Chapter 04 - PhotosynthesisPhotosynthesis is the process of converting E in sunlight to E in chem. bonds, esp-glucose. 6CO2 + 6 H2O + Light -> C6H12O6 + 6O2 Photosynthesis begins w/light-absorbing pigments in plant cells. Pigment molecule absorbs E from light w/in a narrow range of wavelengths. To be efficient as possible, dif. pigments absorb dif. wavelengths and act together to optimize E absorption. These pigments = chlorophyll a and b and carotenoids-which are red, orange or yellow. 1. Light absorbed by pigments -> E from light is incorp. into e` w/in atoms that make up the molecule. 2. Energized e` are unstable and immediately re-emit absorbed E. 3. E is reabsorbed by e` of nearby pigment molecule. 4. Process re-cycles w/energy bouncing from one pigment mol. to another. 5. Process ends when E is absorbed by chlorophyll a P680 or P700. Noncyclic Photophosphorylation-process of making ATP from ADP+Pi, using E frm light. Begins w/PS II: 1. PS II. e` trapped by P680 in PS II are energized by light. 2. Primary e` acceptor. Two energized e` passed to a molecule called primary e` acceptor. It's 1st in chain of e` acceptors. 3. ETC. e` pass through ETC. ETC consists of proteins that pass e` from one carrier to the next. Some carriers, like ferredoxin & cytochrome include nonprotein parts containing Fe. ETCs in photosynthesis = those in oxidative phosphorylation. 4. Phosphorylation. As 2 e` move "down" the ETC, they lose E. E lost by the e` as they pass along the ETC is used to phosphorylate about 1.5 ATP molecules. 5. Photosystem I. ETC terminates w/PS I (w/P700). Here e` are again energized by sunlight and passed to a primary electron acceptor (dif. than the one in PS II) 6. NADPH. The two e` pass through a short ETC...at end of chain, the two e` combine w/NADP+ and H+ to form NADPH-which is a coenzyme and energy-rich. 7. Splitting of H2O. The two e` that originated in PS II are now w/NADPH. Loss of these two e` from PS II is replaced when H2O is split into two e`, 2 H+ and 1/2 O2. A manganese-containing protein complex catalyzes the rxn. The two e` from H2O replace the lost e` from PS II, one of the H+ provides H in HADPH and the 1/2 O2 contributes to the O2 gas that is released. Summary: Photophosphorylation takes E in light and e` in H2O to make energy-rich molecules ATP & NADPH. B/C the rxns require light, they're often called light-dependent rxns/light rxns. H2O + ADP + Pi + NADP+ + light -> ATP + NADPH + O2 + H+ Cyclic Photophosphorylation: The 2nd photophosphorylation sequence occurs when e` energized in PS I are "recycled." In this sequence, energized e` from PS I join w/protein carriers and generate ATP as they pass along the ETC. In contrast to noncyclic photophosphorylation where e` become incorporated into NADPH, e` in cyclic photophosphorylation return to PS I. They can be energized again to participate in cyclic or noncyclic photophosphorylation. Cyclic occurs simultaneously w/noncyclic photophosphorylation to generate additional ATP. Two e` passing through cyclic photophosphorylation generate on avg. 1 ATP.

Calvin Cycle: Fixes CO2. It takes chemically unreactive, inorganic CO2 and incorporates it into an organic molecule that can be used in bio systems. Function of pathway is to produce a single glucose molecule (C6H12O6). To accomplish this, Calvin cycle must go x6, and use 6 CO2 molecules. 1. Carboxylation: 6 CO2 combine w/6 RuBP to produce 12 PGA. PGA is first product formed. Enzyme used is: ribulose bisphosphate carboxylase/oxygenase or rubisco. 2. Reduction: 12 ATP and 12 NASDPH are used to convert 12 PGA to 12 G3P. E in ATP and NADPH is incorporated into G3P, making G3P very energy-rich. ADP, Pi, and NADP+ are released and reenergized in noncyclic photophosphorylation. 3. Regeneration: 6 ATP are used to convert 10 G3P to 6 RuBP. Allows the cycle to repeat (step 1). 4. Carbohydrate synthesis. The remaining 2 G3P are used to build glucose & other monosaccharide's. No light is directly used in Calvin Cycle. These rxns are called light-independent rxns/dark rxns. CAREFUL! The process cannot occur in absence of light...b/c it's dependent upon E from ATP & NADPH...which are ONLY created during photophosphorylation...which ONLY occurs w/light. Summary: Calvin cycle takes CO2 from atmosphere and E in ATP and NADPH to create glucose. Of course, E in ATP and NADPH represents E from sun captured during photophosphorylation. 6CO2 + 18ATP + 12NADPH + H+ -> 18 ADP +18Pi + 12NADP+ + 1 glucose Chloroplasts: Site where both light-dependent/independent rxns of photosynthesis occur. 1. 2. 3. 4. Outer membrane. Consists of double layer phospholipids. Intermembrane space. Narrow area between inner/outer membranes. Inner membrane. Also a double phospholipid bilayer. Stroma. Fluid material that fills the area inside inner membrane. Calvin cycle occurs here, fixing carbon from CO1 to generate G3P 5. Thylakoids. Suspended w/in the stroma are stacks of pancake-like membranes. Individual (pancake) layers are thylakoids. an entire stack of thylakoids is a granum. The membranes of the thylakoids contain protein complexes-including PS I and PS II, cytochromes, and other e` carriers of the light-dependent rxn occur here. 6. Thylakoid lumen. This is the inside, or lumen, of the thylakoid. H+ ions accumulate here. Chemiosmosis in Chloroplasts: The mechanism of ATP generation that occurs when E is stored in form of proton [] gradient across a membrane. Process is analogous to ATP generation in Mitochondria. 1. H+ ions accumulate inside thylakoids. H+ are released into lumen of thylakoid when water is split by PS II. Also, H+ care carried from stroma into lumen by cytochrome in ETC between PS II & PS I. 2. A pH & electrical gradient across the thylakoid membrane is created. As H+ accumulate inside thylakoid, pH decreases. Since some of these H+ come from outside the thylakoids (from stroma), H+ [] decreases in stroma and its pH increases. Thereby creating a pH gradient-stroma pH 8 to thylakoid pH 5. Also electric/voltage gradient. 3. ATP synthases generate ATP. pH and electrical/voltage gradient represent potential E. Channel proteins-ATP synthases, allow H+ to flow through thylakoid membrane and out to stroma...allowing ATP synthases to phosphorylate ADP to ATP. Three H+ to generate one ATP. 4. The Calvin cycle produces G3P using NADPH and CO2 & ATP. At end of ETC following PS I, e` combine w/NADP+ and H+ to produce NADPH. W/NADPH, ATP and CO2, two G2P are generated and used to make glucose and other carbs.

Photorespiration: Is the "fixing" of O2 w/RuBP catalyzed w/rubisco.

Problem: Competing rxn w/Calvin cycle due to rubisco non-specificity Solution: Peroxisomes inside thylakoid near chloroplasts break down waste product to rid cell of it

C4 Photosynthesis: Moves CO2 to bundle sheath cell which increases efficiency by minimizing photorespiration and reducing H2O loss. C4 plants are found in hot, dry climates. Examples: Sugarcane, corn and crab grass. CAM photosynthesis: Very similar to C4 photosynthesis. Advantage of CAM is that photosynthesis can proceed during the day while stomata are closed, greatly reducing H2O loss. As a result, CAM provides adaptation for plants that grow in hot, dry environments w/cool nights (ex: deserts). Examples: Cacti

Stomata are open at NIGHT and malic acid accumulates in cell's vacuole. Stomata are closed during the DAY (reverse of other plants).