AQA · A-Level · Biology
Course version: 7402 · Version from Sept 2016
Students should be able to: recognise correlations and causal…
Full specification wording
Mass transport in plants: Students should be able to: recognise correlations and causal relationships interpret evidence from tracer and ringing experiments and to evaluate the evidence for and against the mass flow hypothesis.
You are given unfamiliar evidence about plant transport and asked to judge it, so structure the answer as evidence for, then evidence against, then what cannot be told.
Checked against the AQA 7402 specification (opens in a new tab) on .
- Point 4 of 4
Study notes
From the checked teaching notes for this specification point.
The idea
You are given unfamiliar evidence about plant transport and asked to judge it, so structure the answer as evidence for, then evidence against, then what cannot be told. Evidence for mass flow includes sap exuding from a cut phloem, showing it is under pressure; a higher sucrose concentration at the source than at the sink; swelling above a ring of removed bark; tracers appearing in phloem; and metabolic inhibitors or lack of oxygen stopping transport, showing loading needs ATP. Evidence against includes sieve plates, which would obstruct bulk flow; solutes travelling at different speeds, which bulk flow cannot explain; and sucrose and amino acids moving in opposite directions within the same sieve tube. Note that opposite directions in different sieve tubes in the same tissue are compatible with mass flow.
What earns the marks
- Identify a point as evidence for the mass flow hypothesis and quote the data that support it.
- Identify a point as evidence against the mass flow hypothesis and quote the data that support it.
- Identify an alternative explanation for a result, such as another variable that changed at the same time.
- Describe a relationship as a correlation rather than a cause where no mechanism has been demonstrated.
- State a limitation of the investigation, such as only one species or one tissue having been tested.
Common mistakes and how to put them right
Writing every point as evidence for and never addressing what the data fail to show.
Put it right: Explicitly consider evidence against and what cannot be concluded.
Concluding that a correlation between sucrose concentration and flow rate proves mass flow.
Put it right: State that the data show a correlation and suggest another variable might be responsible unless a causal mechanism is proven.
Quoting sieve plates as evidence for mass flow when they are normally raised as evidence against it.
Put it right: Sieve plates are an obstacle to bulk flow and are used as evidence against.
Treating sucrose and amino acids moving in opposite directions in the same tissue as evidence against mass flow.
Put it right: This is only evidence against if they are in the same sieve tube; opposite directions in different sieve tubes are compatible with mass flow.
Exam tips
- Split the answer physically into for and against; the mark scheme is usually split the same way.
- Every point should carry a figure or an observation, not just a claim.
- Testing only one species, or only one site, is nearly always worth a limitation mark.
Try the lesson
The start of the Mass transport in plants: Students should be able to: recognise correlations and causal relationships interpret evidence from tracer and ringing experiments and to evaluate the evidence for and against the mass flow hypothesis. lesson, made from checked teaching notes for this specification point.
Have a go first
A student finds that the rate of sugar transport in a plant increases at the same time as the rate of respiration in the leaves increases. What type of relationship is this?
Listen
You are given unfamiliar evidence about plant transport and asked to judge it, so structure the answer as evidence for, then evidence against, then what cannot be told. Evidence for mass flow includes sap exuding from a cut phloem, showing it is under pressure; a higher sucrose concentration at the source than at the sink; swelling above a ring of removed bark; tracers appearing in phloem; and metabolic inhibitors or lack of oxygen stopping transport, showing loading needs ATP. Evidence against includes sieve plates, which would obstruct bulk flow; solutes travelling at different speeds, which bulk flow cannot explain; and sucrose and amino acids moving in opposite directions within the same sieve tube. Note that opposite directions in different sieve tubes in the same tissue are compatible with mass flow. A strong answer earns marks for these points. Identify a point as evidence for the mass flow hypothesis and quote the data that support it. Identify a point as evidence against the mass flow hypothesis and quote the data that support it. Identify an alternative explanation for a result, such as another variable that changed at the same time. Describe a relationship as a correlation rather than a cause where no mechanism has been demonstrated. A common slip: Writing every point as evidence for and never addressing what the data fail to show. Instead, explicitly consider evidence against and what cannot be concluded. Exam tip: Split the answer physically into for and against; the mark scheme is usually split the same way.
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