Water, Carbohydrates, Lipids and Steroids: Biochemistry Basics

WATER: Constitutes 70% of living beings (up to 98% in some organisms in the cell, 40% intracellular, 15% extracellular, and 8% in blood plasma). Freezing point: 0 °C. Boiling point: 100 °C. Density (liquid at 4 °C): 1 g/cm³. Gas: 0.

Anomalous characteristics:

• Solid is less dense than liquid.
• Dissolves polar substances, salts and many organic compounds; does not dissolve apolar substances.
• High specific heat and high dielectric constant.
• Thermal conductivity.
• High cohesion force and capillarity.
• High surface tension.
• Practically incompressible.

MINERAL SALTS: Functions in living beings: regulation of osmotic pressure (intervenes in transport through membranes; prevents hemolysis (cell rupture) and plasmolysis (cell shrinkage)), and pH regulation. Formation of skeletal organs (bones, teeth, shells).

CARBOHYDRATES: Polyhydroxyaldehydes and polyhydroxyketones, and compounds derived from them, linked to each other by glycosidic bonds.

Types:

• Monosaccharides: cannot be broken down further by hydrolysis. Classified as aldoses (aldehyde group at C1) or ketoses (ketone group at C2).
• Oligosaccharides: 2 to 10 monosaccharides (disaccharides, trisaccharides, etc.).
• Polysaccharides: more than 10 monosaccharides; homopolysaccharides (one type of monomer) and heteropolysaccharides (several types).
• Heterosides: compounds of carbohydrates bound to non-carbohydrate molecules.

Monosaccharide features: Aldoses have the aldehyde group at C1; ketoses have the ketone group at C2. Asymmetric carbon: 4 different substituents. Position of OH: D (right) or L (left) configuration, depending on the OH on the last asymmetric carbon. Cyclization: aldopentoses form a hemiacetal between C1 and C4; aldohexoses between C1 and C5; ketohexoses between C2 and C5. α and β anomers: depend on the position of the OH on C1 (trans/cis).

Main monosaccharides:

• Pentoses: aldopentoses such as ribose (RNA) and deoxyribose (DNA); ketopentose such as ribulose.
• Hexoses: aldohexoses such as glucose (blood sugar), galactose (milk sugar, lactose); ketohexose such as fructose (fruit sugar).

Glycosidic link: joins two monosaccharides. Defined by the type (α or β), the carbon of the first monosaccharide involved (1st C), and the carbon of the second (e.g., 1→4 or 1→6).

Main disaccharides:

• Maltose: 2 α-D-glucose units, α1→4 linked. Reducing sugar. Produced by hydrolysis of glycogen and starch.
• Isomaltose: 2 α-D-glucose units, α1→6 linked. Produced by hydrolysis of starch; not found freely in nature. Reducing sugar.
• Cellobiose: 2 β-D-glucose units, β1→4 linked. Forms cellulose. Reducing sugar.
• Chitobiose: 2 β-N-acetylglucosamine units, β1→4 linked. Forms chitin.
• Sucrose: glucose and α-D-fructose, α1→2 linked. Non-reducing sugar (table sugar, cane sugar).

Main polysaccharides:

• Starch: α-D-glucose, α1→4 linked, with branches (α1→6). Found in plants. Energy reserve. Broken down by amylases and α1→6 glucosidase.
• Glycogen: α-D-glucose, α1→4 linked, with branches (α1→6). Found in liver and muscle. Energy reserve. Broken down by amylases and α1→6 glucosidase.
• Chitin: β-D-N-acetylglucosamine, β1→4 linked, linear. Exoskeleton of insects; structural.
• Cellulose: β-D-glucose, β1→4 linked, linear. Found in plants; structural. Broken down by cellulase enzymes.
• Amylose: a linear component of starch with α1→4 linkages.

LIPIDS: Organic molecules formed mainly by C, H, O, N, P (and S). Insoluble in water; soluble in organic solvents. Heterogeneous group of substances.

Fatty acids (part of many lipids) are hydrocarbon derivatives with a carboxyl group, CH₃-(CH₂)n-COOH. They are apolar at one end and polar at the other.

By length of carbon chain: short (up to 8 C); medium (8–12 C); long (more than 12 C).

By number of double bonds: saturated (none); monounsaturated (one double bond); polyunsaturated (two or more). Example: oleic acid (18 C, one double bond at C9).

Physical properties: low solubility in water; melting point depends on chain length (longer chain, higher melting point) and double bonds (more double bonds, lower melting point).

Chemical properties:

• Esterification: acid + alcohol → ester + H₂O.
• Saponification: fat + base (NaOH) → soap + glycerol.
• Autoxidation: unsaturated fatty acids react with oxygen (rancidity).

Simple lipids (saponifiable):

• Fats: glycerol + 3 fatty acids. Energy reserve; stored without water.
• Waxes: long-chain fatty acid + long-chain monohydric alcohol. Insoluble in water; structural function (protection).

Complex lipids (saponifiable):

• Phospholipids: contain C, H, O, N, P. Composed of glycerol, 2 fatty acids, phosphoric acid and an amino alcohol. Polar head and apolar tails. In water, they form micelles or bilayers (cell membranes).

Terpenes or isoprenoids (non-saponifiable): built from isoprene units (C5). Monoterpenes (2 units, 10 C), diterpenes (4 units), triterpenes (6 units), tetraterpenes (8 units, e.g., beta-carotene), and polyterpenes (e.g., rubber). Examples: menthol, cholesterol precursors, vitamin A.

Steroids: derivatives of the sterane nucleus.

• Sterols: hydroxyl group at C3 and an aliphatic chain at C17. Include cholesterol (cell membranes), bile acids and salts (emulsify fats, aiding digestion), and vitamin D (calcium fixation in bones). Estradiol is a female sex hormone.
• Steroid hormones: ketone group at C3. Sex hormones (testosterone: masculinity; progesterone: pregnancy preparation). Adrenal hormones (aldosterone: salt balance; cortisol: regulates glycogen synthesis).