Photosynthesis: Using Light to Make Food

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Lecture 13

• Chapter 7
• Photosynthesis Using Light to Make Food

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Learning Objectives (1 of 3)

• Define
• Autotrophs
• Chemotrophs
• Algae
• Discuss photosynthesis in a leaf
• Recall
• Complete photosynthesis equation
• Source of oxygen produced

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Learning Objectives (p. 2 of 3)

• Draw, label, and discuss a picture of a
  chloroplast
• Compare/contrast
• Balanced photosynthetic equation vs. previous
  classical view from school texts
• Chemiosmosis in mitochondria thylakoids
• Explain why plants are green

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Learning Objectives (3 of 3)

• Discuss
• The Calvin Cycle
• Water-saving measures of desert plants
• Photosynthesis stopping global warming
• Contrast
• Calvin cycle light reactions

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Autotrophs

• From the Greek self-feeder
• These are the producers for every ecosystem
• Vast majority use the sun
• Photosynthetic

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Who are the Autotrophs?

• Two types
• Chemosynthetic organisms
• Live in total darkness near hydrothermal vents in
  ocean bottom
• Photosynthetic organisms
• Live where they can get sunlight
• Most plants
• Some bacteria
• Some Archaea
• Some protists (what?)

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Photosynthetic Protists?

• In a word, the algae!
• Not plants!
• Like plants, they do photosynthesize
• But, unlike them, some can even swim!

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Photosynthesis in a Leaf

• Stomata pores in leaf, let CO2 in, O2 out
• Chloroplasts

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Chloroplast Structure

• Occur in mesophyll (middle layer of leaf)
• Have double membrane inner outer (similar to
  mitochondria)
• Stroma thick liquid, where CO2 turned into
  sugars
• Have many grana

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Details on Grana

• Each granum stack of thylakoids
• Each thylakoid is a disk-like sac of membrane
• Chlorophyll exists in membranes

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Classical View of Photosythesis

• CO2 water ? sugars oxygen
• 6 CO2 6 H20 ? C6H12O6 6O2
• Problem These equations leave out 6 water
  molecules!

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More Complete Photosynthesis Equation

• 6CO2 12H2O ? C6H12O6 6H2O 6O2
• This equation includes 6 additional water
  molecules on each side
• Water is actually both a product and a reactant!
• Less complete equation only concerned with net
  gain or loss of water

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Oxygen Origins

• Fact Photosynthesis produces oxygen gas (O2)
• So, where do oxygen atoms come from?
• Two possibilities CO2 or H20

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O2 Comes from Water

• Shown by experiment!
• Radioactive oxygen put into CO2
• Radioactivity in reappears in sugar, water
• Radioactive oxygen put into H20
• Radioactivity reappears in oxygen gas

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Follow the Oxygen
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Follow the Oxygen
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Why Plants are Green

• White light has all colors
• Chlorophyll absorbs mostly blue, yellow, orange,
  red
• Green is reflected

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Where is this Happening?
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Chemiosmosis in Thylakoids
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Chemiosmosis in Thylakoids

• Chemiosmosis Production of ATP using H
  gradients (either mitochondria or chloroplasts)
• Photophosphorylation Attachment of a 3rd
  phosphate to ADP, using light energy
• Occurs in the thylakoid membrane within
  chloroplasts

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Graphical Comparison
Mitochondrion
Thylakoid
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Comparison of Chemiosmosis
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Light Reactions Video
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7th Inning Stretch
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Photosynthesis Overview

• Photosynthesis in two parts
• Light reactions produce ATP, NADPH
• Calvin cycle produces G3P (sugar precursor)

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Question Where Does Each Occur?
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Question What are Plants Doing with Sugars?
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The Calvin Cycle

• CO2 is made into sugars
• Input CO2, ATP, and NADPH
• Output G3P (Sugar precursor molecule)
• Cell processes it into sugar

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Calvin Cycle Video
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Plants in Deserts

• Normally, plants must use light reactions
  Calvin cycle together
• Problem this puts desert plants in danger
• CO2 is needed for Calvin cycle
• Must come through stomata
• But, open stomata lose water!

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Water-saving Measures

• Cacti well-adapted to dry environs
• Need to keep stomata closed during day
• Advantage Prevents desiccation
• Disadvantage Also prevents CO2 fixation

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Solution CAM

• Crassulacean Acid Metabolism
• Allows cacti friends to run some reactions
  during day, others at night
• Can open stomata at night, when desiccation risk
  is reduced

Regular Plant
CAM Plant
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CO2 A Greenhouse Gas

• CO2 is a greenhouse gas
• Helps to trap heat in atmosphere
• Too much is bad
• We may have too much

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Question for Discussion

• How many ways do we have to remove carbon dioxide
  from the air?

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Photosynthesis a Global Perspective

• Photosynthesis removes carbon dioxide for free!
• Deforestation destroys this abilityhow does it
  occur?
• Developing nations have very high birth rates
• Need for food great
• Slash and burn agriculture practiced
• Destroys forest
• Releases carbon dioxide