Download ES9_CHAP_Ca6 discovering photosynthesis

9Ca(6)
Discovering photosynthesis
In the seventeenth century many people thought that plants sucked up soil from the ground to eat.
Jean-Baptiste van Helmont (1579–1644) did an experiment to show that this was wrong.
?
Van Helmont said that his experiment showed that a plant could not be made from earth because
the mass of the soil decreased only a little bit but the mass of the willow tree increased a lot. He
said that the extra mass of the tree must be made from water. In 1771, Joseph Priestly did an
experiment that showed that plants produced a gas in which a candle would burn. He called this
‘dephlogisticated air’ (dee-floj-jiss-tick-ay-ted).
In 1754, Charles Bonnet (1720–1793) had shown that bubbles were produced by leaves placed
underwater. He said that this was due to dissolved air in the water sticking to the leaves. However
in 1779, Jan Ingenhousz (1730–1799) showed that leaves placed underwater produced bubbles of
‘dephlogisticated air’ but only when the Sun was shining. He also showed that only the green parts
of plants would do this.
Later in 1782, Jean Senebier (1742–1809) showed that ‘fixed air’ contained carbon dioxide and
that it was used up by the plant. During this time the idea that mass was conserved in reactions
was put forward. In 1804, Theodore de Saussure (1767–1845) found that the mass of carbon
dioxide used up by a plant was not enough to account for the increase in mass of the plant. Since
the only other thing needed to keep the plant alive was water, he suggested that both carbon
dioxide and water were changed by the plant into new material.
Page 1 of 2
94
9Ca(6) Discovering photosynthesis (continued)
1 a Write the word equation for photosynthesis.
b Which substances are the reactants?
c Which scientist showed that light was needed for photosynthesis to happen?
d What observation led him to develop this idea?
2 a What is the modern word for the gas produced by the candle to make ‘fixed air’?
b What is the modern word for the gas produced by the plant to make ‘dephlogisticated
air’?
3 a Suggest how Ingenhousz may have collected a sample of ‘dephlogisticated air’.
b How do you think he showed that this was ‘dephlogisticated’ and not ‘fixed’ air?
4 a Do you think van Helmont was successful in what he set out to prove? Explain your
answer.
b Explain why van Helmont’s conclusion was only half right.
c Van Helmont’s tree had a mass of 76.74kg after 5 years. Why do you think that the total
mass of material produced by the plant was actually greater than this?
I CAN... ••
extract relevant information from a text
evaluate a conclusion based on the available evidence.
95
Page 2 of 2