8.1 Photosynthesis and Energy

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1 BIOL 100 Ch Photosynthesis and Energy Photosynthesis and Energy Photosynthesis Making food from light energy Photoautotrophs Use CO2 and water to make sugars Made life possible as we know it Provides carbohydrates (cellulose) for all higher levels of the food chain Primary production Liberates atmospheric oxygen O 2 is a waste product of photosynthesis

2 Photosynthesis and Energy Carbon dioxide converted to sugar Process called carbon fixation Series of redox reactions Water (hydrogen atoms) loses electrons Oxidized (OIL) Carbon dioxide gains electrons Reduced (RIG) Photosynthesis and Energy Generalized equation for photosynthesis CO H 2 O + photons (CH 2 O)n + H 2 O + O 2 carbon dioxide + water + light energy carbohydrate + oxygen + water Photosynthesis and Energy Somewhat opposite of cellular respiration Photosynthesis CO H 2 O + photons (CH 2 O)n + H 2 O + O 2 carbon dioxide + water + light energy carbohydrate + oxygen + water Cellular Respiration (CH 2 O) C 6 H 12 O 6 + O 2 CO 2 + H 2 O + energy

3 Photosynthesis and Energy Energy trapped by photosynthesis approx 100 terawatts six times larger than the yearly global power consumption of human civilization In all, photosynthetic organisms convert around 100,000,000,000 (billion) tons of carbon into biomass per year Three Types of Photosynthesizers Domain: Eukaryota Kingdom: Plantae Domain: Eukaryota Domain: Prokaryota Kingdom: Protista Kingdom: Eubacteria Figure The Components of Photosynthesis

4 The Components of Photosynthesis Chloroplasts Organelles in plants and algae, where photosynthesis takes place Light captured here Exist in great abundance in the mesophyll cells of plant leaves Double membrane bound like mitochondria Contain thylakoids Stacked as grana In the liquid filled stroma Photosynthesis Captured energy comes mostly from blue and red wavelengths of visible sunlight Absorbed by pigments in the thylakoids By chlorophyll a And accessory pigments Why plants look green The Electromagnetic Spectrum high energy low energy short wavelength long wavelength gamma rays x-rays ultraviolet infrared microwaves radiowaves visible light

5 Stomata Microscopic pores in leaves On the underside Can open and close letting carbon dioxide in and water vapor out Controlled by guard cells Site of Photosynthesis petiole blade epidermis mesophyll cells epidermis stomata nucleus chloroplast cell wall vacuole thylakoids stroma granum inner membrane outer membrane Energy from thylakoid sunlight is thylakoid membrane absorbed by thylakoid compartment pigments in the thylakoid membrane. 1. Leaf The primary site of photosynthesis in plants, leaves have a two-part structure: a petiole (or stalk) and a blade (normally thought of as the leaf). 2. Leaf cross section In cross section, leaves have a sandwichlike structure, with epidermal layers at top and bottom and mesophyll cells in between. Most photosynthesis is performed within mesophyll cells. Leaf epidermis is pocked with a large number of microscopic openings, called stomata, that allow carbon dioxide to pass in and water vapor to pass out. 3. Mesophyll cell A single mesophyll cell within a leaf contains all the component parts of plant cells in general, including the organelles called chloroplasts that are the actual sites of photosynthesis. 4. Chloroplast Each chloroplast has an outer membrane at its periphery; then an inner membrane; then a liquid material, called the stroma, that has immersed within it a network of membranes, the thylakoids. These thylakoids sometimes stack on one another to create A Granum Electrons used in photosynthesis will come from water contained in the thylakoid compartment, and all the steps of photosynthesis will take place either within the thylakoid membrane, or in the stroma that surrounds the thylakoids. Stages of Photosynthesis Two primary stages Light reactions Strips electrons from water oxidation Boosts these electrons to higher energy levels Makes ATP and NADPH Calvin cycle Makes carbohydrates from high energy electrons and atmospheric CO 2

6 The Light Reactions Electrons derived from water are energetically boosted by the power of sunlight Electrons physically move in this process: passed along through a series of electron carriers NADPH carries them to the second stage of photosynthesis Stages of Photosynthesis Two primary stages (continued) Light reactions Strips electrons from water Boosts these electrons to higher energy levels Calvin cycle Light independent reactions Makes carbohydrates from high energy electrons and atmospheric CO 2 reduction The Calvin Cycle Electrons carried by NADPH combined with carbon dioxide The product is a high-energy sugar combined into complex carbohydrates powered by ATP produced in the light reactions Occurs in the stroma of the chloroplast First described by Melvin Calvin

7 8.3 Stage 1: The Steps of the Light Reactions Light Reactions Photosynthesis Works through a pair of molecular complexes Photosystems II and I Located thylakoid membranes Composed partly of antennae molecules chlorophyll and some accessory molecules»absorb and transmit solar energy Photosystem Structure Photosystem antennae molecules A few hundred chlorophyll a and some accessory molecules (pigments) Pigments Carotene - an orange pigment Xanthophyll - a yellow pigment Phaeophytin a[1] - a gray-brown pigment Phaeophytin b[1] - a yellow-brown pigment Chlorophyll a - a blue-green pigment Chlorophyll b - a yellow-green pigment transmit solar energy to reaction centers

8 Photosystem Structure Reaction center molecules Pair of modified chlorophyll a molecules pheophytin Receive the light energy Transform it to chemical energy Splits water, strips the electrons off freed hydrogen atoms» also energizes the stripped electrons Includes the primary electron acceptors Receive these high energy electrons Photosystem Structure primary electron acceptor e sunlight reaction center antennae pigments photosystem The Working Units of Photosynthesis primary electron acceptor primary electron acceptor e sunlight NADP + sunlight NADPH electrons photosystem I to Calvin cycle photosystem II electron fall supplies energy that will lead to ATP synthesis Figure 8.5

9 Photosystems Light reactions in detail Photosystem II one chlorophyll molecule Absorbs one photon and loses one electron In the antennae pigments electron passed to a modified form of chlorophyll a called pheophytin» passes the electron to a quinone molecule» allowing the start of a flow of electrons down an electron transport chain» Ultimately drives the production of ATP Photosystems Light reactions in detail (continued) Photosystem I Receives electron from photosystem II electron transport chain Electron energy boosted by light again Like photosystem II but to a higher energy level electron enters a second electron transport chain» leads to the ultimate reduction of NADP to NADPH» creates a proton (H+) gradient across the chloroplast membrane» used by ATP synthase to make ATP Chlorophyll molecule regains the lost electron» from a water molecule through a process called photolysis» which releases an oxygen (O2) molecule Photosystems

10 8.4 What Makes the Light Reactions So Important? Two Key Actions in Light Reaction 1. Water is split Yielding both electrons and oxygen Electrons move through the light reactions We breathe the oxygen Two Key Actions in Light Reaction 2. Electrons derived from water Given an energy boost by the sun s rays Transferred to a different molecule: the initial electron acceptor This is the means through which the sun s energy is transferred into the living world

11 Importance of Light Reactions The energetic fall of electrons through the electron transport chain between photosystems II and I Also yields energy that produces ATP which is used to power the second stage of photosynthesis Light Reactions PLAY Light Reactions 8.5 Stage 2: The Calvin Cycle

12 The Calvin Cycle Carbon dioxide from the atmosphere Combined with a sugar, RuBP (Ribulose-1,5-bisphosphate) By RuBisCo (Ribulose-1,5-bisphosphate carboxylase/oxygenase) Enzyme that catalyzes RuBP and CO 2 The resulting compound is energized with addition of electrons supplied by the first stage of photosynthesis Makes 3-carbon sugars Later combined into carbohydrates The Calvin Cycle G3P Glyceraldehyde 3-phosphate High-energy sugar The result of the Calvin cycle Final product of photosynthesis Can be used for energy or plant structure The Calvin Cycle

13 The Calvin Cycle All these steps are powered by ATP produced in the light reactions G3P Everything in the plant ultimately is derived from this sugar in association with minerals and water that the plant absorbs through its roots Summary

14 8.6 Photorespiration and the C4 Pathway Photorespiration and the C4 Pathway In plants, the enzyme rubisco frequently binds with oxygen Rather than with carbon dioxide Called photorespiration undercuts photosynthesis Photorespiration This problem increases as the temperature rises because as plants close their stomata to keep in moisture they also keep out carbon dioxide thus increasing the likelihood that rubisco will bind with oxygen

15 O 2 CO 2 Photorespiration Some warm-climate plants have evolved a means of dealing with photorespiration C 4 photosynthesis C 4 Photosynthesis C 4 photosynthesis employs an enzyme that binds with carbon dioxide but not with oxygen Forms a 4-carbon compond Oxaloacetic acid From the combination of CO 2 with a 3-carbon molecule phosphoenolpyruvate (PEP) Hence C 4 Occurs in the mesophyll cells Oxaloacetic acid is then shuttled to bundle sheath cells to Calvin cycle Where CO 2 is released to bind with rubisco C 4 Photosynthesis C 4 pathway CO 2 Calvin cycle sugar mesophyll cells bundle-sheath cells vein cells Figure 8.10

16 C 4 Photosynthesis With high levels of carbon dioxide in the bundle-sheath cells Rubisco binds with carbon dioxide and not oxygen thus greatly reducing photorespiration 8.7 Another Photosynthetic Variation: CAM Plants CAM Plants Dry-weather plants such as cacti employ another form of photosynthesis CAM photosynthesis Crassulacean acid metabolism

17 CAM Photosynthesis In CAM photosynthesis the plant s stomata open only at night letting in and fixing carbon dioxide Carbon dioxide is then banked until sunrise when the sun s rays will supply the energy needed to power the Calvin cycle Photosynthesis Photosynthesis Song