PHOTOSYNTHESIS KHADIJAH HANIM BT ABDUL RAHMAN SEM 1, 2013/2014 email@example.com
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 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 • 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) • 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 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 • 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 • 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).
Reduction • 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.
Regeneration • 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.