Biology Now, 2e

86 ■ CHAPTER 05 How Cells Work

CELLS

A proton pump moves

hydrogen ions (H+)

across the membrane

using energy from the

electrons.

Light

Calvin

cycle

Calvin

cycle

Water in

Oxygen out

Glucose out

Sugar (glucose)

Rubisco CO 2

1 Photosystem II^

absorbs light,

energizes electrons,

and splits water into

H+, electrons, and

oxygen.

2 High-energy electrons

move along the electron

transport chain, or ETC

(blue arrows).

In the process of photosynthesis,

carbon dioxide and water in the

presence of sunlight are converted

into glucose (sugars) and oxygen.

The Calvin cycle is a series of

enzyme-driven reactions in which

H+-rich NADPH and energy-rich ATP

from the light reactions help convert

carbon dioxide into sugars. The

enzyme rubisco catalyzes the first

step of many, converting or “fixing”

carbon dioxide into organic

molecules and ultimately sugar.

3 Light energizes

electrons in PS I

and fuels the

synthesis of

NADPH.

4 Hydrogen ions

build up and flow

through ATP

synthase,

generating ATP.

5

NADP+

ADP

ATP

NADPH

Thylakoid Carbon dioxide in

membrane

H+

H+

H+

H+

H+

H+

H+ H+

H+

H H+

+ H

+

H+

H+

H+ H+

PS I

PS II

Light

Light H+

H+

ATP

ATP

P

ADP

ADP

ADP

H 2 O

O 2

ATP

synthase

e–

e–

e–

e–

e–

e–

NADP+

NADP+

NADPH

NADPH

ATP

Light reactions Light-independent reactions,

or Calvin cycle

Figure 5.7

Photosynthesis occurs in two stages

The light reactions generate energy carriers; the light-independent Calvin cycle reactions create sugars. Rubisco contributes to

fixing carbon during the Calvin cycle, but enzymes are critical throughout both stages of photosynthesis. M

Q1: What is the source of the carbon dioxide used for photosynthesis?

Q2: Which products of the light reactions of photosynthesis does the Calvin cycle use?

Q3: What are the two major products of photosynthesis?