Photosynthesis. Nearly all of the usable energy on this planet came, at one time or another, from the sun by the process of photosynthesis

Transcription

1 Photosynthesis Nearly all of the usable energy on this planet came, at one time or another, from the sun by the process of photosynthesis

2 Photosynthesis 6CO H 2 O C 6 H 12 O 6 + 6O 2 + 6H 2 O

3 Pigments The process of photosynthesis begins when light is absorbed by pigments Pigments are colored substances that absorb or reflect light The most famous of these pigments (found in large amounts in green plants) is called

4 Chlorophyll Pigments

5 Separating Pigments

6 Light Absorption

7 Energy-storing compounds When sunlight is absorbed by matter, it is either used to raise the kinetic energy (temperature) of the matter, Or it is transferred to the electrons in matter (by raising the electrons to a higher energy level) If the electrons are raised to a new energy level, the trapping of the energy in chemical bonds happens in one of 2 ways:

8 The 2 Ways to Store Energy 1. A pair of high energy electrons is passed directly to an electron carrier (who can then pass them along with most of their energy) Plants use an electron carrier called NADP + (NADPH) 2. Energy is trapped in the creation of Adenosine Triphosphate AMP ADP ATP Each new phosphate addition stores more energy This is generally the goal of living organisms HOWEVER, ATP is not a good STORAGE unit for more than a few seconds

9 NAD NADP NADPH

10 ATP

11 ATP stores energy in the bonds

12 ATP & NADPH Even though these compounds are high energy and easily accessible to plants, they are not very good at storing this energy The goal of the NADPH molecules is use redox reactions to move the energy to stable carbons Therefore, the plants use Dark Reactions in which they store these high energy compounds as glucose (which is far more stable)

13

14 Chloroplasts Photosynthesis takes place within the chloroplast and consists of two interconnected sets of reactions: The Light Reactions: A set of steps that transfer the energy of sunlight to the covalently bonded electrons of NADPH and ATP The Dark Reactions (aka Calvin Cycle): A cyclical process that incorporates atmospheric CO2 into carbon compounds within the cell and transfers the energy from the light reactions

15

16 Photosystems II & I A photosystem is composed of a protein complex called a reaction center complex It is surrounded by several light harvesting complexes (Photon absorbing complexes) Light absorbing complexes can be chlorophyll A, chlorophyll B, and/or accessory pigments) When the light harvesting complex absorbs a photon, it is passed to the reaction center Within the reaction center exists a molecule called a primary electron receptor capable of accepting electrons and becoming reduced

17

18 8 Steps of the Light Reactions 1. Light Absorption (PSII) 2. Electron Transport (PSII) 3. Oxygen Production (PSII) 4. Cytochrome Electron Transport 5. ATP Formation (Stroma) 6. Light Absorption (PSI) 7. Electron Transport (PSI) 8. NADPH Formation (PSI)

19 1. Light Absorption (PSII) To start photosynthesis light reactions, first a photon of light strikes a pigment molecule in a light harvesting complex This boosts one of its electrons to a higher energy level (increasing its potential energy) As it falls back to its ground state, it excites the electron of a neighboring pigment This relay of passing the photon will continue until it excites the electron of a pair of p680 chlorophyll A molecules in the PS II Rxn Center The p680 chlorophyll A molecules of the rxn center of photosystem II absorbs light at 680nm

20 Thylakoids

21 2. Electron Transport (PSII) The excited electron from P680 is transported to the primary electron receptor Because the P680 chlorophyll A molecules now lack an electron, it is referred to as P680 + Now the issue becomes getting new electrons back to the P680 + molecules so the process can continue

22 3. Oxygen Production (PSII) An enzyme catalyzes the splitting of Water into Oxygen, 2 H +, and 2 electrons The electrons are then used to replace the lost electron from the P680 molecule The Oxygen molecule joins with another oxygen molecule (from a previous split) forming O 2 This is the Oxygen that is released during photosynthesis

23

24 4. Cytochrome Electron Transport Each photo-excited electron is passed from the primary electron acceptor in PSII to PSI via an electron transport chain The electron carriers are: Plastoquinone (Pq)-the higher energy carrier A Cytochrome complex (energy transfer-er) Plastocyanine (Pc)-the lower energy carrier The Cytochrome facilitates the exergonic fall of the electron from Pq Pc

25 5. ATP Formation (Stroma) The energy lost from the fall of the electron in the cytochrome complex is used to pump the H+ ions into the Stroma The Stroma is the fluid space outside of the thylakoid The build up of H+ ions causes a chemiosmosis gradient difference This difference is referred to as an energy of repulsion A protein channel called ATPase is used to convert the energy into the bond needed to make ADP into ATP

26 6. Light Absorption (PSI) Meanwhile, inside of photosystem I, the light harvesting complex is absorbing photons (just like in step 1 PSII) The photon is passed from the various PSI pigment molecules until it reaches the electrons of P700 pair of chlorophyll a molecules (absorb energy in the 700nm) The excited electrons are transferred to the PSI Primary electron receptor creating P700+ The P700+ then accepts the electron from PSII

27

28 8. NADPH Formation (PSI) An enzyme called NADP + reductase catalyses the transfer of electrons from Fd to NADP + It takes a total of 2 electrons from Ferredoxin pathway to reduce NADP + to NADPH NADPH is able to then be used in the Calvin Cycle (Dark Reactions) to transfer energy to the Carbons of Glucose By the END of the light reactions, ATP and NADPH have been created

29 Summary of Light Reactions light reactions use: water, ADP, and NADP+ light reactions produce: O2, ATP, and NADPH

30 Dark Reactions Dark reactions use the energy stored in ATP & the reducing power of NADPH to produce G3P G3P = Glyceraldehyde-3-Phosphate G3P is a building block of glucose Glucose can store more energy than either NADPH or ATP Glucose is also much more stable and safer Dark reactions don t require light, but they can occur in light The organic molecule carbon dioxide is reduced to larger carbon molecules (specifically G3P) Dark reactions provide the raw material needed for the cell

31 Calvin Cycle Background aka Dark Reactions aka Light Independent Reactions Dark reactions form a cycle, or a circular series of reactions The American chemist is credited with the cycle known as the Calvin Cycle (chemists name was Dr. Melvin Calvin)

32 Calvin Cycle The Calvin Cycle can be divided into 3 steps: 1. Carbon Fixation Attaching CO 2 to Ribulose Biphosphate (RuBP) 2. Reduction Using ATP and NADPH (from Light Rxn) to create G3P 3. Regeneration of the CO 2 acceptor RuBP Spending ATP to recreate the RuBP molecule

33 Phase 1: Carbon Fixation -To begin the Calvin Cycle, a 5 carbon sugar called Ribulose Biphosphate (RuBP) is attached to CO 2 -This is catalyzed by an enzyme called Rubisco -The product of the reaction is so unstable, that it immediately breaks in half -The resulting 2 molecules are called 3- phosphoglycerate

34 Phase 2: Reduction Each molecule of 3-phosphoglycerate receives an additional phosphate from ATP Making 1,3-biphosphoglycerate Next, a pair of electrons are donated from NADPH, converting a carboxyl group to a higher energy aldehyde group This reduction turns the molecule into G3P For every 3 CO2 molecules that enter, 6 G3P molecules are made but only 1 is truly gained (the other five get recycled) This 1 G3P molecule gained is kicked out

35 Phase 3: Regeneration of RuBP In the last step of the Calvin Cycle, the 5 remaining molecules of G3P are rearranged into 3 molecules of RuBP In order to rearrange the bonds of these molecules, 3 molecules of ATP must be used The RuBP is now prepared to accept CO2 again, and the cycle can continue The cycle will continue as long as there is a supply of ATP & NADPH

36 Making G3P For every 1 G3P molecule made, the Calvin Cycle consumes: 9 ATP molecules (Making 9 ADP) 6NADPH molecules (Making 6 NADP + ) The light reaction will recharge the carriers, and the cycle will continue The G3P molecule is used to make glucose, as well as several other important carbohydrates for the cell

37 The Wasteful Photorespiration Plants that carry out the photosynthesis just discussed are called C 3 plants because the 3CO 2 molecules that start C3 plants are very efficient at using CO 2 to create G3P (and O 2 ) when conditions are favorable However, when dry or hot conditions occur, the stomata close and CO 2 becomes scarce Rubisco will then add O 2 to the RuBP instead of CO 2, creating a molecule that can be broken into CO 2 by peroxisomes (Destructive and makes no ATP) This process is called Photorespiration

38 C 4 Plants Some plants have the ability to avoid the affects of photorespiration by using a preliminary process This process uses bundle-sheathe cells which are tightly packed sheaths around the veins of the leaf to carry out the Calvin Cycle. It also uses Mesophyll cells which are located between the leaf surface and the bundle-sheathe cells Here, an enzyme called PEP carboxylase makes a molecule called oxaloacetate, which is used to procure CO2 into the bundle-sheathe cells The PEP carboxylase has no affinity for O2 and therefore decreases the photorespiration

39 CAM Plants In extremely arid conditions, many cacti and succulents use another type of photosynthesis These plants open their stomata at night and store the CO2 in a variety of organic molecules (that resist photorespiration) This mode of respiration is called CAM (Crassulacean Acid Metabolism) Named after the family of succulents Crassulacea where it was discovered