PHOTOSYNTHESIS. VAN HELMONT’S EXPERIMENT (1649). 5 years only water. = 77 kg tree + 90,8 kg soil. 2,3 kg shoot + 90,9 kg soil. JOSEPH PRIESTLEY’S EXPERIMENT 1771. EXP. 2. EXP. 1. What do you think happened to the mouse in experiment 1?.
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VAN HELMONT’S EXPERIMENT (1649) 5 years only water = 77 kg tree + 90,8 kg soil 2,3 kg shoot + 90,9 kg soil
JOSEPH PRIESTLEY’S EXPERIMENT 1771 EXP. 2 EXP. 1 What do you think happened to the mouse in experiment 1? What do you think happened to the mouse in experiment 2? Why do you think that happened? Why do you think that happened?
In the early 1900`s scientists believed that light reactions split carbon dioxide and release oxygen. In 1930 van Neil studied bacterial photosynthesis. CO2 + 2 H2S CH2O + H2O + 2S C Dark reactions + H2O C H2 O CO2 O2
In bacterial photosynthesis: • Oxygen is not released • This proves that light does not split carbon dioxide in plant photosynthesis. • Light reactions split water and release oxygen. Anabaena sp.
WHAT IS PHOTOSYNTHESIS? Photosynthesis is theprocess which converts light energy into chemical energy.
WHICH ORGANISMS DO PHOTOSYNTHESIS? • Photosynthesis occurs in some bacteria (blue green algae), algae (green , golden-yellow, red, brown) and in plants. • In autotrophic eukaryotes, photosynthesis occurs inside chloroplast. • In autotrophic prokaryotes photosynthesis occurs in the cytoplasm.
STRUCTURE OF CHLOROPLAST • All chloroplasts contain the green pigment chlorophyll which is found in the thylakoid membranes and absorbs the light energy that initiates photosynthesis. • Chloroplasts like mitochondria contain DNA, RNA and ribosome and can duplicate themselves
OVERALL EQUATION OF PHOTOSYNTHESIS Light energy 6CO2 + 12 H2 O C 6 H 12 O 6 + 6 H2 O + 6 O 2 Enzymes, ETS
STAGES OF PHOTOSYNTHESIS LIGHT ENERGY WATER CARBON DIOXIDE ADP + Pi GRANA STROMA Light reactions convert light energy into chemical energy Dark reactions result in the reduction of carbon dioxide into glucose ATP NADP+ NADPH2 GLUCOSE OXYGEN
STAGES OF PHOTOSYNTHESIS There are two, linked stages of photosynthesis: • The light reactions in the grana produce ATP by photophosphorylation and split water, evolving oxygen and forming NADPH2 by transferring electrons from water to NADP+. 2. The dark reactions (Calvin Cycle) occur in the stroma and use the energy of ATP and the reducing power of NADPH2 to form sugar from CO2. Dark reactions don’t require light directly, it usually occurs during the day, when the light reactions are providing ATP and NADPH2.
LIGHT AND PHOTOSYNTHETIC PIGMENTS • Light falling on an object may, • pass through it (be transmitted) • be reflected (seen as colour) • be absorbed (has its energy converted into the energy of motion) • Only absorbed light is available for photosynthesis
LIGHT AND PHOTOSYNTHETIC PIGMENTS • Photosynthetic pigments are organic molecules that absorb light. • Main plant pigments are chlorophyll and carotenoids with several forms of each type. • The pigments absorb the visible light wavelengths. 380nm 750nm violetgreen red
PHOTOSYSTEMS • Chlorophyll a and one or more types of accessory pigments such as chlorophyll b and various carotenoids surround a single molecule of specialized chlorophyll a (P680 and P700), forming a “photo-system”. • Photo-system I (PSI) contains P700 and photo-system II (PSII) contains P680 at the reaction center.
PHOTOSYNTHETIC PIGMENTS • Chlorophyll contains C, H, O, N and Mg in its structure. (Mg containing protein). • Its synthesis requires the presence of light, Fe, and K. Chlorophyll b • absorbs red and blue light, reflects green • transfers the absorbed light to the chlorophyll a • molecular formula is C55 H70 O6 N 4 Mg Chlorophyll a • absorbs red and blue light • is the primary photsynthetic pigment • is involved directly in converting of light energy into chemical energy • presence of chlorophyll a hides the effect of carotenes and xanthophyll in leaves • molecular formula is C55 H72 O5 N 4 Mg
ACCESSORY PHOTOSYNTHETIC PIGMENTS Caroten(orange) Xantophyll (yellow) Phycoerythrin (red) Phycocyanin (blue) They absorb light energy and transfer it to the chlorophyll.
REACTIONS OF PHOTOSYNTHESIS LIGHT REACTIONS DARK REACTIONS Cyclic photophosphorylation Non-Cyclic photophosphorylation
LIGHT REACTIONS Cyclic photophosphorylation • Various pigments in PSI collect light, passing the energy on to P700 • An electron with raised energy levels is accepted by ferredoxin and passed onto an ETS where ATP is produced as the energy level falls back to the starting point.
LIGHT REACTIONS Cyclic photophosphorylation è Plastoquinone (PQ) Ferredoxine (Fd) ADP + Pi è ATP è Cytochrome b6 ADP + Pi Photosystem I PSI ( Chl a) è ATP è Cytochrome f light è Plastocyanine
The overall equation for cyclic electron transport light 2ADP + 2Pi2ATP chlorophyll
LIGHT REACTIONS Non-Cyclic photophosphorylation 1. When PSII absorbs light, an electron is removed from chlorophyll. This hole in PSll must be filled. 2. Water is split by photolysis. 3. Electrons from water molecule are passed to PSII and then onto PQ (plastoquinon). 4. As in cyclic photophosphorylation, ATP is produced via the ETS, with the electron dropping down to PSI. 5. Light energy also causes the release of an electron from PSI which is accepted by ferrodoxin. 6. Electrons pass from ferrodoxinto NADP leading to the production of NADPH2, with hydrogen coming from the separation of water into ions. 7. Electrons lost by PSI are replaced with the electrons coming from the ETS (PSII).
LIGHT REACTIONS Non-Cyclic photophosphorylation 2è Cytochrome f 2è 2NADP+ Ferredoxine (Fd) ADP + Pi 2è Plastocyanine 2NADPH + H2 ATP 2è 2è Cytochrome b6 2è PSI ( Chl a(P700)) Plastoquinone (PQ) è source light 2e- 2e- H2O PSII (Chl a (P680)) photolysis ½ O2 2H+ light Byproduct
The products of the two types of light reactions are ATP, NADPH2 and oxygen. • The first two products enter the dark reactions of photosynthesis, where they become involved in the Calvin Cycle and the synthesis of PGAL and eventually of glucose. • Oxygen is diffused into the air. LIGHT REACTIONS
Non-Cyclic photophosphorylation 2e- 2NADP+ 4 3 2NADPH + H2 2 1 To dark reactions
Pathway of electron transport PSII PSI 2e- 2e- 2e- To dark reactions 2e- H2O 2NADP+ The overall equation for non-cyclic electron transport 2H2O + ADP+ Pi + 2NADP+ ATP + 2NADPH2 +O2 Dark reactions By product
COMPARISON OF CYCLIC AND NON-CYCLIC PHOTOPHOSPHORYLATION
DARK REACTIONS(CALVIN CYCLE) • Dark reactions involve a series of chemical reactions, first described by Melvin Calvin. • CO2 is incorporated into more complex molecules and eventually carbohydrate. • Energy for the reactions is supplied by ATP with NADPH2 acting as a reducing agent, both coming from the light reactions. • As long as CO2, ATP and NADPH2 are present light is not required for the Calvin cycle to continue. That’s why they are called dark reactions.
DARK REACTIONS(CALVIN CYCLE) • Every turn of the cycle fixes one molecule of CO2 by producing two molecules of PGA and then two molecules of PGAL. • Thus six turns produce sufficient quantities of PGAL for the production of one molecule of glucose. • During dark reactions, for the incorporation of one carbon dioxide molecule into the process 3 ATP and 2 NADPH2 are used. • Therefore, for the synthesis of a hexose (glucose) 18 ATP and 12 NADPH2 are used.
DARK REACTIONS (CALVIN CYCLE) 6CO2 6 RuDP 6 (6C)UNSTABLE MOLECULE 6H2 O 6ADP + 6Pi 12 PGA(3C) 12ATP 6ATP 12ADP + 12Pi 12 NADPH2 12 DPGA 6 RuMP 12H2 O 12NADP+ With series of reactions 12 PGAL 10 PGAL 2 PGAL 2Pi GLUCOSE(6C)
FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS PRINCIPLE OF LIMITING FACTOR (1905 –Blackman) When a chemical process is affected by more than one factors, its rate is limited by the factor which is nearest its minimum value. (The rate of a biochemical process is limited by the factor which is nearest its minimum value.)
INTERNAL (GENETIC) FACTORS 1. Anatomy of leaves Surface area Thickness of cuticle Number of stomata Volume of airspace Thickness of epidermis and mesophyll Number of chloroplasts in mesophyll 2. Amount of chlorophyll 3. Amount of enzymes 4. Accumulation of end products
EXTERNAL (ENVIRONMENTAL) FACTORS 1.Light intensity Relative rate of photosynthesis Foot candles