Photosynthesis Chapter 10 Part 2

The Light Reactions Driven by visible light – light is electromagnetic radiation – only small fraction of radiation perceived by organisms different wavelengths=different colors

The Light Reactions Leaf absorbs some wavelengths (red-orange and blue-violet) and reflects others (green) In plants, light absorbed by chlorophyll a, chlorophyll b and carotenoids

The Light Reactions Chlorophyll a main photosynthetic pigment Chlorophyll b + other accessory pigments act as “antenna” molecules to broaden range of energy absorbed Carotenoids absorb excessive light that would damage chlorophyll

The Photosystems Light behaves like particles (photons) When pigment absorbs photon, energy level of one electron is raised to excited, unstable state In chloroplasts, chlorophyll molecules grouped with proteins to form antenna assembly around two chlorophyll a molecules A photosystem consists of a reaction-center complex (a type of protein complex) surrounded by light-harvesting complexes The light-harvesting complexes (pigment molecules bound to proteins) transfer the energy of photons to the reaction center

(a) How a photosystem harvests light Thylakoid membrane Photon Photosystem STROMA Light- harvesting complexes Reaction- center complex Primary electron acceptor Transfer of energy Special pair of chlorophyll a molecules Pigment molecules THYLAKOID SPACE (INTERIOR OF THYLAKOID) ee The Photosystems  Excited electrons passed from antenna chlorophylls to reaction center chlorophylls then to primary electron acceptor  Series of redox reactions  Final is oxidation of reaction center chlorophyll and reduction of primary electron acceptor

The Photosystems Two photosystems (antenna assembly + primary electron acceptor) identified – Absorb at different wavelengths photosystem II-absorbs maximally at 680nm (P680) photosystem I-absorbs maximally at 700nm (P700) – Function together to carryout linear electron flow Produces ATP and NADPH using light energy – Photosystem I can also carryout cyclic electron flow Thought to be the earliest form of photosynthesis – present in many primitive photosynthetic bacteria Synthesizes only ATP

The Photosystems

Chemical Energy Generation Electron transport chains generate ATP, NADPH and O 2 – kinetic energy of light absorbed and excites electrons – excited electrons passed along electron transport chain – released energy used to generate ATP, NADPH and O 2 Production of NADPH requires 2 electrons supplied to PS I by PS II replaced in PS II by splitting water H 2 O ---> 1 / 2 O 2 + 2H + + 2e -

Chemiosmosis Powers ATP synthesis – H + ions from splitting water and those pumped across thylakoid membrane by electron transport chain form gradient across thylakoid membrane (inside to outside) – ATP synthase provides port for H + to diffuse back into stroma releases energy and phosphorylates ADP to ATP similar process to ATP generation in mitochondria known as photophosphorylation

Carbon Fixation ATP and NADPH from light-dependent reactions power Calvin cycle – Occurs in chloroplast stroma – net result of Calvin cycle is 3C molecules from CO 2 using energy and electrons in ATP and NADPH from light-dependent reactions – CO 2 added to 5C intermediate ribulose-1,5- bisphosphate (RuBP) catalyzed by RuBP carboxylase/oxygenase (rubisco)

 Number of rearrangements occur, using energy in ATP and oxidation of NADPH  Last step in cycle regenerates RuBP  Carbon enters the cycle as CO 2 and leaves as a sugar named glyceraldehyde 3- phospate (G3P)  3C molecules exported to cytoplasm  used to synthesize glucose and other organic molecules For net synthesis of 1 G3P, the cycle must take place three times, fixing 3 molecules of CO 2

Alternative Mechanisms Dehydration is a problem for plants – Water conservation involves trade-offs with other metabolic processes, especially photosynthesis On hot, dry days, plants close stomata, which conserves H 2 O but also limits photosynthesis The closing of stomata reduces access to CO 2 and causes O 2 to build up These conditions favor photorespiration

C 3 plants Plants that use only Calvin cycle to fix carbon called C 3 plants – first identifiable product of carbon fixation is 3C molecule Photorespiration – C 3 plants conserve water by closing stomata Allows buildup of O 2 in leaves Rubisco fixes O 2 rather than CO 2 Uses ATP and NADPH but makes no sugars Photorespiration limits damaging products of light reactions that build up in the absence of the Calvin cycle

C 4 plants C 4 plants adapted to conserve water and prevent photorespiration CO 2 incorporated into 4C molecule in mesophyll cells diffuses into bundle sheath cells, releases CO 2 Enters Calvin cycle in bundle sheath chloroplasts

CAM plants CAM (crassulacean acid metabolism) plants incorporate carbon during night Common in succulent plants Stomata open at night, closed during day – CO 2 incorporated in 4C molecule (organic acid) and stored in vacuole at night – During day, 4C molecules exported into cytoplasm and CO 2 released – CO 2 enters Calvin cycle

C 4 separate carbon incorporation and fixation spatially CAM plants separate carbon incorporation and carbon fixation temporally

Summary of Photosynthesis The energy entering chloroplasts as sunlight gets stored as chemical energy in organic compounds Sugar made in the chloroplasts supplies chemical energy and carbon skeletons to synthesize the organic molecules of cells Plants store excess sugar as starch in structures such as roots, tubers, seeds, and fruits In addition to food production, photosynthesis produces the O 2 in our atmosphere