Photosynthesis Chapter 10 It’s not simple being green

Objectives Understand the difference between autotroph and heterotroph Describe the location and structure of a chloroplast. Explain how chloroplast structure is related to its function Recognize and explain the summary equation for photosynthesis Understand the role of REDOX reactions in photosynthesis Understand the properties of light discussed in class Describe the relationship between action and absorption spectrum Explain what happens when chlorophyll or accessory pigments absorb photons

Objectives continued List the function and components of a photosystem Compare cyclic and noncyclic electron flow and explain the relationship between these components of the light reactions Summarize the light reactions of photosynthesis Summarize the carbon fixing reactions of the Calvin cycle Describe the role of NADPH and ATP in the Calvin cycle Understand why variations of photosynthesis evolved

Overview of Photosynthesis Process by which chloroplast bearing organisms transform solar light energy into chemical bond energy 2 metabolic pathways involved Light reactions: convert solar energy into cellular energy Calvin Cycle: reduce CO 2 to CH 2 O Organisms that can perform photosynthesis are called autotrophs whereas those that cannot are called heterotrophs

Photosynthesis Equation Reduction of carbon dioxide into carbohydrate via the oxidation of energy carriers (ATP, NADPH) Light reactions energize the carriers Dark reactions (Calvin Cycle) produce PGAL (phosphoglyceraldehyde) Photosynthesis 6CO 2 +6H light  C 6 H O 2

Where is all this happening?

Structure of the Chloroplast Thylakoid: membranous system within the chloroplast (site of light reactions). Segregates the chloroplast into thylakoid space and stroma. Grana stacks of thylakoids in a chloroplast Stroma: region of fluid between the thylakoids and inner membrane where Calvin Cycle occurs

Light Electromagnetic energy traveling in waves Wavelength ( ): distance from peak of one wave to the peak of a second wave Inverse relationship between wavelength and energy   energy

Visible Spectrum The portion of the electromagnetic spectrum that our eyes can see White light contains all of the visible spectrum Colors are the reflection of specific within the visible spectrum not reflected are absorbed Composition of pigments affects their absorption spectrum

Absorption vs. Action Absorption spectrum is the range of wavelengths that can be absorbed by a pigment Action spectrum means the wavelengths of light that trigger photosynthesis

Why are plants green? Pigments contained within the chloroplast absorb most of light but absorb the green the least Pigments include –Chlorophyll a –Chlorophyll b –Carotenoids Carotenes Xanthophylls

Chlorophyll a Is only pigment that directly participates in the light reactions Other pigments add energy to chlorophyll a or dissipate excessive light energy Absorption of light elevates an electron to a higher energy orbital (increased potential energy)

Photosystems Collection of pigments and proteins found associated with the thylakoid membrane that harness the energy of an excited electron to do work Captured energy is transferred between photosystem molecules until it reaches the chlorophyll molecule at the reaction center

What Next? At the reaction center are 2 molecules –Chlorophyll a –Primary electron acceptor The reaction-center chlorophyll is oxidized as the excited electron is removed through the reduction of the primary electron acceptor Photosystem I and II

Electron Flow Two routes for the path of electrons stored in the primary electron acceptors Both pathways –begin with the capturing of photon energy –utilize an electron transport chain with cytochromes for chemiosmosis Noncyclic electron flow –uses both photosystem II and I –electrons from photosystem II are removed and replaced by electrons donated from water –synthesizes ATP and NADPH –electron donation converts water into ½ O 2 and 2H + Cyclic electron flow –Uses photosystem I only –electrons from photosystem I are recycled –synthesizes ATP only

Noncyclic Electron Flow 1Electrons at reaction- center are energized 2H 2 O split via enzyme catalysed reaction forming 2H +, 2e -, and 1/2 O 2. Electrons move to fill orbital vacated by removed electrons 3,4 Each excited electron is passed along an electron transport chain fueling the chemiosmotic synthesis of ATP

5The electrons are now lower in energy and enters photosystem I via plastocyanin (Pc) where they are re-energized 6The electrons are then passed to a different electron transport system that includes the iron containing protein ferridoxin. The enzyme NADP + reductase assists in the oxidation of ferridoxin and subsequent reduction of NADP + to NADPH Noncyclic Electron Flow

Non-cyclic Electron Flow

Cyclic Electron Flow Electrons in Photosystem I are excited and transferred to ferredoxin that shuttles the electron to the cytochrome complex. The electron then travels down the electron chain and re-enters photosystem I

Where are the photosystems found on the thylakoid membrane?

Chemiosmosis in 2 Organelles Both the Mitochondria and Chloroplast generate ATP via a proton motive force resulting from an electrochemical inbalance across a membrane Both utilize an electron transport chain primarily composed of cytochromes to pump H + across a membrane. Both use a similar ATP synthase complex Source of “fuel” for the process differs Location of the H + “reservoir” differs Stroma

Calvin Cycle Starts with CO 2 and produces Glyceraldehyde 3- phosphate Three turns of Calvin cycle generates one molecule of product Three phases to the process –Carbon Fixation –Reduction of CO 2 –Regeneration of RuBP

1A molecule of CO 2 is converted from its inorganic form to an organic molecule (fixation) through the attachment to a 5C sugar (ribulose bisphosphate or RuBP). –Catalysed by the enzyme RuBP carboxylase (Rubisco). The formed 6C sugar immediately cleaves into 3- phosphoglycerate

2Each 3- phosphoglycerate molecule receives an additional phosphate group forming 1,3- Bisphosphoglycerate ( ATP phosphorylation ) NADPH is oxidized and the electrons transferred to 1,3- Bisphosphoglycerate cleaving the molecule as it is reduced forming Glyceraldehyde 3- phosphate

3The final phase of the cycle is to regenerate RuBP Glyceraldehyde 3-phosphate is converted to RuBP through a series of reactions that involve the phosphorylation of the molecule by ATP

Variations Anyone? In hot/arid regions plants may run short of CO 2 as a result of water conservation mechanisms C 4 Photosynthesis CO 2 may be captured by conversion of PEP (Phosphoenolpyruvate) into oxaloacetate and ultimately malate that is exported to cells where the Calvin cycle is active CAM Photosynthesis CO 2 may be captured as inorganic acids that my liberate CO 2 during times of reduced availability

Why are CAM and C 4 versions necessary?