Section 4:
The chemistry of photosynthesis
The need for light, chlorophyll and carbon dioxide
Problem to solve
 What substances are required for photosynthesis?
-
light
chlorophyll
carbon dioxide
 How would you prove chlorophyll and carbon dioxide are
necessary for photosynthesis using the apparatus on the
desk? (Hint: you have completed a similar practical in S1)
 Carry out experiment
What are we learning today?
 We are learning to:
-
state that photosynthesis is a two stage process
explain why light and chlorophyll are necessary for
photosynthesis
explain why carbon dioxide is necessary for
photosynthesis
 Success criteria
Two stages photolysis and carbon fixation
Photolysis, light trapped by chlorophyll to split water in to
hydrogen and oxygen
Carbon fixation, carbon dioxide source of carbon for
glucose
The need for light
 From S1 Physical science
you learned that visible
light is split in to the
colours you can see here
 During photosynthesis
plants convert light
energy in to chemical
energy (glucose)
How do plants trap light?
reflected
absorbed
The absorbed
light is used for
photosynthesis
Video clip - Light and starch Science & Plants for Schools
transmitted
Structure of a leaf
phloem
xylem
Guard cells
Structure of the leaf continued
 The palisade mesophyll
and spongy mesophyll
cells have chloroplasts
 Chloroplasts contain the
green pigment
chlorophyll which is
required to trap light
Chloroplasts
 Chloroplasts are the site
of photosynthesis
 Video clip - chloroplasts and starch Science & Plants for Schools
Stroma –
liquid filling
Starch grains –
store of glucose
Grana membranes
containing pigments
e.g. chlorophyll
Two stages of photosynthesis
 Photosynthesis is more than the one line word
equation we know so far.
 Photosynthesis is a series of enzyme controlled
reactions that enable plants to make their own food.
 There are two stages of photosynthesis.
 The two stages of photosynthesis are:
-
photolysis
carbon fixation
Photolysis – occurs in grana
Light energy is used to split
water in to hydrogen and
oxygen
NADP is a
hydrogen
acceptor that is
used to transport
the hydrogen to
the stroma for
carbon fixation
Waste product of
photosynthesis
Light energy is use to make ATP (energy
molecule) also used in carbon fixation
= photophosphorylation
Why is carbon dioxide required for
photosynthesis?
 Revision!!
 Carbon dioxide is required as the source of carbon to
make glucose
Carbon dioxide + Water
CO2
H2O
Glucose + Oxygen
C6H12O6 O2
Carbon fixation – occurs in stroma
 Several enzyme controlled reactions
 Requires energy in the form of ATP from photolysis
 Carbon (from CO2) becomes ‘fixed’ into carbohydrate
 The carbon combines with hydrogen from photolysis
to form glucose
Carbon fixation
NADPH2
NADP
ATP
Carbon
dioxide
(CO2)
ADP + Pi
Glucose
What happens to the glucose?
 Revision!!
 Glucose converted into storage carbohydrate – starch
 Glucose also converted into cellulose - a component
of cell walls
What have I learned today?
 Exit ticket
 On your exit ticket write down (if you can) the summary
of photolysis and carbon fixation