B2.1 Membrane and membrane transport
Lipid bilayers as the basis of cell membranes:
Membranes: essential components of cells
Bilayer of phospholipids and other amphipathic molecules (forms a continuous sheet and controls
the passage of molecules)
Plasma membrane: border between a cell and its environment
Membranes inside eukaryotic cells: divide the cytoplasm into compartments
Phospholipids: phosphate head (hydrophilic) & 2 hydrocarbon tails (hydrophobic)
The tails interact with each other to form the core of biological membranes
There are usually aqueous solutions on either side of cell membranes. The solutes nearest to the membrane
surface might penetrate between the hydrophilic phosphate heads of phospholipids, but if they reach the
hydrophobic core of the membrane they are drawn back to the aqueous solution outside the membrane.
Hydrophobic hydrocarbon chains (core of the membrane): don’t repel hydrophilic solutes but they
are more attracted to each other, and the solutes are more attracted to water outside the
membrane.
Permeability: ability of a molecule to pass through a membrane
The larger the molecule, the lower the permeability
Depends on hydrophilic / hydrophobic nature
Simple diffusion of molecules:
Diffusion: spreading out of particles in liquids and gases that happens because the particles are in a
continuous random motion
Net movement of particles from the higher to the lower concentration (movement down the
concentration gradient)
Passive process: no energy needed
Movement of molecules results in equilibrium.
Simple diffusion across membranes: due to particles passing between phospholipids in the membrane
Only happens if the phospholipid bilayer is permeable to the particles.
Non-polar particles (oxygen): diffuse through easily
If oxygen concentration lowers inside cell due to aerobic respiration (use of oxygen) and the
concentration outside is higher, oxygen will pass into the cell by diffusion
, Ions (positive or negative charges): can’t easily diffuse as the center of the membrane is hydrophobic
Ions are hydrophilic
Polar molecules (partial positive and negative chargers over their surface): diffuse at low rates
Small polar particles (urea, ethanol) pass through easier than large particles
Integral and peripheral proteins in membranes:
Integral proteins:
Hydrophobic on at least part of their surface (amphipathic)
Embedded in the hydrocarbon chains in the center of the membrane
May fit in 1 or both of the phospholipid layers
Many are transmembrane proteins: they extend across the membrane, with hydrophilic parts
projecting through the regions of phosphate heads on either side
Hydrophobic regions: interact with the hydrophobic interior of the lipid bilayer, causing them
to be embedded in the bilayer.
Hydrophilic regions: interact with the hydrophilic heads of the lipid bilayer or the aqueous
environment
Peripheral proteins
Hydrophilic on their surface
Not embedded in the membrane: found in the surface
Most of them are attached to the hydrophilic regions of integral proteins (this attachment is often
reversible)
Some have a single hydrocarbon chain attached to them: is inserted into the membrane, anchoring
the protein to the membrane surface.
Lipid bilayers as the basis of cell membranes:
Membranes: essential components of cells
Bilayer of phospholipids and other amphipathic molecules (forms a continuous sheet and controls
the passage of molecules)
Plasma membrane: border between a cell and its environment
Membranes inside eukaryotic cells: divide the cytoplasm into compartments
Phospholipids: phosphate head (hydrophilic) & 2 hydrocarbon tails (hydrophobic)
The tails interact with each other to form the core of biological membranes
There are usually aqueous solutions on either side of cell membranes. The solutes nearest to the membrane
surface might penetrate between the hydrophilic phosphate heads of phospholipids, but if they reach the
hydrophobic core of the membrane they are drawn back to the aqueous solution outside the membrane.
Hydrophobic hydrocarbon chains (core of the membrane): don’t repel hydrophilic solutes but they
are more attracted to each other, and the solutes are more attracted to water outside the
membrane.
Permeability: ability of a molecule to pass through a membrane
The larger the molecule, the lower the permeability
Depends on hydrophilic / hydrophobic nature
Simple diffusion of molecules:
Diffusion: spreading out of particles in liquids and gases that happens because the particles are in a
continuous random motion
Net movement of particles from the higher to the lower concentration (movement down the
concentration gradient)
Passive process: no energy needed
Movement of molecules results in equilibrium.
Simple diffusion across membranes: due to particles passing between phospholipids in the membrane
Only happens if the phospholipid bilayer is permeable to the particles.
Non-polar particles (oxygen): diffuse through easily
If oxygen concentration lowers inside cell due to aerobic respiration (use of oxygen) and the
concentration outside is higher, oxygen will pass into the cell by diffusion
, Ions (positive or negative charges): can’t easily diffuse as the center of the membrane is hydrophobic
Ions are hydrophilic
Polar molecules (partial positive and negative chargers over their surface): diffuse at low rates
Small polar particles (urea, ethanol) pass through easier than large particles
Integral and peripheral proteins in membranes:
Integral proteins:
Hydrophobic on at least part of their surface (amphipathic)
Embedded in the hydrocarbon chains in the center of the membrane
May fit in 1 or both of the phospholipid layers
Many are transmembrane proteins: they extend across the membrane, with hydrophilic parts
projecting through the regions of phosphate heads on either side
Hydrophobic regions: interact with the hydrophobic interior of the lipid bilayer, causing them
to be embedded in the bilayer.
Hydrophilic regions: interact with the hydrophilic heads of the lipid bilayer or the aqueous
environment
Peripheral proteins
Hydrophilic on their surface
Not embedded in the membrane: found in the surface
Most of them are attached to the hydrophilic regions of integral proteins (this attachment is often
reversible)
Some have a single hydrocarbon chain attached to them: is inserted into the membrane, anchoring
the protein to the membrane surface.
