Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
Answers to self-assessment questions
Answers to SAQs
Chapter 14
1 a The immediate surroundings of most cells in
the body is tissue fluid. Blood cells, however,
are surrounded by plasma. The composition
of tissue fluid is very similar to that of blood
plasma. See pages 164–165 for more about
tissue fluid.
b Cells function efficiently if they are kept in a
constant environment. Maintaining constant
conditions, such as pH, temperature and
water potential, ensures that enzymes within
cells may function at a constant rate.
c Changes in physiological factors, such
as temperature, are the stimuli that are
detected by receptors. Each homeostatic
mechanism has a central control. The
hypothalamus is the central control for many
homeostatic mechanisms. Coordination
systems transfer information from
receptors to the central control and from
the central control to effectors. Information
is transferred as nerve impulses travelling
along nerve cells and as hormones in the
blood. Effectors are muscles and glands that
respond to information from the central
control by changing the physiological factor.
d i homeostasis: the maintenance of (near)
constant internal conditions or the
maintenance of internal conditions within
narrow limits
negative feedback: a control process
in which a change in a factor, such as
body temperature, stimulates corrective
actions to return it to its normal value or
its set point
ii input: sensory information from receptors
about changes in physiological factors; this
information goes to a control centre
output: corrective actions carried out by
effectors that return the physiological
factor to its normal value
2 Your diagram should look like this one.
Temperature rises
above set point.
Muscles shiver,
erector muscles in
skin contract, sweat
glands stop producing
sweat and arterioles
contract.
Temperature receptors
in hypothalamus and
skin sense change
in temperature.
Muscles, sweat glands
and arterioles in the
skin receive information
from hypothalamus.
Temperature falls
below set point.
Sweat glands
secrete more
sweat, arterioles
dilate.
3 a urea, uric acid, creatinine
b Metabolic waste products are toxic
(poisonous) and would damage the body if
they accumulated.
4 a glomerular capillaries via the efferent arteriole
b proteins
c It will increase the solute concentration of
the blood plasma, therefore lowering its
water potential and increasing the water
potential gradient between the filtrate and
the blood.
d osmosis
5 a 80 cm 3
b A large percentage of the water in the fluid
is reabsorbed in the proximal convoluted
tubule, thus the volume of water in which the
urea is dissolved decreases. This increases
the concentration of urea in the fluid.
c i Microvilli give the membrane in contact
with filtrate a large surface area for
absorption of solutes and water.
ii There are many mitochondria to supply
the ATP required by membrane proteins
that pump sodium ions out of the cells.
iii The basal membranes are folded to give
a large surface for the many pump and
carrier proteins in the basal membranes
that move substances out of the cells and
into the blood.
Cambridge International AS and A Level Biology © Cambridge University Press 2014