Biochemistry Lesson 2: Lipids and Biological Molecules

Overview of Macromolecules

• Macromolecules are large molecules consisting of repeated subunits that are covalently linked together.
• There are four primary types of biological macromolecules:
  • Carbohydrates
  • Lipids
  • Proteins
  • Nucleic Acids

Properties and Functions of Lipids

• General Examples: Lipids include fats, triglycerides, phospholipids, steroids, and waxes.
• Elemental Composition: Lipids contain carbon ($C$), hydrogen ($H$), and oxygen ($O$). They contain significantly less oxygen than carbohydrates, making them nonpolar molecules.
• Chemical Property (Hydrophobicity): Lipids are hydrophobic and do not dissolve in water. This characteristic is critical for their specific roles in cell function.
• Monomer-Polymer Relationship: Unlike other macromolecules, lipids do not follow a standard monomer and polymer relationship.
• Energy Density: Lipids contain twice as much energy per gram than carbohydrates.
• Biological Functions:
  • Long-term energy storage.
  • Serving as a source of less accessible energy.
  • Providing insulation and physical protection for organisms.

Triglycerides: Structure and Formation

• Components: A triglyceride consists of one glycerol molecule bonded to three fatty acids.
• Glycerol Structure: Glycerol is a three-carbon chain possessing three hydroxyl ($OH$) groups, with one group located at each carbon.
• Fatty Acid Structure: Fatty acids are hydrocarbon chains featuring a carboxyl ($COOH$) group at one end.
• Chemical Variation: A single fat molecule can consist of three identical fatty acids or a combination of different fatty acids bonded to the glycerol backbone.
• Formation Reaction:
  • The reaction occurs between the carboxyl group of each fatty acid and the hydroxyl group of the glycerol.
  • This process produces three water ($H_2O$) molecules.
  • The bond formed between the glycerol and the fatty acids is known as an ester linkage.

Classification and Impacts of Fatty Acids

• The shape of a fatty acid is directly related to its chemical bonds, and shape fundamentally affects function.

Saturated Fatty Acids

• Bonds: All carbons are connected by single bonds.
• Structure: The fatty acid chain is flat.
• Room Temperature State: Solid (e.g., lard and butter).
• Health Implications: Diets high in saturated fats are linked to heart disease in humans.

Unsaturated Fatty Acids

• Bonds: Contains at least one double bond between carbon atoms.
• Structure: There is a "kink" in the chain wherever a double bond occurs.
• Monounsaturated: Contains exactly one double bond.
• Polyunsaturated: Contains two or more double bonds.
• Cis double bonds: Naturally occurring unsaturated fatty acids with cis bonds are liquid at room temperature (e.g., olive oil and canola oil).
• Health Implications: Diets containing unsaturated fatty acids, specifically polyunsaturated ones, decrease the risk of heart disease.

Hydrogenation

• Hydrogenation is the industrial process of adding hydrogen to unsaturated fatty acids.
• This process converts naturally occurring cis bonds into trans bonds.

Phospholipids and the Lipid Bilayer

• Composition: A phospholipid consists of one glycerol molecule bonded to two fatty acids and a phosphate group. The phosphate group is also attached to an "R" group.
• Identification: The specific R group attached to the phosphate determines the identity of the phospholipid.
• Dual Character:
  • Hydrophilic Head: Formed by the phosphate group, the R group, and the glycerol. This part is attracted to water.
  • Hydrophobic Tails: Formed by the fatty acid backbones. These parts repel water.
• Cell Membrane Function: Phospholipids are the primary component of cell membranes, forming a double-sided wall called the lipid bilayer.
• Bilayer Arrangement: The hydrophilic heads face the aqueous environments (the interior and exterior of the cell), while the hydrophobic tails face each other, sequestered from the water.

Steroids and Waxes

Steroids

• Structure: Steroids consist of a four-carbon ring structure. While they share this base, they appear in different orientations.
• Functions and Categories:
  • Cholesterol: Found in the cell membranes of animals, present in animal blood, and acts as a precursor molecule for other steroids.
  • Hormones: Steroids like estrogen control the development of sexual traits.
  • Medical Applications: Used to decrease inflammation in the body. Examples include treatments for skin conditions like eczema and inclusion in inhalers for asthma or allergic reactions.
  • Signaling: Involved in hormonal signaling and the cell's response to the environment and growth.

Waxes

• Structure: Lipids consisting of long carbon-based chains or long fatty acid chains linked to alcohol or carbon rings. They are solid at room temperature.
• Plant Function: Waxes coat leaves to prevent water loss and provide a defense against insect damage.
• Animal Function: Found on the skin, fur, and feathers of animals, as well as the exoskeletons of insects, providing water resistance and protection.

Summary Table of Lipid Structure and Function

Questions & Discussion

Inquiry and Thought Prompts

• What do you think might happen to the bilayer if there were many unsaturated fatty acids in the phospholipids compared to saturated fatty acids?
• Can you think of any uses for steroids?
• Can you think of any examples of waxes?

Practice Questions

• Explain why lipids are efficient energy-storage molecules.
• Sketch and describe the basic structure of a triglyceride, and explain how the presence of double bonds affects its properties.
• What is meant by the dual character of a phospholipid molecule, and how is this essential to its function in living systems?