OCR GCSE Biology A: Practical skills II: photosynthesis, physiology, microbiology and transport in cells (PAG 5-8) (J247)

This guide covers Practical Activity Groups (PAGs) 5 to 8 in Topic B7 (Practical skills) of the OCR GCSE (9-1) Biology A Gateway Science specification (J247), Version 4.0, for first assessment in 2018. The specification says 15% of the questions in the written examinations assess practical skills, in both tiers. No Higher Tier only labels apply to this unit. Use it with the revision notes and the practice questions. The course hub is at OCR GCSE Biology and the printable list is the OCR GCSE Biology checklist. To find your weak spots first, try the free diagnostics.

What this unit covers

The specification requires at least eight practicals across the whole course, with every learner given the chance to use all the listed apparatus and techniques. Centres may substitute their own activities.

PAG	Group	Example activity in the specification	Apparatus and techniques listed
5	Photosynthesis	Factors affecting the rate of photosynthesis on Cabomba	Observe and measure biological changes; safe and ethical use of living organisms; measure rates; heating devices (Bunsen burner, water bath, electric heater); measurements
6	Physiology, responses, respiration	Effect of exercise on pulse rate or ventilation rate and recovery	Safe and ethical use of living organisms; safe use of heating devices
7	Microbiological techniques	Effectiveness of antimicrobial agents on a bacterial lawn	Observe and measure biological changes; microscopes, magnification and labelled scientific drawings; heating devices; measurements
8	Transport in and out of cells	Effect of different water potentials on the length and mass of potato chips	Observe and measure biological changes; measurements

The measurements named across the groups are length, area, mass, time, temperature, volume of liquids and gases, and pH. Safety is an overriding requirement, and scientific diagrams are expected wherever appropriate. The related content is in B1.4 (photosynthesis), B2.1a (osmosis) and B6.3p (aseptic techniques); see the photosynthesis guide, the membrane transport guide and the microscopy guide.

Skills that run through all four groups

Variables. The independent variable is the one you change, the dependent variable is the one you measure, and control variables are kept the same.

Quality of data. Accuracy describes how near a result is to the true value, while precision describes how tightly repeated readings cluster. Repeatability means the same method gives the same result again; reproducibility means a different person or equipment does. Random error scatters readings both ways; systematic error shifts them all one way, such as a balance reading 0.05 g too high.

Handling results. Repeat and find the mean, leaving out a clear anomaly and saying so. Put units in table headings and plot the independent variable on the horizontal axis.

Percentage change. Percentage change = (final value - initial value) / initial value x 100. A negative answer is a loss.

Scientific diagrams. Draw apparatus in section with a sharp pencil and no shading. Label every part that matters and show where measurements are taken. For a specimen drawing, use clear single lines, correct proportions and ruled label lines that do not cross, and give the magnification: magnification = image size / actual size, with both lengths in the same unit.

PAG 5: Photosynthesis with Cabomba

Cabomba is a water plant. In bright light it gives off a stream of oxygen bubbles from the cut end of a stem. Counting bubbles in a fixed time, or collecting the gas, measures the rate of photosynthesis.

Method in outline.

1. Cut a 10 cm piece of healthy Cabomba under water and invert it in a boiling tube of water. Add sodium hydrogencarbonate so carbon dioxide is not limiting.
2. Stand the tube in a beaker of water at a set temperature, with a thermometer in the beaker.
3. Place a lamp a measured distance away. Leave it two minutes to adjust.
4. Count bubbles for two minutes. Repeat three times at each distance.

 lamp ---- ruler: d cm ----> [ beaker of water (heat shield + thermometer)
                              [  boiling tube: Cabomba, cut end up,
                              [  bubbles rise from the stem        ]

Moving the lamp changes light intensity. Intensity is inversely proportional to distance squared, so doubling the distance cuts it to one quarter. A beaker of water absorbs lamp heat and keeps the temperature steady.

Worked example 1. A student counts bubbles for 2 minutes at three distances (three repeats each).

Distance (cm)	Bubbles	Mean	Rate (per min)
10	52, 56, 54	54	27.0
20	14, 12, 13	13	6.5
40	4, 3, 2	3	1.5

Predict the rate at 20 cm from the rate at 10 cm. Intensity at 20 cm = (10/20)² = 1/4 of the intensity at 10 cm. Predicted rate = 27.0 x 1/4 = 6.75, about 6.8 per minute, close to the measured 6.5. At 40 cm the intensity is (10/40)² = 1/16, giving 27.0 / 16 = 1.69, about 1.7 per minute, near the measured 1.5. The agreement shows light intensity is the limiting factor over this range. Bubble size varies, so counting bubbles is a crude measure of volume.

Better measures. Collect the gas in a capillary tube or gas syringe and calculate rate = volume / time. For example, 4.5 cm³ in 15 minutes is 0.30 cm³ per minute.

Why Cabomba practicals go wrong. A hot lamp warms the water. The plant needs time to adjust after each change. Room light adds extra illumination. Keep mains equipment away from spilled water, and handle the plant gently.

PAG 6: Physiology, responses and respiration

This group uses living organisms, most often the learner. The example activity measures pulse rate or ventilation rate before and after exercise, and recovery time.

Method in outline.

1. Sit for three minutes, then find the pulse at the wrist with two fingers, not the thumb.
2. Count beats for 30 seconds and double, or count for 60 seconds. Repeat three times and take the mean.
3. Do a fixed exercise, such as stepping onto a bench for two minutes, and measure the pulse at once.
4. Measure every minute until it returns to the resting value. The time taken is the recovery time.

For ventilation rate, count chest movements (one inhalation and one exhalation is one breath) over a minute.

Worked example 2. A volunteer counts 36 beats in 30 seconds at rest and 69 beats in 30 seconds straight after exercise. Resting rate = 36 x 2 = 72 beats per minute. Rate after exercise = 69 x 2 = 138 beats per minute. Percentage increase = (138 - 72) / 72 x 100 = 91.7%, so about 92%. The rate rises because muscles respire faster and need more oxygen and glucose delivered, and carbon dioxide removed.

Safe and ethical use. For human volunteers: explain the task and get consent, let anyone decline or stop at any time, exclude anyone with a medical condition that exercise could worsen, stop at once if someone feels dizzy or faint, keep exercise moderate and have space to work safely. For animals (for instance woodlice in a choice chamber): handle gently, limit the time, avoid extremes of heat or light, and return them to where they were collected. For plants: take no more material than needed.

Controls. Use the same exercise for everyone. Pulse also varies with caffeine, anxiety and fitness, so a class mean is more reliable than one person’s result.

PAG 7: Microbiological techniques

The example activity tests antimicrobial agents on a bacterial lawn. A lawn is an even layer of bacteria spread across the whole agar surface. A paper disc soaked in an antibacterial agent is placed on the lawn. If the agent kills or stops the bacteria growing, a clear zone appears round the disc.

Aseptic technique stops unwanted microorganisms entering the culture and stops the culture escaping.

• Sterilise glassware and growth media in an autoclave (steam under pressure) before use.
• Wipe the bench with disinfectant or alcohol.
• Work close to a Bunsen burner flame, because rising warm air carries contaminants away from the open dish.
• Flame an inoculating loop until it glows, then cool it. Dip forceps in alcohol and flame them before placing discs.
• Lift the lid of the dish only as far as needed and for as short a time as possible.
• Label the base, tape the lid on with two short strips (not sealed all round), invert the dish so condensation does not drip on the agar, and incubate at a temperature no higher than about 25 °C in school. This makes it less likely that bacteria harmful to humans grow, because those grow best near body temperature. Do not open the dish afterwards.

Controls. Include a disc soaked in sterile water. It should show no clear zone, proving that the disc and the agar are not responsible.

Worked example 3. Four discs (each 6 mm across) are placed on a lawn. After incubation, the clear zones, measured across their widest point including the disc, are: agent A 18 mm, agent B 24 mm, agent C 10 mm, sterile water 6 mm.

Area of a circle = πr². The radius is half the diameter.

• A: r = 9 mm, area = π x 9² = 254 mm² (3 s.f.)
• B: r = 12 mm, area = π x 12² = 452 mm²
• C: r = 5 mm, area = π x 5² = 78.5 mm²
• Water: r = 3 mm, area = π x 3² = 28.3 mm², which is just the disc, so no inhibition.

B is the most effective of the three. Compared with A, B’s cleared area is (452 - 254) / 254 x 100 = 78% larger. To compare fairly, use same-size discs and equal concentrations. If a zone is not circular, take two diameters at right angles and use the mean. A clear zone shows that growth was prevented, not necessarily that bacteria were killed. Bacteria can also be observed as a stained prepared slide under a microscope and drawn, with the magnification stated.

PAG 8: Transport in and out of cells

The example activity uses potato chips (cylinders) in solutions of different concentration. Osmosis is the net diffusion of water across a partially permeable membrane, down a water potential gradient (towards the lower water potential). When the outside solution is dilute, its water potential exceeds that of the potato cells, so the chip takes in water and gains mass and length. In a strong solution the outside water potential is the lower one, so the chip loses water, loses mass and shortens. No calculation using water potential is required.

Method in outline.

1. Cut chips of the same length (for example 30 mm) with a cork borer and a ruler; remove the skin.
2. Make a dilution series. To make 50 cm³ of 0.4 M sucrose from 1.0 M stock, use 20 cm³ of stock plus 30 cm³ of distilled water (0.4 x 50 = 20).
3. Blot each chip dry, weigh it, and place it in a labelled tube for a set time at constant temperature.
4. Blot dry again and reweigh. Calculate percentage change in mass, because the starting masses differ.

Worked example 4.

Sucrose (M)	Start (g)	End (g)	Change (g)	Change (%)
0.0	2.48	2.86	+0.38	+15.3
0.2	2.52	2.69	+0.17	+6.7
0.4	2.50	2.45	-0.05	-2.0
0.6	2.46	2.17	-0.29	-11.8
0.8	2.51	2.09	-0.42	-16.7

For 0.0 M: (2.86 - 2.48) / 2.48 x 100 = 15.3%. The graph of percentage change against concentration crosses zero between 0.2 M and 0.4 M. Assuming a straight line between those points, the zero is at 0.2 + 0.2 x (6.7 / (6.7 + 2.0)) = 0.35 M. At this concentration the water potential of solution and potato cells is about equal, so there is no net movement.

Common errors

• Counting bubbles straight after moving the lamp.
• Comparing raw changes in mass for chips of different starting mass.
• Not blotting chips dry.
• Writing “the bacteria were killed” when the evidence only shows growth was stopped.

Official syllabus

OCR GCSE (9-1) Biology A (Gateway Science) specification J247, Version 4.0, for first assessment in 2018, published by OCR: Topic B7 Practical skills (PAGs 5 to 8), with links to B1.4, B2.1a, B3.1 and B6.3p, and the Working scientifically section.