Chapter 7

1. Chapter 7 • Capturing Solar Energy: Photosynthesis

2. 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

3. An Overview of Photosynthetic Structures cuticle upper epidermis mesophyll cells (a) Leaves lower epidermis stoma chloroplasts stoma bundle sheath cells outer membrane vascular bundle (vein) inner membrane (b) Internal leaf structure thylakoid stroma channel interconnecting thylakoids (d ) Chloroplast Fig. 7-1 (c) Mesophyll cell containing chloroplasts

4. 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

5. 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)

6. 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

7. 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

8. 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)

9. An Overview of the Relationship Between the Light Reactions and the Calvin Cycle Fig. 7-3

10. 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 three-carbon sugar that will be used to make glucose

11. 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 short-wavelength gamma rays, through ultraviolet, visible, and infrared light, to long-wavelength radio waves

12. 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

13. 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

14. 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

15. 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

16. 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

17. Events of the Light Reactions thylakoid Fig. 7-7 chloroplast (stroma) CO2 light energy H+ are pumped to the thylakoid space 1 H+ electron transport chain I Calvin cycle electron transport chain II NADP+  NADPH e– H+ sugar e– e– e– ATP synthase C6H12O6 e– ADP photosystem II + e– P photosystem I H+ H+ H+ ATP H+ 2 H+ H2O H+ 1/2 O2 H+ H+ H+ Flow of H+ down their concentration gradient powers ATP synthesis 3 High H+ concentration is created 2 thylakoid membrane (thylakoid space)

18. Review • Why is photosynthesis important? • What is the basic equation for photosynthesis? • What is the main light-capturing molecule in chloroplasts? • What are the two main end products from the light reactions? • How and where are they created?

19. 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)

20. The Calvin Cycle Fixes Carbon from CO2 and Produces G3P Fig. 7-9 Carbon fixation combines three CO2 with three RuBP using the enzyme rubisco 1 C 3 CO2 H2O CO2 C C C C C C C C 6 3 ATP PGA RuBP Calvin cycle light reactions NADPH ADP ATP 6 Calvin cycle NADP 6 ADP sugar 3 ADP 6 NADPH ATP 3 O2 C6H12O6 NADP+ 6 C C C 5 C C C 6 G3P Energy from ATP and NADPH is used to convert the six molecules of PGA to six molecules of G3P 2 G3P Using the energy from ATP, five of the six molecules of G3P are converted to three molecules of RuBP 3 C C C 1 G3P 4 One molecule of G3P leaves the cycle 4 C C C C C C C C C C C C 1 + 1 1 G3P G3P glucose Two molecules of G3P combine to form glucose and other molecules 5