Where It Starts – Photosynthesis Chapter 7 Part 1 Let’s start with Mr. Anderson….

7.1 Sunlight as an Energy Source  Photosynthetic organisms use pigments to capture the energy of sunlight  Photosynthesis The synthesis of organic molecules from inorganic molecules using the energy of light

Properties of Light  Visible light is part of an electromagnetic spectrum of energy radiating from the sun Travels in waves Organized into photons  Wavelength The distance between the crests of two successive waves of light (nm)

The Rainbow Catchers  Pigment = An organic molecule that selectively absorbs light of specific wavelengths  Chlorophyll a The most common photosynthetic pigment Absorbs violet and red light (appears green)

Photosynthetic Pigments  Collectively, chlorophyll and accessory pigments absorb most wavelengths of visible light  Certain electrons in pigment molecules absorb photons of light energy, boosting electrons to a higher energy level  Energy is captured and used for photosynthesis

Fig. 7-4c, p. 110

Fig. 7-5a, p. 111

Summary: Photosynthesis

7.3 Overview of Photosynthesis  Chloroplast = An organelle that specializes in photosynthesis in plants and many protists  Stroma = A semifluid matrix surrounded by the two outer membranes of the chloroplast Sugars are built in the stroma

Fig. 7-5b, p. 111

Overview of Photosynthesis  Thylakoid membrane Folded membrane that make up thylakoids Contains clusters of light-harvesting pigments that absorb photons of different energies  Photosystems (type I and type II) Groups of molecules that work as a unit to begin the reactions of photosynthesis Convert light energy into chemical energy

Overview of Photosynthesis  Light-dependent reactions Light energy is transferred to ATP and NADPH Water molecules are split, releasing O 2  Light-independent reactions Energy in ATP and NADPH drives synthesis of glucose and other carbohydrates from CO 2 and water

Fig. 7-5c, p. 111

7.4 Light-Dependent Reactions  In the first stage of photosynthesis, light energy drives electrons out of photosystems  The electrons may be used in a noncyclic or cyclic pathway of ATP formation

Capturing Energy for Photosynthesis  Photons boost electrons in pigments to higher energy levels  Light-harvesting complexes absorb the energy  Electrons are released from special pairs of chlorophyll a molecules in photosystems

The Thylakoid Membrane

Cyclic and Noncyclic Pathways  Electrons from photosystems take noncyclic or cyclic pathways, forming ATP

Noncyclic Pathway of Photosynthesis

Electron Flow in a Noncyclic Pathway  Electrons lost from a photosystem enter an electron transfer chain in the thylakoid membrane  Electron transfer chains Organized arrays of enzymes, coenzymes, and other proteins that accept and donate electrons in a series

Energy Flow in Light-Dependent Reactions

7.6 Light-Independent Reactions: The Sugar Factory  The cyclic, light-independent reactions of the Calvin-Benson cycle are the “synthesis” part of photosynthesis  Calvin-Benson cycle Enzyme-mediated reactions that build sugars in the stroma of chloroplasts

Carbon Fixation  Carbon fixation Extraction of carbon atoms from inorganic sources (atmosphere) and incorporating them into an organic molecule Builds glucose from CO 2 Uses bond energy of molecules formed in light- dependent reactions (ATP, NADPH)

Inputs and Outputs of the Calvin-Benson Cycle

The Calvin-Benson Cycle

Different Food Sources  Autotrophs Organisms that make their own food using energy from the environment and inorganic carbon  Heterotrophs Organisms that get energy and carbon from organic molecules assembled by other organisms

Two Kinds of Autotrophs  Chemoautotrophs Extract energy and carbon from simple molecules in the environment (hydrogen sulfide, methane) Used before the atmosphere contained oxygen  Photoautotrophs Use photosynthesis to make food from CO 2 and water, releasing O 2 Allowed oxygen to accumulate in the atmosphere

7.7 Adaptations: Different Carbon-Fixing Pathways  Environments differ, and so do details of photosynthesis C3 plants C4 plants CAM plants

C3 Plants  C3 plants Plants that use only the Calvin–Benson cycle to fix carbon Forms 3-carbon PGA in mesophyll cells Used by most plants, but inefficient in dry weather when stomata are closed

Photorespiration  When stomata are closed, CO 2 needed for light- independent reactions can’t enter, O 2 produced by light-dependent reactions can’t leave  Photorespiration At high O 2 levels, rubisco attaches to oxygen instead of carbon CO 2 is produced rather than fixed

C4 Plants  C4 plants Plants that have an additional set of reactions for sugar production on dry days when stomata are closed; compensates for inefficiency of rubisco Forms 4-carbon oxaloacetate in mesophyll cells, then bundle-sheath cells make sugar Examples: Corn, switchgrass, bamboo

Fig. 7-12a, p. 116

Fig. 7-12b, p. 116

CAM Plants  CAM plants (Crassulacean Acid Metabolism) Plants with an alternative carbon-fixing pathway that allows them to conserve water in climates where days are hot Forms 4-carbon oxaloacetate at night, which is later broken down to CO 2 for sugar production Example: succulents, cactuses

A CAM Plant  Jade plant (Crassula argentea)

C3, C4, and CAM Reactions