Plant Reactions
Students’ Sheet
Introduction
Photosynthesis and respiration
Vast numbers of chemical reactions happen in plants to make compounds and bring about
energy transfers needed for life processes. They happen in the cytoplasm in plant cells.
Photosynthesis and respiration are examples.
Chemical reactions
Chemical reactions involve the rearrangement of atoms. For any chemical reaction, the reactants
and products are made from the same types and number of atoms, but arranged differently.
Atoms can be neither destroyed nor created in a reaction. It’s which atoms bind to which others
that changes.
A chemical reaction can be
represented by a word and a symbol
equation. For example,
H
hydrogen + oxygen  water
H
H
O
O
H
H
O
H

+
H
H
O
2H2 + O2  2H2O
Circle diagrams showing how four hydrogen atoms
and two oxygen atoms are used to make the reacting
molecules and the product molecules.
Energy stores and transfers
Energy is stored in substances by the
forces of attraction between its
particles and the movement of those particles. When
chemical changes happen energy is transferred between
energy stores. The energy stores are the reactants, products
and surroundings.

If more energy stored in reactants than in products, the
difference is transferred from the surroundings to the
reaction mixture. This is an endothermic reaction.

If less energy stored in reactants than in products, the
difference is transferred from the reaction mixture to the
surroundings. This is an exothermic reaction.
Reaction surroundings
In a chemical reaction energy
is transferred from or to a
reaction mixture and its
surroundings. For example,
the air, water (if the reaction
happens in solution) or the
container in which the
reactions happens.
Energy transfer diagrams for chemical reactions
High
reactants
Energy
stored in
reactants
and products
Low
difference in
stored energy
is transferred to
surroundings
products
EXOTHERMIC REACTION
products
reactants
difference in
stored energy is
transferred from
surroundings
ENDOTHERMIC REACTION
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 1
Activities
Activity 1: Ammonia
Ammonia is a gas. Its particles are molecules, each one made from 1 nitrogen atom and 3
hydrogen atoms.
80% of the ammonia manufactured is used to make nitrogen-based fertilisers. It’s made by the
Haber process.
nitrogen + hydrogen  ammonia
Nitrogen and hydrogen are reacted at a carefully controlled temperature and pressure. A catalyst
is also used.
Questions
1. Write the chemical formula for ammonia.
2. Draw circle diagrams to represent nitrogen, hydrogen and ammonia molecules, putting an
arrow between the nitrogen and hydrogen diagrams and the ammonia diagram.
3. Atoms cannot be created or destroyed in a chemical reaction. Work out how many nitrogen,
hydrogen and ammonia molecules are needed to get a ‘balanced equation’, in other words, the
same number and type of atoms on either side of the arrow.
4. Write a balanced symbol equation for the reaction.
5. Which bonds are broken in the reaction and which new ones form?
6. When ammonia is made from nitrogen and hydrogen
energy is transferred from the surroundings to the reaction
mixture.
(a) What does this tell you about the strength of the bonds in
nitrogen and hydrogen compared to the strength of the bonds
in ammonia?
Bond breaking and making
Breaking chemical bonds is
an endothermic change.
Making chemical bonds is an
exothermic change.
(b) Is this an (i) exothermic reaction, or (ii) endothermic
reaction?
Activity 2: Urea
Urea is used in some fertilisers. It is broken down in wet soil to make ammonia and carbon
dioxide. Urease is an enzyme that catalyses the reaction.
(NH2)2CO + H2O  2NH3 + CO2
Ammonia is converted to nitrates by nitrifying bacteria in the soil.
Questions
1. In a urea molecule, a carbon atom bonds to one oxygen atom and two nitrogen atoms. Each
nitrogen atoms also forms bonds with two hydrogen atoms.
Draw circle diagrams to represent the rearrangement of bonded atoms in (a) the reactants, (b)
the products of the reaction.
2. How many molecules of ammonia are produced from one molecule of urea?
3. Which bonds are broken in the reaction and which new bonds form?
4. The strength of the bonds in reacting molecules is greater than in the molecules that form.
(a) Is energy transferred (i) from the reaction mixture to the surroundings, or (ii) from the
surroundings to the reaction mixture?
(b) Is this an (i) exothermic reaction, or (ii) endothermic reaction?
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 2
Activity 3: Photosynthesis and respiration
Photosynthesis happens in green leaves. It is a
series of chemical reactions that may be summarised
by this word equation:
carbon dioxide + water  glucose + oxygen
Energy is transferred from the Sun and absorbed by
the green leaves.
Respiration is the reverse of photosynthesis. It is also
a series of chemical reactions, but not the same ones
as happen in photosynthesis. These may be
summarised by this word equation:
Photosynthesis or respiration?
Plants photosynthesise during the
day.
They respire all the time, not just
at night.
Photosynthesis is faster than
respiration, which means that
over 24 hours there is a net
intake of carbon dioxide and a
net output of oxygen.
glucose + oxygen  carbon dioxide + water
Questions
1. Glucose is a solid. Its particles are molecules. A glucose molecule is made from 6 carbon
atoms, 12 hydrogen atoms and 6 oxygen atoms.
Write the chemical formula of glucose.
2. A glucose molecule has a ring made from 5 carbon atoms and 1 oxygen atom. All of the
carbon atoms have 1 hydrogen atom bonded to them. Four of them also have an oxygen atom
bonded to them, with a hydrogen atom bonded to it. The fifth carbon atom in the ring has another
carbon atom bonded to it which has 2 hydrogen atoms and an oxygen atom bonded to it - and the
oxygen atom has 1 hydrogen atoms bonded to it.
You can probably see now why scientists use diagrams and models rather than trying to describe
molecules!
Try to draw a circles diagram to represent a glucose molecule. Start by making the ring of 5
carbons and 1 oxygen. Then take it from there.
3. Everything gets rather crowded doesn’t it? Here is a drawing of the molecule that uses just the
element symbols and lines to show the bonds between atoms.
H
O
H
C
H
O
C
H
O
H
C
H
O
H
H
H
C
H
O
C
C
O
H
H
Using molecular modelling kits to make 3D models, like the one pictured on the right, with can
help.
(a) In the 3D model, which atoms are presented by the three different colours?
(b) What does the model tell you about a glucose molecule that the drawing doesn’t?
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 3
(c) Why do you think the 3D model shown is often called a ‘ball-and-stick’ or an ‘exploded’
model?
4. Which bonds between atoms are broken and which new bonds form during photosynthesis?
5. Photosynthesis is an endothermic reaction. Energy, transferred from the Sun by light waves, is
absorbed.
What does this tell you about the strength of the bonds in carbon dioxide and water molecules
compared to the strength of the bonds in glucose and oxygen?
6. Respiration is the reverse of photosynthesis: glucose + oxygen  carbon dioxide + water
(a) Is respiration endothermic or exothermic?
(b) Explain why glucose is a plant food.
(c) What do think happens to the energy that is released during respiration?
7. Do you think that the energy transferred during respiration of 1 g glucose is

the same,

greater, or

less than the energy transferred when 1 g glucose forms by photosynthesis?
8. Draw energy transfer diagrams to summarise
(a) photosynthesis
(b) respiration.
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 4