Chapter 10 Photosynthesis

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Chapter 10 Photosynthesis
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Photosynthesis

• Process by which plants use light energy to make
  food.
• A reduction process that makes complex organic
  molecules from simple molecules.

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Ps General Equation

• 6 CO2 6 H2O ---gt C6H12O6 6 O2
• Requires
• Chlorophyll
• Light

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Early Question

• Does the Oxygen in sugar come from the CO2 or
  from the H2O ?
• Model
• CO2 2 H2O ----gt CH2O O2

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Proof

• Used 18O as a tracer.
• CO2 2 H2O ----gt CH2O O2
• CO2 2 H2O ----gt CH2O O2
• Both experiments confirm that water is split.
• O2 is a waste product of Ps that altered life on
  earth.

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Ps a redox process

• Hydrogens are added to Carbons.
• Water is a source for the Hydrogens.
• Complex covalent bonds are made.

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Ps

• Has two chemical reactions
• 1. Light Reaction
• 2. Dark Reaction
• Names are from light as a requirement, not
  where or when they occur.

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Light

• A form of electromagnetic radiation.
• Visible light has the right energy for use in Ps.

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Too Hot
Too Cold
Just Right
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Action Spectrum

• Not all colors are useable to the same degree for
  Ps.
• Red and Blue light - absorbed and used in Ps.
• Green light - reflected or transmitted.

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Comment

• In oceans, red light is lost or filtered out
  early because it has lower energy.
• Only blue light which has higher energy can reach
  the lower depths.

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Result

• Many deep ocean fish are bright red in color.
• Why?
• They cant be seen because there is no red light
  to reflect their color.

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Photosynthesis Pigments

• 1. Chlorophylls
• 2. Accessory Pigments

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Chlorophylls

• Has CHON and Mg.
• Several types possible.
• Molecule has a lipophilic tail that allows it to
  dissolve into membranes.
• Contains Mg in a reaction center.

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Accessory Pigments

• Absorb light energy and transfer it to
  chlorophyll.
• Ex Carotene (orange). Xanthophyll
  (yellow)

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Fall Leaf Colors

• Chlorophyll breaks down.
• N and Mg salvaged and moved into the stem for
  next year.
• Accessory pigments remain behind, giving the
  various fall leaf colors.

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Chloroplast Structure

• Double outer membrane.
• Inner membrane folded and stacked into grana.
• Stroma - liquid that surrounds the thylakoid
  membranes.

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Photosystems

• Collection of pigments that serve as a light
  trap.
• Made of chlorophyll and the accessory pigments.
• Two types known PSI, PSII

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Cyclic Photophosphorylation

• Uses PSI only.
• Produces ATP.
• Requires light.

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Noncyclic Photophsphorylation

• Uses PSI and PSII.
• Splits water, releasing H, a pair of e-,
  and O2.
• Produces ATP and NADPH. (e- carrier similar to
  NADH)

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Light Reaction
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Light Reaction

• Same thing as Noncyclic Photophsphorylation.
• Location - grana of the chloroplast.
• Function - to split water and produce ATP and
  NADPH.

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Light Reaction
Requirements
Products

• Light
• Water
• ADP Pi
• NADP

• O2
• ATP
• NADPH

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Chemiosmosis Model

• The chloroplast produces ATP in the same manner
  as the mitochondria in Rs.
• Light energy is used to pump H across a
  membrane.
• When the H diffuses back, ATP is generated.

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Chemiosmosis

• H are pumped into the thylakoid space.
• ATP and NADPH are made when the H diffuse into
  the stroma.

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Comment

• There can be a 3 pH unit difference between the
  thylakoid space and the stroma.

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Dark Reactions

• How plants actually makes food (carbohydrates).
• Don't require light directly to run.
• Also known as the Calvin cycle or C3
  Ps.

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Dark Reaction

• Function - to use ATP and NADPH to build food
  from CO2
• Location - stroma of the chloroplast.

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Rubisco

• Ribulose BisPhosphate Carboxylase.
• Enzyme that adds CO2 to an acceptor molecule.
• Most important enzyme on earth.

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C3 Ps
Requirements
Products

• 6 CO2
• 18 ATP
• 12 NADPH

• C6H12O6
• 18 ADP 18 Pi
• 12 NADP

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Photorespiration

• When Rubisco accepts O2 instead of CO2 as the
  substrate.
• Generates no ATP.
• Decreases Ps output by as much as 50.

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Photorespiration

• May reflect a time when O2 was less plentiful and
  CO2 was more common.

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Homework

• Read Chapter 10, 48
• Lab this week Ps lab
• Chapter 10 Mon. 11/5

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Alternate Ps Methods

• 1. C4 Ps
• 2. CAM Ps

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C4 Ps

• Uses a different enzyme to initially capture CO2
• Separates CO2 capture from carbon fixation into
  sugar.
• Still uses C3 Ps to make sugar, but only does so
  in the bundle sheath cells.

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PEP Carboxylase

• Enzyme used for CO2 capture in C4 Ps.
• Can use CO2 down to 0 ppm.
• Prevents photorespiration.

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C4 Ps

• Found in 19 plant families.
• Characteristic of hot regions with intense
  sunlight.
• Examples - sugarcane, Bermuda grass, crab grass

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C3 Ps vs C4 Ps

• Photorespiration
• Shade to full sun
• High water use
• Cool temperatures
• Slow to moderate growth rates
• Cool season crops

• No Photorespiration
• Full sun only
• Moderate water use
• Warm temperatures
• Very fast growth rates
• Warm season crops

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CAM Ps

• Crassulacean Acid Metabolism
• Found in plants from arid conditions where water
  stress is a problem.
• Examples - cacti, succulents, pineapples, many
  orchids.

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CAM Ps

• Open stomata at night to take in CO2.
• The CO2 is stored as a C4 acid.
• During the day, the acid is broken down and CO2
  is fixed into sugar.

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CAM plants

• Tissues decrease in pH over night, rise in pH
  during day.
• Avoid H2O stress by keeping stomates closed
  during the day.
• Generally have slow growth.

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PsRs Ratios

• Reflect a plants balance in making food and
  using food.
• 1. Ps gt Rs, energy available for growth and
  reproduction.
• 2. Ps Rs, no growth, but dont die either.
• 3. Ps lt Rs, death by starvation

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Comments - PsRs

• Rs happens 24 hours a day.
• Ps only in light.
• Plants overwinter on stored food made when Ps gt
  Rs.
• If Ps lt Rs, best solution is to increase the
  amount of light.

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Factors That Affect Ps

• 1. Light - quantity and quality.
• 2. Temperature - too hot or too cold.
• 3. CO2 - often limits C3 plants.
• 4. Minerals - especially NPK and Mg.

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Importances of Ps

• 1. Food - either directly or indirectly comes
  from plants.
• 2. Oxygen in the air.
• 3. CO2 balance.
• 4. Plant products.
• 5. Life on Earth.

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Summary

• Know the main Ps equation.
• Know Light Reaction.
• Know Dark Reaction.
• Alternate Ps forms.
• PsRs ratios.

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Practice Question

• Explain how the molecular reactions of
  photosynthesis transform light energy into
  chemical bond energy. Include in your discussion
  the relationship between chloroplast structure
  and light and dark reactions.