Chapter 7 photosynthesis using light to make food
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CHAPTER 7 Photosynthesis: Using Light to Make Food. Overview: Photosynthesis Light Reactions Calvin Cycle Review of photosynthesis & C3, C4, CAM plants Greenhouse effect Ozone Layer . Life in the Sun. Light is central to the life of a plant

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Chapter 7 photosynthesis using light to make food l.jpg

CHAPTER 7Photosynthesis:Using Light to Make Food



Light Reactions

Calvin Cycle

Review of photosynthesis & C3, C4, CAM plants

Greenhouse effect

Ozone Layer

Life in the sun l.jpg
Life in the Sun

  • Light is central to the life of a plant

  • Photosynthesis is the most important chemical process on Earth

    • It provides food for virtually all organisms

  • Plant cells convert light into chemical signals that affect a plant’s life cycle

  • All of the food consumed by humans can be traced back to photosynthetic plants

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  • Too much sunlight can damage a plant

    • Chloroplasts and carotenoids help to prevent such damage

    • Light can influence the architecture of a plant

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    • Almost all plants are photosynthetic autotrophs (=self-feeders; they make their own food), as are some bacteria and protists

    • They generate their own organic matter through photosynthesis

    All organisms that produce org molecules from inorg molecules using light: photoautotrophs

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    • On land green leaves , plants such as oak trees and cacti are the predominant producers (produce their food supply)

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    • In aquatic green leaves environments, algae and photosynthetic bacteria are the main food producers

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    Overview of Photosynthesis green leaves

    • Photosynthesis is the process by which autotrophic organisms use light energy to make sugar and oxygen gas from carbon dioxide and water

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    Chloroplasts: Sites of Photosynthesis green leaves

    • Occurs in chloroplasts

    • Photosynthesis

    • Chloroplasts:

    • Are found in the cells of the mesophyll (green tissue in the interior of the leaf)

    • Contain stroma, a thick fluid

    • Contain thylakoids, membranous sacs (The thylakoids contain chlorophyll - Chlorophyll is the green pigment that captures light for photosynthesis)

    • (grana- singular granum-, stacks of thylakoids)

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    Plants produce O2 gas by splitting water green leaves

    • The O2 liberated by photosynthesis is made from the oxygen in water

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    Photosynthesis is a redox process, as is cellular respiration

    • Water molecules are split apart and electrons and H+ ions are removed, leaving O2 gas

      • These electrons and H+ ions are transferred to CO2, producing sugar

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    Photosynthesis is a two-step process – An Overview: Light Reaction & Calvin cycle



    • The Calvin cycle makes sugar from carbon dioxide

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    • Sunlight is a type of energy called radiation, or electromagnetic energy

    • The full range of radiation is called the electro-magnetic spectrum

    • Certain wavelengths of visible light drive the light reactions of photosynthesis

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    • Chloroplasts absorb light energy and convert it to chemical energy

    • Electrons in the pigment (chlorophyll) gain energy

    • Light behaves as photons, discrete packets of energy

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    Chloroplast Pigments ENERGY

    • Chlorophyll a

    • Chlorophyll b

    • Carotenoids

    • Chloroplasts contain several pigments

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    Photosystems capture solar power ENERGY

    • Photosystems consist of many light-harvesting complexes (containing chlorophyll a, b, and carotenoid that function as a light-gathering antenna) surrounding a reaction center.

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    • Photosystems are 2 types that corporate in the light reaction; (photosystem II functions first)

    • Photosystem I:Chlorophyll a of the reaction center is P700 (absorbs light with wavelength 700 nm)

    • Photosystem II: Chlorophyll a of the reaction center is P680 (absorbs light with wavelength 680 nm)

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    • Two types of photosystems cooperate in the light reactions

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    • An electron transport chain

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    In the light reactions, electron transport chains generate ATP, NADPH, and O2

    • Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons

    • The excited electrons are passed from the primary electron acceptor to electron transport chains

      • Their energy ends up in ATP and NADPH

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    • Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product

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    To Calvin Cycle pass into its P700 chlorophyll

    • The light reactions in the thylakoid membrane


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    Chemiosmosis powers ATP synthesis in the light reactions pass into its P700 chlorophyll

    • The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane

      • The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP

      • In the stroma, the H+ ions combine with NADP+ to form NADPH

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    ATP and NADPH power sugar synthesis in the Calvin cycle pass into its P700 chlorophyll

    • The Calvin cycle occurs in the chloroplast’s stroma

      • This is where carbon fixation takes place and sugar is manufactured

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    • The Calvin cycle

    • Functions like a sugar factory within a chloroplast

    • Regenerates the starting material with each turn

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  • Energy-rich sugar is then converted into glucose

    • The Calvin cycle constructs G3P using

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    PHOTOSYNTHESIS REVIEWED AND EXTENDED pass into its P700 chlorophyll

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    Review: Photosynthesis uses light energy to make food molecules

    • A summary of the chemical processes of photo-synthesis

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    • Many plants make more sugar than they need

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    • C molecules3 plants

    • Use CO2 directly from the air

    • Are very common and widely distributed

    • Most plants are C3 plants, which take CO2 directly from the air and use it in the Calvin cycle

      • In these types of plants, stomata on the leaf surface close when the weather is hot

      • This causes a drop in CO2 and an increase in O2 in the leaf

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    C molecules4 and CAM plants have special adaptations that save water

    • C4 plants

    • Close their stomata to save water during hot and dry weather

    • Can still carry out photosynthesis

  • CAM plants

    • Open their stomata only at night to conserve water

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    • Special cells in C molecules4 plants—corn and sugarcane—incorporate CO2 into a four-carbon molecule

    • This molecule can then donate CO2 to the Calvin cycle

    • Some plants have special adaptations that enable them to save water

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    • The CAM plants—pineapples, most cacti —employ a different mechanism


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    Human activity is causing global warming; photosynthesis moderates it

    • Due to the increased burning of fossil fuels, atmospheric CO2 is increasing

      • CO2 warms Earth’s surface by trapping heat in the atmosphere

      • This is called the greenhouse effect


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    How Photosynthesis Moderates the Greenhouse Effect moderates it

    • Old-growth forests

    • Are important for lumber

    • Are important for moderating world climates

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    • Destruction of forests may be increasing this effect

    • Because photosynthesis removes CO2 from the atmosphere, it moderates the greenhouse effect

    • It swaps O2 for CO2

    • Unfortunately, deforestation may cause a decline in global photosynthesis

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    • Solar radiation converts O warming, a slow but steady rise in the Earth’s surface temperature2 high in the atmosphere to ozone (O3)

    • Ozone shields organisms on the Earth’s surface from the damaging effects of UV radiation

    • The O2 in the atmosphere results from photosynthesis

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    • International restrictions on these chemicals are allowing recovery