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Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Chapter 7 Capturing Solar Energy: Photosynthesis
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e An Overview of Photosynthetic Structures cuticle (b) Internal leaf structure upper epidermis mesophyll cells lower epidermis chloroplasts stoma bundle sheath cells vascular bundle (vein) stoma outer membrane inner membrane thylakoid stroma channel interconnecting thylakoids (d) Chloroplast (a) Leaves (c) Mesophyll cell containing chloroplasts Fig. 7-1
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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 CO 2 for photosynthesis from the air through pores in the epidermis called stomata (singular, stoma)
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e What Is Photosynthesis? Photosynthesis consists of the light reactions and the Calvin cycle –Starting with carbon dioxide (CO 2 ) and water (H 2 O), photosynthesis converts sunlight energy into chemical energy stored in bonds of glucose and releases oxygen (O 2 ) as a by-product 6 CO 2 + 6 H 2 O + light energy C 6 H 12 O 6 + 6 O 2 carbon water sunlight glucose oxygen dioxide (sugar)
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e An Overview of the Relationship Between the Light Reactions and the Calvin Cycle Fig. 7-3
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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 O 2 –In the reactions of the Calvin cycle, enzymes in the stroma use CO 2 from the air and chemical energy from the energy-carrier molecules to synthesize a three- carbon sugar that will be used to make glucose
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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 Light Reactions: Light Energy Converted to Chemical Energy
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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 Light Reactions: Light Energy Converted to Chemical Energy
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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 Light Reactions: Light Energy Converted to Chemical Energy
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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 Light Reactions: Light Energy Converted to Chemical Energy
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Energy is released as water flows downhill Energy is harnessed to rotate a turbine The energy of the rotating turbine is used to generate electricity 1 2 3 Energy Stored in a Water “Gradient” Can Be Used to Generate Electricity Fig. 7-8
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e H + are pumped to the thylakoid space ATP synthase photosystem I photosystem II thylakoid membrane light energy + P ATP NADP + ADP NADPH Calvin cycle CO 2 C 6 H 12 O 6 sugar e–e– e–e– e–e– e–e– e–e– e–e– 1/21/2 2 H2OH2O chloroplast electron transport chain II electron transport chain I (stroma) (thylakoid space) thylakoid O2O2 High H + concentration is created Flow of H + down their concentration gradient powers ATP synthesis H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ 1 2 3 Fig. 7-7 Events of the Light Reactions
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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?
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e 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)
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e C 3 CO 2 CCC 6 PGA NADP + ATP ADP NADPH 6 6 6 6 CCC 1 G3P CCC 5 ATP ADP 3 3 CCC 3 RuBP CCC 1 G3P + CCC 1 CCC 1 glucose CCC CCC 6 G3P Calvin cycle Energy from ATP and NADPH is used to convert the six molecules of PGA to six molecules of G3P Carbon fixation combines three CO 2 with three RuBP using the enzyme rubisco Using the energy from ATP, five of the six molecules of G3P are converted to three molecules of RuBP 4 One molecule of G3P leaves the cycle Two molecules of G3P combine to form glucose and other molecules CC 5 3 4 2 1 The Calvin Cycle Fixes Carbon from CO 2 and Produces G3P Fig. 7-9 NADP ATP ADP NADPH Calvin cycle light reactions H2OH2O O2O2 CO 2 C 6 H 12 O 6 sugar
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Why Do Some Plants Use Alternate Pathways for Carbon Fixation? When stomata are closed to conserve water, wasteful photorespiration occurs –When plant stomata are closed in hot environments to prevent water loss, oxygen builds up in the plant cells and RuBP combines with it, rather than CO 2, in a wasteful process called photorespiration –This process prevents the Calvin cycle from synthesizing sugar, and plants may die under these circumstances
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Why Do Some Plants Use Alternate Pathways for Carbon Fixation? CAM plants capture carbon and synthesize sugar at different times –Although they use the C 4 cycle to generate CO 2, CAM plants do not use different cell types to capture carbon and to synthesize sugar –Instead, they perform both these activities in the same mesophyll cells, but at different times –Carbon fixation occurs at night, when the stomata are open and CO 2 is available –Sugar synthesis occurs during the day
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e CO 2 Mesophyll cell PEP carboxylase Calvin cycle sugar CO 2 rubisco central vacuole oxalo- acetate (4C) PEP (3C) pyruvate (3C) malic acid malate (4C) malate (4C) nightday (b) CAM plants The C 4 Pathway and the CAM Pathway Fig. 7-11b
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Why Do Some Plants Use Alternate Pathways for Carbon Fixation? Alternate pathways adapt plants to different environmental conditions –C 4 plants and CAM plants consume more energy than do C 3 plants; they have an advantage when light energy is abundant but water is not –In environments where water is abundant or light levels are low, plants using the C 3 carbon fixation pathway dominate
Copyright © 2011 Pearson Education Inc.Biology: Life on Earth, 9e Light and Chloroplast Pigments Fig. 7-4 gamma rays higher energylower energy X-raysUVinfrared micro waves radio waves wavelength (nanometers) chlorophyll b carotenoids chlorophyll a
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