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PHOTOSYNTHESIS. KHADIJAH HANIM BT ABDUL RAHMAN SEM 1, 2013/2014 What is photosynthesis?. Photosynthesis is a process where plants through specialized pigment molecules absorb light energy, consume CO2 and H20 to produce O2 and carbohydrate.

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SEM 1, 2013/2014

what is photosynthesis
What is photosynthesis?
  • Photosynthesis is a process where plants through specialized pigment molecules absorb light energy, consume CO2 and H20 to produce O2 and carbohydrate.

12H2O + 6CO2 ----- light -----> 6O2+ C6H12O6 + 6H20

  • It involves oxidation-reduction reaction (reverse of carbohydrate metabolism) which produces carbohydrate.
  • In plant cell- the main site where this reaction occurs is in chloroplast.
  • This reaction happen in 2 stages:

2H2O ----light---> O2 + 4[H]


The electrons thereby obtained subsequently reduce CO2:

4[H] + CO2 (CH2O) + H2O

  • These 2 stages reaction normally referred as light reaction and dark reaction.
photosynthetic pigments
  • The essential features of photosynthesis is the absorption of light energy by specialized pigment molecules.
  • Pigment is a substance that absorbs visible light that behave as packets of energy called photons.
  • There are 3 types of primary photosynthetic pigments.
  • Chlorophylls- the green pigments that absorb blue-violet and red wavelengths of light
  • Carotenoids- the orange pigments molecules which serves as antenna pigments and protect from ROS (reacting oxygen species)
  • Xanthophylls- oxygenated derivatives of the carotenes which also serves as antenna pigments.

Two types of chlorophyll in plants.

  • Chlorophyll a plays the principle role in eukaryotes photosynthesis because its absorption of light energy directly drives photochemical events.
  • Chlorophyll b- light harvesting pigments by absorbing light energy and pass on to chlorophyll a.
photosynthesis occurs in chloroplasts
Photosynthesis occurs in chloroplasts

Chloroplasts have two parts:

1. A double membrane encloses a fluid-filled space called the stroma  or ground substance  

2. Thylakoids = flattened sacs organized into stacks called grana

3. Chlorophylls and other pigments involved in absorption of solar energy are embedded within thylakoid membranes; these pigments absorb solar energy responsible for light-dependent reaction.

chloroplast structure
Chloroplast Structure
  • Outer membrane
  • Inner membrane systems
  • Thylakoid membranes
  • Thylakoid space (within the thylakoids)
  • Granum(a) (stack(s) of thylakoidsmembranes)
  • Stroma (the liquid area outside the thylakoid membranes)
reactions in photosynthesis

Photosynthesis has two sets of reactions

1. Light-dependent reactions = the energy from the sun is captured in energy carrying molecules

2. Light-independent reactions = energy carrying molecules from the light-dependent reactions are used to make carbohydrates

photosystem i psi
Photosystem I (PSI)
  • Energizes and transfers electrons that are donated to NADP+
  • PSI is a protein-pigment complex that composed of several polypeptides.
  • It possess over 200 molecules of chlorophyll a
  • Their essential role is performed by 2 special chlorophyll a molecules that reside within the reaction centre.
  • These molecules referred as a special pair located in the core complex of PSI, AB dimer.
  • AB dimer absorb light at 700nm, the special pair referred as P700.
photosystem ii psii
  • PSII oxidizes water molecules and donates energized electrons to electron carriers that eventually reduce PSI
  • PSII is a large membrane-spanning protein-pigment complex believed to possess at least 23 components
  • The most prominent- reaction centre- protein-pigment complex composed of 2 polypeptide subunits known as D1 and D2, cytochrome b559, and special pair chlorophyll a molecules that absorb light at 680nm (P680).
cytochrome b 6 f
Cytochrome b6f
  • Found throughout thylakoid membrane- similar structure and fuction to cytochrome bc1 complex in mitochondrial inner membrane.
  • Plays important role in the transfer of electrons from PSII to PSI.
  • An iron-sulfur side on the complex accepts electrons from the membrane-soluble electron carrier plastoquinoneand donates them to a small water-soluble copper-containing protein called plastocyanin.
  • The mechanism of transport appears to be similar to the Q cycle in mitochondria.
atp synthase
ATP synthase
  • Structurally similar to ATP synthase of mitochondria
  • The CF0 component is a membrane-spanning protein complex that contains a protein-conducting channel
  • The CF1 head piece, which project into the stroma- possess an ATP synthesizing activity
  • A transmembrane proton gradient produced during light driven electron transport drives ADP phosphorylation.
light reactions
  • Mechanism by which electrons are energized and subsequently used in ATP and NADPH synthesis.
  • In O2 evolving species- PSI and PSII are required
  • The process of light-driven photosynthesis begins with the excitation of PSII by light energy.
  • One electron at a time is transferred to a chain of electron carriers that connects the 2 photosystems.
  • As electrons are transferred from PSII to PSI, protons are pumped across the thylakoid membrane from the stroma to thylakoid space.
  • ATP is synthesized as protons flow back into the stroma through the ATP synthase.

When P700 absorbs additional photon it releases an energized electron.

  • The newly energized electron is passed through a series of iron-sulfur proteins and falovoprotein to NADP+, the final electron acceptor.
  • Illustrated as the Z scheme.
light independent reaction calvin cycle
  • The process of CO2 incorporation into carbohydrate which occur within chloroplast stroma- Calvin cycle.
  • This process can occur without light if sufficient ATP and NADPH are supplied and can occur only when the plant is illuminated.
  • The net equation for Calvin cycle:

3 CO2 + 6 NADPH + 9 ATP -----> Glyceraldehyde-3-phosphate + 6 NADP+ + 9 ADP + 8 Pi

the reaction can be divided into 3 phases
The reaction can be divided into 3 phases
  • Carbon fixation
  • The mechanism, by which inorganic CO2 is incorporated into organic molecules, consists of a single reaction.
  • Ribulose-1,5-bisphosphate (RuBP) carboxylase catalyzes the carboxylation of ribulose-1,5-bisphosphate to form two molecules of glycerate-3-phosphate.
  • Plants that produce glycerate-3-phosphate (G3P) as the first stable product of photosynthesis are referred to as C3 plants (e.g. soya beans and oats).


  • Next, six molecules of glycerate-3-phosphate are phosphorylated at the expense of six ATP molecules to form glycerate-1,3-bisphosphate.
  • The latter molecules are then reduced by NADP+-glyceraldehyde-3-phosphate dehydrogenase to form six molecules of glyceraldehyde-3-phosphate.


  • The net production of fixed carbon in the Calvin cycle is one molecule of glyceraldehyde-3-phosphate.
  • The other five glyceraldehyde-3-phosphate molecules are processed in the remainder of the Calvin cycle reactions to regenerate three molecules of ribulose-1,5-bisphosphate.