Biology Reference
In-Depth Information
B. GENERAL MEMBRANE FUNCTIONS
It is now generally agreed that biological membranes are probably somehow involved in
all cellular activities. The most obvious function of any membrane is separating two aqueous
compartments. For the plasma membrane this involves separation of the cell contents from
the very different extra-cellular environment. Membranes are therefore responsible for
containment, ultimately delineating the cell. Separation, however, cannot be absolute, as
the cell must be able to take up essential nutrients, gases, and solutes from the exterior, while
simultaneously removing toxic waste products from the interior. A biological membrane
therefore must be selectively permeable, possessing the ability to distinguish many chemi-
cally different solutes and knowing in which direction to redistribute them. Biological
membranes must therefore house a variety of specific, vectorial transport systems (discussed
in Chapter 14).
A characteristic of all living cells is the establishment and maintenance of trans-membrane
gradients of all solutes. Of particular interest are large ion gradients typically associated with
the plasma membrane.
Table 1.1
is a comparison of the mean concentration of selected ions
inside and outside a typical mammalian cell, and the magnitude of each gradient. To main-
tain gradients of this size, efficient energy-dependent transport systems must be employed
(discussed in Chapter 14). Directional trans-membrane structure is required to generate these
ion gradients.
In addition to trans-membrane structure, it is now believed that biological membranes are
composed of countless numbers of very small, transient, lateral lipid microdomains. Each of
these domains is proposed to have a different lipid and resident protein composition. Thus
the activity of any membrane must reflect the sum of the activities of its many specific
domains. One type of lipid microdomain, termed a 'lipid raft', has received a lot of recent
attention as it is reputed to be involved in a variety of important cell signaling events. If the
lipid raft story (discussed in Chapter 8) holds up, this new paradigm for membrane struc-
ture/function may serve as a model for other types of as yet undiscovered non-raft domains.
Each of these domains might then support a different collection of related biochemical activ-
ities. Therefore, membranes have both trans-membrane and lateral structures that are just
beginning to be understood.
All membranes possess an extreme water gradient across their very thin (~5 nm) structure.
In the membrane aqueous bathing solution water concentration is ~55.5 M water in water
(1,000 g of water per liter divided by 18, the molecular weight of water), while the membrane
TABLE 1.1
Trans-membrane Ion Gradients of a 'Typical' Mammalian Cell.
Ion
Inside
Outside
Gradient
Na
þ
10 mM
140 mM
14-fold
K
þ
140 mM
4 mM
35-fold
Ca
2
þ
1.0
m
M
1.0 mM
1,000-fold
Cl
100 mM
4.0 mM
25-fold
