A group of structures that has the general name
plastid. Plastids are membrane-bound organelles found only in plant cells.
Usually plastids are spherical bodies that float freely in the cytoplasm,
holding pigment molecules or starch. Chloroplasts contain the green pigment
chlorophyll, a substance that gives plants the green color. Chlorophyll is a
special molecule that has the ability to trap light and to convert it to a form
of energy that plants can use in carrying out the chemical steps of the
food-making process known as photosynthesis. Each chloroplast is surrounded by a
double membrane. Inside the chloroplast are numerous flattened membranous sacs
called thylakoids (formerly called grana). The thylakoids are the structures
that contain the chlorophyll and it is within these sacs that photosynthesis
takes place. Stroma is the name given to the dense ground substance that
cushions the thylakoids. Animal cells do not have chloroplasts and therefore
cannot make their own food. The figure shows the fine structure of a
chloroplast.
