Photosynthesis: Using Light to Make Food Chapter 7 Photosynthesis: Using Light to Make Food

The Light Reactions: Converting Solar Energy to Chemical Energy Figure 7.0_1 Chapter 7: Big Ideas The Light Reactions: Converting Solar Energy to Chemical Energy An Overview of Photosynthesis Figure 7.0_1 Chapter 7: Big Ideas The Calvin Cycle: Reducing CO2 to Sugar Photosynthesis Reviewed and Extended 2

Can these organisms hold the key to our future energy needs? Figure 7.0_2 Single-celled algae 3

AN OVERVIEW OF PHOTOSYNTHESIS © 2012 Pearson Education, Inc. 4

All Living Organisms Need Energy and Nutrients Autotrophs = self sufficient; able to convert inorganic nutrients to organic molecules Autotrophs are primary producers of all ecosystems!! Photoautotrophs use the energy of light to produce organic molecules from CO2 and water. Plants, algae (type of protist), some prokaryotes Convert light energy to chemical energy Chemoautotrophs are prokaryotes that use inorganic chemicals as their energy source. Heterotrophs are consumers that feed on organic compounds Animals, fungi, some prokaryotes and protists © 2012 Pearson Education, Inc. 5

Figure 7.1A-D Figure 7.1A-D Photoautotroph diversity 6

Photosynthesis occurs in chloroplasts in plant cells Leaf Leaf Cross Section Mesophyll Vein CO2 O2 Stoma Mesophyll Cell Chloroplast Figure 7.2 Zooming in on the location and structure of chloroplasts Inner and outer membranes Granum Thylakoid Thylakoid space Stroma 7

Stomata are tiny pores in the leaf that allow carbon dioxide to enter and oxygen to exit. Chloroplasts are concentrated in the cells of the mesophyll, the green tissue in the interior of the leaf. Leaf Cross Section Leaf Veins in the leaf deliver water absorbed by roots. Mesophyll Vein Mesophyll Cell Figure 7.2_1 Zooming in on the location and structure of chloroplasts (part 1) CO2 O2 Stoma Chloroplast 8

Inner and outer membranes Chloroplast = double membrane Inner and outer membranes Granum = stack of thylakoids Thylakoid Thylakoid space Stroma Figure 7.2_2 Zooming in on the location and structure of chloroplasts (part 2) Stroma = thick fluid surrounding thylakoids = site of Calvin cycle!!! Thylakoids = interconnected system of membranous sacs Thylakoid membranes = site of chlorophyll and light reactions!!! 9

Summary Equation for Photosynthesis Light energy 6 CO2 6 H2O C6H12O6 6 O2 Carbon dioxide Water Oxygen gas Photosynthesis Glucose Water is the source of O2 product 6 CO2  12 H2O → C6H12O6  6 H2O  6 O2 Scientists traced the process of photosynthesis using isotopes Student Misconceptions and Concerns Students may not connect the growth in plant mass to the fixation of carbon during the Calvin cycle. It can be difficult for many students to appreciate that molecules in air can contribute significantly to the mass of plants. Teaching Tips Many students do not realize that glucose is not the direct product of photosynthesis. Although glucose is often shown as a final product of photosynthesis, a three-carbon sugar is directly produced (G3P, as the authors note later in Module 7.10). A plant can use G3P to make many types of organic molecules, including glucose. © 2012 Pearson Education, Inc. 10

7.4 Photosynthesis is an endergonic, redox process Reduced = gain of electrons; oxidized = loss of electrons CO2 becomes reduced to sugar. Water molecules are oxidized to O2 Photosynthesis is endergonic: light energy is captured by chlorophyll molecules to excite electrons light energy is converted to chemical energy, and chemical energy is stored in the chemical bonds of sugars. Becomes reduced Teaching Tips In our world, energy is frequently converted to a usable form in one place and used in another. For example, electricity is generated by power plants, transferred to our homes, and used to run computers, create light, and help us prepare foods. Consider relating this common energy transfer to the two-stage process of photosynthesis. Becomes oxidized 11

Photosynthesis occurs in two stages Light NADP+ ADP P Light Reactions The light reactions occur in the thylakoid membranes. Light energy splits H2O to O2, releasing high energy electrons Movement of electrons used to ATP is generate from ADP and P Electrons end up on NADP+ reducing it to NADPH. (in thylakoids) Figure 7.5_s1 An overview of the two stages of photosynthesis in a chloroplast (step 1) Chloroplast 12

Photosynthesis occurs in two stages Light NADP+ ADP P Light Reactions The light reactions occur in the thylakoid membranes. Light energy splits H2O to O2, releasing high energy electrons Movement of electrons used to ATP is generate from ADP and P Electrons end up on NADP+ reducing it to NADPH. (in thylakoids) ATP Figure 7.5_s2 An overview of the two stages of photosynthesis in a chloroplast (step 2) NADPH Chloroplast O2 13

Calvin cycle occurs in the stroma of the chloroplast. H2O Calvin cycle occurs in the stroma of the chloroplast. CO2 Light Cyclic series of reactions that assembles sugar molecules using CO2 and ATP, NADPH of the light reactions. NADPH provides the electrons for reducing carbon in the Calvin cycle. NADP+ ADP P Calvin Cycle Light Reactions (in stroma) (in thylakoids) ATP Figure 7.5_s3 An overview of the two stages of photosynthesis in a chloroplast (step 3) Carbon fixation = incorporation of C into organic compounds NADPH Chloroplast O2 Sugar 14

EATING THE SUN - THE LIGHT REACTIONS © 2012 Pearson Education, Inc. 15

7.6 Visible radiation drives the light reactions Increasing energy 105 nm 103 nm 1 nm 103 nm 106 nm 1 m 103 m Gamma rays Micro- waves Radio waves X-rays UV Infrared Visible light Student Misconceptions and Concerns 1. The authors note that electromagnetic energy travels through space in waves that are like ripples made by a pebble dropped in a pond. This wave imagery is helpful, but can confuse students when energy is also thought of as discrete packets called photons. The dual nature of light, which exhibits the properties of waves and particles, may need to be discussed further, if students are to do more than just accept definitions. 2. The authors note that sunlight is a type of radiation. Many students think of radiation as a result of radioactive decay, a serious threat to health. The diverse types of radiation and the varying energy associated with each might need to be explained. Teaching Tips Consider bringing a prism to class and demonstrating the spectrum of light. Depending on what you have available, it can be a dramatic reinforcement. 380 400 500 600 700 750 Wavelength (nm) The shorter the wavelength, the greater the energy. 650 nm 16

Animation: Light and Pigments What pigments and wavelengths are responsible for photosynthesis? Pigments = compounds that absorb light Chloroplasts contain 4 major pigments: Chlorophyll a absorbs blue-violet and red light and reflects green. Chlorophyll b absorbs blue and orange and reflects yellow-green. Carotenoids broaden the spectrum of colors that can drive photosynthesis provide photoprotection Animation: Light and Pigments

Light Reflected light Chloroplast Absorbed light Thylakoid Figure 7.6B Light Reflected light Figure 7.6B The interaction of light with a chloroplast Chloroplast Absorbed light Thylakoid Transmitted light 18

What happens when a pigment absorbs light? When pigments absorb photons: potential energy of electrons increases; sending electrons to unstable state As electrons drop back down to ‘ground state’, they release excess energy as heat Excited state Heat Ground state Photon of light Photon (fluorescence) Chlorophyll molecule What if this energy could be harnessed? Student Misconceptions and Concerns Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. Teaching Tips The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. © 2012 Pearson Education, Inc. 19

Photosynthesis in Two Acts Light energy 6 CO2 6 H2O C6H12O6 6 O2 Carbon dioxide Water Oxygen gas Photosynthesis Glucose Act 1 - The Light Reactions GOAL: Transform light energy into chemical energy of ATP and NADPH Harvest electrons from H2O, forming O2 Use these excited electrons to generate ATP Then transfer these electrons to NADP+, forming NAPDH. Light energy H2O + ADP + P + NADP+ O2 + ATP + NADPH

Light Reactions Take Place in the Thylakoids Cast of characters 2 Photosystems: Photosystem II (P680) Reacts with light to oxidize H2O Photosystem I (P700) Reacts with light to transfer electrons to NADP+ Student Misconceptions and Concerns Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. Teaching Tips The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. © 2012 Pearson Education, Inc. 21

7.7 Photosystems capture solar energy Pair of chlorophyll a molecules surrounded by pigments and various enzyme Capture solar energy to excite electrons in chlorophyll a Light Light-harvesting complexes Reaction-center complex Primary electron acceptor Thylakoid membrane Student Misconceptions and Concerns Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. Teaching Tips The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. Pigment molecules Pair of chlorophyll a molecules Transfer of energy 22

7.7 Two Photosystems Cooperate in the Light Reactions Photosystem II (P680) - oxidizes H2O Photosystem I (P700) - reduces NADP+ Electron transport chain Provides energy for synthesis of ATP by chemiosmosis NADP  Light H NADPH Light Photosystem I 6 Photosystem II Stroma 1 Primary acceptor Primary acceptor Student Misconceptions and Concerns Even at the college level, students struggle to understand why we perceive certain colors. The authors discuss the specific absorption and reflection of certain wavelengths of light, noting which colors are absorbed and which are reflected (and thus available for our eyes to detect). Consider spending time to make sure that your students understand how photosynthetic pigments absorb and reflect certain wavelengths. Teaching Tips The authors discuss a phenomenon that most students have noticed: dark surfaces heat up faster in the sun than do lighter-colored surfaces. This is an opportunity to demonstrate to your students the various depths of scientific explanations and help them appreciate their own educational progress. In elementary school, they might have learned that the sun heats darker surfaces faster than lighter surfaces. In high school, they may have learned about light energy and the fact that dark surfaces absorb more of this energy than lighter surfaces. Now, in college, they are learning that at the atomic level, darker surfaces absorb the energy of more photons, exciting more electrons, which then fall back to a lower state, releasing more heat. 2 Thylakoid membrane 4 5 P680 P700 Thylakoid space 3 H2O 2 1 O2 H  2 23

Cast of Characters, Cont’d Electron Transport Chain Complex of proteins that transfer electrons between PSII and PSI And actively transport H+ from stroma into thylakoid space ATP synthase Enzyme responsible for ATP synthesis

7.8 Two photosystems connected by an electron transport chain generate ATP and NADPH Light Stroma Photosystem II Thylakoid space Thylakoid membrane Primary acceptor P680 P700 Photosystem I NADP  NADPH Electron transport chain Provides energy for synthesis of ATP by chemiosmosis 2 1 H2O 3 4 5 6 H O2  Figure 7.8A Electron flow in the light reactions: light energy driving electrons from water to NADPH Between the two photosystems, the electrons move down an electron transport chain and provide energy for the synthesis of ATP. Electrons are removed from water, passed from PS II to PSI and accepted by NADP+, reducing it to NADPH. 25

ATP NADPH Mill makes ATP Photosystem II Photosystem I Photon Photon Figure 7.8B ATP NADPH Mill makes ATP Photon Figure 7.8B A mechanical analogy of the light reactions Photon Photosystem II Photosystem I 26

7.9 Chemiosmosis powers ATP synthesis Chemiosmosis (chemiosmostic theory) Peter Mitchell ATP is generated because the electron transport chain produces a concentration gradient of hydrogen ions (H+) across a membrane In the Light Reactions: Light energy is used to allow ETC to pump H+ into the thylakoid space the resulting concentration gradient drives H+ back through ATP synthase, producing ATP. Photophosphorylation = using light energy to drive H+ pumps and ATP synthesis by chemiosmosis Same general process is used to produce ATP in cellular respiration, expect oxidation of glucose is used to generate H+ gradient Teaching Tips Module 7.9 notes the similarities between oxidative phosphorylation in mitochondria and photophosphorylation in chloroplasts. If your students have not already read or discussed chemiosmosis in mitochondria, consider assigning Modules 6.6 and 6.10 to show the similarities of these processes. (As noted in Module 7.2, the thylakoid space is analogous to the intermembrane space of mitochondria.) © 2012 Pearson Education, Inc. 27

Electron transport chain Figure 7.9 Chloroplast To Calvin Cycle H+ ATP Light Light ADP P Stroma (low H+ concentration) H+ NADP+ H+ NADPH H+ H+ Thylakoid membrane Figure 7.9 The production of ATP by chemiosmosis H+ H+ H+ H+ H2O H+ 1 2 H+ Thylakoid space (high H+ concentration) O2 + 2 H+ H+ H+ H+ H+ H+ Electron transport chain H+ H+ Photosystem II Photosystem I ATP synthase 28

Electron transport chain Figure 7.9_1 To Calvin Cycle H+ ATP Light Light ADP P H+ NADPH NADP+ H+ H+ H+ H+ H+ H+ H+ H2O H+ H+ Figure 7.9_1 The production of ATP by chemiosmosis (partial) 1 2 O2 2 H+ H+ H+ H+ H+ H+ Electron transport chain H+ H+ Photosystem II Photosystem I ATP synthase 30