Reactions of Photosynthesis I Reactions of Photosynthesis A

Reactions of Photosynthesis

I. Reactions of Photosynthesis • A. Photosynthesis occurs in 2 parts – 1. Light-dependent reactions • a. take place in thylakoids – 2. Light-independent reactions (Calvin Cycle) • b. takes place in the stroma outside thylakoid

II. Electron Carriers • A. When sunlight hits chlorophyll in a chloroplast electrons become excited. – 1. When electrons become excited they are considered high-energy electrons • B. Electron carriers are like pans that transport a pair of high-energy electrons throughout the cell, this process is known as electron transport. – 1. Electron carries: NADP+

Where do they get the electrons that are excited? ? ? Water is needed for photosynthesis H 2 O O 2 + 2 H+ + 2 e. NADP+ + 2 H+ + 2 e- 2 NADPH

III. Light-Dependent Reactions “The Light Side” • A. These reactions require light • B. These reactions produce oxygen gas and convert ADP and NADP+ into the energy carriers ATP and NADPH.

III. Light-Dependent Reactions “The Light Side” • C. Along the thylakoids, antenna complexes made from chlorophyll a, b & carotenoids, gather light and send it to the reaction center that can excite electrons. What is e- acceptor? Donor?

III. Light-Dependent Reactions “The Light Side” • D. There are 2 types of reaction centers that differ on the type of chlorophyll & the wavelength they absorb – 1. Photosystem II (PS II) – P 680 maximum absorption is 680 nm. – 2. Photosystem I (PS I) - P 700 absorbs best at a wavelength of 700 nm • E. They are responsible for photophosphorylation (using light to make ATP)

IV. Noncyclic Photophosphorylation • A. Uses both photosystems. First photosystem II (P 680) absorbs sunlight splitting water into Hydrogen ions & excited electrons that are transferred to a primary acceptor when the electrons go down the electron transport chain to photosystem I. Some of the energy is used to pump hydrogen(protons) across the thylakoid membrane. – 1. The accumulation of hydrogen ions in the thylakoids create a proton gradient. As hydrogen ions move through membrane ATP is created. – 2. Photosystem I gains more energy with sunlight activating electrons to be passed to NADP+ to form NADPH

V. Cyclic Photophosphorylation • A. The simpler way to generate ATP. Only uses photosystem I(P 700) to excite electrons and are passed down the ETC to create ATP through the use of a hydrogen gradient. – 1. Does not create any NADPH & water is not split

VI. Light-Independent Reactions (Calvin Cycle) “The Dark Side” • A. These reactions do not require light and take place in the stroma

VI. Light-Independent Reactions (Calvin Cycle) “The Dark Side” • B. The Calvin cycle uses ATP and NADPH from the light-dependent reactions to produce high-energy sugars – 1. Since ATP & NADPH are not very stable and do not store a lot of energy – 2. The Calvin converts 6 CO 2 molecules into 1 molecule of sugar C 6 H 12 O 6 (carbon fixation)

VI. Light-Independent Reactions (Calvin Cycle) • C. Calvin Cycle: C 3 Pathway – 1. Ribulose bisphosphate (Ru. BP) combines with CO 2 to make unstable G 3 P to eventually make a six-carbon compound. (Ru. BP carboxylase, rubisco, catalyzes rxn. ) 6 Ru. BP + 6 CO 2 + 12 ATP + 12 NADPH convert 12 PGA to 12 G 3 P glucose molecule(2 G 3 P) + 10 G 3 P (back into 6 Ru. BP) drive Calvin cycle again + 12 ADP + 12 NADP+

VII. Photorespiration • A. When plants receive extremely bright light plants do not continue to grow due to photorespiration being triggered. • 1. Photorespiration leads to the fixation of oxygen. Ru. BP carboxylase reduces the CO 2 concentration to the point that it starts incorporating O 2 instead.

VIII. C 4: Alternative • A. Calvin Cycle: C 4 Pathway – 1. Plants found in extremely hot/dry conditions go through the C 4 pathway to fix CO 2 to glucose even when supply is low • B. Phosphoenolpyruvate (PEP) combines with CO 2 to form oxaloacetate, a four carbon molecule. (PEP carboxylase an enzyme that helps collect CO 2) – 1. This then is converted into malate which eventually changes form once more releasing CO 2 to drive Calvin cycle forward

IX. C 3 vs. C 4 • A. Similar: – 1. Occur in stroma – 2. Require ATP & NADPH – 3. Fix CO 2 to make glucose • B. Differences: – 1. C 3 uses a a 3 carbon molecule to begin making glucose while C 4 uses a 4 carbon molecule to initially make glucose

X. CAM Photosynthesis • A. Desert plants need to keep their stomates closed during the day to reduce transpiration. • B. CAM (crassulacean acid metabolism) photosynthesis uses C 4 with PEP carboxylase to fix CO 2 to oxaloacetate but instead of converting into malate its converted into malic acid which is placed into a vacuole and converted back into oxaloacetate and CO 2 to be used for Calvin cycle

XI. Factors Affecting Photosynthesis • A. Many factors affect the rate of photosynthesis that include: – 1. Water supply – 2. Temperature (0°C-35°C) – 3. Intensity of light