mechanism of photosysthesis PPT, SSC AP srinivas nallapu

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Information about mechanism of photosysthesis PPT, SSC AP srinivas nallapu

Published on April 27, 2014

Author: SrinivasNallapu



SSC Andhra Pradesh Board

• Almost all plants are photosynthetic autotrophs, – Autotrophs generate their own organic matter through photosynthesis – Sunlight energy is transformed to energy stored in the form of chemical bonds (a) Mosses, ferns, and flowering plants (b) Kelp (c) Euglena (d) Cyanobacteria THE BASICS OF PHOTOSYNTHESIS

6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2

WHYWHY AREARE PLAPLANTSNTS GREGREEN?EN? Plant Cells have Green Chloroplasts The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).

• Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water AN OVERVIEW OF PHOTOSYNTHESIS Carbon dioxide Water Glucose Oxygen gas PHOTOSYNTHESIS

• Chloroplasts absorb light energy and convert it to chemical energy through a serious of reactions that occur in chloroplast. • The reactions of photosynthesis or two types. Light Reflected light Absorbed light Transmitted light Chloroplast 1.3 Mechanism of Photosynthesis

Types of reactions Light Reaction (Hill reaction) Dark Reaction The reactions of photosynthesis that occur in the presence of light is called light reaction During this phase the energy required for Carbon fixation is generated. The reactions of photosynthesis that occur in the absence of light is called dark reaction .

• To understand photosynthesis let us know the reactions that occur in the presence of light. i:e Oxidation and reduction process Oxidation: 1. Addition of oxygen atom 2. removal of hydrogen atom 3. removal of electrons from atom Reduction: 1. Removal of oxygen atom 2. Addition of hydrogen atom 3. Addition of electrons from atom 1.Light Reactions

• When a substance is oxidised the hydrogen and electrons removed from this substances are transformed to another substance. • The substance which accepts hydrogen and electrons get reduced. • The substances which gives electrons are called DONAR and accepts electrons are called ACCEPTOR. • DH2 + A D + AH2 Reduced Donar Oxidized acceptor Oxidized Donar Reduced Acceptor

• These acceptors which accept electrons are called ELECTRON ACCEPTORS. • Eg: Ferridoxins Adenine dinucleotide ( NAD) Nicotinamide adenine dinucleotide phosphate ( NADP) Plastoquinones Cytochromes (Fan in Personal computer) When an electron acceptor is reduced it may transfer the Hydrogen and electrons to another acceptor molecule

Mechanism of photosynthesis 1.During Ps Chlorophyll molecule traps solar energy and gets oxidized • When light falls on the chlorophyll, energy present in the photons is absorbed by the chlorophyll molecule. • This energy pushes an electron in the chlorophyll molecule to a higher energy level. • This electron from chlorophyll is transferred to an electron acceptor as a result chlorophyll gets oxidized by loosing an electron and the acceptor gets reduced by accepting the electron from chlorophyll. • Thus the energy present in the photon of the sun light is used to eject an electron from chlorophyll. This is the basic mechanism by which solar energy is trapped by chlorophyll. • Chlorophyll Chlorophyll* chlorophyll + ẽ • A + ẽ A-- + ve (Low energy state ) (High energy state) (Oxidized) ( Electron) (Acceptor oxidized) (electron) (Acceptor reduced

2.Chlorophyll splits water molecule and gets back its electron from water • With the removal of electron, chlorophyll is in oxidation state. • This oxidising power of chlorophyll is used for splitting of water molecule to liberate electrons. • By accepting these electrons, chlorophyll returns to its original state. • Oxygen is formed when the chlorophyll splits water molecule. • This oxygen escapes in to atmosphere. • 2 H2O 4 H+ + 4 ẽ - + O2 • 4 ẽ - + 4 chl 4 chl • 2 H2O + 4 Chl 4 Chl + 4H+ O2 • In this reaction water molecule is split by light activated chlorophyll . Hence this process is called PHOTOLYSIS of water • Light does not split water molecule directly it acts through chlorophyll molecule in PS II (Water) (Protons) (Electrons) (oxygen) (Water) (Chlorophyll oxidised) (Chlorophyll reduced)

3.ATP AND NADPH are formed during photosynthesis • Protons (derived from splitting of water molecule) are left behind are accumulate in thylokoids. • When their concentrations becomes sufficiently high they are transported across the thylokoid membrane in to stroma. • The energy in the movement of protons is used to produce ATP. • The electrons from PSII are taken up by PS I through a serious of other acceptor molecules in the PS I, these electrons are transferred to NADP to produce NADPH. • Up to this stage all the reactions occur only when light is present. There fore all these reactions are called LIGHT REACTIONS. • Oxygen , ATP and NADPH are the end products of light reactions.

Light NADPH OH- 2 H+ CARBOHYDRATES Co2 ½ O2NADP Formation of carbohydrates and NADPH

1.3.2 The Dark reaction or Carbon fixation • Co2 Glucose in a series of reactions that occur in stroma(Chloroplast). • ATP & NADPH produced in the light reactions are used in these reactions. • The entire reaction from Co2 to productions of glucose is observed by American scientist Melvin Kelvin. • So it is called “Calvin Cycle. Calvin received Nobel prize fro this work.

Calvin Cycle 6 Co2 ( 1 mol) 6 Ribulose 1, 5 Di phosphate 6 Hexose Sugar 1,5 Diphosphate 2 PGA Glyceraldehyde 3 – Phosphate Glucose Corbon Sugar (6 mol) + 2 Phosphates Attached to it 6 molecules of 6 carbon sugar phosphate Is formed 6 HS 1,5 D is highly unstable compound and it Breaks down in to 2 Phosphoglyceric Acid (Each PGA has 3 Carbon atoms) ATP & NADPH are produced in Light reaction Are used up at this stage (3 Carbon atoms) 2 mol of G 3 P (2 x 3=6 C atoms are used to produce 1 mol Glucose ( 6 c atoms) (Glucose is converted to Starch) 10 mol of G 3 P (10 x 3 = 30 C ) are used to regenerate 6 mol of R 1, 5 D ( 6 x 5 = 30 C atoms)

2 H+ + OH - 1 /2 Water-splitting photosystem Reaction- center chlorophyll Light Primary electron acceptor Energy to make Electron transport chain Primary electron acceptor Primary electron acceptor NADPH-producing photosystem Light NADP+ 1 2 3 How the Light Reactions Generate ATP and NADPH

• The production of ATP by chemiosmosis in photosynthesis Thylakoid compartment (high H+ ) Thylakoid membrane Stroma (low H+ ) Light Antenna molecules Light ELECTRON TRANSPORT CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE

• The Calvin cycle makes sugar from carbon dioxide – ATP generated by the light reactions provides the energy for sugar synthesis – The NADPH produced by the light reactions provides the electrons for the reduction of carbon dioxide to glucose Light Chloroplast Light reactions Calvin cycle NADP+ ADP + P • The light reactions convert solar energy to chemical energy – Produce ATP & NADPH AN OVERVIEW OF PHOTOSYNTHESIS

PHOTOSYNTHESIS • Sunlight provides ENERGY CO2 + H2O produces Glucose + Oxygen 6CO2 + 6H2O C6H12O6 + 6O2

Steps of Photosynthesis • Light hits reaction centers of chlorophyll, found in chloroplasts • Chlorophyll vibrates and causes water to break apart. • Oxygen is released into air • Hydrogen remains in chloroplast attached to NADPH • “THE LIGHT REACTION”

Steps of Photosynthesis • The DARK Reactions= Calvin Cycle • CO2 from atmosphere is joined to H from water molecules (NADPH) to form glucose • Glucose can be converted into other molecules with yummy flavors!

• In most plants, photosynthesis occurs primarily in the leaves, in the chloroplasts • A chloroplast contains: – stroma, a fluid – grana, stacks of thylakoids • The thylakoids contain chlorophyll – Chlorophyll is the green pigment that captures light for photosynthesis Photosynthesis occurs in chloroplasts

• The location and structure of chloroplasts LEAF CROSS SECTION MESOPHYLL CELL LEAF Chloroplast Mesophyll CHLOROPLAST Intermembrane space Outer membrane Inner membrane Thylakoid compartmentThylakoidStroma Granum StromaGrana

• Chloroplasts contain several pigments Chloroplast Pigments – Chlorophyll a – Chlorophyll b – Carotenoids – Xanthophyll Figure 7.7

Chlorophyll a & b •Chl a has a methyl group •Chl b has a carbonyl group Porphyrin ring delocalized e- Phytol tail

Different pigments absorb light differently

Cyclic Photophosphorylation • Process for ATP generation associated with some Photosynthetic Bacteria • Reaction Center => 700 nm

Photon Photon Water-splitting photosystem NADPH-producing photosystem ATP mill • Two types of photosystems cooperate in the light reactions

Primary electron acceptor Primary electron acceptor Electron transport chain Electron transport Photons PHOTOSYSTEM I PHOTOSYSTEM II Energy for synthesis of by chemiosmosis Noncyclic Photophosphorylation • Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product

• The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e- ) Plants produce OPlants produce O22 gas by splitting Hgas by splitting H22OO

• Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons • The excited electrons are passed from the primary electron acceptor to electron transport chains – Their energy ends up in ATP and NADPH In the light reactions, electron transportIn the light reactions, electron transport chains generate ATP, NADPH, & Ochains generate ATP, NADPH, & O22

• The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane – The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP – In the stroma, the H+ ions combine with NADP+ to form NADPH Chemiosmosis powers ATP synthesis in the light reactions

Summary—Light Dependent Reactions a. Overall input light energy, H2 O. b. Overall output ATP, NADPH, O2 .

• Animation is of the Calvin Cycle Note what happens to the carbon dioxide and what the end product is. • Second animation of the Calvin Cycle is very clear and even does the molecular bookkeeping for you.

Light Independent Reactions aka Calvin Cycle Carbon from CO2 is converted to glucose (ATP and NADPH drive the reduction of CO2 to C6H12O6.)

Light Independent Reactions aka Calvin Cycle CO2 is added to the 5-C sugar RuBP by the enzyme rubisco. This unstable 6-C compound splits to two molecules of PGA or 3-phosphoglyceric acid. PGA is converted to Glyceraldehyde 3-phosphate (G3P), two of which bond to form glucose. G3P is the 3-C sugar formed by three turns of the cycle.

Summary—Light Independent Reactions a. Overall input CO2 , ATP, NADPH. b. Overall output glucose.

Review: Photosynthesis uses light energy to make food molecules Light Chloroplast Photosystem II Electron transport chains Photosystem I CALVIN CYCLE Stroma Electrons LIGHT REACTIONS CALVIN CYCLE Cellular respiration Cellulose Starch Other organic compounds • A summary of the chemical processes of photosynthesis

Types of Photosynthesis C3 C4 CAM Rubisco: the world’s busiest enzyme!

Competing Reactions • Rubisco grabs CO2, “fixing” it into a carbohydrate in the light independent reactions. • O2 can also react with rubisco, inhibiting its active site – not good for glucose output – wastes time and energy (occupies Rubisco)

Photorespiration • When Rubisco reacts with O2 instead of CO2 • Occurs under the following conditions: – Intense Light (high O2 concentrations) – High heat • Photorespiration is estimated to reduce photosynthetic efficiency by 25%

Why high heat? • When it is hot, plants close their stomata to conserve water • They continue to do photosynthesis  use up CO2 and produce O2  creates high O2 concentrations inside the plant  photorespiration occurs

C4 Photosynthesis • Certain plants have developed ways to limit the amount of photorespiration – C4 Pathway* – CAM Pathway* * Both convert CO2 into a 4 carbon intermediate  C4 Photosynthesis

Leaf Anatomy • In C3 plants (those that do C3 photosynthesis), all processes occur in the mesophyll cells. Image taken without permission from| Mesophyll cells Bundle sheath cells

C4 Pathway • In C4 plants photosynthesis occurs in both the mesophyll and the bundle sheath cells. Image taken without permission from

C4 Pathway • CO2 is fixed into a 4- carbon intermediate • Has an extra enzyme– PEP Carboxylase that initially traps CO2 instead of Rubisco– makes a 4 carbon intermediate

C4 Pathway • The 4 carbon intermediate is “smuggled” into the bundle sheath cell • The bundle sheath cell is not very permeable to CO2 • CO2 is released from the 4C malate  goes through the Calvin Cycle C3 Pathway

How does the C4 Pathway limit photorespiration? • Bundle sheath cells are far from the surface– less O2 access • PEP Carboxylase doesn’t have an affinity for O2  allows plant to collect a lot of CO2 and concentrate it in the bundle sheath cells (where Rubisco is)

CAM Pathway • Fix CO2 at night and store as a 4 carbon molecule • Keep stomates closed during day to prevent water loss • Same general process as C4 Pathway

How does the CAM Pathway limit photorespiration? • Collects CO2 at night so that it can be more concentrated during the day • Plant can still do the calvin cycle during the day without losing water

Summary of C4 Photosynthesis • C4 Pathway – Separates by space (different locations) • CAM Pathway – Separates reactions by time (night versus day)

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