Chapter 17 Active Transport Across Intestinal Epithelia and Other Bifacial Cell Systems

Cellular Asymmetry, Energetic Cycles, and the Role of ATP in Epithelial Active Transport

As always, this is not medical advice, and reading this does not form a client relationship with me - your health is your responsibility.

Many have asked for my “thoughts” on things. The best place to start would be some of my older Substacks - especially the ones on supplementation (1 and 2) and the 4 LEA types.

Today’s Substack will continue with Chapter 17, Active Transport Across Intestinal Epithelia and Other Bifacial Cell Systems. Chapter 16 discussed how muscle contraction is not driven by mechanical sliding of filaments but by quasi-phase/ functional phase transitions in the muscle cell’s water and protein states via the association of ATP leading to a protein-electrostatic rearrangement.

Summary

Epithelial tissues and vacuolated plant cells carry out true active transport, which involves creating a solute gradient across two dilute solutions. Epithelial cells form an interface between the body and the external/ internal environment. They serve a barrier and protective function, are used for absorption, can secrete things, etc. The traditional model relying on membrane pumps is questionable because any valid theory of active transport must assume that cells have an inherent asymmetry, which acts like a "one-way valve" for solute movement - the conventional view uses the ATPase for this. The association-induction hypothesis can be used to describe active transport. Active transport likely involves cyclic changes between high- and low-energy states, driven by ATP interactions. According to the association-induction hypothesis, ATP doesn't provide energy directly but triggers the cyclic adsorption and desorption of solutes and associated water structure changes. The abundance of ATPase in sodium-transporting tissues reflects a need for continual cycling, not more energy supply - the conventional view sees the hydrolysis of ATP to ADP + Pi as providing energy, whereas in Gilbert Ling's view, ATP serves to maintain the polarized, active state of the proteins and structured water; its eventual breakdown to ADP signifies a loss of this organized state rather than a release of usable energy.

17.1 Unifacial and Bifacial Cells

As a reminder, there is an asymmetry in the intracellular vs. extracellular solutes - e.g., at any one time and for any given cell, there is typically more K⁺ and Mg+2 in the cell and more Na⁺ and Ca+2 out of the cell. Chapter 11 argued this was not due to pumps or active transport but due to a metastable equilibrium due to selective absorption/ partial exclusion of solutes and water.

Metastable equilibrium is when a system is stable under small perturbations/ disturbances, but is not the most stable possible state - it is a non-equilibrium state that mimics equilibrium, so it is in local, not global stability. The system is in a state of local stability because there is some amount of activation energy/ energy barrier that must be achieved/ overcome to go to the true global energy minimum. My Substack on supplements discusses stability a little. Thermodynamically, death is at true equilibrium when all chemical gradients collapse, there is no net energy flow/ metabolic activity, the body is at maximum entropy (disorder) with a minimum free energy (no further work can be done); the ultimate steady state. The living state is metastable - the body maintains low entropy (order) via energy input (from food, sunlight, etc.). One might rightly ask, what is the activation energy to death? It is more that the body prevents collapse into equilibrium by sustaining gradients, order, function, etc. it’s as if the walls of the “well” (parabola) shown in my supplements substack decrease (e.g., energy depletion, ROS, homeostatic feedback failure, protein denaturation, etc.) until there is no longer any wall holding the ball in the well, removing the metastable equilibrium state resulting in uncontrolled collapse - why many experience death as falling off a cliff. I know this was how it was with my dad. So here the activation energy to death is what is maintaining order and life - I think of it as a particle in a box/ well in quantum mechanics that is in a quasi-bound state in a deep but finite potential well. The particle could of course tunnel - I will save quantum tunneling and my view of its scaling to this line of thought for another time! Anyways, the deeper and more stable the well, the more resistant to collapse. This is ultimately why addressing the lower energy availability state is crucial for life and why reading Dr. Gilbert Ling’s work was so freeing to me when I was going through my own major health issues and provided a clear way out vs. the constant wack-a-mole most find themselves in nowadays.

* Yes, I am trying to introduce as many terms/ definitions to you as possible so you know what people are talking about when these things are discussed! Keep reading, the more you do, the more these things make sense and you see how they fit in the bigger picture! *

Many cells, like red blood cells, are considered unifacial where there is one interface with the external environment - so red blood cells are suspended in plasma, the liquid component of the blood.

Other cells exist between two aqueous phases. For example, a frog’s skin in contact with the water external to it (e.g., in a pond), and the interstitial (tissue) fluid internal to the frog. The human skin barrier displays this ability to an extent as well - eventually becoming “pruny.” These cells also maintain intracellular and extracellular solute concentrations - e.g., the frog skin does not bring to equal concentration solutes in the interstitial fluid to the external water and vice versa.

17.2. Concepts of Active Solute Transport Based on the Membrane Pump Theory

Cell physiologists have not taken into account the difference between unifacial and bifacial cells - they have assumed what applies to unifacial must for bifacial. Unifacial cells have one “face” that interacts with the environment - e.g., the red blood cell and muscle cell. Bifacial cells have two distinct “faces”, one facing the external environment (apical) and the other, the internal (serosal/ basal) - e.g., the gut and blood so the enterocytes and colonocytes.

17.2.1. The "Two-Membrane Theory" of Koefoed-Johnson and Ussing

The apical surface of the frog’s skin is highly permeable to Na⁺ and the basal surface to K⁺, leading researchers to believe the apical has a Na⁺-pump and the basal, a K⁺-pump.

17.2.2. The Standing Osmotic Gradient Theory of Diamond and Bossert

The gallbladder, intestinal mucosa, and renal proximal tubule transport salt and water as an isotonic solution. An isotonic solution has the same osmotic pressure as the solution within the cells. This means there is no solute concentration gradient which means there is no net movement of water, etc. It’s important to note, no net movement. Researchers argued that isotonic solution transport is driven by ion pumping into intercellular spaces creating a hypertonic environment. A hypertonic solutions is one that has a higher solute concentration (= higher osmotic pressure) than the fluid inside cells. This then causes the water to move out of cells into the hypertonic environment making it isotonic as it moves to the basal side.

17.2.3. The Pericellular Pump Theory of Cereijido and Rotunno

Researchers postulated that the transport of Na⁺ by frog skin involves the migration of Na+ along an array of fixed negative sites on the apical side.

17.2.4. The Na + Gradient Hypothesis of Sugar and Amino Acid Transport

Intestinal glucose transport requires Na⁺ in the mucosal fluid. The mucosal fluid is the fluid that interacts with the apical surface, so here, the fluid in the intestine. Researchers proposed a co-transport model where Na⁺, glucose, and a carrier cross the mucosal membrane as a complex. This complex then dissociates, delivering Na⁺ and glucose to the other side - this is probably the sodium-glucose-linked transporter 1 (SGLT1). The Na⁺ gradient through the cell interior is what provides the energy for glucose transport. Amino acid transport in various cells also similarly depends on Na⁺. Studies using isolated microvilli vesicles show transient "overshoot" uptake of sugar/amino acids when exposed to high Na⁺, supporting the Na⁺ gradient hypothesis. This is why those with hypoglycemia tend to feel better with salt and why many crave salt nowadays - it is a sign of the low energy availability state and is acting as a bandaid.

17.3. Cooperative Adsorption-Desorption Model of Active Transport across Epithelia and Other Bifacial Cell Systems

Hemoglobin (a protein inside red blood cells that binds and transports oxygen and carbon dioxide) in a dialysis bag (semi-permeable pouch used by researchers to separate molecules based on size via diffusion) takes up oxygen at equilibrium. When ATP is added, the oxygen moves out into the external solution. This is because ATP is exerting a long-range non-hydrolytic allostatic effect (binding of something to a non-active site, so it’s not broken down like via hydrolysis - when a molecule is split by the addition of water). When the ATP is removed, the oxygen reenters the bag against the concentration gradient - this should require energy. However, hemoglobin has no ATPase. Therefore, adding or removing ATP can cyclically drive oxygen movement, and a biological system could replicate this using ATPase, ATP-regenerating enzymes, and an ATP recycling mechanism. Cyclic ATP binding and hydrolysis can lead to selective solute uptake and release, providing a mechanistic basis for active transport - movement against the concentration gradient which usually requires ATP in the body. An additional requirement for active transport is a one-way valve mechanism. Structured water layers at the cell surface acting as selective permeability gates might be this mechanism. If the serosal surface undergoes cyclic water polarization changes and the mucosal side has fixed high-permeability Na⁺ sites, then all elements for bifacial active transport are present.
Step 1: Na⁺ enters from the mucosal side, binds to ATP-controlled anionic (negatively charged) sites on cytoplasmic proteins, and accumulates in a cooperative high-concentration state. Although ATP promotes Na⁺ adsorption, in other systems it may favor K⁺ adsorption due to cationic sites.

Step 2: Na⁺ binding near the cardinal ATPase site activates ATP hydrolysis, triggering desorption of Na⁺ and water, depolarization, and conformational changes in the protein.

Step 3: Because “only water in the state of polarized multilayers offers selective resistance to ion permeation”, water depolarization increases serosal permeability, allowing Na⁺ and water to exit the cell through the serosal.

Step 4: Regenerated ATP rebinds, restoring the protein to its initial state with polarized water and Na⁺ adsorption, completing the active transport cycle.

17.4. Application of the Model to Experimental Findings

17.4.1. Cyclic Changes of Adsorption- Desorption as the Basis for Active Transport

Single-celled algae like Nitella show evidence of cyclic ion accumulation and release, with ions first accumulating in the protoplasm and later appearing in the vacuole in a pattern that supports adsorption-desorption cycling.

17.4.2. Location of the Pumping Mechanism

Ling’s model includes the different permeability characteristics of the serosal and mucosal membranes. However, his model diverges by emphasizing protoplasmic involvement in cyclic adsorption-desorption beyond just the serosal surface. Experimental data from insect tubules, amphibian kidneys, and rabbit renal tissue support the idea that ion exchange and transport correlate with total intracellular ion levels, indicating that transport is not confined to thin membrane surfaces alone.

17.4.3. The Source of Energy for Active Transport

The association induction (Ling’s) model proposes that energy for transport is stored in structured protein-water-ion complexes that are derived from ATP synthesis. This links energy use to oxygen and substrate consumption - we obviously see this in the more “macro” with the Bohr/ Haldane effect. Evidence of direct correlation between Na⁺ transport and oxygen use varies across tissues, some data show close links between transport rates and substrate metabolism, especially in systems like perfused kidneys.

17.4.4. Coupling of Ion and Water Transport

Low-resistance epithelia (e.g., gallbladder, proximal kidney tubules, and small intestine) transport isotonic fluid, with osmotic coupling through polarized water layers. However, the required membrane permeability for classical models is unrealistically high. Isotonic fluid secretion occurs when ATP-driven dephosphorylation triggers Na⁺ and water release at the serosal surface, leading to polarization state changes causing osmotic flow. Hormones like anti-diuretic hormone increase water permeability by depolarizing membrane water layers, which can cause permeability cycling at mucosal or serosal surfaces depending on tissue type. “Solute and water transport may be rate-limited either by the frequency of the cycles of adsorption and desorption or by the mucosal surface permeability.” - needs a recharge time. Time at the cellular level is an interesting consideration.

17.4.5. The Relation between "Homocellular" Regulation of Cell K⁺ and Na⁺ Composition and "Homoepithelial" Na⁺ Transport

Some experiments contradict the double membrane model - Ling believes they fail due to incorrect assumptions of pump-based Na⁺/K⁺ regulation - the AI model does not fail because it integrates homocellular (same cell) mechanisms.

17.4.6. Coupling of Na⁺ Transport with Sugar and Amino Acid Transport

Sugars and amino acids are cotransported with Na⁺ - again, one reason why many feel better with more salt.

17.4.6.1. Surface Adsorption Sites

Na⁺-sugar (protein)-carrier are able to permeate the cell readily - Ling argues because in the conventional view, “the fact that the cell membrane physiological barrier is now known not to be due to phospholipid itself demands a new theoretical model”. In the AI model, surface proteins change conformation in the presence of both Na⁺ and D-glucose, creating new binding sites that promote their coupled entry via adsorption-desorption rather than traditional carrier-mediated diffusion - this brings to question the GLUTs and SGLTs.

17.4.6.2. Studies of Microvilli Isolated from Intestinal and Kidney Epithelia

Isolated microvilli allow for analysis of solute uptake dynamics and show that D-glucose accumulation peaks ("overshoots") are not explained by traditional pump theory due to the absence of outward pumps. Data show overshoots reflect transient adsorption in polarized cytoplasmic water layers, with cooperative Na⁺ and D-glucose binding facilitating entry into microvilli. Multiple studies confirm that “overshoots” depend on cytoplasmic proteins, ion-specific interactions, and the regulatory effects of compounds like valinomycin and FCCP, indicating a protein-water-ion-based control system rather than membrane pumps. Ling acknowledges the issue of the transient nature of overshoots - they may be part of a cyclic process or system decline, and further studies are required to fully understand the underlying mechanisms. As an aside, further studies have shown that microvilli contain a core of actin - Ling believe actin was one of the matrix proteins that polarizes water into multilayers. Microvilli are finger-like structures found on the apical surface of epithelial cells, especially in the intestines and kidneys, that increase surface area for absorption and secretion. Microvilli are crucial for absorption and make up the brush border of the small intestine where most of the macronutrient and micronutrient absorption occurs. The microvilli in the large intestine are more for water, electrolyte, and short-chain fatty acid absorption. The large intestine surface is smoother as most nutrient digestion is completed before - this is why I constantly question the focus on the microbiome (in the large intestine).

The next section discusses unsolved problems in Biology and Medicine, starting with protein synthesis. Before diving into this, I plan to release a Substack discussing some of the things I have been up to and the comments/ questions on my last Substack. Then I will continue with reviewing the final chapters in Ling’s “In Search of the Physical Basis of Life”. After that, I think I am going to go through Roeland Van Wijk’s “Light in Shaping Life” as this is another book people namedrop and it is clear they have not read it.