Plant Reactions
Technical & Teaching Notes
Related sheets
Plants, matter and energy; Plant nutrients; Making and testing nutrients; Homemade fertilisers; Soil
culture; Water culture; Floating culture
Introduction and context
Reactions that happen in plant cells make countless numbers of compounds needed for all the
plant’s life processes. They obey the same rules of chemistry as any others that happen in
laboratories or industrial plants. The difference is that they happen in plant cells rather than
glassware or stainless steel reactors. Students use the ideas of atom rearrangements and energy
transfers to understand reactions that happen in plants.
Although not essential to do so, this sheet might be best used after the sheet Plants, matter and
energy.
Possible barriers to learning
The session allows a number of common misconceptions and other hindrances to learning
associated with matter and energy to be addressed. These include:

energy is a material and has mass (like a Mars bar)

there are different forms of energy

plants breathe in oxygen at night and carbon dioxide during the day

plants photosynthesise but do not respire.
Other concepts that may be addressed are ideas about:

energy stores, energy transfers and reaction surroundings

exothermic and endothermic reactions

making chemical bonds is an exothermic change; breaking chemical bonds is an endothermic
change.
Links to National curriculum for science in England at key stage 3
In biology pupils should be taught about photosynthesis and cellular respiration. This includes:

the reactants in, and products of, photosynthesis, and a word summary for photosynthesis

aerobic and anaerobic respiration in living organisms, including the breakdown of organic
molecules to enable all the other chemical processes necessary for life

a word summary for aerobic respiration
In chemistry pupils should be taught about atoms, elements and compounds, chemical reactions
and energetics. This includes:

a simple (Dalton) atomic model

differences between atoms, elements and compounds

chemical symbols and formulae for elements and compounds

chemical reactions as the rearrangement of atoms

representing chemical reactions using formulae and using equations

exothermic and endothermic chemical reactions (qualitative).
In physics pupils should be taught about energy changes and transfers, forces, static electricity
and energy in matter. This includes:
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 1

processes that involve energy transfer: metabolism of food, burning fuels

separation of positive or negative charges when objects are rubbed together: transfer of
electrons, forces between charged objects

forces as pushes or pulls, arising from the interaction between two objects

internal energy stored in materials.
Teaching Notes
Activity 1: Ammonia
In this activity students are introduced to energy transfers that happen when bonds are broken or
made:
Breaking chemical bonds is an endothermic change
Making chemical bonds is an exothermic change.
As an alternative to drawing circle diagrams, coloured discs or counters could be used. To be
consistent with the convention commonly used these are colours used to represent atoms:
Hydrogen, white; Carbon, black; Nitrogen, blue; Oxygen, red; Chlorine, green; Metals, grey
Answers
1.
N
N
nitrogen
H
H
hydrogen
H
N
H
H
ammonia
2. It takes 1 nitrogen molecule and 3 hydrogen molecules to make 2 ammonia molecules.
3. N2 + 3H2  2NH3
4. 1 x N-N and 3 x H-H broken; 3 x N-H formed.
Note: The hydrogen-hydrogen bond in a hydrogen molecule is a single covalent bond: H-H; the
nitrogen-nitrogen bond in a nitrogen molecule is a triple covalent bond: N≡N. However, at this
stage the nature of the bond could be introduced but it is not necessary for the activity.
5. The combined strength of the bonds in the reactants (1 nitrogen molecule and 3 hydrogen
molecules) is greater than the strength of the bonds in the products (3 ammonia molecules).
Activity 2: Urea
This activity allows students to following a more complex chemical reaction that involves
molecules. It’s relevance to plants is that urea is a fertiliser which is broken down in wet soil to
make ammonia and carbon dioxide. Urease is an enzyme that catalyses the reaction.
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 2
Answers
1.
N
H
H
C
H
N
O
H
O
H
N
H
H
H
ammonia
ammonia
N
H
H
H
water
H
O
urea
C
O
carbon dioxide
2. Two
3. 4 x N-H, 2 x C-N, 1 x C-O and 2 x O-H broken, 6 x N-H and 2 x C-O formed.
Some students might simplify this to 2 x C-N, C-O and 2 x O-H broken, 2 x N-H and C-O formed.
However, you need to be wary. This might suggest that we know how the reaction happens, but
the purpose of the circle diagrams is simply to allow some ‘atom accountancy’, in other words,
keeping track of the number and types of atoms. It tells us nothing about the reaction mechanism.
4. (a) Energy is transferred to the surroundings, (b) Exothermic.
Activity 3: Photosynthesis and respiration
This activity illustrates the two types of energy transfers during chemical reactions: endothermic
and exothermic.
The fact that energy is not matter and does not have mass can be pointed out. However,
quantities of energy that are transferred can be measured; this is important in question 8.
It provides an opportunity to introduce or reinforce the idea that plants respire all the time, not just
at night, and photosynthesise during the day. 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.
The activity may be quite demanding and students may need some guidance and support.
Answers
1. C6H12O6.
2. Students may attempt this. The starting point is:
C
O
C
C
C
C
However, it is very difficult to continue simply because everything becomes so crowded.
This is the time to introduce other ways of modelling molecules. It can also be pointed out that
ball-and-stick models can show more than which and how many atoms are used to make a
molecule and which atoms are bonded to which. Models can also show how the bonded atoms
are arranged in space, in other words, represent the structure and shape of a molecule.
3. (a) Black = carbon, red= oxygen, white = hydrogen
(b) How the bonded atoms in a molecule are arranged in space, in other words, its shape
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 3
(c) Because it shows the bonded atoms a distance apart, whereas they are (more or less)
touching.
Note: This is consistent with the convention most commonly used (see answer to question 1).
4. Reactants: In each carbon dioxide molecule 2 x C-O bonds are broken and in each water
molecule 2 x O-H bonds are broken.
Products: In a glucose molecule, 5 x C-C, 7 x C-O, 7 x C-H and 5 x O-H bonds are formed and in
each oxygen molecule an O-O bond is formed.
5. The total strength of bonds in the reactants (6CO2 + 6H2O) is less than the total strength of
bonds in the products (C6H12O6 + 6O2).
6. (a) Exothermic
(b) It’s an energy store as well as a substance that can be converted into other important
compounds needed by the plant
(c) It is transferred to reactants involved in other chemical reactions that are happening in plant
cells.
7. The same. The reaction stages are not simply the same ways for photosynthesis and
respiration but in reverse order (in other words, the forward and back stages are different to one
another). However, the overall change is the same in both directions. So, for specific quantity of
glucose, the total amount of energy transferred from the Sun in photosynthesis is the same as the
amount of energy transferred to the surroundings in respiration.
8. (a) Photosynthesis: 6CO2 + 6H2O  C6H12O6 + 6O2
High
C6H12O6 + 6O2
6H O
Energy stored
in reactants
and products
Low
Energy
transferred
from the Sun
6CO2 + 6H2O
(b) Respiration: C6H12O6 + 6O2  6CO2 + 6H2O
High
Energy stored
in reactants
and products
Low
C6H12O6 + 6O2
6H O
Energy
transferred to
the surroundings
6CO2 + 6H2O
Notes
Students might write the names of the reactants and products rather than their chemical
formulae.
It is important to stress that the energy change is the same only when the amount of glucose
produced by photosynthesis is the same as the amount of glucose used in respiration.
Science & Plants for Schools: www.saps.org.uk
Plant Reactions: p. 4