Chapter 6: Cell Membranes
Introduction
The cell membrane regulates what enters and leaves the cytoplasm
6.1 What Is the Structure of a Biological Membrane?
The general structure of biological membranes is known as the fluid mosaic model.
Phospholipids form a bilayer, which is like a “lake” in which a variety of proteins “float.”
Phospholipids have a polar, hydrophilic “head” and hydrophobic fatty acid “tails.”
In an aqueous environment, phospholipids form a bilayer.
Artificial bilayers can be made in the laboratory.
Lipids maintain a bilayer organization spontaneously. This helps membranes fuse during
phagocytosis, vesicle formation, etc.
Lipid composition of membranes vary.
Phospholipids vary in fatty acid chain length, degree of saturation, and phosphate groups.
Animal cell membranes may be up to 25% cholesterol, which is important for membrane
integrity.
The fatty acid tails make the interior somewhat fluid, allowing lateral movement of
molecules.
Fluidity depends on temperature and lipid composition.
Cholesterol and long-chain, saturated fatty acids pack tightly, making a less-fluid
membrane.
As temperature decreases, movement of molecules and cellular processes slow. Some
organisms change the lipid content of the cell membranes when they get cold.
Membranes also contain proteins; the number varies depending on membrane function.
Peripheral membrane proteins lack exposed hydrophobic groups and do not penetrate the
bilayer.
Integral membrane proteins have hydrophobic and hydrophilic regions or domains.
Some extend across the lipid bilayer; others are partially embedded.
Freeze-fracturing is a technique that reveals proteins embedded in the phospholipid
bilayers of cellular membranes.
The proteins and lipids interact noncovalently.
© 2014 Sinauer Associates, Inc.
, But some membrane proteins have lipid groups covalently attached and are tethered to
the lipid bilayer.
Transmembrane proteins extend all the way through the phospholipid bilayer.
They have one or more transmembrane domains, and the domains on the inner and outer
sides of the membrane can have specific functions.
Peripheral membrane proteins are located on one side of the membrane.
Some membrane proteins can move freely within the bilayer, while some are anchored to
a specific region.
When cells are fused experimentally, some proteins from each cell distribute themselves
uniformly around the membrane.
Membranes are dynamic and are constantly forming, transforming, fusing, and breaking
down.
Membranes also have carbohydrates on the outer surface that serve as recognition sites
for other cells and molecules.
Glycolipids—carbohydrate + lipid
Glycoproteins—carbohydrate + protein
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Cells arrange themselves in groups by cell recognition and cell adhesion.
These processes can be studied in sponge cells—the cells are easily separated and will
come back together again.
Molecules involved in cell recognition and binding are glycoproteins.
Binding of cells is usually homotypic: The same molecule sticks out from both cells and
forms a bond.
Some binding is heterotypic: The cells have different proteins.
Cell junctions are specialized structures that hold cells together:
Tight junctions
Desmosomes
Gap junctions
Tight junctions help ensure directional movement of materials.
Desmosomes are like “spot welds.”
Gap junctions allow communication.
Cell membranes also adhere to the extracellular matrix.
© 2014 Sinauer Associates, Inc.
Introduction
The cell membrane regulates what enters and leaves the cytoplasm
6.1 What Is the Structure of a Biological Membrane?
The general structure of biological membranes is known as the fluid mosaic model.
Phospholipids form a bilayer, which is like a “lake” in which a variety of proteins “float.”
Phospholipids have a polar, hydrophilic “head” and hydrophobic fatty acid “tails.”
In an aqueous environment, phospholipids form a bilayer.
Artificial bilayers can be made in the laboratory.
Lipids maintain a bilayer organization spontaneously. This helps membranes fuse during
phagocytosis, vesicle formation, etc.
Lipid composition of membranes vary.
Phospholipids vary in fatty acid chain length, degree of saturation, and phosphate groups.
Animal cell membranes may be up to 25% cholesterol, which is important for membrane
integrity.
The fatty acid tails make the interior somewhat fluid, allowing lateral movement of
molecules.
Fluidity depends on temperature and lipid composition.
Cholesterol and long-chain, saturated fatty acids pack tightly, making a less-fluid
membrane.
As temperature decreases, movement of molecules and cellular processes slow. Some
organisms change the lipid content of the cell membranes when they get cold.
Membranes also contain proteins; the number varies depending on membrane function.
Peripheral membrane proteins lack exposed hydrophobic groups and do not penetrate the
bilayer.
Integral membrane proteins have hydrophobic and hydrophilic regions or domains.
Some extend across the lipid bilayer; others are partially embedded.
Freeze-fracturing is a technique that reveals proteins embedded in the phospholipid
bilayers of cellular membranes.
The proteins and lipids interact noncovalently.
© 2014 Sinauer Associates, Inc.
, But some membrane proteins have lipid groups covalently attached and are tethered to
the lipid bilayer.
Transmembrane proteins extend all the way through the phospholipid bilayer.
They have one or more transmembrane domains, and the domains on the inner and outer
sides of the membrane can have specific functions.
Peripheral membrane proteins are located on one side of the membrane.
Some membrane proteins can move freely within the bilayer, while some are anchored to
a specific region.
When cells are fused experimentally, some proteins from each cell distribute themselves
uniformly around the membrane.
Membranes are dynamic and are constantly forming, transforming, fusing, and breaking
down.
Membranes also have carbohydrates on the outer surface that serve as recognition sites
for other cells and molecules.
Glycolipids—carbohydrate + lipid
Glycoproteins—carbohydrate + protein
6.2 How Is the Plasma Membrane Involved In Cell Adhesion and
Recognition?
Cells arrange themselves in groups by cell recognition and cell adhesion.
These processes can be studied in sponge cells—the cells are easily separated and will
come back together again.
Molecules involved in cell recognition and binding are glycoproteins.
Binding of cells is usually homotypic: The same molecule sticks out from both cells and
forms a bond.
Some binding is heterotypic: The cells have different proteins.
Cell junctions are specialized structures that hold cells together:
Tight junctions
Desmosomes
Gap junctions
Tight junctions help ensure directional movement of materials.
Desmosomes are like “spot welds.”
Gap junctions allow communication.
Cell membranes also adhere to the extracellular matrix.
© 2014 Sinauer Associates, Inc.
