| Chloroplasts, a structure in the cells of plants and algae
where photosynthesis takes place. Chloroplasts are
mostly disk-shaped organelles, 4 to 6 micrometers in
diameter. They occur most abundantly in leaf cells,
where they can apparently orient themselves to light.
Perhaps 40 to 50 chloroplasts exist in one cell and
500,000 in each sq mm (0.06 sq in) of leaf surface.
Each chloroplast is enclosed in a double membrane.
Internally, it consists of a ground substance called the
stroma, which is traversed by a complex network of
interconnected disks called thylakoids. Many of the
thylakoids are stacked like saucers; the stacks are
called grana. Molecules of chlorophyll, which absorb
light for photosynthesis, are attached to the
thylakoids. The light energy captured by the
chlorophyll is converted to adenosine triphosphate, or
ATP, in a series of chemical reactions that take place
in the grana. Chloroplasts also contain small starch
grains that temporarily store the products of
photosynthesis.
In plants, chloroplasts develop in the presence of light
from small, colorless organelles called proplastids. As
cells divide in the growing parts of a plant, the
proplastids inside them divide by fission. Thus, the
daughter cells have the ability to produce chloroplasts.
In algae, chloroplasts divide directly, without
developing from proplastids. The self-reproducing
ability of chloroplasts, their bacteria-like DNA and
ribosomes, and their close similarity regardless of the
type of cell they inhabit, suggest that they were once
independent organisms that come to exist in symbiosis
with the plant cell as host.
Contributed By:
Marshall R. Crosby, Ph.D.
Director of Botanical Information Resources, Missouri
Botanical Garden. Adjunct Professor of Biology,
Washington University.
© Microsoft
Peter H. Raven, Ph.D.
Director, Missouri Botanical Garden. Engelmann
Professor of Botany, Washinton University. President,
Organization for Tropical Studies. Coauthor of Biology
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