Plastids

Plastids are another unique feature of plant cells. Plastids are organelles that, like mitochondria, are surrounded by a double membrane and contain their own DNA. There are several types of plastids, including chloroplasts, chromoplasts, and leucoplasts.

Chloroplasts

Chloroplasts use light energy to make carbohydrates from carbon dioxide and water. As Figure 4-23 shows, each chloroplast contains a system of flattened, membranous sacs called thylakoids. Thylakoids contain the green pigment chlorophyll, the main molecule that absorbs light and captures light energy for the cell. Chloroplasts can be found not only in plant cells but also in a wide variety of eukaryotic algae, such as seaweed.

Chloroplast DNA is very similar to the DNA of certain photosyn-thetic bacteria. Plant cell chloroplasts can arise only by the division of preexisting chloroplasts. These facts may suggest that chloroplasts are descendants of ancient prokaryotic cells. Like mitochondria, chloroplasts are also thought to be the descendants of ancient prokaryotic cells that were incorporated into plant cells through a process called endosymbiosis.

Chromoplasts

Chromoplasts are plastids that contain colorful pigments and that may or may not take part in photosynthesis. Carrot root cells, for example, contain chromoplasts filled with the orange pigment carotene. Chromoplasts in flower petal cells contain red, purple, yellow, or white pigments.

Other Plastids

Several other types of plastids share the general features of chloro-plasts but differ in content. For example, amyloplasts store starch. Chloroplasts, chromoplasts, and amyloplasts arise from a common precursor, called a proplastid.

Word Roots and Origins chloroplast from the Greek chloros, meaning "pale green," and plastos, meaning "formed"

figure 4-23

A chloroplast captures energy from sunlight and uses that energy to convert carbon dioxide and water into sugar and other carbohydrates.

Outer \ Inner membrane membrane

Outer \ Inner membrane membrane

figure 4-24

figure 4-24

Prokaryotes (a) can be distinguished from eukaryotes (b and c) in that prokaryotes lack a nucleus and membrane-bound organelles. Plant cells (c) have the same organelles that animal cells do and have a cell wall, a central vacuole, and plastids.

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(c)
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