1.4: Electrolyte Balance - Biology

1.4: Electrolyte Balance - Biology

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Learning Objectives

By the end of this section, you will be able to:

  • List the role of the six most important electrolytes in the body
  • Name the disorders associated with abnormally high and low levels of the six electrolytes
  • Identify the predominant extracellular anion
  • Describe the role of aldosterone on the level of water in the body

The body contains a large variety of ions, or electrolytes, which perform a variety of functions. Some ions assist in the transmission of electrical impulses along cell membranes in neurons and muscles. Other ions help to stabilize protein structures in enzymes. Still others aid in releasing hormones from endocrine glands. All of the ions in plasma contribute to the osmotic balance that controls the movement of water between cells and their environment.

Electrolytes in living systems include sodium, potassium, chloride, bicarbonate, calcium, phosphate, magnesium, copper, zinc, iron, manganese, molybdenum, copper, and chromium. In terms of body functioning, six electrolytes are most important: sodium, potassium, chloride, bicarbonate, calcium, and phosphate.

Roles of Electrolytes

These six ions aid in nerve excitability, endocrine secretion, membrane permeability, buffering body fluids, and controlling the movement of fluids between compartments. These ions enter the body through the digestive tract. More than 90 percent of the calcium and phosphate that enters the body is incorporated into bones and teeth, with bone serving as a mineral reserve for these ions. In the event that calcium and phosphate are needed for other functions, bone tissue can be broken down to supply the blood and other tissues with these minerals. Phosphate is a normal constituent of nucleic acids; hence, blood levels of phosphate will increase whenever nucleic acids are broken down.

Excretion of ions occurs mainly through the kidneys, with lesser amounts lost in sweat and in feces. Excessive sweating may cause a significant loss, especially of sodium and chloride. Severe vomiting or diarrhea will cause a loss of chloride and bicarbonate ions. Adjustments in respiratory and renal functions allow the body to regulate the levels of these ions in the ECF.

The following table lists the reference values for blood plasma, cerebrospinal fluid (CSF), and urine for the six ions addressed in this section. In a clinical setting, sodium, potassium, and chloride are typically analyzed in a routine urine sample. In contrast, calcium and phosphate analysis requires a collection of urine across a 24-hour period, because the output of these ions can vary considerably over the course of a day. Urine values reflect the rates of excretion of these ions. Bicarbonate is the one ion that is not normally excreted in urine; instead, it is conserved by the kidneys for use in the body’s buffering systems.

Table 1. Electrolyte and Ion Reference Values
NameChemical symbolPlasmaCSFUrine
SodiumNa+136.00–146.00 (mM)138.00–150.00 (mM)40.00–220.00 (mM)
PotassiumK+3.50–5.00 (mM)0.35–3.5 (mM)25.00–125.00 (mM)
ChlorideCl98.00–107.00 (mM)118.00–132.00 (mM)110.00–250.00 (mM)
BicarbonateHCO322.00–29.00 (mM)————
CalciumCa++2.15–2.55 (mmol/day)——Up to 7.49 (mmol/day)
Phosphate0.81–1.45 (mmol/day)——12.90–42.00 (mmol/day)


Sodium is the major cation of the extracellular fluid. It is responsible for one-half of the osmotic pressure gradient that exists between the interior of cells and their surrounding environment. People eating a typical Western diet, which is very high in NaCl, routinely take in 130 to 160 mmol/day of sodium, but humans require only 1 to 2 mmol/day. This excess sodium appears to be a major factor in hypertension (high blood pressure) in some people. Excretion of sodium is accomplished primarily by the kidneys. Sodium is freely filtered through the glomerular capillaries of the kidneys, and although much of the filtered sodium is reabsorbed in the proximal convoluted tubule, some remains in the filtrate and urine, and is normally excreted.

Hyponatremia is a lower-than-normal concentration of sodium, usually associated with excess water accumulation in the body, which dilutes the sodium. An absolute loss of sodium may be due to a decreased intake of the ion coupled with its continual excretion in the urine. An abnormal loss of sodium from the body can result from several conditions, including excessive sweating, vomiting, or diarrhea; the use of diuretics; excessive production of urine, which can occur in diabetes; and acidosis, either metabolic acidosis or diabetic ketoacidosis.

A relative decrease in blood sodium can occur because of an imbalance of sodium in one of the body’s other fluid compartments, like IF, or from a dilution of sodium due to water retention related to edema or congestive heart failure. At the cellular level, hyponatremia results in increased entry of water into cells by osmosis, because the concentration of solutes within the cell exceeds the concentration of solutes in the now-diluted ECF. The excess water causes swelling of the cells; the swelling of red blood cells—decreasing their oxygen-carrying efficiency and making them potentially too large to fit through capillaries—along with the swelling of neurons in the brain can result in brain damage or even death.

Hypernatremia is an abnormal increase of blood sodium. It can result from water loss from the blood, resulting in the hemoconcentration of all blood constituents. Hormonal imbalances involving ADH and aldosterone may also result in higher-than-normal sodium values.


Potassium is the major intracellular cation. It helps establish the resting membrane potential in neurons and muscle fibers after membrane depolarization and action potentials. In contrast to sodium, potassium has very little effect on osmotic pressure. The low levels of potassium in blood and CSF are due to the sodium-potassium pumps in cell membranes, which maintain the normal potassium concentration gradients between the ICF and ECF. The recommendation for daily intake/consumption of potassium is 4700 mg. Potassium is excreted, both actively and passively, through the renal tubules, especially the distal convoluted tubule and collecting ducts. Potassium participates in the exchange with sodium in the renal tubules under the influence of aldosterone, which also relies on basolateral sodium-potassium pumps.

Hypokalemia is an abnormally low potassium blood level. Similar to the situation with hyponatremia, hypokalemia can occur because of either an absolute reduction of potassium in the body or a relative reduction of potassium in the blood due to the redistribution of potassium. An absolute loss of potassium can arise from decreased intake, frequently related to starvation. It can also come about from vomiting, diarrhea, or alkalosis.

Some insulin-dependent diabetic patients experience a relative reduction of potassium in the blood from the redistribution of potassium. When insulin is administered and glucose is taken up by cells, potassium passes through the cell membrane along with glucose, decreasing the amount of potassium in the blood and IF, which can cause hyperpolarization of the cell membranes of neurons, reducing their responses to stimuli.

Hyperkalemia, an elevated potassium blood level, also can impair the function of skeletal muscles, the nervous system, and the heart. Hyperkalemia can result from increased dietary intake of potassium. In such a situation, potassium from the blood ends up in the ECF in abnormally high concentrations. This can result in a partial depolarization (excitation) of the plasma membrane of skeletal muscle fibers, neurons, and cardiac cells of the heart, and can also lead to an inability of cells to repolarize. For the heart, this means that it won’t relax after a contraction, and will effectively “seize” and stop pumping blood, which is fatal within minutes. Because of such effects on the nervous system, a person with hyperkalemia may also exhibit mental confusion, numbness, and weakened respiratory muscles.


Chloride is the predominant extracellular anion. Chloride is a major contributor to the osmotic pressure gradient between the ICF and ECF, and plays an important role in maintaining proper hydration. Chloride functions to balance cations in the ECF, maintaining the electrical neutrality of this fluid. The paths of secretion and reabsorption of chloride ions in the renal system follow the paths of sodium ions.

Hypochloremia, or lower-than-normal blood chloride levels, can occur because of defective renal tubular absorption. Vomiting, diarrhea, and metabolic acidosis can also lead to hypochloremia. Hyperchloremia, or higher-than-normal blood chloride levels, can occur due to dehydration, excessive intake of dietary salt (NaCl) or swallowing of sea water, aspirin intoxication, congestive heart failure, and the hereditary, chronic lung disease, cystic fibrosis. In people who have cystic fibrosis, chloride levels in sweat are two to five times those of normal levels, and analysis of sweat is often used in the diagnosis of the disease.

Practice Question

Watch this video to see an explanation of the effect of seawater on humans. What effect does drinking seawater have on the body?

A YouTube element has been excluded from this version of the text. You can view it online here:

[reveal-answer q=”805964″]Show Answer[/reveal-answer]
[hidden-answer a=”805964″]Drinking seawater dehydrates the body as the body must pass sodium through the kidneys, and water follows.[/hidden-answer]


Bicarbonate is the second most abundant anion in the blood. Its principal function is to maintain your body’s acid-base balance by being part of buffer systems. This role will be discussed in a different section.

Bicarbonate ions result from a chemical reaction that starts with carbon dioxide (CO2) and water, two molecules that are produced at the end of aerobic metabolism. Only a small amount of CO2 can be dissolved in body fluids. Thus, over 90 percent of the CO2 is converted into bicarbonate ions, HCO3, through the following reactions:

CO2+ H 2 ↔ H2 + CO3 ↔ H2 + CO3 + H +

The bidirectional arrows indicate that the reactions can go in either direction, depending on the concentrations of the reactants and products. Carbon dioxide is produced in large amounts in tissues that have a high metabolic rate. Carbon dioxide is converted into bicarbonate in the cytoplasm of red blood cells through the action of an enzyme called carbonic anhydrase. Bicarbonate is transported in the blood. Once in the lungs, the reactions reverse direction, and CO2 is regenerated from bicarbonate to be exhaled as metabolic waste.


About two pounds of calcium in your body are bound up in bone, which provides hardness to the bone and serves as a mineral reserve for calcium and its salts for the rest of the tissues. Teeth also have a high concentration of calcium within them. A little more than one-half of blood calcium is bound to proteins, leaving the rest in its ionized form. Calcium ions, Ca2+, are necessary for muscle contraction, enzyme activity, and blood coagulation. In addition, calcium helps to stabilize cell membranes and is essential for the release of neurotransmitters from neurons and of hormones from endocrine glands.

Calcium is absorbed through the intestines under the influence of activated vitamin D. A deficiency of vitamin D leads to a decrease in absorbed calcium and, eventually, a depletion of calcium stores from the skeletal system, potentially leading to rickets in children and osteomalacia in adults, contributing to osteoporosis.

Hypocalcemia, or abnormally low calcium blood levels, is seen in hypoparathyroidism, which may follow the removal of the thyroid gland, because the four nodules of the parathyroid gland are embedded in it. Hypercalcemia, or abnormally high calcium blood levels, is seen in primary hyperparathyroidism. Some malignancies may also result in hypercalcemia.


Phosphate is present in the body in three ionic forms: , , and . The most common form is . Bone and teeth bind up 85 percent of the body’s phosphate as part of calcium-phosphate salts. Phosphate is found in phospholipids, such as those that make up the cell membrane, and in ATP, nucleotides, and buffers.

Hypophosphatemia, or abnormally low phosphate blood levels, occurs with heavy use of antacids, during alcohol withdrawal, and during malnourishment. In the face of phosphate depletion, the kidneys usually conserve phosphate, but during starvation, this conservation is impaired greatly. Hyperphosphatemia, or abnormally increased levels of phosphates in the blood, occurs if there is decreased renal function or in cases of acute lymphocytic leukemia. Additionally, because phosphate is a major constituent of the ICF, any significant destruction of cells can result in dumping of phosphate into the ECF.

Regulation of Sodium and Potassium

Sodium is reabsorbed from the renal filtrate, and potassium is excreted into the filtrate in the renal collecting tubule. The control of this exchange is governed principally by two hormones—aldosterone and angiotensin II.


Recall that aldosterone increases the excretion of potassium and the reabsorption of sodium in the distal tubule. Aldosterone is released if blood levels of potassium increase, if blood levels of sodium severely decrease, or if blood pressure decreases. Its net effect is to conserve and increase water levels in the plasma by reducing the excretion of sodium, and thus water, from the kidneys. In a negative feedback loop, increased osmolality of the ECF (which follows aldosterone-stimulated sodium absorption) inhibits the release of the hormone.

Angiotensin II

Angiotensin II causes vasoconstriction and an increase in systemic blood pressure. This action increases the glomerular filtration rate, resulting in more material filtered out of the glomerular capillaries and into Bowman’s capsule. Angiotensin II also signals an increase in the release of aldosterone from the adrenal cortex.

In the distal convoluted tubules and collecting ducts of the kidneys, aldosterone stimulates the synthesis and activation of the sodium-potassium pump. Sodium passes from the filtrate, into and through the cells of the tubules and ducts, into the ECF and then into capillaries. Water follows the sodium due to osmosis. Thus, aldosterone causes an increase in blood sodium levels and blood volume. Aldosterone’s effect on potassium is the reverse of that of sodium; under its influence, excess potassium is pumped into the renal filtrate for excretion from the body.

Regulation of Calcium and Phosphate

Calcium and phosphate are both regulated through the actions of three hormones: parathyroid hormone (PTH), dihydroxyvitamin D (calcitriol), and calcitonin. All three are released or synthesized in response to the blood levels of calcium.

PTH is released from the parathyroid gland in response to a decrease in the concentration of blood calcium. The hormone activates osteoclasts to break down bone matrix and release inorganic calcium-phosphate salts. PTH also increases the gastrointestinal absorption of dietary calcium by converting vitamin D into dihydroxyvitamin D (calcitriol), an active form of vitamin D that intestinal epithelial cells require to absorb calcium.

PTH raises blood calcium levels by inhibiting the loss of calcium through the kidneys. PTH also increases the loss of phosphate through the kidneys.

Calcitonin is released from the thyroid gland in response to elevated blood levels of calcium. The hormone increases the activity of osteoblasts, which remove calcium from the blood and incorporate calcium into the bony matrix.

Chapter Review

Electrolytes serve various purposes, such as helping to conduct electrical impulses along cell membranes in neurons and muscles, stabilizing enzyme structures, and releasing hormones from endocrine glands. The ions in plasma also contribute to the osmotic balance that controls the movement of water between cells and their environment. Imbalances of these ions can result in various problems in the body, and their concentrations are tightly regulated. Aldosterone and angiotensin II control the exchange of sodium and potassium between the renal filtrate and the renal collecting tubule. Calcium and phosphate are regulated by PTH, calcitrol, and calcitonin.

Self Check

Answer the question(s) below to see how well you understand the topics covered in the previous section.

Critical Thinking Questions

  1. Explain how the CO2 generated by cells and exhaled in the lungs is carried as bicarbonate in the blood.
  2. How can one have an imbalance in a substance, but not actually have elevated or deficient levels of that substance in the body?

[reveal-answer q=”763044″]Show Answers[/reveal-answer]
[hidden-answer a=”763044″]

  1. Very little of the carbon dioxide in the blood is carried dissolved in the plasma. It is transformed into carbonic acid and then into bicarbonate in order to mix in plasma for transportation to the lungs, where it reverts back to its gaseous form.
  2. Without having an absolute excess or deficiency of a substance, one can have too much or too little of that substance in a given compartment. Such a relative increase or decrease is due to a redistribution of water or the ion in the body’s compartments. This may be due to the loss of water in the blood, leading to a hemoconcentration or dilution of the ion in tissues due to edema.



dihydroxyvitamin D: active form of vitamin D required by the intestinal epithelial cells for the absorption of calcium

hypercalcemia: abnormally increased blood levels of calcium

hyperchloremia: higher-than-normal blood chloride levels

hyperkalemia: higher-than-normal blood potassium levels

hypernatremia: abnormal increase in blood sodium levels

hyperphosphatemia: abnormally increased blood phosphate levels

hypocalcemia: abnormally low blood levels of calcium

hypochloremia: lower-than-normal blood chloride levels

hypokalemia: abnormally decreased blood levels of potassium

hyponatremia: lower-than-normal levels of sodium in the blood

hypophosphatemia: abnormally low blood phosphate levels

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Electrolytes are essential for basic life functioning, such as maintaining electrical neutrality in cells, generating and conducting action potentials in the nerves and muscles. Sodium, potassium, and chloride are the significant electrolytes along with magnesium, calcium, phosphate, and bicarbonates. Electrolytes come from our food and fluids.

These electrolytes can have an imbalance, leading to either high or low levels. High or low levels of electrolytes disrupt normal bodily functions and can lead to even life-threatening complications. This article reviews the basic physiology of electrolytes and their abnormalities, and the consequences of electrolyte imbalance.

Sodium, which is an osmotically active anion, is one of the most important electrolytes in the extracellular fluid. It is responsible for maintaining the extracellular fluid volume, and also for regulation of the membrane potential of cells. Sodium is exchanged along with potassium across cell membranes as part of active transport.

Sodium regulation occurs in the kidneys. The proximal tubule is where the majority of the sodium reabsorption takes place. In the distal convoluted tubule, sodium undergoes reabsorption. Sodium transport takes place via sodium-chloride symporters, which is by the action of the hormone aldosterone.

Among the electrolyte disorders, hyponatremia is the most frequent. Diagnosis is when the serum sodium level less than 135 mmol/L. Hyponatremia has neurological manifestations. Patients may present with headache, confusion, nausea, deliriums. Hypernatremia presents when the serum sodium levels greater than145 mmol/L. Symptoms of hypernatremia include tachypnea, sleeping difficulty, and feeling restless. Rapid sodium corrections can have serious consequences like cerebral edema and osmotic demyelination syndrome.

Potassium is mainly an intracellular ion. The sodium-potassium adenosine triphosphatase pump has the primary responsibility for regulating the homeostasis between sodium and potassium, which pumps out sodium in exchange for potassium, which moves into the cells. In the kidneys, the filtration of potassium takes place at the glomerulus. The reabsorption of potassium takes place at the proximal convoluted tubule and thick ascending loop of Henle. Potassium secretion occurs at the distal convoluted tubule. Aldosterone increases potassium secretion. Potassium channels and potassium-chloride cotransporters at the apical membrane also secrete potassium.

Potassium disorders are related to cardiac arrhythmias. Hypokalemia occurs when serum potassium levels under 3.6 mmol/L—weakness, fatigue, and muscle twitching present in hypokalemia. Hyperkalemia occurs when the serum potassium levels above 5.5 mmol/L, which can result in arrhythmias. Muscle cramps, muscle weakness, rhabdomyolysis, myoglobinuria are presenting signs and symptoms in hyperkalemia.

Calcium has a significant physiological role in the body. It is involved in skeletal mineralization, contraction of muscles, the transmission of nerve impulse, blood clotting, and secretion of hormones. The diet is the predominant source of calcium. It is mostly present in the extracellular fluid. Absorption of calcium in the intestine is primarily under the control of the hormonally active form of vitamin D, which is 1,25-dihydroxy vitamin D3. Parathyroid hormone also regulates calcium secretion in the distal tubule of kidneys. Calcitonin acts on bone cells to increase the calcium levels in the blood.

Hypocalcemia diagnosis requires checking the serum albumin level to correct for total calcium, and the diagnosis is when the corrected serum total calcium levels are less than 8.8 mg/dl, as in vitamin D deficiency or hypoparathyroidism. CHecking serum calcium levels is a recommended test in post-thyroidectomy patients. Hypercalcemia is when corrected serum total calcium levels exceed 10.7 mg/dl, as seen with primary hyperparathyroidism. Humoral hypercalcemia presents in malignancy, primarily due to PTHrP secretion.

The acid-base status of the blood drives bicarbonate levels. The kidneys predominantly regulate bicarbonate concentration and are responsible for maintaining the acid-base balance. Kidneys reabsorb the filtered bicarbonate and also generate new bicarbonate by net acid excretion, which occurs by excretion of both titrable acid and ammonia. Diarrhea usually results in loss of bicarbonate, thus causing an imbalance in acid-base regulation.

Magnesium is an intracellular cation. Magnesium is mainly involved in ATP metabolism, contraction and relaxation of muscles, proper neurological functioning, and neurotransmitter release. When muscle contracts, calcium re-uptake by the calcium-activated ATPase of the sarcoplasmic reticulum is brought about by magnesium. Hypomagnesemia occurs when the serum magnesium levels are less under 1.46 mg/dl. It can present with alcohol use disorder and gastrointestinal and renal losses—ventricular arrhythmias, which include torsades de pointes seen in hypomagnesemia.

Chloride is an anion found predominantly in the extracellular fluid. The kidneys predominantly regulate serum chloride levels. Most of the chloride, which is filtered by the glomerulus, is reabsorbed by both proximal and distal tubules (majorly by proximal tubule) by both active and passive transport.

Hyperchloremia can occur due to gastrointestinal bicarbonate loss. Hypochloremia presents in gastrointestinal losses like vomiting or excess water gain like congestive heart failure.

Phosphorus is an extracellular fluid cation. Eighty-five percent of the total body phosphorus is in the bones and teeth in the form of hydroxyapatite the soft tissues contain the remaining 15%. Phosphate plays a crucial role in metabolic pathways. It is a component of many metabolic intermediates and, most importantly of adenosine triphosphate(ATPs) and nucleotides. Phosphate is regulated simultaneously with calcium by Vitamin D3, PTH, and calcitonin. The kidneys are the primary avenue of phosphorus excretion.

Phosphorus imbalance may result due to three processes: dietary intake, gastrointestinal disorders, and excretion by the kidneys.


Sodium is the major cation of the extracellular fluid. It is responsible for one-half of the osmotic pressure gradient that exists between the interior of cells and their surrounding environment. People eating a typical Western diet, which is very high in NaCl, routinely take in 130 to 160 mmol/day of sodium, but humans require only 1 to 2 mmol/day. This excess sodium appears to be a major factor in hypertension (high blood pressure) in some people. Excretion of sodium is accomplished primarily by the kidneys. Sodium is freely filtered through the glomerular capillaries of the kidneys, and although much of the filtered sodium is reabsorbed in the proximal convoluted tubule, some remains in the filtrate and urine, and is normally excreted.

Hyponatremia is a lower-than-normal concentration of sodium, usually associated with excess water accumulation in the body, which dilutes the sodium. An absolute loss of sodium may be due to a decreased intake of the ion coupled with its continual excretion in the urine. An abnormal loss of sodium from the body can result from several conditions, including excessive sweating, vomiting, or diarrhea the use of diuretics excessive production of urine, which can occur in diabetes and acidosis, either metabolic acidosis or diabetic ketoacidosis.

A relative decrease in blood sodium can occur because of an imbalance of sodium in one of the body’s other fluid compartments, like IF, or from a dilution of sodium due to water retention related to edema or congestive heart failure. At the cellular level, hyponatremia results in increased entry of water into cells by osmosis, because the concentration of solutes within the cell exceeds the concentration of solutes in the now-diluted ECF. The excess water causes swelling of the cells the swelling of red blood cells—decreasing their oxygen-carrying efficiency and making them potentially too large to fit through capillaries—along with the swelling of neurons in the brain can result in brain damage or even death.

Hypernatremia is an abnormal increase of blood sodium. It can result from water loss from the blood, resulting in the hemoconcentration of all blood constituents. This can lead to neuromuscular irritability, convulsions, CNS lethargy, and coma. Hormonal imbalances involving ADH and aldosterone may also result in higher-than-normal sodium values.

CIE A level biology notes

- small molecules
- water soluble
- 1 gram releases 16 KJ of energy
- disturbs osmotic balance
- can easily cross through plasma membranes
- can be used for release of energy
- can be stored in form of starch and glycogen

Structure of glucose:
- glucose molecules can be formed in 3 forms,
1) straight chain
2) α - glucose
3) β - glucose
- both alpha and beta glucose have 5 carbons present in the hexagon while the 6th carbon lies outside, attached to carbon 5
- in alpha glucose, C-1 hydroxyl (-OH) group is bellow the plain. while in beta glucose (-OH) is placed above the plain

2- Fructose:
It is monosaccharide and a hexose sugar

β Fructose

2CH12O6---------> C12H22O11+ H2O

glycosidic bond between them

*many monomers bound together in glycosidic units
*all polysaccharides are made from repeated same monomers
*present in human muscle
*glycosidic bonds

1- starch
2- glycogen
3- cellulose

1) Starch: unbranched component
- final amylose/ amylopectin complex is insoluble and doesn't affect the osmotic properties of the cell

*starch and glycogen are both storage carbohydrates while cellulose is a structural carbohydrate

- linear helix
- alpha helix
- (1-4) links

- (1-4 & 1-6) links

- Starch is a polysaccharide made from 2 molecules, amylose and amylopectin

Amylose Amylopectin
- α - glucose - 1-4 and 1-6 link
- glycosidic bond - branched
- α helix
- 1-4 link
- unbranched
- is the reason for blue/black
colour change

2) Glycogen:
- glycogen and amylopectin both have 1-4 glycosidic bonds, glycogen contains more 1-6 branches than amylobectin
- it is a storage polysaccharide for animal cells and fungi
- exactly like amylopectin, but branching is more extensive

(cellulose and amylopectin both have C-O-C)

3) Cellulose: dependent on hydrogen bond
- structural polysaccharide
- made from β- glucose to form a straight chain
- has 1-4 links ( β 1-4 only)
- these links are formed alternately bellow and above the plain thus cancelling out the effect of coiled shape and resulting in a straight chain
- being a straight chain it attains strength
- many cellulose molecules are linked together to form a micro fibril this enhances the strength
- many micro fibrils are arranged together to form fibres
- cell wall is made from a mesh like arrangement of these fibres
- because of cellulose the cell wall is able to give a definite shape to the plant cell and helps in maintaining turgidity of cell
- synthesized from identical sub-units

PROTEINS: 20 different amino acids
when a peptide bond is formed 1 amino acid loses a hydroxyl group from its carboxyl group
- weak H+ bond, strong disulfide bond and weak ionic bond hold a molecule of protein in shape
- the functional group 'R' defines its chemical nature
- they can be polar, non-polar, hydrophilic or hydrophobic
- there can be the presence of the sulfide group

Significance of proteins:
- make up a large component of the living body, for example: enzyme, cells etc.

Primary structure of proteins: (peptide bond)
- made by sequencing of specific amino acids
- bonded by peptide bond (covalent bond)
- all polysaccharides joined by peptide bonds are covalent. for example, a polypeptide chain is made from 200 amino acids

Secondary structure of protein: (hydrogen bond)
- a secondary structure results from many -H bonds made among the amino acids of same polypeptide chain
- there are different coiling which results due to -H bonds
- most common are α helix and β pleats sheets
- α helix reults due to more organized hydrogen bonds when they are between the oxygen of carboxylic group with H of amino group of amino acids placed 4 distance apart

Tertiary structure of protein: (disulfide bond)
- less organised beta pleats from folds
- defined by formation of different bonds among functional group of different amino acids
for example:
1- hydrogen
2- ionic
3- hydrophobic interactions
4- disulfide

Ionic bond: when 'R' group has a polar group then an ionic bond is formed

hydrophobic interactions: takes place between non- polar groups

Disulfide bonds: (glycoprotein) very strong and can only be broken down by using reducing agents like urea

- ionic bonds break due to change in pH and temperature
- hydrophobic interaction are broken down by a change in temperature
-- Hydrogen can be broken by both --

*tertiary structure is formed as a 3D structure of a polypeptide chain

Quaternary structure of protein:
- if a protein is made of 2 or more polypeptide chains then it is said to have quaternary structure
- protein can be of 2 types

Fibrous Globular
- linear structure - spherical structure
- structural - functional protein
- keratin, collagen - haemoglobin
- they are all water insoluble - always hydrophobic groups present
clustered on the inside while
hydrophilic groups are on
the outside
- all water soluble

*Keratin: forms hair, nail, and the outer
layer of skin, making these structure water proof.

Haemoglobin: (depends on hydrogen bond)
- globular protein
- has a quaternary structure
- made from a 4 polypeptide chains of which 2 are identical α chains and 2 β chains
- each chain has amino acids with hydrophobic -R- groups contained in the centre shielded by hydrophilic -R- groups on the outer sides
- each chains contain a haem as a prosthetic group (made from IRON) [non protein part]
- haem can be bind with an oxygen molecules
- 4O2 molecules can bind with 4 haem groups
- consists of 4 polypeptide, each with a prosthetic group

- insoluble fibrous protein
- found in skin, tendons, cartilage, bones teeth and walls of blood vessels
- structural protein
- has a quaternary structure
- consist of 3 polypeptide chain each in shape of a loose helix [not alpha helix]
- almost every third amino acid in each polypeptide is glycine
- it can with stand large pulling forces without structure or breaking and is also flexible
- many collagen molecules are laid close together to enhance the strength and make covalent bonds among C-N of 2 different chains
- the molecules don't start and end at the same point rather have staggered ends so that there is not a weak point in the structure

[collagen and deoxyribonucleic acid contains, carbon, hydrogen, oxygen and nitrogen]

Lipids: (contains C=O bonds)
- fats and oil
- fats are solid at room temperature whereas oils are liquid at room temperature
- lipid is a polymer made from condensation of 3 fatty acids and 1 glycerols releasing 3 water molecules
- high energy density (38KJ/mol)
- helps in insulation
- make up blubber
- used as energy storage molecules

Fatty acids:
they can be saturated when they have all single bonds in the hydrocarbon chains

[ a kink is a bend shown due to a bend in hydrocarbon chain]

- they have lower ratio of O2 to carbon compared with carbohydrates
- they are non- polar
- less denser than water
- higher energy value than carbohydrates
- higher proportion of H+ than in carbohydrates

- phospholipid molecule is a polymer made from replacement of 1 hydrogen chain by 1 phosphate
- make plasma membrane
- cell membrane/ plasma membrane are made from a bi-layer of phospholipids in such a way that the hydrocarbon tails [hydrophobic] are sandwiched between the phosphate heads
- in such an arrangement the phosphate heads being hydrophilic stay compatible with water on the outside and inside
- the placement gives cell membrane their property of partial permeability
- since much of the width of the membrane is hydrophobic so small and non- polar molecules can easily pass through the small gaps formed in the phospholipid molecules

Water: (dependent on hydrogen bond)
- because of partial positive and partial negative charges on the hydrogen and oxygen, water acts as solvent for charged ions
- it also act as solvent for transport of dissolved glucose and urea
- it has a high specific heat capacity needed to change state, either into vapour or into ice
- water shows an odd quality at 4 °C thus ice floats on water
- ice insulates the water underneath due to convection current which maintain the aquatic life even in cold temperature, water molecules have strong inter molecular attraction due to which cohesion and adhesion results in water bodies so that small insects can walk on
- due to Cohesion forces, water molecules make a column to be pulled up through the xylem vessels
- Adhesion means water molecules tend to cling on to the surface along which they pass and this property helps in movement of water molecules along the cell wall of the plant calls

sodium(Na)/ potassium(K)= needed for maintaining electrolyte
balance and stability of membrane
Magnesium(Mg)= control of chlorophyll
Iron(Fe)= red blood cells
Calcium(Ca)= 1- bones and teeth
2- at synapses
Chloride ions(Cl − )= needed to dilute the mucus formed in respiratory tract

Electrolyte Replacement and Illness

One of the most vital times to ensure the body has adequate electrolytes is during illness. Illnesses that cause diarrhea can result in electrolyte loss, which can quickly reach dangerous levels. During this time, it is best to seek medical attention immediately. If medical attention is not immediately available, a pharmacist at the drug store may be able to provide advice about drinks with electrolytes or other products such as sports drinks.


Although many of the treatment for electrolyte imbalance mirror those for prevention, there are additional treatments available for severe cases. In general, treatment includes identifying and treating the underlying problem causing the electrolyte imbalance, providing intravenous fluids and providing the specific electrolyte replacement. Minor electrolyte imbalances may be corrected by diet changes. However, in more severe cases, diet alone will not work. It is important to note that low sodium levels must be restored slowly, as rapid changes in sodium concentrations can cause brain cell shrinkage and other damage to the brain. Sodium levels can be repaired by restricting fluids, using intravenous saline solutions or consuming salt tablets. There are also drugs that work by increasing fluid retention and decreasing urination. Treatment for low potassium levels include intravenous potassium solutions or giving the patient potassium supplements. As with sodium, potassium must be administered slowly to avoid complications. Insulin is often given with glucose to help potassium absorption, and albuterol may be also added to increase absorption as well.

Safety and efficacy of intravenous hypotonic 0.225% sodium chloride infusion for the treatment of hypernatremia in critically ill patients

Background: The purpose of this study was to evaluate the safety and efficacy of central venous administration of a hypotonic 0.225% sodium chloride (one-quarter normal saline [¼ NS]) infusion for critically ill patients with hypernatremia.

Methods: Critically ill, adult patients with traumatic injuries and hypernatremia (serum sodium [Na] >150 mEq/L) who were given ¼ NS were retrospectively studied. Serum sodium, fluid balance, free water intake, sodium intake, and plasma free hemoglobin concentration (fHgb) were assessed.

Results: Twenty patients (age, 50 ± 18 years Injury Severity Score, 29 ± 12) were evaluated. The ¼ NS infusion was given at 1.5 ± 1.0 L/d for 4.6 ± 1.6 days. Serum sodium concentration decreased from 156 ± 4 to 143 ± 6 mEq/L (P < .001) over 3-7 days. Total sodium intake was decreased from 210 ± 153 to 156 ± 112 mEq/d (P < .05). Daily net fluid balance was not significantly increased. Plasma fHgb increased from 4.9 ± 5.4 mg/dL preinfusion to 8.9 ± 7.4 mg/dL after 2.6 ± 1.3 days of continuous intravenous (IV) ¼ NS in 10 patients (P = .055). An additional 10 patients had a plasma fHgb of 10.2 ± 9.0 mg/dL during the infusion. Hematocrit and hemoglobin decreased (26% ± 3% to 24% ± 2%, P < .001 and 9.1 ± 1.1 to 8.2 ± 0.8 g/dL, P < .001, respectively).

Conclusions: Although IV ¼ NS was effective for decreasing serum sodium concentration, evidence for minor hemolysis warrants further research to establish its safety before its routine use can be recommended.

Keywords: critical care hemolysis hypernatremia sodium water-electrolyte balance.

41.1 Osmoregulation and Osmotic Balance

By the end of this section, you will be able to do the following:

  • Define osmosis and explain its role within molecules
  • Explain why osmoregulation and osmotic balance are important body functions
  • Describe active transport mechanisms
  • Explain osmolarity and the way in which it is measured
  • Describe osmoregulators or osmoconformers and how these tools allow animals to adapt to different environments

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’s 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’t dissociate into ions during water dissolution. Both electrolytes and non-electrolytes contribute to the osmotic balance. The body’s 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 41.2, a cell placed in water tends to swell due to gain of water from the hypotonic or “low salt” 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 “high salt” 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.

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 – ). 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 - ), 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 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 freshwater 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 seawater. 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 freshwater, their bodies tend to take up water because the environment is relatively hypotonic, as illustrated in Figure 41.3a. 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 seawater they excrete the excess salts through their gills and their urine, as illustrated in Figure 41.3b. Most marine invertebrates, on the other hand, may be isotonic with seawater ( osmoconformers ). Their body fluid concentrations conform to changes in seawater concentration. Cartilaginous fishes’ 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.

Career Connection

Dialysis Technician

Dialysis is a medical process of removing wastes and excess water from the blood by diffusion and ultrafiltration. When kidney function fails, dialysis must be done to artificially rid the body of wastes. This is a vital process to keep patients alive. In some cases, the patients undergo artificial dialysis until they are eligible for a kidney transplant. In others who are not candidates for kidney transplants, dialysis is a life-long necessity.

Dialysis technicians typically work in hospitals and clinics. While some roles in this field include equipment development and maintenance, most dialysis technicians work in direct patient care. Their on-the-job duties, which typically occur under the direct supervision of a registered nurse, focus on providing dialysis treatments. This can include reviewing patient history and current condition, assessing and responding to patient needs before and during treatment, and monitoring the dialysis process. Treatment may include taking and reporting a patient’s vital signs and preparing solutions and equipment to ensure accurate and sterile procedures.

Watch the video: Electrolyte Disorders - Complete Lecture. Health4TheWorld Academy (September 2022).


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