Chapter 10: Photosynthesis IB Biology Review 12. Why is the

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IB Biology Review

• Chapter 10 Photosynthesis

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Relationship Between Photosynthesis and
Respiration

• Products of photosynthesis are reactants in
  respiration

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What is Photosynthesis?

• The conversion of light energy to chemical energy
  in the chemical bonds of glucose
• Which organisms perform photosynthesis?
• Autotrophs / Producers
• Some bacteria, some protists, plants

4
Where Does Photosynthesis Occur?

• In the chloroplast
• What is a chloroplast?
• An organelle found only in plants and
  photosynthetic protists
• absorbs sunlight and uses that energy to drive
  the synthesis of glucose from carbon dioxide and
  water
• Where does photosynthesis occur in the
  chloroplast?
• Thylakoid
• What is a thylakoid?
• A flattened membrane sac inside the chloroplast

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What chemical does photosynthesis need to occur?

• Chlorophyll
• What is chlorophyll?
• The main photosynthetic pigment that absorbs
  solar energy in photosynthesis
• Two types
• Chlorophyll A
• Chlorophyll B
• both absorb in the blue/violet wavelengths, but
  at slightly different wavelengths

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Chloroplast Structure

• Be able to draw these structures
• Outer membrane
• Inner membrane
• Stroma
• Granum
• Lumen
• Thylakoid

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The Reactions of Photosynthesis

• Two main sets of reactions
• Light Dependent Reactions
• The stage of photosynthesis that occurs in the
  thylakoid membranes
• Converts solar energy to the chemical energy of
  ATP and NADPH
• Light Independent Reactions / Calvin Reactions
• The stage of photosynthesis which combines CO2
  and the chemical energy obtained from the light
  reactions (ATP and NADPH)
• Creates 3-carbon sugars which are then combined
  to produce the 6-carbon glucose

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Overview of Photosynthesis
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Photosynthesis Step 1Light Absorption
Photolysis

• Chlorophyll absorbs sunlight / energy from proton
  in Photosystem II
• Electron donated by donated by the splitting of
  water (photolysis) is given energy from sunlight
• Oxygen (O2) released

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Photosynthesis Step 2Chemiosmosis, Excitation
and Photophosphorylation

• Electron moves through the electron transport
  chain
• Electron gives energy to proteins to pump protons
  into the thylakoid space
• This proton gradient is used by ATPase to make
  ATP
• Electron goes to photosystem I at end of electron
  transport chain
• This electron is re-excited by another burst of
  light
• Electron leaves by photosystem I
• This re-excited electron is then passed to NADP
• Produces NADPH

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Comparison of Chemiosmosis in Mitochondria and
Chloroplasts

• In both organelles
• Electron transport chains pump protons (H)
  across a membrane
• Protons go from a region of low H concentration
  (light gray in this diagram) to one of high H
  concentration (dark gray)
• Protons then diffuse back across the membrane
  through ATP synthase
• Produces ATP

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Overview of Light Dependent Reactions
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Photosynthesis Step 3Calvin Cycle Part 1
Carbon Fixation

• Three different carbon molecules from CO2 from
  air combine with three different RuBP molecules
  in stroma of chloroplast
• Produces six 3-phosophoglycerate molecules

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Photosynthesis Step 3Calvin Cycle Part 2
Reduction

• Molecule gets phosphorylated by ATP
• Molecule is reduced (gains an electron) by NADPH
• Becomes G3P
• For every 6 G3P molecules formed,one leaves the
  Calvin Cycle to make glucose
• 2 G3P (3 carbon) 1 glucose (6 carbon)

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Photosynthesis Step 3Calvin Cycle Part 3
Regeneration of RuBP

• The other five G3P molecules stay in the Calvin
  Cycle
• Get phosphorylated by ATP
• Regenerates RuBP

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Review of Calvin Cycle

• RuBP combines with CO2 to produce 3
  phosophoglycerate
• After being phosphorylated by ATP, this resultant
  molecule is reduced by NADPH to become a
  molecule called G3P
• For every 6 G3P molecules formed, only one leaves
  the Calvin Cycle to make glucose
• It takes two G3P (3-carbon) molecules to make
  one glucose (6-carbon) molecule
• The other G3P molecules stay in the Calvin Cycle
  and are phosphorylated by ATP to regenerate RuBP

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Review of Photosynthetic Processes
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Chloroplast Structure and Function

• The thylakoids produce a lot of membrane surface
  area within the chlorophlast
• Increased surface area increases space for the
  light dependent reactions to occur
• More light dependent reactions produce more ATP
  and NADPH
• More ATP and NADPH increase the rate of glucose
  production in the Calvin Cycle

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What three factors limit the rate of
photosynthesis?

• Temperature
• Gradual rise in rate
• Then sudden drop as proteins denature
• Light Intensity
• Gradual rise in rate
• Followed by a leveling off as maximum rate of
  photosynthesis reached
• CO2
• Gradual rise in rate
• Followed by a leveling off as maximum rate of
  photosynthesis reached

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Absorption Spectrum

• A graph showing how well the three photosynthetic
  pigments (chlorophyll a and b, and carotenoids)
  absorb various wavelengths of light
• Shows what waves of light plants absorb
• This graph has a similar overall trend as the
  Action Spectrum graph

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Action Spectrum

• A graph that shows how effective different
  wavelengths of light are in driving
  photosynthesis
• Shows what waves of light plants use for
  photosynthesis
• This graph has a similar overall trend as the
  Absorption Spectrum graph

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Measuring Photosynthesis

• Four main ways
• Production of oxygen because
• Oxygen is a byproduct of photosynthesis
• Measure carbon dioxide uptake because
• carbon dioxide is a reactant of photosynthesis
• Measure increase in biomass because
• products of photosynthesis are used in production
  of cell walls and new tissue
• Measure electron flow in light reaction using
• DPIP as an electron acceptor
• Measuring the color change as it is reduced with
  a spectrophotometer

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1. Which two colours of light does chlorophyll
absorb most? (1 mark)
IB Exam Question

• A. Red and yellow
• B. Green and blue
• C. Red and green
• D. Red and blue
• Correct answer D

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2. Explain photophosphorylation in terms of
chemiosmosis. (4 marks)
IB Exam Question

• Electron transport chain (ETC) causes
  proton/hydrogen ion pumping
• This creates a high proton gradient inside
  thylakoids
• Protons then pass by diffusion into the stroma
  through ATP synthase
• ATP synthase catalyses phosphorylation of ADP to
  produce ATP

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3. What is needed in photosynthesis to convert
carbon dioxide into organic molecules?

(1 mark)
IB Exam Question

• A. Light and hydrogen from the splitting of
  water
• B. Light and oxygen from the splitting of water
• C. ATP and hydrogen from the splitting of water
• D. ATP and oxygen from the splitting of water
• Correct answer C

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4. Describe the reactions in the light
independent part of photosynthesis.
IB Exam Question

• Carbon Fixation - RuBP combines with CO2 to
  produce 3 phosophoglycerate
• After being phosphorylated by ATP, this resultant
  molecule is reduced by NADPH to become a
  molecule called G3P
• For every 6 G3P molecules formed, only one leaves
  the Calvin Cycle to make glucose
• It takes two G3P (3-carbon) molecules to make
  one glucose (6-carbon) molecule
• The other G3P molecules stay in the Calvin Cycle
  and are phosphorylated by ATP to regenerate RuBP

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5. Explain how the light-independent reactions of
photosynthesis rely on light-dependent reactions.
(8 marks)
IB Exam Question

• light-independent reaction fixes CO2
• to make glycerate 3-phosphate
• glycerate 3-phosphate / G3P becomes reduced
• phosphoglyceraldehyde / glyceraldehyde
  3-phosphate
• using NADPH
• using ATP
• ATP needed to regenerate RuBP
• ATP is made in light-dependent reactions
• light causes excitation of electrons
• flow of electrons causes pumping of protons into
  thylakoid
• ATP formation when protons pass back across
  thylakoid membrane
• electrons are passed to NADP/NADP
• NADPH produced in the light dependent reactions

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6. Describe the relationship between chloroplast
structure and function. (4 marks)
IB Exam Question

• The thylakoids produce a lot of membrane surface
  area within the chloroplast
• This increased surface area increases the rate of
  light dependent reactions in the electron
  transport chain (ETC)
• Faster light dependent reactions produce more ATP
  and NADPH
• These molecules in turn- increase the rate of
  glucose production in the Calvin Cycle

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7. Explain the reasons for a shape of the
thylakoid membranes in the chloroplast.
(4 marks)
IB Exam Question

• Shape leads to more surface area on the surface
  of the thylakoids
• This leads to more photosynthetic pigments and
  higher rate of light reactions (photosystem 1 and
  2)
• This leads to more ATP and NADPH produced for the
  Calvin Cycle
• This leads to more glucose (sugar) produced in
  the Calvin Cycle

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8. Outline the effect of temperature, light
intensity and carbon dioxide concentration on the
rate of photosynthesis. (6 marks)
IB Exam Question

• light
• rate of photosynthesis increases as light
  intensity increases
• photosynthetic rate reaches plateau at high light
  levels
• CO2
• photosynthetic rate rises as CO2 concentration
  rises
• up to a maximum when rate levels off
• temperature
• rate of photosynthesis increases with increase in
  temperature
• up to optimal level / maximum
• high temperatures reduce the rate of
  photosynthesis
• Some of the above points may be achieved by means
  of annotated diagrams or graphs.

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9. Explain three ways in which the rate of
photosynthesis can be measured.



(4 marks)
IB Exam Question

• Production of oxygen because oxygen is a by
  product of the reaction
• Measure carbon dioxide uptake because carbon
  dioxide is a raw material of the reaction
• Measure increase in biomass because products are
  used in production of cell walls and new tissue
• Measure electron flow in light reaction using
  DPIP as an electron acceptor and measuring the
  color change as it is reduced with a
  spectrophotometer

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10. Explain the reactions involving the use of
light energy that occur in the thylakoids of the
chloroplast. (8
marks)
IB Exam Question

• Splitting of water/ photolysis provides an
  electron
• Chlorophyll in Photosystem II absorbs light
• Electron is raised to a higher energy level (is
  excited)
• Excited electron passes from photosystem II to
  carriers in electron transport chain (in
  thylakoid membrane)
• Energy from this excited electron is used to pump
  hydrogen ions from stroma into thylakoid space,
  creating high hydrogen ion concentration inside
  thylakoid.

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11. Pigments are extracted from the leaves of a
green plant. White light is then passed through
the solution of pigments. What effect do the leaf
pigments have on the white light?
(1 mark)
IB Exam Question

• A. Green wavelengths are absorbed and red and
  blue wavelengths are transmitted.
• B. Red and blue wavelengths are absorbed and
  green wavelengths are transmitted.
• C. Blue wavelengths are absorbed and green and
  red wavelengths are transmitted.
• D. Green and red wavelengths are absorbed and
  blue wavelengths are transmitted.
• Correct answer B

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12. Why is the action spectrum for photosynthesis
similar to the absorption spectra of
photosynthetic pigments? (1 mark)
IB Exam Question

• A. Photosynthetic pigments have the same optimum
  temperature as the enzymes used in
  photosynthesis.
• B. Plants absorb the same photosynthetic
  pigments for use in photosynthesis.
• C. Only wavelengths of light absorbed by
  pigments can be used in photosynthesis.
• D. The amount of energy absorbed by
  photosynthetic pigments is equal to the
  activation energy for photosynthesis.
• Correct answer C