CH3 CELL PT 5 - CellTransport Mechanisms Notes -

Plasma Membrane and Transport: Key Concepts

• Purpose of cell transport: movement of substances between extracellular fluid and intracellular fluid across the plasma membrane.

• Plasma membrane composition: phospholipid bilayer with integral/transmembrane proteins.

• Selective permeability: some substances pass, others do not.

• Determinants of membrane permeability:

  • Lipid solubility: lipid-soluble (nonpolar) substances pass readily through the lipid bilayer.

  • Water solubility: water-soluble substances require a carrier protein or a channel protein.

  • Size: smaller substances pass more easily than larger ones.

  • Charge: inside of the membrane is typically slightly negative; negatively charged substances face more difficulty entering, while positively charged substances may move in more easily due to the membrane potential.

Passive Transport (no ATP required)

• General rule: moves down a concentration gradient (high to low concentration).

• Diffusion (simple diffusion)

  • Mechanism: movement from high concentration to low concentration across the lipid bilayer for nonpolar/lipid-soluble substances.

  • Examples: oxygen, carbon dioxide, fatty acids, steroids, other nonpolar molecules.

  • Biological relevance: oxygen diffuses into cells for cellular respiration to produce ATP; as internal O2 is used, external O2 keeps diffusing in.

  • Key phrase: diffusion occurs down a concentration gradient.

• Facilitated diffusion

  • Mechanism: diffusion down a concentration gradient that requires a membrane protein (carrier or channel) for polar or charged substances.

  • Distinctions:

  • Ion channels: protein channels that allow specific ions (e.g., Na^+, K^+) to move down their gradient.

  • Carrier proteins: bind specific solutes (e.g., glucose) and change shape to shuttle them across the membrane.

  • Examples:

  • Glucose: polar, cannot diffuse through lipid bilayer; uses carrier protein to move down the gradient.

  • Ions and polar amino acids: often move via specific transporters.

  • Energy: no ATP required.

• Osmosis

  • Definition: diffusion of water across a semipermeable membrane; moves down a gradient until equilibrium.

  • Conceptual view: water movement is often described with solutes; water tends to move toward regions of higher solute concentration (more salt, sugar, or protein).

  • Aquaporins: protein channels in the membrane that allow water to move across; water movement is still driven by solute gradients (osmotic gradients).

  • Clinical relevance: osmosis is foundational to IV fluid planning and fluid balance.

Osmosis in IV Fluids: Isotonic vs Hypotonic vs Hypertonic

• Isotonic solution to a red blood cell (RBC): no net water movement; RBC size remains constant.

  • Practical isotonic IV examples stated:

  • 0.9% sodium chloride (normal saline) = isotonic to RBCs.

  • 5% glucose solution (D5 in some contexts) = isotonic to RBCs in the bag and considered isotonic for the circulatory system.

  • Mechanism: water moves in and out through aquaporins at equal rates when the surrounding solution is isotonic.

• Hypotonic solution

  • Definition: lower solute concentration than inside the cell (more water relative to solute outside).

  • Effect on RBCs: water moves into the cell, cells swell; may cause hemolysis (cell bursting) if excessive.

  • Practical note: hypotonic IV fluids are generally avoided in clinical settings.

• Hypertonic solution

  • Definition: higher solute concentration than inside the cell (more salt/solutes outside).

  • Effect on RBCs: water leaves the cell, RBCs shrink (crenation).

  • Practical note: hypertonic IV fluids can be used in specific situations, but are used cautiously due to osmotic effects.

• Clinical takeaway

  • Most IV fluids used clinically are isotonic to avoid osmotic shifts that alter cell shape and volume.

  • Real-world isotonic reference points include 0.9% NaCl (normal saline) and sometimes 5% glucose solutions as isotonic preparations.

• Water movement via aquaporins is a key mechanism for osmosis in cells, including RBCs.

Active Transport

• Definition: transport that requires energy (ATP) to move substances against their concentration gradient (low to high).

• Sodium-Potassium ATPase pump (Na^+/K^+ ATPase)

  • Function: maintains ion gradients essential for cell function (membrane potential, nerve impulses, muscle contraction).

  • Directionality and stoichiometry:

  • Moves Na^+ out of the cell and K^+ into the cell against their gradients.

  • Stoichiometry: 3 Na^+ are pumped out and 2 K^+ are pumped in per ATP hydrolyzed.

  • Energy source: ATP hydrolysis.

  • Biochemical steps (conceptual):

  • 3 Na^+ bind to the pump on the inside.

  • ATP is hydrolyzed to ADP + Pi, providing energy for conformational change.

  • Na^+ is released outside; 2 K^+ bind on the outside and are transported inside.

  • The cycle resets, maintaining the gradient: high Na^+ outside, high K^+ inside.

  • Physiological significance: maintains ionic gradients critical for heart function and nerve impulses; disruption can impair contraction and signaling (e.g., excessive extracellular K^+ can stop the heart and nerve impulses).

  • Note: other ion pumps exist (e.g., calcium pumps) that also use ATP to move ions against gradients.

• Other notes on active transport

  • Pumps are exemplary of moving against a gradient; they require ATP.

Vesicular Transport: Endocytosis and Exocytosis

• Vesicular transport overview: endocytosis (in) and exocytosis (out) involve membrane-bound vesicles and require ATP.

• Endocytosis (taking substances into the cell)

  • Types:

  • Receptor-mediated endocytosis: ligand binds a specific receptor, triggering vesicle formation. Example: uptake of LDL (lipoprotein with cholesterol) via LDL receptor.

  • Phagocytosis: "cell eating"; solid particles such as bacteria; macrophages/white blood cells surround and engulf the particle with extensions to form a vesicle (phagosome).

  • Pinocytosis: "cell drinking"; uptake of fluids and solutes in vesicles.

  • Vesicular transport: all endocytosis forms involve vesicle formation and ATP.

  • Post-endocytosis fate: vesicles can fuse with lysosomes for digestion (e.g., digestion of phagocytosed bacteria).

• Exocytosis (releasing substances from the cell)

  • Mechanism: vesicles (e.g., secretory vesicles containing protein) fuse with the plasma membrane and release contents outside the cell.

  • Context: proteins synthesized in the rough ER move to Golgi, are packaged into secretory vesicles, and are secreted via exocytosis; ATP is required.

• Overall role in physiology

  • Vesicular transport enables selective import (e.g., receptor-bound ligands) and regulated secretion (e.g., neurotransmitters, hormones).

Connections to Foundational Principles and Real-World Relevance

• Core distinction: passive vs. active transport

  • Passive (diffusion, facilitated diffusion, osmosis): no ATP; down concentration gradient.

  • Active (pumps, vesicular transport): ATP required; gradients moved against.

• Role of Na^+/K^+ gradient in physiology

  • Maintains resting membrane potential, enables action potentials, and supports heart muscle contraction and nerve signaling.

  • Excess extracellular K^+ can disrupt electrical activity and stop heart/nerve impulses; instructional example given about euthanizing animals with potassium, illustrating gradient importance.

• Practical clinical implications

  • IV fluid selection relies on osmosis: isotonic solutions minimize shifts in cell volume and maintain homeostasis.

  • Understanding osmosis explains why high salt intake can influence blood pressure: increased extracellular osmolality can draw water from tissues, increasing blood volume and pressure; also potential hypertonic effects on cells in tissues.

• Ethical/philosophical/practical implications

  • Careful management of IV fluids reflects broader patient safety and homeostasis principles; mismanagement of tonicity can cause cellular swelling or shrinkage with systemic consequences.

Quick Reference: Key Terms and Concepts

• Lipid solubility: ability to cross the lipid bilayer without transport proteins.

• Carrier proteins: integral proteins that bind specific solutes and change shape to shuttle them across (facilitated diffusion).

• Ion channels: channels that allow specific ions to pass down their gradient.

• Diffusion: movement from high to low concentration; no energy input.

• Facilitated diffusion: diffusion with help from proteins; no energy input.

• Osmosis: diffusion of water across a semipermeable membrane; down a gradient of solute concentration.

• Isotonic solution: equal solute concentration to the cell; no net water movement.

• Hypotonic solution: lower solute concentration outside than inside; water moves into the cell; possible swelling/hemolysis.

• Hypertonic solution: higher solute concentration outside than inside; water moves out of the cell; possible crenation.

• Aquaporins: water channel proteins that allow rapid water movement.

• Endocytosis: uptake of material via vesicle formation; includes receptor-mediated endocytosis, phagocytosis, and pinocytosis.

• Exocytosis: release of materials from the cell via vesicle fusion with the plasma membrane.

• Vesicular transport: term often used for endocytosis and exocytosis.

• Na^+/K^+ ATPase pump: active transport pump; exchanges 3 Na^+ out and 2 K^+ in per ATP hydrolyzed, maintaining gradient.

• Hemolysis: rupture of red blood cells due to excessive swelling in hypotonic solution.

• Crenation: shrinkage of red blood cells in a hypertonic solution.

• ATP hydrolysis: the energy-releasing step that powers pumps and vesicular transport; general form: ext{ATP}
  ightarrow ext{ADP} + ext{P_i}

• Sodium outside, potassium inside: the resting ion distribution maintained by the Na^+/K^+ pump; essential for electrical activity and cell function.

Numerical and Equipment References (as stated in the transcript)

• Isotonic IV solutions to RBCs:

  • 0.9% sodium chloride (NaCl) solution

  • 5% glucose solution

• Sodium-Potassium Pump (Na^+/K^+ ATPase):

  • 3 Na^+ moved out per cycle

  • 2 K^+ moved in per cycle

  • Energy source: ATP hydrolysis, i.e., ext{ATP}
    ightarrow ext{ADP} + ext{P_i}

• Conceptual concentrations:

  • Sodium: higher extracellularly

  • Potassium: higher intracellularly

• Practical outcomes: osmotic balance prevents undesired cell shrinkage or swelling; IV fluids match cell osmolality to prevent cellular volume changes.