Bio 3465 - Lab 8 - Photosynthesis - Light reactions

Bio 3465 - Lab 8 - Photosynthesis - Light reactions - BIO...

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BIO 3465 – Plant Physiology Laboratory 8 PHOTOSYNTHESIS – LIGHT REACTIONS. Goal : Photosynthesis involves two major steps known as 1) light reactions and 2) dark reactions. In light reactions chlorophyll absorb light energy and passes electrons through a chain of bio molecules until finally they are being captured by NADP + to produce NADPH. Electron vacuum created in photosynthetic reaction center is replenished by splitting water into H + and O 2 . Rate of photosynthesis can be measured by many ways. Today’s experiment will allow you to understand the effects of light intensity and herbicides on photosynthesis by measuring the changes of an artificial electron acceptor. INTRODUCTION The process by which plants make carbohydrates using CO 2 , H 2 O and light energy is known as photosynthesis . In this process, plants produce oxygen as a byproduct. The generalized overall reaction for photosynthesis is as follows: However, photosynthesis actually occurs in two distinct steps that are separated both spatially and temporaly. The first step is the Light Reactions , and the second is the Calvin Cycle (sometimes called the Dark Reactions because it does not directly require light). The first step of photosynthesis, the light reactions, occurs in the thylakoid membranes. This process produces two important molecules, ATP and NADPH. Light powers the addition of a phosphate group to ADP, producing ATP. The light also drives a transfer of electrons and hydrogen to an electron acceptor NADP, which subsequently becomes reduced to NADPH (Figure 1). Figure 1: Sunlight excites electrons in chrlorophyll molecules in PS II and PSI and transfers electrons through plastoquinone (PQ), Cyto-b 6 f, plastocyanin (PC), photosystem I (PS I), ferrodoxin, ferrodoxin-NADP reductase (FNR) and finally to NADP + to produce NAPDH. Splitting of water will fill the electron vacuum created in the PS II system while increasing H + concentration in the thylakoid lumen, thus generating an electrochemical gradient.
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The second step, the Calvin Cycle, occurs in the stroma of the chloroplasts, and begins with the “fixing” of CO 2 . Using the ATP and NADPH generated by the light reactions, the Calvin Cycle reduces the fixed CO 2 to carbohydrate (CH 2 O). In summary then, it is the Calvin Cycle that actually produces the sugar; however, it cannot do so without the ATP and NADPH generated by the light reactions. In today’s class we are interested in the ability of chlorophyll in the PSII
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This note was uploaded on 11/09/2011 for the course BIO 101 taught by Professor Pesthy during the Fall '07 term at Texas State.

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Bio 3465 - Lab 8 - Photosynthesis - Light reactions - BIO...

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