Osmosis, Buffers, Colloids, and Carbohydrates: A Concise Review

Osmosis, Buffers, Colloids, and Carbohydrates

Osmosis: Osmosis occurs when two solutions with different ionic concentrations are separated by a semipermeable membrane that allows water to pass through but restricts the passage of ions. There are three types of solutions: isotonic, hypertonic, and hypotonic.

Buffer Systems

In organisms, buffer systems are crucial for maintaining a stable pH in fluids. These systems prevent abrupt pH changes. Buffers are based on the properties of weak acids, which do not dissociate completely. Within a specific pH range, they act as proton donors and acceptors, maintaining a constant pH within certain limits. Certain salts and corresponding acids balance the pH. For example, the HCO-3/H2CO3 (bicarbonate buffer) balances the extracellular environment.

Colloidal Nature of Matter

In living beings, water acts as a solvent (dispersing phase), and various molecules of the solute form the dispersed phase. If the dispersed molecules have diameters less than 10^-7 cm, the mixtures are solutions. If the diameters are between 10^-7 and 2 x 10^-5 cm, they form a heterogeneous mixture called colloidal dispersions (e.g., protein). When molecular aggregates are invisible to the dispersant, the colloidal dispersion is called an emulsion. Depending on the particle concentration, they can be in Sol or Gel form.

Carbohydrates

Carbohydrates are polyhydroxyaldehydes and their derivatives, as well as substances resulting from condensation.

Properties:

• Physical: White, soluble solids that can form hydrogen bonds with water.
• Chemical: The presence of a carbonyl group makes them reducing agents. They form internal hemiacetals and glycosidic bonds, and can react with phosphoric acid to form esters.

Isomerism

Isomerism is an important feature of monosaccharides. Types of isomerism include:

1. Functional isomerism.
2. Stereoisomerism: Atoms have different spatial arrangements due to asymmetric carbons (carbon bonded to 4 different radicals).
3. Stereoisomers: The presence of asymmetric carbon determines optical activity in solution. D and L stereoisomers exist.
4. Anomers: In solutions, linear structures are not present. Cyclization occurs because aldehyde or ketone groups react with a hydroxyl group. Stable rings can form with >5 carbon atoms. This bond is covalent and hemiacetal.

Classification of Carbohydrates:

1. Osas: Monosaccharides.
2. Osides: Complex carbohydrates derived from osas.

Holosides:

Formed by the union of osas, including:

• Oligosaccharides: Formed by the union of a few molecules, such as disaccharides.
• Polysaccharides: Formed by the union of many monosaccharide molecules.

Distinction: Homopolysaccharides (1 monosaccharide) vs. Heteropolysaccharides (+1 monosaccharide).

Heterosides:

Formed by two types of components: carbohydrates and other components of different composition.

Disaccharides:

Disaccharides are formed by the union of two monosaccharide molecules through a glycosidic bond and the loss of a water molecule (H2O). The O-glycosidic bond, which is covalent, is established between the OH of the first monosaccharide and the second, with the loss of OH from a water molecule. Examples include sucrose, maltose, lactose, and cellobiose.

Polysaccharides:

Polysaccharides are high molecular weight carbohydrates, insoluble in water, derived from the polymerization of monosaccharides.

Homopolysaccharides:

Formed by the union of only one type of monosaccharide.

Starch:

A polysaccharide reserve stored in amyloplasts. It is an effective way to store glucose. Hydrolyzing starch with specific enzymes yields dextrins, amylase, and glucose. It contains a polymer of alpha D glucose 1.4 linkages that form linear chains arranged in a helical structure. It is soluble in water. Amylopectin also has linear chains but with 1-6 branches and is insoluble in water. Glycogen has a structure similar to amylopectin but is more branched. Cellulose is a linear polymer, while chitin is a polymer of N-acetyl glucosamine with beta 1-4 linkages and is a constituent of insect exoskeletons.