Cell Membranes
- Life at the Edge
a.) The plasma membrane is the boundary that separates the living cell from its
surroundings.
b.) The plasma membrane exhibits selective permeability, allowing some substances to
cross it more easily than others.
- Cellular membranes are fluid mosaics of lipids and proteins
a.) Phospholipids are the most abundant lipid in the plasma membrane
b.) Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic
regions
c.) A phospholipid bilayer can exist as a stable boundary between two aqueous
compartments
- The fluid mosaic model states that a membrane is a fluid-structure with a “mosaic” of
various proteins embedded in it.
- Proteins are not randomly distributed in the membrane
- The Fluidity of Membranes
a.) Phospholipids in the plasma membrane can move within the bilayer
b.) Most of the lipids and some proteins, drift laterally
c.) Rarely, a lipid may flip-flop transversely across the membrane
- As the temperatures cool, the membranes switch from a fluid state to a solid state
- The temperature at which a membrane solidifies depends on the types of lipids
- Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty
acids
- Membranes must be fluid to work properly; they are usually about as fluid as a salad oil
- Membrane Proteins and their Functions
a.) A membrane is a collage of different proteins, often grouped together, embedded in
the fluid matrix of the lipid bilayer
b.) Proteins determine most of the membrane’s specific functions
- Peripheral proteins are bound to the surface of the membrane
- Integral proteins penetrate the hydrophobic core
- Integral proteins that span the membrane are called transmembrane proteins
- The hydrophobic regions of an integral protein consist of one or more stretches of
nonpolar amino acids, often coiled into alpha-helices
, - Six major functions of membrane proteins
- Transport
- Enzymatic activity
- Signal transduction
- Cell-cell recognition
- Intercellular joining
- Attachment to the cytoskeleton and extracellular matrix (ECM)
- The role of membrane carbohydrates in cell-cell recognition
- cells recognize each other by binding to molecules, often containing carbohydrates, on
the extracellular surface of the plasma membrane
- membrane of carbohydrates may be covalently bonded to lipids (forming glycolipids) or
more commonly to proteins (forming glycoproteins)
-Exocytosis
- Membrane structure results in selective permeability
a.) A cell must exchange materials with its surroundings, a process controlled by the
plasma membrane
b.) Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
- Transport proteins
- allow passage of hydrophilic substances across the membrane
- some transport proteins, called channel proteins, have a hydrophilic channel that certain
molecules or ions can use as a tunnel
- channel proteins called aquaporins facilitate the passage of water
- Other transport proteins, called carrier proteins, bind to molecules and change shape to
shuttle them across the membrane’
- A transport protein is specific for the substance it moves
- Passive transport is the diffusion of a substance across a membrane with no
energy investment
- Diffusion is the tendency for molecules to spread out evenly into the available
space
- Although each molecule moves randomly, diffusion of a population of molecules
may be directional
- At dynamic equilibrium, as many molecules cross the membrane in one direction
as in the other
- Life at the Edge
a.) The plasma membrane is the boundary that separates the living cell from its
surroundings.
b.) The plasma membrane exhibits selective permeability, allowing some substances to
cross it more easily than others.
- Cellular membranes are fluid mosaics of lipids and proteins
a.) Phospholipids are the most abundant lipid in the plasma membrane
b.) Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic
regions
c.) A phospholipid bilayer can exist as a stable boundary between two aqueous
compartments
- The fluid mosaic model states that a membrane is a fluid-structure with a “mosaic” of
various proteins embedded in it.
- Proteins are not randomly distributed in the membrane
- The Fluidity of Membranes
a.) Phospholipids in the plasma membrane can move within the bilayer
b.) Most of the lipids and some proteins, drift laterally
c.) Rarely, a lipid may flip-flop transversely across the membrane
- As the temperatures cool, the membranes switch from a fluid state to a solid state
- The temperature at which a membrane solidifies depends on the types of lipids
- Membranes rich in unsaturated fatty acids are more fluid than those rich in saturated fatty
acids
- Membranes must be fluid to work properly; they are usually about as fluid as a salad oil
- Membrane Proteins and their Functions
a.) A membrane is a collage of different proteins, often grouped together, embedded in
the fluid matrix of the lipid bilayer
b.) Proteins determine most of the membrane’s specific functions
- Peripheral proteins are bound to the surface of the membrane
- Integral proteins penetrate the hydrophobic core
- Integral proteins that span the membrane are called transmembrane proteins
- The hydrophobic regions of an integral protein consist of one or more stretches of
nonpolar amino acids, often coiled into alpha-helices
, - Six major functions of membrane proteins
- Transport
- Enzymatic activity
- Signal transduction
- Cell-cell recognition
- Intercellular joining
- Attachment to the cytoskeleton and extracellular matrix (ECM)
- The role of membrane carbohydrates in cell-cell recognition
- cells recognize each other by binding to molecules, often containing carbohydrates, on
the extracellular surface of the plasma membrane
- membrane of carbohydrates may be covalently bonded to lipids (forming glycolipids) or
more commonly to proteins (forming glycoproteins)
-Exocytosis
- Membrane structure results in selective permeability
a.) A cell must exchange materials with its surroundings, a process controlled by the
plasma membrane
b.) Plasma membranes are selectively permeable, regulating the cell’s molecular traffic
- Transport proteins
- allow passage of hydrophilic substances across the membrane
- some transport proteins, called channel proteins, have a hydrophilic channel that certain
molecules or ions can use as a tunnel
- channel proteins called aquaporins facilitate the passage of water
- Other transport proteins, called carrier proteins, bind to molecules and change shape to
shuttle them across the membrane’
- A transport protein is specific for the substance it moves
- Passive transport is the diffusion of a substance across a membrane with no
energy investment
- Diffusion is the tendency for molecules to spread out evenly into the available
space
- Although each molecule moves randomly, diffusion of a population of molecules
may be directional
- At dynamic equilibrium, as many molecules cross the membrane in one direction
as in the other
