CHAPTER 10 - Photosynthesis

1. CHAPTER 10 - Photosynthesis

2. The chloroplasts of plants use a process called photosynthesisto capture light energy from the sun and convert it to chemical energy stored in sugars and other organic molecules. • Autotrophsproduce organic molecules from CO2 and other inorganic raw materials obtained from the environment. • Heterotrophslive on organic compounds produced by other organisms.

3. Leaf Structure • Plant cells have chloroplasts. The structure of a leaf is to the left. Structures to note: • Stoma (stomata) • Cuticle • Veins (xylem/ phloem) • Guard Cells • Epidermis • Mesophyll (in this pic it is called parenchyma – that is the type of cell) Photosynthesis mainly occurs in mesophyll cells

4. Chloroplasts Chloroplasts are the organelles where photosynthesis takes place. Know the following structures: stroma, thylakoid, grana, lumen (inside thylakoid) Chlorophyll, the green pigment in chloroplasts, is found in the thylakoid membranes. Stomata microscopic pores in the bottom of the leaf; used for gas exchange Veins  run throughout the plant to deliver water from the roots and carry sugar from the leaves to the rest of the plant IN THE CHLOROPLAST…. Stroma fluid in the chloroplast; Calvin cycle happens here Thylakoids membrane sacs where the light reaction happens Thylakoid Space/ Lumen  inside the thylakoids Grana stacks of thylakoids

5. Background of Photosynthesis • Process that converts sunlight (light energy) into food (chemical energy) • 6CO2 + 6 H2O + Light → C6H12O6 + 6O2 • The oxygen given off by plants comes from WATER, not CARBON DIOXIDE Make sure you know this picture!

6. C.B. Van Niel Cornelis B. Van Niel was a teacher and made great contributions to the world of science as we know it. He was a pioneer in the field of microbiology and he also discovered some of the chemistry behind photosynthesis. He did research with plants and figured out that water was split to give the ETC a source of hydrogen (and electrons) and that oxygen was given off as a byproduct. Thus, the oxygen that is released during PS is from water, not the CO2that is consumed. Twenty years later (in the 1950’s), this theory was confirmed by doing research using a heavy isotope of oxygen (18O) as a tracer to follow the path of the oxygen. The radioactive label (isotope) was only found in the released oxygen if the source of the 18O was originally in the water (not the CO2).

7. Photosynthesis is a redox reaction. • Both photosynthesis and aerobic respiration involve redox reactions. • Photosynthesis reverses the direction of electron flow. • Water is split and electrons are transferred with H+ from water to CO2, reducing it to sugar. • Oxidized = water • Reduced = CO2

8. PREVIEW  Light Reaction(first stage of photosynthesis) • The light reactions(photo) convert solar energy to chemical energy. • In the light reactions, water is split, providing a source of electrons and protons (H+ ions) and giving off O2 as a by-product. • Light absorbed by chlorophyll drives the transfer of electrons and hydrogen ions from water to NADP+(nicotinamide adenine dinucleotide phosphate), forming NADPH. • The light reactions also generate ATP using chemiosmosis, in a process called photophosphorylation(comparable to oxidative phosphorylation in cellular respiration.)

9. PREVIEW  Calvin Cycle(second stage of photosynthesis) • The Calvin cycle(synthesis) uses energy from the light reactions to incorporate CO2 from the atmosphere into sugar (named for Melvin Calvin ~1940’s.) • The Calvin cycle begins with the incorporation of CO2 into organic molecules, a process known as carbon fixation. • The fixed carbon is reduced with electrons provided by NADPH. • ATP from the light reactions also powers parts of the Calvin cycle. • Thus, it is the Calvin cycle that makes sugar (G3P), but only with the help of ATPand NADPHfrom the light reactions.

10. Light • Light is a form of electromagnetic energy or radiation. • Light travels in waves and the distance between crests of electromagnetic waves is called the wavelength. • The entire range of electromagnetic radiation is the electromagnetic spectrum. • The most important segment of the electromagnetic spectrum for life is a narrow band between 380 and 750 nm, the band of visible lightdetected as colors by the human eye. (ROY.G.BIV!) • Although light travels as a wave, many of its properties are those of a discrete particle, a photon.

11. Light can be transmitted, reflected, or absorbed. Substances that absorb visible light are called PIGMENTS. Whatever color wavelength is reflected is the color that the pigment appears to be. This color wavelength is NOT helpful in providing energy because none of it is absorbed (therefore green is NOT helpful for photosynthesis because chlorophyll reflects green light). Visible Light This is the visible light spectrum. The shorter the wavelength (violet), the more energy it has.

12. The spectrophotometer is a machine that measures a pigments ability to absorb at different wavelengths of light. The more light that is absorbed, the less that gets through, so the lower the transmittance. Low Transmittance = High Absorption Spectrophotometer • An absorption spectrumplots a pigment’s light absorption versus wavelength. • An overall action spectrumfor photosynthesis profiles the relative effectiveness of different wavelengths of light in driving the process.

13. Thomas Engelmann Engelmann’s Experiment → Engelmann shone light of different colors (wavelengths) of light onto filaments of Spirogyra (through a prism). Spirogyra is an alga made up of filaments (chains) of cells. He added motile bacteria (which could swim) and which needed oxygen and observed where they went. He found that the bacteria clustered around the blue and red colors of the spectrum, where the Spirogyra were producing the most oxygen. So he concluded that these two colors were the most important for photosynthesis. RESULT: Red and Blue are the most effective colors for photosynthesis!

14. Accessory Pigments: • Chlorophyll b → similar to chlorophyll a, but traps a slightly different wavelength of energy • Carotenoids→ yellow/orange color; absorb and pass on energy; function in photoprotection Chlorophyll and Accessory Pigments Only Chlorophyll a can participate directly in the light reactions. However, it gets energy from other pigments called accessory pigments. These pigments absorb wavelengths of light that chlorophyll cannot, and then pass along that energy to the chlorophyll a.

15. Part 1 – Light Reactions

16. Light Reaction • Light Reaction makes ATP and NADPH and sends it to the Calvin Cycle • It occurs in the thylakoid • Makes ATP by Photophosphorylation(similar to oxidative phosphorylation with the ETC and chemiosmosis) • Energy carrier = NADPH (reduced from NADP+) • Gets electrons from H2O and gives off O2

17. Reaction Center Chlorophyll and Photosystems Photosystems composed of a reaction center complex and several light harvesting complexs that contain pigments that absorb light energy Reaction Center → made up of the special chlorophyll a molecule and a primary e- acceptor Photosystem II → comes first in the ETC, but was discovered second; called P680 because has an absorption peak at 680nm Photosystem I → comes second in the ETC, but was discovered first; gives e- to NADP+; called P700 because has an absorption peak at 700nm

18. There are two ways electrons can pass through the ETC: Cyclic e- Flow → this is when the e- go down PS II to the next photosystem, but then get passed from ferredoxin BACK to the cytochrome complex; this makes more ATP and makes NO NADPH Linear (Non-Cyclic) e- Flow → this passes through both electrons transport chains and makes both ATP and also NADPH Linear Electron Flow through the ETC Because the Calvin uses more ATP than NADPH, there is a way to make more ATP (and less NADPH) → cyclic e- flow!

19. Linear (Non-Cyclic) Electron Flow Makes NADPH and ATP Electron Flow: Water  PSII  primary e- acceptor  plastoquinone (Pq)  cytochromes  plastocyanin (Pc)  PSI  primary e- acceptor  Ferredoxin (Fd)  NADP+ reductase  NADPH • Discuss: • Water → donates e- to PSII, releases O2 as waste, contributes H+ to concentration gradient • Electrons get energized by light and then get trapped by a primary e- acceptor and passed down the ETC (energy released pumps H+ INTO the lumen) • First ETC→ made up of Plastoquinone, Cytochromes, and Plastocyanin • Electrons from 1st ETC supply e- to PSI where they get re-energzizd by light and passed down the second ETC (ferredoxin and NADP+ reductase) to NADP+ to make NADPH

20. Cyclic Electron Flow • Makes ATP only • Ferredoxin returns the electrons back to the cytochrome complex in the first ETC rather than passing them to NADP+ reductase; no NADPH is made here • Both cyclic and non-cyclic are used because the Calvin cycle uses more ATP than NADPH……so we need more ATP than NADPH

21. Chemiosmosis Makes both ATP (via ATP Synthase) and NADPH (via NADP+ reductase) Uses the proton gradient and ATP synthase to make ATP via photophosphorylation During the Light Reactions, protons (H+) are pumped from the stroma into the lumen (thylakoid space) against their concentration gradient. The want to diffuse back out and the only way they can get out of the thylakoid is to go through ATP SYNTHASE. This is an enzyme that works like a water wheel and converts ADP to ATP. This is utilized in cellular respiration as well.

22. Occurs in the stromaof the chloroplast • Carbon enters as CO2 and leaves as sugar • Produces a sugar called glyceraldehyde-3-phosphate (G3P) • Each turn “fixes” one molecule of carbon, so one G3P takes 3 turns of the Calvin Calvin Cycle

23. Phase 1: Carbon fixing  • Each CO2 molecule is attached to a five-carbon sugar, ribulose bisphosphate (RuBP). This reaction is catalyzed by RuBP carboxylase-oxygenase, or rubisco. The six-carbon intermediate is unstable and splits in half to form two molecules of 3-phosphoglycerate for each CO2 fixed. • Phase 2: Reduction  • During reduction, each 3-phosphoglycerate receives another phosphate group from ATP to form 1,3-bisphosphoglycerate. • A pair of electrons from NADPH reduces each 1,3-bisphosphoglycerate to G3P. • For every three molecules of CO2 that enter the cycle, there are six molecules of G3P formed…but 5 have to be recycled so we only get net gain of one G3P. • This molecule exits the cycle to be used by the plant cell, while the other five molecules are recycled to regenerate the three molecules of RuBP Phase 3: Regeneration of RuBP In a complex series of reactions, the carbon skeletons of five molecules of G3P are rearranged by the last steps of the Calvin cycle to regenerate three molecules of RuBP.

24. Alternate Methods of Carbon Fixing In hot and dry environments, plants close their stomata so that they don’t lose too much water (dehydrate). However, closed stomata lead to no gas exchange, which inhibits photosynthesis. This leads to….. Photorespiration → makes no ATP, makes no food, wastes the plants materials (takes away from the Calvin Cycle. In C3 plants, with the higher levels of O2 (byproduct it can’t get rid of) and low levels of CO2, rubisco (enz) actually adds OXYGEN to the Calvin instead of CO2…..it wastes all the raw materials. Evolutionary baggage  early environment didn’t have a lot of O2 so the inability of rubisco to exclude O2 wouldn’t have made a difference. In order to minimize photorespiration, plants have adapted a C4pathwayor a CAMpathway

25. The key enzyme, phosphoenolpyruvate carboxylase, adds CO2 to phosphoenolpyruvate (PEP) to form the four-carbon product oxaloacetate. • PEP carboxylasehas a very high affinity for CO2 and no affinity for O2. Therefore, PEP carboxylase can fix CO2 efficiently when rubisco cannot (that is, on hot, dry days when the stomata are closed). In effect, the mesophyll cells pump CO2 into the bundle-sheath cells, keeping CO2 levels high enough for rubisco to accept CO2 and not O2. C4photosynthesis minimizes photorespiration and enhances sugar production. • C4 plants thrive in hot regions with intense sunlight. C4 plants → the first product of carbon fixing is a 4C molecule (instead of a 3C molecule); it occurs in the mesophyll cells and sends the product to the bundle sheath cells and the calvin cycle happens there (Calvin cycle is confined to bundle sheath cells!) C4 Plants PEP Carboxylase(enzyme) has a high affinity for CO2 so it can work when rubisco can’t

26. CAM Plants CAM(crassulacean acid metabolism) Plants→ They open their stomata at night and store the CO2 in organic acids; then during the day they break those acids down to use the CO2

27. C4 vs. CAM plants • Both C4 and CAM plants add CO2 to organic intermediates before it enters the Calvin cycle. • In C4 plants, carbon fixation and the Calvin cycle are structurally separated (bundle sheath cells vs. mesophyll cells) • In CAM plants, carbon fixation and the Calvin cycle are temporally separated (day vs. night)