Plasma Membrane

• The plasma membrane is a thin and fragile membranous layer that separates the internals of the cell from the surroundings

Composition §

• The structure of most membranes consists of a lipid bilayer where the hydrophilic ends are directed outwards
  • The membrane is held together by a thin sheet of noncovalent bonds.
  • The bilayer prevents any water-soluble molecules from entering or leaving the cell.
  • The membrane’s lipid bilayer primarily consists of phosphoglycerides but also larger sphingolipids and cholesterols.
  • The membrane itself arises as a consequence of the lipid bilayer forming a thin film around the cell. Thus, they are always continuous but at the same time easily deformable.
  • Lipid bilayers can self-assemble. This is apparent in the formation of liposomes.
  • The lipid bilayer is composed of two more or less stable monolayers that may have different physical and chemical properties. The lipid composition of the lipid bilayer changes the properties of the cell and how other cells interact with it.
• It also consists of various membrane carbohydrates.
  • Typically these carbohydrates are various oligosaccharides.
  • They play a role in mediating the interactions of a cell with its environment and sorting membrane proteins to different compartments.
  • Most of these carbohydrates are linked to proteins to form glycoproteins. These carbohydrates face away from the cytosol.
• Membranes also contain membrane proteins which serve different purposes.
  • These are typically oriented in a specific way relative to the cytoplasm. Usually, this orientation is indicative of their purpose.
  • Integral Proteins are those that can penetrate the lipid bilayer.
    • These tend to be amphipathic globular proteins
    • Some integral proteins contain channels that can allow molecules to pass through.
    • Their transmembrane domains span the core of the lipid bilayer, often in a helical manner. They contain hydrophobic nonpolar molecules.
  • Peripheral proteins are those that are located entirely outside the lipid bilayer and noncovalently bonded to the membrane surface.
    • They usually function to make the membrane’s skeleton, as special enzymes and coats, or transmission of transmembrane signals.
    • They can be recruited or released by the membrane as needed.
  • Lipid Anchored proteins are located outside but are covalently linked to a lipid molecule within the bilayer.
• The Plasma membrane is usually not homogeneous. Depending on its composition and structure, different parts of it can be used for different purposes even in the same cell.

Function §

• Compartmentalization to separate a cellular component from its surroundings.
  • Related to this is the fact that they help demarcate cells from their surroundings even after cell fusion.
• Scaffolds for enzymes and substrates to facilitate metabolic reactions.
• Transportation of molecules and solutes into and out of the cell (see Movement Across Membranes).
• Allows responses to stimuli in a process called signal transduction. Ligands and kinases that facilitate these reactions may alter the internal activities of the cell..
• Allows intracellular interaction by adhering to each other and facilitating the transfer of signals and materials.
• Involved in energy transduction by containing the machinery for metabolic pathways.

Membrane Fluidity and Dynamics §

• Membranes are characterized by their fluidity. At certain temperatures, the membrane will either be in a liquid or a gel like state.
• The fluidity of a membrane determines the movement of material across the lipid bilayer, and also the assembly of the membrane.
• The transition temperature of a bilayer depends on the ability of the lipid molecules to be packed together based on their construction.
  • The greater the unsaturation of fatty acids, the lower the temperature needed before the bilayer becomes a gel.
  • The shorter the fatty acid chains, the lower the melting temperature becomes.
• Cholesterols tend to increase the durability while decreasing the permeability of the membrane by creating regions of intermediate fluidity.
• Maintenance of membrane fluidity is an example of homeostasis. Cells can respond appropriately by changing the composition of the membrane (i.e., making it more cold or heat resistant). This is done via desaturases and phospholipases.
• Lipid rafts are self-assembled lipid membranes from sphingolipids and cholesterols that are more gel-like and highly ordered than the surrounding regions.
  • They tend to float within the more fluid and disordered environment of an artificial bilayer.
  • Certain proteins become concentrated to these liquid rafts.
  • They have not been observed within natural cells, but are promising since they can introduce order in the lipid bilayer.
• The most restrictive motion for a membrane lipid is to move from one membrane to another
  • Flippases can facilitate this reaction much quicker.
  • The lateral movement of lipids allows the membrane to maintain a uniform distribution of them.
  • The diffusion of proteins laterally along the membrane is mostly Brownian, although some proteins are immobile or move in a non-random direction as directed by other proteins.
  • Findings using optical tweezers suggest that the plasma membrane is compartmentalized and contains “barriers” within the cytoplasmic side which restricts movement of integral proteins

Movement Across Membranes §

• On their own, solutes prefer to diffuse from high to low concentrations due to entropy.
• On their own, electrolytes prefer to diffuse based on an electrochemical gradient which accounts for the difference in charge and concentration of electrolyte on either side.

Passive Transport §

• Passive transport can occur naturally as the solute passes through the bilayer.
  • The more soluble the solute is to a lipid, the faster the transport.
  • The smaller the molecule is the easier it will pass through the lipid bilayer
  • Uncharged, nonpolar molecules penetrate easier.
• Osmosis involves the transfer of solvents such as water through the membrane. In such a case, water the solvent moves towards the side with higher concentration (the hypertonic side).
  • Osmosis and solute concentrations have an effect on the shape of the cell.
  • Cells in hypertonic solutions experience turgid pressure which causes them to bloat.
  • Cells in hypotonic solutions experience shriveling, and in the case of plant cells plasmolysis.
  • Osmosis can be facilitated through integral proteins called aquaporins that act as small channels for water to travel through.
• Ion Channels are involved in the transport of ions since the plasma membrane is not permeable to most ions.
  • These ion channels are selective to a particular ion.
  • These ion channels can be gated that are open or closed based on different factors.
    • voltage-gated channels act based on differences in potential.
    • ligand-gated channels act based on ligands (distinct from the solute) clamping onto the channel
    • mechano-gated channels act based on physical forces (such as stretching the membrane)
• Facilitated Diffusion is diffusion which involves a facilitative transporter to which the solute binds to. The transporter then exposes the solute to the other side of the gradient because of a conformational change.
  • The transporters are usually enzymes and so are subjected to the limitations of enzymes.
  • This process can be regulated by the cell.

Active Transport §

• Active transport pertains to the use of integral membrane proteins (called pumps) to selectively move a solute across the membrane using an energy input.
• Active transport usually involves complex conformational changes. Thus, they are slower than passive transport.
• Some examples of pumps:
  • Sodium-Potassium ATP pump
  • P-type pumps
  • V-type pumps.(V = Vacuole)
  • ATP-binding cassette transporters.
  • Light-driven proton pumps
• Cotransport or secondary transport involves the use of gradients formed by active transport to transport solute. These reactions are facilitated by cotransporters.
  • A symport occurs when the ion and the solute move in the same direction
  • An antiport occurs when the ion and solute move in opposite directions (i.e., they are exchanged).

Membrane Potentials §

• Membrane Potentials arise from voltage differences inside and outside the plasma membrane depending on the presence of cations and anions.
• Cells in an unexcited state are said to be at resting potential. However, if the cell receives a strong stimulus, it triggers active potential marked by a sharper potential gradient.
• These potentials are governed and maintained by active and passive transport mechanisms.
  • In the resting state, most of these are closed so the flow of ions is minimal.
  • In the active state, all of these transport mechanisms are activated which causes rapid depolarization to the point the potential flips sign.
  • After these changes, the cell enters a refractory period during which it cannot be further stimulated because the sodium channels must close first before they can be reopened.
• The membrane potential can be calculated using the Nernst Equation.

Links §

• Enzymes as Machines for Cellular Processes - to see more about enzymes and metabolic processes.

• Karp Ch. 4

  • 4.5 - for a discussion on how Integral Proteins were studied.
  • 4.7 - for more about membrane dynamics
  • 4.11 - a discussion on the potassium ion channel and its parts.
  • 4.14 - some examples of pumps
  • 4.16 -4.18 - how membrane potentials are used in nerve cells.