Membrane Transport I

Course and Administrative Details

• Instructor: Doug Keen, Ph.D.

• Office Locations: Gittings 34 or Gittings 115

• Office Hours:

  • Mondays: $9:00\text{ AM}$ to $10:00\text{ AM}$

  • Thursdays: $1:00\text{ PM}$ to $2:00\text{ PM}$

  • Additional times available by appointment

Primary Functions of the Plasma Membrane

• Barrier: Provides a selective physical boundary separating the intracellular environment (cytosol) from the extracellular fluid.

• Organization: Compartmentalizes cellular processes and organizes biochemical pathways and structural frameworks within the cell.

• Transport: Regulates the movement of ions, organic molecules, nutrients, and waste products across the hydrophobic membrane core.

• Reaction and Response: Mediates signal transduction by sensing environmental stimuli, binding signaling molecules, and triggering downstream physiological responses.

Fundamental Classification of Transport Mechanisms

• Passive Transport:

  • Solutes move down their concentration gradient (from an area of high concentration to an area of low concentration).

  • Requires no input of metabolic energy (ATP) from the cell.

• Active Transport:

  • Solutes move against their concentration gradient (from an area of low concentration to an area of high concentration).

  • Requires cellular energy expenditure (ATP hydrolysis or utilization of stored electrochemical gradients).

Dynamics of Passive Transport and Simple Diffusion

Principles of Simple Diffusion

• Definition: The spontaneous net movement of solute particles down a concentration gradient due to random thermal motion.

• Mathematical and Physical Properties:

  • Net diffusion flux ($J$) is directly proportional to the concentration gradient ($[S]$):

$J = P \times [S]$

• Rate vs Concentration Relationship: A plot of diffusion rate versus solute concentration yields a straight linear curve that increases indefinitely without reaching a maximum plateau.

• Distance Dynamics:

  • Diffusion occurs rapidly over short, cellular distances ($\sim \mu\text{m}$ scale).

  • Diffusion occurs extremely slowly over long, organismic distances ($\text{cm}$ to $\text{m}$ scale), necessitating circulatory systems in complex organisms.

Lipid Bilayer Barrier and Solute Permeability

• Hydrophilic (Polar) Solutes: Cannot dissolve into or pass through the non-polar hydrophobic core of the lipid bilayer directly by simple diffusion.

• Hydrophobic (Non-polar / Lipophilic) Solutes: Can partition directly into the lipid phase of the plasma membrane, allowing them to freely cross the membrane via simple diffusion.

• Substances Utilizing Simple Diffusion:

  • Small gases: Oxygen ($O_2$), Carbon Dioxide ($CO_2$)

  • Fatty acids

  • Steroid hormones

  • Fat-soluble vitamins

Osmosis, Water Activity, and Osmotic Pressure

Mechanisms of Osmosis

• Definition: Osmosis is the net passive flow of water across a semipermeable membrane down its chemical activity gradient (from a region of higher water activity/concentration to a region of lower water activity/concentration).

• Pure Water Equilibrium: When pure water is present on both sides of a semipermeable membrane, unidirectional water fluxes across the membrane are equal in both directions, resulting in no net water flux ($\text{net flux} = 0$).

• Effect of Solute Addition: Adding non-penetrating solute to one side (e.g., Side 2) lowers the thermodynamic chemical activity of water on that side. This creates a chemical activity gradient, drawing a net flux of water toward the solute-containing side.

Osmotic Pressure Definition and Measurement

• Osmotic Pressure: The exact hydrostatic pressure that must be applied to a solution to prevent the net flux of pure water across a semipermeable membrane into that solution.

Aquaporins

• Water molecules cross the hydrophobic lipid core of cellular plasma membranes rapidly through specialized transmembrane channel proteins called aquaporins.

Solution Tonicity and Cellular Morphology

• Isotonic Solution:

  • Osmotic concentration equal to intracellular fluid.

  • Net water movement is zero.

  • Red Blood Cells (RBCs) retain their normal biconcave disc shape.

• Hypotonic Solution:

  • Solute concentration is lower outside the cell than inside (higher extracellular water activity).

  • Net water flux flows rapidly into the cell.

  • Red Blood Cells swell and burst, undergoing hemolysis.

  • Example: Distilled water ($0\%\,\text{NaCl}$).

• Hypertonic Solution:

  • Solute concentration is higher outside the cell than inside (lower extracellular water activity).

  • Net water flux flows out of the cell into the extracellular fluid.

  • Red Blood Cells shrink and form spiky projections, undergoing crenation.

  • Example: $5\%\,\text{salt solution}$.

Facilitated Transport Mechanisms

Channel-Mediated Facilitated Diffusion

• Characteristics:

  • Involves transmembrane channel proteins containing central aqueous pores.

  • Allows selective passage of small charged inorganic ions down their electrochemical gradients.

  • Can exist in gated configurations (switching between open and closed conformational states) or non-gated configurations.

• Transported Ions: Potassium ($K^+$), Sodium ($Na^+$), Calcium ($Ca^{2+}$), and Chloride ($Cl^-$).

Carrier-Mediated Facilitated Diffusion

• Characteristics:

  • Involves specific transmembrane carrier/transporter proteins.

  • Substrate binding triggers reversible conformational shape changes in the carrier protein to shuttle molecules across the membrane.

  • Passive transport mechanism; cannot catalyze net transport against an electrochemical gradient ($[X]_{\text{in}} = [X]_{\text{out}}$ at equilibrium).

• Transported Substrates: Monosaccharides such as glucose, fructose, galactose, and certain vitamins.

Diagnostic Characteristics of Carrier-Mediated Transport

Saturability

• Cause:

  • Membranes contain a finite, fixed number of transporter proteins.

  • Each individual transporter protein has a finite maximum substrate turnover rate.

• Kinetics Equation:

$J = \frac{J_{\text{max}} \times [S]}{K_t + [S]}$

• $J$ = Rate of substrate uptake / flux

• $J_{\text{max}}$ = Maximal transport rate capacity

• $[S]$ = Substrate concentration

• $K_t$ = Transport constant (substrate concentration at half-maximal transport rate, $\frac{1}{2} J_{\text{max}}$)

Selectivity

• Cause:

  • Each transport protein possesses a unique three-dimensional binding domain determined by structural folding hierarchy:

  • Primary Structure: Specific linear sequence of amino acids (e.g., $\text{Val-Gly-Ser-Leu}$).

  • Secondary Structure: Hydrogen-bonded patterns forming $\alpha$-helices or $\beta$-pleated sheets.

  • Tertiary Structure: 3D folding of a single polypeptide strand.

  • Quaternary Structure: Multi-subunit spatial assembly of two or more folded polypeptides.

• Consequence: Transporters accept only a highly specific, limited range of chemical structures as substrates (e.g., Lacy sugar transporter).

Comparison Matrix of Transport Mechanisms

• Simple Diffusion:

  • Driven by: Concentration gradient

  • Active/Passive: Passive

  • Saturable: No (Linear rate curve)

  • Selective: No (Property of lipid solubility/partitioning)

• Channel-Mediated Facilitated Diffusion:

  • Driven by: Electrochemical gradient

  • Active/Passive: Passive

  • Saturable: Yes (At extreme ion fluxes)

  • Selective: Yes (Selectivity filter pore dimensions/charge)

• Carrier-Mediated Facilitated Diffusion:

  • Driven by: Concentration gradient

  • Active/Passive: Passive

  • Saturable: Yes ($J_{\text{max}}$ reached at high substrate concentration)

  • Selective: Yes (Specific structural binding site required)

• Active Transport:

  • Driven by: ATP hydrolysis or coupled ion gradient

  • Active/Passive: Active

  • Saturable: Yes

  • Selective: Yes