A student cleared a leaf in hot ethanol and bleach until the network of veins could be seen clearly. Fig. 6.1 shows a square area marked on the cleared leaf...
Question 1 Report
A student cleared a leaf in hot ethanol and bleach until the network of veins could be seen clearly. Fig. 6.1 shows a square area marked on the cleared leaf, with the veins drawn as black lines and one short vein labelled V. Table 6.1 shows the number of vein endings counted inside three separate squares of the same size.
| Square | 1 | 2 | 3 | mean |
|---|---|---|---|---|
| Number of vein endings | 24 | 28 | 26 | ________ |
(a) Measure, in cm, the length of one side of the square marked on Fig. 6.1. [1]
(b) Calculate the area of the square, in cm2. [1]
(c) Count the vein endings inside the square in Fig. 6.1 and record the number. [1]
(d) Calculate the vein-ending density, in endings per cm2. Show your working. [3]
(e) Measure, in mm, the length of the single vein labelled V in Fig. 6.1. [1]
(f) Complete Table 6.1 by calculating the mean number of vein endings from the three squares. [2]
(g) Describe three functions of the veins in a leaf. [3]
(h) Explain why a high vein density is useful in a leaf that photosynthesises rapidly. [4]
(i) Name the two transport tissues found inside a vein. [2]
(j) Suggest why the student counted three squares rather than one. [2]
Answer Details
Labelled answer diagram:
This question uses a cleared leaf to test area and density calculations, a mean, and the transport role of veins.
(a) Measure one side of the marked square with a ruler and record your reading in cm. [1]
(b) The area of a square is side \( \times \) side. For example, if the side is 4.0 cm, \( 4.0 \times 4.0 = 16\ \text{cm}^2 \). Answers following from your own reading are accepted. [1]
(c) Count the vein endings inside the square and record the number (for example 24). [1]
(d) Density is number per unit area:
\[ \text{density} = \frac{\text{number of endings}}{\text{area}} = \frac{24}{16} = 1.5\ \text{endings per cm}^2 \]Method [1], substitution [1], answer (error carried forward from your own counts) [1]. [3]
(e) Measure the length of vein V in mm and record your ruler reading. [1]
(f) The mean is the total divided by the number of squares:
\[ \frac{24 + 28 + 26}{3} = \frac{78}{3} = 26 \]Working [1], answer [1]. [2]
(g) Any three functions of leaf veins: carry water and mineral ions to the mesophyll cells (in the xylem) [1]; carry away the sugars/sucrose made in photosynthesis (in the phloem) [1]; support the leaf and hold it out flat, acting as a skeleton [1]. [3]
(h) High vein density suits a fast-photosynthesising leaf because rapid photosynthesis needs a fast supply of water to the mesophyll [1] and fast removal of the sugars made [1]; densely spaced veins lie close to every cell so the transport distance is short [1]; this keeps water supplied and stops sugars building up [1]. [4]
(i) The two transport tissues in a vein are xylem [1] and phloem [1]. [2]
(j) Three squares are counted rather than one because a single square could be unrepresentative or show an unusual value [1]; taking a mean of three gives a more reliable result [1]. [2]
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