to the nuclear membrane and, in some cases, it is connected directly through small
openings to the exterior of the cell. A second function of the endoplasmic reticulum is to
transport various substances, through the vast network of tubules, from one part of the
cell to another area of the cell. A third function of the endoplasmic reticulum is to store
various substances within the cell.
h. Ribosomes (Figure 2-2). Ribosomes are small particles that are usually
attached to the endoplasmic reticulum. Ribosomes are the site of protein synthesis and
are referred to as "protein factories" of the cell. Ribosome is composed mainly of
ribonucleic acid (RNA).
Pinocytosis is the engulfing of small particles or fluids by the cell. That is, when
these substances meet the cell membrane, they cause the membrane to form a
channel. At the end of this channel, small vesicles form. These vesicles contain the
substance and some extracellular fluid. The vesicle then breaks away from the rest of
the membrane and migrates toward the center of the cell. Figure 2-3 illustrates the
process of pinocytosis.
1. Particles contact cell
2. Vesicle (saclike structure) is
3. Vesicle containing the
particles passes into the cell.
Figure 2-3. Pinocytosis.