Photosynthesis

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Photosynthesis

• Section 5-2

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Photosynthetic Organisms Use the Energy in
Sunlight

• Plants capture energy from sunlight
• Most of the time when you trace your food back to
  its origin, you end up with plants
• Plants, algae, and some bacteria capture about 1
  of the energy in the sunlight that reaches Earth
  and convert it to chemical energy through the
  process of photosynthesis.

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Photosynthesis

• The process that provides energy for almost all
  life.
• Has three stages
• OVERALL CHEMICAL FORMULA
• 3 CO2 3 H2O ? C3H6O3 3 O2

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3 Stages of Photosynthesis

• Stage 1
• Energy is captured from sunlight.
• Stage 2
• Light energy is converted to chemical energy,
  which is temporarily stored in ATP and the energy
  carrier molecule NADPH.

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3 Stages of Photosynthesis (cont)

• Stage 3
• The chemical energy stored in ATP and NADPH
  powers the formation of organic compounds, using
  CO2

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Photosynthesis

• Occurs in the chloroplast of plant cells and
  algae and in the cell membrane of certain
  bacteria
• Plants use the organic compounds they make during
  photosynthesis to carry out their life processes.

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Stage One of Photosynthesis

• Light energy is absorbed
• These reactions are often called light
  reactions or light-dependent reactions because
  the absorption of light has to occur for the
  reactions to proceed
• Light energy is used to make energy-storing
  compounds

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Stage One of Photosynthesis

• Pigments absorb different wavelengths of light.
• Pigments light-absorbing substances that absorb
  only certain wavelengths
• Chlorophyll primary pigment involved in
  photosynthesis. Absorbs mostly blue and red
  light. Reflects green and yellow. This is what
  makes a plant look green.
• Two types of chlorophyll Chlorophyll A and
  Chlorophyll B

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Stage One of Photosynthesis

• Carotenoids pigments that produce yellow and
  orange fall leaf colors.
• Produces the colors of many fruits, vegetables,
  and flowers.
• Absorb different wavelengths of light from those
  absorbed by chlorophyll

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Stage One of Photosynthesis

• By using chlorophyll and carotenoids, plants
  absorb more light energy during photosynthesis.
• Pigments are located in the chloroplasts of leaf
  cells.
• Thylakoids clusters of pigments are embedded in
  the membranes of disk-shaped structures called
  thylakoids

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Stage One of Photosynthesis

• When light strikes thylakoid, energy is
  transferred to electrons in chlorophyll and other
  pigments.
• This energy transfer causes electrons to jump to
  a higher energy level.
• These electrons are said to be excited.
• This is how plants first capture energy from
  sunlight.

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Stage One of Photosynthesis

• Excited electrons jump from chlorophyll molecules
  to other nearby molecules in the thylakoid
  membrane, where the electrons are used to power
  the second stage of photosynthesis.
• The excited electrons that leave chlorophyll
  molecules must be replaced by other electrons.
  Replacement electrons come from water molecules.
  The water molecules are split by an enzyme inside
  the thylakoid.

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Stage One of Photosynthesis

• Water molecules split chlorophyll molecules
  take electrons from hydrogen atoms leaving
  hydrogen ions.
• Remaining oxygen atoms from disassembled water
  molecules combine to form oxygen gas.

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Stage Two of Photosynthesis

• In this stage, light is converted to chemical
  energy.
• Electron Transport Chains series of molecules
  through which excited electrons are passed along
  a thylakoid membrane
• Excited electrons leave chlorophyll molecules are
  used to produce new molecules that temporarily
  store chemical energy, including ATP.

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Stage Two of Photosynthesis

• An excited electron jumps to a nearby molecule in
  a thylakoid membrane.
• The electron is passed through a series of
  molecules along the thylakoid membrane like a
  ball being passed down a line of people.
• Figure 5-8 p. 100

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Stage Two of Photosynthesis Action of Electron
Transport Chains

• One type of ETC contains a protein that acts as a
  membrane pump.
• Excited electrons lose some of their energy as
  they pass through this protein.
• Energy lost by electrons used to pump hydrogen
  ions (H) into the thylakoid.
• Recall H produced when water molecules are
  split inside the thylakoid.

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Stage Two of Photosynthesis Action of Electron
Transport Chains

• H concentration higher on inside of thylakoid
  produces concentration gradient across thylakoid
  membrane.
• Results in H having the tendency to diffuse back
  out of thylakoid down their concentration
  gradient
• Occurs through specialized carrier proteins
  unique function as enzyme and carrier
  protein/ion channel

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Stage Two of Photosynthesis Action of Electron
Transport Chains

• As H ions pass through channel portion protein
  catalyzes a reaction in which a phosphate group
  is added to ADP makes ATP.
• Movement of H across thylakoid membrane through
  carrier proteins provides energy to make ATP used
  to power the third stage of photosynthesis

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Stage Two of Photosynthesis Action of Electron
Transport Chains

• A second ETC provides energy used to make NADPH
• NADPH an electron carrier that provides the
  high-energy electrons needed to make
  carbon-hydrogen bonds in third stage of
  photosynthesis
• Excited electrons combine with H as well as an
  electron acceptor called NADP forming NADPH.

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Stage Two of Photosynthesis Light-Dependent
Reactions

• Pigment molecules in thylakoids of chloroplast
  absorb light energy.
• Electrons in pigments excited by light and move
  through ETC in thylakoid
• These electrons replaced by electrons from water
  molecules, which are split by an enzyme
• Oxygen atoms from water molecules combine to form
  oxygen gas
• H ions accumulate inside thylakoids, setting up
  a concentration gradient that provides the energy
  to make ATP and NADPH.

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Stage Three of Photosynthesis

• Carbon atoms from CO2 in the atmosphere are used
  to make organic compounds which store chemical
  energy.
• Carbon dioxide fixation transfer of CO2 to
  organic compounds
• The reactions that fix CO2 often called dark
  reactions or light-independent reactions.

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Stage Three of Photosynthesis Calvin Cycle

• Calvin Cycle series of enzyme-assisted chemical
  reactions that produce a 3-carbon sugar. Figure
  5-9 p. 102
• STEP 1 Each molecule of CO2 is added to a
  5-carbon compound by an enzyme
• STEP 2 The 6-carbon compound splits into 2
  3-carbon compounds. Phosphate groups from ATP
  and electrons from NADPH are added to the
  3-carbon compounds. Forms a 3-carbon sugar.

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Stage Three of Photosynthesis Calvin Cycle

• STEP 3 1 3-carbon sugar is used to make an
  organic compound
• STEP 4 The other 3-carbon sugar is used to
  regenerate the 3-carbon compound used in the
  beginning of the cycle
• The reactions are cyclic because they recycle the
  5-carbon compound.
• 3 CO2 molecules must enter the cycle to make
  3-carbon molecules (6 are made)
• Energy is supplied by 6NADPH and 9 ATP molecules.

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• http//www.science.smith.edu/departments/Biology/B
  io231/calvin.html
• http//www.phschool.com/science/biology_place/bioc
  oach/photosynth/calvin1.html
• http//www.emc.maricopa.edu/faculty/farabee/BIOBK/
  BioBookPS.html

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Factors that Affect Photosynthesis

• Light the rate of photosynthesis increases as
  light intensity increases until all pigments are
  being used. Then the rate of photosynthesis
  levels off.
• Most efficient within a range of temperatures.
• Like all metabolic processes, involves many
  enzyme-assisted chemical reactions. Enzymes only
  react properly within certain temperature ranges.
• CO2 concentration once certain concentration of
  CO2 is present, cannot proceed any faster.