Water, Lipids and Carbohydrates: Biochemistry Basics

Water & mineral salts were abundant within living organisms, which obtain their substances from the environment (exogenous) or produce other organic molecules (endogenous).

Characteristics:

• Formed by two hydrogen atoms and one oxygen atom (H₂O), linked by covalent bonds.
• Electrically neutral overall, but with a slight positive charge on the hydrogen side and a slight negative charge on the oxygen side.
• Polar, because of its angular structure and its ability to form hydrogen bridges between molecules.

Next:

• High molecular cohesion: it is what allows water to form continuous columns inside vessels and to resist tension.
• High surface tension: the attraction between surface molecules makes possible capillary action, which allows water to penetrate large parts of conducts against gravity.
• Latent heat: molecules absorb or release heat during physical changes of state (fusion from solid to liquid, vaporization from liquid to gas).
• Specific heat: water absorbs heat as temperature rises, which helps stabilize environmental temperatures.
• Heat of vaporization: eliminating much heat with small water loss allows cooling by sweating or transpiration.
• Density: H₂O is denser as a liquid than as a solid, so ice floats and forms a layer over bodies of water, insulating the water below.
• Relative dielectric constant: water tends to oppose attractions between ions, which favors the dissolution of ionic crystalline substances.
• Low degree of ionization: a small quantity of molecules is ionized (dissociated into ions).

Ionic product: the product of ion concentrations in pure water at 25 °C, in aqueous solution, is a constant that also determines the acidity of the solution:

• Neutral: the H₃O⁺ and OH^- concentrations are equal.
• Acidic: the H₃O⁺ concentration is greater than the OH^- concentration.
• Basic: the OH^- concentration is greater than the H₃O⁺ concentration.

Biological importance:

• Main biological solvent: it intervenes in reactions such as hydrolysis, and most metabolic chemical reactions are carried out in it.
• Structural function: it provides volume to cells and DNA, gives turgidity to plants, explains the deformations of the cell cytoplasm, and acts as a hydrostatic skeleton in invertebrates.
• Shock-absorbing mechanical function in vertebrates.
• Transport function: it allows the transport of substances, and sap rises through woody vessels.
• Thermoregulatory function: its high specific heat allows temperature to be maintained.
• Allows aquatic life in cold climates.

Dissolved mineral salts: inorganic molecules present in all living beings, dissolved or in solid state, dissociated into:

• Anions: chlorides, phosphates, monohydrogen phosphates, carbonates, bicarbonates, nitrates.
• Cations: Na⁺, Ca²⁺, Mg²⁺, Fe²⁺ or Fe³⁺, K⁺.

Mineral salts dissolved in water: water-soluble, dissociated ions.

Functions: they contribute to maintenance and homeostasis through: maintaining the degree of salinity in organisms, regulating enzymatic activity, regulating pressure, cell osmotic and volume stabilization, colloidal dispersions, electrical potentials, regulating electrical activity, and acting as buffers to keep pH stable.

Precipitated mineral salts: water-insoluble solid state.

Functions: they form protective or supporting structures through calcium carbonate, silicate (sponges) and phosphate crystals (bone tissue in vertebrates).

• Calcium carbonate: forms shells in marine protozoa and invertebrates, endoskeleton-bones in some vertebrates, rigidity and form in spines of echinoderms, and the exoskeleton of molluscs and arthropods.
• Silicates: structures sustaining the endoskeleton of vegetables and sponges, are spicules formed in protective shells in some microorganisms.
• Calcium phosphate: forms the mineral matrix that makes up bones in vertebrates.

Inorganic biomolecules, heavyweight in living beings: they form part of colloidal phases: a dispersant (water) and a dispersed phase (solid mineral salts and organic molecules).

Properties of colloidal dispersions:

• Tyndall effect: colloidal dispersions are translucent.
• Brownian motion: colloidal particles show disordered movement.
• Sedimentation: colloidal particles settle only under strong centrifugal force.
• High viscosity: the viscosity increases as the quantity of dispersed molecules increases.
• High adsorption: the solid surface has the capacity to attract molecules of a liquid or gas.
• Dialysis: separation process by which molecules that are not part of a colloidal dispersion are separated according to size.

Osmosis: the phenomenon in which solvent molecules diffuse through a semipermeable membrane from a diluted solution to a concentrated one, equalizing concentrations on both sides. Solutions are classified as hypertonic (high solute concentration) or hypotonic (low solute concentration) relative to each other.

Behavior of semipermeable membranes, such as cell membranes:

• In plant cells, when the external environment is hypotonic relative to the internal environment, water enters, volume rises and osmotic pressure increases, which can cause cell rupture; when the external environment is hypertonic, the cell loses water and undergoes plasmolysis.
• In animal cells, when the external environment is hypotonic, water enters and cells can burst, producing hemolysis of blood cells; in an isotonic environment there is no net water movement.

Lipids: Chemically Very Heterogeneous Substances, Structurally and Functionally

Lipids, whatever their composition, have in common their oily texture, their insolubility in water, and their solubility in organic solvents.

Functions:

• Structural: most cell membrane components are lipids.
• Energetic: some are reserves, such as triglycerides, which store energy.
• Hormonal and vitamin functions: many vitamins and hormones are lipids.

Classification:

• Saponifiable: contain fatty acids in their molecules, which may be monocarboxylic and may have double bonds, and form soaps upon saponification (waxes, fats, phospholipids and sphingolipids).
• Unsaponifiable: hydrocarbons and derivatives of cyclic unsaturated fatty acids, containing several molecular formations and associations (terpenes, steroids and prostaglandins).

Fatty Acids

Fatty acids are organic monocarboxylic acids with a number of carbon atoms (n) generally between 10 and 22.

• Saturated: no double bonds, usually solid. Palmitic acid (16 C) is abundant in cocoa butter, stearic acid (18 C) and decanoic acid (10 C) are found in mammalian milk.
• Unsaturated: have one or more double bonds in their carbon chain. They are distinguished as monounsaturated (for example, oleic acid, C18, with one double bond between carbons 9 and 10) and polyunsaturated (for example, linoleic acid, C18, with two double bonds). Some of these cannot be synthesized by mammals; they are called essential fatty acids, also known as vitamin F.

Physicochemical properties:

• They have two zones: they are amphipathic, with a polar carboxyl group, which is hydrophilic, and a nonpolar carbon chain, which is hydrophobic.
• The carboxyl group is stabilized by its polar nature. The aliphatic chain interacts through van der Waals forces with adjacent chains.
• Fatty acids react with alcohols to form esters, releasing water.
• The length and level of unsaturation of the aliphatic chain determine the melting point: the longer the chain, the higher the melting point, since van der Waals forces increase.

Fats and Waxes

Fats: saponifiable lipids that undergo alkaline hydrolysis (saponification). They are compounds formed by glycerin esterified with 1, 2 or 3 fatty acid molecules, giving monoacylglycerides, diacylglycerides or triacylglycerides, the last being the most abundant.

• They are nonpolar molecules, insoluble in water.
• Classification by origin: plant fats usually have a low melting point and are unsaturated, liquid at room temperature (sunflower, corn). Animal fats usually have a high melting point and are saturated, solid at room temperature (butter).
• Functions: energy source, reserve, insulation and food storage.

Waxes: esters formed by a fatty acid with a long chain (between 14 and 36 carbons) and a long-chain monoalcohol (16 to 36 carbons).

• Hydrophobic nature, insoluble in water.
• Functions: protective coatings, covering skin, hair, feathers and leaves in animals and plants, exoskeletons of insects, and fruits and flowers.
• Industrial uses: lanolin, extracted from sheep wool, and spermaceti, used in softeners and lubricants.

Phospholipids

Saponifiable lipids also called phosphoglycerides, the main components of biological membranes. They consist of a glycerol molecule with one phosphate group at C3, a saturated fatty acid at C1, and an unsaturated fatty acid at C2. They are also amphipathic. An example is phosphatidic acid, the simplest.

• In biological membranes, phospholipids form a bilayer, with polar heads facing outward and hydrophobic chains oriented inward.
• In water, phospholipids form spherical structures called liposomes, which contain water inside.

Sphingolipids

Similar to phospholipids, sphingolipids are amphipathic and form bilayers in water. Chemically, they are constructed from sphingosine, an amino alcohol with 18 carbons, normally with a saturated fatty acid chain of 18 to 26 carbons and a polar group of phosphoric acid. The amino group of sphingosine is linked to a fatty acid, forming ceramide, which contributes the hydrophobic layers.

Classification:

• Sphingomyelins: their polar group, linked to ceramide, is phosphocholine or phosphoethanolamine. They are abundant in the myelin sheath surrounding axons, produced by Schwann cells.
• Glycosphingolipids: their polar group is a monosaccharide or oligosaccharide carbohydrate, classified as:
  • Cerebrosides: a union of ceramide and a monosaccharide such as glucose or galactose, called glucocerebrosides and galactocerebrosides, abundant in nerve cells.
  • Gangliosides: ceramide linked to a branched oligosaccharide, found in the outer surface of neurons.

Functions of glycosphingolipids: gangliosides intervene in the recognition of nerve impulses and in the regulation of some synapses. They are related to blood-group specificity, and some gangliosides act as sites of anchorage for viruses, allowing their entry into cells.

Unsaponifiable Lipids (Terpenes, Steroids and Prostaglandins)

These lipids do not contain fatty acids and are not saponifiable, but they abound in biologically important functions, such as hormones and vitamins.

Terpenes: formed by the union of isoprene units, and therefore called isoprenoids.

• Monoterpenes: two isoprene molecules. Found in higher plants, they are volatile and characteristic of aromas (for example, limonene, menthol and geraniol).
• Diterpenes: four isoprene molecules. Some are plant pigments, others are vitamins such as vitamin A, and others take part in cell division processes.
• Triterpenes: six isoprene molecules (squalene, a precursor of cholesterol).
• Tetraterpenes: eight isoprene molecules. Found in plants, they include pigments that participate in photosynthesis, such as the yellow xanthophylls.
• Polyterpenes: multiple isoprene units, for example regularly arranged long linear chains, such as rubber from latex in some plants.

Steroids: cyclic compounds derived from cyclopentanoperhydrophenanthrene, which differ from one another in the number and position of double bonds and functional groups.

• Sterols: contain a hydroxyl group at C3 and a chain of 17 carbons. Cholesterol is of great biological interest, since it helps maintain membrane fluidity and is a precursor of vitamin D and of the regulation of calcium and phosphorus absorption.
• Steroid hormones: derived from cholesterol, they have characteristic sex-related hormones, such as testosterone, progesterone, estrogens, and cortisol produced by the adrenal cortex.
• Bile acids: such as deoxycholic acid in the human colon, which help emulsify fats.

Prostaglandins: formed in animal tissues, independent of sex. Functions: vasodilation, regulation of blood pressure, intervention in inflammatory processes, stimulation of mucus-producing processes, and participation in blood clotting.

Carbohydrates

Carbohydrates are biomolecules that contain carbon, hydrogen and oxygen, generally in the proportion (CH₂O)_n, which is why they are also called hydrates of carbon.

Structural functions: components of membrane receptors, of nucleotides and of plant and bacterial cell walls, and of the exoskeleton of arthropods and connective tissues.

Energetic functions: nutrients, intermediates in metabolism, and energy reserves.

Other functions: industrial, defensive and hormonal.

Classification:

• Holosides: carbohydrates formed only by monosaccharides, which are divided into:
  • Oligosaccharides: 2 to 10 monosaccharides.
  • Polysaccharides: many monosaccharides. Within these, homopolysaccharides are formed by the repetition of a single monomer, and heteropolysaccharides by more than one type of monomer.
• Heterosides: complex compounds formed by the combination of monosaccharides with non-carbohydrate molecular fractions.

Monosaccharides

They are simple sugars, classified by functional group (aldoses or ketoses) and by number of carbon atoms (trioses, tetroses, pentoses, hexoses).

• Solid, crystalline, soluble, colorless, with a sweet taste.
• Optical activity: dextrorotatory (+) and levorotatory (-) isomers.
• Stereochemistry: D and L forms, enantiomers. Epimers differ in the position of one OH group. Anomers are the alpha and beta forms, defined by the position of the hydroxyl at the anomeric carbon.
• Cyclic forms: aldopentoses and aldohexoses form cycles of furanose or pyranose type, in chair or boat conformations.
• Derivatives of monosaccharides: deoxysugars, sugar phosphates, amino sugars, and sugar alcohols (polyalcohols).

Oligosaccharides

Short chains formed by the union of 2 to 10 monosaccharides. Disaccharides are formed by the union of two monosaccharides through a glycosidic bond. Biologically important examples include:

• Maltose: malt sugar.
• Lactose: milk sugar.
• Sucrose: common sugar in plants.
• Cellobiose: does not exist freely in nature.

Polysaccharides

Polymers formed by the union of many monosaccharides through O-glycosidic bonds, forming long molecular chains.

• Homopolysaccharides with structural function: cellulose and chitin, which form the skeleton of plants and the exoskeleton of arthropods.
• Homopolysaccharides with reserve function: starch, glycogen and dextran.
• Properties: generally not sweet, non-reducing, insoluble in water, or forming colloidal dispersions.
• Heteropolysaccharides: formed by different monosaccharides. Examples include pectins, hemicelluloses, agar, gums, mucilages, peptidoglycans and glycosaminoglycans.

Heterosides

These molecules have a huge variety, formed by carbohydrates bonded to non-carbohydrate aglycones.

• Glycolipids: carbohydrate + lipid, such as cerebrosides and gangliosides.
• Glycoproteins: carbohydrate + protein, such as prothrombin and immunoglobulins.
• Active heterosides: hormones and other compounds with pharmacological effects, such as cardiotonic glycosides (digitalis), glycyrrhizin, tannosides, cyanogenic glycosides and anthracenic glycosides.