Week 3: Membranes
Action at the boundaries:
o Receiving informtaion
o Import and export of small molecules
o Capacity for movement and expansion
Membranes allow compartmentalization, creates different
microenvironments
Membranes physically separate reactions, but should also allow
communication between compartment
Membrane structure
Osmosis: the presence or absence of cell walls
influences how cells respond to osmotic
fluctuations in their environment
Isotonic solutions:
o When two environments are isotonic, the total molar
concentration of dissolved solutes is the same in both of them
o When cells are in isotonic solution, movement of water out the
cell is exactly balanced by movement of water into cell.
Hypotonic solutions:
o Hypotonic comes form the Greek “hypo” meaning under, and
tonos meaning stretching. In a hypotonic solution the total molar
concentration of all dissolved solute particlews is less than that of
another solution or less than that of a cell
o If concentrations of dissolved solutes are less outside the cell
than inside, the concentration of water outside is correspondingly
greater. When a cell is exposed to such conditions, there is net
water movement onto the cell (they can burst without a cell wall
(lysis) or turgor)
Hypertonic solutions:
o In a hypertonic solution the total molar concentration of all
dissolved solute particles is greater than that of another solution,
or greater than the concentration in a cell
o If concentrations of dissolved solutes are greater outside the cell,
the concentration of water outside is correspongingly lower. As a
result water inside the cell will flow outwards to attain
equilibrium, causing the cell to shrink (plant cell = plasmolysis)
Membranes are amphipathic
o Membranes mostly consist of phospholipids, molecules with a
hydrophilic and hydrophobic end
o A typical phospholipid has a polar, hydrophilic head group, and
two hydrophobic fatty acid tails
Membranes also contain other lipids
o The exact groups differ among membrane lipids, but all are
amphipathic
, o Chemical properties of these different lipids are diverse, as are
the membranes that carry them
Behavior of different molecules in water
o Hydrophilic molecules attract water, while hydrophobic molecules
tend to avoid water
Lipids naturally organize into bilayers and compartments
o Phospholipids organize such that the hydrophobic tails stick
together, and polar head groups face the water phase
o Energy minimization leads to the formation of spherical
compartments
Chemical composition determines membrane properties
o Cholesterol is structurally more rigid than phospholipids,
and tends to create stiffer membrane domains
The cell can control the composition of the membrane,
enzymes such as scramblase and flippase can help generate
uniform or asymmetric bilayers
Both layers of the phospholipid bilayers can have different
composition, the different properties of head groups can give
both sides of the membrane unique properties
Membranes have and orientation (picture)
Membranes have a high protein concentration
They contain various different integral proteins, they can be inserted
in the membrane in different ways
o In addition to
transmembrane
helices, proteins
can be inserted in
the membrane
through lipid
modification or by
binding to a
membrane protein
Hydrophobic and
amphipathic helices
o A transmembrane helix is usually hydrophobic, as it is surrounded
by the phospholipid tail groups.
o In some proteins, amphipathic helices allow making a hydrophilic
pore
Transport across membranes
Membranes have limited, selective permeability:
o Small, nonpolar molecules like O2, CO2, N2, steroid and
hormones, get true easily
o Small, uncharged polar molecules like H2O, ethanol and glycerol
can get true but not a lot
Action at the boundaries:
o Receiving informtaion
o Import and export of small molecules
o Capacity for movement and expansion
Membranes allow compartmentalization, creates different
microenvironments
Membranes physically separate reactions, but should also allow
communication between compartment
Membrane structure
Osmosis: the presence or absence of cell walls
influences how cells respond to osmotic
fluctuations in their environment
Isotonic solutions:
o When two environments are isotonic, the total molar
concentration of dissolved solutes is the same in both of them
o When cells are in isotonic solution, movement of water out the
cell is exactly balanced by movement of water into cell.
Hypotonic solutions:
o Hypotonic comes form the Greek “hypo” meaning under, and
tonos meaning stretching. In a hypotonic solution the total molar
concentration of all dissolved solute particlews is less than that of
another solution or less than that of a cell
o If concentrations of dissolved solutes are less outside the cell
than inside, the concentration of water outside is correspondingly
greater. When a cell is exposed to such conditions, there is net
water movement onto the cell (they can burst without a cell wall
(lysis) or turgor)
Hypertonic solutions:
o In a hypertonic solution the total molar concentration of all
dissolved solute particles is greater than that of another solution,
or greater than the concentration in a cell
o If concentrations of dissolved solutes are greater outside the cell,
the concentration of water outside is correspongingly lower. As a
result water inside the cell will flow outwards to attain
equilibrium, causing the cell to shrink (plant cell = plasmolysis)
Membranes are amphipathic
o Membranes mostly consist of phospholipids, molecules with a
hydrophilic and hydrophobic end
o A typical phospholipid has a polar, hydrophilic head group, and
two hydrophobic fatty acid tails
Membranes also contain other lipids
o The exact groups differ among membrane lipids, but all are
amphipathic
, o Chemical properties of these different lipids are diverse, as are
the membranes that carry them
Behavior of different molecules in water
o Hydrophilic molecules attract water, while hydrophobic molecules
tend to avoid water
Lipids naturally organize into bilayers and compartments
o Phospholipids organize such that the hydrophobic tails stick
together, and polar head groups face the water phase
o Energy minimization leads to the formation of spherical
compartments
Chemical composition determines membrane properties
o Cholesterol is structurally more rigid than phospholipids,
and tends to create stiffer membrane domains
The cell can control the composition of the membrane,
enzymes such as scramblase and flippase can help generate
uniform or asymmetric bilayers
Both layers of the phospholipid bilayers can have different
composition, the different properties of head groups can give
both sides of the membrane unique properties
Membranes have and orientation (picture)
Membranes have a high protein concentration
They contain various different integral proteins, they can be inserted
in the membrane in different ways
o In addition to
transmembrane
helices, proteins
can be inserted in
the membrane
through lipid
modification or by
binding to a
membrane protein
Hydrophobic and
amphipathic helices
o A transmembrane helix is usually hydrophobic, as it is surrounded
by the phospholipid tail groups.
o In some proteins, amphipathic helices allow making a hydrophilic
pore
Transport across membranes
Membranes have limited, selective permeability:
o Small, nonpolar molecules like O2, CO2, N2, steroid and
hormones, get true easily
o Small, uncharged polar molecules like H2O, ethanol and glycerol
can get true but not a lot
