Chloroplasts
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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Printed: November 11, 2015
AUTHORS
Douglas Wilkin, Ph.D.
Jean Brainard, Ph.D.
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C HAPTER
Chapter 1. Chloroplasts
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Chloroplasts
• Summarize the chloroplast structure.
• Describe the role of the chloroplast in photosynthesis.
• Explain the significance of an electron transport chain.
What do pancakes and chloroplasts have in common?
The chloroplast is the site of photosynthesis. Part of the photosynthesis reactions occur in an internal membrane
within the organelle. The chloroplast contains many of these internal membranes, making photosynthesis very
efficient. These internal membranes stack on top of each other, just like a stack of pancakes.
Stages of Photosynthesis
Photosynthesis occurs in two stages, which are shown in Figure 1.1.
1. Stage I is called the light reactions. This stage uses water and changes light energy from the sun into chemical
energy stored in ATP and NADPH (another energy-carrying molecule). This stage also releases oxygen as a
waste product.
2. Stage II is called the Calvin cycle. This stage combines carbon from carbon dioxide in the air and uses the
chemical energy in ATP and NADPH to make glucose.
Before you read about these two stages of photosynthesis in greater detail, you need to know more about the
chloroplast, where the two stages take place.
The Chloroplast
Chloroplasts: Theaters for Photosynthesis
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FIGURE 1.1
The two stages of photosynthesis are the
light reactions and the Calvin cycle. Do
you see how the two stages are related?
Photosynthesis, the process of turning the energy of sunlight into ”food,” is divided into two basic sets of reactions,
known as the light reactions and the Calvin cycle, which uses carbon dioxide. As you study the details in other
concepts, refer frequently to the chemical equation of photosynthesis: 6CO2 + 6H2 O + Light Energy → C6 H12 O6 +
6O2 . Photosynthesis occurs in the chloroplast, an organelle specific to plant cells.
If you examine a single leaf of a Winter Jasmine leaf, shown in Figure 1.2, under a microscope, you will see within
each cell dozens of small green ovals. These are chloroplasts, the organelles which conduct photosynthesis in
plants and algae. Chloroplasts closely resemble some types of bacteria and even contain their own circular DNA
and ribosomes. In fact, the endosymbiotic theory holds that chloroplasts were once independently living bacteria
(prokaryotes). So when we say that photosynthesis occurs within chloroplasts, we speak not only of the organelles
within plants and algae, but also of some bacteria –in other words, virtually all photosynthetic autotrophs.
Each chloroplast contains neat stacks called grana (singular, granum). The grana consist of sac-like membranes,
known as thylakoid membranes. These membranes contain photosystems, which are groups of molecules that
include chlorophyll, a green pigment. The light reactions of photosynthesis occur in the thylakoid membranes. The
stroma is the space outside the thylakoid membranes, as shown in Figure 1.3. This is where the reactions of the
Calvin cycle take place. In addition to enzymes, two basic types of molecules - pigments and electron carriers –are
key players in this process and are also found in the thylakoid membranes.
You can take a video tour of a chloroplast at Encyclopedia Britannica: Chloroplast: http://www.britannica.com/EB
checked/media/16440/Chloroplasts-circulate-within-plant-cells .
Electron carrier molecules are usually arranged in electron transport chains (ETCs). These accept and pass
along energy-carrying electrons in small steps ( Figure 1.4). In this way, they produce ATP and NADPH, which
temporarily store chemical energy. Electrons in transport chains behave much like a ball bouncing down a set of
stairs –a little energy is lost with each bounce. However, the energy “lost” at each step in an electron transport chain
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Chapter 1. Chloroplasts
FIGURE 1.2
High power microscopic photo of the upper part of a Winter Jasmine leaf. Viewed
under a microscope, many green chloroplasts are visible.
FIGURE 1.3
A chloroplast consists of thylakoid membranes surrounded by stroma. The thylakoid membranes contain molecules of
the green pigment chlorophyll.
accomplishes a little bit of work, which eventually results in the synthesis of ATP.
Summary
• Photosynthesis occurs in the chloroplast, an organelle specific to plant cells.
• The light reactions of photosynthesis occur in the thylakoid membranes of the chloroplast.
• Electron carrier molecules are arranged in electron transport chains that produce ATP and NADPH, which
temporarily store chemical energy.
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FIGURE 1.4
This figure shows the light reactions of photosynthesis. This stage of photosynthesis begins with photosystem II
(so named because it was discovered after photosystem I). Find the two electrons (2 e− ) in photosystem II, and
then follow them through the electron transport chain (also called the electron transfer chain) to the formation of
NADPH. Where do the hydrogen ions (H+ ) come from that help make ATP?
Explore More
Use this resource to answer the questions that follow.
• http://www.hippocampus.org/Biology → Non-Majors Biology → Search: Photosynthetic Structures
1.
2.
3.
4.
5.
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What are the functions of a plant’s leaves?
Where do the photosynthetic reactions occur?
What is a stomata? What is their role?
Describe the internal structure of a chloroplast.
What reactions occur in the thylakoid membranes?
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Chapter 1. Chloroplasts
Review
1. Describe the chloroplast’s role in photosynthesis.
2. Explain how the structure of a chloroplast (its membranes and thylakoids) makes its function (the chemical
reactions of photosynthesis) more efficient.
3. Describe electron carriers and the electron transport chain.
References
1. Mariana Ruiz Villarreal (LadyofHats) for CK-12 Foundation. CK-12 Foundation . CC BY-NC 3.0
2. Image copyright Jubal Harshaw, 2014. Chloroplasts under a microscope . Used under license from Shutterstock.com
3. CK-12 Foundation. The interior of a chloroplast, showing chlorophyll . CC BY-NC 3.0
4. Hana Zavadska. The light reactions of photosynthesis . CC BY-NC 3.0
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