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Title: BIO 2, Lecture 14


1
BIO 2, Lecture 14
  • FIGHTING ENTROPY III
  • PHOTOSYNTHESIS

2
  • Autotrophs sustain themselves without eating
    anything derived from other organisms
  • Autotrophs are the producers of the biosphere,
    producing organic molecules from CO2 and other
    inorganic molecules
  • Almost all plants are photoautotrophs, using the
    energy of sunlight to make complex organic
    molecules from H2O and CO2

3
  • Photosynthesis occurs in plants, algae, certain
    other protists, and some prokaryotes
  • These organisms feed not only themselves but also
    most of the living world

4
(a) Plants
(c) Unicellular protist
10 µm
(e) Purple sulfur bacteria
1.5 µm
(b) Multicellular alga
(d) Cyanobacteria
40 µm
5
  • Heterotrophs obtain their organic material from
    other organisms
  • Heterotrophs are the consumers of the biosphere
  • Almost all heterotrophs, including humans, depend
    on photoautotrophs for food and O2

6
  • In plants, the work of photosynthesis is done by
    organelles called chloroplasts
  • Chloroplasts are structurally similar to and
    likely evolved from photosynthetic bacteria

7
  • In plants, leaves are the major locations of
    photosynthesis
  • Their green color is from chlorophyll, the green
    pigment within chloroplasts
  • Light energy absorbed by chlorophyll drives the
    synthesis of organic molecules in the chloroplast
  • CO2 enters and O2 exits the leaf through
    microscopic pores called stomata

8
  • Chloroplasts are found mainly in cells of the
    mesophyll, an interior tissue of the leaf
  • A typical mesophyll cell has 3040 chloroplasts
  • The chlorophyll is in the membranes of thylakoids
    (connected sacs in the chloroplast) thylakoids
    may be stacked in columns called grana
  • Chloroplasts also contain stroma, a dense fluid

9
Leaf cross section
Vein
Mesophyll
Stomata
CO2
O2
Chloroplast
Mesophyll cell
5 µm
10
Chloroplast
Outer membrane
Thylakoid
Intermembrane space
Thylakoid space
Granum
Stroma
Inner membrane
1 µm
11
  • Chloroplasts split H2O into hydrogen and oxygen,
    incorporating the electrons of hydrogen into
    sugar molecules
  • Photosynthesis can be summarized as the following
    equation

6 CO2 12 H2O Light energy ? C6H12O6 6 O2
6 H2O
12
  • Photosynthesis is a redox process in which H2O is
    oxidized (to O2) and CO2 is reduced (to C6H12O6)

13
  • Photosynthesis consists of two parts
  • Photo part light-dependent reactions
  • Require light only occur in the daytime
  • 2.Synthesis (of sugar) part light independent
    reactions
  • Also called the Calvin cycle
  • Occur both in the daytime and at night
  • Plant switches most energy to Calvin Cycle at
    night

14
  • The light reactions
  • Occur in the thylakoid membrane and thylakoid
    space (inside the thylakoid)
  • Split H2O into H and O2 (gas)
  • Reduce NADP to NADPH
  • Generate ATP from ADP and Pi
  • The Calvin cycle
  • Occurs in the stroma
  • Forms sugar from CO2, using the ATP and NADPH
    generated in the light reactions
  • Begins with carbon fixation, incorporating CO2
    into organic molecules

15
Dark (light-independent) reactions (occur in
the presence and absence of light)
16
  • Light is a form of electromagnetic energy, also
    called electromagnetic radiation
  • Like other electromagnetic energy, light travels
    in rhythmic waves
  • Wavelength is the distance between crests of
    waves
  • Wavelength determines the type of electromagnetic
    energy
  • Shorter wavelength higher energy

17
  • The electromagnetic spectrum is the entire range
    of electromagnetic energy, or radiation
  • Visible light consists of wavelengths (including
    those that drive photo-synthesis) that produce
    colors we can see
  • Light also behaves as though it consists of
    discrete particles, called photons

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19
  • Pigments are substances that absorb visible light
  • Different pigments absorb different wavelengths
  • Wavelengths that are not absorbed are reflected
    back (and seen by observers)
  • Leaves appear green because chlorophyll reflects
    green light back to our eyes
  • It is actually the combined wavelengths not
    absorbed by chlorophyll that collectively appear
    green

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  • Chlorophyll a is the main photosynthetic pigment
  • Accessory pigments, such as chlorophyll b,
    broaden the spectrum used for photosynthesis
  • Accessory pigments called carotenoids absorb
    excessive light that would damage chlorophyll

22
  • When a pigment absorbs light, it goes from a
    ground state to an excited state, which is
    unstable
  • When excited electrons fall back to the ground
    state, photons are given off, an afterglow called
    fluorescence
  • If illuminated, an isolated solution of
    chlorophyll will fluoresce, giving off light and
    heat

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  • Photosynthesis begins at photosystems located in
    the thylakoid membrane
  • A photosystem consists of a reaction-center
    complex (comprised of several proteins)
    surrounded by light-harvesting complexes
  • The light-harvesting complexes (pigment molecules
    bound to proteins) funnel the energy of photons
    to the reaction center

25
  • There are two types of photosystems in the
    thylakoid membrane
  • Photosystem II (PS II) functions first (the
    numbers reflect order of discovery) and is best
    at absorbing a wavelength of 680 nm
  • Photosystem I (PS I) functions second and is best
    at absorbing a wavelength of 700 nm

26
  • At the beginning of photosynthesis, a primary
    electron acceptor in the reaction center of
    Photosystem II accepts an electron from
    chlorophyll a that has been excited by a photon
  • Solar-powered transfer of an electron from a
    chlorophyll a molecule to the primary electron
    acceptor is the first step of the light reactions

27
  • The chlorophyll molecule that is now missing an
    electron is a very strong oxidizing agent
  • It grabs an electron from H2O (in the thylakoid
    space) and is reduced
  • O2 is released as a by-product of this splitting
    of H2O
  • H is also formed, which begins to build up in
    the thylakoid space (sound familiar?)

28
  • The electron falls down an electron transport
    chain from the primary electron acceptor of PS II
    to PS I
  • Energy released by the fall is used by proteins
    in the electron transport chain to drive H ions
    from the stroma into the inner space of the
    thylakoid
  • Thus, there are two sources of build-up of H
    ions in the inner space from the splitting of
    water and from the electron transport chain

29
  • ATP synthase, embedded in the thylakoid membrane,
    coverts ADP Pi to ATP using the energy
    generated by the rush of the H ions in the inner
    compartment out into the stroma (with their
    concentration gradient)

30
  • Meanwhile, the electrons passing through the
    electron transport chain of Photo-system II are
    eventually dumped off at Photosystem I
  • When photons hit chlorophyll molecules in
    Photosystem I, electrons are kicked off
    chlorophyll to a second electron transport chain
  • Chlorophyll molecules in Photosystem I (now
    strong oxidixing agents) grab electrons dumped
    dumped off from Photosystem II to return to their
    reduced state
  • Water is not split at Photosystem I

31
  • Electrons excited by photons at Photosystem I
    fall down a second electron transport chain and
    are eventually dumped onto NADP, reducing it to
    NADPH
  • The electrons of NADPH are available for the
    reactions of the Calvin cycle (that drive the
    endergonic reactions that create sugar and
    starch)
  • ATP produced through the proton motive force are
    also used in the Calvin Cycle

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34
  • Chloroplasts and mitochondria generate ATP by
    chemiosmosis, but use different sources of energy
  • Mitochondria transfer chemical energy from food
    to ATP chloroplasts transform light energy into
    the chemical energy of ATP, which is used to
    produce food
  • Spatial organization of chemiosmosis differs
    between chloroplasts and mitochondria but also
    shows similarities

35
  • In mitochondria, protons are pumped out of the
    inner space (matrix) and into the intermembrane
    space
  • ATP synthesis occurs as they diffuse back into
    the mitochondrial matrix
  • In chloroplasts, protons are pumped into the
    inner thylakoid space and out of the stroma
  • ATP synthesis occurs as they diffuse back out of
    the inner thylakoid space (into the stroma)

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37
  • The Calvin cycle, like the citric acid cycle,
    regenerates its starting material after molecules
    enter and leave the cycle
  • The cycle builds sugar from smaller molecules by
    using ATP and the reducing power of electrons
    carried by NADPH
  • The ATP and NADPH come from the light-dependent
    reactions

38
  • Carbon enters the cycle as CO2 and leaves as a
    sugar named glyceraldehyde-3-phospate (G3P)
  • For net synthesis of 1 G3P, the cycle must take
    place three times, fixing 3 molecules of CO2
  • The Calvin cycle has three phases
  • Carbon fixation (catalyzed by rubisco)
  • Reduction
  • Regeneration of the CO2 acceptor (RuBP)

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