Chemistry Reference
In-Depth Information
Figure 11.4
This algae is related to that early organism produced all
those years ago!
These organisms used the light from the Sun
to produce their own food, and oxygen was
released into the atmosphere as a waste product of
photosynthesis
. This process also acted to reduce
the amount of CO
2
in the atmosphere.
The ultraviolet radiation now broke down some of the
oxygen molecules in the stratosphere (the level above the
troposphere, see below), and some of the oxygen atoms
produced formed ozone molecules, O
3
(
g
). Ozone is an
unstable molecule which readily decomposes under the
action of ultraviolet radiation. However, some of these
ozone molecules then re-form.
Ozone is an important gas in the upper atmosphere
because it prevents harmful ultraviolet radiation from
reaching the Earth. Over many millions of years, the
amount of ultraviolet radiation reaching the Earth's
surface was reduced significantly.
About 400 million years ago the first land plants
appeared on the Earth and so the amount of oxygen,
and hence ozone, increased.
Figure 11.3
Volcanic activity like this expelled gases through the crust
to form a secondary atmosphere.
About 3800 million years ago, when the Earth
had cooled below 100 °C, the water vapour in this
atmosphere condensed and fell as rain. This caused
the formation of the first oceans, lakes and seas on
the now rapidly cooling Earth. Eventually, early
forms of life developed in these oceans, lakes and
seas at depths which prevented potentially harmful
ultraviolet light from the Sun affecting them.
About 3000 million years ago the first forms of
bacteria appeared, followed by algae-like organisms
(Figure 11.4).
Oxygen is a reactive gas and, over millions of
years, organisms have adapted to make use of it.
The oxygen from the atmosphere was used, along
with the carbon they obtained from their food, to
produce energy in a process known as
respiration
.
The process of respiration can be represented as:
The process of photosynthesis can be described
by the following equation:
glucose
+ oxygen
→ carbon
+
water+
energy
dioxide
C
6
H
12
O
6
(
aq
)
+
6O
2
(
g
)
→
6CO
2
(
g
)
+
6H
2
O(
l
)
+ energy
sunlight
chlorophyll
carbon
+ water
●
glucose
+ oxygen
dioxide
6CO
2
(
g
) + 6H
2
O(
l
)
The atmosphere as we know it therefore took a very
long time to develop to what it is today.
C
6
H
12
O
6
(
aq
) + 6O
2
(
g
)
