Photosynthesis - PDF

Photosynthesis • Photosynthesis uses light, carbon dioxide (CO2), and water (H2O) to produce energy-storing sugars/carbohydrates. • Conversion of light energy into the chemical potential energy of carbohydrates. • Oxygen is generated as a waste product of photosynthesis. OpenStax Biology 2e An equ...

Photosynthesis • Photosynthesis uses light, carbon dioxide (CO2), and water (H2O) to produce energy-storing sugars/carbohydrates. • Conversion of light energy into the chemical potential energy of carbohydrates. • Oxygen is generated as a waste product of photosynthesis. OpenStax Biology 2e An equation, but not the equation. = glucose or equivalent Photosynthesis Autotrophs and Heterotrophs • Autotrophs do not need a supply of organic carbon; they can build their organic molecules. • Photosynthetic organisms are photoautotrophs; -photoautotrophs use light energy and inorganic molecules to produce all of the organic molecules that they need -in photoautotrophs, light energy drives the formation of organic molecules from inorganic molecules light + CO2 organic molecules Most green algae are photoautotrophs. This is Chlorococcum sp. from Pasqua Lake, SK. H. Weger Photosynthesis • Photoautotrophs can sustain themselves without external provision of organic molecules, but need light and inorganic molecules. • In contrast, animals are heterotrophs  need external organic molecules in order to survive; -animals “feed” on organic matter; they need to “eat” -the organic matter is ultimately produced by the photoautotrophs (directly or indirectly, depending on the position in the food chain) • Herbivores eat the plants, and carnivores eat the herbivores, etc.; -have a food chain -the energy in the food chain is ultimately derived from photosynthesis Herbivores (which are heterotrophs) such as koalas gain energy and organic matter by eating photoautotrophs (plants). https://www.savethekoala.com/about-koalas/ Photosynthesis • The photoautotrophs provide the bulk of the energy for the majority of the ecosystems on earth: sunlight + CO2 organic C (chemical potential energy) • The “primary producer” is photosynthetic. • That energy works its way through the various levels of the ecosystem, though the various “trophic levels”. A food chain that is typical of Lake Ontario. The “primary producer” is photosynthetic (a photoautotroph), and is the source of the chemical potential energy that works its way through heterotrophs in the system. OpenStax Biology 2e Photosynthesis Definition of Photosynthesis • We will focus on how plants do photosynthesis, which may be defined as: “light-dependent, oxygenic CO2 fixation” • Light-dependent means that photosynthesis requires energy from light. • Oxygenic = O2-producing. • CO2 fixation = conversion of atmospheric CO2 to organic molecules; reduction (addition of electrons) to CO2 to produce carbohydrate. • Photosynthesis is a reductive process (electrons are added to C). • Need to add high energy electrons (HEE) to CO2. You do not need to be able to draw • Produce C-H and C-C bonds. the structure of sucrose. CO2 has no C-H or C-C bonds. Implied C & H O=C=O Sucrose, which is a product of photosynthesis, has lots of C-H and C-C bonds. By NEUROtiker - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2951918 Photosynthesis Light-Dependent Oxygenic CO2 Fixation • This type of photosynthesis is done by all of the familiar plants (e.g. elm trees, dandelions, mosses), and also by eukaryotic algae; -and also by cyanobacteria (= blue-green algae), which are prokaryotic All Organisms that do Plant-Type Photosynthesis have Chlorophyll a (Chla): By Yikrazuul - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7974375 You do not need to know the structure of Chla Photosynthesis • All organisms that perform light-dependent oxygenic CO2 fixation possess the green pigment chlorophyll a. • The global distribution of chlorophyll a can be used as an approximation of global patterns of photosynthetic activity. You do not need to know this. OpenStax Biology 2e This world map shows Earth’s distribution of photosynthetic activity determined by chlorophyll a concentrations. On land, chlorophyll is evident from terrestrial plants, and within oceanic zones, from chlorophyll from phytoplankton. (credit: modification of work by SeaWiFS Project, NASA/Goddard Space Flight Center and ORBIMAGE) Photosynthesis - Overview Photosynthesis is typically conceptually divided into two stages: 1) The light reactions (= light-dependent reactions) convert light energy into the CPE of ATP and NADPH. 2) The enzymatic reactions use the ATP and NADPH to produce carbohydrate (sucrose, starch); the enzymatic reactions include the Calvin cycle. + other enzymatic reactions Chloroplast 3C sugars Starch Modified from OpenStax Biology 2e Sucrose Photosynthesis takes place in chloroplasts in two stages: light dependent reactions and the Calvin cycle and related enzymatic reactions. Light-dependent reactions, which take place in the thylakoid membrane, use light energy to make ATP and NADPH. The Calvin cycle, which takes place in the stroma, uses energy derived from these compounds to make GA3P from CO2. Photosynthesis Compared to Respiration • At a superficial biochemical level, photosynthesis can be considered to be the opposite of respiration. OpenStax Biology 2e Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic cellular respiration consumes oxygen and produces carbon dioxide. Photosynthesis Overview of Photosynthetic Biochemistry • At the simplest level, photosynthesis is the opposite of aerobic cellular respiration: C6H12O6 + 6O2  6CO2 + 6H2O ΔG = -686 kcal/mole Photosynthesis: 6CO2 + 6H2O  C6H12O6 + 6O2 ΔG = +686 kcal/mole Respiration: -as drawn, photosynthesis is non-spontaneous -need an energy input to make it happen  energy input provided by light • There is a biochemically more accurate way of summarizing photosynthesis: 6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O This is our equation. Photosynthesis Overview of Photosynthetic Biochemistry 6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O Oxidized C This is our equation. Reduced C Source of electrons • Photosynthesis is a reductive process (= addition of electrons): -reduces CO2 (inorganic carbon, zero chemical potential energy) to carbohydrate (organic carbon, has chemical potential energy) -it takes 4e- to reduce a CO2 to carbohydrate  to produce the equivalent of one glucose (C6H12O6), therefore need 24e-s • Oxidized C has no C-H or C-C bonds. • Reduced C has C-H and/or C-C bonds. Carbon dioxide, CO2. Photosynthesis 6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O • The 12H2O on the left provide the electrons (24) for the reduction of 6C in 6CO2. • The 6H2O on the right are the products of later condensation reactions. • Photosynthesis is a reductive process (= addition of electrons):  to produce the equivalent of one glucose (C6H12O6) need 24e-s • H2O is the source of the e-s -each water molecule provides 2e-s  12 H2O = 24 e-s Water provides ALL of the electrons for reduction of CO2  carbohydrate Water-Splitting Reaction: 2H2O  4e- + 4H+ + O2 Must run this reaction 6x in order to provide the 24 electrons for producing one C6H12O6. Photosynthesis 6CO2 + 12H2O  C6H12O6 + 6O2 + 6H2O • Glucose (C6H12O6) works as a theoretical photosynthetic product, even though it’s not really a product. • The major products of photosynthesis are: 1) sucrose – highly water-soluble, mobile (moved around within a plant) 2) starch – insoluble starch grains, storage You do not need to know these structures. Sucrose is a disaccharide: glucose-fructose. It is highly water soluble. By NEUROtiker - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2052232 Starch is an insoluble glucose polymer. By NEUROtiker - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=2951918 The products of photosynthesis have C-H and C-C bonds; CO2 does not. Photosynthesis Water is the Electron Source for Photosynthesis • Need an electron source: photosynthesis creates C-H and C-C bonds. • Each C-H and C-C bond represents 2e-. • The water-splitting reaction (= water photolysis) is part of the “light reactions”; -extracts the e-s from H2O, and also releases H+s and produces O2 2H2O  4e- + 4H+ + O2 not important/ it’s a by-product important • The e-s are very important: -will be used to reduce CO2 -but they are low energy e-s when extracted from water  need to become high energy e-s • One important role of light is to convert low energy e-s to high energy e-s. • The H+s produced in water splitting are also important (producing H+s from water is the other important role of light). • O2 is just a by-product (plays no direct role in photosynthesis). Photolysis = to break apart using light Photosynthesis – Leaf Anatomy Chloroplasts are in the periphery/edge of the mesophyll cells (not shown in this diagram). CO2 Vacuole (stomates) Modified from Baillie & Fleming (2019) New Phytologist 225: 1120-1126 • The photosynthetic cells are the mesophyll cells (palisade and spongy). • The mesophyll cells contain chloroplasts, which are the organelles that do photosynthesis; -the chloroplasts are not shown in this diagram -the chloroplasts are at the periphery (at the cell edges) of the mesophyll cells • Stomates (= stomata) are adjustable pores that allow CO2 to diffuse into the leaf. • The epidermal cells are not photosynthetic (colourless & transparent). • (the red lines in this diagram outline air spaces in the leaf; there are many air spaces) By User:Zephyris, modified by Kelvinsong. - File:Leaf Tissue Structure.svg, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=28878567 Leaf Anatomy The epidermal cells are not actually green. CO2 These are the photosynthetic cells. (stomate) CO2 diffuses into a leaf via the stomates A cross section of a leaf, showing chloroplasts in its mesophyll cells. Guard cells are specialized epidermal cells that control the size of the stomatal opening. The only green part of a leaf is the thylakoids within the chloroplasts. Photosynthesis • The mesophyll cells (both palisade and spongy) are the photosynthetic cells; -the other cells in a leaf do not contain chloroplasts. • A typical mesophyll cell will have a few dozen chloroplasts (membrane-delimited organelles), mostly arranged at the edge (periphery) of the cell. • The chloroplasts are the site of photosynthesis, and are the only part of a plant that is green. Protoplasts (plant cells with the cell wall removed) generated from mesophyll cells. The chloroplasts are clearly visible at the cell periphery. Martin von Rüden, own work, Creative Commons Attribution 4.0 International license. Photosynthesis - Chloroplasts • Chloroplasts are found in palisade mesophyll cells and spongy mesophyll cells. • Thylakoids are the only part of a plant that is green. inner membrane (no porins, specific transport proteins) outer membrane (has porins) thylakoids (green) chloroplast envelope (double membrane; no pigments) thylakoid lumen (inside of thylakoid) stroma (location of the Calvin cycle) Image from Biodidac Photosynthesis - Chloroplasts 1. outer membrane • Thylakoids enclose a space (= the lumen, or 2. intermembrane space 3. inner membrane (1+2+3: envelope) the thylakoid space). 4. stroma (aqueous fluid) • Some of the thylakoids are arranged in stacks. 5. thylakoid lumen (inside of thylakoid) • One stack = a granum (singular); 6. thylakoid membrane Plural = grana. 7. granum (stack of thylakoids) 8. thylakoid (lamella) 9. starch (not actually purple) 10. ribosome 11. plastidial DNA Don’t worry about this stuff. 12. plastoglobule (drop of lipids) By: SuperManu, Creative Commons Attribution-Share Alike 3.0 Photosynthesis - Chloroplasts Modified from Chow (2003) Journal of Biological Physics 29: 447-459 starch grains chloroplast envelope (double membrane; no pigments) thylakoid lumen (= the space within the thylakoids) • • • • • Grana are simply stacked thylakoids. Some thylakoids run between grana. Thylakoids are the exclusive location of chlorophyll (green photosynthetic pigment). Thylakoids are the only part of a plant that is green. The stroma is a water-based gel-like fluid that is the location of the Calvin cycle and (metabolic pathway that converts CO2 into carbohydrate) ad other reactions. • Starch is a polysaccharide (and insoluble glucose polymer) and a product of photosynthesis. Stored in the chloroplast as starch grains. Digression – What is Light? • “Light” is part of the electromagnetic spectrum. • The electromagnetic spectrum includes visible light, radio waves, micro waves, X-rays, and others. • The different parts of the electromagnetic spectrum are characterized by having different frequencies or wavelengths. • Wavelength = 1/frequency. Visible light is just a tiny slice of the You will not be asked to describe the electromagnetic spectrum OpenStax Physics electromagnetic spectrum. Digression – What is Light? • Light can be characterized by its wavelength (usually given the symbol “λ” = lower case lambda, a Greek letter). • λ is the distance to complete one wave/cycle. • The wavelengths of visible light are typically expressed in nanometres (nm). • 1 nm = 1 x 10-9 m. By Dicklyon (Richard F. Lyon) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=7184592 • Wavelength = 1/frequency. Digression – What is Light? Another view of the electromagnetic spectrum Increasing wavelength Increasing frequency https://hubblesite.org/contents/articles/theelectromagnetic-spectrum Wavelength = 1/frequency You do not need to know this formula Photosynthesis - Light Photosynthesis is Light-Dependent • Photosynthetically active radiation (PAR) is the wavelengths of light that power photosynthesis. • PAR is usually defined as 400 nm – 700 nm (an approximation; not a sharp cut-off). • 1 nm = 1 x 10-9 m. • PAR is quite similar to the human visible spectrum; -there are various definitions of the human visible spectrum -one definition of the human visible spectrum is 380 nm – 720 nm (there are other definitions) One definition/view of the human visible spectrum (380 – 720 nm). Plants and humans use similar portions of the electromagnetic spectrum. PAR is defined as 400 – 700 nm, which is very similar to the human visible spectrum. Modified from: Bhutajata, own work. Creative Commons Attribution-Share Alike 4.0 International license. Photosynthesis - Light Light has Both Wave-Like and Particle-Like Properties • PAR is based on wave-like properties. • Particles of light are photons or quanta. https://www.sciencecalculators.org/optics/light/ • At the level of the chloroplast, photosynthesis operates on a per photon basis, as long as the photons are within the PAR wavelengths. • In photosynthesis, every photon that is absorbed by the photosynthetic pigments causes the same amount of photosynthesis. Photosynthesis – Digression about Absorbance Spectra • “White light” is composed of various wavelengths of light. • A prism can be used to separate the various wavelengths. http://hyperphysics.phyastr.gsu.edu/hbase/hframe.html You do not need to know about prisms for a test. • Why is this apple red? • If you shine white light on it, the red wavelengths are not absorbed (other wavelengths are absorbed). • The red wavelengths are reflected and can reach our eyes. • The other wavelengths are absorbed, and don’t reach out eyes. • This apple is red because red light is not absorbed by the apple. https://www.applesfromny.com/varieties/red-delicious/ Photosynthesis – Digression about Absorbance Spectra • Chlorophyll does not absorb green light very well, thus it appears are green to our eyes. By Nefronus - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=75964559 By Yikrazuul - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=7974375 Photosynthesis - Pigments Don’t worry about knowing the structures. • Photons are absorbed by photosynthetic pigments. • Individual pigments have unique absorbance/absorption spectra. OpenStax Biology 2e Chlorophyll a Chlorophyll b β-carotene (a) Chlorophyll a, (b) chlorophyll b, and (c) β-carotene are hydrophobic organic pigments found in the thylakoid membrane. Chlorophyll a and b, which are identical except for the part indicated in the red box, are responsible for the green color of leaves. β-carotene is responsible for the orange colour in carrots, and is also found in chloroplasts. Photosynthesis - Pigments • Chlorophylls (Chl) are the main photosynthetic pigments. • All plants have Chla and Chlb (more Chla). • All plants also have carotenoids, of several types. • Carotenoids tend to be yellow  orange  red) • β-carotene is a carotenoid found in all plants. • The colour of a pigment is determined by the wavelengths of light that are not wellabsorbed by that pigment. • Chls are green because they do not absorb green photons very well. • Leaves are green because: 1) there is a lot more Chl than carotenoid 2) Chls don’t absorb green photons very well 3) human eyes are especially sensitive to green light OpenStax Biology 2e Photosynthesis - Pigments Wimpfheimer (2015) Journal of Laboratory Chemical Education 3(2): 19-21. By domdomegg - Own work, CC BY 4.0, https://commons.wikimedia.org/w/index.php ?curid=46011213 Orange light is at ~590-635 nm The absorbance spectra of pure β-carotene (a carotenoid) and a carrot extract. Carrots are orange because β-carotene doesn’t absorb red, orange or yellow light. Photosynthesis – Summary So Far • • • • Photosynthesis is a reductive process. The source of electrons is water. The water-splitting reaction provides low energy electrons. The energy of light (photons) is used to convert low energy electrons to high energy electrons. • Photosynthetically active radiation (PAR) is defined as 400 – 700 nm; -this is similar to the visible spectrum for humans • Photosynthetic pigments (chlorophylls, carotenoids) absorb photons for use in photosynthesis. • The colour of an object (based on the eyesight of humans) is dependent on what wavelengths of light are not absorbed; e.g. objects that appear as red do not absorb red photons Photosynthesis - Photons https://www.symmetrymagazine.org/article/eight-things-you-might-not-know-about-light An artistic view of particles of light (photons).