Equilibrium in Biological Systems

Cellular Equilibrium and Diffusion

• Sodium (Na+) concentration is higher outside the cell compared to the inside.
• This difference in Na+ concentration generates potential for electrical impulses.
• Equilibrium is reached when there is an even distribution of molecules throughout the space.
• Potassium (K+) concentration is higher inside the cell than outside, maintaining charge balance.

Types of Equilibrium

• Dynamic equilibrium occurs when molecules move equally in both directions across the membrane, resulting in no net change in concentration.
• Molecule movement across membranes occurs through diffusion.

Na+/K+ ATPase Pump

• The Na+/K+ ATPase pump is crucial for maintaining the concentration gradients of sodium and potassium ions across the plasma membrane.
• It operates actively against the concentration gradient, requiring energy (ATP).

Mechanism of Passive Diffusion

• Passive diffusion allows molecules to move down their concentration gradient, from areas of high concentration to areas of low concentration, without energy expenditure.
• The mechanism is essential for processes such as oxygen transport across lung membranes.

Osmolarity

• Defined as the measure of solute concentration per unit volume of solvent.
• Different from tonicity, which only considers the effects of non-penetrating solutes on cell volume.
• Osmolarity accounts for all solute concentrations, both penetrating and non-penetrating.### Membrane Transport Overview
• Membrane transport includes unassisted and assisted mechanisms.
• Membrane permeability is influenced by selective permeability, where some substances can cross while others cannot.

Unassisted Membrane Transport

• Occurs when molecules penetrate the plasma membrane without assistance.
• Driven by:
  • Diffusion down a concentration gradient.
  • Movement along an electrical gradient.
• Particles that can easily pass: small, lipid-soluble molecules (e.g., O2, CO2, fatty acids).

Assisted Membrane Transport

• Utilizes carrier proteins for substances that cannot passively diffuse through the membrane.
• Two primary forms:
  • Active Transport:
    • Requires energy (ATP) to move substances against their concentration gradient.
    • Important for charged or polar molecules (e.g., ions, glucose).
    • Can involve saturation, competition, and specificity.
    • Carrier proteins change shape to bind and transport molecules.
  • Passive Transport:
    • Does not require energy; moves substances along their concentration gradient.
    • Example: facilitated diffusion where molecules pass through channel proteins.

Tonicity

• Refers to the effect of a solution on cell volume, determined by nonpenetrating solute concentration.
• Types of tonicity:
  • Isotonic: No net movement, cell volume remains constant.
  • Hypotonic: Solutions cause cells to swell and possibly burst due to water influx.
  • Hypertonic: Solutions cause cells to shrink as water exits.

Concentration and Affinity

• Concentration gradients influence the movement of solutes; substances move toward areas of lower concentration.
• Affinity plays a crucial role in the binding of molecules to carrier proteins.
• The higher the affinity of a carrier for a molecule, the more likely it is to transport that molecule across the membrane.

Forces in Membrane Transport

• Active Forces: Require cellular energy to induce movement and include all forms of active transport.
• Passive Forces: Utilize natural gradients to facilitate movement without energy expenditure.

Primary Active Transport

• Movement of substances against concentration gradients using ATP for energy.
• Often involves carriers known as "pumps," which exhibit ATPase activity to facilitate the transport process.
• Pumps adjust their affinity based on concentration gradients to either pick up or release solutes accordingly.

Vesicular Transport Overview

• Essential for the transport of large polar molecules and multi-molecular materials into and out of the cell.
• Examples include protein hormones secreted by endocrine cells and bacteria ingested by white blood cells.
• Non-polar molecules can cross membranes easily without assistance.

Energy Requirements and Mechanism

• Active transport mechanism, requiring cellular energy for vesicle formation and movement.
• No concentration gradient is necessary for this process.
• Lysosomes contain hydrolytic enzymes for digestion.

Types of Active Transport

• Endocytosis: Cellular uptake mechanism categorized into three forms based on material absorbed.
  • Pinocytosis: Non-selective fluid uptake, described as "cell drinking." Specialized cells can consume surrounding fluid irrespective of its content.
  • Receptor-mediated Endocytosis: Selective uptake involving receptors and specific ligands, e.g., insulin, iron, vitamin B12. Receptors must engage with ligands for vesicle formation.
  • Phagocytosis: Selective engulfment of large particles (cell eating) performed by phagocytes, primarily immune cells.

Pinocytosis Details

• Micropinocytosis involves the consumption of large volumes of extracellular fluid, especially by dendritic cells to activate the immune response.
• Dendritic cells sample their environment for pathogens, operating in tissues like the liver, skin, lungs, and gut.

Receptor-mediated Endocytosis Insights

• Relies on receptor-ligand specificity for uptake; viruses can exploit this mechanism to enter cells.
• Vesicle formation initiates only after binding between receptor and ligand occurs.

Phagocytosis Functionality

• Conducted exclusively by immune cells (phagocytes) for the selective uptake of pathogens or debris.
• Macrophages act as phagocytes, coating pathogens with proteins to facilitate receptor-mediated uptake.

Exocytosis Functionality

• Opposite of endocytosis; does not involve fusion with lysosomes.
• Serves two main purposes: secretion of large polar molecules (like hormones and enzymes) and adding membrane components (like channels and receptors).
• Vesicles formed in the Golgi apparatus fuse with the cell membrane, allowing for component release or retention as part of the membrane.
• The equal balance with endocytosis prevents cell volume increase despite vesicle addition to the membrane.

Exocytosis and Secretory Vesicles

• Secretory vesicles feature a protein coating:
  • v-SNARE Proteins: Serve as docking markers binding to acceptor sites on the inner membrane, facilitating vesicle docking and fusion.
• Membrane components from exocytosis may remain embedded rather than released, depending on design.

This quiz explores the concepts of steady state and equilibrium in biological systems, focusing on sodium concentration differences across membranes. Test your understanding of how these conditions affect cellular functions and overall homeostasis.