Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
5 a Name three cell types found in:
i xylem [3]
ii phloem. [3]
b State the functions of the cell types you have named. [4]
[Total: 10]
6 Arrange the following in order of water potential. Use the symbol > to mean ‘greater than’.
dry atmospheric air; mesophyll cell; root hair cell; soil solution; xylem vessel contents [1]
7 Figure a shows changes in the relative humidity of the atmosphere during the daylight hours of one day.
Figure b shows changes in the tension in the xylem of a tree during the same period.
a
a
b
b
100 100
−500 −500
154
Relative humidity / %
Relative humidity / %
0 0
6
6
8 8 10 10 12 12 14 14 16 16 18 18
Time Time of day of day / hours / hours
Tension in xylem / kPa
Tension in xylem / kPa
−1000 −1000
−1500 −1500
−2000 −2000
−2500 −2500
6 6
bottom bottom of tree of tree
top top of tree of tree
8 8 10 10 12 12 14 14 16 16 18 18
Time Time of day of day / hours / hours
a Describe and explain the relationship between relative humidity and xylem tension. [4]
b Describe and explain the differences observed in xylem tension between the top of the tree and the bottom
of the tree. [3]
[Total: 7]
8 An instrument called a dendrogram can be used to measure small changes in the diameter of a tree trunk.
Typically, the instrument reveals daily changes, with the diameter at its lowest during daylight hours and at its
greatest at night.
Suggest an explanation for these observations. [3]