02.08 Alcohols and Esters
Alcohols
- Definition: Organic molecules containing one or more hydroxyl groups (–OH) attached to carbon atoms.
- Example: Glycerol is a simple alcohol with three hydroxyl groups, making it an important building block in lipid formation.
Esters and Ester Bonds
- Definition: Esters are formed by a reaction between an acid and an alcohol.
- The bond created between the acid and alcohol is known as an ester bond (–COO–).
- Reaction Type:
- This reaction is a condensation reaction, as a water molecule (H₂O) is produced as a by-product.
- Reverse Reaction (Hydrolysis): Esters can be broken down into the original acid and alcohol by adding water.
Formation of Triglycerides
- Triglyceride Structure:
- A triglyceride is a type of lipid formed when glycerol (an alcohol) bonds with three fatty acids.
- Each fatty acid bonds with one of glycerol’s hydroxyl groups, creating three ester bonds in total.
Process:
- Glycerol reacts with three fatty acids.
- Each –COOH (carboxyl) group on a fatty acid reacts with a –OH (hydroxyl) group on glycerol, forming an ester bond and releasing water.
- The resulting molecule, a triglyceride, has three hydrocarbon tails attached to a glycerol “head.”
- Key Points:
- Condensation Reaction: Water is produced during the formation of each ester bond.
- Hydrolysis: Triglycerides can be broken down back into glycerol and fatty acids by adding water, an essential process in lipid digestion and metabolism.
Practise Questions
Question 1
Describe the structure of glycerol and its role in triglyceride formation. (5 marks)
Mark Scheme:
- Glycerol is a simple alcohol with three carbon atoms. (1 mark)
- Each carbon atom is attached to a hydroxyl group (–OH). (1 mark)
- Glycerol serves as the alcohol component in triglyceride formation. (1 mark)
- Each hydroxyl group reacts with the carboxyl group (–COOH) of a fatty acid, forming an ester bond. (1 mark)
- This process results in a triglyceride, with three fatty acid tails attached to the glycerol backbone. (1 mark)
Question 2
What is an ester bond, and how is it formed? (5 marks)
Mark Scheme:
- An ester bond is a covalent bond formed between the hydroxyl group (–OH) of an alcohol and the carboxyl group (–COOH) of an acid. (1 mark)
- It is created during a condensation reaction, where a molecule of water (H₂O) is released. (1 mark)
- Example: In triglyceride formation, ester bonds connect fatty acids to glycerol. (1 mark)
- Each fatty acid reacts with a hydroxyl group of glycerol, forming three ester bonds in a triglyceride. (1 mark)
- Ester bonds can be broken down via hydrolysis, where water is added to reverse the reaction. (1 mark)
Question 3
Explain the process of triglyceride formation and identify the type of reaction involved. (6 marks)
Mark Scheme:
- Triglycerides form when glycerol reacts with three fatty acids. (1 mark)
- Each fatty acid’s carboxyl group (–COOH) reacts with a hydroxyl group (–OH) of glycerol. (1 mark)
- This reaction forms an ester bond and releases a water molecule, a type of condensation reaction. (1 mark)
- In total, three ester bonds and three water molecules are produced per triglyceride molecule. (1 mark)
- The resulting triglyceride consists of three hydrocarbon tails (from fatty acids) attached to a glycerol backbone. (1 mark)
- The reaction is essential for lipid synthesis, providing a compact and efficient energy storage molecule. (1 mark)
Question 4
Describe the role of hydrolysis in triglyceride metabolism. (6 marks)
Mark Scheme:
- Hydrolysis is the process of breaking down triglycerides into their components: glycerol and fatty acids. (1 mark)
- Water is added, breaking the ester bonds between glycerol and fatty acids. (1 mark)
- Enzymes like lipase catalyze this reaction during digestion. (1 mark)
- The released fatty acids can be used for energy production via beta-oxidation. (1 mark)
- Glycerol is converted into intermediates of glycolysis or gluconeogenesis. (1 mark)
- Hydrolysis is crucial for accessing stored energy in adipose tissue during fasting or exercise. (1 mark)
Question 5
Compare the processes of condensation and hydrolysis in lipid metabolism. (6 marks)
Mark Scheme:
| Feature | Condensation | Hydrolysis |
|---|---|---|
| Definition | Joins molecules by removing water. | Breaks molecules by adding water. |
| Reaction | Formation of ester bonds between glycerol and fatty acids. | Breakdown of ester bonds in triglycerides. |
| Products | Triglycerides and water. | Glycerol and fatty acids. |
| Enzymes | Synthase enzymes catalyze lipid formation. | Lipase enzymes catalyze lipid breakdown. |
| Energy | Stores energy in triglycerides. | Releases energy for metabolism. |
| Biological Role | Lipid synthesis for energy storage. | Lipid digestion and energy release. |
Question 6
Explain the significance of triglycerides in energy storage. (6 marks)
Mark Scheme:
- Triglycerides are efficient energy storage molecules due to their high energy density. (1 mark)
- They contain long hydrocarbon chains that yield a large amount of ATP upon oxidation. (1 mark)
- Being hydrophobic, triglycerides do not attract water, making them compact and lightweight for storage. (1 mark)
- Stored in adipose tissue, they provide long-term energy reserves for organisms. (1 mark)
- During fasting or exercise, triglycerides are hydrolyzed into fatty acids and glycerol, which are metabolized for energy. (1 mark)
- They also serve as insulation and protection for vital organs in animals. (1 mark)
Question 7
Why is glycerol important in both triglyceride formation and metabolism? (5 marks)
Mark Scheme:
- Glycerol acts as the alcohol component in triglycerides, forming ester bonds with fatty acids. (1 mark)
- It provides a structural backbone for the triglyceride molecule. (1 mark)
- During metabolism, glycerol is released through hydrolysis of triglycerides. (1 mark)
- Glycerol can enter the glycolysis pathway as glyceraldehyde-3-phosphate, contributing to ATP production. (1 mark)
- Alternatively, glycerol can be used in gluconeogenesis to produce glucose during fasting. (1 mark)
Question 8
What role do ester bonds play in lipid structure and function? (5 marks)
Mark Scheme:
- Ester bonds link the fatty acid tails to the glycerol backbone in triglycerides. (1 mark)
- They are formed during condensation reactions, which release water. (1 mark)
- The bonds are strong, providing stability to the triglyceride molecule. (1 mark)
- During lipid metabolism, ester bonds are broken by hydrolysis, releasing fatty acids and glycerol for energy use. (1 mark)
- The formation and breakdown of ester bonds are crucial for lipid synthesis, storage, and energy release. (1 mark)
Question 9
Explain how the structure of triglycerides makes them suitable for energy storage. (5 marks)
Mark Scheme:
- Triglycerides have three long hydrocarbon tails, which are energy-rich due to numerous C–H bonds. (1 mark)
- These bonds release a large amount of energy when oxidized during metabolism. (1 mark)
- Triglycerides are hydrophobic, allowing them to be stored without water, reducing their weight. (1 mark)
- They are stored in compact forms within adipose tissue, maximizing energy density. (1 mark)
- This structure makes them ideal for long-term energy reserves compared to carbohydrates, which store less energy per gram. (1 mark)
Question 10
Describe the biological importance of hydrolysis in triglyceride digestion. (6 marks)
Mark Scheme:
- Hydrolysis breaks triglycerides into glycerol and fatty acids. (1 mark)
- This reaction is catalyzed by enzymes like lipase in the digestive system. (1 mark)
- Fatty acids are absorbed and metabolized via beta-oxidation to generate ATP. (1 mark)
- Glycerol enters the glycolysis or gluconeogenesis pathways, providing additional energy or glucose. (1 mark)
- Hydrolysis allows stored triglycerides in adipose tissue to be mobilized during fasting or high energy demand. (1 mark)
- This process ensures a continuous energy supply when carbohydrates are unavailable. (1 mark)