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    Photosynthesis (BIO CH 13)

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    tamadesanmi1
    CIE A Level Notes on Photosynthesis

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    Photosynthesis (BIO CH 13)

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    tamadesanmi1
    2 views3 pages
    CIE A Level Notes on Photosynthesis

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    © © All Rights Reserved

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    CIE A Level Notes on Photosynthesis

    Copyright:

    © All Rights Reserved
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    2 views3 pages

    Photosynthesis (BIO CH 13)

    Uploaded by

    tamadesanmi1
    CIE A Level Notes on Photosynthesis

    Copyright:

    © All Rights Reserved
    Download as DOCX, PDF, TXT or read online from Scribd
    Download as docx, pdf, or txt
    of 3

    Structure and Function of the Chloroplast

    A. Stroma: It contains the enzymes for the light independent reactions. It also has 70s
    ribosomes to synthesize proteins for the reaction. They also contain starch grains which can
    store all the starch made in photosynthesis.
    B. Lamellae & Thylakoid: Lamellae are arranged to produce fluid filled sacs called thylakoids.
    These contain the electron carriers for the light dependent stage.
    C. Photosynthetic pigments (chlorophyll a and b, carotene and xanthophyll): These are
    embedded tightly in the thylakoid membrane. They absorb the light energy for the light
    dependent phase.
    The pigments are stacked in clusters to form photosystems. The part of the photosystems
    where the light energy is transferred to chemical energy in form of electrons is called the
    reaction center.
    NB: Both chlorophyll a and b absorb blue and red-light wavelengths and reflect green.
    Chlorophyll a has a peak in the violet and red regions and chlorophyll b in the blue and
    orange regions.

    Absorption and Action Spectra:

    An absorption spectrum shows all the absorbance of different wavelengths of light by photosynthetic
    pigments. An action spectrum, meanwhile, shows the effect of different types of light on the rate of
    photosynthesis.

    Photosynthesis General Equation

    C O2 + H 2 O→ Carbonhydates ( C6 H 12 O6 ) +O2

    Types of Light Dependent Stages of Photosynthesis:

    All the stages of light dependent photosynthesis occur in the thylakoid membrane.

    Photophosphorylation is the producing of ATP using energy that originated from light.

    1. Cyclic Photophosphorylation:
     Light energy is absorbed by photosystem I and is passed to the reaction centre. The
    energy excites an electron in chlorophyll a to a higher energy level.
     The excited electrons are captured by an electron acceptor in the thylakoid
    membrane.
     The electron carriers pass the electrons across an electron chain, in the process the
    electrons gradually loose (release) energy as they pass across the chain.
    NB: As electrons enter the carrier they are reduced and as they are transferred to
    the next carrier they are oxidised.
     The energy released moves protons actively from the stroma across the thylakoid
    membrane and into the intermembrane space.
     The protons move down the concentration gradient through facilitated passive
    diffusion from the intermembrane space through an ATP synthase molecule to make
    ATP through chemiosmosis.
     The electrons return to the photosystem I.
    2. Non-cyclic Photophosphorylation:
     Water is photolyzed (split) into oxygen, hydrogen protons and electrons.
     The electrons replenish the lost electrons from the photosystem II as the system is
    non-cyclic (the electrons are not reused)
     Electrons from the photosystems I and II are excited by light energy and excite the
    electrons moving them to a higher energy level. They then reduce the electron
    carriers just like in cyclic phosphorylation.
    NB: The electron carriers carry the electrons to photosystem I to for the same
    reaction to occur (photosystem I does not have its own natural source of electrons)
     As energy is released through the transport of electrons from one electron carrier to
    the next in the photosystem II to produce ATP through chemiosmosis.
     In contrast, the electrons released in the same process in photosystem I combine
    with the hydrogen protons from the photolysis of water to form hydrogen once
    again.
     The hydrogen combines with a coenzyme NADP (NAD Phosphate) to make reduced
    NADP.
     The reduced NADP as well as the ATP formed are transferred to the light
    independent phase.

    Stages of the Light Independent Stage of Photosynthesis (Calvin Cycle)

     This stage takes place in the stroma of the chloroplast.


     Carbon dioxide is combined to a 5-carbon compound called Ribulose biphosphate (RuBP).
    The reaction is catalysed by an enzyme called Ribulose Biphosphate Carboxylase (rubisco).
     This produces 2 glycerate-3-phosphate (GP) molecules which are 3 carbon molecules.
    NB: This reaction is referred to as carbon dioxide fixation as carbon dioxide has been
    removed from the environment and has become part of the plant cell.
     The energy from the ATP and the hydrogen molecules from the reduced NADP are used to
    reduce the glycerate-3-phosphate into a carbohydrate called triose phosphate (TP). 2
    molecules of TP are made during the Calvin Cycle.
     Most of the triose phosphate are used to regenerate RuBP (although it requires more ATP)
    but a good amount condenses to from hexose phosphates that are used to produce starch.
    NB: Glycerate-3-phosphate can also be used to produce glycerol and fatty acids to form
    lipids.
    Fatty Acids

    Nitrogen

    Limiting Factors of Photosynthesis

    A limiting factor is any requirement for a process in the shortest supply.

    1. Light Intensity: An increase in light intensity leads to a general increase in light intensity till
    the graph levels out (when other factors become the limiting factor). This is because an
    increase in light intensity increases the energy to create more ATP and reduced NADP in the
    light dependent phases.
    2. Temperature: An increase in temperature does not affect the light dependent phase though
    it does affect the light independent phase. This is because the light independent phase has
    enzymes unlike the light dependent phase. Hence an increase in kinetic energy leads to a
    more frequent number of collisions and hence an increase in rate of photosynthesis. At
    higher temperatures the enzyme rubisco can denature.
    3. Carbon Dioxide: An increase in carbon dioxide concentration leads to an increase in
    photosynthesis as it is the raw material in the Calvin Cycle.

    NB: When other factors are limiting factors, the graph is less steep, and the rate of photosynthesis is
    still low. When the other factors aren’t limiting factors, the graph is steeper, and the rate of
    photosynthesis is higher.

    4. Water: A shortage of water usually affects other process in plants before it affects
    photosynthesis. But a decrease in water in extreme circumstances leads to a decrease in
    photosynthesis.
    5. Prescence of Photosynthetic Pigments: a decrease in photosynthetic pigments reduces the
    rate of absorption of light energy. and hence a decrease in the rate of the light dependent
    reactions and hence reduce the rate of photosynthesis.

    PRACTICAL WORK

    A. Using Chromatography to identify chloroplast pigments.


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