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CHAPTER 6 PHOTOSYNTHESIS: ACQUIRING ENERGY FROM THE SUN.

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Presentation on theme: "CHAPTER 6 PHOTOSYNTHESIS: ACQUIRING ENERGY FROM THE SUN."— Presentation transcript:

1 CHAPTER 6 PHOTOSYNTHESIS: ACQUIRING ENERGY FROM THE SUN

2 AN OVERVIEW OF PHOTOSYNTHESIS Most of the energy used by almost all living cells ultimately comes from the sun. Plants, algae, and some bacteria capture the sunlight energy by a process called photosynthesis. Only about 1% of the available energy in sunlight is captured. Cross-section of leaf Cuticle Epidermis Mesophyll Vascular bundle Bundle sheath Stoma Vacuole Nucleus Cell wall Chloroplasts Inner membrane Outer membrane Granum Stroma Chloroplast Thylakoid Mesophyll cell

3 AN OVERVIEW OF PHOTOSYNTHESIS The leaf cells of plants contain chloroplasts. The chloroplast contains internal membranes called thylakoids. The thylakoids are stacked together in columns called grana. The stroma is a semi liquid substance that surrounds the thylakoids.

4 AN OVERVIEW OF PHOTOSYNTHESIS The photosystem is the starting point of photosynthesis. It is a network of pigments in the membrane of the thylakoid. The primary pigment of a photosystem is chlorophyll. The pigments act as an antenna to capture energy from sunlight. Individual chlorophyll pigments pass the captured energy between them.

5 AN OVERVIEW OF PHOTOSYNTHESIS Photosynthesis takes place in three stages: 1.Capturing energy from sunlight. 2.Using the captured energy to produce ATP and NADPH. 3.Using the ATP and NADPH to make carbohydrates from CO 2 in the atmosphere.

6 AN OVERVIEW OF PHOTOSYNTHESIS The process of photosynthesis is divided into two types of reactions. Light-dependent reactions take place only in the presence of light and produce ATP and NADPH. Light-independent reactions do not need light to occur and result in the formation of organic molecules. More commonly known as the Calvin cycle.

7 AN OVERVIEW OF PHOTOSYNTHESIS The overall reaction for photosynthesis may be summarized by this simple equation: 6 CO 2 carbon dioxide + 6 H 2 O water + Light energy C 6 H 12 O 6 glucose + 6 O 2 oxygen

8 HOW PLANTS CAPTURE ENERGY FROM SUNLIGHT Light is comprised of packets of energy called photons. Sunlight has photons of varying energy levels. The possible range of energy levels is represented by an electromagnetic spectrum. Human eyes only perceive photons of intermediate energy levels. This range of the spectrum is known as visible light.

9 Increasing energy of photon Increasing wave length nm 1 nm1,000 nm 10 nm0.01 cm 1 cm 1 m100 m Radioactive elements X-ray machines Light bulbs People Radar Microwave ovens FM radio AM radio Gamma rays X rays UV light InfraredMicrowavesRadio waves Visible light 400 nm430 nm500 nm560 nm600 nm650 nm740 nm

10 HOW PLANTS CAPTURE ENERGY FROM SUNLIGHT Pigments are molecules that absorb light energy. The main pigment in plants is chlorophyll. Chlorophyll absorbs light at the ends of the visible spectrum, mainly blue and red light.

11 HOW PLANTS CAPTURE ENERGY FROM SUNLIGHT Plants also contain other pigments, called accessory pigments, that absorb light levels that chlorophyll does not. These pigments give color to flowers, fruits, and vegetables. They are present in leaves too, but are masked by chlorophyll until the fall when the chlorophyll is broken down.

12 1. All wave lengths except 500- to600-nanometer wave lengths (green) absorbed by leaf 4. Brain perceives “green” 2. Green reflected by leaf 3. Green absorbed by eye pigment WHY ARE PLANTS GREEN?

13 CarotenoidsChlorophyll bChlorophyll a Relative light absorption Wavelength (nm) ABSORPTION SPECTRA OF CHLOROPHYLLS AND CAROTENOIDS

14 ORGANIZING PIGMENTS INTO PHOTOSYSTEMS In plants, the light-dependent reactions occur within a complex of proteins and pigments called a photosystem.

15 ORGANIZING PIGMENTS INTO PHOTOSYSTEMS Light energy is first captured by any one of the chlorophyll pigments. The energy is passed along to other pigments until it reaches the reaction center chlorophyll molecule. The reaction center then releases an excited electron, which is then transferred to an electron acceptor. The excited electron that is lost is then replaced by an electron donor. Photon Photosystem e–e– e–e– Reaction center chlorophyll Electron donor Chlorophyll molecules Electron acceptor

16 ORGANIZING PIGMENTS INTO PHOTOSYSTEMS The light-dependent reactions in plants and algae use two photosystems. Photosystem II Captures a photon of light and releases an excited electron to the electron transport system (ETS). The ETS then produces ATP.

17 Energy of electrons Electron transport system Photon Photosystem II NADP + + H + Photosystem I 2H 2 O 4H + + O 2 H+H+ P 700 P 680 e–e– Electron transport system e–e– e–e– e–e– e–e– Excited reaction center Excited reaction center Reaction center Proton gradient formed for ATP synthesis Water-splitting enzyme ATP Reaction center NADPH ORGANIZING PIGMENTS INTO PHOTOSYSTEMS Photosystem I Absorbs another photon of light and releases an excited electron to another ETS. The ETS produces NADPH. The electron from photosystem II replaces the electron from the reaction center.

18 HOW PHOTOSYSTEMS CONVERT LIGHT TO CHEMICAL ENERGY Plants produce both ATP and NADPH by noncyclic photophosphorylation. The excited electrons flow through both photosystems and end up in NADPH. High energy electrons generated by photosystem II are used to make ATP and are then passed along to photosystem I to drive the production of NADPH.

19 THE PHOTOSYNTHETIC ELECTRON TRANSPORT SYSTEM Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Thylakoidspace 2H 2 O 4 H + H + + NADP + O2O2 e–e– H+H+ Light-dependent reactions Calvin cycle ATP Photon Thylakoid membrane Photosystem IIElectron transport systemPhotosystem IElectron transport system Water-splitting enzyme Stroma NADP Photon Antenna complex Proton gradient Thylakoid space H+H+ H+H+ H+H+ H+H+ H+H+ e–e– e–e– e–e– NADP

20 HOW PHOTOSYSTEMS CONVERT LIGHT TO CHEMICAL ENERGY As a result of the proton pump of the ETS, a large concentration of protons builds up in the thylakoid space. The thylakoid membrane is impermeable to protons. Protons can only re-enter the stroma by traveling through a protein channel called ATP synthase. As the protons follow their concentration gradient and pass through ATP synthase channels, ADP is phosphorylated into ATP. This process is called chemiosmosis.

21 CHEMIOSMOSIS IN A CHLOROPLAST Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. H+H+ 2H 2 O 4 H + O2O2 Light-dependent reactions ATP Calvin cycle NADPH Thylakoid space Thylakoid space Stroma Photon ADPATP Photosystem IIElectron transport systemATP synthase e–e– e–e– H+H+ H+H+ H+H+ H+H+ H+H+ H+H+

22 BUILDING NEW MOLECULES Cells use the products of the light-dependent reactions to build organic molecules. ATP is needed to drive endergonic reactions. NADPH is needed to provide reducing power in the form of hydrogens.

23 BUILDING NEW MOLECULES The synthesis of new molecules employs the light-independent, or Calvin cycle, reactions. These reactions are also known as C 3 photosynthesis.

24 BUILDING NEW MOLECULES The Calvin cycle reactions occur in 3 stages 1. Carbon fixation Carbon from CO 2 in the air is attached to an organic molecule, RuBP. 2. Making sugars The carbons are shuffled about through a series of reactions to make sugars. 3. Reforming RuBP The remaining molecules are used to reform RuBP.

25 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. P P The Calvin cycle begins when a carbon atom from a CO 2 molecule is added to a five-carbon molecule (the starting material). The resulting six-carbon molecule is unstable and immediately splits into three-carbon molecules. (Three “turns” of the cycle are indicated here with three molecules of CO 2 entering the cycle.) Then, through a series of reactions, energy from ATP and hydrogens from NADPH (the products of the light- dependent reactions) are added to the three-carbon molecules. The now- reduced three-carbon molecules either combine to make glucose or are used to make other molecules. Most of the reduced three-carbon molecules are used to regenerate the five-carbon starting material, thus completing the cycle. 6 3-phosphoglycerate 6 6 Glucose 3-phosphoglycerate CO P P P RuBP (Starting material) ATP Glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate RuBP (Starting material) NADPH

26 BUILDING NEW MOLECULES The Calvin cycle must “turn” 6 times in order to form a new glucose molecule. Only one carbon is added from CO 2 per turn. The Calvin cycle also recycles reactants needed for the light-dependent reactions. It returns ADP so that it is available for chemiosmosis in photosystem II. It returns NADP+ back to the ETS of photosystem I.

27 REACTIONS OF THE CALVIN CYCLE Rubisco is thought to be the most abundant protein on earth 3 Rubisco Carbon fixation 3-phosphoglycerate CO 2 6 ADP 1,3-bisphosphoglycerate 6 NADP Glyceraldehyde 3-phosphate Glyceraldehyde 3-phosphate 3 ADP RuBP Stroma of chloroplast Glyceraldehyde 3-phosphate 2 P i 6 Making sugars P i P P P P P P Glucose and other sugars 3 ATP 6 ATP Reforming RuBP Thylakoid space Light-dependent reactions AT P Calvin cycle NADPH 6 NADPH

28 PHOTORESPIRATION: PUTTING THE BRAKES ON PHOTOSYNTHESIS Many plants have trouble carrying out C 3 photosynthesis when it is hot. Plants close openings in their leaves, called stomata (singular, stoma), in order to prevent water loss. The closed stoma also prevent gas exchange. O 2 levels build up inside the leaves while the concentrations of CO 2 fall. The enzyme rubisco fixes oxygen instead of carbon. This process is called photorespiration and short- circuits the Calvin cycle and photosynthesis.

29 PLANT RESPONSE IN HOT, ARID WEATHER H2OH2O O2O2 Under hot, arid conditions, leaves lose water by evaporation through openings in the leaves called stomata. The stomata close to conserve water but as a result, O 2 builds up inside the leaves, and CO 2 cannot enter the leaves. This leads to photorespiration. H2OH2O O2O2 CO 2 Leaf epidermis Heat Stoma

30 CARBON FIXATION IN C 4 PLANTS Some plants have adapted to hot climates by performing C 4 photosynthesis. These C 4 plants include sugarcane, corn, and many grasses. They fix carbon using different types of cells and reactions than C 3 plants and do not run out of CO 2 even in hot weather. CO 2 becomes trapped in cells called bundle-sheath cells. CO 2 Phosphoenol- pyruvate (PEP) Oxalo- acetate Mesophyll cell PyruvateMalate PyruvateMalate ATP PP i + AMP P i + Bundle- sheath cell Calvin cycle Glucose

31 PHOTORESPIRATION: PUTTING THE BRAKES ON PHOTOSYNTHESIS Another strategy to avoid a reduction in photosynthesis in hot weather occurs in many succulent (water-storing) plants, such as cacti and pineapples. These plants undergo crassulacean acid metabolism (CAM). Photosynthesis occurs via the C 4 pathway at night and the C 3 pathway during the day.

32 COMPARING CARBON FIXATION IN C 4 AND CAM PLANTS C 4 plants Glucose CO 2 CAM plants Night Day CO 2 Glucose CO 2 C 4 pathway C 4 pathway Calvin cycle Calvin cycle Mesophyll cell Mesophyll cell Bundle- sheath cell


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