Edexcel A-Level Biology: Synoptic & Practical Skills

During strenuous exercise, the skeletal muscles of a mammal respire rapidly and the oxygen they receive from haemoglobin must increase sharply.

Explain how the structure of haemoglobin, together with the conditions generated by respiration in exercising muscle, allows oxygen to be unloaded efficiently where it is needed. In your answer you should link the structure of the haemoglobin molecule to the products and conditions of respiration in the working tissue.

(9 marks)

A farmer is advised that, to produce the most human food energy from a fixed area of land, growing a cereal crop and eating it directly is always far more efficient than feeding that crop to cattle and eating the beef.

A study on one mixed farm collected the data below for a single growing season on equal land areas.

Pathway	Net primary production of crop / MJ per m² per year	Energy reaching humans / MJ per m² per year
Crop eaten directly by humans	30.0	18.0
Crop fed to cattle, beef eaten by humans	30.0	1.2

Evaluate the claim that growing crops for direct human consumption is always the better choice. In your answer you should use the data and draw on your knowledge of energy transfer through ecosystems and at least one other relevant area of biology.

(9 marks)

In a dihybrid cross between two heterozygous pea plants, the offspring were expected to show four phenotypes in a 9:3:3:1 ratio if the two genes assort independently. A student scored 320 offspring and recorded the numbers below.

Phenotype	Observed number (O)	Expected ratio
Tall, round seed	165	9
Tall, wrinkled seed	70	3
Short, round seed	62	3
Short, wrinkled seed	23	1

A chi-squared test is used to compare the observed and expected results, where

$\chi^2 = \sum \frac{(O - E)^2}{E}$χ2=∑E(O−E)2​

The critical value of chi-squared at the 0.05 probability level for these data is 7.82.

Calculate and interpret the value of chi-squared for these results. State the degrees of freedom and what your result tells you about whether the genes assort independently.

(6 marks)

The following is adapted from a scientific article about a newly studied antibiotic, zeltomycin.

Researchers grew a strain of the bacterium Staphylococcus aureus in liquid culture and exposed separate samples to a range of zeltomycin concentrations for 24 hours. They measured the percentage of bacteria surviving and, in a parallel experiment, the activity of an enzyme that builds the bacterial cell wall. The data are summarised below.

Zeltomycin concentration / μg cm⁻³	Bacteria surviving / %	Cell-wall enzyme activity / arbitrary units
0	100	50
2	96	49
8	71	34
16	38	19
32	9	6

The article concludes that zeltomycin kills S. aureus by inhibiting the enzyme that builds the bacterial cell wall.

Analyse the data to assess how well they support the conclusion drawn in the article.

(6 marks)

A student investigated the effect of temperature on the rate of respiration of yeast. They set up a single boiling tube containing 10 cm³ of glucose solution and 1 g of dried yeast, placed it in a water bath, and counted the number of gas bubbles released from a delivery tube in one minute. They repeated this once at each of five temperatures (20, 30, 40, 50 and 60 °C) and concluded that yeast respires fastest at 40 °C.

Evaluate this experimental method. In your answer you should identify the independent, dependent and control variables, comment on the reliability and validity of the data, identify sources of error and suggest improvements, and state what would be needed for a valid conclusion.

(6 marks)

A single base substitution in the gene that codes for an enzyme can change one amino acid in the enzyme's primary structure. In some cases this leaves the enzyme working normally, but in others it stops the enzyme from catalysing its reaction.

Explain how a change in a single DNA base can stop an enzyme from working. In your answer, link the gene mutation to the structure and function of the enzyme.

(5 marks)