Scientific Inquiry & Diversity of Life — Print · Life Science & Scientific Inquiry, Grade 7

1. Name what the investigator meant to change — the daily water volume, 10 mL against 5 mL — and what was measured, height gain in centimetres over seven days.

2. Find the second factor that changed with it: light differed too, bright against dim, so two things varied at once.

3. State the consequence precisely: the 3 cm difference is real, but it cannot be divided between water and light, so neither can be credited with it.

4. Propose the fix: hold light identical for both groups and vary only water, with several plants per group, the same species, soil, pot size and starting height.

The intended independent variable is the daily volume of water, 10 mL against 5 mL, and the dependent variable is height gain in centimetres over seven days. Group A gained 4 cm and Group B gained 1 cm, a difference of 3 cm. But light was also different — bright for A, dim for B — and that is a confound: two factors changed together, so the record cannot say whether the extra growth came from the water, the light, or some combination. What it does establish is that the whole package of more water plus more light was associated with more height gain here, which is worth knowing and is not the same as a claim about water. A fairer comparison would place both groups under identical light, hold species, soil, pot size, starting height and temperature the same, vary only the water, and use several plants in each group rather than one, so that ordinary variation between individuals can be seen and allowed for. It would also be worth recording the temperature and rotating the pots, since the brighter position is usually the warmer one.

1. Compare the cell with the scale bar on the page: 1.2 cm ÷ 4 cm = 0.3, so the cell is three tenths of the bar's length.

2. Convert using the bar's stated value: 0.3 × 200 micrometres = 60 micrometres, which is the estimated length of the specimen.

3. Calculate the magnification separately by multiplying the lenses: 10 × 40 = 400×, which describes the image, not the cell.

4. State the assumption: the calculation holds only if the 4 cm bar genuinely represents 200 micrometres for this reproduction of the image.

The cell is approximately 60 micrometres long and the total magnification is 400×, and the two numbers answer different questions. The length comes from the scale bar: the cell spans 1.2 cm against the bar's 4 cm, which is 0.3 of it, and 0.3 × 200 micrometres gives 60 micrometres. That is a property of the specimen. The magnification comes from the lenses: a 10× eyepiece multiplied by a 40× objective gives 400×, and that is a property of the image, describing how much the view was enlarged rather than how big anything is. You could photograph a much smaller cell at the same 400× and a much larger one too. The calculation rests on the stated calibration being right for the image as printed — if the reproduction was scaled unevenly, or the bar was added to the wrong version of the image, the 60 micrometre estimate carries that error. It is also worth saying what the record cannot establish: the cell's depth, whether preparation distorted its shape, and anything about structures smaller than the roughly 200 nanometre resolution limit of a light microscope.

1. Mean of A: (3 + 4 + 5) ÷ 3 = 12 ÷ 3 = 4 cm. Mean of B: (1 + 4 + 7) ÷ 3 = 12 ÷ 3 = 4 cm.

2. Range of A: 5 − 3 = 2 cm. Range of B: 7 − 1 = 6 cm, three times as wide.

3. State the likeness and the difference: the averages are identical, while B's results are far more spread out than A's.

4. State the limits: three plants per treatment is a very small set, and nothing here explains why B varied more or whether the pattern would hold again.

Both treatments have a mean of 4 cm: A gives (3 + 4 + 5) ÷ 3 = 4 and B gives (1 + 4 + 7) ÷ 3 = 4. They are alike in their averages and quite unlike in their spread. A's range is 5 − 3 = 2 cm; B's is 7 − 1 = 6 cm, three times as wide. That difference changes what each mean is worth. A's plants behaved consistently, so 4 cm describes them well and a repeat would probably land near it. B's results scattered from 1 to 7 cm, so its mean of 4 cm describes no individual plant particularly and the next result could fall anywhere across that span. Reporting only the means would have hidden this entirely, which is why a mean should always travel with a measure of spread. What the summaries cannot establish is anything about cause. Three plants per treatment is a very small set, and the records do not say why B varied more — it could be uneven seed quality, uneven watering, differences in position, or chance. Nor can they support a prediction outside the conditions that were run. The useful next step would be more plants per treatment and a record of the conditions each one experienced, so that the source of B's variability can be looked for rather than guessed at.

1. Calculate the difference the record shows: 12 − 5 = 7 g less water lost from the covered sample over the stated day.

2. Correct the claim against the investigator's own data: the covered sample lost 5 g, so "prevents all evaporation" is contradicted; the supported word is reduced.

3. Supply the mechanism: water leaves as vapour from the exposed surface, and a cover reduces that exposed area and traps humid air above the soil, so fewer molecules escape per unit time.

4. Name a limit and a rival worth checking: one comparison over one day in one setting, and the record does not confirm that the two containers had identical surface areas or air movement.

A better claim is that covering the soil was associated with 7 g less water loss over a day under these conditions — 12 g uncovered against 5 g covered — rather than that a cover prevents evaporation. The investigator's own figure refutes the stronger wording, because the covered sample still lost 5 g. The mechanism that would explain the reduction is straightforward: water leaves soil as vapour from the exposed surface, and the rate depends on how much surface is exposed, how dry the air above it is, and how fast that air is replaced. A cover reduces the exposed area and holds a layer of humid air in place, so fewer molecules escape per minute, and the loss that remains can happen at the edges or through the cover itself. That mechanism makes testable predictions: a perforated cover should fall between the two, a draught should widen the gap, and a wider container should lose more. The limits are worth stating plainly. This is one comparison over one day in one setting, so it does not establish how the effect would scale with time, temperature or wind. The record states that starting water, container and temperature were comparable, but not that the exposed surface areas were equal, and a difference there would be a rival explanation for part of the 7 g.

1. Apply the set to A: made of cells, with the organisation to grow, use energy and reproduce, which germination later demonstrates — dormancy is a state, not evidence against life.

2. Apply the set to B: it moves and uses stored energy, and it is not made of cells, does not grow, does not reproduce and regulates nothing, so it fails nearly every characteristic.

3. Apply the set to C: it is a cell with a membrane and DNA, so it is an organism; the absence of an enclosing membrane around the DNA classifies it as prokaryotic rather than placing it outside life.

4. State why single tests fail: movement is shown by B and absent in most plants, and a nucleus is absent in every bacterium and in mature mammalian red blood cells.

A and C are living under these records; B is not. A is made of cells and carries the organisation needed to grow, use energy and reproduce, and its later germination demonstrates that directly — dormancy is a state an organism can be in, not evidence against being one, as a date palm grown in 2005 from a two-thousand-year-old seed showed. C is a cell with a membrane and its own DNA, which makes it an organism; the fact that the DNA is not enclosed in a nucleus is a classification result, placing it among the prokaryotes, and not a reason to exclude it. Bacteria are enormously successful organisms and the most numerous on Earth. B moves and draws on stored energy, and that is all: it is not made of cells, does not grow or develop, does not reproduce, and maintains no internal conditions. The two tempting single tests both fail. Movement is shown by B, which is not alive, and is largely absent in plants, which are. A nucleus is absent in every prokaryote and in mature mammalian red blood cells, so its absence classifies rather than disqualifies. The records also leave things open: they do not say whether A's cells were still carrying out any measurable metabolism while dormant, and for C they report structure without reporting reproduction, which would be the direct evidence.