Biological molecules

• Condensation removes water and makes the bond; hydrolysis adds water and breaks it. Shape decides function: a coiled, branched store; a straight, cross-bonded fibre; a folded globular carrier.
• α below, β above. Condensation out, hydrolysis in. Coiled and branched to store; straight and hydrogen-bonded to support. Backbone for secondary, R groups for tertiary.

• 2.1.1 I can describe — describe and carry out the Benedict's test for reducing sugars, the iodine test for starch, the emulsion test for lipids and the biuret test for proteins
• 2.1.2 I can describe — 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
• 2.1.3 I can describe — describe and carry out a test to identify the presence of non-reducing sugars, using acid hydrolysis and Benedict's solution
• 2.2.1 I can describe — describe and draw the ring forms of α-glucose and β-glucose
• 2.2.2 I can define — define the terms monomer, polymer, macromolecule, monosaccharide, disaccharide and polysaccharide
• 2.2.3 I can state — state the role of covalent bonds in joining smaller molecules together to form polymers
• 2.2.4 I can state — state that glucose, fructose and maltose are reducing sugars and that sucrose is a non-reducing sugar
• 2.2.5 I can describe — describe the formation of a glycosidic bond by condensation, with reference to disaccharides, including sucrose, and polysaccharides
• 2.2.6 I can describe — describe the breakage of a glycosidic bond in polysaccharides and disaccharides by hydrolysis, with reference to the non-reducing sugar test
• 2.2.7 I can describe — describe the molecular structure of the polysaccharides starch (amylose and amylopectin) and glycogen and relate their structures to their functions in living organisms
• 2.2.8 I can describe — describe the molecular structure of the polysaccharide cellulose and outline how the arrangement of cellulose molecules contributes to the function of plant cell walls
• 2.2.9 I can state — state that triglycerides are non-polar hydrophobic molecules and describe the molecular structure of triglycerides with reference to fatty acids (saturated and unsaturated), glycerol and the formation of ester bonds
• 2.2.10 I can relate — relate the molecular structure of triglycerides to their functions in living organisms
• 2.2.11 I can describe — describe the molecular structure of phospholipids with reference to their hydrophilic (polar) phosphate heads and hydrophobic (non-polar) fatty acid tails
• 2.3.1 I can describe — describe and draw the general structure of an amino acid and the formation and breakage of a peptide bond
• 2.3.2 I can explain — explain the meaning of the terms primary structure, secondary structure, tertiary structure and quaternary structure of proteins
• 2.3.3 I can describe — describe the types of interaction that hold protein molecules in shape: • hydrophobic interactions • hydrogen bonding • ionic bonding • covalent bonding, including disulfide bonds
• 2.3.4 I can state — state that globular proteins are generally soluble and have physiological roles and fibrous proteins are generally insoluble and have structural roles
• 2.3.5 I can describe — describe the structure of a molecule of haemoglobin as an example of a globular protein, including the formation of its quaternary structure from two alpha (α) chains (α–globin), two beta (β) chains (β–globin) and a haem group
• 2.3.6 I can relate — relate the structure of haemoglobin to its function, including the importance of iron in the haem group
• 2.3.7 I can describe — describe the structure of a molecule of collagen as an example of a fibrous protein, and the arrangement of collagen molecules to form collagen fibres
• 2.3.8 I can relate — relate the structures of collagen molecules and collagen fibres to their function
• 2.4.1 I can explain — explain how hydrogen bonding occurs between water molecules and relate the properties of water to its roles in living organisms, limited to solvent action, high specific heat capacity and latent heat of vaporisation

The importance of iron. Oxygen binds to the iron in haem, not to the globin chains. The globin holds each haem in the right position, but without iron there is no binding site. A diet short of iron means less haemoglobin is made, so less oxygen is carried by the blood.

• “Benedict’s solution tests for glucose.” Correct Benedict’s solution detects any reducing sugar — glucose, fructose and maltose all give a positive result. Sucrose, a non-reducing sugar, gives none until it has been hydrolysed.
• “In α-glucose the OH on carbon 1 points up.” Correct In α-glucose the carbon 1 OH is below the ring, on the opposite side to the CH2OH; in β-glucose it is above, on the same side. Starch and glycogen are α; cellulose is β.
• “Condensation adds water to join the molecules.” Correct Condensation removes a molecule of water for every bond formed; hydrolysis adds one for every bond broken. A chain of n monomers has n − 1 bonds.
• “Cellulose is strong because its glycosidic bonds are strong.” Correct The strength comes from the hydrogen bonds between many parallel straight chains, which group into microfibrils and fibres laid in layers.
• “Secondary structure is held by bonds between R groups.” Correct The α-helix and β-pleated sheet are held by hydrogen bonds between the C=O and N–H groups of the backbone. Interactions between R groups hold the tertiary structure.
• “Oxygen binds to the globin chains of haemoglobin.” Correct Oxygen binds to the iron ion (Fe2+) in each of the four haem groups, so one molecule carries up to four O2.
• “A hydrogen bond in water is a bond between two hydrogen atoms.” Correct A hydrogen bond is the attraction between a δ+ hydrogen of one water molecule and the δ− oxygen of another.
• “Benedict’s is a test for glucose.” Repair It detects any reducing sugar; glucose, fructose and maltose all give a positive result.
• “Sucrose gives a positive Benedict’s test after heating.” Repair Sucrose is non-reducing. It must first be hydrolysed by boiling with dilute acid and then neutralised, because Benedict’s test works only in alkaline conditions.
• “A positive result after acid hydrolysis proves a non-reducing sugar.” Repair Only if the iodine test is negative: boiling acid hydrolyses starch to reducing sugars as well.
• Drawing α-glucose with the carbon 1 OH above the ring. Repair In α it is below, opposite the CH2OH; in β it is above.
• “Starch is made of β-glucose and cellulose of α-glucose.” Repair Starch and glycogen are α-glucose; cellulose is β-glucose.
• “Amylose is branched.” Repair Amylose is unbranched and helical; amylopectin and glycogen have α-1,6 branches.
• “Glycogen is a good store because it is soluble.” Repair It is insoluble, so it stays in the cell and has no effect on water potential.
• “Cellulose is strong because glycosidic bonds are strong.” Repair Its strength comes from hydrogen bonds between many parallel chains, grouped into microfibrils and fibres.
• “A triglyceride has one ester bond.” Repair Three, one per fatty acid, with three water molecules removed.
• “Unsaturated fatty acids have no double bonds.” Repair Unsaturated means one or more C=C; saturated means none.
• “Phospholipids have a hydrophobic head and hydrophilic tails.” Repair The other way round: the phosphate head is hydrophilic, the fatty acid tails hydrophobic.
• “Secondary structure is held by bonds between R groups.” Repair By hydrogen bonds between backbone C=O and N–H; R groups hold the tertiary structure.
• “Haemoglobin has one haem group.” Repair Four, one per chain, so it carries up to four O2.
• “Oxygen binds to the globin.” Repair To the Fe2+ in each haem group.
• “Collagen is strong because it has a quaternary structure.” Repair Name the features and the reasons: triple helix with hydrogen bonds, covalent cross-links between molecules, staggered ends, fibrils bundled into fibres.
• “Water heats up slowly because it is dense.” Repair Energy goes into breaking hydrogen bonds before the molecules gain kinetic energy, so the temperature rises slowly.

• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.
• Say “reducing sugar”, never “glucose”. A brick-red result shows that a reducing sugar is present; it does not tell you which one. Glucose, fructose and maltose all give it (2.2.4). Only Benedict’s test is heated: heating the iodine or biuret test is a method error.
• Interleave with the chapters that use this one. Topic 3 (enzymes are globular proteins, and denaturation is the loss of tertiary structure): re-answer why a change in pH alters a protein’s shape. Topic 4 (the phospholipid bilayer): re-answer why phospholipids form a bilayer in water. Topic 6 (the gene sets the primary structure): re-answer what primary structure is. Topic 8 (haemoglobin and oxygen): re-answer why the iron in haem matters. Topic 12 (respiratory substrates): re-answer why a triglyceride is an energy store. Recalling a topic inside a new context is worth more than another pass over this chapter on its own; at A Level, Paper 4 assumes the whole of the AS content, so nothing here is ever finished with.

• Cambridge International AS & A Level Biology 9700 has five components. Topic 2 is AS Level content, so it is examined in Papers 1, 2 and 3. AS Level content: examined in Paper 1 (multiple choice), Paper 2 (AS structured) and, as practical context, Paper 3. Assumed knowledge for Papers 4 and 5. AS Level candidates take Papers 1, 2 and 3; A Level candidates take all five, either staged over two years (Papers 1–3 in year one, Papers 4 and 5 in year two) or together in one series. Examinations are available in the June and November series, and in March in India.
• Across both the AS Level and the A Level the assessment objectives are weighted AO1 40% (knowledge and understanding), AO2 40% (handling, applying and evaluating information) and AO3 20% (experimental skills and investigations). AS candidates are graded a–e; A Level candidates A*–E. There is no data booklet in Biology. At A Level, the statistical formulae (Hardy–Weinberg, the Lincoln index, Simpson’s index, standard deviation, standard error, 95% confidence intervals, the χ² test, the t-test, and Pearson’s and Spearman’s correlation) are printed in a question when it needs them, and so are the tables of critical values; degrees of freedom you must work out yourself. Everything else — magnification, surface area to volume, RQ, Rf, rates — you must recall, and this chapter says which is which.
• A multiple-choice item on this topic can turn on one exact detail: which side of the ring the carbon 1 OH is on, whether a bond is 1,4 or 1,6, whether a sugar is reducing, which interaction a change in pH breaks. A structured question asks you to describe a structure, draw a ring or a peptide bond, explain how a structure suits a function, or identify a molecule from a set of test results.
• Structural diagrams of sugars, lipids and amino acids to recognise or complete; tables of test results to interpret; a colour scale or a time-to-colour-change graph to read an unknown from. The numbers are AS Level: rate = 1 / time, a proportional dilution, the number of bonds and water molecules in a chain, a relative molecular mass from supplied values, and energy from mass and a supplied specific heat capacity or latent heat.
• The syllabus names three practicals here: the four tests, the semi-quantitative Benedict’s test and the non-reducing sugar test. The independent variable is sugar concentration, made by proportional dilution; the dependent variable is the time to the first colour change or the colour after a fixed time; volumes, temperature and heating time are standardised; distilled water is the control; judging a colour by eye is the main random error.
• Read the command word before you decide how much to write. This syllabus has seventeen of them: assess, calculate, comment, compare, contrast, define, describe, determine, discuss, explain, give, identify, outline, predict, sketch, state and suggest. State, give and identify want a fact and nothing more. Define wants a precise meaning. Outline wants the main points only; describe wants the points or the features in full — and when you describe a graph, the trend with figures quoted from it. Explain wants the reasons and the mechanism — a describe-level answer to an explain question is incomplete however well written it is. Compare wants similarities and differences, each stated for both things side by side; contrast wants differences only. Discuss wants the issue written about in depth, in a structured way; assess wants an informed judgement. Suggest asks you to apply what you know to a situation where there is a range of valid responses, making proposals or putting forward considerations, so any sound biological reasoning is creditable.