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.
Chloroplasts are one of several different types of plastids. Plastids are plant cell organelles that
store energy and create materials for the cell. For instance, the colorless leucoplasts create starch,
oils, and proteins. Yellow-to-red colored chromoplasts manufacture carotenoids. Finally green
colored chloroplasts contain the pigments chlorophyll a and chlorophyll b, which absorb the light
energy needed for photosynthesis to occur.
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
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Observing Chloroplasts
What do chloroplasts look like? Most ecoystems and organisms in the world depend on tiny organelles called
chloroplasts. Discover what chloroplats look like in this investigation.
Procedure:
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Observe the pictures of plant tissue in the article and the slides of plant tissue in the classroom.
Identify the chloroplasts in the cells you observe.
Make a table or sketch to record your observations about the parts of chloroplasts in a cell.
Answer the analysis questions when you have finished.
Observations: Draw your chloroplast sketch here. Label as many parts of the chloroplast as you can.
Analysis Questions:
1.
Compare and contrast the appearance of the chloroplasts you observed in the the different pictures and slides.
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2.
Hypothesize why green plant leaves vary in color.
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3.
Explain one theory about how cells might have obtained chloroplasts orignially?
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4.
Justify the argument that chloroplasts were once free-living prokaryotic cells with evidence from the article.
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