Cambridge International A Level Biology Revision Guide

Cambridge International A Level Biology
304
Excretion
Many of the metabolic reactions occurring within the
body produce unwanted substances. Some of these are
toxic (poisonous). The removal of these unwanted products
of metabolism is known as excretion.
Many excretory products are formed in humans, but
two are made in much greater quantities than others.
These are carbon dioxide and urea. Carbon dioxide
is produced continuously by cells that are respiring
aerobically. The waste carbon dioxide is transported
from the respiring cells to the lungs, in the bloodstream
(page 170). Gas exchange occurs within the lungs, and
carbon dioxide diffuses from the blood into the alveoli; it is
then excreted in the air we breathe out.
Urea is produced in the liver. It is produced from
excess amino acids and is transported from the liver to the
kidneys, in solution in blood plasma. The kidneys remove
urea from the blood and excrete it, dissolved in water, as
urine. Here, we will look more fully at the production and
excretion of urea.
Deamination
If more protein is eaten than is needed, the excess cannot
be stored in the body. It would be wasteful, however,
simply to get rid of all the excess, because the amino acids
provide useful energy. To make use of this energy, the liver
removes the amino groups in a process known
as deamination.
Figure 14.4a shows how deamination takes place. In
the liver cells, the amino group (−NH 2
) of an amino acid
is removed, together with an extra hydrogen atom. These
combine to produce ammonia (NH 3
). The keto acid that
remains may enter the Krebs cycle and be respired, or it
may be converted to glucose, or converted to glycogen or
fat for storage.
Ammonia is a very soluble and highly toxic compound.
In many aquatic animals, such as fish that live in fresh
water, ammonia diffuses from the blood and dissolves
in the water around the animal. However, in terrestrial
animals, such as humans, ammonia would rapidly build
up in the blood and cause immense damage. Damage is
prevented by converting ammonia immediately to urea,
which is less soluble and less toxic. Several reactions,
known as the urea cycle, are involved in combining
ammonia and carbon dioxide to form urea. These are
simplified as shown in Figure 14.4b. An adult human
produces around 25–30 g of urea per day.
Urea is the main nitrogenous excretory product
of humans. We also produce small quantities of other
nitrogenous excretory products, mainly creatinine and uric
acid. A substance called creatine is made in the liver, from
certain amino acids. Much of this creatine is used in the
muscles, in the form of creatine phosphate, where it acts as
an energy store (Chapter 15). However, some is converted
to creatinine and excreted. Uric acid is made from the
breakdown of purines from nucleotides, not from
amino acids.
Urea diffuses from liver cells into the blood plasma.
All of the urea made each day must be excreted, or
its concentration in the blood would build up and
become dangerous. As the blood passes through the
kidneys, the urea is filtered out and excreted. To explain
how this happens, we must first look at the structure of
a kidney.
QUESTION
14.3 a Name the nitrogenous waste substances excreted
by mammals.
b Explain why it is important that carbon dioxide
and nitrogenous wastes are excreted and not
allowed to accumulate in the body.
a
NH 2
R
C
H
amino acid
R
COOH
–2H + H 2 O
C COOH
O
NH 3
ammonia
keto acid (respired
or converted to
glucose or fat)
b
2NH 3 + CO 2
CO(NH 2 ) 2 + H 2 O
urea
Figure 14.4 a Deamination and b urea formation.