CoverModule 1.0. Homeostasis, Membranes, Electrophysiology and ANS1.1. Homeostasis1.1.1. Homeostasis Defined1.1.2. Homeostatic Control Systems1.1.3. Feedback Response Loop1.2. Cell Transport; Water & Solutes1.2.1. Fluid Compartments1.2.2. Osmosis1.2.3. Diffusion of Solutes1.2.4. Active Transport1.2.5. Bulk Transport1.3. Electrophysiology1.3.1. Ions and Cell Membranes1.3.2. Membrane Potentials1.3.3. Graded Potential1.3.4. Action Potentials1.3.5. Refractory Periods1.3.6. Propagation of an Action Potential1.4. The Synapse1.5. The Autonomic Nervous System1.5.1. Organization of the Nervous System1.5.2. Structural Organization of the ANS1.5.3. The SNS and the PNS1.5.4. The Enteric Nervous System1.5.5. Physiology of the ANS1.5.6. Neurotransmitters of the ANS1.5.7. Receptors of the ANS1.5.8. Actions of the Autonomic Nervous System1.5.9. Table of Actions for the SNS and PNS and Some Common DrugsModule 2.0. Skeletal Muscle and Special Senses2.1. Structural Organization of Skeletal Muscle2.2.1. Neuromuscular Junction, Excitation-Contraction Coupling2.2.2. Muscle Contractures and Cramps2.3. Whole Muscle Contraction, Fiber Type, Fatigue and Muscle Pharmacology2.3.1. Motor Units2.3.2. Factors that Influence the Force of Contraction2.3.3. Energy Source for Muscle Contraction2.3.4. Skeletal Muscle Fiber Types2.3.5. Fatigue2.3.6. Muscle Pharmacology2.4. Smooth Muscle2.4.1. Smooth Muscle Contraction2.5. Control of Body Movement2.5.1. Voluntary Control of Muscle2.5.2. Reflexes2.6. Taste and Smell2.6.1. Taste2.6.2. The Sense of Smell2.7. Vision2.7.1. Structure of the Eye2.7.2. Focusing Light on the Retina2.7.3. Converting Light to Action Potentials2.7.4. The Retina2.7.5. Phototransduction2.7.6. Receptive Fields2.8. Hearing and Equilibrium2.8.1. The Nature of Sound2.8.2. The Hearing Apparatus2.8.3. Sound Vibrations to Action Potentials2.8.4. The Sense of Balance and EquilibriumModule 3.0. Cardiovascular System3.1. Structure of the Heart3.1.1. Chambers and Circulation3.2. Cardiac Cell Action Potentials3.2.1. Action Potentials in Cardiac Muscle Cells3.2.2. Action Potentials in Cardiac Autorhythmic cells3.2.3. Cellular Mechanisms of Inotropy and Chronotropy3.3. Electrophysiology of Heart Muscle3.3.1. Heart Conduction System3.3.2. Electrocardiogram (ECG)3.3.3. Abnormal ECG - Current of Injury3.4. The Cardiac Cycle3.4.1. Cardiac cycle3.4.2. Cardiac Measurements and Pressure Volume Loops3.5. Blood vessels and Blood Pressure3.5.1. Arteries and Veins3.5.2. Capillaries3.5.3. Blood Pressure Regulation and Shock3.5.4. Capillary Exchange3.5.5. Myogenic and Paracrine Regulation of Vasoconstriction and Vasodilation3.6. Blood3.6.1. Composition of Blood3.6.2. Hematopoeisis3.6.3. Breaking Down Red Blood Cells3.6.4. HemostasisModule 4.0. Urinary and Respiratory Systems4.1. Function and Structure of the Kidney4.1.1. Urinary System Function4.1.2. Functional Anatomy of the Urinary System4.1.3. The Nephron: Functional Unit of the Kidney4.1.4. The Renal Corpuscle: Bowman's Capsule4.2. Physiology of Urine Production4.2.1. Filtration4.2.2. Renal Clearance4.2.3. Tubular Reabsorption4.2.4. Urine Concentration and Dilution4.2.5. Hormonal Regulation of Urine Production4.3. Acid/Base Balance4.3.1. Buffers4.3.2. Acid/Base Disturbances4.4. The Respiratory System4.4.1. Respiratory System Structure and Function4.4.2. Respiratory Membrane4.4.3. Respiratory pressures and Inspriation/Expiration4.4.4. Alveoli and Surfactant4.4.5. Pneumothorax4.4.6. Pressure-Volume Loops and the Work of Breathing4.5. Gas Exchange and Transport4.5.1. Gas Laws4.5.2. Partial Pressure Gradients in the Lung4.5.3. Alveolar Gas Equation4.5.4. Oxygen and Carbon Dioxide Transport in the Blood4.5.5. Alveolar Ventilation4.5.6. Ventilation/Perfusion Ratio4.6. Chronic Bronchitis and Emphysema4.6.1. Respiratory Control by the Medulla Oblongata4.6.2. Chemicals that Regulate VentilationModule 5.0. Digestive, Endocrine and Reproductive Systems5.1. Functional Anatomy of the Digestive System5.1.1. Layers of the Digestive Tract5.1.2. Enteric Nervous System5.1.3. Organs of the Digestive System5.2. Digestion5.2.1. Carbohydrates5.2.2. Proteins5.2.3. Lipids5.2.4. Lipoproteins5.3. Regulation of Digestive Secretions5.4. Endocrine System5.4.1. Overview of the Endocrine System5.4.2. Hormone Receptors5.4.3. Hormones of the Body5.4.4. Other Hormones: Melatonin and Pheromones5.5. The Hypothalamus and Pituitary Gland5.5.1. Structure and Function of the Hypothalamus and Pituitary Gland5.5.2. The Posterior Pituitary5.5.3. The Anterior Pituitary5.5.4. Growth Hormone5.5.5. Prolactin5.5.6. Thyroid Hormones5.5.7. Adrenal Hormones5.6. Pancreas5.6.1. Insulin and Glucagon5.6.2. Diabetes Mellitus5.7. Reproductive System Anatomy5.7.1. Female Reproductive Anatomy5.7.2. Male Reproductive Anatomy5.7.3. Sexual Development at Puberty5.7.4. Male Reproductive Endocrine Axis5.7.5. Spermatogenesis5.7.6. Female Reproductive System: Oogenesis5.7.7. Ovulation and Fertilization5.7.8. The Ovarian Cycle5.7.9. The Uterine Cycle5.7.10. PregnancyAppendix A. GenderAppendix B. The Placebo EffectB.2.1. The Placebo EffectB.2.2. Examples of the Placebo EffectB.2.3. How do Placebos Work?B.2.4. Are Placebos Ethical?B.2.5. How do we validate actual effectiveness of placebosB.2.6. Tips for evaluating scientific evidenceB.2.7. What about Faith Healings

Diffusion of Solutes

Because the hydrophobic core of cell membranes creates a barrier; (preventing hydrophilic substances, such as ions, water and large polar molecules, from moving across the membrane), the membrane makes use of proteins to facilitate movement of most solutes and water. Processes that move substances (solutes) across membranes can be grouped into two general categories based on whether the process requires an input of cellular energy or not. If no energy input is required for the transport, then we say particles move via a passive transport process. On the other hand, if the process requires cellular energy, usually in the form of ATP, then it is an active transport process.

Simple Diffusion

Diffusion is a process that results from the fact that all molecules, including solids, liquids, and gasses, are constantly in a state of random movement. This motion causes collisions between neighboring molecules, thus altering directions and creating a state of “random” motion. This random motion can be further altered by temperature, with increases in temperature stimulating a more rapid random movement. If there is an initial, unequal distribution of the molecules (i.e. more concentrated in one area than another), the constant random movement and collisions cause them to eventually become equally distributed. This process of gradual movement from more concentrated to less concentrated is called diffusion. We refer to the concentration difference as the concentration gradient.

Substances diffuse down their concentration gradients (from high to low concentration). Once the molecules are evenly distributed, we say that we have reached a state of diffusion equilibrium, and even though the molecules are still moving, there is no longer any net change in concentration. You can observe this phenomenon by carefully placing a drop of food coloring into a glass of water. The dye gradually moves through the liquid until it is evenly dispersed in the water. In the body, if the material in question can pass through the cell membrane without the aid of a membrane protein, we refer to the process as simple diffusion. Solutes that cross the membrane by simple diffusion tend to be hydrophobic. Examples of substances that cross the membrane by simple diffusion are the gasses CO2 and O2.

Simple Diffusion: Process of Moving from High to Low Concentration to Reach Equilibrium
Image created by BYU-Idaho student, Hannah Crowder 2013.

The top panel shows the diffusion of solute from left (high concentration) to the right (low concentration) until an equilibrium is established. Once a diffusion equilibrium exists, there will no longer be any net movement of solute (lower panel).

Factors That Affect the Rate of Diffusion

The speed at which a molecule moves across a membrane depends in part on the mass, or molecular weight, of the molecule. The higher the mass, the slower the molecule will diffuse (rate is proportional to 1/MW1/2). Another factor that affects the rate of diffusion across the membrane is the solubility of the substance. Nonpolar substances, such as oxygen, carbon dioxide, steroids and fatty acids, will diffuse rapidly while polar substances, having a much lower solubility in the membrane phospholipids move through more slowly, or not at all. Ions, such as Na+ and Cl-, tend to diffuse across a membrane rather rapidly. The diffusion rate across a membrane is proportional to the area of the membrane and to the difference in concentration of the diffusing substance on the two sides of the membrane. This relationship can be demonstrated by Fick’s first law of diffusion, which states that:

J = -DA(∆C/∆X)

J = net rate of diffusion in moles or grams per unit time

D = diffusion coefficient of the diffusing solute in the membrane (this coefficient takes into account the size of the substance as well as its solubility in the membrane)

A = surface area of the membrane

∆C = concentration difference across the membrane

∆X = thickness of the membrane. Diffusion is quite rapid over short distances but gets slower the further it goes. The time it takes for something to diffuse is proportional to the square of the distance. Therefore, if it takes one second to diffuse one centimeter, it would take 100 seconds to diffuse 10 cm and 10,000 seconds to diffuse 100 cm. So, to go 100 times further takes 10,000 times longer. In the body, diffusion is quite sufficient to cross the thin cell membrane, but traveling long distances by diffusion would be very slow. This is why we have other mechanisms, like blood circulation, for moving substances long distances.

Facilitated Diffusion

Facilitated diffusion represents the movement of substances across the membrane that are too big and/or too polar to pass through the membrane. This type of movement is mediated by integral membrane proteins called transport proteins. Unlike simple diffusion, this process of diffusion exhibits saturation, and its rate is directly related to the concentration of specific transport proteins within the membrane. In addition, this type of transport, like simple diffusion, does not require an input of energy.  Facilitated diffusion can occur in two different ways, through channel proteins and carrier proteins.

Channel proteins resemble fluid filled tubes through which the solutes can move down their concentration gradients across the membrane. These channels are often responsible for helping ions, such as Na+, K+, Ca2+, and Cl-, cross the membranes. Even though they are open tubes, they often only allow very specific ions to pass through them. For instance, a K+ channel may allow K+ to pass through but not Na+ or Cl-. This is due to the presence of a selectivity filter that selects for hydrated or dehydrated states of the specific ion. Also, as we will learn later, the regulation of the movement of the various ions across the membranes is crucial for many important cellular functions. These channels, therefore, are often gated (they have doors or gates that can be opened or closed). Depending on the channel, these gates may respond to voltage differences across the membrane (voltage-gated channels), specific signal molecules (ligand-gated channels), or even stretching or compressing of the membrane (mechanically-gated channels).

Voltage Gated Channel
Author: OpenStax College; Site:  License: Licensed under a Creative Commons Attribution 4.0 License

Voltage-gated channels (shown above) open when membrane voltage changes. The concentration of ions in the intracellular fluid create the voltage. Amino acids in the protein transporter are sensitive to charge and cause the channel to open for a specific ion.

In ligand-gated channels the pore opens to ions when the ligand binds to a specific location on the extracellular surface of the channel protein. Acetylcholine is the ligand shown in the example below.

Ligand-Gated Channels  License: Licensed under a Creative Commons Attribution 4.0 License

When a mechanical change such as pressure, touch, or a change in temperature happens, mechanically-gated channels open.

Mechanical-Gated Channels

Author: OpenStax College; Site:   License: Licensed under a Creative Commons Attribution 4.0 License

Another example of a gated channel protein is the K+ leak channel which opens and closes intrinsically and contributes to the cells electrical potential (discussed in more detail later).

Leak Channel

Author: OpenStax College; Site:  License: Licensed under a Creative Commons Attribution 4.0 License

The second type of facilitated diffusion utilizes carrier proteins in the membrane and is known as carrier-mediated transport. Unlike the channel proteins, these carriers do not open to both sides of the membrane simultaneously. Instead, they bind to a specific solute on one side of the membrane. This binding causes the carrier to change shape, which moves the solute to the other side of the membrane (think of a revolving door).

Carrier Proteins

By LadyofHats Mariana Ruiz Villarreal [Public domain], via Wikimedia Commons

Like the channel proteins, these carriers can be very specific in the solute they transport since the solute must bind to a receptor site that is designed to fit a specific solute. Another interesting characteristic of these carriers is; similar to all channel proteins, that they have a maximum rate of transport and can thus become saturated if the solute concentration is high enough. An important family of carrier proteins transports glucose across cell membranes. To date, 12 carriers in this family have been identified. They are simply identified as GLUT1 through GLUT12, and their distribution and specificity vary. For example, GLUT2 is found in the liver and pancreatic islets, while GLUT4 is found in skeletal muscle and fat tissue. Interestingly, GLUT4 is the only one of these carriers that requires insulin for maximal activity.

GLUT 4 Carrier Protein

By CNX OpenStax [CC BY 4.0 (], via Wikimedia Commons

As mentioned above, one of the characteristics of carrier proteins is that they can become saturated. One of the symptoms of uncontrolled diabetes is the presence of glucose in the urine. This is due to the fact that so much glucose is entering the kidney tubules, so the transporters that normally move the glucose back into the blood become saturated, and the excess glucose ends up in the urine.

End-of-Chapter Survey

: How would you rate the overall quality of this chapter?
  1. Very Low Quality
  2. Low Quality
  3. Moderate Quality
  4. High Quality
  5. Very High Quality
Comments will be automatically submitted when you navigate away from the page.
Like this? Endorse it!