Plants move mineral ions: from a low concentration in the soil to a high concentration in the root cells - AQA - GCSE Biology - Question 9 - 2018 - Paper 1

Water moves from the roots to the leaves through the process of transpiration. In the roots, water is absorbed from the soil and enters the xylem vessels. As water evaporates from the stomata in the leaves, it creates a negative pressure that pulls more water up from the roots through the xylem. This continuous flow helps maintain the necessary hydration and nutrient transport throughout the plant.

To collect data in Table 5, the student might have followed these steps:

1. Prepare a microscope slide with the leaf epidermis mounted.
2. Count the number of stomata in a single area of the slide.
3. Repeat the counting in at least four more areas to ensure accuracy and create a comprehensive data set.
4. Calculate the mean number of stomata based on the counts from multiple areas for more reliable results.

To determine the median number of stomata, arrange the counts from the upper surface leaf in numerical order and find the middle value. If there is an even number of counts, the median will be the average of the two middle numbers.

Given the values listed in Table 5 for stomata counts: 44, 41, 40, 42, and 39, the calculation for the mean is:

$X = \frac{(44 + 41 + 40 + 42 + 39)}{5} = 41.2$

Thus, when rounded to 2 significant figures, X = 41.

Having fewer stomata on the upper surface of the leaf is advantageous because it reduces water loss through transpiration. This is particularly important in arid conditions or during drought, as it helps the plant conserve water while still allowing for gas exchange when necessary.