Plastid  Cell Wall Vacuole Centriole Ciliate o Found in plants Stores food or pigments o Found in plants o Supports and protects the cell o Found in plants and some protists o Stores enzymes and waste products o Found in animal cells and some protists o Protein fibers that aid in cell division o Usually found in protists o Hair-like microtubules attached to the plasma membrane that often help in movement

Specialized Cells

Cell specialization in blood: red blood cells, platelets (yellow) and T lymphocytes (green)

Multicellular organisms exhibit much greater specialization. RED BLOOD CELLS, for instance, have disklike flattened membranes. This shape gives them a maximum amount of surface area while still remaining smooth enough to slide through the smallest capillaries. Because red blood cells are manufactured for limited-time use, they have lost nearly all of their internal organelles, including nuclei.

Plant cell showing Golgi apparatus, vacuole and chloroplast (a plastid)

NERVE CELLS are responsible for the rapid conduction of messages throughout the body. Consequently, they are very long and have branches that enable them to connect to other nerve cells.

GLAND CELLS, like the cells found in the pancreas, are filled with Golgi apparati. And muscle cells, which must generate large amounts of force, have huge mitochondria and many microfibers.

Nerve cells, or neurons, have many long branches to deliver messages quickly.

CELL SPECIALIZATION occurs because many forms of life have many levels of organization. In most plants, animals, and fungi, cells are organized into different types of tissues. TISSUES are groups of cells that carry out a common function.

Because cells in tissues perform specific functions, they often contain organelles that are different from the ones found in a "typical" cell. That's why the factory analogy breaks down as cells become more specialized. Not all cells have to perform all of the functions of a factory if they are found in specialized tissues or organ systems.