BIOL1020 Chap.3 Carbohydrates, Lipids and Proteins
Carbohydrates, Lipids and Proteins
Macromolecule questions ask how chemistry becomes function. This chapter compares alpha and beta glycosidic linkages, branching in storage polysaccharides, hydrogen-bonded cellulose fibres, triacylglycerols, amphipathic phospholipids and the four levels of protein structure.
The recurring method is to identify a monomer or component, name the bond or interaction, describe the three-dimensional arrangement and predict a property. That method explains why related molecules can serve storage, structural, catalytic or membrane roles.
What this chapter covers
- 01
Glycosidic bond orientation
- 02
Starch, glycogen and cellulose
- 03
Triacylglycerol and phospholipid structure
- 04
Peptide bond formation
- 05
Four levels of protein structure
- 06
Side-chain chemistry and folding
Worked example · free
Explaining a destabilised protein core
- +1The substitution changes the primary amino-acid sequence.
- +1The new charged side chain interacts differently with the surrounding hydrophobic core and with water.
- +1The tertiary fold may rearrange or become less stable as the side chain seeks a more favourable environment.
- +1A changed active-site or binding-surface shape can alter protein activity.
Key terms
- Glycosidic Bond
- A covalent linkage joining sugar residues within a carbohydrate.
- Peptide Bond
- The covalent bond connecting the carboxyl group of one amino acid to the amino group of another.
- Primary Structure
- The ordered amino-acid sequence of a polypeptide.
- Tertiary Structure
- The overall three-dimensional fold produced largely by interactions among side chains.
Carbohydrates, Lipids and Proteins FAQ
Why can humans digest glycogen but not cellulose?
Glycogen uses alpha glycosidic linkages that human digestive enzymes can hydrolyse, while cellulose uses beta linkages in a different orientation. Beta-linked chains align into hydrogen-bonded fibres, and humans lack the enzyme needed to break that linkage efficiently.
Why do phospholipids form membranes but triacylglycerols do not?
Phospholipids have a polar head and non-polar tails, so they can expose water-compatible surfaces while hiding tails inside a bilayer. Triacylglycerols are dominated by three hydrophobic fatty-acid chains and collect into storage droplets rather than stable two-sided sheets.
How does primary structure control protein folding?
The amino-acid sequence determines where non-polar, polar and charged side chains occur. As the chain folds in water, those groups make different interactions, with many non-polar residues buried and water-compatible residues exposed. Their combined pattern selects the tertiary structure.
How do dehydration and hydrolysis differ?
Dehydration joins monomers by forming a covalent bond while releasing the elements of water. Hydrolysis reverses that relationship by using water to split a bond. Naming the reacting functional groups is more informative than merely saying that one process builds and the other breaks a polymer.
Why does branching change carbohydrate storage?
Branching creates many terminal sites where enzymes can add or remove glucose, allowing a storage polymer to be mobilised rapidly. It also changes how molecules pack. The explanation should connect glycosidic-bond pattern to three-dimensional organisation and then to access by the relevant enzymes.
How do unsaturated fatty acids alter packing?
A cis double bond introduces a bend into a fatty-acid tail, preventing neighbouring tails from packing as closely as straight saturated tails. Weaker packing generally increases membrane fluidity under comparable conditions. The causal chain is bond geometry, tail arrangement, intermolecular contact and membrane behaviour.
Why can one amino acid substitution alter activity?
A substitution changes side-chain chemistry at one position. If that position supports the hydrophobic core, an ionic interaction, a binding surface or the active site, altered contacts can shift folding or local geometry. The functional effect depends on location and chemistry, not simply on the existence of a mutation.
Why do phospholipids and triacylglycerols behave differently?
Both contain fatty-acid chains, but a phospholipid also has a polar head group and is amphipathic. It can orient at a water boundary and assemble into a bilayer. A triacylglycerol is predominantly non-polar, so it clusters as an energy-storage droplet rather than presenting stable hydrophilic faces to surrounding water.
Exam move
Revise macromolecules in matched pairs: starch versus cellulose, glycogen versus cellulose, triacylglycerol versus phospholipid, and primary versus tertiary protein structure. For each pair, complete four prompts: components, bonds, three-dimensional arrangement and function. Draw a simple dehydration reaction for a peptide or glycosidic bond and reverse it as hydrolysis.
When predicting a mutation, classify the changed side chain before discussing folding. Avoid vague words such as stronger unless you can name the bond, contact or packing arrangement. Finish by explaining one biological property from structure without using the molecule's name until the last sentence. Check that the final function follows from the named molecular arrangement rather than from memorised association.
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