The Molecules of Cells: Structure and Function of Biological Macromolecules

The Molecules of Cells

Overview

Cells are composed of a diverse array of molecules, many of which are large and complex. These molecules are essential for the structure and function of all living organisms. The study of these molecules focuses on their chemical composition, structure, and the roles they play in biological systems.

Molecular Diversity is Based on Carbon

The Role of Carbon in Biological Molecules

• Carbon is the backbone of most biological molecules due to its ability to form four covalent bonds with other atoms, including other carbon atoms.

• This versatility allows carbon to form chains, branched molecules, and rings, leading to a vast diversity of organic compounds.

• Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen. They are nonpolar and can store significant amounts of energy.

• Other elements commonly found in biological molecules include hydrogen (H), oxygen (O), and nitrogen (N).

Example: Methane (CH4) is the simplest hydrocarbon, while more complex hydrocarbons form the basis of fats and other macromolecules.

Functional Groups in Biological Molecules

Definition and Importance

• Functional groups are specific groups of atoms attached to carbon skeletons that confer particular chemical properties to molecules.

• Common functional groups include hydroxyl (-OH), carbonyl (C=O), carboxyl (-COOH), amino (-NH2), phosphate (-PO4), and methyl (-CH3).

• Functional groups are often polar, making molecules hydrophilic (water-soluble) and reactive in biological systems.

Example: The carboxyl group acts as an acid, while the amino group acts as a base in amino acids.

Formation and Breakdown of Polymers

Polymerization and Hydrolysis

• Polymers are large molecules made by joining many smaller units called monomers.

• Dehydration reactions (condensation reactions) link monomers together by removing a molecule of water, forming a covalent bond.

• Hydrolysis is the process of breaking down polymers into monomers by adding water, reversing the dehydration reaction.

Equation for Dehydration Reaction:

$\text{Monomer}_1 + \text{Monomer}_2 \xrightarrow{\text{Dehydration}} \text{Polymer} + H_2O$

Equation for Hydrolysis:

$\text{Polymer} + H_2O \xrightarrow{\text{Hydrolysis}} \text{Monomer}_1 + \text{Monomer}_2$

The Four Classes of Biological Macromolecules

Overview

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

Each class has unique monomers, structures, and functions in the cell.

Carbohydrates

Structure and Function

• Monosaccharides are the simplest carbohydrates (e.g., glucose, fructose), typically with the formula (CH2O)n.

• Disaccharides are formed by joining two monosaccharides (e.g., sucrose, maltose, lactose).

• Polysaccharides are long chains of monosaccharide units (e.g., starch, glycogen, cellulose).

• Carbohydrates serve as energy sources and structural materials.

Example: Glucose (C6H12O6) is a primary energy source for cells.

Comparison of Starch, Glycogen, and Cellulose

Polysaccharide	Function	Structure	Location
Starch	Energy storage	Branched/unbranched chains of glucose	Plants
Glycogen	Energy storage	Highly branched chains of glucose	Animals (liver, muscle)
Cellulose	Structural support	Unbranched chains, hydrogen-bonded	Plant cell walls

Lipids

Structure and Types

• Lipids are hydrophobic molecules, not true polymers, mainly composed of carbon and hydrogen.

• Major types include fats (triglycerides), phospholipids, and steroids.

Fats (Triglycerides)

• Composed of glycerol and three fatty acids linked by ester bonds.

• Function as energy storage molecules.

• Saturated fatty acids have no double bonds; solid at room temperature (e.g., butter).

• Unsaturated fatty acids have one or more double bonds; liquid at room temperature (e.g., vegetable oil).

Phospholipids

• Similar to fats, but one fatty acid is replaced by a phosphate group.

• Have hydrophilic (phosphate head) and hydrophobic (fatty acid tails) regions.

• Form the basis of cellular membranes (phospholipid bilayer).

Steroids

• Lipids with a carbon skeleton of four fused rings.

• Cholesterol is a common steroid and a precursor for other steroids, such as hormones.

Proteins

Structure and Function

• Proteins are polymers of amino acids, essential for structure and function in cells.

• Amino acids have a central carbon, amino group, carboxyl group, hydrogen atom, and variable R group.

• Amino acids are linked by peptide bonds formed via dehydration reactions.

Levels of Protein Structure

• Primary structure: Sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape of a polypeptide, stabilized by interactions between R groups (hydrogen bonds, ionic bonds, disulfide bridges).

• Quaternary structure: Association of multiple polypeptide subunits.

Denaturation is the loss of a protein's native structure due to changes in temperature, pH, or other environmental factors, resulting in loss of function.

Functions of Proteins

• Structural support (e.g., collagen, keratin)

• Movement (e.g., actin, myosin)

• Storage (e.g., ovalbumin)

• Defense (e.g., antibodies)

• Transport (e.g., hemoglobin)

• Signaling (e.g., hormones, neurotransmitters)

• Catalysis (e.g., enzymes)

Nucleic Acids

Structure and Function

• Nucleic acids are polymers of nucleotides, which store and transmit genetic information.

• Two main types: DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• Nucleotides consist of a phosphate group, a pentose sugar (deoxyribose in DNA, ribose in RNA), and a nitrogenous base.

• DNA bases: adenine (A), thymine (T), guanine (G), cytosine (C).

• RNA bases: adenine (A), uracil (U), guanine (G), cytosine (C).

Structure of DNA and RNA

• DNA is double-stranded, forming a double helix; strands are held together by hydrogen bonds between complementary bases (A with T, G with C).

• RNA is usually single-stranded.

The Central Dogma of Molecular Biology

• Genetic information flows from DNA to RNA to protein.

• Transcription: DNA is transcribed into messenger RNA (mRNA).

• Translation: mRNA is translated by ribosomes to synthesize proteins.

Example: The sequence of nucleotides in a gene determines the sequence of amino acids in a protein, which in turn determines the protein's structure and function.

Carbohydrates	MonSummary Table: Major Classes of Biological Moleculesosaccharide	Polysaccharide	Energy storage, structure
Lipids	Fatty acids, glycerol	Triglycerides, phospholipids, steroids	Energy storage, membranes, signaling
Proteins	Amino acid	Polypeptide	Structure, catalysis, transport, signaling
Nucleic Acids	Nucleotide	DNA, RNA	Genetic information storage and transfer