chlorophyll, a special compound

in chloroplasts, absorbs incoming visible light

mostly in the violet and red wavelengths. The

green light that is not absorbed is refl ected back,

which is why photosynthetic plants look green.

The absorbed wavelengths of solar energy initi-

ate a sequence of chemical reactions with other

molecules in what are called light-dependent

reactions.

This series of reactions splits water into

hydrogen ions (H) and oxygen (O 2 ) which

is released into the atmosphere. Small ADP

molecules in the cells absorb the energy released

and store it as chemical energy in ATP mol-

ecules (Figure 13). The chemical energy released

by the ATP molecules drives a series of light-

independent (dark) reactions in the plant cells. In

this second sequence of reactions, carbon atoms

stripped from carbon dioxide combine with

hydrogen and oxygen to produce sugars such

as glucose (C 6 H 12 O 6 , Figure 7, p. S42) that plant

cells can use as a source of energy and carbon.

Chemists Balance Chemical

Equations to Keep Track

of Atoms

Chemists use a shorthand system to

represent chemical reactions. These chemi-

cal equations are also used as an accounting

system to verify that no atoms are created or

destroyed in a chemical reaction as required

by the law of conservation of matter (p. 39

and Concept 2-3, p. 40). As a con-

sequence, each side of a chemical

equation must have the same number of atoms

or ions of each element involved. Ensuring that

this condition is met leads to what chemists call

a balanced chemical equation. The equation for

the burning of carbon (C  O 2 CO 2 ) is bal-

anced because one atom of carbon and two at-

oms of oxygen are on both sides of the equation.

Consider the following chemical reaction:

When electricity passes through water (H 2 O),

the latter can be broken down into hydrogen

(H 2 ) and oxygen (O 2 ), as represented by the fol-

lowing equation:

H 2 O H 2  O 2

2 H atoms 2 H atoms 2 O atoms

1 O atom

This equation is unbalanced because one

atom of oxygen is on the left side of the equa-

tion but two atoms are on the right side.

We cannot change the subscripts of any of

the formulas to balance this equation because

that would change the arrangements of the

atoms, leading to different substances. Instead,

we must use different numbers of the molecules

involved to balance the equation. For example,

we could use two water molecules:

2 H 2 O H 2  O 2

4 H atoms 2 H atoms 2 O atoms

2 O atoms

This equation is still unbalanced. Although

the numbers of oxygen atoms on both sides of

the equation are now equal, the numbers of

hydrogen atoms are not.

We can correct this problem by having the

reaction produce two hydrogen molecules:

2 H 2 O 2 H 2  O 2

4 H atoms 4 H atoms 2 O atoms

2 O atoms

ATP synthesis:

Energy is stored in ATP

A P P + P

A P P P

Energy

ADP

ATP

Phosphate

ATP breakdown:

Energy stored in ATP is released

Energy

A P P P

ATP

A P P + P

ADP Phosphate

Figure 13
Energy storage
and release in
cells.

Light-independent

reaction

Energy storage

and release

(ATP/ADP)

Chlorophyll

Light-dependent

reaction

H 2 O

Sunlight

CO 2

6CO 2 + 6H 2 OC 6 H 12 O 6 + 6O 2

O 2

Chloroplast

in leaf cell

Glucose

Sun

Figure 14 Simplified overview of photosynthesis.

In this process, chlorophyll molecules in the chlo-

roplasts of plant cells absorb solar energy. This

initiates a complex series of chemical reactions in

which carbon dioxide and water are converted to

sugars such as glucose and oxygen.