Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
190
The alveoli have extremely thin walls, each
consisting of a single layer of squamous epithelial cells
no more than 0.5 μm thick. Pressed closely against
the alveoli walls are blood capillaries, also with very
thin single-celled walls. Oxygen and carbon dioxide
molecules diffuse quickly between the air and the blood
because the distance is very small.
You will remember that diffusion is the net movement
of molecules or ions down a concentration gradient.
So, for gas exchange to take place rapidly, a steep
concentration gradient must be maintained. This is done
by breathing and by the movement of the blood. Breathing
brings supplies of fresh air into the lungs, with a relatively
high oxygen concentration and a relatively low carbon
dioxide concentration. Blood is brought to the lungs with a
lower concentration of oxygen and a higher concentration
of carbon dioxide than the air in the alveoli. Oxygen
therefore diffuses down its concentration gradient from
the air in the alveoli to the blood, and carbon dioxide
diffuses down its concentration gradient in the opposite
direction. The blood is constantly flowing through and
out of the lungs, so, as the oxygenated blood leaves, more
deoxygenated blood enters to maintain the concentration
gradient with each new breath.
QUESTIONS
9.2 a Describe the pathway taken by a molecule of
oxygen as it passes from the atmosphere to the
blood in the lungs.
b Explain how alveoli are adapted for gas exchange.
9.3 a Explain the advantage of being able to adjust the
diameter of bronchioles.
b How many times does an oxygen molecule cross
a cell surface membrane as it moves from the air
into a red blood cell?
Smoking
The World Health Organization (WHO) considers
smoking to be a disease. Until the end of the 19th century,
tobacco was smoked almost exclusively by men and in
pipes and cigars, involving little inhalation. Then the
manufacture of cigarettes began. Smoking cigarettes
became fashionable for European men during the First
World War and in the 1940s women started smoking
in large numbers too. The numbers of smokers in many
countries, such as the UK and the USA, has decreased
sharply in recent decades. Meanwhile, there has been a
huge rise in the number of people smoking in countries
such as China and Pakistan. It is estimated that in
Pakistan as many as 40% of men and 8% of women are
regular smokers. The smoking of a flavoured tobacco
called shisha has become fashionable among young people
across the world, with many thinking that as it is smoked
through a water pipe it is safer than smoking cigarettes.
Tobacco smoke
The tobacco companies do not declare the ingredients
in their products, but it is known by analysis that there
are over 4000 different chemicals in cigarette smoke,
many of which are toxic. Tobacco smoke is composed
of ‘mainstream’ smoke (from the filter or mouth end)
and ‘sidestream’ smoke (from the burning tip). When a
person smokes, about 85% of the smoke that is released is
sidestream smoke. Many of the toxic ingredients are in a
higher concentration in sidestream than in mainstream
smoke, and any other people in the vicinity are also
exposed to them. Breathing in someone else’s cigarette
smoke is called passive smoking.
The main components of cigarette smoke pose a threat
to human health. These are:
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tar, which contains carcinogens (cancer-causing
compounds)
carbon monoxide
nicotine.
In general, tar and carcinogens damage the gas exchange
system; carbon monoxide and nicotine damage the
cardiovascular system (page 171).
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Tar is a mixture of compounds that settles on the
lining of the airways in the lungs and stimulates a
series of changes that may lead to obstructive lung
diseases and lung cancer.
Carcinogens are cancer-causing compounds. These
cause mutations in the genes that control cell division.
Lung diseases
Lung diseases are a major cause of illness and death
worldwide. Air pollution, smoking and allergic reactions
are the causes of almost all cases.
The gas exchange system is naturally efficient and
adaptable. Healthy people breathe with little conscious
effort; for people with lung disease, every breath may be
a struggle.