Biological Macromolecules: Carbohydrates and Nucleic Acids

Biological Macromolecules

Overview

Biological macromolecules are large, complex molecules essential for life. The four major classes are carbohydrates, nucleic acids, proteins, and lipids. This guide focuses on carbohydrates and nucleic acids, their structure, function, and significance in cell biology.

• Carbohydrates

• Nucleic Acids

• Proteins

• Lipids

Carbohydrates

Structure and Classification

Carbohydrates are organic molecules containing carbon (C), hydrogen (H), and oxygen (O) in a 1:2:1 ratio, generally represented as $(CH_2O)_n$. They serve as energy sources and structural components in cells.

• Monosaccharides: Simple sugars, such as glucose, fructose, and galactose. They are the basic building blocks of carbohydrates.

• Polysaccharides: Macromolecules composed of monosaccharide subunits joined by glycosidic linkages. Examples include starch, glycogen, and cellulose.

Polysaccharides: Storage and Support

Polysaccharides differ in their chemical structure and biological function. They are crucial for energy storage and providing structural support in cells.

Polysaccharide	Chemical Structure	Three-dimensional Structure	Function
Starch	α-glucose, mostly unbranched	Helical	Energy storage in plants
Glycogen	α-glucose, highly branched	Branched helices	Energy storage in animals
Cellulose	β-glucose, unbranched	Parallel strands joined by hydrogen bonds	Structural support in plant cell walls

Synthesis of Glycogen

Glycogen synthesis is a multi-step process involving the activation of glucose and its polymerization into glycogen for energy storage in animal cells.

• Activation: Glucose is activated by reaction with UTP (uridine triphosphate) to form UDP-glucose.

• Polymerization: Glycogen synthase catalyzes the addition of UDP-glucose to the growing glycogen chain.

• Branching: Branching enzyme introduces α-1,6 linkages, creating a branched structure.

Key Equations:

• $ext{UTP} + ext{Glucose-1-phosphate} ightarrow ext{UDP-glucose} + ext{PP}_i$

Additional info: ATP (adenosine triphosphate) and UTP (uridine triphosphate) are energy carriers involved in the activation of glucose.

Nucleic Acids

Types and Functions

Nucleic acids are polymers that store, transmit, and express genetic information. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA: Stores genetic information.

• RNA: Involved in gene expression and regulation. Types include mRNA, rRNA, tRNA, miRNAs, and siRNAs.

Nucleotides: Building Blocks of Nucleic Acids

Nucleotides are the monomers of nucleic acids, each consisting of a phosphate group, a five-carbon sugar, and a nitrogenous base.

• Phosphate group

• Sugar: Ribose in RNA, deoxyribose in DNA

• Nitrogenous base: Purines (adenine, guanine) and pyrimidines (cytosine, thymine in DNA, uracil in RNA)

Base	RNA Nucleoside	RNA Nucleotide	DNA Nucleoside	DNA Nucleotide
Adenine	Adenosine	Adenosine monophosphate (AMP)	Deoxyadenosine	Deoxyadenosine monophosphate (dAMP)
Guanine	Guanosine	Guanosine monophosphate (GMP)	Deoxyguanosine	Deoxyguanosine monophosphate (dGMP)
Cytosine	Cytidine	Cytidine monophosphate (CMP)	Deoxycytidine	Deoxycytidine monophosphate (dCMP)
Uracil	Uridine	Uridine monophosphate (UMP)	-	-
Thymine	-	-	Deoxythymidine	Deoxythymidine monophosphate (dTMP)

Polymerization of Nucleotides

Nucleotides are joined by phosphodiester bonds to form nucleic acid polymers. The polymerization is directional, proceeding from the 5' to the 3' end.

• Phosphodiester bond: Links the 3' carbon of one sugar to the 5' carbon of the next.

• Directionality: Nucleic acids are synthesized and read in the 5' to 3' direction.

Key Equation:

• $ext{Nucleotide}_n + ext{Nucleotide} ightarrow ext{Nucleotide}_{n+1} + ext{H}_2 ext{O}$

DNA Structure

DNA is a double-helix with two antiparallel strands. The sugar-phosphate backbone is on the outside, and nitrogenous bases are on the inside, paired by hydrogen bonds.

• Antiparallel strands: One runs 5' to 3', the other 3' to 5'.

• Complementary base pairing: Purines pair with pyrimidines (G-C and A-T).

RNA Structure

RNA differs from DNA in two key ways: it contains uracil instead of thymine, and ribose instead of deoxyribose, making it more reactive. RNA can form various secondary structures, such as hairpins, and also tertiary and quaternary structures.

• Uracil replaces thymine

• Ribose sugar

• Secondary structure: Hairpin loops are common

Additional info: RNA's ability to form complex structures allows it to perform diverse functions, including catalysis (ribozymes) and regulation (miRNAs, siRNAs).