Photosynthesis

You must know… How photosystems convert solar energy to chemical energy… How linear electron flow in the light reactions results in the formation of ATP, NADPH, and 02 How chemiosmosis generates ATP in the light reactions How the Calvin cycle uses energy molecules of the light reactions to produce G3P The metabolic adaptions of C4

Context Autotrophs Heterotrophs Self Feeders Producers Consumers Depend on autotrophs for food and oxygen

Context Chloroplasts The site of photosynthesis in plants Mesophyll tissue in interior of leaf Chlorophyll Located in thylakoid membranes Light absorbing pigment

Chloroplast

Plant cell structure Stomata: allow CO2 to enter and O2 and water vapor to exit

Photosynthesis Reaction 6CO2 + 6H2O + Light Energy  C6H12O6 + 6O2 Reverse of respiration Water is split for electrons and H+ ions It is a two step process Light Reactions The Calvin Cycle

Light Reactions Occurs in thylakoid membranes Solar energy converted to chemical energy Electrons transferred from water to NADP+ Oxygen released ATP generated via photophosphorylation Net products are… NADPH ATP Oxygen

Calvin Cycle Occurs in stroma Sugar formed from CO2 aka carbon fixation Powered by NADPH and ATP from light reactions

Big Picture

Light Reaction Details Light is electromagnetic energy, photons Pigments: substances that absorb light.

Light Reactions Photons absorbed by pigments called photosystems with 2 parts Light Harvesting complex Photon excites an electron, moves to higher orbital Reaction center 2 Chlorophyll a molecules accept energy Energy transferred to primary electron acceptor At this point, light energy has been converted to chemical energy

Photosystems Photosystem 2 (P680) and 1(P700) Major concept is the linear electron flow Flow of electrons through photosystems in the thylakoid membrane

Major steps of light reactions PS2 absorbs light energy, reaction center (2 chlorophylls) donate e- to primary electron receptor. Chlorophyll is now oxidized. An enzyme splits water into 2 H+ and O and electrons go to PS2 Electron travels down electron transport chain from PS2 to PS1 The energy from this electron sets up proton gradient used for chemiosmosis to power ATP production. ATP used later to make carbohydrates in Calvin cycle

Major steps of light reaction 5. Meanwhile light also activated PS1, electrons donated to primary acceptor. What replaces these electrons? What is the source of these electrons? 6. Electrons go down another transport chain to NADP+, NADP+ and H form NADPH for Calvin cycle.

An alternative Cyclic electron flow: PS1 only. Cycles electrons, produces ATP but no NADPH

Chloroplasts VS Mitochondria Both generate ATP by chemiosmosis Both generate proton motive force Differences Spatial Mitochondria transfer energy from food to ATP Chloroplasts use light energy to transfer to ATP

Calvin Cycle Uses ATP and NADPH to convert CO2 to sugar To make one G3P you must complete the cycle 3 times. Don’t memorize all intermediate steps… understand concept of reducing CO2 to sugar.

Calvin Cycle 3 CO2’s attach to 3 molecules of 5-carbon ribulose bisphosphate (RuBP) Catalyzed by rubisco Intermediate is unstable, breaks into 3 carbon compounds 6 NADPH reduce the intermediate One G3P molecule exits cycle 2 G3P molecules combine to form glucose Last step, RuBP regenerated

Calvin Cycle Nine molecules of ATP consumed along with 6 molecules of NADPH

Alternate mechanisms of carbon fixation Evolved in hot and arid climates The stomata is the problem CO2 enters the same place that H2O exits Plant has to keep stomata closed hence CO2 shortage for Calvin cycle Photorespiration: Oxygen binds and reduces RuBP, blocks up CO2 binding. This can drain 50% of carbon fixed by Calvin cycle.

Adaptations 2 Types C4 CAM Plants

C4 Plants C4 plants have bundle-sheath cells and mesophyll cells. CO2 is added to PEP to form 4 C compound oxaloacetate. PEP carboxylase can not bind oxygen so photorespiration is not an issue Mesophyll cells export oxaloactate to bundle sheath cells releasing CO2 to the normal Calvin cycle.

CAM Plants Adaption to hot dry climates Stomata closed during day, prevents water loss What else will this prevent? Open stomata at night, fix CO2 in acids Where do you think this is stored? CAM plants separate stages of PS temporally

Adaptive Values C3 more efficient than C4 and CAM plants under “normal” conditions requires less machinery, enzymes and anatomy C4 PS faster than C3 plants under high light intensity and high temperatures. Prevents photorespiration. Has better water use PEP Carboxylase brings in CO2 faster and so does not need to keep stomata open as much (less water lost by transpiration) CAM Better water use efficiency than C3 plants under arid conditions due to opening stomata at night when transpiration rates are low.

References http://wc.pima.edu/~bfiero/tucsonecology/plants/plants_photosynthesis.htm