Biology (Holt)

Stage Two: Conversion of Light Energy

Excited electrons that leave chlorophyll molecules are used to

produce new molecules, including ATP, that temporarily store

chemical energy. First an excited electron jumps to a nearby mol-

ecule in the thylakoid membrane. Then the electron is passed

through a series of molecules along the thylakoid membrane like a

ball being passed down a line of people. The series of molecules

through which excited electrons are passed along a thylakoid mem-

brane are called .Trace the path taken by

excited electrons in the electron transport chains shown in Figure 8.

Electron Transport Chains

How are electron transport chains used to make molecules that

temporarily store energy in the cell? The first electron transport

chain shown in Figure 8 lies between the two large green clusters of

pigment molecules. This type of electron transport chain contains a

protein (the large purple molecule) that acts as a membrane pump.

Excited electrons lose some of their energy as they each pass through

this protein. The energy lost by the electrons is used to pump hydro-

gen ions, H, into the thylakoid. Recall that hydrogen ions are also

produced when water molecules are split inside the thylakoid.

As the process continues, hydrogen ions become more concen-

trated inside the thylakoid than outside, producing a concentration

gradient across the thylakoid membrane. As a result, hydrogen ions

have a tendency to diffuse back out of the thylakoid down their

electron transport chains

100 CHAPTER 5Photosynthesis and Cellular Respiration

H+ H+ H+

H+

H+

H+

H+

H+ H+

H+ H+

H+

H+ H+

H+

H+

H+

H+

P

e-

e-

e-

Light

Pigments

Thylakoid

ATP-producing

carrier protein

2 H 2 O

O 2

4 H+

Path of

electrons

Hydrogen

ions, H+

NADP+ +

Light

Water-splitting

enzyme

NADPH

ADP + AT P

Electron transport chains (represented by the red lines) convert light energy to chemical energy.

Figure 8 Electron transport chains of photosynthesis

Interpreting Graphics

Look closely at Figure 8.

Electrons are represented

by the symbol e–. The red

arrows show the path of

excited electrons. Hydrogen

ions are represented by the

symbol H+. The blue arrows

show the path of hydrogen

ions that cross the thylakoid

membrane.