Carbohydrates, Lipids and ProteinsOxford AQA IGCSE Biology: Revision notes
Section 1
What are carbohydrates and how are they structured?
Carbohydrates are organic compounds made of carbon, hydrogen and oxygen in the general formula Cn(H2O)m. They are classified into two main types:
- Monosaccharides (simple sugars): Single sugar units with the formula C6H12O6, including glucose, fructose and galactose
- Polysaccharides (complex carbohydrates): Many monosaccharides joined together by glycosidic bonds
Glucose is a particularly important monosaccharide because it:
- Serves as a substrate for respiration, providing energy for cells
- Acts as the monomer (building block) for three major polysaccharides: starch, glycogen and cellulose
When monosaccharides join together, they form larger carbohydrate molecules through condensation reactions, releasing water molecules in the process. This allows cells to store energy efficiently and build structural components.
Think of glucose molecules like Lego bricks: individual bricks (monosaccharides) can be snapped together to build larger structures (polysaccharides). The same bricks can be arranged differently to make starch, glycogen or cellulose.
Examiners expect you to explain that glucose is both a substrate for respiration AND a monomer of three polysaccharides. Link these ideas together in your answer to show understanding.
Section 2
How are lipids structured and what do they do?
Lipids are organic compounds made of carbon, hydrogen and oxygen. Unlike carbohydrates, lipids contain much less oxygen relative to carbon and hydrogen, making them less soluble in water.
Structure of lipids: Lipids are composed of:
- Glycerol: A three-carbon molecule with three hydroxyl (-OH) groups
- Fatty acids: Long hydrocarbon chains (typically 16–18 carbons) with a carboxyl group (-COOH) at one end
One glycerol molecule bonds with three fatty acids through ester bonds (formed during condensation reactions) to create a triglyceride.
Functions of lipids:
| Function | Detail |
|---|---|
| Energy storage | Lipids store more than twice the energy per gram compared to carbohydrates; used when glucose is unavailable |
| Insulation | Stored under the skin as fat, reducing heat loss in warm-blooded animals |
| Cell membranes | Phospholipids form the bilayer structure of all cell membranes, controlling what enters and leaves cells |
Lipids are essential for survival and play a vital role in protecting organs and regulating body temperature.
Students often confuse lipids and carbohydrates. Remember: lipids have less oxygen, are insoluble in water, and store more energy per gram. Carbohydrates are soluble and have a 1:2 ratio of carbon to hydrogen.
A triglyceride is made when one glycerol molecule bonds with three fatty acids. During this process, three water molecules are released (condensation). This is why it's called a 'tri'-glyceride—three fatty acids attach.
Section 3
What are proteins and how are they made?
Proteins are polymers made up of many amino acid monomers linked together. There are approximately 20 different amino acids found in living organisms, and the order in which they are arranged determines a protein's specific structure and function.
How proteins are built:
- Amino acids contain carbon, hydrogen, oxygen and nitrogen (and sometimes sulphur)
- Amino acids join together through peptide bonds formed during condensation reactions
- A chain of amino acids is called a polypeptide
- One or more polypeptide chains fold into a three-dimensional structure to form a functional protein
The variety of proteins: The thousands of different proteins in living organisms are created from combinations of just 20 amino acids. The specific sequence of amino acids (the primary structure) determines how the protein folds and what it can do.
Proteins are the most structurally and functionally diverse molecules in cells, making them essential for virtually all biological processes.
Amino acids are like the 20 letters of an alphabet. Different sequences create different 'words' (proteins), each with a different meaning (function). The 26 letters of English create thousands of words in the same way.
Remember that proteins contain nitrogen while carbohydrates and lipids do not. Use this to distinguish proteins in exam questions: 'Proteins are polymers of amino acids joined by peptide bonds.'
Section 4
What do proteins do in living organisms?
Proteins have four main functions in living organisms:
| Function | Examples and Details |
|---|---|
| Enzymes | Biological catalysts that speed up metabolic reactions; each enzyme is specific to one substrate (e.g. amylase breaks down starch) |
| Structural proteins | Provide strength and support in cells and tissues (e.g. collagen in tendons, keratin in hair and nails) |
| Hormones | Chemical messengers that regulate body processes (e.g. insulin controls blood glucose, growth hormone promotes growth) |
| Antibodies | Immune proteins that recognise and destroy pathogens and foreign antigens; part of the immune response |
Other important protein functions include transport proteins (carrying oxygen or ions), storage proteins (storing amino acids), and contractile proteins (enabling muscle contraction).
The shape of a protein is crucial because it determines how it can function. An enzyme's active site must be the correct shape to fit its substrate. If a protein is denatured (its shape is permanently changed by heat or chemical damage), it can no longer perform its function.
When describing protein functions, link structure to function: 'Enzymes have a specific active site shaped to fit their substrate' or 'Antibodies have a variable region shaped to fit specific antigens.'
Section 5
How does protein structure relate to protein function? (Higher Tier)
The relationship between protein structure and protein function is fundamental to understanding how proteins work. This is tested at Higher Tier.
Levels of protein structure:
-
Primary structure: The sequence of amino acids in the polypeptide chain. This is determined by DNA and cannot be changed without mutation. Different sequences produce different proteins with different functions.
-
Secondary and tertiary structures: The polypeptide folds into three-dimensional shapes due to bonds between amino acids (hydrogen bonds, disulphide bonds). This 3D shape is critical for function.
-
Structure determines function:
- Enzymes: The 3D shape creates an active site that is complementary to a specific substrate. Only the correct shape allows substrate binding and catalysis.
- Antibodies: The variable regions of the protein are shaped to fit specific antigens. A change in shape means the antibody cannot recognise its target.
- Structural proteins: Their shape and strength (e.g. collagen's triple helix) provide mechanical support.
- Hormones: Their shape allows them to bind to specific receptors on target cells.
Consequences of structural change: If a protein is denatured, its three-dimensional shape is destroyed. Even if the primary structure (amino acid sequence) is unchanged, the protein cannot function. This is why high temperatures or extreme pH cause permanent loss of protein activity.
An enzyme like amylase has a specific active site shaped to fit starch molecules. If amylase is heated above its optimal temperature, hydrogen bonds break and the 3D shape collapses. The active site is destroyed, and even cooled amylase cannot bind starch anymore because its shape is permanently altered.
Higher Tier answers must explain HOW structure links to function, not just state that it does. Say: 'The amino acid sequence determines the 3D shape, which creates the active site, allowing the enzyme to catalyse its specific reaction.'
Must Know
- Carbohydrates are made of carbon, hydrogen and oxygen; monosaccharides (like glucose) join via glycosidic bonds to form polysaccharides (starch, glycogen, cellulose)
- Glucose is both a substrate for respiration and the monomer from which starch, glycogen and cellulose are made
- Lipids contain carbon, hydrogen and oxygen and are made of glycerol and fatty acids joined by ester bonds; they store more energy per gram than carbohydrates and provide insulation and form cell membranes
- Proteins are polymers of amino acids (which contain nitrogen) joined by peptide bonds; they function as enzymes, structural proteins, hormones and antibodies
- Protein structure determines function: the primary structure (amino acid sequence) determines 3D shape and function; denaturation destroys the 3D shape and permanently destroys function (HT)
- Enzymes work because their active site is shaped complementary to their specific substrate; antibodies work because their shape is complementary to specific antigens (HT)
That's the notes covered.
Carry on to the next subtopic.