Biology Reference
In-Depth Information
interior is quite dry (
1 mMwater). The aqueous interface provides a charged or polar phys-
ical surface to help arrange related functional enzymes, known as pathways, in one plane for
increased efficiency. In contrast, the dry interior provides an environment for dehydration
reactions. Both the aqueous interface and dry interior are responsible for maintaining the
proper conformation of membrane proteins (discussed in Chapter 6).
In addition to transport, biological membranes are also the site of many other biochemical
or physiological processes including: inter-cellular communication, cell
<
cell recognition and
adhesion, cell identity and antigenicity, regulation (resident home of many receptors), intra-
cellular signaling, and some energy transduction events.
e
C. EUKARYOTE CELL STRUCTURE
While the plasma (cell) membrane defines cell boundaries, internal membranes define
a variety of cell organelles. In eukaryotes, the internal membranes also separate very
different internal aqueous chambers resulting in compartmentation into membrane-
bordered packets called organelles. Each organelle supports different sets of biological
functions.
Figure 1.2
is a cartoon depiction of a 'typical' animal (eukaryote) cell
[2]
.All
of the important membrane-bound organelles are depicted. Throughout this topic, specific
examples demonstrating aspects of membrane structure or function will be selected from
these organelle types. Below is a very brief description of the major cellular membranes.
More detailed descriptions can be readily found in many cell biology
[3
6]
,andbiochem-
e
istry
[7,8]
textbooks.
Endomembrane System
With the exception of mitochondria, peroxisomes and, in plants, chloroplasts, the intracel-
lular membranes are suspended together in the cytoplasm where they form an intercon-
nected complex. Although membrane types are unique and can be separated from one
another, all are related structurally, chemically, functionally, and developmentally. This
strong inter-relationship is referred to as the 'endomembrane system'. The endomembrane
system divides the cell into many compartments, primarily organelles, which are distinct
yet interconnected. The connected membranes include the nuclear envelope, the rough
and smooth endoplasmic reticulum, the Golgi apparatus, lysosomes, vacuoles, many types
of small vesicles, and the plasma membrane (
Figure 1.3
)
[9]
. Transport back and forth
between these various membranes and compartments is continuous and under strict regula-
tion
[10]
. The endomembrane system is the foundation of cell trafficking.
Plasma Membrane
All cells are surrounded by a plasma membrane (PM) that separates the cell contents
from the rest of the world
[11]
. The PM is the most dynamic and busiest of all cellular
membranes and more is known about the PM than any other membrane. The majority of
examples described in this topic were obtained from PM studies. The PM defines the cell's
