Photosynthesis: Converting Sunlight into Energy

2. Overview • All organisms need a constant supply of energy to survive. • For most life on earth, the ultimate source of energy is the sun. • Converting that energy source into something usable is accomplished by photosynthesis.

3. Overview • Photosynthesis: the overall process by which sunlight (solar/light energy) chemically converts water and carbon dioxide into chemical energy stored in glucose (a sugar/carbohydrate.) • Water is absorbed in roots • CO2 is absorbed through stomata • It can be represented by the following chemical equation: 6CO2 + 6H2O C6H12O6 + 6O2 Solar energy

4. Overview Solar energy 6CO2 + 6H2O C6H12O6 + 6O2 • Reactants= ingredients • CO2(carbon dioxide) and H2O (water) • Products = results • C6H12O6(glucose) and O2 (oxygen) • Note: Solar energy from the sun is necessary for photosynthesis to happen (as well as some enzymes) but isn’t considered a reactant or product.

5. Structure of Chloroplast • Photosynthesis takes place in the chloroplastwhich has 2 main parts: • Grana: pancake-like stacks of thylakoid membrane • Stroma: fluid-like substance that fills the space between the grana grana thylakoid membrane

6. Why are plants green? • The presence of the pigment chlorophyll • Chlorophyll a, chlorophyll b, and other pigments called carotenoids absorb every color of light in sunlight exceptgreen • Therefore, green is leftover and is reflected and is what we see

7. Two Stages of Photosynthesis Photosynthesis can be divided into two sets of reactions: 1. Light-dependent (“photo”) Reaction Requires solar energy. AKA the Electron Transport Chain (or light rxn) 2. Light-independent(“synthesis”) Reaction Does not require any solar energy. AKA the Calvin Cycle (or dark rxn)

8. Light-Dependent Reaction • Purpose = Capture energy from the sun and store energy in “energy-carrying molecules” (ATP and NADPH) • Location= occurs in the grana (specifically the thylakoid membrane) where the chlorophyll is stored.

9. Light-Dependent Reaction Summary: • Watermolecules are split into hydrogen and oxygen. • Oxygen is released as a waste product. • ATP and NADPH are charged up by the sun.

10. Light-Dependent Reaction Details: • Energy from sun is passed down the Electron Transport Chainand is stored in the bonds of ATP and NADPH • Light energy excites e- (electrons) • e- move down ETC • At end they combine with “final electron acceptors/carriers” of NADP+ and ADP, making NADPH and ATP • Chemiosmotic process b/c H+ ions move down the gradient to make ATP • ATP, NADPH, and H+ leave the grana and go into the stroma for the next stage!

11. How is light absorbed? • Photosystems absorb light • They are clusters of chlorophyll and proteins that trap energy from the sun • Chlorophyll is a pigment that can absorb sunlight • Energy is transferred to electrons  makes “excited” electrons

12. What are electron carriers? • Molecules that carry electrons in order to pass on their energy • Ex. Compound (NADP+) that can accept a pair of high-energy electrons and transfer them to another molecule • NADP+ grabs/carries 2 electrons and a H+ becomes NADPH • ATP and NADPH carry energy from the light-dependent rxn to the light-independent rxn.

13. Light-Dependent Reaction

14. Light-Independent Reaction • Purpose = use the energy from the “energy-carrying molecules” from the light-dependent reaction to make sugar (glucose) • Location= occurs in the stroma

15. Light-Independent Reaction Summary = Calvin Cycle • Series of enzyme-assisted chemical reactions powered by ATP and NADPH that produce three-carbon (3-C) sugars from CO2 and the H+ from water. • The cycle happens twice and then these 3-C sugars combine to make glucose = C6H12O6

16. Light-Independent Reaction Details • Grab • CO2 diffuses into stroma • Enzyme attaches CO2 to 5-C RuBP • Produce unstable 6-C molecules • Split • Energy from ATP and NADPH and an enzyme break the 6-C molecule into 2 3-C molecules (PGA)

17. Light-Independent Reaction Details • Leave • Each 3-C molecule (PGA) is converted to a different 3-C molecule (G3P) • One G3P leaves the cycle to become glucose • The other G3P moves on to next step • Switch • Remaining G3P converts back to 5-C RuBP by using a phosphate from ATP and the cycle starts again!

18. Light-Independent Reaction

19. Rate of Photosynthesis • Speed is affected by 3 factors: • Light intensity • Excites more e- causing light reactions to happen faster • Amount of CO2 • More ingredients to work with and process through cycle • Temperature • Increased temperature accelerates chemical reactions to a degree

20. Why do root cells in a plant not need chloroplasts? • Chloroplasts catch sunlight! Since roots are underground, they are not exposed to the sun! • So they can’t do photosynthesis.

21. Alternate Pathways • Stomata = pores on underside of leaf. Where… • Plants lose water • CO2 enters • O2 exits • If it is too hot or dry out, the plant will close its stomata so that it doesn’t lose too much water and become dehydrated • However this eliminates the gas exchange!! • SO  the levels of CO2 drop and the levels of O2 increase • This results in…. PHOTORESPIRATION • Photorespiration adds oxygen to the Calvin Cycle instead of carbon dioxide - This makes NO sugar or ATP - This wastes all of the plants resources! • Two types of alternative pathways in plants to avoid this: 1. CAM 2. C4

22. Alternate Pathways • CAM • Done by cacti and pineapples • Open stomata at night and close during day • Opposite of normal plants • Causes them to grow slowly

23. Alternate Pathways • C4 • Done by corn and sugarcane • Partially close stomata during hottest part of day • Allows them to only need ½ as much water as normal plants!