Substance transport

All processes in the body require the interaction of various cells, necessitating effective communication. Cell communication occurs through electrochemical means (e.g., ion flow) and chemical means via signaling molecules, which involve signal transduction pathways. Substance transport is crucial for both types of communication. In addition to the passive, energy-independent forms like diffusion and osmosis, there are specific active transport processes that require energy investment. Generally, the transport of substances into cells necessitates crossing a biological membrane, either the outer plasma membrane or the membranes of cellular compartments.

Since biological membranes are composed of lipid bilayers, only small lipophilic substances can permeate freely. Other substances, including larger molecules, hydrophilic, or charged particles, require aid from transmembrane proteins. These proteins include channel proteins, which create pores through the membrane, and carrier proteins, which act as transporters that alter their shape to facilitate the movement of substances across the membrane.

Most biological processes take place in solutions that contain water. In these solutions, every molecule or ion is surrounded by solvent molecules. The concentration of a dissolved substance in a solution can be quantified in various ways.

• Molarity (molar concentration): c = n_solute/V_solution
  • Unit: mol/L or M
  • c = molar concentration (M), n_solute = moles of solute (mol), V_solution = volume of solution (L)
• Molality: b = n_solute/m_solvent
  • Unit: mol/kg
  • b = molality (mol/kg), n_solute = moles of solute (mol), m_solvent = mass of solvent (kg)
  • Useful in situations where temperature changes may affect solution volumes.
• Activity
  • Defined as the "effective concentration" of a species within a solution, considering non-ideal behavior that often occurs in concentrated solutions
  • Illustrates how a substance interacts during a chemical reaction, offering a perspective beyond simple molar concentration
  • Important when analyzing real (non-ideal) mixtures

While molarity is based on volume and can be influenced by temperature changes that affect solution volumes, molality is based on mass, providing consistent measurements regardless of temperature fluctuations.

Solubility behavior of molecules in multiphase systems

• Multiphase system: consists of several immiscible layers (or phases) with differing polarities, such as water (polar) and oil (nonpolar)
• Concentration of dissolved molecules: The concentration of molecules in a given phase is greater when the phase's polarity aligns with that of the molecules.
  • Polar molecules and ions: readily dissolve in polar solvents, e.g., water
  • Nonpolar molecules: dissolve more effectively in nonpolar solvents, e.g., oil
  • Amphiphilic molecules: exhibit both polar and nonpolar characteristics

The solubility behavior in multiphase systems can be quantitatively described using the Nernst partition coefficient, which helps predict how a substance distributes itself between two phases based on their polarity.

Like dissolves like!

Diffusion

Diffusion is the process of spontaneous mixing within a solution, characterized by the net movement of solute particles from an area of high concentration to an area of low concentration.

• Cause: arises from the random thermal motion of particles, which is non-directional
• 1. Fick's law of diffusion: The rate of diffusion (particle flux) is proportional to the concentration gradient, the difference in concentration between two areas.
• Effect
  • Particles naturally diffuse from regions of high concentration to regions of low concentration.
  • The steeper the concentration gradient, the greater the rate of particle flux.
• Nernst partition coefficient: quantifies the distribution of a substance between two immiscible phases (e.g., water and oil, or water and ether) at equilibrium
  • Formula: K = c_{A in phase 1}/c_{Ain phase 2}
  • K = partition coefficient (unitless), c_{A in phase 1} = concentration of substance A in phase 1 (mol/L), c_{A in phase 2}= concentration of substance A in phase 2 (mol/L)

In a closed system, diffusion ultimately results in the equalization of concentrations, eliminating any concentration differences across phases.

Diffusion through membranes

Diffusion occurs not only freely in solutions but also through permeable membranes.

• Diffusion through a membrane: D = P × A × Δc (derived from Fick's law)
  • Unit: mol/s
  • D = diffusion rate (mol/s), P = permeability coefficient (specific to the substance and membrane), A = area of the membrane (cm²), Δc = concentration difference across the membrane (mol/cm³)
  • Sample calculation
    • A cell membrane has a permeability of 1×10⁻⁷ cm/s for the molecule tryptophan.
    • What is the diffusion rate if the concentration difference across the membrane is 500 μmol/cm³ and the membrane area is 2 cm²?
      • Find: diffusion rate D
      • Given: permeability P, membrane area A, concentration difference Δc
        • D = P × A × Δc = 1 × 10⁻⁷cm/s × 2 cm² × 500 μmol/cm³ = 1 × 10⁻⁴ μmol/s
        • Tryptophan is transported through the membrane at a diffusion rate of 1 × 10⁻⁴ μmol/s.

Even substances that cannot freely cross a membrane can "diffuse" with the assistance of specialized transport proteins, such as carriers or channels. This process is known as facilitated diffusion. In this scenario, the number of available transporters limits the rate of particle flux, which can reach saturation. Consequently, the simple diffusion equation becomes inadequate, as facilitated diffusion involves specific interactions between the solute and the transport proteins.

Osmosis

• Osmosis: a specialized form of diffusion that occurs across semipermeable membranes, characterized by the net movement of water from areas of lower solute concentration to areas of higher solute concentration
• Osmolarity (osmotic concentration): the number of osmotically active particles per volume of solution, analogous to molarity; the value is temperature-dependent
  • Formula: c_osmotic = n_osmotic/V_solution
    • Unit: osmol/L
    • c_osmotic = osmotic concentration (osmol/L); n_osmotic = number of osmotically active particles (moles); V = volume of the solution (L)
  • The osmolarity varies with the number of dissolved particles; for example, NaCl dissociates into Na⁺ and Cl⁻, making its osmolarity approximately twice its molarity.
• Osmolality: the number of osmotically active particles relative to the mass of the solvent, similar to molality; the value is not temperature-dependent
  • Formula: b_osmotic = n_osmotic/m_solvent
    • Unit: osmol/kg
    • b_osmotic = osmolality (osmol/kg); n_osmotic = number of osmotically active particles (moles); m = mass of solvent (kg)
• Colligative properties: depend on the concentration of dissolved solute particles rather than their identity; crucial in understanding how solutes affect the physical properties of solutions
  • van 't Hoff factor (i): represents the number of particles a single formula unit of solute dissociates into in solution
    • Non-dissociating solutes: e.g., i = 1 for glucose
    • Dissociating solutes: e.g., , i ≈ 2 for NaCl, which dissociates into Na+ and Cl− or i ≈ 3 (1 + 2) for MgCl₂, which dissociates into Mg²⁺ and 2Cl^-
  • Vapor pressure depression: solutes lower the vapor pressure of the solvent
  • Boiling point elevation: solutes raise the boiling point, calculated as: ΔT_b = i × K_b × m
    • ΔTb = change in boiling point (°C), i = van 't Hoff factor (unitless), K_b = ebullioscopic constant (°C kg/osmol), m = molality (mol/kg)
  • Freezing point depression: solutes decrease the freezing point, calculated as: ΔT_f = i × K_f × m
    • ΔT_f = change in freezing point (°C), i = van 't Hoff factor (unitless), K_f = cryoscopic constant (°C kg/osmol), m = molality (mol/kg)
• Osmotic pressure: the pressure required to prevent the inward flow of water across a semipermeable membrane
  • Formula (van 't Hoff equation): π = i × c × R × T
    • Unit: Pa
      • π = osmotic pressure (Pa), i = van 't Hoff factor (unitless), c = molar concentration of solute (mol/L), R = ideal gas constant (8.314 Pa⋅m³/K⋅mol), T = absolute temperature (K)
    • As the concentration of particles increases on one side of the membrane, osmotic pressure increases, resulting in water moving toward the higher concentration area until equilibrium is reached.
• Oncotic pressure: the osmotic pressure exerted by colloids (like proteins) in solution
  • Reflection coefficient σ: describes the selectivity of a membrane for a solute
    • σ = 0: membrane is fully permeable (no osmotic effect)
    • σ = 1: membrane is impermeable (maximum osmotic effect)
    • Semipermeable membrane: allows the solvent (e.g., water) to pass while restricting certain solutes (with σ = 1)

Osmosis and cells

Biological membranes are semipermeable, making osmosis a crucial phenomenon in medicine. The movement of water from areas of lower solute concentration to areas of higher solute concentration has significant effects on cells and tissues.

• Tonicity: measures the effective osmotic pressure gradient between two systems separated by a semipermeable membrane, describing how a solution affects cell volume
  • Isotonic solutions
    • Cell: The effective osmotic pressure inside the cell equals that of the surrounding medium.
    • Environment: The osmotic pressure of an isotonic environment matches that in the cell.
    • Effect: When a cell is placed in an isotonic medium, there is no net osmosis, and cell volume remains stable.
  • Hypertonic solutions
    • Cell: The effective osmotic pressure in the cell is lower than in the surrounding medium.
    • Environment: The osmotic pressure of a hypertonic environment is greater than that in the cell.
    • Effect: In a hypertonic medium, water exits the cell, causing it to shrink (crenate).
  • Hypotonic solutions
    • Cell: The effective osmotic pressure in the cell is higher than that in the surrounding medium.
    • Environment: The osmotic pressure of a hypotonic medium is less than that in the cell.
    • Effect: When placed in a hypotonic medium, water enters the cell, causing it to swell and potentially burst (lysis). Therefore, administering distilled water via infusion must be avoided.
      • Protective mechanism: The potassium-chloride symporter can help transport K+ and Cl− ions out of the cell, reducing intracellular osmotic pressure to prevent lysis.

Edema
Edema refers to the accumulation of fluid in tissues (interstitium) when fluid leaks from capillaries. The flow is regulated by a balance between hydrostatic pressure (pushing fluid out) and oncotic pressure (pulling fluid in). Edema can result from: high hydrostatic pressure in capillaries, low oncotic pressure due to reduced protein levels (e.g., albumin) in blood, or high oncotic pressure in tissues due to increased osmotically active substances.

Osmotic diuresis
The principles of osmosis also apply to fluid excretion in the body. When excessive osmotically active substances (e.g., glucose) are present in the renal tubules, water is retained due to osmosis, leading to increased urine output. This condition is common in diabetes mellitus and is utilized in therapy with osmotic diuretics.

Biological membranes are composed of lipid bilayers, creating a predominantly nonpolar environment. This structural characteristic allows for the permeability of nonpolar molecules, such as carbon dioxide and steroid hormones. However, the lipid bilayer is largely impermeable to charged ions, polar molecules (e.g., sugars), and large molecules.

Consequently, cells rely on controlled transport mechanisms for these substances. There are two main types of transport: vesicular transport, where segments of the membrane move with the substance, and membrane protein-mediated transport, where specialized proteins (such as channels or carriers) facilitate the movement.

In membrane-displacing transport, portions of the cell membrane or organelle membranes are pinched off to form vesicles containing the substances to be transported, a process known as vesicular transport. Transport into cells is termed endocytosis, while transport out of cells is referred to as exocytosis. In transcytosis, substances undergo endocytosis, are transported across the cell, and are released via exocytosis. These forms of transport are regulated by a specific subset of G-proteins known as Rab proteins, which ensure that vesicles reach their designated target membranes.

Endocytosis

Endocytosis involves the uptake of molecules into cells through vesicle formation. It can be categorized into three main types: pinocytosis, phagocytosis, and receptor-mediated endocytosis. All three processes result in the eventual formation of vesicles containing the particles; however, their mechanisms are distinct.

Phagocytosis

Phagocytosis ("cell eating") is performed by specialized phagocytic cells such as macrophages and dendritic cells. These cells express opsonin receptors that bind opsonins (e.g., complement factors or antibodies) that mark particles (e.g., bacterial cells) for ingestion, acting as a bridge between the phagocyte and the target particle. This process allows for the uptake of large particles and entire cells.

• Process
  1. Binding of a particle marked with opsonins to the opsonin receptor of the phagocytic cell
  2. Formation of pseudopods: The cell membrane extends outward, fully encircling the particle, facilitated by actin polymerization.
  3. Pinching off the membrane to form a phagosome, which is an endocytic vesicle

Pinocytosis

Pinocytosis ("cell drinking") entails the uptake of extracellular fluid and dissolved solutes. Unlike phagocytosis and receptor-mediated endocytosis, classic pinocytosis is not receptor-specific and occurs spontaneously in various cell types.

• Process
  1. Spontaneous invagination of the cell membrane around the fluid to be internalized
  2. Actin remodeling assists in the invagination and vesicle formation.
  3. The vesicle pinches off inwardly to transport the fluid.

Receptor-mediated endocytosis

In receptor-mediated endocytosis, specific small to medium-sized particles (ligands) are internalized with the aid of membrane-bound receptors. This process is crucial for functions such as cholesterol metabolism (e.g., LDL uptake) and the assimilation of transferrin-bound iron.

• Process
  1. Binding of ligands to specific receptors, leading to clustering and interaction with adaptin proteins
  2. Invagination of the membrane with intracellular attachment of clathrin molecules, forming a "coated pit"
  3. Further invagination around the ligand-receptor complex occurs
  4. The vesicle pinches off into the cell's interior via the protein dynamin.
  5. The now uncoated vesicle fuses with an early endosome.
  6. An acidic environment within the endosome causes the ligand to dissociate from the receptor.
  7. The receptor is recycled back to the membrane, while the ligand is transported to the lysosome for degradation or to the Golgi apparatus for processing.

Exocytosis

In exocytosis, medium to large molecules are released into the extracellular environment, including products synthesized by the cell, such as hormones or indigestible breakdown products.

• Process
  1. The molecule to be released is packaged into a vesicle, typically formed in the Golgi apparatus.
     1. These vesicles are surrounded by a single lipid bilayer and contain specific coat proteins that vary depending on their position in the secretory pathway.
  2. The vesicle then fuses with the cell membrane.
  3. As the vesicle merges with the membrane, the molecule is released into the extracellular space.
• Regulation: often triggered by an increase in intracellular calcium concentrations or through receptor-mediated signals
• Apocytosis (or apocrine secretion): a specialized form of exocytosis where the cell releases its contents along with a portion of the cell membrane, resulting in loss of cytoplasm
  • Example: This process is particularly prominent in the lactating mammary gland (in the release of fat droplets) and in apocrine sweat glands.

Transcytosis

Transcytosis refers to the receptor-mediated transport of extracellular molecules through a cell, effectively combining endocytosis and exocytosis. This mechanism is crucial in cell layers with tight junctions, such as the enterocytes of the intestine, allowing substances to efficiently traverse barriers.

Transport through membrane proteins

Membrane-displacing transport processes are primarily employed for transporting large substances, whereas the rapid transport of small molecules or ions typically occurs through membrane proteins. There are two main categories of membrane proteins: channel proteins and carrier proteins.

• Passive transport: occurs without direct energy expenditure and moves substances down their electrochemical gradients
  • Simple diffusion: involves the net movement of small, nonpolar molecules (such as O₂ and CO₂) directly through the lipid bilayer without the help of proteins
  • Facilitated diffusion
    • Used for large, polar, or charged substances (e.g., glucose, ions) that cannot cross the membrane unaided
    • Requires transmembrane proteins (channels or carriers)
    • Process is specific for the substance being transported
    • Subject to saturation
  • Osmosis: can occur via simple diffusion or be facilitated by specific channels called aquaporins
• Active transport: requires energy to move substances against their electrochemical gradients
  • Primary active transport: directly utilizes energy from ATP hydrolysis to drive transport
  • Secondary active transport: couples the transport of one substance down its gradient to drive the transport of another against its gradient, indirectly consuming ATP

Channel proteins

Channel proteins are stationary transmembrane proteins that create a water-filled pore, facilitating the transport of small particles, mainly ions.

• Mechanisms
  • Channels can be leak channels (always open) or gated channels (open/close in response to stimuli).
    • Voltage-gated channels
      • Activation: open in response to changes in membrane potential
      • Duration: remain open for just a few milliseconds before transitioning to an inactive state that prevents activation
      • Resting state: after the inactive state, the channel returns to a closed (activatable) state
      • Ion diffusion through these channels is essential for establishing the resting membrane potential, which is crucial for the electrical excitability of cells.
    • Ligand-gated channels: open upon binding of a specific ligand, allowing selective ion flow
  • Selectivity filter: Channel proteins possess selectivity filters, narrow regions that ensure specificity for particular ions.
    • Example: A voltage-gated potassium channel allows only dehydrated K⁺ ions to pass through its narrowest section, while Na⁺ ions, being larger even when dehydrated, are barred due to the channel's selectivity.

Aquaporins are specialized transmembrane proteins that serve as water channels, facilitating rapid water transport across biomembranes. They can transport up to 3 billion water molecules per second, playing a vital role in cellular hydration and homeostasis.

Membrane potential calculations

• Nernst equation: calculates the equilibrium potential (E_ion) for a single ion
  • This is the membrane potential at which the chemical gradient (diffusion) and the electrical gradient (electrostatic force) for that ion are perfectly balanced, resulting in no net ion flow
  • Formula: E_ion = (RT/zF) × ln([ion]_outside/[ion]_inside)
    • Unit: volts (V)
    • E_ion = electric potential (V) R = ideal gas constant (J/(mol·K)); T = absolute temperature (K); z = ion's charge (dimensionless); F = Faraday's constant (C/mol), [ion]_outside: the concentration of the ion outside the cell (mol/L), [ion]_inside: the concentration of the ion inside the cell (mol/L)
• Goldman-Hodgkin-Katz equation: calculates the overall resting membrane potential (V_m) of the cell
  • Considers the contributions of all major ions that can cross the membrane (primarily K⁺, Na⁺, and Cl⁻)
  • Accounts for the relative permeability (P) of the membrane to each of these ions
  • The resting potential is determined by the concentration gradients and relative permeabilities of these ions
  • Formula: V_m = (RT/F) × ln((P_K[K⁺]_out + P_Na[Na⁺]_out + P_Cl[Cl⁻]_in)/(P_K[K⁺]_in + P_Na[Na⁺]_in + P_Cl[Cl⁻]_out))
    • Unit: volts (V)
    • V_m = the membrane voltage across the cell membrane (V), R = universal gas constant (∼ 8.314 J/(mol·K)), T = absolute temperature (K), F =Faraday's constant (∼ 96485 C/mol), P_K,P_Na,P_Cl: permeabilities of the respective ions (potassium, sodium, and chloride; dimensionless), [K⁺/Na⁺/Cl⁻]_out: the concentrations of potassium/sodium/chloride ions outside the cell (mol/L), [Cl⁻/K⁺/ Na⁺]_in: the concentrations of chloride/potassium/sodium ions inside the cell (mol/L)
  • Note: Chloride (Cl⁻) concentrations are inverted (in/out) because of its negative charge (z = -1).

Carrier proteins

Carrier proteins (or transporters) are transmembrane proteins that bind specific substances and change shape to transport them across the membrane.

• Classification
  • By number of substances transported and direction of transport
    • Uniport: transports one substance in one direction
    • Symport: transports two different substances simultaneously in the same direction
    • Antiport: transports two different substances in opposite directions
  • By transport mechanism
    • Facilitated diffusion (passive)
    • Primary active transport
    • Secondary active transport

Primary active transport

In primary active transport, energy from ATP directly drives the conformational changes needed for transport against concentration gradients.

• Na⁺/K⁺-ATPase (sodium-potassium pump): antiport
  • Transport process: 3Na⁺_{intracellular} + 2K⁺_{extracellular} + ATP + H₂O → 3Na⁺_{extracellular} + 2K⁺_{intracellular} + ADP + P_i
  • Location: plasma membrane of most animal cells
  • Function
    • Osmotic control: regulates water content by maintaining low intracellular Na⁺ concentration, preventing the cell from becoming hypertonic and swelling
    • Establishment of concentration gradient
      • Creates the Na⁺ gradient essential for secondary active transport
      • Maintains Na⁺ and K⁺ gradients crucial for the resting membrane potential
  • Mechanism
    1. The pump opens to the inside and binds three intracellular sodium ions.
    2. ATP binds intracellularly, and ATP hydrolysis occurs, converting ATP to ADP and adding a phosphate group to the pump.
    3. This phosphorylation induces a conformational change, opening the pump outward, leading to the release of sodium ions outside.
    4. Two extracellular potassium ions then bind to the pump, initiating dephosphorylation.
    5. This dephosphorylation causes another conformational change, opening the pump back to the inside, allowing potassium ions to enter the cytosol.

• Ca²⁺-ATPase (calcium pump): uniport
  • Location: plasma membrane and sarcoplasmic/endoplasmic reticulum (SERCA)
  • Function: lowers the cytosolic Ca²⁺ concentration by pumping it out of the cell or into the SR/ER, essential for muscle contraction
• H⁺/K⁺-ATPase (proton-potassium pump): antiport
  • Transport process: K⁺_lumen + H⁺_cytoplasm + ATP + H₂O → K⁺_cytoplasm + H⁺_lumen + ADP + P_i
  • Location: parietal cells of the stomach
  • Function: gastric acid secretion

"3 sodiums out, 2 potassiums in", as the Na⁺/K⁺-ATPase pumps three sodium ions out for every two potassium ions in.

Gastric ulcers
Gastric ulcers are ulcers that form in the stomach wall, potentially affecting all layers based on their severity. They can arise from chronic inflammation or excessive gastric acid production. One effective therapeutic strategy targets the H⁺/K⁺-ATPase in parietal cells, which regulates the low pH of gastric acid. Drugs like omeprazole irreversibly inhibit this pump, reducing gastric acid production. These medications are commonly referred to as proton pump inhibitors (PPIs).

Secondary active transport

Secondary active transport utilizes the electrochemical gradient established by primary active transport to move another substance against its gradient.

• Electrochemical gradient: a concentration difference of a solute that also has a charge difference across a membrane.
  • This gradient represents stored potential energy, which can be converted into work, reflecting principles from entropy and Gibbs free energy.
    • Establishment of the gradient
      • The gradient is established through primary active transport, which requires energy (derived from ATP).
      • The breakdown of the gradient supplies energy that can be utilized in several ways:
        • Driving secondary active transport to move additional substances against their gradients.
        • Supporting the synthesis of ATP from ADP and inorganic phosphate (Pi).
• Examples of secondary active transporters
  • Glucose transporter (SGLT1): symport
    • Transport process: Glucose_lumen + 2Na⁺_lumen → Glucose_{intracellular} + 2Na⁺_{intracellular}
      • Energy for the transport of glucose against its gradient comes from the breakdown of the Na⁺ gradient (movement of Na⁺ into the cell)
    • Location: intestinal epithelial cells (membrane on the luminal side)
    • Function: uptake of glucose and galactose from the intestine
  • ATP synthase
    • Transport process: ADP + P_i + H⁺_{(intermembrane space)} → ATP + H₂O + H⁺_(matrix)
      • Energy for the synthesis of ATP comes from the movement of H⁺ down its electrochemical gradient
    • Location: inner mitochondrial membrane
    • Function: enzyme that catalyzes the synthesis of ATP (oxidative phosphorylation)

Primary active transport directly uses energy, such as ATP, to move ions against their concentration gradient. In contrast, secondary active transport relies on the energy stored in concentration gradients created by primary active transporters, resulting in indirect ATP use.

Cystic fibrosis
Cystic fibrosis is a common example of a channelopathy caused by a malfunction of the CFTR chloride channel. This defect prevents chloride ions (Cl⁻) from being properly transported outside the cells onto epithelial surfaces. As a result, water cannot be osmotically pulled out of the cells effectively. This problem mainly affects exocrine glands, leading to thick and sticky secretions. In the lungs, for instance, the thick mucus cannot be cleared properly, causing recurrent and severe infections in the bronchi and lungs.

Tertiary active transport

• Definition: involves the transport of substances using the concentration gradient generated by secondary active transport mechanisms
  • Examples
    • Oligopeptide transporter 1 (PEPT1) in the kidney and intestine, which utilizes an H⁺ gradient established by secondary active transport (a Na⁺/H⁺ exchanger)

In tissues, continuous transport of substances occurs between adjacent cells through various mechanisms: paracellular transport, intercellular transport, and transcellular transport.

• Paracellular transport
  • Mechanism
    • Involves movement exclusively through the spaces between cells, as opposed to crossing two membranes (apical and basolateral) like in transcellular transport
    • Primarily functions in epithelial tissues
  • Regulation: extent of paracellular transport is influenced by the tightness of tight junctions between cells
    • Leaky epithelia: In regions with poorly developed sealing strands, such as the proximal tubule of the kidney and the small intestine, paracellular transport is significant.
      • Driving force: A special driving force known as solvent drag occurs during paracellular water transport, where dissolved particles are dragged along with the water.
    • Tight epithelia: In structures like the blood-brain barrier and the distal tubule of the kidney, paracellular transport plays a minor role due to more tightly sealed junctions.
    • Impermeable barriers: In highly impermeable regions, such as the epidermis, paracellular transport is not possible.
• Intercellular transport: direct transport between adjacent cells via gap junctions
  • Occurrence: e.g., cardiac muscle cells, epithelial cells, and glial cells
• Transcellular transport: transport through a cell, crossing both the apical and basolateral membranes (e.g., via transcytosis or through channels/carriers located on opposite membranes, enabling efficient substance movement across cellular barriers)