Photosynthesis: Chapt. 8

Transcription

1 Respiration vs. Photosynthesis Photosynthesis and respiration as complementary processes in the living world. Photosynthesis uses the energy of sunlight to produce sugars and other organic molecules. These molecules in turn serve as food. Respiration is a process that uses O2 and forms CO2 from the same carbon atoms that had been taken up as CO2 and converted into sugars by photosynthesis. In respiration, organis ms obtain the energy that they need to survive. Photosynthesis preceded respiration on the earth for probably billions of years before enough O2 was released to create an atmosphere rich in oxygen. (The earth's atmosphere presently contains 20% O2.) The Early Years: von Helmont (1600 s) Priestley (1700 s) Photosynthesis: Chapt. 8 From these studies it was concluded: PS converts H 2 0 & CO 2 to organic matter & 0 2 Joseph Priestley born 1733 Priestley became the first person ever to observe photosynthesis in plants - the fact that they take in carbon dioxide and release oxygen. In 1772, Priestley placed a shoot of a green plant into a container of water. He then covered the container and lit a candle...the candle burned longer than without the plant. Priestley also was able to keep mice alive under the jar. Priestley had just discovered what would later be known as oxygen. He called the gas dephlogisticated air, based on the phlogiston theory (the idea that combustion was essentially the process of losing a hypothetical substance known as phlogiston) of the day. Also: Invented soda pop and the rubber eraser The injury which is continually done to the atmosphere by the respiration of such a large number of animals... is, in part at least, repaired by the vegetable creation. 1

2 Jan Baptista van Helmont ( ) Van Helmont describes his own experiment: I took an earthen vessel, in which I put 200 pounds of earth that had dried in a furnace, which I moistened with rainwater, and I implanted therein the trunk or stem of a willow tree, weighing five pounds. And at length, five years being finished, the tree spring from thence did weigh 169 pounds and about three ounces. Lest the dust that flew about should be mingled with the earth, I covered the lip or mouth of the vessel with an iron plate covered with tin and easily passable with many holes. I again dried the earth up in the vessel, and there was found the same 200 pounds, wanting about two ounces. Therefore, 164 pounds of wood, bark, and roots, arose out of water only. Photosynthesis Life is powered by sunlight. The energy used by most living cells comes ultimately from the sun. Plants, algae, and some bacteria use energy from sunlight, particularly blue and red wavelengths, to build molecules which later can be split through cellular respiration to retrieve some of that energy. Storing energy in molecules and then oxidizing those molecules to retrieve the stored energy maintains all life on Earth. Plants are often called producers because they produce energy-storing molecules used by almost all other organisms on Earth. By eating plants, herbivores and carnivores steal these energy-storing molecules to maintain their own life processes. Ultimately, the process of photosynthesis is the most important chemical reaction on Earth. Photosynthetic Equation 6CO 2 + 6H 2 O C 6 H 12 O 6 + 6O 2 six molecules of carbon dioxide plus six molecules of water produce one molecule of sugar plus six molecules of oxygen 2

3 Where does PS occur? Green leaves More Specifically: The Chloroplasts Chloroplast Structure surrounded by double membrane Contains additional internal membranes termed thylakoids which may be unstacked or stacked (grana ) internal region termed stroma Text pg 69 3

4 Photosynthesis: in two distinct reactions Light Reactions absorb light produce oxygen Dark Reactions fix CO 2 Photosynthesis: involves 2 separate pathways Light reactions.. light absorption, oxygen production, uses sunlight E to produce ATP & NADPH Dark reactions..co 2 uptake and conversion to glucose (CO 2 fixation). Contains cyclic pathway to fix CO 2 The Light Reactions requires light input (light absorption) produces oxygen, NADPH & ATP occur on thylakoid membranes 4

5 A Closer look Light Absorption: by Chlorophyll Evidence: Light absorption spectrum of chlorophyll matches the effective light wavelengths for rates of photosynthesis Text pgs Chlorophyll lipid molecule with a porphyrin ring structure and a long HC tail HC tail embeds chlorophyll in lipid bilayer of thylakoid has a central Mg + atom Text pg

6 Chlorophyll Types a in all photosynthetic eukaryotes and cyanobacteria a & b b in higher plants & green algae c in brown algae, diatoms & dinoflagellates a & c a & b a & c a & c Accessory Pigments Increase efficiency of photosynthesis by absorbing light of different wavelengths and passing e- on to Chlorophyll Most common... the carotenoids: lipids which absorb in blue/green light and thus reflect in red/yellow. Add to Fall colors Text pg 140 Photosystems Chlorophylls & accessory pigments group together to form a cohesive unit: a Photosystem of two components: 1. Light-harvesting component: gathers light E. and passes it around 2. Reaction center: Specific Chl a molecule which passes electrons on. Text pg

7 Photon Reaction Center Light Harvesting Pigments From the Photosystem, e- are passed along an Electron Transport Chain.. The Photosynthetic Electron Transport Chain (PETC) Photon PETC 7

8 Photosynthetic Electron Transfer Chain (PETC) series of electron carriers which take electrons from photosystem, and.. ultimately carry electrons to NADP + Photosystems Expts. in the 1940 s suggested that light photons are absorbed at 2 different points along the same PETC. In fact, there are two Photosystems in operation Text pg. 143 Photon PETC Photon PETC 8

9 Two Photosystems operate in light absorption PS I max. light absorption at 700nm (P700) PS II max. abs. at 680nm (P680) So how does it all work? 2 separate photosystems (I, II) PETC: groups of electron acceptors Final e- acceptor is NADP + which is reduced to NADPH The Z Scheme e- acceptors Photon e- acceptors NADPH Photon PS I H + + O 2 PS II H 2 O 9

10 The Z Scheme The Z Scheme: 1. PS II : absorbs light at 680nm. Chl a at reaction center becomes activated & passes on e-. Lost e- from Chl a is replaced by water, releasing O 2 2. e- carried on to PS I 3. PSI: passes e- onto NADP +. Requires additional Energy to do this. Energy comes from light of 700nm 4. NADP+ is reduced to NADPH Text pg. 143 So what have we done? Chl a reaction center (P680) gets hit by light Passes e- to PETC P680 replenishes lost e- by splitting H 2 0 H P700 picks up e-, and gets hit by more light Passes e- further along PETC Finally e- used to reduce NADP + 10

11 End Result of Light Reactions Split water Formed oxygen Reduced NADP + to NADPH What about ATP production? The Light Reactions requires light input (light absorption) occur on thylakoid membranes produces oxygen, NADPH & ATP Photophosphorylation: as e- are passed from one part of PETC to next, H+ are pumped across thylakoid membrane and out an ATP synthase particle (CF1) Light Reactions: ATP Synthesis Text pg

12 Photophosphorylation Occurs in two ways: Non-cyclic Photophosphorylation Cyclic Photophosphorylation Text pg 144 Noncyclic Photophosphorylation produces: oxygen, 2 ATP s, NADPH H 2 O PSII PSI NADP + NADPH H + H + Cyclic Photophosphorylation ATP only produced In evolution, probably first PS PSI PETC ATP 12

13 Remember Photosynthesis occurs in two distinct reactions Light Reactions absorb light produce oxygen Dark Reactions fix CO 2 The Dark Reactions Energy stored in ATP and NADPH used to drive CO 2 fixation to carbohydrates may go on in light or dark, provided sufficient energy is available requires a molecule which will attach CO 2 to it occurs in a cyclic pattern (The Calvin Cycle) Text pg 148 Dark Reactions: Calvin Cycle Involves a 5C sugar... RuBPcombining w. carbon dioxide.. CO2 + (5C) RuBP and ending up as: two (3C)phosphoglycerate (3PG) formed 13

14 Along Calvin Cycle: Phosphoglycerate (3PG) transforms to: Glyceraldehyde 3- phosphate (G3P) some G3P goes to remake RuBP excess G3P goes to make sugars (glucose) Text p 148 Calvin Cycle combines 1 CO 2 with 5C sugar (RuBP) to produce 2 PGA Enzyme named Rubisco does step 1 above Cycle s main product is 3C molecule G3P Cycle requires 3 ATP and 2 NADPH 6 turns of cycle required to produce a 6C compound such as glucose Text pg 148 G3P = Glyceraldehyde 3-Phosphate 3 Carbon compound Main product of Calvin cycle Used for synthesis of starch in plastids 14

15 How does G3P make glucose? 3 turns of Calvin cycle produces 6 G3P molecules 6 G3P = 18 Carbon atoms fixed (6x 3C = 18C fixed). Of these, 15C go to reform 3 new RuBP, while 3C remaining (1 G3P) goes to produce glucose 6 turns of Calvin required to make 1 glucose Calvin Cycle: Summation Every 6 turns: Produces 12 (3C) G3P = 36C of this 30 C Regenerate 6 (5C) RuBP and 6C produce 1 (6C) glucose And uses 18 ATP & 12 NADPH 6CO ATP+ 12 NADPH C 6 H 12 O 6 Calvin Cycle: Summation 15

16 Rubisco Ribulose 1,5-bisphosphate carboxylase enzyme which catalyzes the joining of CO 2 with RuBP makes up ~20% plant protein most abundant protein on planet (by far!) in low CO 2 conditions (high O 2 ), oxygen interferes with Rubisco...Photorespiration Photorespiration may waste 20-50% of C fixed Rubisco Inside plant cells rubisco forms the bridge between life and the lifeless, creating organic carbon from the inorganic carbon dioxide in the air. Rubisco takes carbon dioxide and attaches it to ribulose bisphosphate, a short sugar with five carbon atoms. Rubisco then clips the lengthened chain into two identical phosphoglycerate pieces (PGA), each with three carbon atoms. Phosphoglycerates are familiar molecules in the cell, and many pathways are available to use it. Most of the phosphoglycerate made by rubisco is recycled to build more ribulose bisphosphate, which is needed to feed the carbon-fixing cycle. But one out of every six molecules is skimmed off and used to make sucrose (table sugar) to feed the rest of the plant, or stored away in the form of starch for later use. Contains 2992 hydrogen bonds, which force the 16 protein chains to assume 208 helices, 248 beta-strands and 456 turns. Enzyme is composed of two subunits: The small subunit is made up of 123 amino acids. The protein contains a fourstranded antiparallel sheet, which is flanked by two helices. Some turns stabilize the loops. The large subunit contains 475 amino acids. 16