Chloroplasts
Name: ________________________________
Date______________________ Section_____
Part 1
Examine the diagrams of chloroplasts in your book (p.222) and in the article provided. Carefully label parts of the
chloroplast below.
Granum
Inner membrane
Intermembrane space
Lamella
Outer membrane
Stroma
Thylakoid
Part 2
Read pages 222 – 224 in your textbook to answer the following questions.
1. Light dependant reactions take place in the _________________________________.
2. Light independent reactions in phase two of photodynthesis occur in the _____________________________.
3. Figure 8.6 is illustrating what point.
4. Why do plant parts that contain chlorophyll appear green to the human eye?
5 . What acessory pigment can be found in most leaves and what why do we see the colors of these pigments in
the fall?
Part 3
Read the attached article about chloroplasts and answer the following analysis questions.
1.
Hypothesize why green plant leaves vary in color.
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2.
Explain one theory about how cells might have obtained chloroplasts orignially?
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3.
Justify the argument that chloroplasts were once free-living prokaryotic cells with evidence from the article.
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Chloroplasts
Adapted from http://micro.magnet.fsu.edu/cells/chloroplasts/chloroplasts.html by Michael W. Davidson and the Florida State
University.
One of the most important characteristic of photoautotrophs is their ability to conduct
photosynthesis, a process by which these organisms make their own food by converting light
energy into chemical energy. This process occurs in most all plants, algae, and even tiny
phytoplankton that live in the ocean. The process is carried out in specialized organelles known as
chloroplasts. All of the green structures in plants, including stems and unripened fruit, contain
chloroplasts, but the majority of photosynthesis activity in most plants occurs in the leaves. On the
average, the chloroplast density on the surface of a leaf is about one-half million per square
millimeter.
Green colored chloroplasts contain the pigments chlorophyll a and chlorophyll b, which absorb the
light energy needed for photosynthesis to occur. Pigments are what dictate the coloration of an
organism. The chief pigments found in animals include guanine, which is expressed as the color
white or as an iridescent hue, the carotenes, which range from yellow to red, the melanins, which are
perceived as brown or black, and the hemes, which are responsible for the reddish hue of blood
hemoglobin. Some of these pigments function in various physiological activities as well as providing
color. Most notable in this regard are the carotenes, which are important in the vitamin synthesis of
animals, as well as chlorophyll synthesis in plants.
The oval-shaped chloroplast is enclosed in a double membrane. The area between these two
membrane layers is called the intermembrane space. The outer layer of the double membrane is
much more permeable than the inner layer. The inner layer contains many embedded membrane
transport proteins. Inside the double membrane of the chloroplast is the stroma, a semi-fluid
material that contains dissolved enzymes and makes up most of the chloroplast's volume. In higher
plants, flat, hollow discs called thylakoids are found throughout the stroma. The thylakoids form
stacks called grana (singular: granum). Each thylakoid is surrounded by its own membrane.
Embedded in this thylakoid membrane are the major light-capturing molecules of the plant. The
inside of a thylakoid is called the lumen. It is believed that the lumen of each thylakoid in a granum
is connected.
Light travels as packets of energy called photons. These photons of light are absorbed by
chlorophyll pigments embedded in the thylakoid membrane. Through the process of photosynthesis,
chloroplasts are able to first convert this absorbed light energy into ATP and other energy-storing
molecules. Finally, ATP and the other energy storing molecules are used to manufacture sugars for
the plant. These sugars can be metabolized for energy or saved for future use.
Plant cells are remarkable in that they have two organelles specialized for energy production:
chloroplasts, which create energy via photosynthesis, and mitochondria, which generate energy
through cellular respiration. Like the mitochondrion, the chloroplast is different from most other
organelles because it has its own DNA. It is an apparent case of endosymbiosis. Scientists
hypothesize that millions of years ago small, free-living prokaryotes were engulfed, but not
consumed, by larger prokaryotes. Perhaps these prokaryotes were able to resist the digestive
enzymes of the engulfing organism. DNA evidence suggests that the ancestors of modern plants
gained the ability to photosynthesize by acquiring a photosynthetic bacterium as an endosymbiont.
As suggested by this hypothesis, the two organisms developed a symbiotic relationship over time.
The larger organism provided the smaller with ample nutrients, and the smaller organism providing
ATP molecules to the larger one. Eventually, the larger organism developed into the eukaryotic cell,
the smaller organism into the chloroplast. Nonetheless, there are a number of prokaryotic traits that
chloroplasts continue to exhibit. Their DNA is circular and their ribosomes and reproductive methods
(binary fission) are more like those of the prokaryotes.
Figure 1: Chloroplasts
Figure 2: Courtesy Dr. L.K. Shumway