Carbohydrates and lipids

Key definitions

· Monomer = small molecule that can join with others to form a larger molecule.
· Polymer = large molecule made from many repeating monomers.
· Macromolecule = very large molecule, often a polymer, e.g. starch, glycogen, cellulose.
· Monosaccharide = single sugar unit, e.g. glucose and fructose.
· Disaccharide = two monosaccharides joined by a glycosidic bond, e.g. maltose and sucrose.
· Polysaccharide = many monosaccharides joined by glycosidic bonds, e.g. starch, glycogen, cellulose.
· Covalent bonds join smaller biological molecules together to form larger molecules and polymers.

α-glucose and β-glucose

· Glucose is a monosaccharide and a reducing sugar.
· α-glucose and β-glucose are ring forms of glucose.
· In α-glucose, the OH group on carbon 1 is below the ring.
· In β-glucose, the OH group on carbon 1 is above the ring.
· This small structural difference is important because it affects the type of polymer formed: α-glucose → starch/glycogen, β-glucose → cellulose.
· Students must be able to describe and draw the ring forms of α-glucose and β-glucose.

This diagram compares the two ring forms of glucose. The key exam point is the position of the OH group on carbon 1, which distinguishes α-glucose from β-glucose. Source

Reducing and non-reducing sugars

· Glucose, fructose and maltose are reducing sugars.
· Sucrose is a non-reducing sugar.
· Non-reducing sugar test links to this topic because acid hydrolysis breaks glycosidic bonds, releasing reducing sugars that can then react with Benedict’s solution.
· Exam tip: do not say all disaccharides are non-reducing; maltose is reducing, but sucrose is non-reducing.

Condensation, hydrolysis and glycosidic bonds

· Condensation reaction = two molecules join together with the removal of water.
· A glycosidic bond forms between monosaccharides during condensation.
· Example: glucose + glucose → maltose + water.
· Example: glucose + fructose → sucrose + water.
· Hydrolysis reaction = bond is broken by the addition of water.
· Hydrolysis breaks glycosidic bonds in disaccharides and polysaccharides.
· In the non-reducing sugar test, acid hydrolysis breaks sucrose into reducing monosaccharides.

This diagram shows how monosaccharides are joined in disaccharides. For CIE, focus on maltose, sucrose and the formation of glycosidic bonds by condensation. Source

Starch

· Starch is a storage polysaccharide in plants.
· Made from α-glucose monomers joined by glycosidic bonds.
· Has two forms: amylose and amylopectin.
· Amylose = long, unbranched chain of α-glucose; coils into a compact helix.
· Amylopectin = branched chain of α-glucose.
· Structure-function links:
· Compact shape allows lots of glucose to be stored in a small space.
· Insoluble so it does not affect water potential.
· Branched amylopectin provides many ends for rapid enzyme action and glucose release.

Glycogen

· Glycogen is a storage polysaccharide in animals and fungi.
· Made from α-glucose monomers joined by glycosidic bonds.
· Similar to amylopectin but more highly branched.
· Structure-function links:
· Very compact for efficient storage.
· Insoluble so it does not affect water potential.
· Many branches allow rapid hydrolysis to release glucose for respiration.
· Glycogen is especially useful in tissues with high energy demand, e.g. liver and muscle.

This diagram compares the branching patterns of major polysaccharides. It is useful for linking starch and glycogen structure to their storage functions. Source

Cellulose

· Cellulose is a structural polysaccharide in plant cell walls.
· Made from β-glucose monomers.
· Alternate β-glucose molecules are rotated 180°, forming long, straight chains.
· Many cellulose chains are held together by hydrogen bonds to form strong microfibrils.
· Microfibrils group together to form strong fibres in the plant cell wall.
· Structure-function links:
· Straight chains allow close packing.
· Hydrogen bonding between chains gives high tensile strength.
· Cellulose helps plant cell walls resist turgor pressure and prevents cells bursting.

This diagram shows how plant cell wall structure supports plant cells. Link it to cellulose forming strong fibres that resist pressure and maintain cell shape. Source

Triglycerides

· Triglycerides are lipids made from one glycerol molecule and three fatty acids.
· Fatty acids join to glycerol by ester bonds.
· Each ester bond forms by condensation, releasing water.
· Triglycerides are non-polar and hydrophobic, so they are insoluble in water.
· Saturated fatty acids have no carbon-carbon double bonds.
· Unsaturated fatty acids have one or more carbon-carbon double bonds, causing bends/kinks in chains.
· Structure-function links:
· High ratio of C-H bonds makes triglycerides a good energy store.
· Insoluble so they do not affect water potential.
· Hydrophobic so they can be stored as droplets.
· Useful for thermal insulation, protection around organs and buoyancy in some organisms.
· Exam tip: triglycerides are not polymers because they are not made of repeating identical monomers.

This diagram shows the basic structure of a triglyceride. Use it to identify glycerol, fatty acids and the idea that unsaturated fatty acids contain double bonds that create kinks. Source

Phospholipids

· Phospholipids are lipids with a hydrophilic phosphate head and hydrophobic fatty acid tails.
· The phosphate head is polar and interacts with water.
· The fatty acid tails are non-polar and avoid water.
· This makes phospholipids amphipathic: they have both hydrophilic and hydrophobic regions.
· Phospholipids form bilayers in cell membranes because the heads face water and the tails face inward away from water.
· This links directly to Topic 4: cell membranes, where phospholipids explain membrane structure and selective permeability.

This diagram shows why phospholipids form bilayers in membranes. The polar phosphate head interacts with water, while the non-polar fatty acid tails avoid water. Source