| Cell membranes are inherited structures
that serve as barriers to the diffusion of most water-soluble
molecules and enable cells to create compartments in which
the chemical composition differs from the surroundings.
They are composed of polar lipids that form a bilayer
continuum and proteins that are responsible for most membrane
functions, including transport, receptor-based signaling,
and the generation of ATP. Membrane proteins are classified
as integral, peripheral, or lipid-anchored based on the
nature of their noncovalent association with the lipid
bilayer. Both lipids and proteins can diffuse laterally
in the plane of the bilayer.
The plasma membrane serves
as the diffusional boundary of individual cells and controls
the transport of molecules into and out of cells. Plasma
membranes contain receptors that participate in the interactions
of cells with their surroundings. The process of endocytosis
participates in the recycling of molecules from the plasma
The endomembrane system
comprises membranes that are continuous with the ER (e.g.,
nuclear envelope) or derived from the ER (e.g., Golgi
apparatus, trans-Golgi network, plasma membrane, vacuole,
transport vesicles). The ER forms a dynamic network that
permeates all regions of the cytosol and is differentiated
into numerous functional domains. The sheet-like rough
ER domains are defined by the presence of bound polysomes
that produce membrane and secretory/ vacuolar proteins,
whereas the tubular smooth ER membranes are mostly involved
in the synthesis of lipidic compounds. The Golgi apparatus
consists of flattened cisternae organized into stacks.
Its principle function is to serve as a carbohydrate factory,
adding sugars to glycoproteins and assembling complex
polysaccharides such as hemicelluloses and pectins. The
trans-Golgi network sorts secretory and vacuolar molecules
and packages them into separate vesicles. Vacuoles enable
plant cells to grow rapidly with expenditure of minimal
amounts of energy and proteins. They perform a multitude
of functions including, storage, digestion, ionic homeostasis,
defense against pathogens and herbivores, and sequestration
of toxic compounds.
The nuclear genome-containing
nucleus is surrounded by an envelope with pore complexes
that regulate trafficking between the nuclear matrix and
the cytosol. The envelope breaks down during mitosis and
reassembles around the daughter nuclei. Ribosomes are
assembled in the nucleolus and are then exported into
the cytosol. Peroxisomes and glyoxysomes are small vesicle-like
organelles that contain many enzymes in their lumen.
Peroxisomes work together
with chloroplasts and mitochondria in the glycolate pathway.
In certain tissues, peroxisomes can be transiently converted
to glyoxysomes, which participate in the mobilization
of fatty acids and in conversion of fixed N2
The term plastid refers
to a family of organelles that are developmentally related
to proplastids, reproduce by fission, and are semiautonomous.
The principal types of plastids are the photosynthetic
chloroplasts, the starch-storing amyloplasts, the carotenoid-forming
and colorful chromoplasts, and the monoterpene-producing
leucoplasts. The aqueous stroma contains the plastid’s
genetic machinery as well as enzymes and variable amounts
of internal membranes. The photosynthetic apparatus of
chloroplasts is located in thylakoid membranes, which
are organized into three-dimensional networks with granal
(stacked) and stroma-expose domains.
Mitochondria, like chloroplasts,
are semiautonomous. organelles that divide by fission.
Mitochondria consist of two membranes and an internal
matrix. Their principal function is to generate ATP, which
is produced by enzymes in the inner membrane and the matrix.