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Membrane Permeability

Notes

Membrane Permeability

Sections

CELL MEMBRANE

  • Separates intracellular and extracellular environments
  • Regulates import and export of molecules
  • Lipid bilayer

SIMPLE DIFFUSION
Molecules diffuse across the cell membrane

Rate of diffusion is determined by:

  • Size: smaller molecules diffuse across faster than large ones
  • Polarity: the less polar, the faster it diffuses across the membrane
  • Charged molecules: do not diffuse across hydrophobic interior

Types of molecules

  • Nonpolar molecules: hydrophobic, diffuse rapidly
    i.e. oxygen, carbon dioxide, nitrogen and steroid hormones
  • Small, uncharged polar molecules: diffuse across
    i.e. water, glycerol and ethanol
  • Large, uncharged polar molecules: do NOT diffuse across
    i.e. amino acids, glucose and nucleosides
  • ions (charged molecules): cannot diffuse across
    ie. Hydrogen, sodium, potassium, calcium and chloride ions

MEMBRANE TRANSPORT PROTEINS
Allow molecules to cross the membrane faster and more efficiently than simple diffusion
i. Channels: form open pores in the membrane
ii. Transporters: undergo conformational changes during transport

  • Solute binds solute binding site
  • Binding produces conformational change in protein
  • Solute leaves transporter (release facilitated by conformational change)

TWO TYPES OF TRANSPORT ACROSS MEMBRANES

Passive transport

  • Molecules move down their electrochemical gradient
  • No energy is required

Active transport

  • Molecules move against their concentration gradient
  • Energy is required

ELECTROCHEMICAL GRADIENT
Accounts for voltage across the membrane (electro-) and concentration gradient (chemical)

Full-Length Text

  • Here we will learn about membrane permeability.
  • To begin, start a table to learn some key features of the cell membrane.
  • Denote that it:
    • Separates the intracellular and extracellular environments.
    • And regulates the import and export of molecules.

Let's illustrate the cell membrane, now.

  • Draw a representative hydrophilic circular head with two hydrophobic fatty acid tails.
    • Form a row of these lipids.
  • Then draw another in opposite orientation underneath it.
  • Indicate that the cell membrane comprises two layers of lipids.
    • It is a lipid bilayer.
  • Label the hydrophilic surfaces of the membrane.
  • Label the hydrophobic interior of the cell membrane; it facilitates its barrier function.
  • Indicate that in this tutorial, we'll illustrate:
    • Simple diffusion the process whereby molecules diffuse across the cell membrane.
    • Membrane transport proteins, which allow molecules to cross the membrane faster and more efficiently than simple diffusion.

Begin with simple diffusion.

  • Write that the rate of diffusion across a membrane depends on three key determinants:
    • Size – Smaller molecules diffuse across the membrane faster than large ones.
    • Polarity – The less polar the molecule, the faster it diffuses across the membrane.
    • Charged molecules – do not diffuse through the membrane's hydrophobic interior.
  • To illustrate this concept, add the following categories to our diagram:
    • Nonpolar molecules, which are hydrophobic
    • Polar, uncharged molecules, which subdivides into small versus large molecules.
    • Ions (which are charged).

Now, let's apply the rules of size, polarity, and charge to these molecules.

  • Show that hydrophobic molecules rapidly diffuse across the membrane.
    • Hydrophobic molecules pass freely through the bilayer's hydrophobic interior.
  • Write that these molecules include oxygen, carbon dioxide, nitrogen and steroid hormones.
    • Oxygen and carbon dioxide are important for respiration, nitrogen is a waste product and steroid hormones deliver signals in the body.
  • Now, illustrate that small, uncharged polar molecules diffuse across the membrane.
  • But that large uncharged polar molecules do NOT: the membrane is impermeable to them.
  • Write that the small, uncharged polar molecules include water, glycerol and ethanol.
    • Although these molecules are polar, they are small, which allows them to diffuse across the membrane.
  • Show that the large polar molecules include amino acids, glucose and nucleosides; these metabolic intermediates are too large to diffuse across freely.
  • Finally, illustrate that ions, which are charged molecules, cannot diffuse freely through the bilayer's hydrophobic interior.
  • Show that these ions include, hydrogen, sodium, potassium, calcium and chloride ions.

We have seen that certain molecules can diffuse freely across the membrane, but what about the molecules that can't?

  • Denote that membrane transport proteins transport molecules across the membrane.
  • Denote that there are two classes of membrane transport proteins:
    • Channels, which form open pores in the membrane.
    • Transporters, which undergo conformational changes during transport.

Now, for the channel protein.

  • Extend the lipid bilayer to include a tunnel-like protein extending through it.
  • Show that a specific molecule of a certain size may pass through it.

Now, the transporter.

  • Draw a C-shaped protein with a large, open pocket embedded in the bilayer.
  • Label its pocket a solute-binding site.
  • For step 1, show a molecule bound to it.
  • Next, redraw the transporter, but show that it opens to the opposite side of the bilayer.
  • Indicate that solute binding produces this conformational change in the protein.
  • For step 2, show that the solute leaves the transporter; its release is facilitated by the conformational change.

Now, start a chart to introduce the two types of transport across the membrane: passive and active transport.

  • Write that in passive transport:
    • Molecules move down their concentration gradient
    • No energy is required.
  • Write that in active transport:
    • Molecules move against their concentration gradient, and energy is required.
  • Note that these are temporary definitions that we use for simplicity.
    • Molecular movement is actually dictated by electrochemical gradients.
  • As we learn elsewhere, an electrochemical gradient accounts for both the voltage across the membrane (electro-) and the concentration gradient (chemical).
  • Now, indicate that channel proteins can only facilitate passive transport.
  • Indicate that transporters can facilitate passive and active transport.