Organization of the Body
In secondary active transport, the energy of an ion gradient
(produced by a primary active transport process) is used to
transport a substance passively. Many active transport systems are
coupled, and cotransported substances move in either the same
(symport) or opposite (antiport) directions across the membrane.
Vesicular transport also requires that energy be provided.
Endocytosis brings substances into the cell, typically in protein-
coated vesicles. If the substance is relatively large particles, the
process is called phagocytosis. If the substance is dissolved
molecules, the process is pinocytosis. Receptor-mediated
endocytosis is selective: Engulfed molecules attach to receptors
on the membrane before endocytosis occurs. Exocytosis, which
uses SNAREs to anchor the vesicles to the plasma membrane,
ejects substances (hormones, wastes, secretions) from the cell.
Most endocytosis (and transcytosis) is mediated by clathrin-
coated vesicles. Other types of protein coating are found in
caveolae and vesicles involved in vesicular traﬃcking. Caveolae
appear to be important as sites that accumulate receptors
involved in cell signaling.
The Plasma Membrane: Generation of a Resting
All cells in the resting stage exhibit a voltage across their
membrane, called the resting membrane potential. Because of the
membrane potential, both concentration and electrical gradients
determine the ease of an ion’s diﬀusion.
Selective Diffusion Establishes Membrane Potential
Te resting membrane potential is generated by concentration
gradients of ions and the diﬀerential permeability of the
plasma membrane to ions, particularly potassium ions. Sodium
is in high extracellular concentration and low intracellular
concentration, and the membrane is poorly permeable to
it. Potassium is in high concentration in the cell and low
concentration in the extracellular ﬂuid. Te membrane is more
permeable to potassium than to sodium. Protein anions in the
cell are too large to cross the membrane and Cl
, the main
anion in extracellular ﬂuid, is repelled by the negative charge on
the inner membrane face.
Active Transport Maintains Electrochemical Gradients
Essentially, a negative membrane potential is established when
the movement of K
out of the cell equals K
movement into the
movements across the membrane contribute minimally
to establishing the membrane potential. Te greater outward
diﬀusion of potassium (than inward diﬀusion of sodium) leads
to a charge separation at the membrane (inside negative). Tis
charge separation is maintained by the operation of the sodium-
Nervous System I; Topics: Ion Channels, pp. 3, 8, 9;
The Membrane Potential, pp. 1–17.
The Plasma Membrane: Cell-Environment
Cells interact directly and indirectly with other cells. Indirect
interactions involve extracellular chemicals carried in body ﬂuids
or forming part of the extracellular matrix.
Molecules of the glycocalyx are intimately involved in cell-
environment interactions. Most are cell adhesion molecules or
Activated membrane receptors act as catalysts, regulate
channels, or, like G protein–linked receptors, act through
second messengers such as cyclic AMP and Ca
The Plasma Membrane: Structure
Te plasma membrane encloses cell contents, mediates exchanges
with the extracellular environment, and plays a role in cellular
The Fluid Mosaic Model
Te plasma membrane is a ﬂuid bilayer of lipids (phospholipids,
cholesterol, and glycolipids) within which proteins are inserted.
Te lipids have both hydrophilic and hydrophobic regions that
organize their aggregation and self-repair. Te lipids form the
structural part of the plasma membrane.
Most proteins are integral transmembrane proteins that extend
entirely through the membrane. Some, appended to the integral
proteins, are peripheral proteins.
Proteins are responsible for most specialized membrane
functions: Some are enzymes, some are receptors, and others
mediate membrane transport functions.
Externally facing glycoproteins contribute to the glycocalyx.
Cell junctions join cells together and may aid or inhibit
movement of molecules between or past cells.
±ight junctions are impermeable junctions. Desmosomes
mechanically couple cells into a functional community. Gap
junctions allow joined cells to communicate.
The Plasma Membrane: Membrane Transport
Te plasma membrane acts as a selectively permeable barrier.
Substances move across the plasma membrane by passive
processes, which depend on the kinetic energy of molecules,
and by active processes, which depend on the use of cellular
Diﬀusion is the movement of molecules (driven by kinetic
energy) down a concentration gradient. Fat-soluble solutes can
diﬀuse directly through the membrane by dissolving in the lipid.
Facilitated diﬀusion is the passive movement of certain solutes
across the membrane either by their binding with a membrane
carrier protein or by their moving through a membrane channel.
As with other diﬀusion processes, it is driven by kinetic energy,
but the carriers and channels are selective.
Osmosis is the diﬀusion of a solvent, such as water, through
a selectively permeable membrane. Water diﬀuses through
membrane channels (aquaporins) or directly through the lipid
portion of the membrane from a solution of lesser osmolarity
(total concentration of all solute particles) to a solution of greater
Te presence of solutes unable to permeate the plasma membrane
leads to changes in cell tone that may cause the cell to swell or
shrink. Net osmosis ceases when the solute concentration on
both sides of the plasma membrane reaches equilibrium.
Solutions that cause a net loss of water from cells are hypertonic.
Tose causing net water gain are hypotonic. Tose causing
neither gain nor loss of water are isotonic.
Active transport (solute pumping) depends on a carrier protein
and energy. Substances transported move against concentration
or electrical gradients. In primary active transport, such as that
provided by the Na
pump, A±P directly provides the energy.