Chapter 5 - Membrane Transport and Cell Signaling
Life at the Edge
Plasma membrane exhibits selective permeability, it allows some substances to cross it more easily than
others. The resulting ability of the cell to discriminate in its chemical exchanges with its environment is
fundamental to life.
5.1 Cellular membranes are fluid mosaics of lipids and proteins
A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic
region.
A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the
molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the
hydrophilic heads to water.
In the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of
phospholipids. Groups of proteins are often associated in long-lasting, specialized patches, as are certain
lipids.
The membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent
bonds.
Cholesterol within the animal cell membrane reduces membrane fluidity at moderate temperatures by
reducing phospholipid movement, but at low temperatures it hinders solicitation by disrupting the regular
packing of phospholipids.
Fishes that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails,
enabling their membranes to remain fluid. Some bacteria and archaea thrive in thermal hot springs and
geysers. Their membranes include unusual lipids that help prevent excessive fluidity at such high
temperatures.
In many plants that tolerate extreme bold, the percentage unsaturated phospholipids increases in autumn,
keeping the membrane from solidifying during winter.
, More than 50 kinds of proteins have been found so far in the plasma membrane of red blood cells, for
example.
Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane
proteins that span the membrane; other integral protein extends only partway into the hydrophobic interior.
The hydrophobic regions of an integral protein consist of one or more stretches of
nonpolar amino acids, usually coiled into α helices.
Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages
loosely bound to the surface of the membrane, often to exposed parts of integral
proteins.
On the cytoplasmic side of the plasma membrane, some membrane proteins are held
in place by attachment to the cytoskeleton. On the extracellular side, certain
membrane proteins are attached to fibers or the extracellular matrix.
Transport protein is a protein that spans the membrane that may provide a hydrophilic
channel across the membrane that is selective for a particular solute. Some of these
proteins hydrolyze ATP as an energy source to actively pump substances across the
membrane.
Enzymatic protein is a protein built into the membrane that may be an enzyme with its active site exposed
to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a
team that carries out sequential steps of a metabolic pathway.
Attachment to the cytoskeleton and extracellular matrix (EMC) are microfilaments or other elements of the
cytoskeleton may be noncovalently bound to membrane proteins. Its function is to help maintain cell shape
and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can
coordinate extracellular and intracellular changes.
Cell-cell recognition is when some glycoproteins serve as identification tags that are specifically recognized
by membrane proteins of other cells. This type of cell-cell binding is usually short-lived compared to
intercellular joining.
Intercellular joining is when membrane proteins of adjacent cells hooking together in various kinds of
junctions, such as gap junctions or tight junctions. This type of biding is more long-lasting than cell-cell
recognition.
Signal transduction is a membrane protein/receptor may have a binding site with a specific shape that fits
the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may
Life at the Edge
Plasma membrane exhibits selective permeability, it allows some substances to cross it more easily than
others. The resulting ability of the cell to discriminate in its chemical exchanges with its environment is
fundamental to life.
5.1 Cellular membranes are fluid mosaics of lipids and proteins
A phospholipid is an amphipathic molecule, meaning it has both a hydrophilic region and a hydrophobic
region.
A phospholipid bilayer can exist as a stable boundary between two aqueous compartments because the
molecular arrangement shelters the hydrophobic tails of the phospholipids from water while exposing the
hydrophilic heads to water.
In the fluid mosaic model, the membrane is a mosaic of protein molecules bobbing in a fluid bilayer of
phospholipids. Groups of proteins are often associated in long-lasting, specialized patches, as are certain
lipids.
The membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent
bonds.
Cholesterol within the animal cell membrane reduces membrane fluidity at moderate temperatures by
reducing phospholipid movement, but at low temperatures it hinders solicitation by disrupting the regular
packing of phospholipids.
Fishes that live in extreme cold have membranes with a high proportion of unsaturated hydrocarbon tails,
enabling their membranes to remain fluid. Some bacteria and archaea thrive in thermal hot springs and
geysers. Their membranes include unusual lipids that help prevent excessive fluidity at such high
temperatures.
In many plants that tolerate extreme bold, the percentage unsaturated phospholipids increases in autumn,
keeping the membrane from solidifying during winter.
, More than 50 kinds of proteins have been found so far in the plasma membrane of red blood cells, for
example.
Integral proteins penetrate the hydrophobic interior of the lipid bilayer. The majority are transmembrane
proteins that span the membrane; other integral protein extends only partway into the hydrophobic interior.
The hydrophobic regions of an integral protein consist of one or more stretches of
nonpolar amino acids, usually coiled into α helices.
Peripheral proteins are not embedded in the lipid bilayer at all; they are appendages
loosely bound to the surface of the membrane, often to exposed parts of integral
proteins.
On the cytoplasmic side of the plasma membrane, some membrane proteins are held
in place by attachment to the cytoskeleton. On the extracellular side, certain
membrane proteins are attached to fibers or the extracellular matrix.
Transport protein is a protein that spans the membrane that may provide a hydrophilic
channel across the membrane that is selective for a particular solute. Some of these
proteins hydrolyze ATP as an energy source to actively pump substances across the
membrane.
Enzymatic protein is a protein built into the membrane that may be an enzyme with its active site exposed
to substances in the adjacent solution. In some cases, several enzymes in a membrane are organized as a
team that carries out sequential steps of a metabolic pathway.
Attachment to the cytoskeleton and extracellular matrix (EMC) are microfilaments or other elements of the
cytoskeleton may be noncovalently bound to membrane proteins. Its function is to help maintain cell shape
and stabilizes the location of certain membrane proteins. Proteins that can bind to ECM molecules can
coordinate extracellular and intracellular changes.
Cell-cell recognition is when some glycoproteins serve as identification tags that are specifically recognized
by membrane proteins of other cells. This type of cell-cell binding is usually short-lived compared to
intercellular joining.
Intercellular joining is when membrane proteins of adjacent cells hooking together in various kinds of
junctions, such as gap junctions or tight junctions. This type of biding is more long-lasting than cell-cell
recognition.
Signal transduction is a membrane protein/receptor may have a binding site with a specific shape that fits
the shape of a chemical messenger, such as a hormone. The external messenger (signaling molecule) may
