PHOTOSYNTHESIS

Outline Photosynthesis in Nature A. Plants and other autotrophs are producers of the biosphere B. Chloroplasts are the sites of photosynthesis in plants

More Outline Pathways of photosynthesis A. Evidence that chloroplasts split water molecules enabled researchers to track atoms through photosynthesis: Science as Process B. The light reactions and the Calvin cycle cooperate in converting light energy to the chemical energy of food: An overview C. The light Rx’s transform solar energy to the chemical energy of ATP and NADPH: A closer look

Outline: The last page More II….. D. The Calvin cycle uses ATP and NADPH to convert CO2 to sugar E. Alternative mechanisms of carbon fixation have evolved in hot, arid climates F. Photosynthesis is the biosphere’s metabolic foundation: A review

PHOTOSYNTHESIS Transforms solar light energy trapped by chloroplasts into chemical bond energy stored in sugar and other organic molecules. This process: Synthesizes energy rich molecules from energy poor CO2 and H2O. Uses CO2 as a C source and light as an energy source Directly or indirectly supplies energy to most living organisms

AUTOTROPHS PHOTOAUTOTROPHS Autotrophic organisms that use light as an energy source to synthesize organic molecules. Examples: Plants Algae Some prokaryotes CHEMOAUTOTROPHS Autotrophic organisms that use the oxidation of inorganic substances such as sulfur or ammonia as an energy source to synthesize organic molecules. Unique to bacteria

Leaves and Chloroplasts

Chlorophyll Green pigment in chloroplasts that absorbs the light energy used to drive photosynthesis. Found in chloroplasts in eukaryotes.

CHLOROPLASTS Mostly found in mesophyll in leaves Lens shaped organelles Usually 2-4 microns by 4-7 microns Divided by a system of membranes into 3 functional compartments: Intermembrane space Thylakoid space Stroma

The Chloroplast

The Structure of Chlorophyll/Chloroplast

Chloroplast Parts Thylakoids: Flattened membranous sacs inside the chloroplasts. The thylakoid membrane segregates the interior of the chloroplast into the thylakoid space and stroma. Chlorophyll is found in the thylakoid membranes. Thylakoids function in the steps of photosynthesis that initially convert light to chemical energy

More Chloroplast Parts Grana: Stacks of thylakoids Stroma: Viscous fluid outside of the thylakoids .Reactions that use chemical energy to convert CO2 to sugar occur here.

Overview of Photosynthesis

The Equation 6CO2 + 12H20  Light Energy C6H12O6 + 6O2 + 6H2O

Tracking atoms through photosynthesis

H20Sugar An important result of photosynthesis is the extraction of H from H20 and its incorporation into sugar. Electrons in H have more potential energy in organic molecules than in water, where the e-’s are closer to a more electronegative atom Energy in sugar and other food molecules is stored in these high energy e-’s

Photosynthesis is a redox process Yup! It’s also endergonic Light is the energy source that boosts the potential energy of e-’s as they are moved from water to sugar When water is split e-’s are transferred from water to CO2 , reducing it to sugar

Smitty’s Patented Photosynthesis Diagram

LIGHT REACTIONS Convert light energy to chemical bond energy in ATP and NADPH. These reactions: Occur in the thylakoid membranes of the chloroplasts Reduce NADP+ (Nicotinamide adenine dinucleotide phosphate) to NADPH Gives off O2 from the splitting of water Generate ATP : Called PHOTOPHOSPHORYLATION

How the Photosystem harvests energy

The Calvin Cycle The assimilation of CO2 and the reduction of it into a carbohydrate. These reactions: Occur in the stroma of the chloroplasts Incorporate atmospheric CO2 into existing organic molecules by CARBON FIXATION and then reduce fixed C into a carbohydrate. Do not require light, but DO require NADPH and ATP from the light reactions

The Nature of Sunlight Light travels as electromagnetic energy: Energy that travels in rhythmic waves The wavelengths of visible light range from about 380-750nm Light ALSO behaves as if it consists of discrete particles or quanta called photons

PIGMENTS Substances which absorb visible light Different pigments absorb different wavelengths of light Each pigment has a characteristic absorption spectrum. Obviously, what wavelengths a pigment reflects is what you see

More Pigment Stuff Chlorophyll a is involved directly in the light reactions, so we can analyze its absorption spectrum to see which works best for photosynthesis A graph of wavelength/rate of photosynthesis is called an ACTION SPECTRUM

Chlorophyll a and accessory pigments Even though only special chlorophyll a molecules can participate directly in the light reactions, accessory pigments can absorb light and transfer the energy to chlorophyll a. These are: Chlorophyll b: yellow/green, structurally similar to chlorophyll a Carotenoids: yellow and orange hydrocarbons that are built into the thylakoid membrane w/ the other two types

When Pigments Get Excited The absorbed photon boosts one of the pigment molecule’s e-’s in its ground state to an excited state The only photons absorbed by a molecule are those with an energy state = to the difference in energy between the ground state and excited state of the electrons. This energy varies from one molecule to another. This photon is converted into the potential energy of the excited e-

Excited Electrons In the thylakoid membranes, primary electron acceptor molecules trap excited state electrons This is a redox rx where chlorophyll is photo-oxidized by the absorption of light energy and the e- acceptor is reduced

PHOTOSYSTEMS Light harvesting complexes of the thylakoid membrane. Each photosystem is composed of: An antenna complex Reaction-center chlorophyll Primary e- acceptor

Antenna Complex Several hundred choropyll a/b and carotenoids are light gathering antennae which absorb photons and pass the energy from molecule to molecule. This process of resonance energy transfer is called INDUCTIVE RESONANCE. Due to differences in pigments, a wider range of the light spectrum can be absorbed

Reaction-Center Chlorophyll Only a specialized chlorophyll a molecule in the reaction center can transfer an excited e- to initiate the light reactions

Primary e- Acceptor Located near the reaction center Traps excited e-’s from reaction-center chlorophyll This transfer is the first step of the light rx and powers the formation of ATP and NADPH in later steps

Photosystem 1 In thylakoid membranes Chlorophyll a: P700 Absorbs best at 700nm (far red)

Photosystem 2 In the thylakoid membranes Chlorophyll a: P680 Absorbs best at 680 nm

Noncyclic Electron Flow

Cyclic Electron Flow

Cyclic Electron Flow Involves only photosystem 1 and generates ATP without producing NADPH or oxygen It’s cyclic because excited e-’s that leave from chlorophyll a at the rx center return to the rx center. As the exergonic flow of e-’s is coupled to ATP production it’s called cyclic photophosphorylation The goal: To produce ATP

A Summary of the light Reactions: Noncyclic The photosystems of the thylakoid membrane transform light energy into chemical energy stored as ATP and NADPH. This Process: Pushes low energy e-’s from water to NADPH, where they are stored at a higher energy NADPH is the e- donor used to reduce CO2 to sugar (in the calvin cycle) Makes ATP and Oxygen

A Summary of the Light Reactions: Cyclic Electrons ejected from P700 reach ferredoxin and flow back to P700. This process: Produces ATP Does NOT make NADPH or Oxygen

Calvin The Calvin Cycle Uses ATP and NADPH to convert CO2 to sugar The Calvin cycle is similar to the Krebs Cycle in that the starting material is regenerated Carbon enters as CO2 and leaves as sugar ATP=energy source NADPH=reducing agent that adds high energy e-’s Produces a 3 carbon sugar: Glyceraldehyde 3-Phosphate

The Calvin Cycle

Alternative Carbon Fixation Photorespiration: A metabolic pathway in plants that consumes O2, evolves CO2, produces NO ATP, and decreases photosynthetic output.(It actually reduces molecules used in the Calvin Cycle. This process is fostered by hot, bright days, when plants close their stomata, CO2 is depleted and O2 builds up in leaves

Alternative Carbon Fixation C4 Plants: Plants that preface the Calvin Cycle with reactions resulting in CO2 incorporating into 4 carbon compounds. (The other “normal” plants can be called C3 plants). This enhances C fixation in hot, arid climates which ordinarily favor photorespiration

C4 Plants Leaf anatomy separates the Calvin Cycle spatially from the initial incorporation of CO2 into organic compounds. There are 2 types of photosynthetic cells: Bundle-sheath cells: Packed into tightly packed sheaths around veins, thylakoids NOT stacked in Grana, Calvin cycle only in chloroplasts of bundle sheath Mesophyll Cells: More loosely arranged in the area between the bundle sheath and the leaf surface (Incorporation of CO2 into organic compounds).

C4

CAM Plants Again, usually in hot, arid climates Plants open stomata primarily at night When stomates are open, CO2 is taken up and incorporated into organic acids. This is called Crussulacean Acid Metabolism (CAM). These acids are stored in vacuoles in the mesophyll until morning During the day, CO2 is released from these organic acids, and made into sugar the usual way.