Objectives
•
•
Describe the structure of a chloroplast.
Identify the overall reactants and products of photosynthesis.
Key Terms
•
•
•
•
•
•
chloroplast
chlorophyll
stroma
thylakoid
light reactions
Calvin cycle
As you read in Chapter 7, photosynthesis is the process by which plants and other producers convert the energy
of sunlight into the energy stored in organic molecules. Just as cellular respiration takes place largely within a
cell's mitochondria, photosynthesis also occurs in a specific organelle.
The Structure of Chloroplasts
The cellular organelle where photosynthesis takes place is called a chloroplast. Chloroplasts contain chemical
compounds called chlorophylls that give these organelles a green color. When you observe a plant, all the green
parts you can see contain cells with chloroplasts and can carry out photosynthesis. In most plants, the leaves
contain the most chloroplasts and are the major sites of photosynthesis.
Within a leaf, the chloroplasts are concentrated in the cells of the mesophyll, the inner layer of tissue (Figure 82). Tiny pores called stomata (singular, stoma) are found on the surface of the leaf. Carbon dioxide enters the
leaf and oxygen exits the leaf through the stomata. Veins carry water and nutrients from the plant's roots to the
leaves. The veins also deliver organic molecules produced in the leaves to other parts of the plant.
Figure 8-2
Photosynthesis takes place in cellular organelles called chloroplasts. In this
sunflower, the greatest numbers of chloroplasts are located in the leaves.
Chlorophylls give the chloroplasts—and in turn the leaves—their green color.
The chloroplast's structure is key to its function. Like a mitochondrion, a chloroplast has an inner and an outer
membrane. The inner membrane encloses a thick fluid called stroma. Suspended in the stroma are many diskshaped sacs called thylakoids. Each thylakoid is enclosed by a membrane. The thylakoids are arranged in stacks
called grana (singular, granum). These various structures within the chloroplast organize the complex series of
chemical reactions that make up the overall process of photosynthesis. Some of the steps take place in the
thylakoid membranes, while others take place in the stroma.
Overview of Photosynthesis
You have read that cellular respiration involves the process of electron transfer. The "fall" of electrons from
glucose to oxygen releases energy, which is then used to make ATP. The opposite occurs in photosynthesis.
Electrons from water are boosted "uphill" by the energy from sunlight. The chloroplast uses these "excited"
electrons, along with carbon dioxide and hydrogen ions, to produce sugar molecules. The reaction steps add up
to the overall chemical equation for photosynthesis shown in Figure 8-3.
Figure 8-3
As in cellular respiration, the chemical equation for
photosynthesis summarizes many reaction steps.
Photosynthesis occurs in two main stages, each with many steps: the light reactions and the Calvin cycle (Figure
8-4).
Figure 8-4
This "road map" shows the two main stages of
photosynthesis: the light reactions, which occur in the
thylakoids, and the Calvin cycle, which occurs in the stroma.
The Light Reactions The light reactions convert the energy in sunlight to chemical energy. These reactions
depend on molecules built into the membranes of the thylakoids. First, chlorophyll molecules in the membranes
capture light energy. Then the chloroplasts use the captured energy to remove electrons from water. This splits
the water into oxygen and hydrogen ions. The oxygen is a "waste product" of photosynthesis. It escapes to the
atmosphere through the stomata of leaves. What becomes of the water's electrons and hydrogen ions?
Chloroplasts use them to make an energy-rich molecule called NADPH. (NADPH is an electron carrier very
similar to the NADH you read about in Chapter 7.) The chloroplasts also use the captured light energy to
generate ATP. The overall result of the light reactions is the conversion of light energy to chemical energy
stored in two compounds: NADPH and ATP.
The Calvin Cycle The Calvin cycle makes sugar from the atoms in carbon dioxide plus the hydrogen ions and
the high-energy electrons carried by NADPH. The enzymes for the Calvin cycle are located outside the
thylakoids and dissolved in the stroma. The ATP made by the light reactions provides the energy to make sugar.
The Calvin cycle is sometimes referred to as the "light-independent reactions" because, unlike the light
reactions, it does not directly require light to begin. However, this doesn't mean that the Calvin cycle can
continue running in a plant kept in the dark. The Calvin cycle requires two inputs supplied by the light
reactions, ATP and NADPH. You'll explore both the light reactions and the Calvin cycle in more detail in
Concept 8.2 and Concept 8.3.
Concept Check 8.1
1. Draw and label a simple diagram of a chloroplast that includes the following structures: outer and inner
membranes, stroma, thylakoids.
2. What are the reactants for photosynthesis? What are the products?
3. Name the two main stages of photosynthesis. How are the two stages related?