1. BIO 2, Lecture 14 FIGHTING ENTROPY III: PHOTOSYNTHESIS

2. Autotrophs sustain themselves without eating anything derived from other organisms Autotrophs are the producers of the biosphere, producing organic molecules from CO 2 and other inorganic molecules Almost all plants are photoautotrophs, using the energy of sunlight to make complex organic molecules from H 2 O and CO 2

3. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes These organisms feed not only themselves but also most of the living world

4. (a) Plants (c) Unicellular protist 10 µm 1.5 µm 40 µm (d) Cyanobacteria (e) Purple sulfur bacteria (b) Multicellular alga

5. Heterotrophs obtain their organic material from other organisms Heterotrophs are the consumers of the biosphere Almost all heterotrophs, including humans, depend on photoautotrophs for food and O 2

6. In plants, the work of photosynthesis is done by organelles called chloroplasts Chloroplasts are structurally similar to and likely evolved from photosynthetic bacteria

7. In plants, leaves are the major locations of photosynthesis Their green color is from chlorophyll, the green pigment within chloroplasts Light energy absorbed by chlorophyll drives the synthesis of organic molecules in the chloroplast CO 2 enters and O 2 exits the leaf through microscopic pores called stomata

8. Chloroplasts are found mainly in cells of the mesophyll, an interior tissue of the leaf A typical mesophyll cell has 30–40 chloroplasts The chlorophyll is in the membranes of thylakoids (connected sacs in the chloroplast); thylakoids may be stacked in columns called grana Chloroplasts also contain stroma, a dense fluid

9. 5 µm Mesophyll cell Stomata CO 2 O2O2 Chloroplast Mesophyll Vein Leaf cross section

10. 1 µm Thylakoid space Chloroplast Granum Intermembrane space Inner membrane Outer membrane Stroma Thylakoid

11. Photosynthesis can be summarized as the following equation: 6 CO 2 + 12 H 2 O + Light energy  C 6 H 12 O 6 + 6 O 2 + 6 H 2 O Chloroplasts split H 2 O into hydrogen and oxygen, incorporating the electrons of hydrogen into sugar molecules

12. Photosynthesis is a redox process in which H 2 O is oxidized (to O 2) and C O 2 is reduced (to C 6 H 12 O 6 ) Reactants: 6 CO 2 Products: 12 H 2 O 6 O 2 6 H 2 O C 6 H 12 O 6 Reduced CO 2 Oxidized H 2 O

13. Photosynthesis consists of two parts: 1.“Photo” part = light-dependent reactions Require light; only occur in the daytime 2.“Synthesis” (of sugar) part = light independent reactions Also called the Calvin cycle Occur both in the daytime and at night Plant switches most energy to Calvin Cycle at night

14. The light reactions –Occur in the thylakoid membrane and thylakoid space (inside the thylakoid) –Split H 2 O into H + and O 2 (gas) –Reduce NADP+ to NADPH –Generate ATP from ADP and P i The Calvin cycle Occurs in the stroma Forms sugar from CO 2, using the ATP and NADPH generated in the light reactions Begins with carbon fixation, incorporating CO 2 into organic molecules

15. Light H2OH2O Chloroplast Light Reactions NADP + P ADP i + ATP NADPH O2O2 Calvin Cycle CO 2 [CH 2 O] (sugar) “Dark” (light- independent) reactions (occur in the presence and absence of light)

16. Light is a form of electromagnetic energy, also called electromagnetic radiation Like other electromagnetic energy, light travels in rhythmic waves Wavelength is the distance between crests of waves Wavelength determines the type of electromagnetic energy Shorter wavelength = higher energy

17. The electromagnetic spectrum is the entire range of electromagnetic energy, or radiation Visible light consists of wavelengths (including those that drive photo- synthesis) that produce colors we can see Light also behaves as though it consists of discrete particles, called photons

18. UV Visible light Infrared Micro- waves Radio waves X-rays Gamma rays 10 3 m 1 m (10 9 nm) 10 6 nm 10 3 nm 1 nm 10 –3 nm 10 –5 nm 380 450 500 550 600 650 700 750 nm Longer wavelength Lower energyHigher energy Shorter wavelength

19. Pigments are substances that absorb visible light Different pigments absorb different wavelengths Wavelengths that are not absorbed are reflected back (and seen by observers) Leaves appear green because chlorophyll reflects green light back to our eyes It is actually the combined wavelengths not absorbed by chlorophyll that collectively appear green

20. Reflected light Absorbed light Light Chloroplast Transmitted light Granum

21. Chlorophyll a is the main photosynthetic pigment Accessory pigments, such as chlorophyll b, broaden the spectrum used for photosynthesis Accessory pigments called carotenoids absorb excessive light that would damage chlorophyll

22. When a pigment absorbs light, it goes from a ground state to an excited state, which is unstable When excited electrons fall back to the ground state, photons are given off, an afterglow called fluorescence If illuminated, an isolated solution of chlorophyll will fluoresce, giving off light and heat

23. (a) Excitation of isolated chlorophyll molecule Heat Excited state (b) Fluorescence Photon Ground state Photon (fluorescence) Energy of electron e–e– Chlorophyll molecule

24. Photosynthesis begins at photosystems located in the thylakoid membrane A photosystem consists of a reaction- center complex (comprised of several proteins) surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) funnel the energy of photons to the reaction center

25. There are two types of photosystems in the thylakoid membrane: Photosystem II (PS II) functions first (the numbers reflect order of discovery) and is best at absorbing a wavelength of 680 nm Photosystem I (PS I) functions second and is best at absorbing a wavelength of 700 nm

26. At the beginning of photosynthesis, a primary electron acceptor in the reaction center of Photosystem II accepts an electron from chlorophyll a that has been excited by a photon Solar-powered transfer of an electron from a chlorophyll a molecule to the primary electron acceptor is the first step of the light reactions

27. The chlorophyll molecule that is now missing an electron is a very strong oxidizing agent It grabs an electron from H 2 O (in the thylakoid space) and is reduced O 2 is released as a by-product of this splitting of H 2 O H+ is also formed, which begins to build up in the thylakoid space (sound familiar?)

28. The electron “falls” down an electron transport chain from the primary electron acceptor of PS II to PS I Energy released by the fall is used by proteins in the electron transport chain to drive H + ions from the stroma into the inner space of the thylakoid Thus, there are two sources of build-up of H + ions in the inner space: from the splitting of water and from the electron transport chain

29. ATP synthase, embedded in the thylakoid membrane, coverts ADP + P i to ATP using the energy generated by the rush of the H + ions in the inner compartment out into the stroma (with their concentration gradient)

30. Meanwhile, the electrons passing through the electron transport chain of Photo- system II are eventually dumped off at Photosystem I When photons hit chlorophyll molecules in Photosystem I, electrons are kicked off chlorophyll to a second electron transport chain Chlorophyll molecules in Photosystem I (now strong oxidixing agents) grab electrons dumped dumped off from Photosystem II to return to their reduced state Water is not split at Photosystem I

31. Electrons excited by photons at Photosystem I “fall” down a second electron transport chain and are eventually dumped onto NADP +, reducing it to NADPH The electrons of NADPH are available for the reactions of the Calvin cycle (that drive the endergonic reactions that create sugar and starch) ATP produced through the proton motive force are also used in the Calvin Cycle

32. Mill makes ATP e–e– NADPH Photon e–e– e–e– e–e– e–e– e–e– ATP Photosystem II Photosystem I e–e–

33. Light Fd Cytochrome complex ADP + i H+H+ ATP P synthase To Calvin Cycle STROMA (low H + concentration) Thylakoid membrane THYLAKOID SPACE (high H + concentration) STROMA (low H + concentration) Photosystem II Photosystem I 4 H + Pq Pc Light NADP + reductase NADP + + H + NADPH +2 H + H2OH2O O2O2 e–e– e–e– 1/21/2 1 2 3

34. Chloroplasts and mitochondria generate ATP by chemiosmosis, but use different sources of energy Mitochondria transfer chemical energy from food to ATP; chloroplasts transform light energy into the chemical energy of ATP, which is used to produce food Spatial organization of chemiosmosis differs between chloroplasts and mitochondria but also shows similarities

35. In mitochondria, protons are pumped out of the inner space (matrix) and into the intermembrane space ATP synthesis occurs as they diffuse back into the mitochondrial matrix In chloroplasts, protons are pumped into the inner thylakoid space and out of the stroma ATP synthesis occurs as they diffuse back out of the inner thylakoid space (into the stroma)

36. Key Mitochondrion Chloroplast CHLOROPLAST STRUCTURE MITOCHONDRION STRUCTURE Intermembrane space Inner membrane Electron transport chain H+H+ Diffusion Matrix Higher [H + ] Lower [H + ] Stroma ATP synthase ADP + P i H+H+ ATP Thylakoid space Thylakoid membrane

37. The Calvin cycle, like the citric acid cycle, regenerates its starting material after molecules enter and leave the cycle The cycle builds sugar from smaller molecules by using ATP and the reducing power of electrons carried by NADPH The ATP and NADPH come from the light- dependent reactions

38. Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde-3- phospate (G3P) For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2 The Calvin cycle has three phases: –Carbon fixation (catalyzed by rubisco) –Reduction –Regeneration of the CO 2 acceptor (RuBP)

39. Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P

40. Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle

41. Ribulose bisphosphate (RuBP) 3-Phosphoglycerate Short-lived intermediate Phase 1: Carbon fixation (Entering one at a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6 1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2: Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar) Glucose and other organic compounds Calvin Cycle 3 3 ADP ATP 5 P Phase 3: Regeneration of the CO 2 acceptor (RuBP) G3P

42. Light Reactions: Photosystem II Electron transport chain Photosystem I Electron transport chain CO 2 NADP + ADP P i + RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage) Sucrose (export) Chloroplast Light H2OH2O O2O2