A student stood several fresh celery stalks upright in a beaker of red dye, as shown in Fig. 2.1, and left them in a warm room. Every ten minutes she remove...
Question 1 Report
A student stood several fresh celery stalks upright in a beaker of red dye, as shown in Fig. 2.1, and left them in a warm room. Every ten minutes she removed a stalk, held a millimetre ruler against it and measured the height that the red colour had risen up the stalk above the dye surface. She recorded the mean height for several stalks at each time in Table 2.1. One value in the table is missing. The dye is carried up the stalk in one of the transport tissues, which becomes stained red.
| Time / min | 0 | 10 | 20 | 30 | 40 |
|---|---|---|---|---|---|
| Mean height of red dye / mm | 0 | 18 | 33 | 54 |
(a) Name the tissue that carries the red dye up the stalk. [1]
(b) Measure, using a ruler, the height risen shown on the stalk in Fig. 2.1, in mm. [1]
(c) Complete Table 2.1 by reading a sensible value for the missing height at 30 minutes. [1]
(d) Describe the trend shown by the results in Table 2.1. [2]
(e) Calculate the mean rate of rise of the dye, in mm per minute, between 10 and 40 minutes. Show your working. [2]
(f) State two variables that should be kept the same for each stalk so that the comparison is valid. [2]
Answer Details
This experiment follows water movement up a stem and tests table reading, trend description and a rate calculation.
(a) Tissue that carries the dye [1] Water and dissolved substances travel up the stem in the xylem, so the xylem becomes stained red. [1]
(b) Height risen on the stalk [1] Hold the ruler against the coloured region and read its height. On Fig. 2.1 this is about \(34\ \text{mm}\); any honest reading of the stained length earns the mark. [1]
(c) Completed table [1] The values rise by a little less each interval (18, then +15, then +12), so a sensible reading at 30 min lies between 33 and 54, about \(45\ \text{mm}\) (accept 42 to 48).
| Time / min | 0 | 10 | 20 | 30 | 40 |
|---|---|---|---|---|---|
| Mean height of red dye / mm | 0 | 18 | 33 | 45 [1] | 54 |
(d) Trend [2] The height risen increases with time [1], but the rate of rise slows down: the steps between readings get smaller as time goes on [1].
(e) Mean rate between 10 and 40 min [2] Rate is change in height divided by change in time: \[\text{rate}=\frac{54-18}{40-10}=\frac{36}{30}=1.2\ \text{mm min}^{-1}\] working [1], answer \(1.2\ \text{mm per minute}\) [1].
(f) Variables to keep the same [2] Any two of: length or age of stalk; number of leaves; concentration of the dye; volume of the dye; same species of plant. [2] Keeping these constant means the only thing being compared is time, making the test valid.
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