Photosynthesis: The Light Dependent Stage
• state that the light-dependent stage takes place in thylakoid
membranes and that the light-independent stage takes place in the
stroma;
• outline how light energy is converted to chemical energy (ATP and
reduced NADP) in the light-dependent stage (reference should be
made to cyclic and non-cyclic photophosphorylation, but no
biochemical detail is required);
• explain the role of water in the light-dependent stage;
• outline how the products of the light-dependent stage are used in
the light-independent stage (Calvin cycle) to produce triose
phosphate (TP) (reference should be made to ribulose bisphosphate
(RuBP), ribulose bisphosphate carboxylase (rubisco) and glycerate
3-phosphate (GP), but no other biochemical detail is required);
Light Dependent Stage
• Takes place in the thylakoid
membranes of the chloroplast
• Photosystems are embedded in
these membranes
• Photosystem I mainly in the
intergranal lamellae
• Photosystem II almost exclusively
on the granal lamellae
• Photosystems trap light energy to
be converted to chemical energy
Photosynthesis
• We will look at what happens with the raw
materials in photosynthesis
Water + carbon dioxide  glucose + oxygen
6H2O +
6CO2
 C6H12O6 + 6O2
Stage 1: Splitting Water
• Known as photolysis
• Enzymes in Photosystem II split water with
light into H+ and e• 2H2O  4H+ + 4e- + O2
• Some oxygen used for respiration
• Some diffuses out into the air
• In this way, water is one of the raw materials
in photosynthesis
Stage 2: Electron Transport
• Just like the one in Respiration, Photosynthesis begins
with an electron transport chain called
photophosphorylation (as opposed to oxidative
phosphorylation in respiration)
• It involves Photosystems I and II
• A photon of light hits a chlorophyll molecule which
excites 2 electrons that have come from water
• The electrons are captured by electron acceptors and
passed along electron carriers embedded in the
thylakoid membranes
• Again, the electron carriers are proteins containing iron
(like in respiration)
Stage 2: Electron Transport
• Energy is released as the electrons are passed from
carrier to carrier
• This pumps protons across the thylakoid membranes
into the thylakoid space where they accumulate
• A proton gradient forms
• Chemiosmosis takes place and the H+ move through
ATP synthase and ATP is generated
• The Hydrogen acceptor is a co-enzyme called NADP
which becomes reduced NADP
• This is called Non- Cyclic Photophosphorylation
Task
• Underneath your copy of the diagram, use the
textbooks to describe the sequence of events
happening in non-cyclic photophosphorylation
Non- Cyclic Photophosphorylation
(Z- Scheme)
• Photons of light hit photosystem II and move
electrons to a higher energy level
• ATP is made as the electrons are passed from carrier
to carrier
• The electrons are passed along a chain of electron
carriers to photosystem I
• Light energy also hits photosystem I which again,
excites electrons which combine with Hydrogen ions
and pass to a different electron acceptor (NADP) to
become NADPH
Cyclic Photophosphorylation
• This is another way that ATP is formed, it involves:
• Only Photosystem I
• Excited electrons from water pass to an electron
acceptor, then back to the chlorophyll molecule from
which they were lost
• Small amounts of ATP are made
• It is known as cyclic photophosphorylation
Photosynthesis: The Light Independent Stage
(Calvin Cycle)
• outline how the products of the light-dependent stage
are used in the light-independent stage (Calvin cycle)
to produce triose phosphate (TP) (reference should be
made to ribulose bisphosphate (RuBP), ribulose
bisphosphate carboxylase (rubisco) and glycerate 3phosphate (GP), but no other biochemical detail is
required);
• explain the role of carbon dioxide in the lightindependent stage (Calvin cycle);
• state that TP can be used to make carbohydrates, lipids
and amino acids;
• state that most TP is recycled to RuBP;
Calvin Cycle
• Takes place in the stroma of the chloroplast
• Products of light dependent stage
(photophosphorylation) are used
Calvin Cycle
• Carbon dioxide diffuses into leaf through stomata on
underside of leaf and enters stomata
• Combines with 5-carbon rubulose bisphosphate (RuBP)
helped by the enzyme rubisco
• Two 3-carbon glycerate 3-phosphates (GP) created
• GP is reduced and phosphorylated by NADP and ATP to
make triose phosphate (TP)
• Triose phosphate goes on to make sugars e.g. glucose
• RuBP is reformed
How are the Products of the Calvin Cycle Used?
• GP used for amino acids and fatty acids
• TP used to make sugars e.g. glucose, sucrose,
fructose
• TP can be converted to glycerol and combined
with fatty acids formed from GP to make lipids
• So...
• TP can be used to make carbohydrates, lipids
and amino acids;
• But... most TP is recycled to RuBP
Questions
1. Where do the light dependent and light independent
reactions take place?
2. Outline how light energy is converted to chemical energy
(cyclic and non cyclic)
3. Explain the role of water in the light-dependent stage
4. Explain the role of carbon dioxide in the light independent
stage (calvin cycle)
5. What happens to most of the triose phosphate produced in
the Calvin Cycle?
6. What else can triose phosphate make?
Questions
1. Where do the light dependent and light independent
reactions take place?
2. Outline how light energy is converted to chemical energy
(cyclic and non cyclic)
3. Explain the role of water in the light-dependent stage
4. Explain the role of carbon dioxide in the light independent
stage (calvin cycle)
5. What happens to most of the triose phosphate produced in
the Calvin Cycle? Most is recycled to RuBP
6. What else can triose phosphate make? Carbohydrates, lipids
and amino acids