as you know life is made of carbon
compounds we will see how the element
carbon is taken in as carbon dioxide by
plants used and reused by all living
things before being respired back into
the environment as carbon dioxide once
again so let's start with the gas carbon
dioxide do you know how much there is of
it in the air
well the air contains nearly 0.04
carbon dioxide but hundreds of years ago
before we started using fossil fuels it
was only 0.28
not much you might think but enough to
enable plants to build up all the
structures they need with a little help
from the water and minerals they take up
from their roots and energy from the sun
plants use the sunlight to pull oxygen
away from water and carbon dioxide
allowing the plant to capture the carbon
this happens in the green chloroplasts
found in the leaves of plants
what is the name of this reaction
perhaps the most important chemical
reaction on the planet pause the video
whilst you think of your answer
it is photosynthesis well done if you
got it right
photo means light and synthesis means
building up
however in its simplest terms we are
actually breaking carbon dioxide apart
releasing the reactive gas oxygen into
the environment
allowing the carbon to be captured by
the plants to build up their structures
as they grow
animals in their turn get carbon by
eating plants or other animals
all this of course requires energy
in order to fuel these living processes
plants and animals take back oxygen from
the air
and rejoin it with the carbon releasing
carbon dioxide back into the environment
do you know the name of this energy
releasing process
pause the video whilst you think of your
answer
well it's called respiration and here is
a simple representation of the two
processes first of all sunlight breaks
the bonds allowing the carbon dioxide
and water to form oxygen and
carbohydrate which is a compound of
carbon and hydro or water
hence c h2o a simple representation of
the carbohydrate molecule
that's photosynthesis
energy is now stored and living things
can use the carbohydrates to build their
bodies and structures
to obtain energy they simply reverse
this reaction letting the oxygen and the
carbohydrate to rejoin forming carbon
dioxide and water all over again and
this is respiration
let's look at these two processes more
closely they both need catalysts to
enable them to work at normal
temperatures
these catalysts are called enzymes
photosynthesis also needs a pigment
called chlorophyll the active site is a
magnesium atom coloured green in this
model which absorbs blue and red light
from the sun spectrum to drive the
photosynthesis reactions
leaving the green light to be reflected
making the leaves look well green
the reaction takes place by a number of
complex stages it's a reaction which
evolved early on in the lifetime of our
planet and the blue-green algae also
known as cyanobacteria are still alive
today living in the sea
the chloroplasts you find in the green
leaves of plants evolved from these
simple bacteria
if only we could find a way to use
sunlight to split carbon dioxide and
water to make fuel and oxygen for
ourselves we could solve our green
energy problems
scientists are trying to find
alternative catalysts to chlorophyll but
if there was a better catalyst maybe
life would have evolved it already
respiration also needs a series of
enzymes to enable oxygen to rejoin with
a carbohydrate fuel a series of
reactions takes place in so-called
mitochondria in the cells of plants and
animals in humans and most other
vertebrates their blood transports the
oxygen from the lungs and the digested
food from the gut to all the cells in
their body there the food and oxygen
react and the blood takes the carbon
dioxide back to the lungs to be excreted
plants also respire but you only notice
it in the dark because in the sunlight
they release more oxygen from
photosynthesis than they use in
respiration but think of the roots and
seeds which are hidden from the sun
pause the video and think how roots and
seeds get energy
the answer is that they need a constant
supply of oxygen which means that the
soil must be full of air and not
waterlogged the roots get sugars sent
down from the leaves and seeds have
enough of their own fuel supply until
they put out green shoots and start
making more respiration of food is
similar to combustion in both cases
oxygen is added to the fuel or the food
and carbon dioxide is built up in
respiration the reaction happens at
normal temperatures using enzymes for
example in the cells of our bodies
whereas combustion having no catalyst
only takes place at high temperatures
but both reactions release energy
you