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The Light Independent Reactions
Objectives
• Outline how the products of the light-dependent
stage are used in the light-independent stage
(Calvin cycle) to produce triose phosphate (TP) –
referring also to ribulose bisphosphate (RuBP),
ribulose bisphosphate carboxylase (rubisco) and
glycerate 3-phosphate (GP).
• Explain the role of carbon dioxide in the lightindependent stage.
• State that TP (and GP) can be used to make
carbohydrates, lipids and amino acids.
• State that most TP is recycled to RuBP.
The Light Independent Reactions
• Occur in the stroma via a cyclic pathway
called the Calvin cycle.
• They are a result of a sequence of
enzyme controlled reactions requiring
ATP and reduced NADP (from light
dependent stage) and CO2.
• CO2 diffuses into the leaf through
open stomata, into the air spaces.
Describe the path taken by CO2 to get to
the chloroplast
• Use these key words:
Stomata
Stroma
Spongy mesophyll
Air space
Palisade mesophyll
Thin cell wall
Chloroplast envelope
Cytoplasm
Cell membrane
• In the stroma CO2 is fixed by the 5 carbon
compound ribulose bisphosphate (RuBP) to
form an unstable 6 carbon compound.
• RuBP is a CO2 acceptor molecule.
• This process is catalysed by the enzyme
RUBISCO and is a carboxylation reaction
• RUBISCO is made in chloroplasts using
chloroplast DNA
• The 6C compound immediately splits into two
molecules of a 3C compound called
glycerate-3-phosphate (GP).
• ATP and reduced NADP from the light
dependent stage are used to reduce
glycerate-3-phosphate to produce
triose phosphate (TP –a 3 carbon
phosphorylated sugar)
• One sixth of TP is used to make
glucose, amino acids, fatty acids,
glycerol or nucleic acids.
• Five sixths are used to regenerate more
RuBP (this also requires ATP).
The Calvin Cycle
The Uses of the Calvin Cycle Products
• Glycerate-3-phosphate can be used to
synthesise fatty acids by entering the
glycolytic pathway and being converted
to acetyl CoA
• TP is used to synthesise glycerol
• Glycerate-3-phosphate and inorganic
salts are used to synthesise amino acids
• Pairs of TP combine to form hexose
sugars e.g. glucose
• Glucose and fructose combine to form
sucrose the main translocatory sugar
• Starch is synthesised in the stroma for
energy storage
• The structural carbohydrate cellulose is
also synthesised