Essentials of Anatomy and Physiology

“switched on,” in a particular cell. These active genes
are the codes for the proteins necessary for the specific
cell type and its functions. For example, the gene for
insulin is present in all human cells, but only in certain
islet cells of the pancreas is the gene active or switched
on. Only in these cells is insulin produced. How the
genetic code in chromosomes is translated into pro-
teins will be covered in a later section.

CYTOPLASM AND CELL ORGANELLES

Cytoplasmis a watery solution of minerals, gases,

organic molecules, and cell organelles that is found

between the cell membrane and the nucleus. Cytosol

is the water portion of cytoplasm, and many chemical

reactions take place within it. Cell organelles are

intracellular structures, often bounded by their own

50 Cells

Nuclear

membrane

Nucleolus

Nucleus

Chromatin

Golgi apparatus

Cilia

Microvilli

Smooth endoplasmic

reticulum

Centrioles

Cell membrane

Rough endoplasmic

reticulum

Ribosomes

Cytoplasm

Mitochondrion

Lysosome

Proteasome

Figure 3–2. Generalized human cell depicting the structural components. See text and

Table 3–1 for descriptions.

QUESTION:How do cilia differ in structure from microvilli?