These slides cover the concepts of Photosynthesis in plants as per A level syllabus

1. Photosynthesis
Review Questins;
1. (a)What are autotrophs?
(b) name two types of autotrophs
2. Define the term photosynthesis? And mention the raw materials,
condition and products of photosynthesis
3. Mention the site for photosynthesis in a leaf.
4. Mention the names of photosynthetic pigments
5. Mention two stages of photosynthesis

2. Photosynthesis
• Light dependent and light independent reactions
objectives;
Light Reaction
1. Concept of photophosphorylation
2. distinguish between cyclic and non cyclic photophosphorylation
3. Explain the role of water as a source of hydrogen (photolysis)
Dark Reaction
1. Meaning of Dark reaction
2. Events which take place during Dark Reaction
3. Outline the role of ribulose biphosphate (RuBP)

3. Objectives cont…
• C₃ and C₄ plants
1. Explain the meaning of C₃ and C₄ plants
2. Distinguish C₃ and C₄ plants
3. Describe C₄ plants pathway (Hatch slack pathway)

4. Stages of photosynthesis
• When chlorophyll a absorbs light energy, an electron gains
energy and is 'excited'.
• The excited electron is transferred to another molecule (called
a primary electron acceptor).
• The chlorophyll molecule is oxidized (loss of electron) and
has a positive charge.
• Photoactivation of chlorophyll results in the splitting of water
molecules (photolysis) and the transfer of energy to ATP
(phosphorylation) and reduced nicotinamide adenine
dinucleotide phosphate (NADP).

5. The chemical reactions involved include:
• condensation reactions - responsible for water molecules splitting
out,
• phosphorylation - the addition of a phosphate group to an organic
compound
• oxidation/reduction (redox) reactions involving electron transfer

6. Photosynthesis is a two stage process.
• The light dependent reactions;
• A light-dependent series of reactions which occur in the grana,
and require the direct energy of light to make energy-carrier
molecules that are used in the second process:
• light energy is trapped by chlorophyll to make ATP
(photophosphorylation)
• at the same time water is split into oxygen, hydrogen ions
and free electrons:
2H2O 4H+ + O2 + 4e- (photolysis)

7. • The electrons then react with a carrier molecule
nicotinamide adenine dinucleotide phosphate (NADP),
changing it from its oxidised state (NADP+) to its reduced
state (NADPH):
NADP+ + 2e- + 2H+ NADPH + H+
• The light-independent reactions, a light-independent
series of reactions which occur in the stroma of the
chloroplasts, when the products of the light reaction, ATP
and NADPH, are used to make carbohydrates from carbon
dioxide (reduction); initially glyceraldehyde 3-phosphate (a
3-carbon atom molecule) is formed.

8. The light-dependent reactions
• When light energy is absorbed by a chlorophyll molecule its electrons gain
energy and move to higher energy levels in the molecule
(photoexcitation).
• Sufficient energy ionises the molecule, with the electron being 'freed'
leaving a positively charged chlorophyll ion.
• This is called photoionisation.
• In whole chloroplasts each chlorophyll molecule is associated with an
electron acceptor and an electron donor.
• These three molecules make up the core of a photosystem.
• Two electrons from a photoionised chlorophyll molecule are transferred
to the electron acceptor.
• The positively charged chlorophyll ion then takes a pair of electrons from
a neighbouring electron donor such as water.

10. • An electron transfer system (a series of chemical reactions)
carries the two electrons to and fro across the thylakoid
membrane.
• The energy to drive these processes comes from two
photosystems:
Photosystem II (PSII) (P680)
Photosystem I (PSI) (P700

11. • It may seem confusing, but PSII occurs before PSI.
• It is named because it was the second to be discovered and
hence named second.
• The energy changes accompanying the two sets of changes
make a Z shape when drawn out.
• This is why the electron transfer process is sometimes called
the Z scheme.
• Key to the scheme is that sufficient energy is released during
electron transfer to enable ATP to be made from ADP and
phosphate.

12. A condensation reaction has led to phosphorylation

14. Non-cyclic phosphorylation (the Z scheme)
Both adenosine triphosphate (ATP) and NADPH are produced.
In the first photosystem (Photosystem II or PSII):
• photoionisation of chlorophyll transfers excited electrons to an electron
acceptor
• photolysis of water (an electron donor) produces oxygen molecules,
hydrogen ions and electrons, and the latter are transferred to the
positively-charged chlorophyll
• the electron acceptor passes the electrons to the electron transport
chain; the final acceptor is photosystem PSI
• further absorbed light energy increases the energy of the electrons,
sufficient for the reduction of NADP+ to NADPH

16. Chemiosmosis and ATP synthesis
• The components of non-cyclic phosphorylation are found in the
thylakoid membranes of the chloroplast.
• Electrons passing through the transport chain provide energy to
pump H+ ions from the stroma, across the thylakoid membrane
into the thylakoid compartment.
• H+ ions are more concentrated in the thylakoid compartment
than in the stroma.
• Therefore, there is an electrochemical gradient.
• H+ ions diffuse from the high to the low regions of concentration.
• This drives the production of ATP.

17. Cyclic phosphorylation
• The net effect of non-cyclic phosphorylation is to pass electrons from
water to NADP.
• Energy released enables the production of ATP.
• But much more ATP is needed to drive the light-independent
reactions.
• This extra energy is obtained from cyclic phosphorylation.
• This involves only Photosystem I which generates excited
electrons.
• These are transferred to the electron transport chain between PSII
and PSI, rather than to NADP+ and so no NADPH is formed.
• The cycle is completed by electrons being transported back to PSI
by the electron transport system.

20. Photosynthesis: An Overview
• The net overall equation for photosynthesis is:
• Photosynthesis occurs in 2 “stages”:
1. The Light Reactions (or Light-Dependent Reactions)
2. The Calvin Cycle (or Calvin-Benson Cycle or Dark
Reactions or Light-Independent Reactions)
20
6 CO2 + 6 H2O C6H12O6 + 6 O2
light
Is photosynthesis
an ENDERGONIC
or EXERGONIC
reaction?

21. Photosynthesis: An Overview
• To follow the energy in photosynthesis,
21
light
light ATP
NADPH
Light
Reactions
thylakoids
Calvin
Cycle
stroma
Organic
compounds
(carbs)

22. Phase 2: The Calvin Cycle
• In the Calvin Cycle, chemical energy (from the
light reactions) and CO2 (from the atmosphere)
are used to produce organic compounds (like
glucose).
• The Calvin Cycle occurs in the stroma of
chloroplasts.
22

23. Phase 2: The Calvin Cycle
• The Calvin Cycle involves the process of
carbon fixation.
• This is the process of assimilating carbon from a
non-organic compound (ie. CO2) and incorporating
it into an organic compound (ie. carbohydrates).
23
CARBON FIXATION

24. Phase 2: The Calvin Cycle
Step 1: Carbon Fixation
• 3 molecules of CO2 (from the atmosphere)
are joined to 3 molecules of RuBP (a 5-carbon
sugar) by Rubisco (an enzyme also known as
RuBP carboxylase)
24
C C
C C
C
C C
C C
C
C C
C C
C
C
C
C
3 carbon dioxide
molecules
3 RuBP molecules
Rubisco
This forms 3
molecules
which each
have 6 carbons
(for a total of 18
carbons!)

25. Phase 2: The Calvin Cycle
Step 2: Reduction
• The three 6-carbon molecules (very unstable)
split in half, forming six 3-carbon molecules.
• These molecules are then reduced by gaining
electrons from NADPH.
• ATP is required for this molecular rearranging
25
C C
C C
C C
C C
C C
C C
C C
C C
C C
C C
C
C C
C
C C
C
C
C C
C
C C
C
C C
NADPH
NADP+
ATP ADP P
Where did the NADPH and
ATP come from to do this?

26. Phase 2: The Calvin Cycle
• There are now six 3-carbon molecules, which
are known as G3P or PGAL.
• Since the Calvin Cycle started with 15 carbons
(three 5-carbon molecules) and there are now
18 carbons, we have a net gain of 3 carbons.
26
C C
C
C C
C
C C
C
C
C C
C
C C
C
C C
Where did these 3 extra
carbons come from?
• One of these “extra” 3-
carbon G3P/PGAL
molecules will exit the
cycle and be used to form
½ a glucose molecule.

27. Phase 2: The Calvin Cycle
• Once the Calvin Cycle “turns” twice (well,
actually 6 times), those 2 molecules of G3P (a
3-carbon carbohydrate) will combine to form 1
molecule of glucose (a 6-carbon carbohydrate
molecule) OR another organic compound.
27
C
C C
G3P
(from 3 turns of
the Calvin Cycle)
C C
C
G3P
(from 3 turns of
the Calvin Cycle)
C
C C C C
C
glucose

28. Phase 2: The Calvin Cycle
Step 3: Regeneration of RuBP
• Since this is the Calvin Cycle, we must end up
back at the beginning.
• The remaining 5 G3P molecules (3-carbons
each!) get rearranged (using ATP) to form 3
RuBP molecules (5-carbons each).
28
C C
C
C C
C
C C
C
C C C
C
C C
5 G3P molecules
Total: 15 carbons
3 RuBP molecules
Total: 15 carbons
Where does the ATP
come from to do this?
ATP
ADP
P

29. Phase 2: The Calvin Cycle
ORGANIC
COMPOUND
NADPH
NADP+
ATP
ADP
P
RuBP
CO2

30. Phase 2: The Calvin Cycle
30

31. Phase 2: The Calvin Cycle
Quick recap:
•In the Calvin Cycle, energy and electrons from the
Light Reactions (in the form of ATP and NADPH) and
carbon dioxide from the atmosphere are used to
produce organic compounds.
•The Calvin Cycle occurs in the stroma inside the
chloroplasts (inside the cells…).
•Carbon dioxide, ATP, and NADPH are required
(reactants).
•Organic compounds (G3P) are produced (products).
31

32. Photosynthesis: A Recap
• So, as a broad overview of photosynthesis,
• The Light Reactions (Phase 1) capture the energy in sunlight and
convert it to chemical energy in the form of ATP and NADPH
through the use of photosystems, electron transport chains, and
chemiosmosis.
• The Calvin Cycle (Phase 2) uses the energy transformed by the light
reactions along with carbon dioxide to produce organic compounds.
32

33. Photosynthesis: A Recap
33
Based on this equation,
how could the rate of
photosynthesis be
measured?
The photosynthetic equation:
light
Excites
electrons
during the
light
reactions
6 H2O
Split during the
light reactions
to replace
electrons lost
from
Photosystem II
6 CO2
Provides the carbon to
produce organic
compounds during the
Calvin Cycle
Produced as a
byproduct of the
splitting of
water during the
light reactions
6 O2 C6H12O6
The organic compound
ultimately produced
during the Calvin Cycle

34. Photosynthesis: A Recap
• Photosynthesis Animation
(click on “Animation” after clicking the link)
34

35. Environmental Factors & Photosynthesis
• The rate (or speed) of photosynthesis can vary, based on
environmental conditions.
• Light intensity
• Temperature
• Oxygen concentration
35

36. Environmental Factors & Photosynthesis
• Light intensity
• As light intensity increases, so too does the rate of
photosynthesis.
36
• This occurs due to increased
excitation of electrons in the
photosystems.
• However, the photosystems
will eventually become
saturated.
• Above this limiting level, no
further increase in
photosynthetic rate will occur.
light
saturation
point

37. Environmental Factors & Photosynthesis
• Temperature
• The effect of temperature on the rate of
photosynthesis is linked to the action of enzymes.
• As the temperature increases up to a certain
point, the rate of photosynthesis increases.
37
• Molecules are moving faster &
colliding with enzymes more
frequently, facilitating chemical
reactions.
• However, at temperatures
higher than this point, the rate
of photosynthesis decreases.
• Enzymes are denatured.

38. Environmental Factors & Photosynthesis
• Oxygen concentration
• As the concentration of oxygen increases, the rate of
photosynthesis decreases.
• This occurs due to the phenomenon of photorespiration.
38

39. Photorespiration
• Photorespiration occurs when Rubisco (RuBP
carboxylase) joins oxygen to RuBP in the first step
of the Calvin Cycle rather than carbon dioxide.
• Whichever compound (O2 or CO2) is present in higher
concentration will be joined by Rubisco to RuBP.
• Photorespiration prevents the synthesis of glucose AND utilizes
the plant’s ATP.
39
More CO2
More O2
Rubisco joins
CO2 to RuBP
Rubisco joins
O2 to RuBP
Photosynthesis
occurs; glucose is
produced
Photorespiration
occurs; glucose is
NOT produced

40. Photorespiration
• Photorespiration is primarily a problem for plants
under water stress.
• When plants are under water stress, their stomata
close to prevent water loss through transpiration.
• However, this also limits gas exchange.
• O2 is still being produced (through the light reactions).
40
• Thus, the concentration of O2
is increasing.
• CO2 is not entering the leaf since
the stomata are closed.
• Thus, as the CO2 is being used
up (in the Calvin Cycle) and not
replenished, the concentration
of CO2 is decreasing.

41. Photorespiration
• As the concentration of O2 increases and the
concentration of CO2 decreases (due to the
closure of the stomata to prevent excessive water
loss), photorespiration is favored over
photosynthesis.
• Some plant species that live in hot, dry climates
(where photorespiration is an especially big
problem) have developed mechanisms through
natural selection to prevent photorespiration.
• C4 plants
• CAM plants 41

42. C3 Plants
• C3 plants, which are “normal” plants, perform
the light reactions and the Calvin Cycle in the
mesophyll cells of the leaves.
42
• The bundle sheath cells of
C3 plants do not contain
chloroplasts
palisade mesophyll
spongy mesophyll
bundle sheath cells

43. C4 and CAM Plants
• C4 plants and CAM plants modify the process of
C3 photosynthesis to prevent photorespiration.
• Overview:
• C4 plants perform the Calvin Cycle in a different
location within the leaf than C3 plants.
• CAM plants obtain CO2 at a different time than C3
plants.
• Both C4 and CAM plants separate the initial fixing
of CO2 (carbon fixation) from the using of CO2 in
the Calvin Cycle.
43

44. C4 Plants: Preventing Photorespiration
• Plants that use C4 photosynthesis include corn,
sugar cane, and sorghum.
• In this process, CO2 is transferred from the
mesophyll cells into the bundle-sheath cells,
which are impermeable to CO2.
44
• This increases the concentration of
CO2.
• Thus, the Calvin Cycle is favored
over photorespiration.
• The bundle-sheath cells of C4
plants do contain chloroplasts.

45. C4 Plants: Preventing Photorespiration
• C4 plants use the Hatch-Slack
pathway prior to the Calvin Cycle:
• PEP carboxylase adds carbon dioxide
to PEP, a 3-carbon compound, in the
mesophyll cells.
• This produces a 4-carbon
compound (which is why it’s
known as C4 photosynthesis).
• This 4-carbon molecule then moves
into the bundle-sheath cells via
plasmodesmata.
45
• In the bundle sheath cells, the CO2 is
released and the Calvin Cycle begins.

46. C4 Plants: Preventing Photorespiration
46
If the Hatch-Slack
pathway helps to
prevent
photorespiration,
why wouldn’t ALL
plants have this
adaptation?

47. CAM Plants: Preventing Photorespiration
• Plants that use CAM photosynthesis include succulent plants
(like cacti) and pineapples.
• In CAM (crassulacean acid metabolism) photosynthesis,
plants open their stomata at night to obtain CO2 and release
O2.
• This prevents them from drying out by keeping their stomata closed
during the hottest & driest part of the day.
47

48. • When the stomata are opened at night, the CO2 is
converted to an organic acid (via the C4 pathway) and
stored overnight.
• During the day – when light is present to drive the Light
Reactions to power the Calvin Cycle – carbon dioxide is
released from the organic acid and used in the Calvin
Cycle to produce organic compounds.
• Remember:
48
• Even though the CO2 is
taken in at night, the Calvin
Cycle cannot occur because
the Light Reactions can’t
occur in the dark!
CAM Plants: Preventing Photorespiration

49. 49

50. Avoiding Photorespiration
• Both C4 and CAM plants – which are primarily
found in hot, dry climates – have evolutionary
adaptations which help prevent photorespiration.
• C4 plants perform the Calvin Cycle in the bundle-
50
sheath cells.
• CAM plants
open their
stomata at
night and
store the CO2
until
morning.

52. Group work
• What are differences between C3 and C4 plants? Six points
• Distinguish cyclic from non-cyclic photophosphorylation
• Explain why dark reaction is said to be dependent on light
reaction?
52