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2. PHOTOSYNTHESIS

3. What is Photosynthesis?
Is a process by which plants used
sunlight, water, and carbon dioxide
to create oxygen and energy in the
form of sugar.
C
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Most life on Earth depends
on photosynthesis
C
P

4. Photosynthesis is an endothermic process that involves the
oxidation-reduction (redox) reactions, as summarized in the
formula shown below;
02 03

5. THREE PRINCIPAL CHEMICAL
ELEMENTS
OXYGEN HYDROGEN
CARBON
DIOXIDE
~ make sugar molecules
and oxygen.
~make plants more
productive.
O2 H CO2
~ Proton gradients and
Plant Respiration.
~Glucose.
~Byproduct

6. MORE IMPORTANT ELEMENTS
~to absorb light.
SUN
~provides necessary light
and energy to plants.
WATER
~provides a hydrogen
CHLOROPHYLL GASES
~carbon dioxide (CO2)
~ help plants grow

7. The two process of
photosynthesis occurs in two
distinct stages;
LIGHT-DEPENDENT REACTIONS LIGHT-INDEPENDENT REACTIONS
- Do not required sunlight
- Required sunlight
- Convert light energy into
chemical energy.

8. PROPERTIES OF
LIGHT
- Light is a form of electromagnetic
radiation.
- Light particles called photon acts like a
discrete bundle of energy called quantum.
- Energy content of a quantum is inversely
proportional to the wavelength of the light.
(figure 1)
- Only visible light is ideal for biological
reactions.
- Light beyond 700 nm has insufficient
quantum yield to drive photosynthesis
(figure 2)

9. PHOTOSYNTHETIC PIGMETS
- Pigment that is present in chloroplast or photosynthetic bacteria and captures
the light energy necessary for photosynthesis.
- Pigments are light –harvesting molecules.
- In plants, they are present in the thylakoid membranes of chloroplast.
5 MAIN TYPES OF CHLOROPHYLLS
- Chlorophyll a
- Chlorophyll b
- Chlorophyll c
- Chlorophyll d
- Bacteriochlorophyll a (molecules found in prokaryotes)
- Chlorophylls absorb photons with narrow energy range.
(figure 4)
- Carotenoids and Xanthophylls serve as antenna pigments, extending the
range of light useful for photosynthesis. (figure 5)

10. LIGHT-DEPENDENT REACTION
- Convert solar energy into chemical energy in the form of
NADPH and ATP.
- Photons are absorbed by the photosynthetic pigments
organized into photocenters in the chloroplast membrane
(figure 6).
- Each photocenter contains a hundreds of pigment molecules
that act as antennae to absorb light and transfer the energy of
their excited electrons to a chlorophyll molecule that serves as
a reaction center (figure 7).

11. Two electron transport pathways used to make ATP molecules
NON-CYCLIC
PHOTOPHOSPHORYLATION
- Happens when an electron passes
through PSII to PSI and drives the
synthesis of ATP and NADPH
(figure 8)
CYCLIC
PHOTOPHOSPHORYLATION
- Light energy harvested at
PSI is used to synthesizing
ATP rather than NADPH,
thereby supplying additional
ATP for other metabolic
processes. ( figure 9)

12. NON-CYCLIC PHOTOPHOSPHORYLATION
Steps in the non-cyclic photophosphorylation
pathway of light-dependent reactions;
1. Once the energy from the absorbed photon reaches
the PS II special pair (P680), the special pair can lose
an electron when excited, passing it to the primary
electron acceptor, pheophytin, an organic molecule
that resembles chlorophyll.
2. The electron will begin its journey through an electron transport chain. Pheophytin transfers the electron first to
plastoquinone (pq). Pq transfers the electron to a cytochrome complex (Cyt), then a cooper-containing protein,
plastocyanin (Pc), accepts the electron.
As this high-energy electron moves through this electron transport chain, it losses or releases energy. It goes from its
high to lower energy state. Some of the released energy is used to pump protons (H+) from the stroma (outside of the
thylakoid) into the thylakoid interior.

13. NON-CYCLIC PHOTOPHOSPHORYLATION
Steps in the non-cyclic photophosphorylation pathway
of light-dependent reactions;
3. After the special pair gives up its electron, it has a
positive charge and needs a new electron. This
electron is replaced by extracting a low-energy
electron from the splitting of water molecules
(photolysis) carried out by a portion of PSII called the
manganese center.
The splitting of one H2O molecules releases two electrons, 2 H+, and one atom of oxygen. Two molecules of H2O are
needed to form one O2 molecule. Mitochondria in the leaf use about 10% of the oxygen to support oxidative
phosphorylation, while the remainder escape to the atmosphere. Aerobic organisms use this oxygen in the atmosphere
for respiration.
This transfer of H+, along with the release of H+ from photolysis/Hill reaction by the manganese center of chlorophyll,
forms a proton gradient. The ATP synthase uses the energy from the proton gradient to add inorganic phosphate (Pi) to
ADP to make ATP. This process of making ATP using energy stored in a chemical gradient is called chemiosmosis.

14. NON-CYCLIC PHOTOPHOSPHORYLATION
Steps in the non-cyclic photophosphorylation pathway
of light-dependent reactions;
4. Once an electron has gone down the first electron
transport chain and arrives at PSI, it joins with the
chlorophyll a special pair called P700. because the
electron has lost energy before its arrival at PSI, it must
be re-energized through absorption of another photon by
PSI antenna pigments.
5. P700 (PS I) is oxidized and sends the high-energy electron to its primary electron acceptor, chlorophyll A0, and down to
a short electron transport chain.
6. The electron is first passed to a protein called ferredoxin (Fd), the transferred to an enzyme called NADP+ reductase.
7. NADP+ reductase transfers electrons to the electron carrier NADP+ to make NADPH. NADPH will travel to the Calvin
cycle, where the synthesis of sugars from carbon dioxide utilizes its electrons.
Thus, PSII captures the energy to create proton gradients to make ATP, and PSI captures the same to reduce NADP+ into
NADPH. The two photosystems work in concert to guarantee the equal production of NADPH and ATP. Other mechanisms
(cyclic photophosphorylation) exist to fine-tune that ratio to exactly match the chloroplast’s constantly changing energy
needs.

15. CYCLIC PHOTOPHOSPHORYLATION
Cyclic photophosphorylation of light-
dependent reactions has the following
steps:
1. The antenna complex captures two photons
from either the red or blue spectrum and
donates them to the P700 reaction center,
which contributes two high-energy electrons
to the primary electron receptor.
2. High-energy electrons are passed to ferredoxin (fd), an iron containing protein which acts as the first electron carrier.
3. A second electron carrier plastoquinone (Pq), carries the electrons to a complex of two cytochromes. In the process,
energy is provided to produce a proton gradient across the membrane, facilitating chemiosmosis that forms additional
ATP molecules.
4. The electrons are returned by plastocyanin (Pc) to the 700 pigment in the reaction center to complete the cycle.

17. LIGHT-INDEPENDENT REACTION
- Do not required sunlight.
- Cell has the fuel needed to make carbohydrate molecules for long-term energy
storage.
- The ATP and NADPH molecules have lifespans in the range of millionths of seconds
whereas, products of the light-independent reactions (carbohydrates and other forms of
reduced carbon) can survive for hundreds of millions of years.
- The carbohydrate molecules made have a backbone of carbon atoms that come from
carbon dioxide, the gas that is a by-product of respiration in microbes, fungi, plants, and
animals.
- In plants, CO2 enters the leaves through stomata, where it diffuses through intercellular
spaces until it reaches the mesophyll cells and into the stoma of the chloroplast, the site of
light-independent reactions.
- The reaction as a whole are also called the Calvin cycle, named after Melvin Calvin.
- Most outdated name is dark reactions

18. Relation of light-dependent and light-independent reactions

19. The light-independent reactions or Calvin cycle consist of three main stages:
fixation, reduction, and regeneration.
STAGE 1: Fixation
- In addition to CO2, an enzyme called ribulose bisphosphate carboxylase (RuBisCO)
and six ribulose bisphosphate (RuBP) initiate the light-independent reactions in the
stroma ( figure 10).
- RuBisCO catalyzes a reaction between CO2 and ruBP. For each CO2 molecule that
reacts with one RuBP, two molecules of another compound (3-PGA) form. PGA has
three carbons and one phosphate. Each turn of the cycle involves only one RuBP and
one carbon dioxide and forms two molecules of 3-PGA. The number of carbon atoms
remains the same, as they move to form new bonds during the reactions (3 atoms from
3CO2 + 15 atoms from 3RuBP is 18 atoms of carbon in 3 atoms of 3-PGA). This
process is called carbon fixation because inorganic CO2 is ‘fixed’ into organic
molecules.

20. The light-independent reactions or Calvin cycle consist of three main stages:
fixation, reduction, and regeneration.
STAGE 2: Reduction
Six ATP and NADPH molecules are used to convert the six molecules of 3-PGA into six
molecules of glyceraldehyde 3-phosphate (G3P). That is a reduction reaction because it
involves the gain of electrons 3-PGA. Recall that a reduction means the gain of an electron
by an atom or molecule. Six molecules of both ATP and NADPH are used. For ATP,
energy is released with the loss of the terminal phosphate atom, converting it into ADP. For
NADPH to become NADP+, both energy and a hydrogen atom are lost. Both ADP and
NADP+ return to the nearby light-dependent reactions to be reused and reenergized.
(figure 10)

21. The light-independent reactions or Calvin cycle consist of three main stages:
fixation, reduction, and regeneration.
STAGE 3: Regeneration
At this point, only one of the G3P molecules leaves the Calvin cycle and is sent to the
cytoplasm to contribute to the formation of other compounds needed by the plant.
Because each G3P exported from the chloroplast has three carbon atoms, it takes
three ‘turns’ of the Calvin cycle to fix enough net carbon to export one G3P. But each
turn makes two G3Ps thus, three turns make six G3Ps. Out of six, one is exported
while the remaining five G3P molecules remain in the cycle and are used to
regenerate RuBP, which allows the system to prepare for more CO2 fixation. The
regeneration reactions involving five G3P use three molecules of ATP. (figure 10)

22. The figure above summarizes the processes of the two reactions of
photosynthesis. Photosynthesis transformed life on earth. By harnessing energy
from the sun, photosynthesis evolved to allow living things access to enormous
amounts of energy. Because of photosynthesis, living things gained access to
sufficient energy sources that allowed them to build new structures and achieve
the evident biodiversity today.

23. GROUP 2
MEMBERS;
~ Abecia, Tejoice
~ Lumakin, Dan C.
~ Lopez, Neil Jean
~ Mellejor, Ma. Kaye Jenissa V.
~ Templanza, Aldrin M. II

24. FIGURE 1: ELECTROMAGNETIC
SPECTRUM

25. FIGURE 2: RELATION OF WAVELENGHT OF
LIGHT,QUANTUM YIELD AND PHOTOSYNTHESIS

26. List of photosynthetic pigments (in order of
increasing polarity):
- CAROTENE: an orange pigment
- XANTHOPHYLL: a yellow pigment
- PHAEOPHYTIN A: a gray- brown pigment
- PHAEOPHYTIN B: a yellow-brown pigment
-CHLOROPHYLL A: a blue-green pigment
- CHLOROPHYLL B: a yellow-green pigment

27. Figure 4: Visible light absorbed by chlorophyll

28. Figure 5: Wavelength of light absorb by photosynthesis

29. Figure 6:
Each photocenter consists of hundreds of antenna pigment
molecules, which absorb photons and transfer energy to a reaction
center chlorophyll. The reaction center chlorophyll then transfers its
excited electrons to an acceptor (pheophytin) in the electron
transport chain.

30. Figure 7:
Each photosystem has light-harvesting complex consist of antenna pigments,
chlorophyll b and carotenoids, that pass the light energy absorbed through resonance
energy transfer until it reaches chlorophyll a, the reaction center.

31. Figure 8

32. Figure 9

33. Figure 10

34. Figure 10

35. Figure 10