Recommended additional practicals: heart dissection, water potential, pollen tubes

Heart dissection (Topic 1)

Observe external features: coronary arteries, aorta, pulmonary artery, vena cava, pulmonary veins, atria and ventricles. Cut to show the thick muscular left ventricle wall, the thinner right ventricle, the atrioventricular valves with tendinous cords ('heart strings') attached to papillary muscles, and the semilunar valves.

January 2025: many candidates could not label structures or draw a sensible cut line, which suggests they had never dissected a heart. Be able to:

• name the four chambers and main vessels;
• draw a labelled diagram of a cut surface;
• write a risk assessment (scalpel, raw tissue: wash hands, gloves) and an ethical point (organs from a butcher).

Tissue water potential (Topic 2)

Put equal-sized pieces of plant tissue (potato cylinders) in a range of sucrose or salt solutions (for example 0 to 1.0 mol dm⁻³), blot and weigh before and after a fixed time.

• Calculate percentage change in mass for each.
• Plot percentage change (y) against concentration (x) and read the concentration where the line crosses zero change: at that point the water potential of the tissue equals that of the solution.
• Controls: same size, same blotting, same temperature, same time.

Pollen tube growth (Topic 3)

Sprinkle pollen into a drop of sucrose solution on a cavity slide; observe under a microscope at intervals and measure tube lengths with a calibrated eyepiece graticule, or count the percentage of grains that have germinated.

• Variables to investigate: sucrose concentration, temperature, calcium or boron ions, time.
• Controls: same species of flower, same age of pollen, same temperature and volume of solution.
• Growth rate = change in length ÷ time, using the correct interval.
• Function: the pollen tube grows down the style, carrying the male nuclei to the ovule.

The January 2025 report found the pollen tube graph well answered, but method marks for standardising were less often seen.

Worked example

Pollen tubes in 10% sucrose had a mean length of 85 µm at 30 minutes and 245 µm at 90 minutes. Calculate the mean growth rate in µm per minute.

1. Change in length = 245 - 85 = 160 µm
2. Time interval = 90 - 30 = 60 min
3. Rate = 160 ÷ 60 = 2.67

Answer: 2.7 µm per minute (2 s.f.: 2.7)

Check yourself

1. Which chamber of a dissected heart has the thickest muscular wall?
   • Right ventricle
   • Left ventricle
   • Left atrium
   • Right atrium
   Show answer

   Left ventricle. It pumps blood around the whole body at high pressure.

2. What are the 'heart strings' seen when the ventricles are opened?
   • Nerves that start the heartbeat
   • Coronary arteries
   • Blood clots
   • Tendinous cords that stop the atrioventricular valves turning inside out
   Show answer

   Tendinous cords that stop the atrioventricular valves turning inside out. They attach valve flaps to papillary muscles.

3. A potato cylinder had a mass of 3.20 g, which became 3.36 g after soaking. Calculate the percentage change in mass.
   Show answer

   5.0 %. (3.36 - 3.20) / 3.20 x 100 = 5%.

4. On a graph of percentage change in mass against sucrose concentration, what does the concentration at zero change show?
   • The tissue has died
   • All the water has left the cells
   • The tissue's water potential equals the water potential of that solution
   • The sucrose has entered the cells
   Show answer

   The tissue's water potential equals the water potential of that solution. No net osmosis, so equal water potentials.

5. A pollen tube grows from 40 µm at 20 minutes to 220 µm at 80 minutes. Calculate its mean growth rate in µm per minute.
   Show answer

   3 µm per minute. 180 / 60 = 3 µm per minute.

6. Which is the most important controlled variable when comparing pollen tube growth in different sucrose concentrations?
   • Colour of the slide
   • Size of the microscope
   • Species and age of the pollen
   • Concentration of sucrose
   Show answer

   Species and age of the pollen. Sucrose is the IV; pollen source must be the same.

7. Why are potato cylinders blotted the same way before each weighing?
   • Surface solution would add to the mass and vary between pieces
   • To remove the cell walls
   • To kill the cells
   • To make them absorb more water
   Show answer

   Surface solution would add to the mass and vary between pieces. Consistent blotting removes surface liquid.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills
• Pearson Examiners' Report WBI13/01, January 2025

Last updated 8 November 2026. Spotted a mistake? Tell us.