Understanding Photosynthesis: From Autotrophs to Photosystems

1. Photosynthesis AP Biology Unit 4

2. Vocabulary Review • What is an autotroph? • Make their own energy • What is a heterotroph? • Aren’t able to make own energy– have to get it from another source

3. Photosynthetic Organisms • Photosynthesis is one way to make your own energy • Who does it? • Plants, algae, some other protists, some prokaryotes (ex. cyanobacteria)

4. Location of Photosynthesis • Photosynthesis occurs in the chloroplasts • Organelles that contain the pigment chlorophyll and other pigments • Openings in the leaf (stomata) allow CO2 (reactant) to enter and O2 (product) to leave

5. Equation for Photosynthesis 6 CO2 + 6 H2O + ENERGY  C6H12O6 + 6 O2 • In terms of reactants and products, it is the reverse of cellular respiration, but process is different.

6. Stages of Photosynthesis • Light Reactions • Captures light energy to produce ATP and NADPH • Calvin Cycle (“Dark” Reactions) • Uses ATP and NADPH to “fix” carbon into a carbohydrate • “fix” means to incorporate the carbon into a compound

7. Light Reactions • Main point = convert sunlight energy into chemical energy (ATP and NADPH) • Sunlight can be divided into different wavelengths of light– only some are useful in photosynthesis

8. Light Reactions • Photosynthetic pigments in the chloroplast absorb different wavelengths • Chlorophyll a • Chlorophyll b • Carotenoids

9. Question… • Why are most plants green? • The chlorophyll and carotenoid molecules absorb the blue-violet and red light and use it to perform photosynthesis • They reflect green light – colors we see are the reflected wavelengths.

10. Light Reactions: Photosystems • Photosystems are the complex of pigments that receives the sunlight to begin the process • Photosystems absorb energy to excite an electron in chlorophyll • There are 2 different photosystems in chlorophyll – I and II.

11. Light Reactions: Noncyclic Flow • Step 1: Light excites electrons in photosystems I and II • Step 2: Excited electrons are passed down an electron transport chain – generates ATP and NADPH

12. Noncyclic Flow • Excited electron from Photosystem II fills the space left in Photosystem I • Water is split (photolysis) to provide an electron to fill the space left in Photosystem II

13. Cyclic Electron Flow • Under certain circumstances, the excited electrons will only flow through Photosystem I (not II)

14. Cyclic Electron Flow • Generates ATP • Doesn’t generate NADPH • Runs because the Calvin Cycle needs more ATP than NADPH • May shift from noncyclic to cyclic flow when NADPH levels are high

15. Electron Transport Chain • very similar to cellular respiration • An H+ concentration gradient across the thylakoid membrane drives ATP synthesis

16. Calvin Cycle • Also called the Dark Reactions, Non-light Dependent Reactions, or the Calvin Benson Cycle • Uses the ATP and NADPH produced in the light reactions to produce glucose using CO2

17. Calvin Cycle: Rubisco • Rubisco is the enzyme that first “traps” CO2 into the cycle • Through a series of reactions, CO2 is used to eventually make glucose

18. Calvin Cycle • CO2 only contributes 1 carbon  6 “turns” of the calvin cycle are needed to produce one glucose. • RuBP also has to be regenerated so that cycle can continue

19. 15 The O2 released during photosynthesis comes from • CO2 • H2O • NADPH • RuBP (RuDP) • C6H12O6

20. 15 Carbohydrate-synthesizing reactions of photosynthesis directly require • Light • Products of the light reactions • Darkness • O2 and H2O • Chlorophyll and CO2

21. 15 The end products of the light-dependent reactions of photosynthesis are • ADP, H2O, NADPH • ADP, PGAL, RuBP (RuDP) • ATP, CO2, H2O • ATP, NADPH, O2 • CO2, H+, PGAL