AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Lipids: Triglycerides and phospholipids are two groups of lipid.

    Lipids contain carbon, hydrogen and oxygen, but with proportionally less oxygen than carbohydrates.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 1 of 8
    • 3 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Lipids contain carbon, hydrogen and oxygen, but with proportionally less oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents like ethanol. The specification names two groups: triglycerides and phospholipids. Triglycerides consist of one glycerol joined to three fatty acids. They are non-polar, hydrophobic molecules, making them ideal energy storage lipids in adipose tissue. Phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group. This structure makes them amphipathic: they have a hydrophilic phosphate head and hydrophobic fatty acid tails. This property is essential for forming the bilayer of cell membranes.

    What earns the marks

    • State that triglycerides consist of one glycerol joined to three fatty acids.
    • State that phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group.
    • Explain that triglycerides are non-polar and hydrophobic, acting as energy storage molecules.
    • Explain that phospholipids are amphipathic, allowing them to form the bilayer of cell membranes.
    • Identify that lipids contain carbon, hydrogen and oxygen, with proportionally less oxygen than carbohydrates.

    Common mistakes and how to put them right

    • Describing a triglyceride as a polymer of fatty acids.

      Put it right: Lipids are macromolecules but not polymers, as they are not made of repeating monomer units.

    • Stating that all lipids contain phosphorus.

      Put it right: Only phospholipids contain a phosphate-containing group; triglycerides contain only C, H, and O.

    • Saying lipids are insoluble in all solvents.

      Put it right: Lipids are insoluble in water but soluble in organic solvents such as ethanol.

    Exam tips

    • Fix the two structures by counting fatty acids: three means a triglyceride (storage), while two plus a phosphate-containing group means a phospholipid (membrane).
    • Never use the words monomer or polymer when describing lipids, as examiners frequently penalise this error.

    Try the lesson

    The start of the Lipids: Triglycerides and phospholipids are two groups of lipid. lesson, made from checked teaching notes for this specification point.

    Have a go first

    A student is classifying different biological molecules. Which of the following pairs consists entirely of two groups of lipid?

    Listen

    Lipids contain carbon, hydrogen and oxygen, but with proportionally less oxygen than carbohydrates. They are insoluble in water but soluble in organic solvents like ethanol. The specification names two groups: triglycerides and phospholipids. Triglycerides consist of one glycerol joined to three fatty acids. They are non-polar, hydrophobic molecules, making them ideal energy storage lipids in adipose tissue. Phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group. This structure makes them amphipathic: they have a hydrophilic phosphate head and hydrophobic fatty acid tails. This property is essential for forming the bilayer of cell membranes. A strong answer earns marks for these points. State that triglycerides consist of one glycerol joined to three fatty acids. State that phospholipids consist of one glycerol joined to two fatty acids and a phosphate-containing group. Explain that triglycerides are non-polar and hydrophobic, acting as energy storage molecules. Explain that phospholipids are amphipathic, allowing them to form the bilayer of cell membranes. A common slip: Describing a triglyceride as a polymer of fatty acids. Instead, Lipids are macromolecules but not polymers, as they are not made of repeating monomer units. Exam tip: Fix the two structures by counting fatty acids: three means a triglyceride (storage), while two plus a phosphate-containing group means a phospholipid (membrane).

    See it

    A top-down flowchart. At the top is a node labelled Lipids, which branches into two nodes: Triglycerides and Phospholipids. From Triglycerides, one arrow leads to a node reading One glycerol joined to three fatty acids, and another arrow leads to a node reading Non-polar and hydrophobic, which in turn leads to a node reading Energy storage in adipose tissue. From Phospholipids, one arrow leads to a node reading One glycerol joined to two fatty acids and a phosphate-containing group, and another arrow leads to a node reading Amphipathic: hydrophilic phosphate head and hydrophobic fatty acid tails, which in turn leads to a node reading Form the bilayer of cell membranes.
    It shows the two groups of lipid, their components, and how each structure links to its role in the cell.

    Key words

    Say what each word means before you turn the card.

    Triglyceride structure

    Try answering aloud or in your head before turning the card.

    Phospholipid structure

    Try answering aloud or in your head before turning the card.

    Hydrophobic and amphipathic properties

    Try answering aloud or in your head before turning the card.

    See every definition
    Triglyceride structure
    One glycerol joined to three fatty acids, forming a non-polar, hydrophobic molecule.
    Phospholipid structure
    One glycerol joined to two fatty acids and a phosphate-containing group.
    Hydrophobic and amphipathic properties
    Triglycerides are non-polar and hydrophobic, while phospholipids are amphipathic with a hydrophilic phosphate head and hydrophobic fatty acid tails.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. At the top is a node labelled Lipids, which branches into two nodes: Triglycerides and Phospholipids. From Triglycerides, one arrow leads to a node reading One glycerol joined to three fatty acids, and another arrow leads to a node reading Non-polar and hydrophobic, which in turn leads to a node reading Energy storage in adipose tissue. From Phospholipids, one arrow leads to a node reading One glycerol joined to two fatty acids and a phosphate-containing group, and another arrow leads to a node reading Amphipathic: hydrophilic phosphate head and hydrophobic fatty acid tails, which in turn leads to a node reading Form the bilayer of cell membranes.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

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