OCR A-Level Biology: Biological Molecules

Triglycerides are the principal long-term energy-storage molecule of many animals and plants.

Describe the structure of a triglyceride and explain how its structural features make it well-suited to long-term energy storage. In your answer you should refer to how a triglyceride is built from its subunits, the type of bond involved, and at least two properties that make it a better long-term store than a storage carbohydrate such as glycogen.

(6 marks)

A technician set up a quantitative Benedict's test to measure the concentration of the reducing sugar glucose in a fruit-juice sample. She prepared standard glucose solutions of known concentration, performed the Benedict's test on each under identical conditions, filtered off the brick-red precipitate, and measured the absorbance of the remaining solution in a colorimeter fitted with a red filter. Her calibration data are shown below.

Glucose concentration / g dm⁻³	Absorbance (arbitrary units)
0	0.80
2	0.66
4	0.52
6	0.38
8	0.24
10	0.10

The unknown fruit-juice sample, treated identically, gave an absorbance of 0.45.

(a) Use the calibration data to estimate the glucose concentration of the fruit-juice sample. Show your working. (3 marks)

(b) Explain why the absorbance decreases as the glucose concentration increases, and why filtering the precipitate is necessary before taking the reading. (3 marks)

A student tested four unknown solutions (W, X, Y and Z) using a series of biochemical tests. The "Benedict's (after hydrolysis)" column shows the result of repeating the Benedict's test after the sample had been boiled with dilute hydrochloric acid and then neutralised. The results are shown below.

Sample	Benedict's (direct)	Benedict's (after hydrolysis)	Iodine in KI	Biuret	Emulsion
W	Brick-red	Brick-red	Yellow-brown	Blue	Clear
X	Blue	Brick-red	Yellow-brown	Blue	Clear
Y	Blue	Blue	Blue-black	Blue	Clear
Z	Blue	Blue	Yellow-brown	Lilac/purple	Cloudy white

(a) Identify the biological molecule(s) present in each of samples W, X, Y and Z. (4 marks)

(b) Explain why sample X gave a negative direct Benedict's test but a positive Benedict's test after hydrolysis. (1 mark)

Researchers studying a jumping insect found a rubber-like structural protein, which they named springin, in the spring that powers its hind legs. They reported that springin:

• is made of several polypeptide chains joined together;
• has a repeating amino-acid sequence containing very few R-groups able to form ionic bonds or disulfide bridges;
• is held together mainly by a small number of covalent cross-links between chains;
• is insoluble in water.

The spring must stretch and recoil many thousands of times without breaking or permanently deforming.

Suggest and explain how the structural features of springin make it suited to this role. (5 marks)

A grower noticed that tomato plants grown in a sandy soil that had been heavily watered showed yellowing of the older leaves and stunted growth. A soil test showed that the soil was very low in both nitrate ions (NO₃⁻) and magnesium ions (Mg²⁺). Adding a fertiliser containing these ions restored normal green growth.

Using your knowledge of the roles of inorganic ions, explain how a shortage of nitrate ions and magnesium ions could cause the yellowing and stunted growth observed. (4 marks)

A phospholipid molecule is described as amphipathic. When phospholipids are mixed with water, they arrange themselves into a bilayer.

Explain why phospholipids form a bilayer when placed in water. In your answer refer to the hydrophilic and hydrophobic parts of the molecule. (3 marks)