Inorganic ions

Biological physical chemistry ?

Inorganic ions are also known as salts, or mineral salts. They are always partnered with other ions, which are often ignored.

Ions are charged particles - often simply a single chemical element or a compound of a few elements. Positive ions have lost one or more electrons, and negative ions have gained one or more electrons. This gain or loss generally means that the outer orbitals (electron shells) of the atoms concerned are full, so they become more stable. Most positive ions originate from metallic elements (exceptions: hydrogen ions, ammonium ions), and negative ions mostly originate from non-metallic elements.

Charged particles dissolve well in water, and these particles are mobile so they spread out and cause effects by their own diffusion, as well as influencing the diffusion of water across membranes - otherwise known as osmosis.

From a biological point of view, the main importance of inorganic ions is the role they play within living organisms, but they may also be considered to be of significance in an environmental context, or as components of diet.

Inorganic ions often affect the functioning of cells, either from inside the cytoplasm or outside of it - in the body fluids such as blood, tissue fluid, lymph and other body secretions such as tears and saliva.

It is said that some are present in fairly high concentrations, and some in much lower concentrations. There are often regulatory (homeostatic) mechanisms ensuring that optimum concentrations are maintained, and kept in balance with other, quite similar, ions.

Collectively, ions affect the osmolarity of solutions. A 1.0 molar solution of sodium chloride (58.5g in 1 litre) has twice the osmotic effect of a 1.0 molar glucose solution (180g in 1 litre), because it dissociates to give both Na⁺ and Cl^-.

Dissolved ions affect the water potential - ψ - of biological fluids which determines the direction of transport of water across membranes by osmosis.

Go with the flow ?

Medically, inorganic ions may also be known as electrolytes.

This reminds us that the ions can be separated out by the application of an electric charge - a voltage - between 2 electrodes. This movement only occurs when the ions are able to move: when dissolved in water (although chemists will know that it can also occur in a molten state). In fact electrolysis means splitting by electricity.

Positive ions (cations) move to the negative electrode (cathode) and negative ions (anions) move to the positive electrode (anode).

In biochemistry, this phenomenon is used in a different context - electrophoresis - to separate different components from a mixture on the basis of their molecular charge.

What about organic ions ?

Certain organic molecules can become ionised.

Fatty acids and lipids
Ethanoate (acetate) - CH₃COO^- is a salt of ethanoic (acetic) acid. Other fatty acids have salts with larger molecules which become ionised, e.g. stearate, palmitate. Neutral lipids (triglycerides) have no groups that can become ionised, so this is described as non-polar, hydrophobic. This situation is reversed in the case of phospholipids which have a phosphate head which ionises, making polar, hydrophilic compounds which are the basis for membranes inside and on the outside of cells.

Amino acids and proteins
Amino acids can also become ionised, depending on the pH of their environment. The amino section (-NH₂) can become positively charged (-NH₃⁺) and the acid section (-COOH) can become negatively charged (-COO^-). Amino acids with these or similar groups in their R- groups can become similarly charged, and this can be the case with the proteins they form.

Nucleic acids
ATP and DNA are organic compounds containing phosphate groups, which can become ionised.

Carbohydrates
Sugars do not become ionised, but are soluble because they have plenty of -OH groups. Derivative compounds such as glucuronic acid have salts (glucuronates) that are ionic.

Examples

Hydrogen ions: H⁺ - also known as protons.

Hydrogen ions are the key to pH - the negative log of the hydrogen ion concentration.
This can have marked effects on metabolism, especially enzyme activity. By interacting with amino acid sidechains of the enzyme's polypeptide chains, H⁺ ions can cause changes in the shape of the active site or binding site which may slow down or speed up the enzyme activity. At the optimum pH, the active site is working at maximum efficiency, so enzyme-substrate complexes form and are processed most quickly.
In the stomach the protease pepsin requires a low pH, whereas in the colon other enzymes function better in higher pH.

Inside cells, hydrogen ions are generated by biological energy transfers, and they build up within mitochondria and chloroplasts. In fact they build up in the spaces between the inner and outer membranes, and their concentration is used to produce ATP - the energy currency of the cell.

Respiration, acids and hydrogen ions

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃^-
carbon dioxide + water ⇌ carbonic acid ⇌ hydrogen ions + hydrogencarbonate ions
Minor variations in the pH of blood as a result of buildup of carbonic acid from respiration will stimulate chemorecptors and affect the respiratory centre in the medulla oblongata, which sends nervous impulses to the diaphragm and other respiratory muscles, increasing the rate of breathing.

A fall in pH of blood plasma also causes oxy-haemoglobin to dissociate (the Bohr effect) and thus release more oxygen from red blood cells.

Both of these responses increase the supply of oxygen to respiring tissue.

Iron ions

- There are two versions (oxidation states):

• iron(II) ions - also known as ferrous ions - (Fe²⁺)
• iron (III) ions- also known as ferric ions - (Fe³⁺)
.

Iron forms part of several important biological chemicals.

Haemoglobin in red blood cells contains iron at the centre of the haem groups where it attracts oxygen molecules. As there are 4 polypeptide chains and 4 haem groups, there are 4 iron atoms per haemoglobin molecule.

Myoglobin in muscles functions similarly but only contains one polypeptide chain and one haem group, so there is one iron atom per myoglobin molecule.
In both of these molecules iron exists in the Fe²⁺ form.

Iron - changing oxidation states

Cytochromes are iron-containing enzymes that undergo redox reactions as they participate in oxidative phosphorylation in mitochondria, and photophosphorylation in chloroplasts, leading in both cases to the generation of ATP.
These iron ions move between oxidation states :
Fe²⁺ ⇌ Fe³⁺

The enzyme catalase - which in many organisms has a protective function against hydrogen peroxide - also contains iron. Interestingly there are some similarities with haemoglobin: Catalase is a tetramer of four polypeptide chains, and it contains four haem groups containing iron. However in a 2-stage conversion process the iron moves from oxidation state III to IV (and higher) before reverting to Fe(III)

'Free iron', in Fe(II) or Fe (III) form, is toxic to cells. In most organisms, iron is safely stored in the form of the protein ferritin and released in a controlled fashion. Ferritin is a hollow globular protein consisting of 24 subunits, and it has an external diameter of about 12 nm (internally 8nm). It is present in practically every cell type.

Sodium ions - Na⁺

Both seawater and blood plasma are mixtures of many ions, but one of the principal ions is sodium. Common table salt is sodium chloride, so chloride ions (Cl^-) are also present in large amounts.

Sodium ions are involved in the uptake of glucose and amino acids in the ileum by co-transport.

Initially, sodium ions are actively transported out of epithelial cells called enterocytes which line the villi (fingerlike structures in the small intestine). 3 sodium ions are pumped out for every 2 potassium ions pumped in. This is an energy-requiring process, driven by ATP, produced as a result of aerobic respiration (oxidative phosphorylation).

As a result, the concentration of sodium ions inside the epithelial cells is lower than the sodium ion concentration in the lumen of the small intestine. There is a concentration gradient between these two areas, separated by the cell membrane.

There are a variety of specific co-transport proteins on the surface membrane of the epithelial cells. Incidentally, there are many intuckings (microvilli) on this surface, forming a 'brush border', and this increases the surface area for absorption. Some co-transport proteins accept amino acids (there are different co-transport proteins for acidic, neutral and basic amino acids). Other co-transport proteins [Na⁺/glucose cotransporter 1 (SGLT1)] accept glucose.

But all these co-transport proteins must also take in sodium ions, and discharge them, together with the other molecules, inside the epithelial cells. They then diffuse around the cell in the cytoplasm. On the other surface of the cell, near to a network of blood capillaries, there are specific carrier proteins forming channels through which amino acids (and glucose) can pass. As the blood is always taking away these nutrients (towards the liver and general circulation) these nutrients are in a lower concentration outside the cell than inside. So the amino acids (and glucose) diffuse down a concentration gradient and out through the carrier protein channels - facilitated diffusion.

Sodium-dependent glucose cotransporters also exist in the proximal convoluted tubule of the kidney and they perform renal glucose reabsorption.
Sodium ions are also involved in the passage of nervous impulses along the axon of a neuron, and at synapses between them.
These processes also depend on the sodium-potassium ion pump.

Sodium ions and co-transport

Co-transport of Na+ ions and amino acids
(different co-transport and carrier proteins exist for glucose)

Phosphate ions:

Because phosphoric acid, H₃PO₄, has 3 x H, it is called polyprotic. Thus there are several versions of its ions: PO₄^3- is the basic version.

Phosphate groups are components of the nucleic acids DNA and RNA, as well as the nucleoside triphosphate ATP, amongst others. All of these are organic compounds involving ester bonds between ribose or dexoxyribose and phosphate, but the phosphate groups may be ionised.
And of course Pi stands for inorganic phosphate, which is given off when ATP yields energy: ATP ⇌ ADP + Pi

Phosphorus has the elemental symbol P, and several biochemical compounds (ATP, NADP, G6P, TP, RuBP) have P in their abbreviated names but these are all phosphates.

The term phosphorous is used as an adjective to describe some compounds of Phosphorus(III), and phosphoric is used for the more common Phosphorus(V) compounds.