Photosynthesis • 6 CO2 + 12 H2O --> C6H12O6 + 6 O2 + 6 H2O • carried out by photoautotrophs • Solar energy --> chemical energy • redox process- water oxidized, CO2 reduced • Photosynthesis occurs in 2 stages: • 1. light reactions (photo) • 2. Calvin cycle (synthesis)
Chloroplasts • found in mesophyll • contain chlorophyll = green pigment • thylakoid membranes in stacks called grana • in prokaryotes, thylakoids on cell membrane
In each chloroplast are functional groups called photosynthetic units • 1. contain about 300 molecules of pigment • 2. contain one specialized chlorophylla molecule called the reaction center • 3. other molecules function like antennae • 4. reaction center can trap energy and pass it along an enzyme-linked series of reactions to convert energy to a usable form
chlorophyll a- • 1. absorbs violet, blue-violet, red • 2. reflects green, yellow, orange • carotenes- • 1. absorb wavelengths not absorbed by chlorophyll • 2. provide photoprotection •**All pigments absorb different wavelengths!**
electrons within chlorophyll are excited to higher energy levels by photons (particle of light) • What happens to excited electron? • 1. energy dissipated as heat • 2. energy reemitted as light =fluorescence (in test tube) • 3. energy may cause a chemical reaction (in chloroplast)
1. Light Reactions • Light energy drives the transfer of e- and H from water to NADP+ ---> NADPH • H2O is split and O2 is given off. • Light reactions also carry out photophosphorylation converting ADP to ATP.
Ground state e- + photon ---> excited e- • unstable • drops back • energy can be trapped by ETC
Photosystem I= P700 • Photosystem II= P680 • Have identical chlorophyll a but different associated proteins
Noncyclic Photophosphorylation • •occurs in green plants and a few bacteria • 1. Photon strikes P680 reaction center. • 2. Electron passed down ETC to P700. • 3. Some energy in chain used to make ATP by chemiosmosis across the thylakoid membrane. • 4. Electrons in P680 replaced by splitting H2O. • ** This is the step where O2 is released!**
5. Photon strikes P700 reaction center. • 6. Electrons passed through a second ETC. • 7. More ATP is made by chemiosmosis. • 7. As e- pass down ETC, NADP+ is reduced -> NADPH. • **NADPH and ATP go on to the Calvin cycle.**
Cyclic Photophosphorylation • •less efficient • •only uses P700 • 1. Photon strikes P700 reaction center. • 2. Electrons passed down ETC back to P700. • 3. Generates ATP. • 4. No production of NADPH or release of O2. • ***Like oxidative phosphorylation in the mitochondria!***
Cyclic Photophosphorylation continued… • May have been the earliest form of ATP production. • Still used by photosynthetic bacteria. • Operates along with noncyclic flow in plants to generate more ATP. (The Calvin Cycle uses more ATP than is produced by noncyclic photophosphorylation.)
2. Calvin Cycle • •Doesn’t require light directly • (occurs during daylight for most plants!) • Begins with Carbon fixation= Carbon from CO2 is incorporated into organic molecules • 3 CO2 enter per cycle
•The Calvin Cycle then reduces the carbon into carbohydrate by the addition of e- from NADPH • •produces one 3C sugar (PGAL or G3P) per cycle
Process: (for each CO2) • 1. CO2 attached to a five C sugar (RuBP) • -catalyzed by enzyme rubisco • (most abundant protein on Earth!!!) • 2. Forms unstable 6-C intermediate which splits into 2 3-C sugars. • 3. 3-C sugars phosphorylated by ATP.
4. 3-C sugars reduced by NADPH. • 5. PGAL produced. • 6 PGALs are produced for each Calvin Cycle • -1 exits cycle • -5 remain in cycle---> regenerate RuBP • 6. Rest of cycle= regenerate RuBP (ATP needed) • It takes 2 cycles to produce one glucose!
Special Cases of Photosynthesis • Most plants= C3 plants • CO2 fixed by rubisco & first product = 3C sugar • In dry, hot weather, plants close stomata to reduce transpiration --> Reduces CO2 intake. • Rubisco binds to O2 when [CO2] is low. • =photorespiration- decreases photosynthesis
C4 plants- form a four C compound as first product • ex- sugar cane, corn & some grasses • Krantz Anatomy- 2 types of photosynthetic cells • a. bundle sheath cells tightly packed around veins • b. mesophyll cells located outside bundle sheath • CO2 is fixed in the mesophyll by the enzyme PEP carboxylase --> 4C compound • 4C compound then enters bundle sheath where CO2 is released and is used by rubisco in the Calvin cycle. • ***C4 pathway minimizes photorespiration and enhances sugar production!***
CAM plants- no special cells, CO2 is trapped during the night (stoma open) for use during the day(stoma closed) • incorporate CO2 into organic acids in night • ex- cactus & pineapple