Higher Level
Photosynthesis (HL)
Metabolism, Respiration and
Photosynthesis
Essential Idea
Light energy is converted into chemical
energy.
Nature of Science
Developments in scientific research follow
improvements in apparatus—sources of 14 C and
autoradiography enabled Calvin to elucidate the
pathways of carbon fixation.
IB Understandings
● Light-dependent reactions take place in the
intermembrane space of the thylakoids.
● Light-independent reactions take place in the
stroma.
● Reduced NADP and ATP are produced in the
light-dependent reactions.
● Absorption of light by photosystems generates
excited electrons.
IB Understandings
● Photolysis of water generates electrons for use in
the light-dependent reactions.
● Transfer of excited electrons occurs between
carriers in thylakoid membranes.
● Excited electrons from Photosystem II are used to
generate a proton gradient.
IB Understandings
● ATP synthase in thylakoids generates ATP using
the proton gradient.
● Excited electrons from Photosystem I are used to
reduce NADP.
● In the light-independent reactions a carboxylase
catalyzes the carboxylation of ribulose
bisphosphate.
IB Understandings
● Glycerate 3-phosphate is reduced to triose
phosphate using reduced NADP and ATP.
● Triose phosphate is used to regenerate RuBP and
produce carbohydrates.
● Ribulose bisphosphate is reformed using ATP.
● The structure of the chloroplast is adapted to its
function in photosynthesis.
IB Application and Skills
● Application: Calvin’s experiment to elucidate
the carboxylation of RuBP.
● Skill: Annotation of a diagram to indicate the
adaptations of a chloroplast to its function.
Photosynthesis Overview
Photosynthesis
Photosynthesis
Photosynthesis is introduced in Core Topic 8.3
Photosynthesis.
1. Define photosynthesis.
2. State the name of the pigment that absorbs
light for photosynthesis.
3. State the colors (wavelengths) of light that are
best absorbed and reflected by the main
photosynthetic pigment.
Diagram of a Chloroplast
Outer
Membrane
Thylakoid
Space
Stroma
Inner
Membrane
Thylakoid
Membrane
Ribosome
Granum
Lipid
Droplet
Starch Grain
The following slides outline the functions of structures found
in chloroplasts.
The functions become clearer as we look at the stages of
photosynthesis.
Chloroplast Structure
The function of chloroplasts is photosynthesis, which uses
light energy to produce organic compounds.
Chloroplasts contain the following key structures:
Stroma - The stroma contains 70S ribosomes, DNA
nucleoid region, and all of the enzymes required for the
Calvin Cycle (light dependent reactions).
Nucleoid region - contains a single circular chromosome,
which includes genes for the synthesis of enzymes
required for photosynthesis
Chloroplast Structure
Ribosomes - synthesize proteins for the chloroplasts
including the enzymes required for photosynthesis.
Thylakoid membranes - are organised into stacks of
grana. The thylakoid membrane contains: photosystems
(containing chlorophyll), electron transport chains and
ATP synthase.
Thylakoid lumen / space - allows for the concentration of
protons (H+) to accumulate quickly.
Stages of Photosynthesis
Stages of Photosynthesis
Light Dependent Reactions
● Non-cyclic photophosphorylation – produces ATP
and NADPH (Reduced NADP)
● Cyclic photophosphorylation – produces ATP
The Calvin Cycle (Light Independent Reactions)
● Uses ATP and NADPH from the light dependent
reaction, and carbon dioxide to produce glucose
and other organic compounds.
Light Dependent Reactions
In the light dependent reactions, sunlight is
absorbed by photosystems (containing chlorophyll)
to release high energy electrons.
● The high energy electrons provide energy that is
used to produce ATP (using an electron transport
chain found in the thylakoid membrane and
chemiosmosis).
● The electron carrier NADP+ is reduced to NADPH
when it gains electrons from chlorophyll.
Light Dependent Reactions
There are two different light dependent pathways:
● Non-cyclic photophosphorylation - which
produces ATP and NADPH.
● Cyclic photophosphorylation - which produces
ATP.
The ATP and NADPH are used in the Calvin cycle
to produce organic compounds.
Noncyclic
Photophosphorylation
Explain Non-Cyclic
Photophosphorylation
The video on the previous slide describes non-cyclic
photophosphorylation. Refer to chemiosmosis and
wavelengths of light absorbed by chlorophyll in your answer.
Chlorophyll
Chlorophyll in photosystems absorbs light energy
to release electrons to electron transport chains.
● Chlorophyll absorbs mainly red and blue light.
● Chlorophyll reflects green light.
● Accessory pigments (carotene and
xanthophyll) can absorb some light of different
wavelengths and pass the energy to
chlorophyll.
Events of Non-cyclic
Photophosphorylation
Photoactivation of Photosystem II (PSII): Light shines on PS II
(within the thylakoid membrane). Electrons absorb light
energy, and escape from chlorophyll.
The electrons move along an electron transport chain within
the thylakoid membrane.
PS II has lost electrons and is positively charged.
Photolysis of water: The positively charged PS II removes
electrons from water to replace electrons lost by
photoactivation.
Water is split into H+ and O2. Oxygen gas is released as a waste
product.
Events of Non-cyclic
Photophosphorylation
Electron transport chain: The electrons released from PS
II travel along an electron transport chain within the
thylakoid membrane.
The energy from the electrons actively pumps protons
(H+) from the stroma into the thylakoid space.
Chemiosmosis: A high concentration of H+ is present in
the thylakoid space. The H+ returns to the stroma through
ATP synthase.
The moving H+ provide energy for ATP synthase to
catalyze the conversion of ADP and phosphate to ATP.
Events of Non-cyclic
Photophosphorylation
Photoactivation of Photosystem II (PSII): Electrons
arrive at PS I from the electron transport chain. Light
shines on PS I, exciting electrons which escape from
chlorophyll.
Reduction of NADP+: The electrons lost from PS I are
accepted by the electron carrier NADP+.
H+ (from photolysis of water), electrons and NADP+
form reduced NADPH.
Non –Cyclic
Photophosphorylation Notes
● The ATP and NADPH produced during non-cyclic
photophosphorylation are used in the Calvin cycle.
● The process involves many oxidation (loss of
electrons) and reduction (gain of electrons)
reactions.
● The process is referred to as non-cyclic
photophosphorylation because the electrons lost
from PS II do not return to PS II. They are replaced
by electrons from water.
Cyclic
Photophosphorylation
Outline Cyclic
Photophosphorylation
The video on the previous slide describes cyclic
photophosphorylation in bacteria. This process also happens in
plants when there is a low concentration of NAD+.
Events of Cyclic
Photophosphorylation
Photoactivation of PS I: Light shines on PS I, exciting
electrons. The electrons gain enough energy to escape from
chlorophyll.
Electron transport chain: The electrons released from PS I
travel along an electron transport chain within the thylakoid
membrane.
The energy from the moving electrons actively pumps
protons (H+) from the stroma into the thylakoid space. The
electrons return to PS1.
Events of Cyclic
Photophosphorylation
Chemiosmosis: A high concentration of protons (H+) is
present in the thylakoid space. The protons return to the
stroma through ATP synthase.
The moving H+ provide enough energy for ATP synthase
to catalyze the conversion of ADP and phosphate to ATP.
1. Explain why this is cyclic photophosphorylation.
2. Distinguish between cyclic and noncyclic
photophosphorylation.
The Calvin Cycle
IB Naming of Chemicals in
the Calvin Cycle
There are different names for the chemicals which
feature in the Calvin cycle.
You need to know the names that the IB prefers:
● RuBP breaks down to an unstable 6-carbon
compound which breaks down to glycerate 3phosphate.
● Glycerate 3-phosphate is reduced to triose
phosphate.
Describe the Calvin Cycle
The Calvin Cycle
The Calvin cycle is also known as the light independent
reaction of photosynthesis, as it does not need light.
● The Calvin cycle occurs in the stroma where the
enzymes required for the cycle are synthesized by
the chloroplasts’ ribosomes.
● The Calvin cycle uses the products of the light
dependent reactions (ATP & NADPH), carbon
dioxide and RuBP (Ribulose Bisphosphate) to
generate glucose and other organic compounds.
The Calvin Cycle
Carboxylation of Ribulose bisphosphate (RuBP):
RuBP reacts with carbon dioxide to produce an
unstable 6 carbon compound. The enzyme Rubisco
(ribulose bisphosphate carboxylase) catalyzes the
reaction.
Production of glycerate 3-phosphate: The
unstable 6 carbon compound breaks down to two
glycerate 3-phosphate molecules.
The Calvin Cycle
Reduction of glycerate 3-phosphate: Glycerate 3phosphate is reduced to triose phosphate by electrons and
H+ from NADPH.
ATP provides the energy for this reaction to happen.
Production of glucose: Some triose phosphate is
converted to glucose.
Regeneration of RuBP: Most triose phosphate is
converted back to RuBP.
Energy from ATP (from light dependent reactions) is
required for this reaction.
Glucose
The glucose produced by the Calvin cycle can be:
● Respired
● Converted to vitamins
● Converted to other
carbohydrates
including starch and
cellulose
● Converted to amino
acids
● Converted to lipids
● Converted to nucleic
acids
Adaptations of Chloroplasts
● The stroma contains DNA which includes the genes for
making the proteins required for photosynthesis.
● Ribosomes synthesize all of the enzymes required for
photosynthesis (including Rubisco).
● The thylakoid membrane provides a large surface area for
light absorption and electron transport chains.
● The small space within the thylakoid membranes allows
for the rapid build up of H+.
Calvin’s Lollipop
Apparatus
Calvin’s Lollipop
Apparatus
Developments in scientific research follow
improvements in apparatus—sources of radioactive
C-14 and autoradiography enabled Calvin to
elucidate (to work out) the pathways of carbon
fixation.
Read the linked article.
Describe the method that Calvin and his team used to
elucidate the sequence of chemicals present in the
Calvin Cycle.
Calvin’s Lollipop
Experiment
Chlorella algae were added to the lollipop apparatus.
The algae were provided with bright light and a radioactive
source of Carbon dioxide.
Samples of the algae were dropped into boiling alcohol at
five second intervals. This kills the cells and stops
photosynthesis.
The dead algae were analysed using two-way
chromatography.
The sequence of chemicals in the Calvin cycle was
determined by the order that radioactive molecules appeared
on the chromatograms.
Review
Essential Idea
Light energy is converted into chemical
energy.
Nature of Science
Developments in scientific research follow
improvements in apparatus—sources of 14 C and
autoradiography enabled Calvin to elucidate the
pathways of carbon fixation.
IB Understandings
● Light-dependent reactions take place in the
intermembrane space of the thylakoids.
● Light-independent reactions take place in the
stroma.
● Reduced NADP and ATP are produced in the
light-dependent reactions.
● Absorption of light by photosystems generates
excited electrons.
IB Understandings
● Photolysis of water generates electrons for use in
the light-dependent reactions.
● Transfer of excited electrons occurs between
carriers in thylakoid membranes.
● Excited electrons from Photosystem II are used to
generate a proton gradient.
IB Understandings
● ATP synthase in thylakoids generates ATP using
the proton gradient.
● Excited electrons from Photosystem I are used to
reduce NADP.
● In the light-independent reactions a carboxylase
catalyzes the carboxylation of ribulose
bisphosphate.
IB Understandings
● Glycerate 3-phosphate is reduced to triose
phosphate using reduced NADP and ATP.
● Triose phosphate is used to regenerate RuBP and
produce carbohydrates.
● Ribulose bisphosphate is reformed using ATP.
● The structure of the chloroplast is adapted to its
function in photosynthesis.
IB Application and Skills
● Application: Calvin’s experiment to elucidate
the carboxylation of RuBP.
● Skill: Annotation of a diagram to indicate the
adaptations of a chloroplast to its function.
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