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

These notes condense Practical Activity Groups 5 to 8 of Topic B7 (Practical skills) in the OCR GCSE (9-1) Biology A Gateway Science specification (J247), Version 4.0, for first assessment in 2018. Topic B7 is assessed in the papers for both tiers, and nothing in this unit is Higher Tier only. For full explanations and longer worked examples, read the study guide, then test yourself with the practice questions. The course page is OCR GCSE Biology, the printable list is the OCR GCSE Biology checklist, and the free diagnostics show which topics to revise first.

The four groups at a glance

PAG	Example activity	Living material	Measured quantity
5 Photosynthesis	Cabomba under a lamp	Plant	Bubbles or gas volume per minute
6 Physiology	Exercise and pulse or ventilation	Human volunteers	Beats or breaths per minute; recovery time
7 Microbiology	Antimicrobial discs on a bacterial lawn	Bacteria	Diameter then area of clear zone
8 Transport	Potato chips in solutions	Plant tissue	Change in mass or length

Techniques that appear across the groups: measuring length, area, mass, time, temperature, volume of liquids and gases and pH; safe use of a Bunsen burner and a water bath or electric heater; measuring rates (gas produced, water taken up, colour change of an indicator); microscopes, magnification and labelled drawings; safe and ethical use of living organisms.

Formulas and conversions

Quantity	Formula	Check
Rate	amount / time	4.5 cm³ in 15 min = 0.30 cm³ per min
Percentage change	(final - initial) / initial x 100	3.10 g to 2.79 g = -10.0%
Area of a circle	πr², r = diameter / 2	d = 14 mm gives 154 mm²
Magnification	image size / actual size	45 mm drawing of 30 µm cell = x1500
Light intensity	proportional to 1 / distance²	15 cm to 45 cm gives 1/9
Mean	total / number of values	ignore a clear anomaly

Conversions: 1 cm = 10 mm; 1 mm = 1000 µm. For magnification, convert both lengths to the same unit first: 45 mm = 45 000 µm, and 45 000 / 30 = 1500.

Method boxes

PAG 5: rate of photosynthesis (Cabomba)

1. Cut the stem under water; invert it in water with sodium hydrogencarbonate added.
2. Put the tube in a water-filled beaker with a thermometer, to hold temperature steady.
3. Set the lamp at a measured distance; wait two minutes.
4. Count bubbles for a fixed time, three repeats, find the mean, divide by time.
5. Change one variable only: distance (light), temperature or hydrogencarbonate (carbon dioxide).

PAG 6: exercise and pulse

1. Rest three minutes; take a pulse at the wrist with two fingers.
2. Count for 30 s and double (21 beats in 15 s means x4 = 84 per min); repeat and average.
3. Same standard exercise for all, then measure straight away and each minute.
4. Recovery time is the time to return to the resting rate.

PAG 7: bacterial lawn

1. Sterilise dishes and agar (autoclave); wipe bench with disinfectant or alcohol.
2. Work near a Bunsen flame; flame the loop and cool it; lid open as little as possible.
3. Spread a lawn; place discs with flamed forceps, including a sterile water control.
4. Tape the lid on with short strips, invert, label the base, incubate at no more than about 25 °C.
5. Measure the clear zone diameter (two directions, mean), then calculate area.
6. Microscope work: view a prepared, stained bacteria slide, draw it in labelled single lines and state the magnification.

PAG 8: potato chips

1. Cut identical chips with a cork borer; same length, no skin.
2. Make a dilution series; 100 cm³ of 0.3 M from 1.0 M stock = 30 cm³ stock + 70 cm³ water.
3. Blot, weigh, soak for a set time at constant temperature, blot, reweigh.
4. Calculate percentage change; plot against concentration; read off where the line crosses zero.

Variables table for each group

PAG	Independent	Dependent	Typical controls
5	Lamp distance, temperature or hydrogencarbonate amount	Bubbles or gas volume per minute	Plant piece and length, other two factors, counting time
6	Exercise (or none)	Pulse or ventilation rate; recovery time	Same exercise, same time of day, same counting method
7	Type or concentration of agent	Area of clear zone	Disc size, volume of agent, bacterial species, agar depth, incubation time and temperature
8	Solution concentration	Percentage change in mass or length	Chip size, volume of solution, soak time, temperature, same potato

Reading graphs and evaluating

• A rate curve that rises then levels off means another factor has become limiting; name it.
• A line of best fit that crosses the horizontal axis (percentage change = 0) gives the concentration with no net water movement.
• A point far from the pattern is an anomaly: check for a recording error, repeat it, and say whether you excluded it from the mean.
• To improve reliability, repeat more times, use more volunteers or more plates, and control the variables better.
• To improve accuracy, use a smaller measuring scale (for example a syringe graduated in 0.1 cm³ rather than 1 cm³), a balance that reads to 0.01 g, and a timer, not a guess.
• Suggest a follow-up: a narrower range of concentrations, another agent, a different plant or a longer soak.

Worked reminders

Gas volume rate. A gas syringe collects 5.4 cm³ in 18 minutes. Rate = 5.4 / 18 = 0.30 cm³ per minute.

Pulse recovery. A resting rate of 66 per minute becomes 110 after exercise. Increase = (110 - 66) / 66 x 100 = 66.7%.

Antimicrobial zones. Zone diameters 14 mm, 16 mm and 15 mm for one agent have a mean of 15 mm. Radius 7.5 mm, area = π x 7.5² = 177 mm².

Chip length. A chip goes from 30.0 mm to 31.5 mm. Change = (31.5 - 30.0) / 30.0 x 100 = +5.0%. It has gained water, so the solution had a higher water potential than the chip.

Reading the zero. If a chip gains 4% in 0.1 M and loses 6% in 0.3 M, the straight line crosses zero at 0.1 + 0.2 x 4 / (4 + 6) = 0.18 M. Roughly, that is the concentration with about the same water potential as the cells.

Must-know distinctions

• Independent, dependent, control variable: change it, measure it, keep it the same.
• Accuracy and precision: nearness to the true value versus tightness of the spread of repeats.
• Repeatable and reproducible: same method again versus a different person or equipment.
• Random and systematic error: scatter versus a consistent shift in one direction.
• Sterilise and disinfect: autoclaving kills all organisms on glassware and media; a disinfectant or alcohol wipe cleans surfaces.
• Clear zone: shows growth was prevented near the disc, not that every cell died.
• Water potential direction: net water movement is down the water potential gradient, across a partially permeable membrane. No calculation with water potential is needed.

Safe and ethical practice

• Volunteers: explain the task, get agreement, allow anyone to stop, exclude anyone with a relevant condition, and stop if someone feels unwell.
• Animals: gentle handling, short time, no extremes, return to the habitat.
• Plants: take only what is needed.
• Microorganisms: cultures are not opened after sealing; dishes are autoclaved or disinfected for disposal.
• Heat and electricity: tie back hair, wear eye protection, use a heatproof mat, turn the Bunsen to the safety flame when not heating, and keep mains equipment dry.
• Diagrams: label the lamp, ruler, thermometer, plant, tube and water level, and show where each measurement is taken.

Quick self-test

1. A volunteer counts 21 beats in 15 s. What is the pulse rate per minute?
2. A lamp is moved from 15 cm to 45 cm. By what fraction does the light intensity change?
3. A chip changes from 3.10 g to 2.79 g. Calculate the percentage change.
4. A clear zone is 14 mm across. Calculate its area (3 s.f.).
5. How would you make 100 cm³ of 0.3 M sucrose from 1.0 M stock?
6. A drawing of a cell is 45 mm long. The cell is 30 µm long. What is the magnification?
7. Why is a bacterial dish inverted?
8. Why include a disc soaked in sterile water?
9. Why is percentage change better than change in mass for chips?
10. Give two ways to make a Cabomba investigation safe and ethical.

Answers

1. 21 x 4 = 84 beats per minute.
2. Intensity = (15/45)² = 1/9, so it falls to one ninth.
3. (2.79 - 3.10) / 3.10 x 100 = -10.0%.
4. r = 7 mm, area = π x 49 = 154 mm².
5. 30 cm³ of stock plus 70 cm³ of distilled water.
6. 45 mm = 45 000 µm; 45 000 / 30 = x1500.
7. So condensation collects in the lid and cannot drip on to the agar.
8. It is a negative control: no clear zone shows the disc and agar do not stop growth.
9. Chips begin with different masses, so percentage change allows a fair comparison.
10. Keep mains equipment away from water and dry; handle the plant gently and take only the material needed (any two sensible answers).

Where marks are usually lost

• Not stating the time with a rate (beats, bubbles or cm³ need “per minute”).
• Writing “fair test” without naming the variable kept the same.
• Using mass change, not percentage change, when starting masses differ.
• Using the diameter in the πr² formula instead of the radius.
• Mixing mm and µm in a magnification calculation.
• Saying the lamp “heats the plant” with no mention of controlling temperature in a water bath.
• Describing osmosis as movement of solute instead of water.
• Saying bacteria were killed when the data show only a clear zone.
• Drawing diagrams with shading, crossing label lines or no labels on the water level and measuring device.

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, Practical Activity Groups 5 to 8, with the Working scientifically section.