In the following lessons we'll cover the structure, formation, function and key properties of triglycerides and phospholipids.

LipidsFats and Oils are composed from 2 types of molecule: -

  1. Glycerol

  2. Fatty acids

 

Condensation Reactions Form Lipids.

 

A “free” Hydroxyl group from the glycerol molecule joins to the OH on the COOH end of a fatty acid. This condensation reaction can also be called “Esterification” due to the resulting bond formed between the glycerol and the fatty acid being called an Ester bond.

 

Fatty Acids can be represented in many ways. You should be able to recognise and describe the type of fatty acid being shown.

 

Remember, saturated fatty acids have NO C-C double bonds.

However, unsaturated fatty acids may have 1 or more C-C double bonds hence they are unsaturated, since the hydrocarbon tail is not completely ‘full’ (saturated) with Hydrogens.

 

Condensation reactions build triglycerides.

 

  • 1 glycerol + 1 fatty acid = 1 monoglyceride + 1 molecule1 of Water.

  • 1 glycerol + 2 fatty acids = 1 diglyceride + 2 molecules of Water.

  • 1 glycerol + 3 fatty acids = 1 triglyceride + 3 molecules of Water.

 

If one of the fatty acids is replaced with a phosphate group, the molecule is a phospholipid.

A Level Biology: Lipids - The Structure & Formation of Triglycerides.

 

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson “Lipids: Triglycerides structure and formation” we begin with an overview of the learning outcomes as usual. Next we emphasise that Lipids are not polymers! From here we go on to describe the structure of triglycerides and the structure of glycerol. Glycerol is attached to fatty acids (which are either saturated or unsaturated) and it is essential you can describe and identify these molecules which make up lipids (triglycerides). The Hydrocarbon tail of lipids is discussed along with the fact that all fatty acids have the functional group COOH (the Carboxyl Group). A Recap of condensation reactions shows exactly how triglycerides are ‘built’ - again another common and by now familiar concept frequently covered and examined in A-Level biology. Glycerol is linked to fatty acids by an ester bond (you’ll need to know this and sometimes be asked to ‘label it up’ in a biology exam)…

So, once again, when you’re confident that you can describe triglycerides and you understand the structure and formation of them (condensation reactions between glycerol and fatty acids) it's time for you to complete the accompanying knowledge check questions. As Always, you’ll be able to check your answers with mine written in the back of the work book and see exactly how you should write your answers in a way that gains maximum marks in the exams!

A-Level biology lipids triglycerides structure and formation Revision Notes with Knowledge Check Questions and Answers pdf
A-Level biology lipids triglycerides structure and formation Revision Notes Poster A3 PDF.

A Level Biology: Lipids - The Properties of Triglycerides.

 

Check Where this Lesson fits into your Exam Specification!

In This A-Level Biology Lesson “Lipids: The Properties of Triglycerides” the learning outcomes lay out the key points covered in the lesson as usual. Then we discuss how the structure of triglycerides are related to their functions - a quick re-emphasis of the point that lipids are not polymers and we move on to the fact that lipids are hydrophobic molecules. The lesson progresses with and outline of why lipids are useful energy storage molecules, lipids are waterproofing agents, lipids are great insulators and protectors and lipids play important roles as steroids. When you’re super confident that you can relate the structure and function of triglycerides to their many functions in nature then you’re ready to have a go at the knowledge check questions. As always, you’ll be able to check your answers with mine written in the back of the work book and see exactly how you should write your answers in a way that gains maximum marks in the exams!

A-Level biology lipids: The Properties of triglycerides Revision Notes with Knowledge Check Questions and Answers pdf
A-Level biology lipids: The Properties of triglycerides Revision Notes Poster A3 PDF.

A-Level Biology "Lipids: The Properties of Triglycerides"

The structure of triglycerides is related to their functions. For example, the long hydrocarbon tail of the fatty acid part of “fats” is an important source of energy. In fact, fatty acids contain around twice as much energy per gram than do carbohydrates.

 

Remember! Lipids are NOT considered Polymers (Why? Because they are NOT composed of many repeating monomers).

 

Lipids are made up from: - Glycerol + Fatty acids.

 

Fats and oils are insoluble in water, and this hydrophobic nature of lipids allows them to “bundle” together forming fat droplets. It is also yet another reason fat is a great energy storage molecule, since they don’t affect the water potential of cells.

 

The hydrophobic properties of lipids make them useful waterproofing agents, e.g. bird feathers, leaves the waxy exoskeletons of many insects.

 

Fats have a low density which makes them useful for buoyancy, insulation and protection.

Lipids play important roles as steroids (e.g. Cholesterol).

 

Lipids are useful respiratory substrates. During Cellular Respiration, lipids are oxidised and the bonds of the hydrocarbon tail are hydrolysed – the products of which enter the ‘link reaction’ to produce ATP.

A Level Biology: Lipids - The Properties of Phospholipids

 

Check Where this Lesson fits into your Exam Specification!

In this A-Level Biology Lesson “Lipids: The Properties of Phospholipids” following an outline of the learning outcomes we’ll learn all about Phospholipids (The 2nd Lipid you need to know all about!). Phospholipids are similar to triglycerides (having a glycerol with fatty acids attached - but here we’ll look at the The Structure of a Phospholipid - a structure you should be able to recognise and label up under exam conditions (the simple version as shown below that is!).

 

You’ll learn that Phospholipids are Ambivalent Molecules and that Phospholipids form monolayers (called micelles). Finally we’ll see that Phospholipids are super important biomolecules in living organisms as Phospholipids form bilayers, that is to say in living organisms Phospholipids form Cell Membranes!

When you’ve finished learning all about Phospholipid structure and function you'll be ready to download the knowledge check PDF - quickly re-cap the notes and test your knowledge with the knowledge check questions. When’ve you answered them make sure you check your answers with mine which are written out in full in the back of the work book (no rubbish one word mark schemes here!). So, you can see exactly how you should write your answers in a way that gains maximum marks in the exams!

A-Level biology Lipids: Phospholipids Revision Notes with Knowledge Check Questions and Answers pdf
A-Level biology Lipids: Phospholipids Revision Notes Poster A3 PDF.

A-Level Biology "Lipids: The Properties of Phospholipids"

Phospholipids are similar to triglycerides, but rather than having 3 fatty acids attached to glycerol, Phospholipids have 2 fatty acids and a phosphate group.

 

Phospholipids are fundamental molecules in all living organisms!

 

Phospholipids are composed of the hydrophilic (water loving) phosphate head and the hydrophobic (water hating) fatty acid tails.

The Structure of a Phospholipid (A-Level Biology)
The Structure of a Phospholipid (A-Level Biology)
 

Phospholipids are ambivalent (i.e. a molecule that has different properties when placed in water). Because of this, when placed in water phospholipids spontaneously form monolayers known as micelles.

As we can see, micelles orientate themselves so that the hydrophilic phosphate head of the phospholipid is “facing outwards” dissolved in the water and protecting the hydrophobic fatty acid tails which are orientated “inwards”.

Phospholipids from Micelles

In living organisms, Phospholipids form Bilayers!

 

The Phospholipid bilayer is the fundamental structure of all cell membranes. 

 

The Phospholipid bilayer separates the internal cellular environment from the external cellular environment.

 
In Living Organisms Phospholipids form Cell Membranes
 
Check Your Exam Specification
 

AQA Specification Reference: - 3.1.3 Lipids. Triglycerides and phospholipids are two groups of lipid. Triglycerides are formed by the condensation of one molecule of glycerol and three molecules of fatty acid. A condensation reaction between glycerol and a fatty acid (R-COOH) forms an ester bond. The R-group of a fatty acid may be saturated or unsaturated.

In phospholipids, one of the fatty acids of a triglyceride is substituted by a phosphate-containing group. Students should be able to recognise, from diagrams, saturated and unsaturated fatty acids. Students should be able to explain the different properties of triglycerides and Phospholipids.

 

CIE Specification Reference: - 2 Biological molecules - 2.2 Carbohydrates and lipids. Describe the molecular structure of a triglyceride with reference to the formation of ester bonds. Must be able to relate the structure of triglycerides to their functions in living organisms. Lipids have important roles in the provision and storage of energy, and for a variety of other functions, such as providing barriers around cells: the phospholipid bilayer. Describe the structure of a phospholipid and relate the structure of phospholipids to their functions in living organisms.

 

Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 1: Lifestyle, Health and Risk. 1.14 i) Know how a triglyceride is synthesised by the formation of ester bonds during condensation reactions between glycerol and three fatty acids. ii) Know the differences between saturated and unsaturated lipids. Understand how the structure of lipids relates to their role in energy storage, waterproofing and insulation. Know the structure and properties of cell membranes.

 

Edexcel (Biology B) Specification Reference: - Topic 1: Biological Molecules. 1.2 Lipids i) Understand how a triglyceride is synthesised, including the formation of ester bonds during condensation reactions between glycerol and three fatty acids. ii) Know the differences between saturated and unsaturated lipids. Understand how the structure of lipids relates to their role in energy storage, waterproofing and insulation. Understand how the structure and properties of phospholipids relate to their function in cell membranes.

 

OCR (Biology A) Specification Reference: - 2.1.2 Biological molecules. Learners should be able to demonstrate and apply their knowledge and understanding of: The structure of a triglyceride. To include an outline of saturated and unsaturated fatty acids. The synthesis and breakdown of triglycerides by the formation (esterification) and breakage of ester bonds between fatty acids and glycerol. Learners should be able to explain how the properties of triglycerides relate to their functions in living organisms and how the properties of phospholipids relate to their functions in living organisms.

 

OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange. (j) The structure of triglycerides and phospholipids. To include reference to fatty acids, glycerol, phosphate groups, ester bonds and hydrophobic/ hydrophilic properties. how the plasma membrane is composed of modified lipids and how the structure of phospholipids is related to their functions. The fluid mosaic model of the typical plasma membrane, to include the location and function of phospholipids.

 

WJEC Specification Reference: - Core Concepts 1. Chemical elements are joined together to form biological compounds: - (f) the structure, properties and functions of lipids as illustrated by triglycerides and phospholipids.

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