Topic 2.9 and 8.3 Photosynthesis

SL Topic 2.9 Photosynthesis

Light striking the plant is converted into chemical energy  glucose Photosynthetic organisms produce foods Primary production

Chloroplasts are the Chlorophyll  photosynthetic pigment only green component of a plant

Light provides energy  photosynthesis, light absorbed by chloroplasts partly determines rate of photosynthesis Useful colors  Red and Blue Absorption correlates to action (run photosynthesis) Green not useful since not absorbed (reflected) Absorption and Action spectra vary for different plants Different pigments Relative amounts of those pigments

Stages of photosynthesis First stage: Light Dependent Reactions Chlorophyll absorbs light energy and convert it to chemical energy (ATP) Photolysis occurs to release needed electrons, separating oxygen (waste) from hydrogen Oxygen ATP Chlorophyll Water Light independent reactions Electrons Hydrogen ions Hydrogen

Stages of Photosynthesis Second stage: Light independent reactions 6 CO2 + 12 H2O  C6H12O6 + 6 H2O + 6 O2 Inorganic compound Organic compound Carbon Fixation ATP and hydrogen used as chemical energy to convert CO2 and H2O into glucose

Measuring the rate of photosynthesis Rate of cell respiration is consistent day and night  at low levels Directly measure the rate of oxygen production or carbon dioxide intake Measure biomass (indirect measure) As a plant photosynthesizes, it should increase in biomass. correction is made for cell respiration

Effects of Environmental Factors Enzymes, etc. already working at their maximum rate Positive correlation between light intensity and photosynthesis rate in this area of the graph

Effects of Environmental Factors Enzymes and other proteins become denatured Increased molecular collisions

Effects of Environmental Factors Rate of reaction reaches a plateau unless light or temperature is also increased. Positive correlation

HL Topic 8.3 Photosynthesis

The Chloroplast All photosynthetic processes occur within the chloroplast Occur only in algae and plant cells Contains double membrane, own DNA and 70s ribosomes Grana Made up of numerous stacked thylakoids Thylakoids Flattened membrane sacs Used in the absorption of light (1st step of photosynthesis) Stroma Fluid (like cytosol), within the chloroplast Contains many enzymes and chemicals necessary for photosynthesis

Process of Photosynthesis 6 CO2 + 12 H2O  C6H12O6 + 6 H2O + 6 O2 Light Water occurs on both sides because 12 molecules are consumed and 6 are produced

Light Dependent Reaction Occurs in the thylakoids/grana Light supplies the energy  Sun Pigments Absorbed light Different pigments absorb different wavelengths of light Chlorophylls Carotenoids Organized on the membranes of the thylakoids Photosystems: Chlorophyll a molecules Accessory pigments A protein matrix

Light Dependent Reaction Reaction Center contains: A pair of chlorophyll molecules A matrix protein A primary electron acceptor

2. Electron is captured by the primary acceptor 1. A photon of light is absorbed by a pigment in photosystem II  transferred to other pigment  reaches chlorophyll a (P680)  causes an excitement of electrons into a higher energy state

3. Photolysis: Driven by energy of light  water is split by an enzyme to produce electrons, H+, and an oxygen atom  electrons supply chlorophyll a

4. Excited electrons pass from primary acceptor down an electron transport chain (losing energy at each exchange) Three electron carriers

5. Energy lost from electron movement  drives chemiosmosis (think respiration)  phosphorylation of ADP to ATP

7. Electrons are passed down a second electron transport chain 8. Enzyme NADP reductase catalyzes the transfer of an electron to NADP+ 2 electrons are required to fully reduce NADP+ to NADPH 7. Electrons are passed down a second electron transport chain 6. Photon of light is absorbed by pigment in photosystem I  chlorophyll a (P700) accepts energy  increases energy in an electron  de-energized electron from photosystem II fills void

Final Products NADPH + ATP Oxygen Release Supply chemical energy for the light-independent reaction Oxygen Release Step 3

Chemiosmosis Comparison Respiration Chemiosmosis Photosynthesis Chemiosmosis 1. Involves an ETC embedded in the membranes of cristae 1. Involves an ETC embedded in the membranes of thylakoids 2. Energy is released when e- are exchanged from one carrier to another 3. Released energy is used to actively pump H+ into the intermembrane space 3. Released energy is used to actively pump H+ into the thylakoid space 4. H+ come from the matrix 4. H+ come from the stroma 5. H+ diffuse back into the matrix through the channels of ATP synthase 5. H+ diffuse back into the stroma through the channels of ATP synthase 6. ATP synthase catalyzes the oxidative phosphorylation of ADP to form ATP 6. ATP synthase catalyzes the photophosphorylation of ADP to form ATP

A mechanical analogy for the light reactions

How Do these Fit Spatially? THYLAKOID MEMBRANE STROMA THYLAKOID SPACE

So how is ATP synthesized?

Light-Independent Reaction Occurs within the stroma (cytosol-like) region of the cholorplast ATP and NADPH produced in light-dependent reaction provide the energy and reducing power Product = GLUCOSE 

Calvin Cycle 1. Ribulose biphosphate (RuBP), 5C compound, binds to CO2 through an enzyme, rubisco  carbon fixation  unstable 6C compound 6 CO2 from the air Rubisco CO2 Fixation 6 6 RuBP Unstable intermediate 2. 6C breaks down into two 3C compounds  glycerate-3-phosphate 12 Glycerate-3-phosphate 12 ATP 3. Glycerate-3-phosphate are acted on by ATP and NADPH (from LD reactions) to form triose phophate (TP)  reduction reaction 12 NADPH 12 ADP 12 Pi 12 NADP+ 12 Triose phosphate

Calvin Cycle 5. In order to regain RuBP, the cycle uses 6 ATP CO2 from the air 5. In order to regain RuBP, the cycle uses 6 ATP Rubisco CO2 Fixation 6 6 4. TP may go one of two directions: 1) leave cycle  sugar phosphates, or 2) continue cycle to reproduce originating compound: RuBP RuBP Unstable intermediate 6 ADP 12 6 ATP Glycerate-3-phosphate 12 ATP 12 NADPH 10 Triose phosphate 12 ADP 12 Pi 12 NADP+ 12 2 Triose phosphate Triose phosphate Sugar

Calvin Cycle TP is the pivotal compound Produce simple sugars (glucose), disaccharides (sucrose),or polysaccharides (cellulose or starch) Most is used to regain starting compound, RuBP

Summary Light-Dependent Light-Independent Occurs in the thylakoids Occurs in the stroma Uses light energy to form ATP and NADPH Uses ATP and NADPH to form TP Splits water in photolysis to provide replacement electrons and H+, and to release O2 to the atmosphere Returns ADP, inorganic phosphate, and NADP to the light-dependent reaction Includes 2 ETC and photosystems I and II Involves the Calvin Cycle

Chloroplast Chloroplast Structure Function Allowed Extensive membrane surface area of the thylakoids Allows greater absorption of light by photosynthesis Small space (lumen) within the thylakoids Allows faster accumulation of protons to create a concentration gradient Stroma region similar to the cytosol of the cell Allows an area for the enzymes necessary for the Calvin cycle to work Double membrane on the outside Isolated the working parts and enzymes of the chloroplast from the surrounding cytosol

Factors Affecting Photosynthesis Potential limiting factors: Temperature Light intensity Carbon dioxide concentration Rate Limiting Step One part of photosynthesis is slowed, all parts are slowed

Light Intensity (why?)

Carbon Dioxide Concentration (why?)

Temperature (why?)

Cyclic Photophosphorylation Light energized electrons from photosystem I flow back to the cytochrome complex  creating more ATP