Photosynthesis: Using Light to Make Food

Transcription

1 Chapter 7 Photosynthesis: Using Light to Make Food BIOL 1408 Dr. Chris Doumen Introduction Understanding of the finer details of Photosynthesis is globally important. Fossil fuels come from plants and the new fuel resources will come from plants derived resources. (forest plantation, algae cultures, sun energy panels, ) 1

2 Future of bio-fuels Algae-Based Jet Fuel Research Gets $25 Million Boost Car completes cross-country trip on algae fuel Future of bio-fuels Joule's bioreactors, mimicking solar panels, host photosynthetic bacteria engineered to secrete diesel-like fuel components 2

3 A little Photosynthesis History Aspects of plant science only became know in the last of hunderd years. The early Greeks thought plants eat soil In the 1600 s (1648) Jan Baptist van Helmont devised a simple experiment that showed how wrong the Greeks were He concluded that plants do not grow by consuming soil, but consuming water. (not really true either) A little Photosynthesis History In 1771, Joseph Priestley finds that air which has been made "noxious" by the breathing of animals or burning of a candle can be restored (i.e., made to support breathing or combustion again) by the presence of a green plant. 3

4 A little Photosynthesis History 1774 : Antoine Lavoisier begins to investigate and later names oxygen. He recognizes that it is consumed in both animal respiration and combustion. His work discredits the theory of "phlogiston," a hypothetical substance then believed to be emitted during respiration or combustion, and lays the foundations of modern chemistry. 1779: Jan Ingenhousz discovers that only the green parts of plants release oxygen and that this occurs only when they are illuminated by sunlight. 1782: Jean Senebier demonstrates that green plants take in carbon dioxide from the air and emit oxygen under the influence of sunlight. A little Photosynthesis History 1791: Comparetti observes green granules in plant tissues, later identified as chloroplasts. 1804: Nicolas de Saussure shows that the carbon assimilated from atmospheric carbon dioxide cannot fully account for the increase in dry weight of a plant. He hypothesized that the additional weight was derived from water. At this point, therefore, the basic equation of photosynthesis was established. It was understood as a process in which a green plant illuminated by sunlight takes in carbon dioxide and water and converts them into organic material and oxygen. 4

5 PhotoSynthesis Photosynthesis is thus the process where light energy is converted to chemical energy the chemical energy is made from carbon dioxide and water. that is turned into sugars, where the sun-energy is now stored in the covalent bonds between the carbon molecules (the chemical form). 6 CO H 2 O Light energy C 6 H 12 O O 2 Carbon dioxide Water PHOTOSYNTHESIS Glucose Oxygen gas Introduction Definitions Autotrophs make their own food through the process of photosynthesis, sustain themselves, and do not usually consume organic molecules derived from other organisms. In other words, autotrophs produce their own food and sustain themselves without eating other organisms Autotrophs are the producers of the biosphere 5

6 Definitions There are two kinds of autotrophs Chemoautotrophs are prokaryotes (bacteria and such) that use inorganic chemicals as their energy source. Photoautotrophs use the energy of light to produce organic molecules. Plants, algae, and some bacteria are photo-autotrophs Introduction The importance of Photo-autotrophs is that they are the Producers of food consumed by virtually all organisms! 6

7 Definitions Organisms that rely on the food sources produced by autotrophs are called heterotrophs A heterotroph is a consumer that feeds on plants (herbivores) or Animals (carnivore), or decompose organic material (decomposers) Food chains Autotroph Heterotroph Food chains in terms of autotrophs and heterotrophs (herbivores/carnivores) 7

8 Photosynthesis Plants execute Photosynthesis in the cellular organelles called chloroplasts! In plants : Chloroplasts are almost always located in the leaves of a plant Thus Photosynthesis occurs primarily in the leaves The chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf. Photosynthesis The exterior leaf cells have specialized cells that form stomata Stomata are tiny pores in the leaf that allow carbon dioxide to enter and oxygen to exit. The leaves also contain veins that deliver water absorbed by roots. 8

9 Introduction Photosynthesis occurs in chloroplasts Leaf Cross Section Leaf Mesophyll Vein Mesophyll Cell CO 2 O 2 Stoma Chloroplast Chloroplasts Chloroplasts consist of an envelope of two phospholipid membranes The inner membrane encloses an inner compartment filled with a thick fluid called stroma and In this stroma is a system of interconnected membranous sacs called thylakoids. Thylakoids are stacked like pancakes into a structure called a granum (plural = grana). 9

10 Figure 7.2_2 Chloroplast Inner and outer membranes Granum Thylakoid Thylakoid space Stroma Chloroplasts Thylakoids have an internal compartment called the thylakoid space, which has functions analogous to the intermembrane space of a mitochondrion. Thylakoid membranes also house much of the machinery that converts light energy to chemical energy. Chlorophyll molecules are built into the thylakoid membrane and capture light energy. are at the basis of the green color of leaves 10

11 Photosynthesis Reactions Scientists thus eventually determined the chemical equation for Photosynthesis shown below (compare with respiration) 6 CO H 2 O Light energy C 6 H 12 O O 2 Carbon dioxide Water PHOTOSYNTHESIS Glucose Oxygen gas But does this oxygen come from carbon dioxide or water? For many years, it was assumed that oxygen was extracted from CO 2 taken into the plant. Photosynthesis Reactions However, later research using a heavy isotope of oxygen, 18 O, confirmed that oxygen produced by photosynthesis comes from H 2 O. Plants thus produce O 2 gas by splitting water ; the O 2 liberated by photosynthesis is made from the oxygen atom in water (H 2 O) 11

12 Photosynthesis Reactions Although we write the reaction of photosynthesis as follows 6 CO H 2 O + Light Energy C 6 H 12 O O 2 The actual reaction of photosynthesis is really 6 CO H 2 O + Light Energy C 6 H 12 O H 2 O + 6 O 2 Photosynthesis Reactions Isotopes can be very instrumental in un-raveling metabolic reactions. The experiment with oxygen isotope, 18 O, was constructed as follows. In experiment 1, the labeled oxygen isotope was only present in carbon dioxide ( labelled with *) In experiment 1, the labeled oxygen isotope was only present in water ( labelled with *) * * * Exp.1: 6 CO H 2 O + Light C 6 H 12 O H 2 O + 6 O 2 * * Exp.2: 6 CO H 2 O + Light C 6 H 12 O H 2 O + 6 O 2 By analyzing the labeled oxygen in the products, one can figure out details of the overall process 12

13 Photosynthesis Reactions * * * Exp.1: 6 CO H 2 O + Light C 6 H 12 O H 2 O + 6 O 2 * * Exp.2: 6 CO H 2 O + Light C 6 H 12 O H 2 O + 6 O 2 In experiment 1, no oxygen became labeled but glucose and water did. In experiment 2 only the released oxygen was labeled Photosynthesis Reactions Reactants: Products: Thus, all the carbons of carbon dioxide end up in glucose and the oxygens from carbon dioxide end up in glucose andwater. All the hydrogens in glucose and water come from water AND the released oxygen comes from the splitting of water. 13

14 Photosynthesis Reactions Photosynthesis, like respiration, is a redox (oxidationreduction) process. In this case CO 2 becomes reduced to sugar as electrons along with hydrogen ions from water are added to it. Water molecules are oxidized when they lose electrons along with hydrogen ions. Becomes reduced Becomes oxidized Photosynthesis vs Respiration The following once again compares the basic redox reaction that occur in plants during photosynthesis and what happens when mitochondria containing cells use these sugars to generate energy. Photosynthesis: the reduction requires energy (= light energy) ( in chloroplasts) Reduction 6 CO H 2 O C 6 H 12 O 6 + Oxidation 6 O 2 Cellular respiration: the oxidation releases energy, captured in ATP ( in mitochondria) Oxidation C 6 H 12 O O 2 6 CO H 2 O Reduction 14