Chapter 5.3

Chapter 5.3: Lipids and Proteins

Overview

This chapter explores the structure, classification, and biological roles of lipids and proteins, two essential classes of biomolecules in living organisms. Lipids are hydrophobic molecules important for energy storage, membrane structure, and signaling, while proteins perform a vast array of functions including catalysis, transport, and structural support.

Lipids

Definition and General Properties

• Lipids are a diverse group of hydrophobic molecules that do not form polymers.

• The defining feature of lipids is their poor solubility in water due to their large proportion of hydrocarbon regions.

• Major biologically important lipids include fats (triglycerides), phospholipids, and steroids.

Fats (Triglycerides)

Fats are constructed from two types of smaller molecules: glycerol and fatty acids.

• Glycerol is a three-carbon alcohol, each carbon bearing a hydroxyl group (-OH).

• Fatty acid consists of a long hydrocarbon chain (typically 16–18 carbons) with a terminal carboxyl group (-COOH).

• Fats are formed by dehydration reactions (condensation), where each fatty acid is joined to glycerol by an ester linkage, releasing a water molecule per bond.

General reaction:

$\text{Glycerol} + 3\ \text{Fatty Acids} \rightarrow \text{Triglyceride} + 3\ \text{H}_2\text{O}$

Saturated vs. Unsaturated Fatty Acids

• Saturated fatty acids have no double bonds between carbon atoms; they are saturated with hydrogen atoms.

• Unsaturated fatty acids have one or more double bonds, causing kinks (bending) in the hydrocarbon chain.

• Saturated fats (e.g., butter) are solid at room temperature; unsaturated fats (e.g., olive oil) are liquid at room temperature.

• Cis double bonds in unsaturated fats prevent tight packing, contributing to their liquid state.

Table: Comparison of Saturated and Unsaturated Fats

Property	Saturated Fat	Unsaturated Fat
Double Bonds	None	One or more (cis or trans)
Physical State at Room Temp	Solid	Liquid
Sources	Animal fats (butter, lard)	Plant oils (olive oil, fish oil)
Health Impact	May contribute to cardiovascular disease	Generally considered healthier

Functions of Fats

• Major function is energy storage; fats store more than twice as much energy per gram as polysaccharides.

• Fats are stored in adipose tissue, which also insulates the body and cushions vital organs.

Phospholipids

Phospholipids are major components of cell membranes.

• Composed of two fatty acids and a phosphate group attached to glycerol.

• The fatty acid tails are hydrophobic, while the phosphate group (often with additional attachments) is hydrophilic.

• In water, phospholipids self-assemble into bilayers, forming the basic structure of biological membranes.

Example: The phospholipid bilayer forms the fundamental architecture of cell membranes, with hydrophobic tails facing inward and hydrophilic heads facing outward.

Steroids

• Steroids are lipids characterized by a carbon skeleton consisting of four fused rings.

• Cholesterol is a crucial steroid in animal cell membranes and a precursor for the synthesis of other steroids, including hormones.

• Excessive cholesterol can contribute to cardiovascular disease.

Example: Hormones such as testosterone and estrogen are derived from cholesterol.

Proteins

Definition and Functions

• Proteins are biologically functional molecules made up of one or more polypeptides, each folded and coiled into a specific three-dimensional structure.

• Proteins account for more than 50% of the dry mass of most cells.

• Functions include catalysis (enzymes), defense, storage, transport, cellular communication, movement, and structural support.

Table: Major Protein Functions and Examples

Function	Description	Example
Enzymatic	Selective acceleration of chemical reactions	Digestive enzymes
Defensive	Protection against disease	Antibodies
Storage	Storage of amino acids	Casein in milk, ovalbumin in egg white
Transport	Transport of substances	Hemoglobin transports oxygen
Hormonal	Coordination of organismal activities	Insulin regulates blood sugar
Receptor	Response to chemical stimuli	Receptors in nerve cells
Contractile and Motor	Movement	Actin and myosin in muscles
Structural	Support	Collagen in connective tissue

Amino Acids and Peptide Bonds

• Amino acids are organic molecules with an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group) attached to a central carbon.

• There are 20 different amino acids, each with unique properties due to their R groups.

• Amino acids are linked by peptide bonds (covalent bonds) to form polypeptides.

• A polypeptide has a unique linear sequence of amino acids, with an N-terminus (amino end) and a C-terminus (carboxyl end).

Peptide bond formation:

$\text{Amino Acid}_1 + \text{Amino Acid}_2 \rightarrow \text{Dipeptide} + \text{H}_2\text{O}$

Protein Structure and Function

• The specific activities of proteins result from their intricate three-dimensional architecture.

• A functional protein consists of one or more polypeptides precisely twisted, folded, and coiled into a unique shape.

• Protein structure is organized into four levels: primary, secondary, tertiary, and quaternary (not fully detailed in the provided notes).

Example: The enzyme amylase catalyzes the hydrolysis of starch into sugars, demonstrating the catalytic function of proteins.

Summary Table: Lipids vs. Proteins

Property	Lipids	Proteins
Monomer	Glycerol and fatty acids (not true polymers)	Amino acids
Bond Type	Ester linkage	Peptide bond
Main Functions	Energy storage, membrane structure, signaling	Catalysis, structure, transport, defense, etc.
Solubility	Hydrophobic	Varies (many are hydrophilic)

Additional info: Some details about protein structure (secondary, tertiary, quaternary) and lipid diversity (e.g., waxes, glycolipids) can be found in extended textbooks but are not included in these notes.