Photosynthesis

1. Photosynthesis

2. Overview • Photosynthesis is the process that converts solar energy (sunlight) into chemical energy (glucose) • Directly or indirectly, photosynthesis nourishes almost the entire living world

3. Autotrophs make their own food • Live without eating anything from other organisms • Are the producers of the biosphere, producing organic molecules from CO2 and other inorganic molecules • Almost all plants are photoautotrophs, using the energy of sunlight to make organic molecules from H2O and CO2

4. Photosynthesis occurs in plants, algae, certain other protists, and some prokaryotes • These organisms feed not only themselves but also most of the living world Plants Multicellular alga Unicellularprotist Cyanobacteria Purple sulfur bacteria BioFlix: Photosynthesis

5. Heterotrophs obtain their energy from the foods they consume • Get their energy from organisms they eat • Are the consumers of the biosphere • Almost all heterotrophs, including humans, depend on photoautotrophs for food and O2

6. How is energy obtained? • High–energy bonds break • Replaced by low–energy bonds • Extra energy is released • Some stored in ATP • Some released as heat • ATP (adenosine triphosphate) • Used to store and release energy in the cell • Made of adenine, ribose (5-carbon sugar), and 3 phosphate groups

7. ADP (adenosine diphosphate) • When energy is available, small amounts are stored by adding a phosphate group to ADP • Making ATP • Releasing energy when needed is the reverse reaction • The high–energy bond is broken between the second (2nd) and third (3rd) phosphates

8. How much ATP does a cell contain? • Only a small amount of ATP is at hand • Enough for a few seconds of activity • Why? • An ATP molecule is good for energy transfer and NOT for storing over long periods of time • What is the energy storage molecule? • Glucose is the storage molecule • More than 90 times the chemical energy of ATP • Cells can regenerate ATP from ADP as needed

9. Photosynthesis • Plants use sunlight (solar energy) to convert water (H2O) and carbon dioxide (CO2) into high–energy carbohydrates (sugars: mainly glucose C6H12O6 and starches: complex sugars) and oxygen (O2) 6 CO2 + 12 H2O Sunlight (Solar energy) C6H12O6 + 6 O2 + 6 H2O

10. How do plants capture solar energy? • Requires chloroplasts and the chlorophyll within • Chlorophyll: a green pigment that absorbs light energy • Pigment: light–absorbing molecules

11. Our eyes see light as “white” light • Light is actually a mixture of different wavelengths • The different wavelengths your eyes can see make up the visible spectrum and produce different colors • Different pigments absorb different wavelengths • Wavelengths not absorbed are reflected/transmitted back

12. Chlorophyll is the major pigment of plants • Two types of chlorophyll • Chlorophyll a • Major photosynthetic pigment • Absorbs well in the blue-violet and red regions • Little is absorbed in the green region • Reflects back (this is why plants are green) • Chlorophyll b • Broadens the absorbing range • Another pigment is carotene • Red and orange pigments • Absorbs light in other regions

13. Any compound that absorb light absorbs the energy from the light • After chlorophyll absorbs light • Much of the energy is transferred directly to electrons • Energy levels are raised making them high–energy electrons Animation: Light and Pigments

14. A spectrophotometer measures a pigment’s ability to absorb various wavelengths • This machine sends light through pigments and measures the fraction of light transmitted at each wavelength

15. Leaves are the major location of photosynthesis • Their green color is from the chlorophyll • Light energy absorbed by chlorophyll drives the synthesis of organic molecules (sugars) • CO2 enters and O2 exits the leaf through microscopic pores called stomata

16. Chloroplasts are found mainly in cells of the mesophyll, the interior tissue of the leaf • A typical mesophyll cell has 30–40 chloroplasts • Photosynthesis occurs inside chloroplasts • The chlorophyll is in the membranes of thylakoids found in the chloroplast • Saclike photosynthetic membranes • Newspapers • Thylakoids are arranged in columns called grana (granum: singular) • Stack of newspapers

17. Proteins within the thylakoid membrane organize pigments, including chlorophyll, into photosystems • Photosystems are the light–collecting units • Chloroplasts also contain stroma • A dense fluid

18. Photosynthesis Reactions • 2 types of reactions • Light–dependent reactions • Photo part • Requires light • Summary: uses light to produce oxygen (O2) gas, ATP and NADPH • Light–independent reactions (Calvin cycle) • Synthesis part • Does not require light • Summary: uses CO2, ATP and NADPH to produce sugar

19. All reactions occur within the chloroplast • Light–dependent reactions: within the thylakoid membrane • Calvin cycle: in the stroma

20. Electron transport chain (ETC) • When electrons (e─) gain energy from sunlight, they require a special carrier • High–energy electrons are too hot to handle • Electron transport: carrier molecules accept a pair of high–energy electrons and transfer (carry) them to other molecules • Electron carriers are the electron transport chain • NADP+(nicotinamide adenine dinucleotide phosphate) • Accepts and holds 2 high–energy e─ s as well as a hydrogen ion (H+) • Converts NADP+ to NADPH • Traps some sunlight into chemical form NADP+ NADPH 2 e─+ H+

21. CO2 H2O Light NADP+ ADP + P i Calvin Cycle Light Reactions ATP NADPH Chloroplast O2 (sugars)

22. Light–dependent reactions • Uses light energy to produce O2 as a by–product and ATP and NADPH for use later • Step 1: pigments in photosytem II absorb light through electrons (increasing their energy) • These high–energy electrons are transported by the ETC • How are electrons available? • Split H2O molecules into 2 electrons, 2 hydrogen ions, and 1 oxygen • Oxygen is released to the atmosphere • Hydrogen ions are released inside the thylakoid membrane • Electrons released replace those electrons lost to the ETC

23. Step 2: high–energy electrons move through the ETC from photosystem II to photosystem I • This energy is used in the ETC to transport H+ ions into the inner thylakoid space

24. Step 3: pigments in photosystem I use light energy to reenergize electrons • NADP+ picks up these high–energy electrons with H+ ions

25. Step 4: as electrons pass from chlorophyll to NADP+ • H+ ions are pumped across the membrane • Making the inside of the membrane fill with a positive (+) charge • Making the outside of the membrane more negatively (-) charged • Differences across the membrane provides energy to make ATP

26. Step 5: H+ cross the membrane only through ATP synthase • As H+ crosses, ATP synthase rotates • As it rotates, ATP synthase binds ADP and a phosphate group to make ATP

27. 6 CO2 12 C-C-C • Calvin cycle • Uses the energy of ATP and NADPH (short–term storage) to build high–energy compounds (sugars) for long time storage • Step 1: 6 CO2 enter from the atmosphere combine with 3–carbon (C) molecules to produce 12 3–C molecules • Step 2: 12 3–C molecules converted to high – energy forms through ATP and NADPH • Step 3: 2 3–C molecules are removed from the 12 3–C molecules • Used to make sugars among other compounds need by the plant for growth and metabolism • Step 4: 10 3–C molecules converted back to the 6 5–C molecules • The cycle continues • The sugars made • Used to meet the energy needs of the plant • Used to build complex macromolecules such as cellulose and starch 12 ATP 6 C-C-C-C-C 12 ADP 6 ADP 12 NADPH 6 ATP 12 NADP+ 10 C-C-C 12 C-C-C 2 C-C-C C-C-C-C-C-C glucose (sugar)

28. Rate of photosynthesis • Water shortage • May slow or even stop photosynthesis • Adaptations: waxy coating to reduce water loss • Temperature • Too low or high: slows down photosynthesis • Enzymes function best between 0°C and 35°C • Light intensity • Increasing light = increased photosynthesis • Until photosynthesis rate is maximized (highest it can go)