Geography Reference
In-Depth Information
comes from the Sun. Around one sixth of the light energy absorbed
by a green plant is used for photosynthesis while the rest is converted
into chemical or potential food energy of the plant tissues. This
energy can be used by other organisms consuming the plant tissue.
The energy is released as heat through respiration in plants and
animals which consume oxygen and release carbon dioxide. Green
plants therefore need light, and the more light they have the more
growth can be expected. Thus faster growth would be expected in
the tropics and slower growth at the poles. The majority of plant
species (C
3
plants) are found to fix carbon dioxide
into '3-carbon'
compounds known as triose phosphates. However, some other
species (C
4
plants) make a '4-carbon' compound instead, known as
oxaloacetic acid. C
4
plants have only evolved recently with many
being grasses, sedges and a few herbs and shrubs. C
4
plants have an
advantage over C
3
plants in that they can utilise high levels of solar
radiation effectively, use water more efficiently and are more drought
tolerant. They may therefore be favoured by climate change over the
next few centuries and are among the fast growing crops of the
world, such as maize, sorghum, millet and sugar cane. Note that it
has only recently been discovered that light is not a prerequisite for
life and ecosystem development. Instead of photosynthesis, some
deep ocean systems have developed
chemosynthesis
(see Box 5.1).
Box 5.1 Deep ocean life in the dark
At mid-ocean ridges (see Chapter 3) over 2 kilometres below the ocean
surface, where light does not penetrate, there is life. This life is sustained
by energy, not from sunlight, but from hot vents in the ocean floor. These
hot vents emit water and many dissolved chemicals and black particles.
Surrounding the vents there are large communities of animals including
tube worms several metres long and blind shrimps. Unlike photosynthe-
sising green plants at the top of the ocean or on land, bacteria around the
vents gain their energy from the chemicals released by the vents such as
hydrogen sulphide or methane, in a process known as chemosynthesis.
These bacteria are then grazed upon by other creatures, creating a
food
chain.
Some of the bacteria even live in the shells of other creatures. The
deep ocean creatures have to avoid being boiled by temperatures from
the vents which can be over 300°C. The discovery of these ecosystems
means that light is no longer considered to be prerequisite for life.
