Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
iii Insulin and glucagon both have liver cells
as their target cells. If they had the same
second messenger, they would both
have the same effect, rather than having
opposite effects on glycogen, for example.
10a Insulin is a protein. Its molecules would be
hydrolysed to amino acids in the digestive
system.
b People with type 1 diabetes are encouraged to
test their blood or urine regularly for glucose.
c They can adjust their diet accordingly. They
should eat small amounts of carbohydrate
fairly regularly, rather than large quantities
at any one time. High-sugar foods such as
confectionery should be avoided, as these may
result in a rapid and dangerous rise in blood
glucose concentrations. People with type 2
diabetes can also take exercise frequently.
11 The biosensor will be able to give a more
precise reading. With the dip stick, judgement
is needed to match the colour on the stick to
a colour on the chart; this is difficult to do,
especially if the colour lies in between two
colours on the chart. The biosensor can be
used over and over again, but once a dip stick
has been used it is discarded.
keeping them open, as photosynthesis
cannot happen and there is no requirement
for a continuous supply of carbon dioxide.
c Abscisic acid (ABA) is produced in times
of water stress. ABA binds with receptors
on the cell surface membrane that leads
to the inhibition of the proton pumps so
that hydrogen ions are not pumped out
of the guard cells. Channel proteins open
to allow negatively charged ions to leave
the cells followed by potassium ions. This
stimulates the closure of channel proteins
that allow potassium to enter the cells. The
concentration of calcium ions within the
cytoplasm increases and it may be that they
act as a second messenger for ABA. The effect
of these changes is that the water potential
increases within the guard cells and water
passes out by osmosis. The cells become
flaccid and close the stomata.
12a Proton pumps in the cell surface membranes
of guard cells transport, hydrogen ions / H + /
protons, out of the guard cell. The decrease
in hydrogen ion concentration increases the
negative charge inside the guard cell. This
stimulates K + channels to open so that K +
diffuse from the surroundings into the guard
cells. The increase in K + concentration inside
the guard cells lowers their water potential.
Water enters by osmosis down a water
potential gradient. The volume of the cells
increases so they expand and bow outwards
so that the stomatal pore opens.
b A plant is under water stress when the loss
of water by transpiration is greater than the
rate of water uptake. The stomata close when
there is more to be gained by doing this than
keeping them open. At night, plants close
stomata to reduce the rate of transpiration
because there is nothing to be gained by
Cambridge International AS and A Level Biology © Cambridge University Press 2014