Transcript 1.3.4 Photosynthesis

Match structure and function
Double membrane
chloroplast envelope
As much light as possible can
be absorbed
Thylakoids have large
surface area
Needed for the lightindependent reaction to take
place
ATP synthase molecules
in thylakoid membrane
Reactants kept close to
reaction sites
Stroma contains
enzymes, sugars and
organic acids
Produce ATP in the lightdependent reaction
Learning Objectives
• Outline how the products of the LDS are used
in the LIS
• Explain the role of carbon dioxide in the LIS
• State what TP is used to make
• State that TP is recycled to make RuBP
Photosynthesis
Photophosphorylation:
‘The making of ATP energy using the energy from
light’
May involve cyclic or non-cyclic phosphorylation
Cyclic – PSI – intergranal lamellae
Non – cyclic PSI & PSII – granal lamellae
Non-cyclic
• Light excites electrons of Mg in PSII & PSI
• Electrons passed along electron carriers
• Energy released drives H+ proton pumps
• H+ accumulates in thylakoid space – gradient forms
• H+ flow down gradient through ATP synthase
• ATP used in light independent stage
• Photolysis of H20 and generation of NADPH
Cyclic
• Only uses PSI
• Excited electrons are passed back to Mg in PSI
• No photolysis or generation of NADPH
• Small amounts of ATP made
Used in light independent or to open stomata
1.3.4 Light independent stage
• Occurs in the stroma – also called the Calvin Cycle
• Uses ATP made in the light dependent stage
• CO2 becomes fixed into complex organic
compounds
• C & O used for structure and energy store
Three phases
1. Carboxylation
Carbon dioxide fixation with ribulose bisphosphate
(RuBP)
2. Reduction
Reduction of glycerate 3-phosphate (GP) to
glyceraldehyde 3-phosphate (GALP)
3. Regeneration
Re-formation of the CO2
acceptor molecules
Rubisco
Regeneration of
RuBP
TP
RUBISCO
1.3.4 Light Independent
• CO2 diffuses into the stroma
• Combines with a 5C acceptor RuBP – uses enzyme
• RuBP has become carboxylated – carboxyl group
• Produces 2 x 3C molecule – glycerate 3-phosphate
• GP is reduced & phosphorylated to TP
• TP recycled by phosphorylation to RuBP
1.3.4 Light Independent stage
Products of the Calvin Cycle
•
•
•
•
•
•
GP used to make amino acids & fatty acids
TP combines to form 6C glucose
Some glucose isomerised to Fructose
Fructose & glucose combine = disaccharide
Some sugars polymerised – cellulose
TP converted to glycerol – makes lipids
The light-independent reaction
Triose phosphates (TPs) are used to form glucose.
3C (TP)
+
3C (TP)
1C (x6)
Hexose (6C)
5C
Regenerate RuBP
6C
ATP
•`6 cycles are required to form 1 molecule of glucose
• RuBP is then joined with carbon dioxide to re-start the cycle.
1C (6 cycles)
5C RuBP) + 1C (CO2) 6C
2 x 3C (TP)
Hexose
6C
5C
Regenerate RuBP
The Maths
• 2 molecules of GALP are made each turn
• 5 out of 6 molecules of GALP are used to
regenerate RuBP
• 2 molecules of GALP are needed to make a
hexose sugar
• How many turns are needed to make 1
molecule of hexose sugar?
• How many molecules of ATP and reduced
NADP are needed?
Products of the Calvin cycle
Products of the light-independent reaction that pass back into the light-dependent
reaction:

NADP

ADP

inorganic phosphate
Products of the light-independent reaction that are used in other processes:

triose phosphate – used to build complex carbohydrates, amino acids and
lipids.
How much can you remember?
Summary of photosynthesis
light
light- dependent
reactions
light- independent
reactions
carbohydrates, other complex
molecules
Photosynthesis: true or false?
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)
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)).
Explain the role of carbon dioxide in the light-independent stage
(Calvin cycle).
Carbon Fixation – Light Independent Stage (Calvin Cycle)
•
The carbon fixation stage occurs in the stroma and results in the production of glucose.
•
It is a result of an enzyme controlled sequence of reactions requiring ATP and
hydrogens (from reduced NADP) from the light stage, and carbon dioxide (“fixed”
from the air).
•
It involves the reduction of carbon dioxide, that is the addition of hydrogen (from
reduced NADP), to form carbohydrate.
•
CO2 is accepted by the 5C compound ribulose 1,5-biphosphate (RuBP) to form an
unstable 6C compound.
•
The 6C compound formed immediately splits into two molecules of a 3C compound
called glycerate 3-phosphate (GP).
•
ATP and reduced NADP is used to convert the two GP molecules into two molecules of
triose phosphate (TP), a 3 carbon compound.
•
TP’s are used in the formation of carbohydrate (glucose) and to regenerate RuBP
•
ATP is required to regenerate RuBP
Examination Questions – Model Answers
1
ATP
Reduced NADP / NADPH / NADPH2 / NADPH + H+
Regenerates / produces, ribulose bisphosphate / RuBP
So cycle can continue / for (further) CO 2 fixation / to combine with CO2
Formation of , sugar / glucose / hexose / sucrose / starch / cellulose
Formation of fat / triglyceride / lipid fatty acids / glycerol / amino
acids / protein / nucleic acids / nucleotides
Most triose phosphate used to produce RuBP and the rest for production
of hexose
Oxygen used and CO2 produced / excreted
Light energy required
Uses Rubisco
Involves calvin cycle
Reduces rate of photosynthesis / increases rate of photorespiration
Less Rubisco available for CO 2 / more oxygen competing with CO2
for Rubisco
More O2 binding to Rubisco
O2 outcompetes CO2 for Rubisco
Less CO2 fixation / for Calvin cycle
CO2 given off
Less GP / TP, produced
Less RuBP, regenerated
O2, not a substrate for / cannot bind to / will not compete for.
PEP carboxylase
PEP carboxylase, is only specific to CO 2
2
(i) A
(ii)D
(iii)A
Flattened membrane bound sacs
Termed thylakoids
Arranged as stacks / grana
Contain pigment / chlorophyll arranged in
photosystems / quantosomes, in membrane
(62.4 / 162) x 100
Answer 38.5 %
Different lighting has little effect
Variation in percentage less than 3
Probably not significant
Yield is less for low pressure sodium lamps
The best yield is metal halide
Crops can be grown out of season /all year round
Plants photosynthesize 24 hours a day
Less physical damage from weather /animals
Pest control easier
Other factors can be controlled – e.g. Temperature,
CO2, water supply, humidity