Photosynthesis 6 CO H 2 O --> C 6 H 12 O O H 2 O carried out by photoautotrophs Solar energy --> chemical energy redox process- water oxidized, CO 2 reduced Photosynthesis occurs in 2 stages: 1. light reactions (photo) 2. Calvin cycle (synthesis)

CO 2 and O 2 enter and leave through stomata

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 chlorophyll a 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 P Some energy in chain used to make ATP by chemiosmosis across the thylakoid membrane. 4. Electrons in P680 replaced by splitting H 2 O. ** This is the step where O 2 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 P Photon strikes P700 reaction center. 2. Electrons passed down ETC back to P 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 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 CO 2 fixed by rubisco & first product = 3C sugar In dry, hot weather, plants close stomata to reduce transpiration --> Reduces CO 2 intake. Rubisco binds to O 2 when [CO 2 ] 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 CO 2 is fixed in the mesophyll by the enzyme PEP carboxylase --> 4C compound 4C compound then enters bundle sheath where CO 2 is released and is used by rubisco in the Calvin cycle. ***C4 pathway minimizes photorespiration and enhances sugar production!***

CAM plants- no special cells, CO 2 is trapped during the night (stoma open) for use during the day(stoma closed) incorporate CO 2 into organic acids in night ex- cactus & pineapple