KHADIJAH HANIM BT ABDUL RAHMAN PHOTOSYNTHESIS KHADIJAH HANIM BT ABDUL RAHMAN WEEK 15: 17/12/2012 khadijahhanim@unimap.edu.my

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 plastoquinone and 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 ans 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.

The Calvin Cycle

Summary of photosynthesis process