AQA A-Level - Biology Biological molecules
AQA | A-Level | Biology | Biological molecules | Lesson 01
Monomers and polymers
AQA | A-Level | Biology | Biological molecules | Lesson 02
Carbohydrates
AQA | A-Level | Biology | Biological molecules | Lesson 03
Lipids
Lipids are a diverse group of organic compounds that are insoluble in water but soluble in non-polar solvents. They play crucial roles in biological systems, including energy storage, structural components of cell membranes, and signalling molecules. Here's a comprehensive overview of lipids.
1. Types of Lipids
- Triglycerides: Consist of one glycerol molecule bonded to three fatty acids. They are the main form of stored energy in animals.
- Phospholipids: Comprise two fatty acids, a phosphate group, and a glycerol molecule. They form the bilayer structure of cell membranes.
- Steroids: Characterized by a carbon skeleton with four fused rings. Cholesterol is a common steroid that stabilizes cell membranes and is a precursor for other steroids, including hormones.
- Waxes: Long-chain fatty acids bonded to long-chain alcohols or carbon rings. They provide protective coatings on plants and animals.
2. Structure and Properties
- Fatty Acids: Long hydrocarbon chains with a carboxyl group at one end. They can be:
- Saturated: No double bonds between carbon atoms; solid at room temperature.
- Unsaturated: One or more double bonds; liquid at room temperature.
- Glycerol: A three-carbon alcohol with hydroxyl groups attached to each carbon.
- Ester Bond Formation: Triglycerides are formed by ester bonds between glycerol and fatty acids through a condensation reaction (esterification).
3. Functions of Lipids
- Energy Storage: Triglycerides store energy efficiently due to their high caloric content.
- Insulation and Protection: Fat deposits provide thermal insulation and protect vital organs.
- Cell Membrane Structure: Phospholipids form the lipid bilayer, creating a semi-permeable membrane around cells.
- Signalling: Steroids and other lipids act as hormones and signaling molecules, influencing physiological processes.
4. Phospholipids and Cell Membranes
- Amphipathic Nature: Phospholipids have hydrophilic (water-attracting) heads and hydrophobic (water-repelling) tails.
- Bilayer Formation: In aqueous environments, phospholipids arrange themselves into a bilayer, with hydrophobic tails facing inward and hydrophilic heads facing outward.
- Fluid Mosaic Model: Describes the cell membrane as a dynamic structure with proteins and cholesterol embedded in or attached to the phospholipid bilayer.
5. Lipid Metabolism
- Digestion and Absorption: Lipids are broken down by enzymes such as lipases into fatty acids and glycerol, which are absorbed in the small intestine.
- Beta-Oxidation: Fatty acids are catabolized in the mitochondria to generate acetyl-CoA, which enters the Krebs cycle for ATP production.
- Lipogenesis: The process of synthesizing fatty acids from acetyl-CoA, typically occurring in the liver.
6. Health Implications
- Cholesterol: Essential for membrane structure but high levels can lead to cardiovascular diseases.
- Saturated and Trans Fats: Linked to increased risk of heart disease; found in processed foods and animal fats.
- Omega-3 and Omega-6 Fatty Acids: Essential fatty acids with anti-inflammatory properties, beneficial for heart health.
7. Laboratory Techniques
- Emulsion Test for Lipids: A qualitative test where lipids, when mixed with ethanol and then water, form a cloudy emulsion.
- Thin-Layer Chromatography (TLC): Used to separate and identify lipids based on their solubility and movement on a chromatographic plate.
Summary
Lipids are a vital class of biomolecules with diverse structures and functions, from energy storage and insulation to forming cell membranes and acting as signalling molecules. Understanding their roles and metabolism is crucial for comprehending many biological processes and their impact on health.
Review Questions:
- Explain the difference between saturated and unsaturated fatty acids.
- Describe the structure and function of phospholipids in cell membranes.
- What are the health implications of high cholesterol levels?
Practical Application:
Conduct an emulsion test to identify the presence of lipids in various food samples. This will reinforce the concept of lipid solubility and detection methods.
AQA | A-Level | Biology | Biological molecules | Lesson 04
Proteins
AQA | A-Level | Biology | Biological molecules | | Lesson 05
Nucleic acids are important information-carrying molecules
AQA | A-Level | Biology | Biological molecules | Lesson 06
ATP
AQA | A-Level | Biology | Biological molecules | Lesson 07
Water
AQA | A-Level | Biology | Biological molecules | | Lesson 08