Unit 1: Acid-Base Chemistry and Carbohydrates - Subjective Questions

Acids, bases, and buffers:

• An acid is a substance that donates protons or hydrogen ions, , in a chemical reaction. According to the Brønsted–Lowry concept, an acid is a proton donor.
• A base is a substance that accepts protons. Hydroxide ions, , and bicarbonate ions, , are examples of biologically important bases.
• A buffer is a solution that resists sudden changes in pH when small amounts of acid or base are added.

Buffers are essential because most biochemical reactions occur only within a narrow pH range. For example, the bicarbonate buffer system helps maintain the pH of blood, while phosphate and protein buffers help regulate the pH of cells and body fluids.

Important properties of water:

• Polarity: Water has partial positive and negative charges because oxygen is more electronegative than hydrogen.
• Hydrogen bonding: Water molecules form hydrogen bonds, which contribute to its high boiling point and cohesive nature.
• Excellent solvent: Water dissolves ionic and polar substances and therefore supports biochemical reactions and transport.
• High specific heat: It absorbs considerable heat with only a small change in temperature, helping regulate body temperature.
• High heat of vaporization: Evaporation of water removes heat and contributes to cooling.
• Cohesion and adhesion: These properties support capillary movement and transport in plants.
• Ionization: Water undergoes limited dissociation into and , which is important in acid–base chemistry.

Thus, water serves as a solvent, reactant, transport medium, and temperature regulator.

Definition of pH:

pH is a measure of the hydrogen-ion concentration of a solution. It is expressed as:

Therefore:

A solution with a high hydrogen-ion concentration has a low pH and is acidic. A solution with a low hydrogen-ion concentration has a high pH and is basic. At , pure water has , which is neutral.

• Acidic solution:
• Neutral solution:
• Basic solution:

The pH scale is logarithmic, so a change of one pH unit represents a tenfold change in hydrogen-ion concentration.

A buffer is a solution containing a weak acid and its conjugate base, or a weak base and its conjugate acid. It resists changes in pH when small amounts of acid or base are added.

Consider an acidic buffer containing a weak acid and its salt containing :

• When a small amount of acid is added, the added combines with to form undissociated . Thus, the increase in free hydrogen ions is minimized.
• When a small amount of base is added, reacts with , and more dissociates to replace the removed hydrogen ions.

The buffer works most effectively when the concentrations of the weak acid and conjugate base are approximately equal.

For a weak acid buffer:

The dissociation constant is:

Rearranging:

Taking the negative logarithm of both sides gives:

This is the Henderson–Hasselbalch equation.

Applications:

• It calculates the pH of a buffer.
• It determines the ratio of conjugate base to weak acid needed to prepare a buffer.
• It predicts how pH changes when acid or base is added.
• It explains why a buffer is most effective when is close to .

For a basic buffer, the corresponding form is .

Buffer capacity is the amount of strong acid or strong base required to change the pH of one litre of a buffer by one unit.

Buffer capacity depends on:

• Total concentration: A concentrated buffer has greater capacity than a dilute buffer because it contains more molecules available to react with added acid or base.
• Ratio of buffer components: Capacity is highest when the concentrations of the weak acid and conjugate base are approximately equal.
• pH relative to : A buffer is most effective when .
• Temperature and ionic strength: These factors can alter dissociation and activity coefficients.

A buffer with a high capacity can neutralize a larger quantity of added acid or base without a significant pH change.

The bicarbonate buffer system consists of carbonic acid and bicarbonate ions:

• When excess acid is present, bicarbonate ions combine with hydrogen ions to form carbonic acid. Carbonic acid can produce carbon dioxide and water, and carbon dioxide is exhaled by the lungs.
• When excess base is present, carbonic acid dissociates and supplies hydrogen ions, reducing the effect of the base.
• The lungs regulate carbon dioxide concentration, while the kidneys regulate bicarbonate concentration and hydrogen-ion excretion.

This system is an important extracellular buffer and helps maintain blood pH near .

Carbohydrates are polyhydroxy aldehydes or ketones, or substances that yield such compounds on hydrolysis. They generally contain carbon, hydrogen, and oxygen, often in the approximate ratio .

General chemical features:

• They contain several hydroxyl groups.
• They may possess an aldehyde group, forming an aldose, or a ketone group, forming a ketose.
• They can exist in open-chain and cyclic forms.

Biological functions:

• Provide energy, especially glucose.
• Store energy as glycogen in animals and starch in plants.
• Form structural materials such as cellulose and chitin.
• Participate in cell recognition and signaling as components of glycoproteins and glycolipids.
• Contribute to nucleic acid structure through ribose and deoxyribose.

Carbohydrates are classified according to the number of sugar units formed on hydrolysis.

• Monosaccharides: Simple sugars that cannot be hydrolyzed into smaller carbohydrates. Examples include glucose, fructose, ribose, and galactose.
• Oligosaccharides: Carbohydrates containing a small number of monosaccharide units, usually two to ten. Examples include sucrose, lactose, and maltose.
• Polysaccharides: Long chains containing many monosaccharide units. Examples include starch, glycogen, cellulose, and hyaluronic acid.

They may also be classified according to the functional group:

• Aldoses: Contain an aldehyde group, such as glucose and ribose.
• Ketoses: Contain a ketone group, such as fructose.

This classification helps relate carbohydrate structure to function and digestibility.

Monosaccharides are the simplest carbohydrates and cannot be hydrolyzed into smaller carbohydrate units. They contain multiple hydroxyl groups and either an aldehyde or ketone group.

Examples:

• Glucose is an aldohexose.
• Fructose is a ketohexose.
• Ribose is an aldopentose.

Important properties:

• They are generally soluble in water because of their hydroxyl groups.
• Many are sweet and crystalline.
• They can form cyclic structures in solution.
• They show optical activity because they contain one or more chiral carbon atoms.
• They may act as reducing sugars if a free anomeric carbon is present.

Monosaccharides serve as energy sources and as building blocks for oligosaccharides, polysaccharides, nucleotides, and glycoproteins.

Stereoisomerism occurs when compounds have the same molecular formula and connectivity but differ in the three-dimensional arrangement of atoms.

• Enantiomers: Non-superimposable mirror images. The two forms are designated and according to the configuration of the chiral carbon farthest from the carbonyl group.
• Epimers: Diastereomers that differ in configuration at only one chiral carbon. For example, glucose and galactose are epimers at carbon 4.
• Anomers: Isomers that differ at the anomeric carbon formed during cyclization. The and forms of glucose are anomers.
• Mutarotation: Interconversion between and anomers through the open-chain form, causing a change in optical rotation.

Stereoisomerism is biologically important because enzymes usually recognize only a particular stereoisomer.

Glucose contains an aldehyde group at carbon 1 and hydroxyl groups at other carbon atoms. In aqueous solution, the hydroxyl group at carbon 5 can react intramolecularly with the aldehyde group at carbon 1. This forms a six-membered cyclic hemiacetal called glucopyranose.

The former carbonyl carbon becomes the anomeric carbon.

• In -D-glucose, the hydroxyl group at the anomeric carbon is opposite to the group.
• In -D-glucose, the hydroxyl group at the anomeric carbon is on the same side as the group.

The two forms can interconvert through the open-chain form. This process is called mutarotation.

Sugar derivatives are compounds formed when one or more functional groups of a monosaccharide are modified or replaced.

• Sugar alcohols: The carbonyl group is reduced to an alcohol group. Examples include sorbitol from glucose and mannitol from mannose.
• Sugar acids: Oxidation produces acids. Glucose can form gluconic acid, glucuronic acid, or glucaric acid depending on the carbon atom oxidized.
• Amino sugars: A hydroxyl group is replaced by an amino group. Examples include glucosamine and galactosamine.
• Deoxy sugars: An oxygen atom is removed from a hydroxyl group. Deoxyribose is an important example.
• Sugar phosphates: A hydroxyl group is esterified with phosphate, as in glucose-6-phosphate.

Sugar derivatives are important in metabolism, nucleic acids, connective tissue, and cell-surface structures.

Oligosaccharides contain a small number of monosaccharide units joined by glycosidic bonds.

• Sucrose: Consists of glucose and fructose joined through their anomeric carbons. It is a non-reducing sugar because both anomeric carbons participate in the glycosidic bond.
• Lactose: Consists of galactose and glucose joined by a glycosidic bond. It is the principal sugar in milk and is a reducing sugar.
• Maltose: Consists of two glucose units joined by an glycosidic bond. It is produced during starch digestion and is a reducing sugar.

The type and position of the glycosidic bond determine the properties and digestibility of each oligosaccharide.

A glycosidic bond is a covalent bond that joins a sugar molecule to another sugar or to a non-carbohydrate group.

During formation between two monosaccharides, the hydroxyl group of one sugar reacts with the anomeric hydroxyl group of another sugar. A molecule of water is eliminated, making the reaction a condensation reaction.

For example, maltose is formed when two glucose molecules are joined by an glycosidic bond. The notation indicates:

• The configuration of the anomeric carbon, such as or .
• The carbon of the first sugar involved in the bond.
• The carbon of the second sugar involved in the bond.

Hydrolysis of a glycosidic bond requires water and may be catalyzed by acids or specific enzymes.

Polysaccharides are polymers composed of many monosaccharide units linked by glycosidic bonds.

Storage polysaccharides:

• Starch: The main storage carbohydrate in plants. It contains amylose with linkages and amylopectin with and linkages.
• Glycogen: The storage polysaccharide in animals. It is highly branched and has frequent branch points.

Structural polysaccharides:

• Cellulose: Contains glucose units linked by bonds. It provides strength to plant cell walls.
• Chitin: Contains modified glucose units called -acetylglucosamine and forms the exoskeleton of arthropods.

Storage polysaccharides are readily mobilized for energy, whereas structural polysaccharides provide mechanical strength and are generally more resistant to digestion.

Starch:

• It is the major storage carbohydrate of plants.
• Amylose is mostly linear and contains linkages.
• Amylopectin is branched and contains linkages with branch points.

Glycogen:

• It is the principal storage carbohydrate of animals.
• It has a structure similar to amylopectin but is more highly branched.
• It is stored mainly in liver and skeletal muscle.

Digestion:

• Salivary and pancreatic amylases hydrolyze internal bonds.
• Brush-border enzymes hydrolyze disaccharides and branch points to produce glucose.
• Glycogen can be mobilized by glycogen phosphorylase to provide glucose-1-phosphate.

Branching allows rapid synthesis and breakdown because many terminal units can be acted upon simultaneously.

Cellulose is a linear homopolysaccharide composed of glucose units joined by glycosidic bonds. Its chains form extensive hydrogen bonds with neighboring chains, producing strong fibers.

Biological importance of cellulose:

• It is the major structural component of plant cell walls.
• It provides rigidity and resistance to mechanical stress.
• It contributes dietary fiber in humans.

Comparison with starch:

• Starch contains -linked glucose, whereas cellulose contains -linked glucose.
• Starch is used for energy storage, whereas cellulose provides structural support.
• Humans digest starch because they possess enzymes for linkages, but they cannot digest cellulose efficiently because they lack cellulase.
• Starch is often helical or branched, while cellulose forms straight, strong fibers.

Mucopolysaccharides, also called glycosaminoglycans, are long, unbranched polysaccharides composed of repeating disaccharide units. Usually, each repeating unit contains an amino sugar and a uronic acid or galactose.

Examples and functions:

• Hyaluronic acid: Present in connective tissue, synovial fluid, and the vitreous humor; it provides lubrication and hydration.
• Chondroitin sulfate: Found in cartilage, tendons, ligaments, and bone; it contributes to strength and resilience.
• Heparin: A highly sulfated glycosaminoglycan that acts as an anticoagulant.
• Keratan sulfate: Present in the cornea, cartilage, and other connective tissues.
• Dermatan sulfate: Found in skin, blood vessels, and heart valves.

Their negative charges attract water and cations, allowing them to form hydrated gels and support extracellular matrix structure.

A reducing sugar has a free anomeric carbon that can open to form an aldehyde or a reactive carbonyl group. It can reduce mild oxidizing agents such as Benedict's or Fehling's reagent.

Examples of reducing sugars:

• Glucose
• Fructose, because it can undergo tautomerization under test conditions
• Lactose
• Maltose

A non-reducing sugar does not have a free anomeric carbon because the anomeric carbon is involved in a glycosidic bond.

Example: Sucrose is non-reducing because the anomeric carbons of both glucose and fructose participate in the glycosidic linkage.

This distinction is useful in carbohydrate analysis and in understanding the chemical behavior of oligosaccharides.