Photosynthesis: Using Light to Introduction Chapter 7 Make Food . Plants, algae, and certain prokaryotes – convert light energy to chemical energy and – store the chemical energy in sugar, made from – carbon dioxide and – water. PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko © 2012 Pearson Education, Inc. Figure 7.0_1 Introduction Chapter 7: Big Ideas . Algae farms can be used to produce – oils for biodiesel or The Light Reactions: – carbohydrates to generate ethanol. An Overview of Converting Solar Energy to Photosynthesis Chemical Energy The Calvin Cycle: Photosynthesis Reviewed Reducing CO2 to Sugar and Extended © 2012 Pearson Education, Inc. Figure 7.0_2 AN OVERVIEW OF PHOTOSYNTHESIS © 2012 Pearson Education, Inc. 1 7.1 Autotrophs are the producers of the biosphere 7.1 Autotrophs are the producers of the biosphere . Autotrophs . Photoautotrophs use the energy of light to produce organic molecules. – make their own food through the process of photosynthesis, . Chemoautotrophs are prokaryotes that use – sustain themselves, and inorganic chemicals as their energy source. – do not usually consume organic molecules derived from . Heterotrophs are consumers that feed on other organisms. – plants or – animals, or – decompose organic material. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 7.1 Autotrophs are the producers of the biosphere Figure 7.1A-D . Photosynthesis in plants – takes place in chloroplasts, – converts carbon dioxide and water into organic molecules, and – releases oxygen. © 2012 Pearson Education, Inc. Figure 7.1A Figure 7.1B 2 Figure 7.1C Figure 7.1D 7.2 Photosynthesis occurs in chloroplasts in plant 7.2 Photosynthesis occurs in chloroplasts in plant cells cells . Chloroplasts are the major sites of photosynthesis . Chloroplasts are concentrated in the cells of the in green plants. mesophyll, the green tissue in the interior of the leaf. Chlorophyll . Stomata are tiny pores in the leaf that allow – is an important light-absorbing pigment in chloroplasts, – carbon dioxide to enter and – is responsible for the green color of plants, and – oxygen to exit. – plays a central role in converting solar energy to chemical energy. Veins in the leaf deliver water absorbed by roots. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 7.2 Figure 7.2_1 Leaf Leaf Cross Section Mesophyll Vein Leaf Cross Leaf Section CO2 Mesophyll O2 Vein Stoma Mesophyll Cell Mesophyll Cell Chloroplast Inner and outer membranes Granum Thylakoid Thylakoid space Stroma CO 2 O2 Stoma Chloroplast 3 7.2 Photosynthesis occurs in chloroplasts in plant 7.2 Photosynthesis occurs in chloroplasts in plant cells cells . Chloroplasts consist of an envelope of two . Thylakoids membranes, which – are often concentrated in stacks called grana and – enclose an inner compartment filled with a thick fluid – have an internal compartment called the thylakoid space, called stroma and which has functions analogous to the intermembrane space of a mitochondrion. – contain a system of interconnected membranous sacs called thylakoids. – 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. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 7.2_2 Figure 7.2_3 Chloroplast Mesophyll Cell Inner and outer membranes Granum Thylakoid Chloroplast Thylakoid space Stroma Figure 7.2_4 7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes . Scientists have known since the 1800s that plants Granum produce O2. But does this oxygen come from carbon dioxide or water? Stroma – For many years, it was assumed that oxygen was extracted from CO2 taken into the plant. – However, later research using a heavy isotope of oxygen, 18O, confirmed that oxygen produced by photosynthesis comes from H2O. © 2012 Pearson Education, Inc. 4 Figure 7.3A 7.3 SCIENTIFIC DISCOVERY: Scientists traced the process of photosynthesis using isotopes . Experiment 1: 6 CO2 12 H2O → C6H12O6 6 H2O 6 O2 . Experiment 2: 6 CO2 12 H2O → C6H12O6 6 H2O 6 O2 © 2012 Pearson Education, Inc. Figure 7.3B 7.4 Photosynthesis is a redox process, as is cellular respiration . Photosynthesis, like respiration, is a redox (oxidation-reduction) process. Reactants: – CO2 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. Products: © 2012 Pearson Education, Inc. Figure 7.4A 7.4 Photosynthesis is a redox process, as is cellular respiration . Cellular respiration uses redox reactions to harvest the chemical energy stored in a glucose molecule. Becomes reduced – This is accomplished by oxidizing the sugar and reducing O2 to H2O. – The electrons lose potential as they travel down the electron transport chain to O . Becomes oxidized 2 – In contrast, the food-producing redox reactions of photosynthesis require energy. © 2012 Pearson Education, Inc. 5 7.4 Photosynthesis is a redox process, as is Figure 7.4B cellular respiration . In photosynthesis, – light energy is captured by chlorophyll molecules to boost the energy of electrons, Becomes oxidized – light energy is converted to chemical energy, and – chemical energy is stored in the chemical bonds of sugars. Becomes reduced © 2012 Pearson Education, Inc. 7.5 Overview: The two stages of photosynthesis 7.5 Overview: The two stages of photosynthesis are linked by ATP and NADPH are linked by ATP and NADPH . Photosynthesis occurs in two metabolic stages. 2. The second stage is the Calvin cycle, which occurs in the stroma of the chloroplast. 1. The light reactions occur in the thylakoid membranes. In these reactions – The Calvin cycle is a cyclic series of reactions that assembles sugar molecules using CO2 and the energy-rich – water is split, providing a source of electrons and giving off products of the light reactions. oxygen as a by-product, – During the Calvin cycle, CO2 is incorporated into organic – ATP is generated from ADP and a phosphate group, and compounds in a process called carbon fixation. – light energy is absorbed by the chlorophyll molecules to drive – After carbon fixation, enzymes of the cycle make sugars by the transfer of electrons and H+ from water to the electron further reducing the carbon compounds. acceptor NADP+ reducing it to NADPH. – The Calvin cycle is often called the dark reactions or light- – NADPH produced by the light reactions provides the independent reactions, because none of the steps requires electrons for reducing carbon in the Calvin cycle. light directly. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 7.5_s1 Figure 7.5_s2 H2O H2O Light Light NADP+ NADP+ ADP ADP P P Light Light Reactions Reactions (in thylakoids) (in thylakoids) ATP NADPH Chloroplast Chloroplast O2 6 Figure 7.5_s3 H2O CO2 Light NADP+ THE LIGHT REACTIONS: ADP P CONVERTING SOLAR ENERGY Calvin TO CHEMICAL ENERGY Light Cycle Reactions (in stroma) (in thylakoids) ATP NADPH Chloroplast O2 Sugar © 2012 Pearson Education, Inc. 7.6 Visible radiation absorbed by pigments 7.6 Visible radiation absorbed by pigments drives the light reactions drives the light reactions . Sunlight contains energy called electromagnetic . Light behaves as discrete packets of energy called energy or electromagnetic radiation. photons. – Visible light is only a small part of the electromagnetic – A photon is a fixed quantity of light energy. spectrum, the full range of electromagnetic – The shorter the wavelength, the greater the energy. wavelengths. – Electromagnetic energy travels in waves, and the wavelength is the distance between the crests of two adjacent waves. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 7.6A 7.6 Visible radiation absorbed by pigments Increasing energy drives the light reactions 105 nm 103 nm 1 nm 103 nm 106 nm 1 m 103 m . Pigments Gamma Micro- Radio X-rays UV Infrared rays waves waves – absorb light and – are built into the thylakoid membrane. Plant pigments Visible light – absorb some wavelengths of light and – reflect or transmit other wavelengths. 380 400 500 600 700 750 . We see the color of the wavelengths that are Wavelength (nm) transmitted. For example, chlorophyll transmits 650 green wavelengths. nm Animation: Light and Pigments © 2012 Pearson Education, Inc. 7 Figure 7.6B Figure 7.6B_1 Light Reflected light Chloroplast Absorbed light Thylakoid Transmitted light 7.6 Visible radiation absorbed by pigments 7.7 Photosystems capture solar energy drives the light reactions . Chloroplasts contain several different pigments, . Pigments in chloroplasts absorb photons (capturing which absorb light of different wavelengths. solar power), which – Chlorophyll a absorbs blue-violet and red light and – increases the potential energy of the pigment’s reflects green. electrons and – Chlorophyll b absorbs blue and orange and reflects yellow-green. – sends the electrons into an unstable state. – Carotenoids – These unstable electrons – broaden the spectrum of colors that can drive photosynthesis and – drop back down to their “ground state,” and as they do, – provide photoprotection, absorbing and dissipating excessive – release their excess energy as heat. light energy that would otherwise damage chlorophyll or interact with oxygen to form reactive oxidative molecules. © 2012 Pearson Education,