490 Part V: Fruits, Vegetables, and Cereals
across the membrane. These differences usually be-
come operational during signal transduction events.
An essential characteristic of the membrane is its
fluidity. The fluid-mosaic model of the membrane
(Singer and Nicholson 1972) depicts the membrane
as a planar matrix comprised of phospholipids and
proteins. The proteins are embedded in the mem-
brane bilayer (integral proteins) or are bound to the
periphery (peripheral proteins). The nature of this
interaction stems from the structure of the proteins.
If the proteins have a much larger proportion of
hydrophobic amino acids, they would tend to be-
come embedded in the membrane bilayer. If the pro-
tein contains more hydrophilic amino acids, it may
tend to prefer a more aqueous environment and thus
remain as a peripheral protein. In addition, proteins
may be covalently attached to phospholipids such
as phosphatidylinositol. Proteins that remain in the
cytosol may also become attached to the membrane
in response to an increase in cytosolic calcium
levels. The membrane is a highly dynamic entity.
The semifluid nature of the membrane allows for
the movement of phospholipids in the plane of the
membrane and between the bilayers of the mem-
brane. The proteins are also mobile within the plane
of the membrane. However, this process is not al-
ways random, and it is regulated by the functional
assembly of proteins into metabolons (photosynthet-
ic units in thylakoid membrane, respiratory com-
plexes in the mitochondria, cellulose synthase on
plasma membrane, etc.), their interactions with the
underlying cytoskeletal system (network of proteins
such as actin and tubulin), and the fluidity of the
membrane.
The maintenance of homeostasis (life processes)
requires the maintenance of the integrity and func-
tion of discrete membrane compartments. This is
essential for the compartmentalization of ions and
metabolites, which may otherwise destroy the cell.
For instance, calcium ions are highly compartmen-
talized within the cell. The concentration of calcium
is maintained at millimolar levels within the cell
wall compartment (apoplast), endoplasmic reticu-
lum, and the tonoplast (vacuole). This is achieved by
energy-dependent extrusion of calcium from the
cytoplasm into these compartments by ATPases. As
a result, the cytosolic calcium levels are maintained
at low micromolar ( 1 M) levels. Maintenance of
this concentration gradient across the membrane is a
key requirement for the signal transduction events,
as regulated entry of calcium into the cytosol can
be achieved simply by opening calcium channels.
Calcium can then activate several cellular biochemi-
cal reactions that mediate the response to the signal.
Calcium is pumped back into the storage compart-
ments when the signal diminishes in intensity. In
a similar manner, cytosolic pH is highly regulated
by the activity of proton ATPases. The pH of the
apoplast and the vacuole is maintained near 4,
whereas the pH of the cytosol is maintained in the
range of 6–6.5. The pH gradient across the mem-
brane is a key feature that regulates the absorption or
extrusion of other ions and metabolites such as sug-
ars. The cell could undergo senescence if this com-
partmentalization is lost.
There are several factors that affect the fluidity of
the membrane. The major factor that affects the flu-
idity is the type and proportion of acyl chain fatty
acids of the phospholipids. At a given temperature, a
higher proportion of unsaturated fatty acyl chains
(oleic, linoleic, linolenic) in the phospholipids can
increase the fluidity of the membrane. An increase in
saturated fatty acids such as palmitic and stearic
acids can decrease the fluidity. Other membrane
components such as sterols, and degradation prod-
ucts of fatty acids such as fatty aldehydes, alkanes
and so on, can also decrease the fluidity. Based on
the physiological status of the tissue, the membrane
can exist in either a liquid crystalline state (where
the phospholipids and their acyl chains are mobile)
or a gel state (where they are packed as rigid ordered
structures and their movements are much restricted).
The membrane usually has coexisting domains of
liquid crystalline and gel phase lipids, depending on
growth conditions, temperature, ion concentration
near the membrane surface, and so on. The tissue
has the ability to adjust the fluidity of the membrane
by altering the acyl lipid composition of the phos-
pholipids. For instance, an increase in the gel phase
lipid domains resulting from exposure to cold tem-
perature could be counteracted by increasing the
proportion of fatty acyl chains having a higher de-
gree of unsaturation, and therefore a lower melting
point. Thus, the membrane will tend to remain fluid
even at a lower temperature. An increase in gel
phase lipid domains can result in the loss of com-
partmentalization. The differences in the mobility
properties of phospholipid acyl chains can cause
packing imperfections at the interface between gel
and liquid crystalline phases, and these regions can