Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 7 Photosynthesis: Using Light to Make Food Biology and Society: Plant Power for Power Plants • On a global scale the productivity of photosynthesis is astounding (=160 billion metric tons) • An “energy plantation (에너지농장)” is a renewable energy source (willow (버드나무) is being tested as an energy source) • The trees are cut once every three years, and harvested wood is sent to power plants to generate electricity © Jong B. Lee, Ph.D. • Burning wood for energy has advantages over burning fossil fuels RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. The Basics of Photosynthesis Chloroplasts: Sites of Photosynthesis • Almost all plants are photosynthetic autotrophs, as are some bacteria and protists. • Photosynthesis – They generate their own organic matter through photosynthesis. – Occurs in chloroplasts. – Leaves have the most chloroplasts and are the major sites of photosynthesis – More specifically, the green color in plants comes from pigment molecules in the chloroplasts called chlorophyll – The chlorophyll molecules that capture light energy are built into the thylakoid membrane RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 1 The Overall Equation for Photosynthesis • Chloroplasts contain stroma, a thick fluid. : Thylakoids, disk-like membranous sacs, are suspended in the stroma • CO2 enters, and O2 exits, by way of tiny pores called stomata, which are found on lower epidermis of leaves • Water is mainly absorbed by the plant’s roots, then travels via veins to the leaves RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. • Photosynthesis takes “exhaust” of cellular respiration and rearranges its atoms to produce food and oxygen RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. In photosynthesis A Photosynthesis Road Map • It is a chemical transformation that requires much energy, and sunlight provides the energy • Photosynthesis is composed of two processes: – Electrons are boosted “uphill” and the energized electrons are added to carbon dioxide to make sugar – This requires the chloroplast to actually split water molecules into hydrogen and oxygen (It takes a lot of energy to split water. the oxygen escapes into the atmosphere as O2 ) – Hydrogen is moved along with the electrons, so the redox process takes hydrogen transfer from water to carbon dioxide to form sugar RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee – The light reactions convert solar energy to chemical energy, ATP & NADPH. – The Calvin cycle makes sugar from carbon dioxide. (Using ATP & NADPH) RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 2 The Light Reactions: Converting Solar Energy to Chemical Energy The Nature of Sunlight • Sunlight is a type of energy called radiation or electromagnetic energy • The full range of radiation is called the electro-magnetic spectrum – Electromagnetic energy travels through space as rhythmic waves – The distance between the crests of two adjacent waves is called wavelength RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. The Process of Science: What Colors of Light Drive Photosynthesis? • In 1883, German biologist Theodor Engelmann performed an experiment using bacteria and algae and determined that certain types of light drive photosynthesis. And then, Experiment Results ? – Observation: certain bacteria tend to cluster in areas with higher oxygen concentrations – Question: which wavelengths of light are best for promoting photosynthesis? – Hypothesis: oxygen-seeking bacteria would congregate near regions of algae undergoing the most photosynthesis RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 3 Chloroplast Pigments • Chloroplasts absorb selected wavelengths of light that drive photosynthesis. 우리가 잎을 보고 녹색으로 느끼는 것은 엽록체의 색소 (pigments)가 다른 색 (blue-violet and redorange)의 빛은 흡수시키지만 녹색은 반사하기 때문이다. 에너지는 파괴되지 않는 것이기 때문에 어떤 색소가 어느 특정 파장의 빛을 흡수하면, 그 에너지는 다른 에너지 형태로 전환된다. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. • Chloroplasts contain several pigments: all of these chloroplast pigments are built into the thylakoid membranes – Chlorophyll a: is the pigment that directly participate in the light reaction, and absorbs mainly blue-violet and red light – Chlorophyll b: absorbs blue and orange light, does not participate directly in the light reactions, but broadens the range of light that a plant can use by conveying absorbed energy to chlorophyll a. – Carotenoids: absorbs mainly blue-green light, some pass energy to chlorophyll a, some have a protective function (absorb and dissipate excessive light energy that would otherwise damage chlorophyll) RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. How Photosystems Harvest Light Energy • When a pigment molecule absorbs a photon, one of the pigment’s electrons gains energy: electrons has been raised from a ground state to an excited state • The excited state is very unstable, and generally the electron loses the excess energy and falls back to its ground state almost immediately. • Colors of fall foliage: due to decreases in green chlorophyll, allowing the yellow-orange hues of longerlasting carotenoids to show through RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee • Some pigments, including isolated chlorophyll that has been extracted from chloroplasts, emit light as well as heat after absorbing photons RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 4 Photosystems (광계) • Chlorophyll molecules absorb photons - Electrons in the pigment gain energy - Light-excited chlorophyll behaves very differently in an intact chloroplast - In its native habitat, chlorophyll is organized with other molecules into photosystems • The excited electrons quickly fall back down to their ground state, releasing energy in the form of fluorescence light RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. • A photosystem is an organized group of chlorophyll and other molecules. - Each photosystem has a cluster of a few hundred pigment molecules including chlorophyll a and b and carontenoids: This cluster of molecules functions as a light-gathering antenna RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. - When a photon strikes one pigment molecule, the energy jump from pigment to pigment until it arrives at the reaction center of the photosystem. Reaction center: chlorophyll a + primary electron acceptor - This primary electron acceptor traps the light-excited electron from the reaction center chlorophyll - Another team of molecules in thylakoid membrane then uses that trapped energy to make ATP and NANPH RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 5 How the Light Reactions Generate ATP and NADPH • Two types of photosystems cooperate in the light reactions. - Water-splitting photosystem (PSII): uses light energy to extract electrons from H2O and releases O2 PSI: P700 How to release oxygen? - NADPH-producing photosystem (PSI): produces NADPH by transferring light-excited electrons from chlorophyll to NADP+ RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. PSII: P680 Z scheme: non-cyclic event • An electron transport chain - Connects the two photosystems. - Releases energy and acts as a proton pumping machine - Chloroplast uses proton gradient energy to make ATP. • The mechanism of ATP production during light reactions is very similar to the ATP production in cellular respiration - An electron transport chain pumps hydrogen ions (H+) across a membrane (inner membrane of mitochondria in cellular respiration vs thylakoid membrane in photosynthesis) - ATP synthases use the energy stored by the H+ gradient Difference: i) food provides the high-energy electrons vs light-excited electrons provides the energy ii) topology of ATP synthase RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 6 The Calvin Cycle: Making Sugar from Carbon Dioxide Enzyme: rubisco • The Calvin cycle Ribulose biphosphate – Functions like a sugar factory within a chloroplast. – Regenerates the starting material with each turn. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. Water-Saving Adaptations of C4 and CAM Plants • C3 plant : because the first organic compound produced is the three-carbon compound (3-PGA) - Are very common and widely distributed (soybeans, oats, wheat, and rice), Use CO2 directly from the air. - Dry weather can reduce the rate of photosynthesis and decrease crop productivity : On a hot, dry day, plants close their stomata. Closing stomata is an adaptation that reduces water loss, but it also prevents CO2 from entering the leaf - As a result, CO2 levels can get very low in the leaf, and sugar production ceases, at least in C3 plants RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee • The Calvin cycle - Regenerates the starting material with each turn. Therefore, it is called “a cycle” - Input : CO2 (carbon source), ATP (energy source), NADPH (high-energy electron source) - The first organic molecule after CO2 fixation: 3-PGA (3-phospho-glyceric acid) : Enzyme called “rubisco”: RuBP + CO2 → two 3-PGA - Output : glyceraldehyde 3-phosphate (G3P) RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. C4 plants : 옥수수 , 사탕수수, 귀리... • When the weather is hot and dry, a C4 plant keeps its stomata closed most of time, thus conserving water. At the same time, it continues making sugars by photosynthesis • A C4 plant has an enzyme that incorporate carbon from CO2 into four-carbon compound (oxaloacetic acid) instead of into 3-PGA PEP carboxylase : PEP (C3) + CO2 → oxaloacetic acid(C4) • This enzyme has an intense affinity for CO2 and can continue to mine CO2 from the air spaces of the leaf even when the stomata are closed The four-carbon compound acts as a carbon shuttle; it donates the CO2 to 3-PGA (Calvin cycle) in a nearby cell, which therefore keeps on making sugars even though the plant’s stomata is closed RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 7 - Two steps are separated spatially: they are separated into two cell types (mesophycells & bundle sheath cells) - PEP carboxylase는 Rubisco 비하여 CO2에 대한 친화력이 매우 강하다. 주변 CO2 농도가 작아도 bundle sheath cells로 CO2 를 농축시킬수 있다 CAM plants ; 다육성 식물 (선인장.파인애플..) • They open their stomata only at night to conserve water. : They are adapted to very dry climates (pineapples, many cacti, and most succulent plants) • When CO2 enter leaf, it is incorporated into a four-carbon compound The four-carbon compound banks CO2 at night (in vacuoles) and release it to the Calvin cycle during the day • This keeps photosynthesis operating during the day, even though the leaf admits no more CO2 because stomata are closed. The two steps are separated in time but not space. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. The Environmental Impact of Photosynthesis Hot and dry regions of tropics: Corn, sorghum, sugarcane etc RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee Extremely dry areas: Cactus, pineapple etc RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 8 How Photosynthesis Moderates Global Warming • Greenhouses used to grow plant indoors • CO2 & other gases trap sunlight that warms the air inside. “greenhouse effect” 온실효과 Greenhouse gases • The gases in the atmosphere absorb heat radiation: Water vapor, CO2, CH4, CFC4 (chlorofluorocarbons: Freon) – CO2 is one of the most important greenhouse gases – Most of that carbon returns to the atmosphere via cellular respiration, the action of decomposers, and fires – But a substantial amount of CO2 remains locked in large tracts of forests – The rise in atmospheric CO2 levels during last century coincided with widespread deforestration. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. • CO2 concentration at air and global warning – CO2 concentration: 280 ppm (0.03%) before 1850 – 380 ppm (2006) – Mostly from the combustion of carbon-based fossil fuels (coal, oil, and gasoline) – Increasing concentrations of CO2 have been linked to global warming, a slow but steady rise in Earth’s surface temperature – What can be done to slow this increase in atmosphere? “ Energy plantation” RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. RPTSE Biology – Fall 2015, Dr. Jong B. Lee RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. Evolution Connection: The Oxygen Revolution • The atmospheric oxygen we breathe is a by-product of photosynthesis. • Cyanobacteria were the first organisms to carry out photosynthesis. • The production of oxygen changed the Earth forever. The “oxygen revolution” was a major episode in the history of life on Earth. RPTSE BIO Fall 2015 Jong B. Lee, PhD, All rights reserved. 9