Postharvest Biology and Technology of Fruits, Vegetables, and Flowers

24 POSTHARVEST BIOLOGY & TECHNOLOGY OF FRUITS, VEGETABLES, & FLOWERS
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 integrity
and function 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 compartmentalized within the cell. The concentration of calcium is
maintained at the millimolar levels within the cell wall compartment (apoplast), endoplasmic
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 cytosolic
calcium levels are maintained at low micromolar (