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41.1B: Transport of Electrolytes across Cell Membranes - Biology

41.1B:  Transport of Electrolytes across Cell Membranes - Biology


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Ions cannot diffuse passively through membranes; instead, their concentrations are regulated by facilitated diffusion and active transport.

Learning Objectives

  • Explain the relationship between osmotic pressure and the transport of electrolytes across cell membranes

Key Points

  • Important ions cannot pass through membranes by passive diffusion; if they could, maintaining specific concentrations of ions would be impossible.
  • Osmotic pressure is directly proportional to the number of solute atoms or molecules; ions exert more pressure per unit mass than do non- electrolytes.
  • Electrolyte ions require facilitated diffusion and active transport to cross the semi-permeable membranes.
  • Facilitated diffusion occurs through protein -based channels, which allow passage of the solute along a concentration gradient.
  • In active transport, energy from ATP changes the shape of membrane proteins that move ions against a concentration gradient.

Key Terms

  • facilitated diffusion: The spontaneous passage of molecules or ions across a biological membrane passing through specific transmembrane integral proteins.
  • passive diffusion: movement of water and other molecules across membranes along a concentration gradient
  • active transport: movement of a substance across a cell membrane against its concentration gradient (from low to high concentration) facilitated by ATP conversion

Transport of Electrolytes across Cell Membranes

A teaspoon of table salt readily dissolves in water. The solubility of sodium chloride results from its capacity to ionize in water. Salt and other compounds that dissociate into their component ions are called electrolytes. In water, sodium chloride (NaCl) dissociates into the sodium ion (Na+) and the chloride ion (Cl). The most important ions, whose concentrations are very closely regulated in body fluids, are the cations sodium (Na+), potassium (K+), calcium (Ca+2),and magnesium (Mg+2); and the anions chloride (Cl-), carbonate (CO3-2), bicarbonate (HCO3-), and phosphate(PO3-). Electrolytes are lost from the body during urination and perspiration. For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.

Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because electrolytes dissociate into ions, adding relatively more solute molecules to a solution, they exert a greater osmotic pressure per unit mass than non-electrolytes such as glucose.

Water passes through semi-permeable membranes by passive diffusion, moving along a concentration gradient and equalizing the concentration on either side of the membrane. Electrolyte ions may not be able to passively diffuse across a membrane, but may instead require special mechanisms to cross the semi-permeable membrane. The mechanisms that transport ions across membranes are facilitated diffusion and active transport. Facilitated diffusion of solutes occurs through protein-based channels. Active transport requires energy in the form of ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.

Transport across cell membranes: Paul Andersen describes how cells move materials across the cell membrane. All movement can be classified as passive or active. Passive transport, such as diffusion, requires no energy as particles move along their gradient. Active transport requires additional energy as particles move against their gradient. Specific examples, such as GLUT and the Na/K, pump are included.


Transport of Ions through Cell Membrane

The living organisms can be resolved into organs, glands, tissues, cells and organelles. It is very inter­esting in biology to know how solutes and water get into and out of cells and organelles. Most at­tention is to be paid to erythrocytes and mitochon­drion. The cell membrane is a complex lipoprotein structure.

Some channels are continuously open, whereas others are gated i.e. they have gates that open or close. Some are gated by alterations in membrane potential (voltage gated) whereas others are opened or closed when they bind a ligand (ligand gated).

The ligand is often external (neurotransmitter or hormone) or internal (intracellular Ca ++ , cAMP). Other transport proteins are carriers that bind ions and other molecules and then change their configuration, moving the bound molecule from one side of the cell membrane to the other.

Molecules move from areas of high concen­tration to areas of low concentration (down their chemical gradient). Cations move to negatively charged areas whereas anions move to positively charged areas (down their electrical gradient), ligand gated chan­nel.

Some of the carrier proteins are called uniports because they transport only one substance. Others are called symports because transport requires the binding of more than one substance to the trans­port protein and the substances are transported across the membrane together.

In the intestinal mucosa that is responsible for the cotransport by facilitated diffusion of Na + and glu­cose from the intestinal lumen into mucosal cells. Other transporters are called anti-ports because they exchange one substance for another. Example: Na + – K + ATPase.

It catalyses the hydrolysis of ATP to ADP and uses the energy to extrude 3Na + from the cell and take 2K + into the cell for each mole of ATP hydrolysed. The pump is said to have a coupling ratio of 3/2. Its activity is inhibited by ouabain and related to digitalis glycosides used in the treatment of heart failure.

Na + -K + ATPase is a heterodimer made up of α and β subunit.

Na + and K + transport occurs through a subunit.

β subunit is a glycoprotein.

Substances passing through the lipid bilayer of the cell membrane by simple diffusion are:

1. All lipid soluble substances.

2. Lipid soluble gases mainly CO2, O2 and N2.

3. Water—though not lipid soluble—passes because of small molecular size and high kinetic energy.

Substances passing through protein channels of cell membrane by simple diffusion are:

1. Ions mainly Na + , K + and Ca ++ .

A. Passive Diffusion:

1. Some solutes pass through cell membrane by simple diffusion with the concentra­tion gradient.

This can be expressed by the modification of Fick’s law:

where, P = the permeability coefficient.

C0 and Ci = the concentration of solution outside and inside the membrane, respec­tively.

ds/dt = rate of movement of solute.

2. Lipid-soluble solutes pass more readily through cell membranes than lipid-insoluble solutes. Because the cell membrane consists of small water-filled pores of ra­dius about 0.4 nm. through which water- soluble solute of suitable molecular size pass, surrounded by lipid areas through which lipid-soluble solutes penetrate.

3. Water diffuses through the cell pores from a solution of low concentration to a solu­tion of high concentration and this “bulk flow” of liquid across the membrane will speed up molecules diffusing in the direc­tion of the flow and slow down those mov­ing in the opposite direction. This “drag” effect is a second force acting in passive diffusion.

4. The third force which may operate is an electric potential across the membrane. Many cell membranes can maintain po­tential difference between their inside and outside and the potential gradient acts as a driving force for passive transport across the cell. The membrane acts as a passive barrier.

B. Facilitated Transfer:

1. Some compounds, e.g., sugar, amino acids, pass through membranes at a greater rate than expectations. This is because of the effect of a carrier.

2. The carrier in the membrane combines with the substance to be transported and in some way ferried through the membrane and released on the other side.

3. In case of enzymic reactions, there is a “saturation effect”. The rate of transport of the solute increases when the carrier, enzyme, is saturated. This type is some­times termed “catalysed diffusion”.

4. Another mechanism is that the substance to be transferred is converted into another which will penetrate the membrane more easily, e.g., the mitochondrial membrane is impermeable to acyl coenzyme A deriva­tives. The acyl group is transferred to car­nitine to form acyl carnitine derivative which can pass through the membrane. The acyl coenzyme A derivative is then reformed on the other side of the mem­brane.

Fatty acids can also be transferred into and out of mitochondria.

Acetyl-CoA within the mitochondria can be transferred to oxaloacetate to yield citrate to which the mitochondrial mem­brane is permeable. The citrate passes out into the cytoplasm where it is split enzymically to give acetyl-CoA again.

1. The cell membrane forms pockets or invaginations which can draw materials on the outside towards the cell interior.

2. The vesicles extend into the cell where they are pinched off and finally release their contents into the cell by some un­known way.

3. This process occurs in the foetal and new­born animals and helps the absorption of intact protein from the gut.

D. Transport of Ions:

1. The membrane itself contains polar groups and is, therefore, electrically charged.

2. The transport of most ions occur more slowly than the non- electrolytes. But H + , OH − penetrate all cell membranes easily. The red cell is easily penetrated by Cl − and HCO − 3.

3. In the case of ions, especially, Na + and K + , the permeability is very small. The high concentration of K + and low concentra­tion of Na + which are often found in cells are maintained by special mechanism which involve the expenditure of energy.

E. Active Transport:

1. The process by which solutes can often pass through membranes against their con­centration gradient requires energy. This process is termed active transport.

2. Active transport is involved in the absorp­tion from the small intestine of glucose and galactose, amino acids and other sub­stances important to the body.

3. An active transport device which forces Na + out and K + in has been referred to as the “Sodium Pump”.

4. The mechanism requires a carrier which can exist in two forms with different af­finities for Na + and K + . ATPase is involved in it (see active transport of glucose).


3. Osmosis

Osmosis (from the Greek osmos = push) is the movement of water across the cell membranes from the solution with lower concentration (hypotonic solution) to the solution with higher concentration of solutes (hypertonic solution). Osmosis is the main mechanism of water distribution in the body [1] .

When you drink usual beverages, like water and fruit juices, the fluid in your intestine will become less concentrated (hypotonic) in relation to the blood plasma, so it will move across the intestinal wall into the blood, by the principle of osmosis.

Osmosis can also cause problems:

When a person with lactose intolerance drinks a lot of milk, the unabsorbed lactose will build up in the intestinal fluid, which will become hypertonic in relation to the fluid in the intestinal wall, so the fluid will start to move from the intestinal wall into the intestine and thus cause diarrhea.

When a person drinks a large amount of water in a short time and consumes no or very little salt, the water absorbed from the intestine into the blood plasma will make blood plasma less concentrated (hypotonic) in relation to the fluid in the brain, so the water will move from the blood into the brain cells resulting in brain swelling. This is called water intoxication.


Transport Across Cell Membranes

Essential and continuous parts of the life of a cell are the taking in of nutrients and the expelling of wastes. All of these must pass through the cell membrane.

Transport may occur by diffusion and osmosis across the membrane. It can also occur when a vescicle attaches to the cell membrane from the inside and then opens to form a pocket, expelling its contents to the outside. This may be called exocytosis. The cell membrane may also envelope something on the outside and surround it, taking it into the cell. This may be called endocytosis or phagocytosis.

There are also examples where molecules move across a membrane from a region of low concentration to an region of high concentration, and this requires a source of energy to "pump" the molecules uphill in concentration. Such processes are called active transport.


Structure and Composition of the Cell Membrane

Figure 1. Phospholipid Structure. A phospholipid molecule consists of a polar phosphate “head,” which is hydrophilic and a non-polar lipid “tail,” which is hydrophobic. Unsaturated fatty acids result in kinks in the hydrophobic tails.

The cell membrane is an extremely pliable structure composed primarily of back-to-back phospholipids (a “bilayer”). Cholesterol is also present, which contributes to the fluidity of the membrane, and there are various proteins embedded within the membrane that have a variety of functions. A single phospholipid molecule has a phosphate group on one end, called the “head,” and two side-by-side chains of fatty acids that make up the lipid tails (Figure 1). The phosphate group is negatively charged, making the head polar and hydrophilic—or “water loving.”

A hydrophilic molecule (or region of a molecule) is one that is attracted to water. The phosphate heads are thus attracted to the water molecules of both the extracellular and intracellular environments. The lipid tails, on the other hand, are uncharged, or nonpolar, and are hydrophobic—or “water fearing.”

A hydrophobic molecule (or region of a molecule) repels and is repelled by water. Some lipid tails consist of saturated fatty acids and some contain unsaturated fatty acids. This combination adds to the fluidity of the tails that are constantly in motion. Phospholipids are thus amphipathic molecules.

An amphipathic molecule is one that contains both a hydrophilic and a hydrophobic region. In fact, soap works to remove oil and grease stains because it has amphipathic properties. The hydrophilic portion can dissolve in water while the hydrophobic portion can trap grease in micelles that then can be washed away.

Figure 2. Phospolipid Bilayer. The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads contact the fluid inside and outside of the cell.

The cell membrane consists of two adjacent layers of phospholipids. The lipid tails of one layer face the lipid tails of the other layer, meeting at the interface of the two layers. The phospholipid heads face outward, one layer exposed to the interior of the cell and one layer exposed to the exterior (Figure 2).

Because the phosphate groups are polar and hydrophilic, they are attracted to water in the intracellular fluid. Intracellular fluid (ICF) is the fluid interior of the cell. The phosphate groups are also attracted to the extracellular fluid. Extracellular fluid (ECF) is the fluid environment outside the enclosure of the cell membrane. Interstitial fluid (IF) is the term given to extracellular fluid not contained within blood vessels. Because the lipid tails are hydrophobic, they meet in the inner region of the membrane, excluding watery intracellular and extracellular fluid from this space. The cell membrane has many proteins, as well as other lipids (such as cholesterol), that are associated with the phospholipid bilayer. An important feature of the membrane is that it remains fluid the lipids and proteins in the cell membrane are not rigidly locked in place.

Membrane Proteins

The lipid bilayer forms the basis of the cell membrane, but it is peppered throughout with various proteins. Two different types of proteins that are commonly associated with the cell membrane are the integral proteins and peripheral protein (Figure 3). As its name suggests, an integral protein is a protein that is embedded in the membrane. A channel protein is an example of an integral protein that selectively allows particular materials, such as certain ions, to pass into or out of the cell.

Figure 3. Cell Membrane. The cell membrane of the cell is a phospholipid bilayer containing many different molecular components, including proteins and cholesterol, some with carbohydrate groups attached.

Another important group of integral proteins are cell recognition proteins, which serve to mark a cell’s identity so that it can be recognized by other cells. A receptor is a type of recognition protein that can selectively bind a specific molecule outside the cell, and this binding induces a chemical reaction within the cell. A ligand is the specific molecule that binds to and activates a receptor. Some integral proteins serve dual roles as both a receptor and an ion channel. One example of a receptor-ligand interaction is the receptors on nerve cells that bind neurotransmitters, such as dopamine. When a dopamine molecule binds to a dopamine receptor protein, a channel within the transmembrane protein opens to allow certain ions to flow into the cell.

Some integral membrane proteins are glycoproteins. A glycoprotein is a protein that has carbohydrate molecules attached, which extend into the extracellular matrix. The attached carbohydrate tags on glycoproteins aid in cell recognition. The carbohydrates that extend from membrane proteins and even from some membrane lipids collectively form the glycocalyx.

The glycocalyx is a fuzzy-appearing coating around the cell formed from glycoproteins and other carbohydrates attached to the cell membrane. The glycocalyx can have various roles. For example, it may have molecules that allow the cell to bind to another cell, it may contain receptors for hormones, or it might have enzymes to break down nutrients. The glycocalyces found in a person’s body are products of that person’s genetic makeup. They give each of the individual’s trillions of cells the “identity” of belonging in the person’s body. This identity is the primary way that a person’s immune defense cells “know” not to attack the person’s own body cells, but it also is the reason organs donated by another person might be rejected.

Peripheral proteins are typically found on the inner or outer surface of the lipid bilayer but can also be attached to the internal or external surface of an integral protein. These proteins typically perform a specific function for the cell. Some peripheral proteins on the surface of intestinal cells, for example, act as digestive enzymes to break down nutrients to sizes that can pass through the cells and into the bloodstream.

Transport across the Cell Membrane

One of the great wonders of the cell membrane is its ability to regulate the concentration of substances inside the cell. These substances include ions such as Ca ++ , Na + , K + , and Cl – nutrients including sugars, fatty acids, and amino acids and waste products, particularly carbon dioxide (CO2), which must leave the cell. The membrane’s lipid bilayer structure provides the first level of control. The phospholipids are tightly packed together, and the membrane has a hydrophobic interior. This structure causes the membrane to be selectively permeable. A membrane that has selective permeability allows only substances meeting certain criteria to pass through it unaided. In the case of the cell membrane, only relatively small, nonpolar materials can move through the lipid bilayer (remember, the lipid tails of the membrane are nonpolar). Some examples of these are other lipids, oxygen and carbon dioxide gases, and alcohol. However, water-soluble materials—like glucose, amino acids, and electrolytes—need some assistance to cross the membrane because they are repelled by the hydrophobic tails of the phospholipid bilayer. All substances that move through the membrane do so by one of two general methods, which are categorized based on whether or not energy is required. Passive transport is the movement of substances across the membrane without the expenditure of cellular energy. In contrast, active transport is the movement of substances across the membrane using energy from adenosine triphosphate (ATP).

Passive Transport

In order to understand how substances move passively across a cell membrane, it is necessary to understand concentration gradients and diffusion. A concentration gradient is the difference in concentration of a substance across a space. Molecules (or ions) will spread/diffuse from where they are more concentrated to where they are less concentrated until they are equally distributed in that space. (When molecules move in this way, they are said to move down their concentration gradient.) Diffusion is the movement of particles from an area of higher concentration to an area of lower concentration. A couple of common examples will help to illustrate this concept. Imagine being inside a closed bathroom. If a bottle of perfume were sprayed, the scent molecules would naturally diffuse from the spot where they left the bottle to all corners of the bathroom, and this diffusion would go on until no more concentration gradient remains. Another example is a spoonful of sugar placed in a cup of tea. Eventually the sugar will diffuse throughout the tea until no concentration gradient remains. In both cases, if the room is warmer or the tea hotter, diffusion occurs even faster as the molecules are bumping into each other and spreading out faster than at cooler temperatures. Having an internal body temperature around 98.6 ° F thus also aids in diffusion of particles within the body.

Whenever a substance exists in greater concentration on one side of a semipermeable membrane, such as the cell membranes, any substance that can move down its concentration gradient across the membrane will do so. Consider substances that can easily diffuse through the lipid bilayer of the cell membrane, such as the gases oxygen (O2) and CO2. O2 generally diffuses into cells because it is more concentrated outside of them, and CO2 typically diffuses out of cells because it is more concentrated inside of them. Neither of these examples requires any energy on the part of the cell, and therefore they use passive transport to move across the membrane. Before moving on, you need to review the gases that can diffuse across a cell membrane. Because cells rapidly use up oxygen during metabolism, there is typically a lower concentration of O2 inside the cell than outside. As a result, oxygen will diffuse from the interstitial fluid directly through the lipid bilayer of the membrane and into the cytoplasm within the cell. On the other hand, because cells produce CO2 as a byproduct of metabolism, CO2 concentrations rise within the cytoplasm therefore, CO2 will move from the cell through the lipid bilayer and into the interstitial fluid, where its concentration is lower. This mechanism of molecules spreading from where they are more concentrated to where they are less concentration is a form of passive transport called simple diffusion (Figure 4).

Figure 4. Simple Diffusion across the Cell (Plasma) Membrane. The structure of the lipid bilayer allows only small, non-polar substances such as oxygen and carbon dioxide to pass through the cell membrane, down their concentration gradient, by simple diffusion.

Solutes dissolved in water on either side of the cell membrane will tend to diffuse down their concentration gradients, but because most substances cannot pass freely through the lipid bilayer of the cell membrane, their movement is restricted to protein channels and specialized transport mechanisms in the membrane. Facilitated diffusion is the diffusion process used for those substances that cannot cross the lipid bilayer due to their size and/or polarity (Figure 5). A common example of facilitated diffusion is the movement of glucose into the cell, where it is used to make ATP. Although glucose can be more concentrated outside of a cell, it cannot cross the lipid bilayer via simple diffusion because it is both large and polar. To resolve this, a specialized carrier protein called the glucose transporter will transfer glucose molecules into the cell to facilitate its inward diffusion.

Figure 5. Facilitated Diffusion. (a) Facilitated diffusion of substances crossing the cell (plasma) membrane takes place with the help of proteins such as channel proteins and carrier proteins. Channel proteins are less selective than carrier proteins, and usually mildly discriminate between their cargo based on size and charge. (b) Carrier proteins are more selective, often only allowing one particular type of molecule to cross.

In some cases, facilitated diffusion might move two substances in the same direction across the membrane, called a “symport.” For example, in intestinal cells, sodium ions and glucose molecules are co-transported into the cells. In other cases, the facilitated diffusion might only require a tunnel-like channel for particular solutes, such as electrolytes (small charged ions), to pass through the membrane (this is called a “uniport”). As an example, even though sodium ions (Na + ) are highly concentrated outside of cells, these electrolytes are polarized and cannot pass through the nonpolar lipid bilayer of the membrane. Their diffusion is facilitated by membrane proteins that form sodium channels (or “pores”), so that Na + ions can move down their concentration gradient from outside the cells to inside the cells. There are many other solutes that must undergo facilitated diffusion to move into a cell, such as amino acids, or to move out of a cell, such as wastes. Because facilitated diffusion is a passive process, it does not require energy expenditure by the cell. Water also can move freely across the cell membrane of all cells, either through protein channels or by slipping between the lipid tails of the membrane itself. Osmosis is the diffusion of water through a semipermeable membrane (Figure 6).

Figure 6. Osmosis. Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient. If a membrane is permeable to water, though not to a solute, water will equalize its own concentration by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In the beaker on the left, the solution on the right side of the membrane is hypertonic.

The movement of water molecules is not itself regulated by cells, so it is important that cells are exposed to an environment in which the concentration of solutes outside of the cells (in the extracellular fluid) is equal to the concentration of solutes inside the cells (in the cytoplasm). Two solutions that have the same concentration of solutes are said to be isotonic (equal tension). When cells and their extracellular environments are isotonic, the concentration of water molecules is the same outside and inside the cells, and the cells maintain their normal shape (and function). Osmosis occurs when there is an imbalance of solutes outside of a cell versus inside the cell. A solution that has a higher concentration of solutes than another solution is said to be hypertonic, and water molecules tend to diffuse into a hypertonic solution (Figure 7). Cells in a hypertonic solution will shrivel as water leaves the cell via osmosis. In contrast, a solution that has a lower concentration of solutes than another solution is said to be hypotonic, and water molecules tend to diffuse out of a hypotonic solution. Cells in a hypotonic solution will take on too much water and swell, with the risk of eventually bursting. A critical aspect of homeostasis in living things is to create an internal environment in which all of the body’s cells are in an isotonic solution. Various organ systems, particularly the kidneys, work to maintain this homeostasis.

Figure 7. Concentration of Solutions. A hypertonic solution has a solute concentration higher than another solution. An isotonic solution has a solute concentration equal to another solution. A hypotonic solution has a solute concentration lower than another solution.

Another mechanism besides diffusion to passively transport materials between compartments is filtration. Unlike diffusion of a substance from where it is more concentrated to less concentrated, filtration uses a hydrostatic pressure gradient that pushes the fluid—and the solutes within it—from a higher pressure area to a lower pressure area. Filtration is an extremely important process in the body. For example, the circulatory system uses filtration to move plasma and substances across the endothelial lining of capillaries and into surrounding tissues, supplying cells with the nutrients. Filtration pressure in the kidneys provides the mechanism to remove wastes from the bloodstream.

Active Transport

For all of the transport methods described above, the cell expends no energy. Membrane proteins that aid in the passive transport of substances do so without the use of ATP. During active transport, ATP is required to move a substance across a membrane, often with the help of protein carriers, and usually against its concentration gradient. One of the most common types of active transport involves proteins that serve as pumps. The word “pump” probably conjures up thoughts of using energy to pump up the tire of a bicycle or a basketball. Similarly, energy from ATP is required for these membrane proteins to transport substances—molecules or ions—across the membrane, usually against their concentration gradients (from an area of low concentration to an area of high concentration). The sodium-potassium pump, which is also called N + /K + ATPase, transports sodium out of a cell while moving potassium into the cell. The Na + /K + pump is an important ion pump found in the membranes of many types of cells. These pumps are particularly abundant in nerve cells, which are constantly pumping out sodium ions and pulling in potassium ions to maintain an electrical gradient across their cell membranes. An electrical gradient is a difference in electrical charge across a space. In the case of nerve cells, for example, the electrical gradient exists between the inside and outside of the cell, with the inside being negatively-charged (at around -70 mV) relative to the outside. The negative electrical gradient is maintained because each Na + /K + pump moves three Na + ions out of the cell and two K + ions into the cell for each ATP molecule that is used (Figure 8). This process is so important for nerve cells that it accounts for the majority of their ATP usage.

Figure 8. Sodium-Potassium Pump. The sodium-potassium pump is found in many cell (plasma) membranes. Powered by ATP, the pump moves sodium and potassium ions in opposite directions, each against its concentration gradient. In a single cycle of the pump, three sodium ions are extruded from and two potassium ions are imported into the cell.

Other forms of active transport do not involve membrane carriers. Endocytosis (bringing “into the cell”) is the process of a cell ingesting material by enveloping it in a portion of its cell membrane, and then pinching off that portion of membrane (Figure 9). Once pinched off, the portion of membrane and its contents becomes an independent, intracellular vesicle. A vesicle is a membranous sac—a spherical and hollow organelle bounded by a lipid bilayer membrane. Endocytosis often brings materials into the cell that must to be broken down or digested. Phagocytosis (“cell eating”) is the endocytosis of large particles. Many immune cells engage in phagocytosis of invading pathogens. Like little Pac-men, their job is to patrol body tissues for unwanted matter, such as invading bacterial cells, phagocytize them, and digest them. In contrast to phagocytosis, pinocytosis (“cell drinking”) brings fluid containing dissolved substances into a cell through membrane vesicles.

Figure 9. Three Forms of Endocytosis. Endocytosis is a form of active transport in which a cell envelopes extracellular materials using its cell membrane. (a) In phagocytosis, which is relatively nonselective, the cell takes in a large particle. (b) In pinocytosis, the cell takes in small particles in fluid. (c) In contrast, receptor-mediated endocytosis is quite selective. When external receptors bind a specific ligand, the cell responds by endocytosing the ligand.

Figure 10. Exocytosis. Exocytosis is much like endocytosis in reverse. Material destined for export is packaged into a vesicle inside the cell. The membrane of the vesicle fuses with the cell membrane, and the contents are released into the extracellular space.

Phagocytosis and pinocytosis take in large portions of extracellular material, and they are typically not highly selective in the substances they bring in. Cells regulate the endocytosis of specific substances via receptor-mediated endocytosis. Receptor-mediated endocytosis is endocytosis by a portion of the cell membrane that contains many receptors that are specific for a certain substance. Once the surface receptors have bound sufficient amounts of the specific substance (the receptor’s ligand), the cell will endocytose the part of the cell membrane containing the receptor-ligand complexes. Iron, a required component of hemoglobin, is endocytosed by red blood cells in this way. Iron is bound to a protein called transferrin in the blood. Specific transferrin receptors on red blood cell surfaces bind the iron-transferrin molecules, and the cell endocytoses the receptor-ligand complexes. In contrast with endocytosis, exocytosis (taking “out of the cell”) is the process of a cell exporting material using vesicular transport (Figure 10).

Many cells manufacture substances that must be secreted, like a factory manufacturing a product for export. These substances are typically packaged into membrane-bound vesicles within the cell. When the vesicle membrane fuses with the cell membrane, the vesicle releases it contents into the interstitial fluid. The vesicle membrane then becomes part of the cell membrane. Cells of the stomach and pancreas produce and secrete digestive enzymes through exocytosis (Figure 11). Endocrine cells produce and secrete hormones that are sent throughout the body, and certain immune cells produce and secrete large amounts of histamine, a chemical important for immune responses.

Figure 11. Pancreatic Cells’ Enzyme Products. The pancreatic acinar cells produce and secrete many enzymes that digest food. The tiny black granules in this electron micrograph are secretory vesicles filled with enzymes that will be exported from the cells via exocytosis. LM × 2900. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Diseases of the Cell: Cystic Fibrosis

Cystic fibrosis (CF) affects approximately 30,000 people in the United States, with about 1,000 new cases reported each year. The genetic disease is most well known for its damage to the lungs, causing breathing difficulties and chronic lung infections, but it also affects the liver, pancreas, and intestines. Only about 50 years ago, the prognosis for children born with CF was very grim—a life expectancy rarely over 10 years. Today, with advances in medical treatment, many CF patients live into their 30s.

The symptoms of CF result from a malfunctioning membrane ion channel called the cystic fibrosis transmembrane conductance regulator, or CFTR. In healthy people, the CFTR protein is an integral membrane protein that transports Cl – ions out of the cell. In a person who has CF, the gene for the CFTR is mutated, thus, the cell manufactures a defective channel protein that typically is not incorporated into the membrane, but is instead degraded by the cell. The CFTR requires ATP in order to function, making its Cl – transport a form of active transport. This characteristic puzzled researchers for a long time because the Cl – ions are actually flowing down their concentration gradient when transported out of cells. Active transport generally pumps ions against their concentration gradient, but the CFTR presents an exception to this rule. In normal lung tissue, the movement of Cl – out of the cell maintains a Cl – -rich, negatively charged environment immediately outside of the cell. This is particularly important in the epithelial lining of the respiratory system.

Respiratory epithelial cells secrete mucus, which serves to trap dust, bacteria, and other debris. A cilium (plural = cilia) is one of the hair-like appendages found on certain cells. Cilia on the epithelial cells move the mucus and its trapped particles up the airways away from the lungs and toward the outside. In order to be effectively moved upward, the mucus cannot be too viscous rather it must have a thin, watery consistency. The transport of Cl – and the maintenance of an electronegative environment outside of the cell attract positive ions such as Na + to the extracellular space. The accumulation of both Cl – and Na + ions in the extracellular space creates solute-rich mucus, which has a low concentration of water molecules. As a result, through osmosis, water moves from cells and extracellular matrix into the mucus, “thinning” it out. This is how, in a normal respiratory system, the mucus is kept sufficiently watered-down to be propelled out of the respiratory system.

If the CFTR channel is absent, Cl – ions are not transported out of the cell in adequate numbers, thus preventing them from drawing positive ions. The absence of ions in the secreted mucus results in the lack of a normal water concentration gradient. Thus, there is no osmotic pressure pulling water into the mucus. The resulting mucus is thick and sticky, and the ciliated epithelia cannot effectively remove it from the respiratory system. Passageways in the lungs become blocked with mucus, along with the debris it carries. Bacterial infections occur more easily because bacterial cells are not effectively carried away from the lungs.


Osmoregulation

Osmoregulation is the process of maintaining salt and water balance (osmotic balance) across membranes within the body. The fluids inside and surrounding cells are composed of water, electrolytes, and nonelectrolytes. An electrolyte is a compound that dissociates into ions when dissolved in water. A nonelectrolyte, in contrast, does not dissociate into ions in water. The body&rsquos fluids include blood plasma, the fluid that exists within cells, and the interstitial fluid that exists in the spaces between cells and tissues of the body. The membranes of the body (both the membranes around cells and the &ldquomembranes&rdquo made of cells lining body cavities) are semipermeable membranes. Semipermeable membranes are permeable to certain types of solutes and to water, but typically cell membranes are impermeable to solutes.

The body does not exist in isolation. There is a constant input of water and electrolytes into the system. Excess water, electrolytes, and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance. Insufficient fluid intake results in fluid conservation by the kidneys. Biological systems constantly interact and exchange water and nutrients with the environment by way of consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences.

Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, interstitial fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Since blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure.

Excretory system

The human excretory system functions to remove waste from the body through the skin as sweat, the lungs in the form of exhaled carbon dioxide, and through the urinary system in the form of urine. All three of these systems participate in osmoregulation and waste removal. Here we focus on the urinary system, which is comprised of the paired kidneys, the ureter, urinary bladder and urethra (Figure 4.1). The kidneys are a pair of bean-shaped structures that are located just below the liver in the body cavity. Each of the kidneys contains more than a million tiny units called nephrons that filter blood containing the metabolic wastes from cells. All the blood in the human body is filtered about 60 times a day by the kidneys. The nephrons remove wastes, concentrate them, and form urine that is collected in the bladder.

Internally, the kidney has three regions&mdashan outer cortex, a medulla in the middle, and the renal pelvis, which is the expanded end of the ureter. The renal cortex contains the nephrons&mdashthe functional unit of the kidney. The renal pelvis collects the urine and leads to the ureter on the outside of the kidney. The ureters are urine-bearing tubes that exit the kidney and empty into the urinary bladder.

Figure 4.1. The human excretory system is made up of the kidneys, ureter, urinary bladder, and urethra. The kidneys filter blood and form urine, which is stored in the bladder until it is eliminated through the urethra. On the right, the internal structure of the kidney is shown. (credit: modification of work by NCI, NIH)

Blood enters each kidney from the aorta, the main artery supplying the body below the heart, through a renal artery. It is distributed in smaller vessels until it reaches each nephron in capillaries. Within the nephron, the blood comes in intimate contact with the waste-collecting tubules in a structure called the glomerulus. Water and many solutes present in the blood, including ions of sodium, calcium, magnesium, and others as well as wastes and valuable substances such as amino acids, glucose, and vitamins, leave the blood and enter the tubule system of the nephron. As materials pass through the tubule much of the water, required ions, and useful compounds are reabsorbed back into the capillaries that surround the tubules leaving the wastes behind. Some of this reabsorption requires active transport and consumes ATP. Some wastes, including ions and some drugs remaining in the blood, diffuse out of the capillaries into the interstitial fluid and are taken up by the tubule cells. These wastes are then actively secreted into the tubules. The blood then collects in larger and larger vessels and leaves the kidney in the renal vein. The renal vein joins the inferior vena cava, the main vein that returns blood to the heart from the lower body. The amounts of water and ions reabsorbed into the circulatory system are carefully regulated and this is an important way the body regulates its water content and ion levels. The waste is collected in larger tubules and then leaves the kidney in the ureter, which leads to the bladder where urine, the combination of waste materials and water, is stored.

The bladder contains sensory nerves, stretch receptors that signal when it needs to be emptied. These signals create the urge to urinate, which can be voluntarily suppressed up to a limit. The conscious decision to urinate sets in play signals that open the sphincters, rings of smooth muscle that close off the opening, to the urethra that allows urine to flow out of the bladder and the body.

Osmosis is the diffusion of water across a membrane in response to osmotic pressure caused by an imbalance of molecules on either side of the membrane. Osmoregulation is the process of maintenance of salt and water balance ( osmotic balance) across membranes within the body&rsquos fluids, which are composed of water, plus electrolytes and non-electrolytes. An electrolyte is a solute that dissociates into ions when dissolved in water. A non-electrolyte, in contrast, doesn&rsquot dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body&rsquos fluids include blood plasma, the cytosol within cells, and interstitial fluid, the fluid that exists in the spaces between cells and tissues of the body. The membranes of the body (such as the pleural, serous, and cell membranes) are semi-permeable membranes. Semi-permeable membranes are permeable (or permissive) to certain types of solutes and water. Solutions on two sides of a semi-permeable membrane tend to equalize in solute concentration by movement of solutes and/or water across the membrane. As seen in Figure 4.2, a cell placed in water tends to swell due to gain of water from the hypotonic or &ldquolow salt&rdquo environment. A cell placed in a solution with higher salt concentration, on the other hand, tends to make the membrane shrivel up due to loss of water into the hypertonic or &ldquohigh salt&rdquo environment. Isotonic cells have an equal concentration of solutes inside and outside the cell this equalizes the osmotic pressure on either side of the cell membrane which is a semi-permeable membrane.

Figure 4.2. Cells placed in a hypertonic environment tend to shrink due to loss of water. In a hypotonic environment, cells tend to swell due to the intake of water. The blood maintains an isotonic environment so that cells neither shrink nor swell. (credit: Mariana Ruiz Villareal)

The body does not exist in isolation. There is a constant input of water and electrolytes into the system. While osmoregulation is achieved across membranes within the body, excess electrolytes and wastes are transported to the kidneys and excreted, helping to maintain osmotic balance.

Need for osmoregulation

Biological systems constantly interact and exchange water and nutrients with the environment by way of consumption of food and water and through excretion in the form of sweat, urine, and feces. Without a mechanism to regulate osmotic pressure, or when a disease damages this mechanism, there is a tendency to accumulate toxic waste and water, which can have dire consequences.

Mammalian systems have evolved to regulate not only the overall osmotic pressure across membranes, but also specific concentrations of important electrolytes in the three major fluid compartments: blood plasma, extracellular fluid, and intracellular fluid. Since osmotic pressure is regulated by the movement of water across membranes, the volume of the fluid compartments can also change temporarily. Because blood plasma is one of the fluid components, osmotic pressures have a direct bearing on blood pressure.

Transport of electrolytes across cell membranes

Electrolytes, such as sodium chloride, ionize in water, meaning that they dissociate into their component ions. In water, sodium chloride (NaCl), dissociates into the sodium ion (Na+) and the chloride ion (Cl&ndash). The most important ions, whose concentrations are very closely regulated in body fluids, are the cations sodium (Na+), potassium (K+), calcium (Ca+2),
magnesium (Mg+2), and the anions chloride (Cl&ndash), carbonate (CO3-2), bicarbonate (HCO3&ndash), and phosphate(PO3&ndash). Electrolytes are lost from the body during urination and perspiration. For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.

Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to
the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because electrolytes dissociate into their component ions, they, in essence, add more solute particles into the solution and have a greater effect on osmotic pressure, per mass than compounds that do not dissociate in water, such as glucose.

Water can pass through membranes by passive diffusion. If electrolyte ions could passively diffuse across membranes, it would be impossible to maintain specific concentrations of ions in each fluid compartment, therefore, they require special mechanisms to cross the semi-permeable membranes in the body. This movement can be accomplished by facilitated diffusion and active transport. Facilitated diffusion requires protein-based channels for moving the solute. Active transport requires energy in the form of ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.

Concept of osmolality and milliequivalent

In order to calculate osmotic pressure, it is necessary to understand how solute concentrations are measured. The unit for measuring solutes is the mole. One mole is defined as the gram molecular weight of the solute. For example, the molecular weight of sodium chloride is 58.44. Thus, one mole of sodium chloride weighs 58.44 grams. The molarity of a solution is the number of moles of solute per liter of solution. The molality of a solution is the number of moles of solute per kilogram of solvent. If the solvent is water, one kilogram of water is equal to one liter of water. While molarity and molality are used to express the concentration of solutions, electrolyte concentrations are usually expressed in terms of milliequivalents per liter (mEq/L): the mEq/L is equal to the ion concentration (in millimoles) multiplied by the number of electrical charges on the ion. The unit of milliequivalent takes into consideration the ions present in the solution (since electrolytes form ions in aqueous solutions) and the charge on the ions.

Thus, for ions that have a charge of one, one milliequivalent is equal to one millimole. For ions that have a charge of two (like calcium), one milliequivalent is equal to 0.5 millimoles. Another unit for the expression of electrolyte concentration is the milliosmole (mOsm), which is the number of milliequivalents of solute per kilogram of solvent. Body fluids are usually maintained within the range of 280 to 300 mOsm.

Osmoregulators and osmoconformers

Persons lost at sea without any fresh water to drink, are at risk of severe dehydration because the human body cannot adapt to drinking seawater, which is hypertonic in comparison to body fluids. Organisms such as goldfish that can tolerate only a relatively narrow range of salinity are referred to as stenohaline. About 90 percent of all bony fish are restricted to either freshwater or seawater. They are incapable of osmotic regulation in the opposite environment. It is possible, however, for a few fishes like salmon to spend part of their life in freshwater and part in sea water. Organisms like the salmon and molly that can tolerate a relatively wide range of salinity are referred to as euryhaline organisms. This is possible because some fish have evolved osmoregulatory mechanisms to survive in all kinds of aquatic environments. When they live in fresh water, their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in Figure 4.3a. In such hypotonic environments, these fish do not drink much water. Instead, they pass a lot of very dilute urine, and they achieve electrolyte balance by active transport of salts through the gills. When they move to a hypertonic marine environment, these fish start drinking sea water they excrete the excess salts through their gills and their urine, as illustrated in Figure 4.3b. Most marine invertebrates, on the other hand, maybe isotonic with sea water (osmoconformers). Their body fluid concentrations conform to changes in seawater concentration. Cartilaginous fishes&rsquo salt composition of the blood is similar to bony fishes however, the blood of sharks contains the organic compounds urea and trimethylamine oxide (TMAO). This does not mean that their electrolyte composition is similar to that of seawater. They achieve isotonicity with the sea by storing large concentrations of urea. These animals that secrete urea are called ureotelic animals. TMAO stabilizes proteins in the presence of high urea levels, preventing the disruption of peptide bonds that would occur in other animals exposed to similar levels of urea. Sharks are cartilaginous fish with a rectal gland to secrete salt and assist in osmoregulation.

Figure 4.3. Fish are osmoregulators, but must use different mechanisms to survive in (a) freshwater or (b) saltwater environments. (credit: modification of work by Duane Raver, NOAA)


Ionic Gradients and an Electric Potential Are Maintained across the Plasma Membrane

The specific ionic composition of the cytosol usually differs greatly from that of the surrounding fluid. In virtually all cells — including microbial, plant, and animal cells — the cytosolic pH is kept near 7.2 and the cytosolic concentration of K + is much higher than that of Na + . In addition, in both invertebrates and vertebrates, the concentration of K + is 20 –� times higher in cells than in the blood, while the concentration of Na + is 8 –� times lower in cells than in the blood (Table 15-1). The concentration of Ca 2+ free in the cytosol is generally less than 0.2 micromolar (2 ×� 𢄧 M), a thousand or more times lower than that in the blood. Plant cells and many microorganisms maintain similarly high cytosolic concentrations of K + and low concentrations of Ca 2+ and Na + even if the cells are cultured in very dilute salt solutions. The ATP-driven ion pumps that generate and maintain these ionic gradients are discussed later.

Table 15-1

Typical Ion Concentrations in Invertebrates and Vertebrates.

In addition to ion pumps, which transport ions against their concentration gradients, the plasma membrane contains channel proteins that allow the principal cellular ions (Na + , K + , Ca 2+ , and Cl − ) to move through it at different rates down their concentration gradients. Ion concentration gradients and selective movements of ions through channels create a difference in voltage across the plasma membrane. The magnitude of this electric potential is � millivolts (mV) with the inside of the cell always negative with respect to the outside. This value does not seem like much until we realize that the plasma membrane is only about 3.5 nm thick. Thus the voltage gradient across the plasma membrane is 0.07 V per 3.5 ×� 𢄧 cm, or 200,000 volts per centimeter! (To appreciate what this means, consider that high-voltage transmission lines for electricity utilize gradients of about 200,000 volts per kilometer!) As explained below, the plasma membrane, like all biological membranes, acts like a capacitor —𠁚 device consisting of a thin sheet of nonconducting material (the hydrophobic interior) surrounded on both sides by electrically conducting material (the polar head groups and the ions in the surrounding aqueous medium) — that can store positive charges on one side and negative charges on the other.

The ionic gradients and electric potential across the plasma membrane drive many biological processes. Opening and closing of Na + , K + , and Ca 2+ channels are essential to the conduction of an electric impulse down the axon of a nerve cell (Chapter 21). In many animal cells, the Na + concentration gradient and the membrane electric potential power the uptake of amino acids and other molecules against their concentration gradient this transport is catalyzed by ion-linked symport and antiport proteins. In most cells, a rise in the cytosolic Ca 2+ concentration is an important regulatory signal, initiating contraction in muscle cells and triggering secretion of digestive enzymes in the exocrine pancreatic cells.

Here we discuss the role of ion channels in generating the membrane electric potential. Later we examine the ATP-powered ion pumps that generate ion concentration gradients, and ion-linked cotransport proteins.


Transport of Electrolytes across Cell Membranes

A teaspoon of table salt readily dissolves in water. The solubility of sodium chloride results from its capacity to ionize in water. Salt and other compounds that dissociate into their component ions are called electrolytes. In water, sodium chloride (NaCl) dissociates into the sodium ion (Na + ) and the chloride ion (Cl – ). The most important ions, whose concentrations are very closely regulated in body fluids, are the cations sodium (Na+), potassium (K+), calcium (Ca+2),and magnesium (Mg+2) and the anions chloride (Cl-), carbonate (CO3-2), bicarbonate (HCO3-), and phosphate(PO3-). Electrolytes are lost from the body during urination and perspiration. For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.

Osmotic pressure is influenced by the concentration of solutes in a solution. It is directly proportional to the number of solute atoms or molecules and not dependent on the size of the solute molecules. Because electrolytes dissociate into ions, adding relatively more solute molecules to a solution, they exert a greater osmotic pressure per unit mass than non-electrolytes such as glucose.

Water passes through semi-permeable membranes by passive diffusion, moving along a concentration gradient and equalizing the concentration on either side of the membrane. Electrolyte ions may not be able to passively diffuse across a membrane, but may instead require special mechanisms to cross the semi-permeable membrane. The mechanisms that transport ions across membranes are facilitated diffusion and active transport. Facilitated diffusion of solutes occurs through protein-based channels. Active transport requires energy in the form of ATP conversion, carrier proteins, or pumps in order to move ions against the concentration gradient.


41.1B: Transport of Electrolytes across Cell Membranes - Biology

The small intestine must absorb massive quantities of water. A normal person or animal of similar size takes in roughly 1 to 2 liters of dietary fluid every day. On top of that, another 6 to 7 liters of fluid is received by the small intestine daily as secretions from salivary glands, stomach, pancreas, liver and the small intestine itself.

By the time the ingesta enters the large intestine, approximately 80% of this fluid has been absorbed. Net movement of water across cell membranes always occurs by osmosis, and the fundamental concept needed to understand absorption in the small gut is that there is a tight coupling between water and solute absorption. Another way of saying this is that absorption of water is absolutely dependent on absorption of solutes, particularly sodium:

  • Sodium is absorbed from the intestinal lumen by several mechanisms, most prominently by cotransport with glucose and amino acids, and by Na+/H+ exchange, both of which move sodium from the lumen into the enterocyte.
  • Absorbed sodium is rapidly exported from the cell via sodium pumps - when a lot of sodium is entering the cell, a lot of sodium is pumped out of the cell, which establishes a high osmolarity in the small intercellular spaces between adjacent enterocytes.
  • Water diffuses in response to the osmotic gradient established by sodium - in this case into the intercellular space. It seems that the bulk of the water absorption is transcellular, but some also diffuses through the tight junctions.
  • Water, as well as sodium, then diffuses into capillary blood within the villus.

As sodium is rapidly pumped out of the cell, it achieves very high concentration in the narrow space between enterocytes. A potent osmotic gradient is thus formed across apical cell membranes and their connecting junctional complexes that osmotically drives movement of water across the epithelium.

Water is thus absorbed into the intercellular space by diffusion down an osmotic gradient. However, looking at the process as a whole, transport of water from lumen to blood is often against an osmotic gradient - this is important because it means that the intestine can absorb water into blood even when the osmolarity in the lumen is higher than osmolarity of blood.

Absorption in the Small Intestine

Absorption of Monosaccharides


What Do Electrolytes Do, How Much Do You Need, and Where Do You Find Them?

Sodium

Main functions in the body: Along with potassium, regulates the fluid volume in cells, interstitial fluid, and blood plasma. Needed for muscle contraction and generating nerve impulses.

Dietary sources: Most sodium in our diet comes from the salt we add to food. Much smaller amounts naturally occur in foods like beets, carrots, celery, and dairy products, and in drinking water. Someone eating a typical modern diet gets the bulk of their sodium from processed, packaged foods.

Recommended intake: In recent decades, doctors and the folks behind our governmental dietary standards have told us to limit sodium intake, mostly in the name of heart health. However, experts are increasingly challenging that advice. Multiple studies point to a greater risk of negative health outcomes with too little sodium 1 2 3 Many believe that the current recommended daily intake of 1,500 mg per day for adults is woefully inadequate.

Instead, the sweet spot seems to be between 4 and 6 grams per day. That’s about 2 teaspoons of fine sea salt like Redmond Real Salt or a heaping tablespoon of kosher salt. (Remember, the salt we eat is not pure sodium, it’s sodium plus chloride—NaCl.) However, individuals with salt-sensitive hypertension or kidney disease will want to consult their doctors, as these populations probably do need to restrict sodium.

Potassium

Main functions in the body: Along with sodium, potassium regulates fluid volume and allows for muscle contraction and nerve impulses. Regulates heartbeat.

Dietary sources: Fruits and vegetables. Bananas have become synonymous with potassium, but a medium potato actually contains twice as much potassium as a medium banana. Avocado is a better source as well. If your diet includes a variety of vegetables and perhaps some fruit, you are probably getting enough potassium.

Recommended intake: Adequate intake (AI) is 2,600 mg per day for adult females and 3,400 mg per day for males. The FDA’s recommended daily intake (RDI) is 4,700 mg per day.

While sodium gets most of the attention when it comes to heart health, potassium is at least as essential, if not more so. People with higher (but not excessive) potassium intake have lower blood pressure, less risk for cardiovascular disease, 4 and lower all-cause mortality. 5

Research also suggests that the relative amounts of sodium and potassium you eat—the sodium:potassium ratio—is as important as the absolute amounts of each. You want to avoid high levels of sodium with low potassium. On the other hand, increasing potassium intake seems to offset the supposed dangers of higher levels of sodium intake (within reason). 6 7 8

Chloride

Main functions in the body: Maintaining fluid balance, which is vital for regulating blood pressure and pH of body fluids. Also a primary component of gastric juice in the form of hydrochloric acid.

Dietary sources: Mostly from added salt—sodium chloride and, to a lesser extent, potassium chloride. Seaweed and many vegetables also contain some chloride. You can also get chloride through the skin if you use a magnesium spray, which is usually magnesium chloride.

Recommended intake: 2.3 grams per day for adults up to 50, 2.0 grams per day up to age 70, 1.8 grams per day thereafter.

Calcium

Main functions in the body: In addition to structural roles (bones and teeth), calcium helps muscles contract and nerves fire. Calcium also has a role in blood clotting.

Dietary sources: Leafy greens, broccoli, nuts and seeds, fish like sardines and anchovies where you eat the bones. Dairy products, if you consume them, are good sources as well despite any controversy about bioavailability.

Recommended intake: For adult females, 1,000 mg per day up to age 50, 1,200 mg per day thereafter. For males, 1,000 mg per day up to age 70, 1,200 mg per day thereafter.

Phosphate

Main functions in the body: Like calcium, most phosphate is stored in bones and teeth, acting as a mineral reserve. The rest is used by cells for energy production and in cell membranes and DNA.

Dietary sources: Derived from phosphorous, which is found most abundantly in animal products—meat, dairy, eggs.

Recommended intake: 700 mg per day for all adults

Bicarbonate

Main functions in the body: Crucial for maintaining extracellular acid-base balance. Moves carbon dioxide through the bloodstream.

Dietary sources: We get bicarbonate from baking soda (sodium bicarbonate), but the body also produces bicarbonate endogenously (on its own), so it’s not necessary to target it in the diet.

Recommended intake: Has not been established

Magnesium

Main functions in the body: Magnesium is involved in over 300 enzymatic reactions, including ones that allow nerves to fire and muscles to contract. Maintains regular heartbeat.

Dietary sources: Leafy greens, dark chocolate, nuts and seeds, fish, avocado

Recommended intake: For adult females, 310 mg per day up to age 30, then increases to 320 per day. For males, 400 mg per day up to age 30, increasing to 420 mg per day.

Natural Electrolyte Supplements

When people talk about supplementing electrolytes, they generally mean sodium, potassium, and magnesium. For the average healthy person, you can meet your electrolyte needs by eating a varied diet rich in different vegetables, perhaps some fruit, and animal products, especially fish.

However, you may need to supplement if you eat a restricted diet or have certain health conditions such as gastrointestinal issues that interfere with your ability to absorb nutrients, or kidney or liver disease. Because supplements can interact with medications, talk to your doctor before starting any kind of supplement regimen.

Obviously, if you get an electrolyte panel done by your doctor, and it shows a deficiency, that’s another good reason to supplement. Likewise, if you’ve had a bout of vomiting or diarrhea, or if you’re having issues such as brain fog or muscle cramping. Don’t go overboard it is certainly possible to have too much of any electrolyte. Drinking some salty bone broth or trying a standard dose of a potassium or magnesium supplement should be safe.

I should note, though, that dietary deficiencies in potassium are uncommon. It’s never a bad idea to track your food for a few days using an app like Cronometer. See how much you’re getting from diet so you can tailor your supplementing appropriately. It’s probably much more likely that you’re getting less sodium than you need if you’re eating mostly close-to-nature foods, especially if you’re hewing to conventional wisdom about restricting salt.

What Are the Best Forms of Electrolytes?

For sodium, all you need is good old salt. Different forms of salt contain varying amounts of sodium, so look at the label.

For potassium, I like potassium citrate. You can also use LoSalt or Nu-Salt, which contain potassium chloride. They are found with the table salt at your local grocery store. Some folks make their own electrolyte blend with cream of tartar (yes, the same stuff you bake with), which is potassium bitartrate. Any of these will work, but I think potassium citrate is the superior option.

For magnesium, the most bioavailable are the chelated forms that end in -ate. Different forms of magnesium are thought to have specific benefits, but magnesium malate or glycinate (also called bis-glycinate) are good all-around options. Magnesium L-threonate is particularly touted for cognitive benefits because it crosses the blood-brain barrier.

Is Potassium Supplementation Safe?

Because potassium is closely linked to heart function, there is a concern that supplementing potassium could lead to arrhythmias or even heart attacks. However, a 2016 meta-analysis of randomized controlled trials found no risk associated with supplementing within normal guidelines in healthy individuals. 9 People with heart or kidney problems should definitely talk to their doctors, though.

Although I think supplementing potassium is generally safe, it’s also reasonably easy to meet your potassium requirements through diet alone. Potassium supplements are limited to 100 mg per dose by the FDA anyway, which is a fraction of what you need.

Considerations for Keto Folks

If you’re following a keto diet, you probably do need to supplement. When you drop your carbs low enough for the liver to start making ketones, this also triggers a (normal) hormonal response that leads the kidneys to dump water. Along with water goes sodium and potassium especially. This can lead to low blood pressure, and it’s the reason why some people feel so crappy when they first go keto—-the dreaded “keto flu.”

If you’re eating a keto diet and your workouts are suffering, or you have low energy, headaches, or brain fog, low sodium and/or potassium is the likely culprit. Some people find that they need to supplement when transitioning into keto but not once they are keto-adapted. Others feel better if they continue supplementing.

In particular, many keto folks feel better when they increase their sodium considerably—3 to 5 grams above what they get from food, or perhaps even more.

Considerations for Athletes

Electrolytes, especially sodium and chloride, are lost through sweat, so many athletes use electrolyte supplements as a part of their training nutrition. This probably isn’t crucial for the average person working up a sweat at the gym. For hard-charging endurance athletes pounding away for hours, especially in intense heat, it might be the difference between making it to the finish line or not.

If you’re taking in a lot of water during a training session, it’s a good idea to add a pinch of salt, and perhaps a bit of carbohydrate, to your water. For one thing, this increases absorption. Drinking too much water without adequately replacing sodium losses can also lead to the dangerous, even fatal, condition of hyponatremia. 10 I’m not a huge fan of most commercial electrolyte drinks due to their high sugar content, but it’s easy to make your own using one of the many online recipes. You can also use salt pills. It might take some tinkering to dial in the amount you need.

Some athletes also take sodium bicarbonate supplements in an attempt to offset exercise-induced acidosis. (Recall that bicarbonate helps maintain acid-base homeostasis.) Research shows that doses of 200 to 500 mg/kg may reduce lactate concentration and improve aerobic exercise performance and hand-eye coordination. 11 Doses at the higher end of the spectrum seem to be more effective, but they can also cause undesirable gastrointestinal symptoms. If you experiment with this, make sure to take into account both the sodium and the bicarbonate you are adding and, if necessary, adjust your additional sodium supplementation accordingly.



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