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Chapter 7 Capturing Solar Energy: Photosynthesis Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. What Is Photosynthesis? For most organisms, energy is derived from sunlight, either directly or indirectly Those organisms that can directly trap sunlight do so by photosynthesis Photosynthesis is the process by which solar energy is trapped and stored as chemical energy in the bonds of a sugar – In water – protists and certain bacteria – On land – plants Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. An Overview of Photosynthetic Structures cuticle upper epidermis mesophyll cells (a) Leaves stoma outer membrane inner membrane thylakoid stroma (d) Chloroplast Biology: Life on Earth, 9e lower epidermis stoma chloroplasts bundle sheath cells vascular bundle (vein) (b) Internal leaf structure Fig. 7-1 channel interconnecting thylakoids Copyright © 2011 Pearson Education Inc. (c) Mesophyll cell containing chloroplasts What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis in plants – Leaves are flat and thin for best light penetration – Takes place in chloroplasts contained within leaf cells – Both the upper and lower surfaces of a leaf consist of a layer of transparent cells, the epidermis Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis in plants – The outer surface of both epidermal layers is covered by the cuticle, a transparent, waxy, waterproof covering that reduces the evaporation of water from the leaf – Leaves obtain CO2 for photosynthesis from the air through pores in the epidermis called stomata (singular, stoma) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis – Inside the leaf are layers of cells called the mesophyll, where the chloroplasts are located and where photosynthesis occurs – Bundle sheath cells surround the vascular bundles, which form veins in the leaf and supply water and minerals to the mesophyll Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. What Is Photosynthesis? Leaves and chloroplasts are adaptations for photosynthesis – Chloroplasts are organelles with a double membrane enclosing a fluid called the stroma – Embedded in the stroma are disk-shaped membranous sacs called thylakoids – Reactions that depend on light take place in the thylakoids – Reactions of the Calvin cycle that capture carbon dioxide and produce sugar occur in the stroma Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. What Is Photosynthesis? Photosynthesis consists of the light reactions and the Calvin cycle – Starting with carbon dioxide (CO2) and water (H2O), photosynthesis converts sunlight energy into chemical energy stored in bonds of glucose and releases oxygen (O2) as a by-product 6 CO2 + 6 H2O + light energy C6H12O6 + 6 O2 carbon water sunlight glucose oxygen dioxide (sugar) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. An Overview of the Relationship Between the Light Reactions and the Calvin Cycle Fig. 7-3 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. What Is Photosynthesis? Photosynthesis consists of the light reactions and the Calvin cycle – In the light reactions, chlorophyll captures light energy and converts some into energy-carrier molecules ATP and NADPH. Water is split releasing O2 – In the reactions of the Calvin cycle, enzymes in the stroma use CO2 from the air and chemical energy from the energy-carrier molecules to synthesize a threecarbon sugar that will be used to make glucose Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Light Reactions: Light Energy Converted to Chemical Energy Light is captured by pigments in chloroplasts – The sun emits energy within a broad spectrum of electromagnetic radiation – This electromagnetic spectrum ranges from shortwavelength gamma rays, through ultraviolet, visible, and infrared light, to long-wavelength radio waves Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Light Reactions: Light Energy Converted to Chemical Energy Light is captured by pigments in chloroplasts – Light is composed of individual packets of energy called photons – Visible light has wavelengths with energies strong enough to alter biological pigment molecules such as chlorophyll a – Chlorophyll a is a key light-capturing pigment molecule in chloroplasts, absorbing violet, blue, and red light – Green light is reflected, which is why leaves appear green Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Light Reactions: Light Energy Converted to Chemical Energy Light is captured by pigments in chloroplasts – Accessory pigments, that absorb additional wavelengths of light energy and transfer them to chlorophyll a – Chlorophyll b - absorbs blue and red-orange light, and appear yellow-green – Carotenoids - absorb blue and green light, and appear yellow or orange – In autumn, more-abundant, green chlorophyll breaks down before the carotenoids do, revealing their yellow color, which in summer is masked Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Light Reactions: Light Energy Converted to Chemical Energy The light reactions occur in association with the thylakoid membranes – Contain many photosystems – each consists of a cluster of chlorophyll and accessory pigment molecules surrounded by various proteins – These electron transport chains (ETC) each consist of a series of electron carrier molecules embedded in the thylakoid membrane Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Light Reactions: Light Energy Converted to Chemical Energy The hydrogen ion gradient generates ATP by chemiosmosis – The energy of electron movement through the thylakoid membrane creates an H+ gradient that drives ATP synthesis in a process called chemiosmosis – The generation of ATP ADP + phosphate resembles the electrical energy obtained from water flowing downhill and driving an electrical turbine Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. Energy Stored in a Water “Gradient” Can Be Used to Generate Electricity 1 Energy is released as water flows downhill 2 Energy is harnessed to rotate a turbine 3 The energy of the rotating turbine is used to generate electricity Fig. 7-8 Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. thylakoid Events of the Light Reactions chloroplast Fig. 7-7 (stroma) light energy 1 H+ are pumped to the thylakoid space H+ electron transport NADP+ chain I H+ electron transport chain II e– e– e– photosystem I 2 H+ 1/ 2 O2 H+ (thylakoid space) Biology: Life on Earth, 9e sugar C6H12O6 ADP + P H+ H+ NADPH ATP synthase e– photosystem II H+ Calvin cycle e– e– H2O CO2 H+ ATP H+ H+ 2 High concentration is created H+ H+ thylakoid membrane 3 Flow of H+ down their concentration gradient powers ATP synthesis Copyright © 2011 Pearson Education Inc. Review 1. Why is photosynthesis important? 2. What is the basic equation for photosynthesis? 3. What is the main light-capturing molecule in chloroplasts? 4. What are the two main end products from the light reactions? 5. How and where are they created? Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Calvin Cycle: Chemical Energy Stored in Sugar Molecules The Calvin cycle captures carbon dioxide – ATP and NADPH synthesized from light reactions are used to power the synthesis of a simple sugar (gyceraldehyde-3-phosphate, or G3P) – A series of reactions occurring in the stroma – In reactions that occur outside the Calvin cycle, two G3P molecules can be combined to form one six-carbon glucose molecule – Glucose may then be converted to the disaccharide sucrose or linked to form starch (a storage molecule) or cellulose (a major component of plant cell walls) Biology: Life on Earth, 9e Copyright © 2011 Pearson Education Inc. The Calvin Cycle Fixes Carbon from CO2 and Produces G3P 3 C CO2 H2O ATP light reactions NADPH Calvin cycle Calvin cycle ADP NADP sugar 3 ATP 6 ADP 6 NADPH ATP 6 NADP+ C6H12O6 5 C C C G3P 3 Using the energy from ATP, five of the six molecules of G3P are converted to three molecules of RuBP 6 C C C G3P 1 C C C G3P 2 Energy from ATP and NADPH is used to convert the six molecules of PGA to six molecules of G3P 4 One molecule of G3P leaves the cycle 1 C C C G3P Biology: Life on Earth, 9e 6 ADP 3 O2 6 C C C PGA 3 C C C C C RuBP CO2 Fig. 7-9 1 Carbon fixation combines three CO2 with three RuBP using the enzyme rubisco + 1 C C C G3P 1 C C C C C C glucose 5 Two molecules of G3P combine to Copyright © 2011 Pearson Education Inc. form glucose and other molecules