Photosynthesis
Photosynthesis (Life from Light) Energy needs of life All life needs a constant input of energy o Heterotrophs (consumers) Animals, fungi, most bacteria Get their energy from other organisms Consume organic molecules o Autotrophs (producers) Plants, photosynthetic bacteria (blue-green algae) Get their energy from self, ultimately from sunlight Use light energy to synthesize organic molecules What does it mean to be a plant? Need to Collect light energy Transform it into chemical energy Store light energy in a stable form to be moved around the plant & also saved for a rainy day Need to get building block atoms from the environment (C,H,O,N,P,S) Produce all organic molecules needed for growth (carbohydrates, proteins, lipids, nucleic acids) Plant structure Obtaining raw materials Sunlight: leaves = solar collectors CO 2 : stomates = gas exchange H2O: uptake from roots Nutrients: uptake from roots Chloroplasts o double membrane o stroma o thylakoid sacs o grana stacks Chlorophyll & ETC in thylakoid membrane o H+ gradient built up within thylakoid sac
Light: Absorption Spectra Photosynthesis performs work only with absorbed wavelengths of light chlorophyll a the dominant pigment absorbs best in red & blue wavelengths & least in green other pigments with different structures have different absorption spectra Chloroplasts are green because they absorb light wavelengths in red & blue and reflect green back out. Photosystems -collections of chlorophyll molecules. 2 photosystems in thylakoid membrane o act as light-gathering antenna complex Photosystem II chlorophyll a P 680 = absorbs 680nm wavelength red light Photosystem I chlorophyll b P 700 = absorbs 700nm wavelength red light Photosynthesis overview Light reactions o Convert solar energy to chemical energy (ATP) Calvin cycle o Uses chemical energy (NADPH & ATP) to reduce CO 2 to build C 6 H 12 O 6 (sugars). ETC of Photosynthesis ETC produces from light energy o ATP & NADPH NADPH (stored energy) goes to Calvin cycle PS II absorbs light o excited electron passes from chlorophyll to primary electron acceptor o need to replace electron in chlorophyll o enzyme extracts electrons from H 2 O & supplies them to chlorophyll o splits H 2 O o combines with another O to form O 2 o O 2 released to atmosphere and we breathe easier! Experimental evidence Where did the O 2 come Radioactive tracer = O 18
2 Photosystems Light reactions elevate electrons in 2 steps (PS II & PS I) o PS II generates energy as ATP o PS I generates reducing power as NADPH Cyclic Photophosphorylation If PS I can t pass electron to NADP, it cycles back to PS II & makes more ATP, but no NADPH o Coordinates light reactions to Calvin cycle o Calvin cycle uses more ATP than NADPH Where did the energy come Where did the H 2 O come Where did the electrons come Where did the O 2 come Where did the H+ come Where did the ATP come Where did the O 2 go? What will the ATP be used for? What will the NADPH be used for?
Photosynthesis: The Calvin Cycle: Life from Air Remember what it means to be a plant Need to produce all organic molecules necessary for growth o carbohydrates, lipids, proteins, nucleic acids Need to store chemical energy o in stable form o can be moved around plant o saved for a rainy day Light reactions Convert solar energy to chemical energy o ATP energy o NADPH reducing power Ultimately build stuff!! From CO 2 C 6 H 12 O 6 CO 2 has very little chemical energy o fully oxidized C 6 H 12 O 6 contains a lot of chemical energy o reduced o endergonic Reduction of CO 2 C 6 H 12 O 6 proceeds in many small uphill steps o each catalyzed by specific enzyme o using energy stored in ATP & NADPH From Light reactions to Calvin cycle Calvin cycle o chloroplast stroma Need products of light reactions to drive synthesis reactions o ATP o NADPH Calvin Cycle PGAL o end product of Calvin cycle o energy rich sugar o 3 carbon compound o C3 photosynthesis PGAL important intermediate PGAL glucose carbohydrates lipids amino acids nucleic acids Rubisco Enzyme which fixes carbon from atmosphere o ribulose bisphosphate carboxylase o The most important enzyme in the world! o It makes life out of air! o Definitely the most abundant enzyme
The accounting is complicated: 3 turns of Calvin cycle = 1 PGAL o 3 CO 2 1 PGAL (3C) 6 turns of Calvin cycle = 1 C 6 H 12 O 6 (6C) o 6 CO 2 1 C 6 H 12 O 6 (6C) 18 ATP + 12 NADPH 1 C 6 H 12 O 6 6 ATP = left over from light reactions for cell to use elsewhere. Photosynthesis summary Light reactions o produced ATP o produced NADPH o consumed H 2 O o produced O 2 as byproduct Calvin cycle o consumed CO 2 o produced PGAL o regenerated ADP o regenerated NADP
Controlling water loss from leaves Hot or dry days stomates close to conserve water Guard cells o gain H 2 O = stomates open o lose H 2 O = stomates close Adaptation to living on land, but creates PROBLEMS! Closed stomates Closed stomates lead to o O 2 builds up (from light reactions) o CO 2 is depleted (in Calvin cycle) causes problems in Calvin Cycle Inefficiency of Rubisco: CO 2 vs. O 2 Rubisco in Calvin cycle o carbon fixation enzyme normally bonds C to RuBP Photosynthesis building sugars When O 2 concentration is high: o Rubisco bonds O to RuBP Photorespiration o O 2 is alternative substrate o The product splits, and a 2-C compound leaves the chloroplast. o This product is broken down by peroxisomes back into CO 2. o It wastes ATP!!! o Why in the world does this happen? Think evolution and atmosphere. Reducing Photorespiration Separate carbon fixation from Calvin cycle o C4 plants physically separate carbon fixation from Calvin cycle different enzyme to capture CO 2 PEP carboxylase instead of Rubisco stores carbon in 4C compounds (Loves CO 2 not O 2 ) Different leaf structure o CAM plants separate carbon fixation from Calvin cycle by time of day fix carbon (capture CO 2 ) during night store carbon in organic acids perform Calvin cycle during day Why the C3 problem? Possibly evolutionary baggage o Rubisco evolved in high CO 2 atmosphere There wasn t strong selection against active site of Rubisco accepting both CO 2 & O 2 Today it makes a difference o 21% O 2 vs. 0.03% CO 2 o Photorespiration can drain away 50% of carbon fixed by Calvin cycle on a hot, dry day. o Strong selection pressure to evolve better way to fix carbon & minimize photorespiration.