Photosynthesis

Learning objectives SWBAT: Describe how organisms capture free energy present in sunlight and convert to the chemical energy of food. SWBAT: Describe the light-dependent and light- independent stages of photosynthesis SWBAT: Compare the processes of cellular respiration and photosynthesis

Heterotroph = consumer Autotroph = producer Make their own fuel/organic molecules, but still need to use cellular respiration to convert that fuel into usable energy Heterotroph = consumer Depend on autotrophs for food and O2

Photosynthesis = the conversion of light energy to chemical energy that is stored in sugars or other organic compounds; occurs in plants, algae, and some prokaryotes

Photo- Light energy from the sun is converted to the chemical energy in the form of chemical bonds. Remember the 1st Law of Thermodynamics! Which product of cellular respiration contains energy in its bonds?

-synthesis Energy from ATP and NADPH powers the synthesis of a 3 carbon molecule (G3P) that can then be used to make glucose (and other simple carbohydrates)

Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Figure 8.3 Leaf cross section Chloroplasts Vein Mesophyll Stomata CO2 O2 Chloroplast Mesophyll cell Figure 8.3 Zooming in on the location of photosynthesis in a plant Outer membrane Thylakoid Granum Thylakoid space Intermembrane space Stroma 20 m Inner membrane 1 m

Photosynthesis equation 6CO2 + 6H2O + light energy C6H12O2 + 6O2 An endergonic redox reaction- electrons gain potential energy as they travel from H2O to C6H12O6

Reactants: 6 CO2 12 H2O Products: C6H12O6 6 H2O 6 O2 Figure 8.4 Figure 8.4 Tracking atoms through photosynthesis 9

2 stages of photosynthesis Light-dependent reactions Light-independent reactions

Light-dependent reactions A series of coordinated pathways that capture free energy present in light. Yields ATP and NADPH which power the production of organic molecules in the light-independent reactions Occurs in the thylakoid membranes of the chloroplast Very similar to the electron transport chain in cellular respiration

More about light-dependent reactions Within the thylakoid membrane are 2 complexes called photosystems: PSI and PSII (though PSII occurs first) During photosynthesis, chlorophylls, within the photosystems, absorb light energy and boost electrons to higher energy levels. PSI and PSII are connected by the transfer of these electrons through an electron transport chain much like that in cellular respiration.

Just like in cellular respiration, this electron transport creates a proton gradient (hydrogen ions) across a membrane. In this case, the thylakoid membrane. The gradient is then responsible, via ATP Synthase, for the formation of ATP from ADP.

Photosystem I (PS I) Photosystem II (PS II) Figure 8.13-5 4 Electron transport chain 7 Electron transport chain Primary acceptor Primary acceptor Fd Pq e− 8 2 NADP e− 2 H H2O e− e− Cytochrome complex H   NADP reductase  2 1 3 O2 NADPH Pc e− P700 P680 1 e− 5 Light Light 6 ATP Figure 8.13-5 How linear electron flow during the light reactions generates ATP and NADPH (steps 7-8) Pigment molecules Photosystem I (PS I) Photosystem II (PS II) 15

Light-independent reactions The Calvin Cycle Production of carbohydrates from CO2 powered by ATP and NADPH Occurs in the stroma of the chloroplast

Light-independent reactions Though sometimes called the “dark reactions”, they occur in the daytime. They, however, do not require light. There are 3 phases of the Calvin Cycle Carbon fixation Reduction- production of G3P (a 3 carbon compound) Regeneration of the CO2 acceptor (RuBP)

Photorespiration CO2enters the plant through stomata in the leaves. H2O also transpires, is lost, through stomata. What happens on hot, dry days? If the plant closes its stomata, it can conserve water, but cannot obtain the CO2 that it needs. In photorespiration O2 enters the Calvin Cycle where CO2 normally would.

When there is not enough CO2, O2 enters the Calvin Cycle in a process called photorespiration. Two problems: This process consumes O2, but uses rather than generates ATP Decreases photosynthetic output by removing carbon from the Calvin cycle and releasing it as CO2 rather than as organic compounds.

Adaptations to optimize photosynthesis in hot climates C4 Plants- fix carbon in a 4-carbon compound then uses it as a “shuttle” to a different location, the bundle- sheath cells, where the Calvin Cycle occurs. CAM Plants- open stomata during the night to let in CO2, closed during the day when the plant then completes the light-independent Calvin Cycle

Calvin Cycle Calvin Cycle Figure 8.18c 1 1 CO2 CO2 C4 CAM Mesophyll cell Organic acid Organic acid Night CO2 2 CO2 2 Bundle- sheath cell Day Calvin Cycle Calvin Cycle Figure 8.18c C4 and CAM photosynthesis compared (part 3: detail) Sugar Sugar (a) Spatial separation of steps (b) Temporal separation of steps 22

Electron transport chain Figure 8.15a MITOCHONDRION STRUCTURE CHLOROPLAST STRUCTURE Inter- membrane space H Diffusion Thylakoid space Electron transport chain Inner membrane Thylakoid membrane ATP synthase Figure 8.15a Comparison of chemiosmosis in mitochondria and chloroplasts (detail) Matrix Stroma Key ADP  P i ATP Higher [H] H Lower [H] 24

Learning objectives SWBAT: Describe how organisms capture free energy present in sunlight and convert to the chemical energy of food. SWBAT: Describe the light-dependent and light- independent stages of photosynthesis SWBAT: Compare the processes of cellular respiration and photosynthesis