Cells and Tissues
I
nmedievaltimes,the city of Carcassonne in
southern France was an entirely self-contained
community. As you can see from this aerial view,
the city was protected by two walls and each was
equipped with guarded gates to control who en-
tered or exited the city. Craftspeople, merchants,
and residents lived and worked in the buildings
inside the city walls. They interacted with each
other, with inhabitants who dwelled outside the
city walls, and with travelers. Residents and visi-
tors traveled along city streets. At the heart of
the city was the castle, where the nobility and the
army lived. The nobles governed, and the army de-
fended the city from attack. Every part of the city
and each individual within it had distinct functions.
Beyond the walls, many medieval cities and their
surroundings formed countries.
Each cell in your body is like the medieval city
of Carcassonne. The cell membrane serves as a
wall that separates the inside from the outside and
has “gates” that control which substances enter
and leave the cell. Just as the buildings within
the city house shops and workplaces, organelles
carry out different functions. Like traffic moving
along the streets, materials move within cells. The
nucleus can be likened to the castle, governing the
cell and coordinating its functions. Finally, many
cells combine to form tissues, similar to the group-
ing of cities and their surroundings into countries.
This chapter examines the structures and func-
tions of various cells and how they are assembled
into tissues.
NATIONAL
GEOGRAPHIC
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