Cells, the basic units of life, require membranes
for their existence. Foremost among these is the
plasma membrane, which defines each cell’s boundary
and helps create and maintain electrochemically
distinct environments within and outside the cell.
Other membranes enclose eukaryotic organelles
such as the nucleus, chloroplasts, and mitochondria.
Membranes also form internal compartments, such
as the endoplasmic reticulum (ER) in the cytoplasm
and thylakoids in the chloroplast (Fig. 1.1).
The principal function
of membranes is to serve as a barrier to the diffusion
of most water-soluble molecules. These barriers
delimit compartments wherein the chemical composition
can differ from the surroundings and can be optimized
for a particular activity. Membranes also serve
as scaffolding for certain proteins. As membrane
components, proteins perform a wide array of functions:
transporting molecules and transmitting signals
across the membrane, processing lipids enzymatically,
assembling glycoproteins and polysaccharides,
and providing mechanical links between cytosolic
and cell wall compounds.
This chapter is
divided into two parts. The first is devoted to
the general features and molecular organization
of membranes. The second provides an introduction
to the architecture and functions of the different
membranous organelles of plant cells. Many later
chapters of this text focus on metabolic events
that involve these organelles.