Biology Reference
In-Depth Information
FIGURE 1.3
The
endomembrane system of
a 'typical' animal cell.
The various membranes
and compartments are
connected to one another
structurally, chemically,
functionally, and devel-
opmentally
[9]
.
boundary and its interaction with the external environment. It is responsible for transport-
ing nutrients into the cell while allowing waste products to leave. Thus, it prevents
unwanted materials from entering the cell while keeping needed materials from escaping.
It maintains the pH of the cytosol, and preserves the proper cytosolic osmotic pressure.
Proteins on the PM surface assist the cell in recognizing and interacting with neighboring
cells. Other proteins on the plasma membrane allow attachment to the cytoskeleton and
extracellular matrix, functions that maintain cell shape and fix the location of membrane
proteins. The PM contains several characteristic functions and structures that can be used
to identify PM fractions (Chapter 12). Tight junctions seal contacts between cells while
desmosomes are adhesion sites between adjacent cells. Gap junctions (Chapter 14) contain
hexagonal arrays of pores that allow communication between adjacent cells. Caveolae and
coated pits (Chapter 14) are similar shaped PM invaginations that are involved in cell
signaling and solute uptake, respectively. Plasma membranes are indeed complex entities.
Nuclear Envelope (Membrane)
The nuclear envelope is a double membrane surrounding a perinuclear space
[12]
. This
space is probably contiguous with the lumen of the endoplasmic reticulum. The envelope
has large nuclear pores (about 600
˚
) that allow passage of large RNA-protein complexes
out of the nucleus into the cytoplasm and movement of regulatory proteins from the cyto-
plasm into the nucleus.
