PHOTOSYNTHESIS SUMMARY NOTES

(a) With reference to the chloroplast structure, explain the light

dependent reactions of photosynthesis (No biochemical details are
needed but will include the outline of cyclic and non-cyclic light
dependent reactions, and the transfer of energy for the
subsequent manufacturing of carbohydrates from carbon dioxide).

Relate chloroplast structure to its function: Electron micrograph of chloroplast

Structural features Functions

 Bound by chloroplast envelope

 Double membrane which partitions its contents  Controls internal movement of CO2, water and minerals
from the cytosol. and outward movement of sugars.

 A system of thylakoid membranes run through Thylakoid membrane is the site for light dependent reactions
the stroma. in photosynthesis:

 Thylakoids are flattened membranous, fluid-  Provide large surface area which holds photosynthetic
filled sacs inside the chloroplast, which are pigments (arranged in photosystems)/ light harvesting
joined to one another by membranes to form complexes for maximum light absorption.
stacks of thylakoids, called grana (singular:  Site of electron transport chain, maintains the electron
granum), which are joined to other grana by carriers in sequence of highest to lowest energy level;
intergranal lamellae  Thylakoid membrane impermeable to ions, enable the
establishment of proton gradient (by maintaining a high
proton concentration in the thylakoid space after protons
have been actively transported from stroma into thylakoid
space)
 Also holds ATP synthase, and is the site of ATP
synthesis by chemiosmosis.

 Separates the interior of chloroplast into two

compartments: thylakoid space and the stroma.

 Thylakoid space 
+
 Space inside the thylakoid.  Contains proton (H ) reservoir used in chemiosmosis.

 Stroma
 Viscous fluid outside the thylakoids  Bathes the membrane of the grana so that it can receive
 the products of the light-dependent reactions. (ATP &
NADPH)

 Site of light independent reactions (Calvin cycle).

 Contains 70S ribosomes, circular chloroplast DNA, lipid

droplets, starch grains, sugars, organic acids, enzymes of
Calvin cycle (e.g. ribulose bisphosphate carboxylase).

Explain the roles of membranes in the chloroplast.

The membranes would refer to the chloroplast envelope and the thylakoid membrane. Refer
to the above table to explain the roles of the relevant membranes.

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 Describe and explain the light dependent reactions of photosynthesis.

Non-cyclic photophosphorylation
Light harvesting step at PSII
 A photon of light strikes a pigment molecule in the light harvesting complex on the
thylakoid membrane and energy is being relayed by resonance from pigment to
pigment until it reaches the P680 pair of chlorophyll a in the photosystem II reaction
centre complex where electrons are excited.

There can be a question to ask you to solely describe the light harvesting stage e.g. Explain the photoactivation of
chlorophyll.
Answer follows the same concept as that of the light harvesting step:
1. A photon of light strikes a pigment molecule in the light harvesting complex on the thylakoid membrane.
2. Light energy absorbed excites electron in chlorophyll to a higher energy level.
3. Energy is passed on from one chlorophyll molecule to another until it reaches the reaction center ( a pair of
chlorophyll a molecules)
4. Excited electron ejected from one of the chlorophyll a molecule;

Transfer of electron from P680 pair of chlorophyll a to primary electron acceptor

 The excited electron from the P680 pair of chlorophyll a is raised to a higher energy
level and captured by the PSII primary electron acceptor.
 Hence, P680 has an electron ‘hole’ which will be filled with an electron from the
photolysis of water.

Photolysis of water
 A water-splitting enzyme catalyses the splitting of each water molecule into 2
protons, 2 electrons and 1 oxygen atom.
 The electrons fill the electron hole in PSII (P680), H+ is used to reduce NADP (in the
stroma) and O atom combines with another resulting in the release of molecular
oxygen.

Chemiosmosis
 Excited electron is passed from primary electron acceptor of PSII to P700 of PSI
down electron transport chain, embedded in the thylakoid membrane.
 Energy released, as electrons moves down the chain of electron carriers, is
harvested by proton pump to pump H+ from the stroma to the thylakoid space.
 Setting up a proton gradient across the thylakoid membrane & its hydrophobic core
prevents charged H+ from passing through
 Protons diffuse down the proton gradient through the hydrophilic channel associated
with ATP synthase, from the thylakoid space back into the stroma.
 This generates a proton motive force for the phosphorylation of ADP to synthesize
ATP.

Light harvesting step at PSI

 Meanwhile, the light harvesting complexes of PSI absorbs light; energy is eventually
transferred by resonance to the P700 chlorophyll a in the reaction center complex.
 Electron from this pair of chlorophyll a molecules is excited to a higher energy level
and captured by the PSI primary electron acceptor.

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  This creates an “electron hole” in the P700. It now acts as an electron acceptor,
accepting an electron that reaches the bottom of the ETC from PSII.

Transfer of electron down second ETC

 The electron is then passed down from the primary electron acceptor of PSI, down
another ETC to ferrodoxin (Fd).
 No ATP is synthesized for this ETC.
 Fd then passes the electron to NADP, acting as a final electron acceptor,
producing NADPH/reduced NADP.
 NADP reductase catalyses this reduction reaction.

* Non-cyclic photophosphorylation produces equal amount of ATP and NADPH. However for every run of the Calvin cycle, more
ATP is required than NADPH. The shift from non-cyclic to cyclic photophosphorylation will produce more ATP to make up these
differences until the ATP supply catches up with the demand.

Cyclic photophosphorylation
 Light harvesting complexes of PSI absorbs light; energy is eventually transferred by
resonance to the P700 chlorophyll a in the reaction center complex.
 Electron from this pair of chlorophyll a molecules is excited to a higher energy level
and captured by the PSI primary electron acceptor.
 This creates an “electron hole” in the P700.

 The electron passed down from the PSI primary electron acceptor, down another
electron transport chain to ferrodoxin (Fd) which passes the electron back down the
first ETC, back to PS I.
 The downhill flow of electrons down the first ETC results in the energy drop being
coupled to the production of ATP by chemiosmosis.

Explain how non-cyclic photophosphorylation differs from cyclic photophosphorylation.

Features Non-cyclic Cyclic
Operates when plant requires ATP and NADPH only ATP
Pathway of electron Linear pathway: electron does not return to Cyclic pathway: electron return to the
(Cyclic or non-cyclic) the same molecule/ electron flow through same molecule (chlorophyll a of PSI)
Photosystems involved
PSII & PS I PS I
(PSI/PSII or both)
First electron donor P700 in PSI (Hence, no photolysis of
Water (Hence, photolysis of water occur)
(source of electrons) water occur)
Last electron acceptor (destination of
NADP P700 in PSI
electrons)
Products ATP, oxygen and NADPH Only ATP

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Explain the importance of the light dependent reaction in photosynthesis.

 Converts light energy to chemical energy;

 Via cyclic and non-cyclic photophosphorylation;
(Non-cyclic photophosphorylation)
 Photoactivation of chlorophyll;
 Light energy used to excite electrons to higher energy level;
 Displaces electron in reaction center (chlorophyll a molecules);
 Electrons pass down ETC, energy release is used to synthesize ATP from ADP and Pi via
chemiosmosis;
 ATP is then used to provide energy needed in carbon reduction and RuBP regeneration
phase of Calvin cycle;
 Photolysis of water to generate electrons and protons;
 Electrons will fill up the elctron hole in chlorophyll a in PSII;
 Electron from PSI donated to NADP, to produce NADPH;
 NADPH is then used to provide reducing power for carbon reduction in Calvin cycle;
(Cyclic photophosphorylation)
 High energy electrons from PS I returns back to first ETC;
 To generate more proton motive force for ATP synthesis;

Describe the role of light in photophosphorylation.

 Source of energy;
 To excite electrons/ raised electrons to higher energy level;
 For synthesis of ATP (from ADP);
 And reduced NADP/ NADPH;
 Ref. photolysis (of water);

Describe the role of the electron transport chain in photosynthesis.

 series of electron carriers;
 embedded in thylakoid membrane in chloroplasts;
 involved in (cyclic and non-cyclic) photophosphorylation ;

 electron carriers are arranged in order of decreasing energy level ;

 series of redox reactions / electron carriers alternate between reduced and oxidized states as
they accept and donate electrons as electrons are transferred down ETC;

 ref. energy released in ETC

 used to pump protons by proton pump
 from stroma to thylakoid space
 generate proton gradient across thylakoid membrane;
 protons diffused back into stroma via hydrophilic channel associated with the ATP synthase
embedded on the membrane;
 generate proton motive force to synthesize ATP;

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Outline the role of NADP in photosynthesis.
 coenzymes of dehydrogenase and function as electron/hydrogen carrier;
 Final electron acceptor in non-cyclic photophosphorylation;
 Drives photolysis of water;
+ -
 2H + 2e + NADP  NADPH or reduced NADP/ hydrogen ions from splitting of water
combine with NADP to form NADPH;
 NADPH passes to Calvin cycle/ light independent reactions;
 NADPH reduces glycerate bisphosphate to triose phosphate in carbon reduction phase;
 and gets reoxidized back to NADP;

DCPIP
The role of NADP can be replaced by DCPIP in the following experimental context:
A suspension of chloroplasts was made by grinding fresh leaves in buffer solution and centrifuging the mixture.
Tubes were then prepared and treated in the following way:
Tubes Content Treatment Colour Explanation
Start After 20 min
A 1 cm3 Illuminated Blue-green Green  DCPIP acts like a final electron acceptor, NADP+;
 in non-cyclic photophosphorylation of the light-dependent reaction;
chloroplast strongly  Light energy is absorbed by pigments in LHC in thylakoid membrane; and
suspension relayed to reaction centers P700 and P680;
5cm3 DCPIP  Chlorophyll a in reaction centers eject an excited electron, which is then
passed down the ETC and finally to DCPIP;
 changing DCPIP from blue to colourless/ since DCPIP is in a mixture of green
chloroplasts, hence the colour change is seemed to change from blue-green
to green;
B 1 cm3 buffer Illuminated Blue Blue Control to show To show that chloroplast is indeed responsible for the colour
change
solution strongly
5cm3 DCPIP
C 1 cm3 Left in the dark Blue-green Blue-green Question asked about tube C:
The chloroplast suspension may be contaminated with mitochondria. Explain the
chloroplast evidence from the investigation that presence of mitochondria was not
suspension responsible for the reduction of DCPIP. [2]
5cm3 DCPIP Respiration in the mitochondria can occur in the presence or absence of light,
producing protons and electrons for the reduction of DCPIP;
However when left in the dark (tube C) DCPIP remained blue-green like at the
start of the expt, indicating that there is no reduction of DCPIP ;

Herbicides
There are also herbicides (weed killers) such as diquat and paraquat that readily accept electrons. Explain how these herbicides
can cause the plants to die.
 No photophosphorylation
 Excited electrons emitted from P680 and P700/ special chlorophyll a/ photosystems are accepted by the herbicides
 No electron flow along ETC
 No ATP and NADPH produced
 Calvin cycle cease, no production of sugar/ glucose
 Respiration cannot occur since there is no respiratory substrate, leading to cell death

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(b) Outline the three phases of the Calvin cycle: (i) CO2 uptake (ii) carbon reduction and (iii) ribulose bisphosphate (RuBP)
regeneration and indicate the roles of ATP and NADP in the process.

Describe the main stages of the Calvin cycle. (Question may be rephrased into: Describe how chemical energy (i.e.
ATP) and reduced NADP are used to convert carbon dioxide to carbohydrates.)
 Light independent;
 Carbon fixation phase
 Carbon dioxide (1C) is accepted by RuBP (5C compound);
 Reaction is catalyzed by ribulose bisphosphate carboxylase (Rubisco);
 To form an unstable 6C compound;
 Which cleaves to form 2 molecules of 3C glycerate-3-phosphate (GP);
 Undergoes reduction phase;
 Where in the presence of ATP and NADPH;
 GP forms 3C triose phosphate/ glyceraldehydr-3-phosphate (G3P);
 For every 6 G3P formed, 1 G3P exits the cycle to produce glucose and other organic compound (e.g. other
carbohydrates, amino acids and lipids);
 5 G3P used to regenerate 3 RuBP;
 This requires ATP to provide the source of phosphate;
 Stoichiometry reference: reference to 3 turns of Calvin cycle to produce1 G3P or 6 turns to produce 1 glucose molecules
or 6 molecules of carbon dioxide to make 2 molecules of G3P;

Experiment related to Calvin cycle

The figure below shows how the pathway of carbon during the
reactions of photosynthesis was discovered by Calvin using the
unicellular green algae Chlorella.
The Chlorella culture was supplied with 14CO2 and illuminated with
varying length of time. Some algae were removed at intervals and
immediately killed by boiling in alcohol. This provided an extract which
was examined using chromatographic techniques and any radioactive
compounds present showed up on X-ray film as blackened areas. X- glycerate-3-phosphate because its earliest appearance
is on the 5-sec chromatogram

In a second series of experiments the algae were illuminated in the presence In another series of experiments, the algae were illuminated in the
of 14CO2 for 15 seconds. Illumination of the algae continues for a further 1 presence of 14CO2 for 15 seconds and then placed in the dark for
minute after all 14CO2 present had been removed. Some algae were then 1 min in the presence of 14CO2. Some algae were removed, killed
removed, killed and the extract examined. A distinct blackened area was and the extract examined. A distinct blackened area was identified
identified on the X-ray film. Name the compound that was found in this on the X-ray film. Name the compound that was found in this
blackened area. Explain your answer. [3] blackened area. Explain your answer. [3]
 Ribulose bisphosphate (RuBP);  Glycerate phosphate (GP/ PGA);
 In the presence of light, light-dependent reaction forms ATP and  Labeled CO2 combines with RuBP to form labeled GP/PGA
NADPH; in the absence of light;
 reducing labelled PGA/GP to glyceraldehyde-3-phosphate (G3P);  However, in the absence of light, light-dependent reaction
 G3P is further regenerated to labeled RuBP using ATP; cannot proceed,
 However in the absence of CO2, RuBP is not utilised in carbon fixation  thus, ATP and NADPH cannot be made;
to form glycerate phosphate (GP/ PGA);  GP/PGA cannot be reduced to G3P
 thus, accumulation of labeled RuBP;  hence accumulation of labeled GP/PGA

Graphs related to Calvin cycle

Fig. 1a and Fig. 1b show the results of two separate experiments on carbon dioxide fixation in photosynthesis using a unicellular
green alga and the radioactive isotope 14C to label carbon dioxide. The algae were actively photosynthesizing before the start of
both experiments.

Fig. 1a

 In the dark, light independent reactions cannot occur;

Fig. 1b
 ATP and NADPH not produced;
 CO2 would still be fixed into GP;
 However, GP cannot be converted to G3P;  Quantity of RuBP increases because less RuBP will be used for fixing
 Hence, rapid/ sharp increase in GP; CO2 ;
 RuBP cannot be regenerated;
 Quantity of GP decreases because at low CO2 concentration, less GP
 Hence sharp decrease in RuBP from light to dark reactions;
would be produced from RuBP;

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Graphs related to photosynthesis
Limiting factor – the factor which is at its, least favourable / nearest its minimum, value if increased
would speed up process; factor which limits the rate of a, reaction / process;

Describe the effect of increasing light intensity on the rate of photosynthesis.

At P: Light intensity is a limiting factor. Rate increases linearly with increasing light intensity. Light saturation point:
(Explanation) As light intensity increases (P), more photons of light fall on a unit area. More light Light intensity beyond which
energy is converted to chemical energy of ATP and NADPH, which is needed for Calvin cycle, an increase in light intensity
increasing rate of photosynthesis. will not increase the rate of
photosynthesis

At Q: Light intensity is not the only limiting factor. Rate increases at a decreasing rate. Some other
factor is also limiting e.g. CO2 concentration.

At R: Light saturation point has been exceeded -light intensity is no longer limiting. When light
intensity is increased, there is no increase in the rate of reaction. Some other factor is limiting.
Compensation point: is the light intensity at which the rate of photosynthesis is equal to the rate of
respiration.

Below compensation point:

 Low/no photosynthesis at lowest light intensity;
 Respiration will still occur;
 O2 is taken in & CO2 still given off by mitochondria in plant;

At compensation point, all the CO2 produced during respiration is used in photosynthesis and all the
oxygen produced in photosynthesis is used in respiration. Hence there is no net gain/loss of CO2.
Significance of compensation point to growth:
No net gain in dry mass and no plant growth as products of photosynthesis (i.e. glucose/
glyceraldehyde-3-phosphate) used up in respiration

Explain why rate levels off at high light intensity. [3]

 Systems involved in photosynthesis (e.g. Photosystems) are saturated with light, such that
photosynthesis is occuring at maximum rate;
 Light intensity is no longer the limiting factor;
 Other factors have become the limiting factor e.g. temperature;

Combined effects of light intensity, CO2 concentration and temperature on the rate of
photosynthesis

At low light intensity, light intensity is a limiting factor. Rate increases linearly with increasing light
intensity. (Explanation) As light intensity increases, more photons of light fall on a unit area. More
light energy is converted to chemical energy of ATP and NADPH, which is needed for Calvin cycle,
increasing rate of photosynthesis.

Plateau regions: Light saturation point has been exceeded -light intensity is no longer limiting. When
light intensity is increased, there is no increase in the rate of reaction. Some other factor is limiting.

Comparing experiments 1 and 2 or 3 and 4:

1. Temperature can be limiting when the light intensity is no longer limiting.
2. Increase in temperature results in an increase in photosynthesis rate.

Comparing experiments 1 and 3 or 2 and 4:

1. Carbon dioxide can be limiting when light intensity is no longer limiting.
2. Increase CO2 results in an increase in the rate of photosynthesis.

Difference between shade and sun plants

Figure shows the net rate of photosynthesis of sun and shade plants in response to increasing light
intensity. The net rate of photosynthesis is defined as:mass of CO2 fixed in photosynthesis minus
mass of CO2 produced in respiration, per unit time
Describe the responses of sun and shade plants to increasing light intensity.
 shade plant more photosynthesis at low light intensities;
 sun plant more photosynthesis at high light intensities;
 shade plant reaches, compensation point / net rate of photosynthesis, at low light intensity;
 shade plants , plateau / max photosynthetic rate, at low light intensities;

For shade plant, by having low compensation point and having reached light saturation at lower light
intensity, they are able to utilise whatever available light to make sugar for growth.

Absorption spectrum: a record of the amount of light absorbed at each wavelength Action spectrum: a record of the amount of
photosynthesis occurring at each wavelength
of light

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