BIO 201 Chapter 4

Plasma Membrane: Overview

• Plasma membrane — a barrier and a gateway between the cytoplasm and extracellular fluid (ECF).

• Selectively permeable — allows some things through, and prevents other things from entering and leaving the cell.

• Passive transport mechanisms require no ATP; energy comes from random molecular motion of particles. Includes filtration, diffusion, osmosis.

• Active transport mechanisms consume ATP; includes active transport and vesicular transport.

• Carrier-mediated mechanisms use a membrane protein to transport substances from one side of the membrane to the other.

Filtration

• Filtration — particles are driven through a selectively permeable membrane by hydrostatic pressure (force exerted on a membrane by water).

• Examples:

  • Filtration of nutrients through gaps in blood capillary walls into tissue fluids.

  • Filtration of wastes from the blood in the kidneys while holding back blood cells and proteins.

• Blood pressure in capillaries forces water and small solutes (e.g., salts) through narrow clefts between capillary cells.

• Capillary wall separates water and solutes from larger particles (e.g., red blood cells), which are held back by clefts.

Simple Diffusion

• Simple diffusion — net movement of particles from an area of high concentration to an area of low concentration.

• Driven by constant, spontaneous motion of particles.

• Also known as movement down the concentration gradient.

• Concentration can differ from one point to another.

• Visual: Down gradient (high to low) vs Up gradient (low to high).

Factors Affecting Diffusion

• Temperature: Higher temperature → higher motion of particles.

• Molecular weight: Larger molecules diffuse more slowly.

• Steepness of concentration gradient: Greater difference → faster rate.

• Membrane surface area: Larger area → faster rate.

• Membrane permeability: Higher permeability → faster rate.

Simple Diffusion through a Cell Membrane

• Diffusion through lipid bilayer: Nonpolar, hydrophobic, lipid-soluble substances diffuse readily through the lipid layer.

• Diffusion through channel proteins: Water and charged/hydrophilic solutes diffuse through channel proteins in the membrane.

• Cells regulate permeability by adjusting the number of channels or by opening/closing gates.

Osmosis

• Osmosis — flow of water from one side of a selectively permeable membrane to the other.

• Water moves from side with higher water concentration to side with lower water concentration.

• Hydration spheres form around solute particles; water associated with solutes is less available to diffuse back to the original side.

• Conceptual depiction: Side A (solutes) | Side B (solutes) with water exchange across the membrane.

Aquaporins

• Aquaporins — channel proteins in the plasma membrane specialized for passage of water.

• Cells can increase the rate of osmosis by adding more aquaporins; decrease rate by removing them.

• Significant amounts of water can diffuse even through the hydrophobic, phospholipid regions of the plasma membrane via aquaporins.

Osmotic Pressure

• Osmotic pressure — amount of hydrostatic pressure required to stop osmosis.

• More solutes → higher osmotic pressure.

• Reverse Osmosis — applying pressure to one side overrides osmotic pressure and drives water against its concentration gradient.

• Example context: the heart can drive water out of capillaries by reverse osmosis, contributing to capillary filtration.

• Related terms: Osmotic pressure, hydrostatic pressure.

Osmolarity and Tonicity

• One osmole = 1 mole of dissolved particles.

• 1 M NaCl (1 mole Na+ + 1 mole Cl−) = 2 osm/L.

• Osmolarity — number of osmoles of solute per liter of solution. Higher solute concentration -> higher osmolarity.

• Osmolality — number of osmoles of solute per kilogram of water.

• Physiological solutions are expressed in milliosmoles per liter (mOsm/L).

• Blood plasma = ~300 mOsm/L.

• Example: a diet high in salt -> higher blood osmolarity/selective effects on volume and pressure.

• Osmolality is similar to osmolarity at physiological concentrations; difference typically < 1%.

Tonicity

• Tonicity — ability of a solution to affect fluid volume and pressure in a cell.

• Depends on concentration and permeability of solute.

• Hypotonic solution: lower concentration of nonpermeating solutes than intracellular fluid (ICF); higher water concentration; cells absorb water, swell, and may burst (lyse).

• Hypertonic solution: higher concentration of nonpermeating solutes; lower water concentration; cells lose water and crenate.

• Isotonic solution: concentrations in the cell and ICF are the same; causes no changes in cell volume or shape; example: normal saline.

Effects of Tonicity on Red Blood Cells (RBCs)

• Hypotonic, isotonic, and hypertonic solutions influence RBC volume.

• Visual outcomes: swelling (hemolysis) in hypotonic solutions; crenation in hypertonic solutions; no volume change in isotonic solutions.

Carrier-Mediated Transport

• Transport proteins in the plasma membrane carry solutes from one side of the membrane to the other.

• Specificity: Transport proteins are typically specific for a certain ligand.

• Solute binds to a specific receptor site on the carrier protein.

• Differs from membrane enzymes because carriers do not chemically modify their ligand; they pick up on one side and release on the other.

• Saturation: As solute concentration rises, transport rate rises but plateaus at the transport maximum (Tm) when all carriers are occupied.

• Two types of carrier-mediated transport: Facilitated diffusion and active transport.

Transport Maximum (Tm)

• Transport maximum — the transport rate when all carrier sites are occupied.

• Conceptual relation: higher solute concentration increases rate until a plateau is reached at Tm.

• Axes (conceptual): Concentration vs. rate of solute transport through the plasma membrane.

Carrier-Mediated Transport: Carrier Types

• Uniport — carries only one solute at a time.

• Symport (cotransport) — carries two or more solutes simultaneously in the same direction.

• Antiport (countertransport) — carries two or more solutes in opposite directions.

• Example: Sodium–potassium pump (Na+/K+ ATPase) brings in K+ and removes Na+ from the cell.

• Carriers employ two methods of transport: Facilitated diffusion (passive) and Active transport (requires ATP).

Facilitated Diffusion

• Facilitated diffusion — carrier-mediated transport of solute through a membrane down its concentration gradient.

• Passive process — does not consume ATP.

• Mechanism: Solute binds to binding site on carrier; carrier changes conformation; solute released on the other side.

• Steps (illustrative):
  1) A solute particle approaches the carrier.
  2) The solute binds to the receptor site on the carrier.
  3) The carrier undergoes a conformational change.
  4) The solute is released on the opposite side of the membrane.

• ECF → ICF or vice versa depending on gradient.

Active Transport

• Active transport — carrier-mediated transport of solute through a membrane up against its concentration gradient.

• ATP energy is consumed to change the carrier.

• Common uses:

  • Sodium–potassium pump maintains higher intracellular K+ and lower Na+ concentration.

  • Bring amino acids into the cell.

  • Pump Ca2+ out of the cell.

Sodium–Potassium Pump (Na+/K+ ATPase)

• Each pump cycle consumes one ATP and exchanges three Na+ for two K+.

• Keeps intracellular K+ concentration higher and Na+ concentration lower than in the ECF.

• Necessary because Na+ and K+ constantly leak through the membrane.

• Note: a substantial portion of daily calories are used by the Na+/K+ pump.

• Typical stoichiometry (conceptual):

  • 3\,\mathrm{Na^+}{\text{out}} \rightleftharpoons 2\,\mathrm{K^+}{\text{in}} per ATP hydrolysis.

  • ATP hydrolysis: \mathrm{ATP} \rightarrow \mathrm{ADP} + \mathrm{P_i}.

• Functional placement: may be depicted in epithelial transport as part of transport mechanisms.

Vesicular Transport

• Vesicular transport — moves large particles, fluid droplets, or many molecules at once through the membrane in vesicles.

• Endocytosis — bringing material into the cell.

  • Phagocytosis — “cell eating”: engulfing large particles.

  • Pinocytosis — “cell drinking”: taking in droplets of ECF containing molecules useful to the cell.

  • Receptor-mediated endocytosis — particles must first bind to specific receptors on the plasma membrane.

• Exocytosis — discharging material from the cell; vesicles fuse with the plasma membrane.

• Mechanism: motor proteins energized by ATP drive vesicular movement and membrane fusion.

Phagocytosis (Process Outline)

• A phagocytic cell encounters a particle of foreign matter.

• The cell surrounds the particle with its pseudopods.

• The particle is phagocytized and contained in a phagosome.

• The phagosome fuses with a lysosome to form a phagolysosome.

• Indigestible residue is voided by exocytosis.

• The phagolysosome fuses with the plasma membrane.

• Enzymes from the lysosome digest the foreign matter.

Pinocytosis (Process Outline)

• Pinocytosis — uptake of droplets of ECF.

• Occurs in all human cells.

• The membrane caves in and pinches off into the cytoplasm as pinocytotic vesicles.

Receptor-Mediated Endocytosis

• More selective form of endocytosis.

• Cells take in specific molecules that bind to extracellular receptors.

• Process: extracellular molecules bind to receptors, receptors cluster, pit forms (clathrin-coated), pit deepens and pinches off to form a clathrin-coated vesicle containing concentrated molecules from the ECF.

• Terminology: coated pit → clathrin-coated vesicle.

Exocytosis

• Exocytosis — secreting material from the cell.

• Also involved in replenishing the plasma membrane after endocytosis.

• Process steps:
  1) A secretory vesicle approaches the plasma membrane.
  2) Vesicle docks on the membrane via linking proteins.
  3) The membrane caves in at the contact point and fusion pore forms.
  4) Vesicle contents are released to the outside.