Agriculture Reference
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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 become embedded in
the membrane bilayer. If the protein 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 membrane. The proteins are also mobile
within the plane of the membrane. However, this process is not always random and is
regulated by the functional assembly of proteins into metabolons (photosynthetic units
in thylakoid membrane, respiratory complexes 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 in-
tegrity and function of discrete membrane compartments. This is essential for the compart-
mentalization of ions and metabolites, which may otherwise destroy the cell. For instance,
calcium ions are highly compartmentalized within the cell. The concentration of calcium is
maintained at the millimolar levels within the cell wall compartment (apoplast), endoplas-
mic reticulum, 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 cytoso-
lic calcium levels are maintained at low micromolar (
<
μ
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 cal-
cium channels. Calcium can then activate several cellular biochemical reactions that mediate
the response to the signal. Calcium is pumped back into the storage compartments 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
membrane is a key feature that regulates the absorption or extrusion of other ions and metabo-
lites such as sugars. The cell could undergo senescence if this compartmentalization is lost.
There are several factors that affect the fluidity of the membrane. The major factor that
affects the fluidity 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 products of fatty acids such as
fatty aldehydes and alkanes can also decrease the fluidity. Based on the physiological
status of the tissue, the membranes 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. The tissue has the
ability to adjust the fluidity of the membrane by altering the acyl lipid composition of
the phospholipids. For instance, an increase in the gel-phase lipid domains resulting from
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