Epithelial Transport

I. Molecules can cross an epithelial layer of cells by two pathways: (1) through the extracellular spaces between the cells—the paracellular pathway, and (2) through the cell, across both the luminal and basolateral membranes as well as the cell's cytoplasm—the transcellular pathway.

II. In epithelial cells, the permeability and transport characteristics of the luminal and basolateral plasma membranes differ, resulting in the ability of the cells to actively transport a substance between the fluid on one side of the cell and the fluid on the opposite side of the cell.

III. The active transport of sodium through an epithelium increases the osmolarity on one side of the cell and decreases it on the other, causing water to move by osmosis in the same direction as the transported sodium.

IV. Glands are composed of epithelial cells that secrete water and solutes in response to stimulation.

a. There are two categories of glands: (1) exocrine glands, which secrete into ducts, and (2) endocrine glands (ductless glands), which secrete hormones and other substances into the extracellular fluid, from which they diffuse into the blood.

b. The secretions of glands consist of (1) organic substances that have been synthesized by the gland, and (2) salts and water, which have been transported across the gland cells from the blood.



plasma membrane





osmotic pressure

net flux

nonpenetrating solute

diffusion equilibrium


permeability constant, kp




membrane potential


electrochemical gradient


channel gating


patch clamping


ligand-sensitive channel


voltage-gated channel

fluid endocytosis

mechanosensitive channel

adsorptive endocytosis



mediated transport


facilitated diffusion


active transport

luminal membrane

primary active transport

basolateral membrane

secondary active transport

paracellular pathway


transcellular pathway


exocrine gland


endocrine gland



What determines the direction in which net diffusion of a nonpolar molecule will occur? In what ways can the net solute flux between two compartments separated by a permeable membrane be increased?

Why are membranes more permeable to nonpolar molecules than to most polar and ionized molecules? Ions diffuse across cell membranes by what pathway?

When considering the diffusion of ions across a membrane, what driving force, in addition to the ion concentration gradient, must be considered? What factors can alter the opening and closing of protein channels in a membrane? Describe the mechanism by which a transporter of a mediated-transport system moves a solute from one side of a membrane to the other. What determines the magnitude of flux across a membrane in a mediated-transport system? What characteristics distinguish diffusion from facilitated diffusion?

What characteristics distinguish facilitated diffusion from active transport?

Contrast the mechanism by which energy is coupled to a transporter during (a) primary active transport and (b) secondary active transport. Describe the direction in which sodium ions and a solute transported by secondary active transport move during cotransport and countertransport. How can the concentration of water in a solution be decreased?

If two solutions having different osmolarities are separated by a water-permeable membrane, why will there be a change in the volumes of the two compartments if the membrane is impermeable to the solutes, but no change in volume if the membrane is permeable to solute? To which solution must pressure be applied to prevent the osmotic flow of water across a membrane separating a solution of higher osmolarity and a solution of lower osmolarity? Why do sodium and chloride ions in the extracellular fluid and potassium ions in the intracellular fluid behave as if they are nonpenetrating solutes? What is the osmolarity of the extracellular fluid? Of the intracellular fluid?

What change in cell volume will occur when a cell is placed in a hypotonic solution? In a hypertonic solution?

Under what conditions will a hyperosmotic solution be isotonic?

Endocytotic vesicles deliver their contents to which parts of a cell?

How do the mechanisms for actively transporting glucose and sodium across an epithelium differ? By what mechanism does the active transport of sodium lead to the osmotic flow of water across an epithelium?

What is the difference between an endocrine gland and an exocrine gland?

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Movement of Molecules Across Cell Membranes CHAPTER SIX


(Answers are given in Appendix A.)

1. In two cases (A and B), the concentrations of solute X in two 1-L compartments separated by a membrane through which X can diffuse are


CONCENTRATION OF X, mM Compartment 1 Compartment 2

a. In what direction will the net flux of X take place in case A and in case B?

b. When diffusion equilibrium is reached, what will be the concentration of solute in each compartment in case A and in case B?

c. Will A reach diffusion equilibrium faster, slower, or at the same rate as B?

When the extracellular concentration of the amino acid alanine is increased, the net flux of the amino acid leucine into a cell is decreased. How might this observation be explained?

If a transporter that mediates active transport of a substance has a lower affinity for the transported substance on the extracellular surface of the plasma membrane than on the intracellular surface, in what direction will there be a net transport of the substance across the membrane? (Assume that the rate of transporter conformational change is the same in both directions.)

Why will inhibition of ATP synthesis by a cell lead eventually to a decrease and, ultimately, cessation in secondary active transport?

5. Given the following solutions, which has the lowest water concentration? Which two have the same osmolarity?





















Assume that a membrane separating two compartments is permeable to urea but not permeable to NaCl. If compartment 1 contains 200 mmol/L of NaCl and 100 mmol/L of urea, and compartment 2 contains 100 mmol/L of NaCl and 300 mmol/L of urea, which compartment will have increased in volume when osmotic equilibrium is reached?

What will happen to cell volume if a cell is placed in each of the following solutions?



(nonpenetrating) (penetrating)

150 100 200 100

100 150 100 50

Characterize each of the solutions in question 7 as to whether it is isotonic, hypotonic, hypertonic, isoosmotic, hypoosmotic, or hyperosmotic. By what mechanism might an increase in intracellular sodium concentration lead to an increase in exocytosis?

Vander et al.: Human I II. Biological Control I 7. Homeostatic I I © The McGraw-Hill

Physiology: The Systems Mechanisms and Cellular Companies, 2001

Mechanism of Body Communication


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  • Kian
    Which pathways do molecules use to cross epithelial layers?
    8 years ago
  • samuel aatifa
    What follows when a solute is actively transported across epithelium?
    7 years ago

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