7 Living things and air
7
Living things and air
7.1
Gases in the air
1
The Earth is surrounded by a layer of air called the atmosphere.
2
Air is a mixture of gases. It contains nitrogen, oxygen, carbon dioxide, noble gases (e.g. neon),
water vapour and other gases.
3
Test for oxygen, carbon dioxide, nitrogen and water:
Gas
4
How to test?
Oxygen
By using a glowing splint.
The glowing splint relights.
Carbon dioxide
By using hydrogencarbonate
indicator or lime water.
The indicator turns yellow;
the lime water turns milky.
Water
By using dry cobalt chloride
paper.
The paper changes from blue to
pink.
The percentages of gases in unbreathed air and breathed air:
Gas in the air
Unbreathed air
Breathed air
Nitrogen
78%
78%
Oxygen
21%
16%
0.03%
4%
Water vapour
Variable
Saturated
Noble gases
0.9%
0.9%
Other gases
Variable
Variable
Carbon dioxide
5
Result
The temperature of breathed air is higher than that of unbreathed air.
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7 Living things and air
7. 2
Air and burning
1
Burning is a chemical reaction between a fuel and oxygen.
2
The word equation of the process of burning:
fuel
3
+ oxygen
high temperature
carbon dioxide + water +
energy (heat and light)
The fire triangle shows the three conditions that are necessary for a fire to occur:
fuel
oxygen
high temperature
4
To put out a fire, we can remove the fuel, cut off the oxygen supply, and/or lower the
temperature.
7.3
How humans obtain energy
1 Food stores chemical energy. We obtain energy from the food we eat.
2 The set-up for comparing the amounts of energy in different foods:
thermometer
water (1/3 full)
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burning spoon
Measure and compare
the temperature rise of
water for the food to be
burnt completely.
burning food
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3 The energy value of food is measured in kilojoules (kJ) or kilocalories (kcal). It is usually
shown on the food labels on food packages.
7 . 4 How green plants obtain energy
1
Green plants can make their own food by photosynthesis.
2
Green plant cells contain chloroplasts. The chlorophyll in the chloroplasts absorbs light
energy for carrying out photosynthesis.
3
The word equation of photosynthesis:
light energy
carbon dioxide + water
food (starch) + oxygen
chlorophyll
4
Test for starch in green leaves:
Step
Purpose
i
Boil the leaf in water.
Destroy the cell membranes of the leaf cells.
ii
Soak the leaf in hot alcohol.
Remove the chlorophyll from the leaf.
iii
Wash the leaf in hot water.
Wash away the alcohol and soften the leaf.
iv
Add iodine solution to the leaf.
Test for the presence of starch.
If the leaf becomes blue-black in the iodine test in step iv, starch is present in the leaf.
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5
Chlorophyll, light, carbon dioxide and water are necessary for photosynthesis.
6
A food chain can represent the feeding relationship among living things.
7
An example of a food chain:
Grass (producer)
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Rabbit (consumer)
Snake (consumer)
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7 Living things and air
7 . 5 Gaseous exchange in animals and plants
1
The process in which living cells use oxygen to release energy from food is called
respiration.
2
The word equation of respiration:
food
Е
+
oxygen
carbon dioxide
+
water
+ energy
3
The exchange of oxygen and carbon dioxide between living things and the surroundings is
known as gaseous exchange.
4
The major parts of the human respiratory system:
nasal cavity
bronchus
bronchiole
trachea
air sac
lung
rib
intercostal muscle
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diaphragm
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5
Gaseous exchange takes place at the surface of the air sac:
carbon dioxide is removed
from the body
oxygen from the air
air sac
direction of blood flow
capillary
Carbon dioxide is produced by
respiration in body cells. The
carbon dioxide gets into the
blood and reaches the air sac.
Oxygen passes into the
blood and is carried to all
body cells for carrying out
respiration.
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6
The breathing mechanism:
Breathing in
Breathing out









Intercostal muscles contract. Ribs move
upwards and outwards. Diaphragm
flattens

Intercostal muscles relax. Ribs move
downwards and inwards. Diaphragm
returns to dome shape.

The volume of the chest cavity
increases. The gas pressure inside the

The volume of the chest cavity
decreases. The gas pressure inside the
chest cavity decreases and becomes
lower than the atmospheric pressure.

chest cavity increases and becomes
higher than the atmospheric pressure.

Air is drawn into the lungs.
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Air is forced out of the lungs.
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7
The rib cage model can show the action of the intercostal muscles during breathing.
B
A
C
B
rubber
band
Part in the model
Represents
strip A
backbone
strips B
ribs
strip C
chest bone
rubber band
intercostal muscles
 Represents the situation of breathing in
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8
 Represents the situation of breathing out
The balloon-bell jar model can show the action of the diaphragm during breathing.
tube A
tubes B
Part in the model
Represents
tube A
trachea
tubes B
bronchi
balloons
lungs
rubber sheet
diaphragm
balloons
rubber sheet
 Represents the situation of breathing in
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 Represents the situation of breathing out
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7 Living things and air
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Gaseous exchange in green plants:
Rates of photosynthesis
and respiration
Results
In the
daytime
Rate of photosynthesis > Rate of respiration
Net release of oxygen;
net uptake of carbon dioxide
At night
Stop taking in carbon dioxide for
photosynthesis; respiration continues
Net release of carbon dioxide;
net uptake of oxygen
10 The balance of oxygen and carbon dioxide in nature:
releases oxygen
taken up during
oxygen in air
respiration of animals and plants
photosynthesis of green plants
releases carbon dioxide
taken up during
carbon dioxide in air
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11 Carbon dioxide can absorb heat energy from the sun. It helps keep the Earth warm. This
natural phenomenon is called the greenhouse effect.
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12
Burning fossil fuels in motor cars, power stations and factories gives out a
lot of carbon dioxide.
The increasing amount of carbon dioxide enhances the greenhouse effect,
leading to global warming.
The rising temperature will speed up the melting of ice at the polar regions.
Living things in the polar regions
may lose their habitats and die.
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Sea levels may rise and flood
the low-lying areas.
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7 Living things and air
7.6
Air pollution and smoking
1 Burning fossil fuels in power stations, factories and motor cars gives out large amounts of
air pollutants.
2 The effects of some air pollutants on health:
Air pollutant
Е
Effect on health
Sulphur dioxide and
nitrogen oxides

Irritate the eyes and throat.

Irritate our lungs and cause bronchitis and asthma.
Carbon particles

Irritate our lungs and cause bronchitis and asthma.
Lead particles

Adversely affect the brain functions.
Carbon monoxide

Lowers the amount of oxygen in the blood, causing dizziness
and headaches.
3 The Air Pollution Index ranges from 0 to 500 and is divided into five levels. The higher the
API, the poorer the air quality.
4 The effects of some harmful substances in cigarette smoke on health:
Harmful substance
Effect on health
Carbon monoxide

Lowers the oxygen content in blood.
Tar

Causes throat and lung cancer.

Causes coughs and other respiratory diseases (e.g. bronchitis).

Stains the teeth and finger nails.

Causes addiction.

Increases heart rate and blood pressure.

Leads to heart diseases.

Irritates the eyes, nose and skin.

Causes headaches and vomiting.
Nicotine
Hydrogen cyanide
5 Non- smokers breathing in the smoke given out from other people’s cigarette is known as
passive smoking.
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