Carbohydrates & lipids
Carbohydrates and lipids are two of the great families of biological molecules — the fuels and building materials of every living thing. In this lesson we start with the vocabulary of size: monomer, polymer and macromolecule, and the sugar words monosaccharide, disaccharide and polysaccharide. We draw the ring forms of α-glucose and β-glucose and see why one flipped hydroxyl group decides whether a molecule becomes a soft energy store or a tough fibre. We build the glycosidic bond by condensation and break it by hydrolysis, then meet the polysaccharides starch (amylose and amylopectin), glycogen and cellulose, relating each structure to its job — including how cellulose microfibrils strengthen plant cell walls. We turn to lipids: the triglyceride (glycerol plus three fatty acids joined by ester bonds) and the phospholipid with its hydrophilic head and hydrophobic tails. Finally we carry out the food tests you are examined on — Benedict's for reducing sugars, iodine for starch, the emulsion test for lipids, the semi-quantitative Benedict's test, and the non-reducing sugar test using acid hydrolysis.
By the end you should be able to (NSSCAS Biology (AS) 2.4.1):
- define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide
- describe and draw the ring forms of α-glucose and β-glucose
- describe the formation of a glycosidic bond by condensation, with reference both to polysaccharides and to disaccharides, including sucrose
- describe the breakage of glycosidic bonds in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test
- describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms
- describe the molecular structure of the polysaccharide cellulose and outline how the arrangement of cellulose molecules contributes to the function of plant cell walls
- describe the molecular structure of a triglyceride with reference to the formation of ester bonds and relate the structure of triglycerides to their functions in living organisms
- describe the structure of a phospholipid and relate the structure to the functions of phospholipids in living organisms
- describe and carry out the Benedict's test for reducing sugars, the iodine test for starch and the emulsion test for lipids
- describe and carry out a semi-quantitative Benedict's test on a reducing sugar solution by standardising the test and using the results (time to first colour change or comparison to colour standards) to estimate the concentration
- describe and carry out a test to identify the presence of non-reducing sugars, using acid hydrolysis and Benedict's solution
- carry out a semi-quantitative Benedict's test on a reducing sugar using dilution, standardising the test and using the results (colour standards or time to first colour change) to estimate the concentration