Carbon, Carbohydrates, and Lipids: Structure and Function in Biology

Carbon, Carbohydrates, and Lipids

Course Objectives

This section outlines the learning goals for understanding the chemistry and biology of carbon, carbohydrates, and lipids. Students should be able to describe the unique properties of carbon, explain how carbohydrates and lipids are assembled, and distinguish their structural and functional features.

• Describe unique properties of the carbon atom

• Explain how carbohydrates and lipids are assembled and disassembled

• Identify and distinguish features of carbohydrates (bonds, monomers, functional groups)

• Identify and distinguish features of lipids (key subunits, functional groups, saturated vs. unsaturated)

• Compare and contrast the structure and functions of carbohydrates and lipids

Properties of Carbon

Unique Chemical Properties

Carbon is the foundational element for organic molecules due to its versatile bonding capabilities. Its ability to form four covalent bonds allows for a diversity of molecular structures.

• Forms four covalent bonds with other atoms, including itself

• Can form double and triple bonds (e.g., ethene, ethyne)

• Creates long, stable hydrocarbon chains and rings

• Skeletons can vary by length, branching, double bond position, and ring presence

Example: Ethane (C2H6), Propane (C3H8), Cyclohexane (C6H12), Benzene (C6H6)

Functional Groups

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

• Hydroxyl group (–OH): Found in alcohols; forms hydrogen bonds

• Carbonyl group (>C=O): Found in ketones and aldehydes

• Carboxyl group (–COOH): Characteristic of organic acids

• Amino group (–NH2): Found in amines and amino acids

• Phosphate group (–OPO32–): Found in organic phosphates

Example: Estradiol and Testosterone differ by functional groups, affecting their biological activity.

Chemical Bonds in Biological Molecules

Types of Bonds

Biological molecules are held together by various types of chemical bonds, which influence their structure and function.

• Ionic bonds: Formed by the transfer of electrons (e.g., Na+ and Cl–)

• Covalent bonds: Formed by sharing electrons; can be polar or non-polar

• Hydrogen bonds: Weak attractions between polar molecules

Example: Water (H2O) forms hydrogen bonds due to its polarity.

Carbohydrates

Structure and Classification

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, typically with a ratio of 1:2:1. They serve as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose)

• Disaccharides: Two monosaccharides joined by a glycosidic bond (e.g., maltose, sucrose)

• Polysaccharides: Long chains of monosaccharides (e.g., starch, glycogen, cellulose, chitin)

Example: Glucose (C6H12O6) is a common monosaccharide.

Functional Groups in Carbohydrates

• Hydroxyl groups (–OH): Contribute to solubility and reactivity

• Carbonyl group (–C=O): Present in aldoses and ketoses

Formation and Breakdown of Carbohydrates

Carbohydrates are assembled and disassembled through dehydration synthesis and hydrolysis reactions.

• Dehydration synthesis: Joins monomers by removing water, forming glycosidic bonds

• Hydrolysis: Breaks polymers into monomers by adding water

Equation for Dehydration Synthesis:

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

Functions of Carbohydrates

• Energy storage: Starch (plants), glycogen (animals)

• Structural support: Cellulose (plants), chitin (arthropods)

Example: Cellulose provides rigidity to plant cell walls; glycogen stores energy in animal muscle tissue.

Lipids

Structure and Classification

Lipids are a diverse group of hydrophobic molecules that are not true polymers. They include fats, phospholipids, and steroids.

• Fats (triglycerides): Composed of glycerol and three fatty acids

• Phospholipids: Composed of glycerol, two fatty acids, and a phosphate group

• Steroids: Characterized by four fused rings (e.g., cholesterol)

Functional Groups in Lipids

• Carboxyl group (–COOH): Present in fatty acids

• Methyl group (–CH3): Common in hydrocarbon chains

• Phosphate group (–OPO32–): Present in phospholipids

Saturated vs. Unsaturated Fatty Acids

The presence or absence of double bonds in fatty acid chains determines their physical properties.

• Saturated fatty acids: No double bonds; straight chains; solid at room temperature

• Unsaturated fatty acids: One or more double bonds; kinked chains; liquid at room temperature

Example: Palmitic acid (saturated), oleic acid (unsaturated)

Functions of Lipids

• Energy storage: Fats store energy efficiently due to many C–H bonds

• Structural roles: Phospholipids form cell membranes; steroids serve as hormones

Comparison Table: Carbohydrates vs. Lipids

Feature	Carbohydrates	Lipids
Monomers	Monosaccharides	Fatty acids, glycerol
Polymerization	Form true polymers (polysaccharides)	Not true polymers
Functional Groups	Hydroxyl, carbonyl	Carboxyl, methyl, phosphate
Functions	Energy storage, structure	Energy storage, membranes, hormones
Solubility	Generally hydrophilic	Hydrophobic

Summary

Carbon's versatility enables the formation of diverse biological molecules. Carbohydrates and lipids are essential macromolecules with distinct structures, functional groups, and biological roles. Understanding their assembly, breakdown, and functions is fundamental to the study of biology.