Macromolecules: Proteins, Nucleic Acids, and ATP

Macromolecules: Proteins, Nucleic Acids & ATP

Overview

Macromolecules are large, complex molecules essential for life. This section focuses on three major types: proteins, nucleic acids (DNA and RNA), and ATP (adenosine triphosphate). Each plays a critical role in cellular structure, function, and energy transfer.

Proteins

Amino Acids and Protein Structure

Proteins are polymers made from amino acids, which are joined together by peptide bonds through dehydration synthesis. There are 20 amino acids used in humans; 11 can be synthesized by the body, while 9 are essential and must be obtained from the diet.

• Amino Acid Structure: Each amino acid contains an amino group (-NH2) and a carboxyl group (-COOH).

• Polypeptides: Chains of 3–100 amino acids; longer chains are called proteins.

• Dehydration Synthesis: The process by which amino acids are joined, releasing water.

Functions of Proteins in Humans

Proteins serve a wide variety of functions in the human body:

• Structural Support: Proteins form the framework of cells and tissues.

• Muscle Contraction: Actin and myosin are key proteins in muscle movement.

• Cell Membrane Components: Proteins facilitate communication and transport across membranes.

• Enzymes: Proteins that catalyze biochemical reactions.

Enzymes: Biological Catalysts

Enzymes are proteins that act as catalysts, speeding up chemical reactions without being consumed. They are essential for metabolic processes, such as digestion and energy production.

• Catalyst: A substance that increases the rate of a reaction.

• Specificity: Enzymes are highly specific for their substrates.

• Example: Humans can digest starch and glycogen but not cellulose due to enzyme specificity.

Nucleic Acids

DNA and RNA: Structure and Function

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

• DNA: Contains instructions for producing RNA; double-stranded, with deoxyribose sugar.

• RNA: Contains instructions for producing proteins; single-stranded, with ribose sugar.

• Interrelation: DNA → RNA → Protein (central dogma of molecular biology).

Nucleotide Structure

Nucleic acids are composed of nucleotides, each consisting of:

• A five-carbon sugar (deoxyribose in DNA, ribose in RNA)

• One or more phosphate groups

• A nitrogenous base (single or double ring)

There are four nucleotides in DNA (adenine, thymine, cytosine, guanine) and four in RNA (adenine, cytosine, guanine, uracil).

DNA Structure

DNA is a double helix formed by two strands of nucleotides held together by hydrogen bonds between paired bases:

• Base Pairing: Adenine pairs with thymine; cytosine pairs with guanine.

• Backbone: Covalent bonds between sugars and phosphates.

• Hydrogen Bonds: Hold the two strands together.

RNA Structure

RNA is typically single-stranded and shorter than DNA. It contains ribose sugar and uracil instead of thymine.

• Function: Acts as a messenger and template for protein synthesis.

• Structure: Single strand, complementary to a segment of DNA.

ATP: The Universal Energy Carrier

Structure and Function of ATP

Adenosine triphosphate (ATP) is a nucleotide that serves as the primary energy carrier in cells. It is structurally similar to an RNA nucleotide but has three phosphate groups.

• Energy Storage: Energy is stored in the bonds between phosphate groups.

• Hydrolysis: Breaking the bond between the last two phosphates releases energy for cellular processes.

• ATP Synthesis: Energy for ATP synthesis comes from food or body stores.

Equation for ATP hydrolysis:

$\mathrm{ATP} + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{ADP} + \mathrm{P}_i + \text{energy}$

Summary Table: Comparison of Macromolecules

Macromolecule	Monomer	Main Function	Key Features
Protein	Amino acid	Structure, enzymes, transport	20 types of amino acids, peptide bonds
DNA	Nucleotide	Genetic information storage	Double helix, base pairing (A-T, C-G)
RNA	Nucleotide	Protein synthesis	Single strand, uracil replaces thymine
ATP	Nucleotide	Energy transfer	Three phosphate groups, hydrolysis releases energy