Photosynthesis

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Photosynthesis
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By the end of this class you should understand

• How energy may be stored in chemical bonds
• Electron transport as a means of transforming
  energy
• The complete process, reactants, and products of
  photosynthesis
• The essential purposes of the light and dark
  reactions

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Transforming Energy

• According to the laws of thermodynamics, energy
  is always lost as we do things
• Moving around, producing body heat, etc.
• The total amount of energy in all living things
  is constantly decreasing
• Where is fresh energy coming from?

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New Energy

• Life gains new energy from the sun!
• Light is a type of energy that can move through
  empty space
• The sun is spraying massive amounts of light in
  all directions, and the Earth is getting
  bombarded with it
• Life took off after developing a way to directly
  convert this light energy

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Photosynthesis

• Photosynthesis is a process where nonliving
  materials are converted into living materials
  using light energy
• Please note the light itself is not turned into
  matter, only the energy is being used to do this
• The first organisms to do this were ancient
  prokaryotes we call cyanobacteria

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Photosynthetic Organisms

• The ocean is still full of cyanobacteria!
• In the ancient past, some eukaryotic cells
  captured cyanobacteria and converted them into
  organelles called chloroplasts
• These were the ancestors of plants and algae
• This probably happened multiple times, since red
  algae and green algae are not closely related!

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Energy Essentials

• The key to understanding energy in living things
  is that chemical bonds have energy stored in them
• Some bonds have much more energy than others
• The third phosphate in ATP is attached to the
  second phosphate with a very high-energy bond
• Many other bonds are also high-energy, such as
  the bonds between carbon atoms in sugar

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Energy Storage

• Photosynthesis is essentially the process of
  turning carbon dioxide (low energy) into sugar
  (high energy)
• This is accomplished by using the energy of
  sunlight
• Water is also a raw material that is used up
• This is why your plants get thirsty!

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Two Tools of Photosynthesis

• Enzymes
• Many reactions, including the actual capture of
  carbon dioxide, are performed with enzymes
• Electron Transport
• This is a special type of process where energy is
  transformed from one kind to another

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Electron Transport Analogy

• An electron transport chain works similarly to a
  water wheel in a river
• The energy of water running downstream is
  transformed into mechanical energy when it pushes
  on the water wheel
• An electron transport chain allows a high energy
  electron to move from one protein group to the
  next, and it transforms the energy in the process
• Allowing the electron to do work!

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Cell Membranes

• Electron Transport Chains have many different
  proteins that must bump into each other in the
  right order
• They are floating in a cell membrane and shaped
  so each will only interface with the next one in
  line
• Remember the cell membrane is a fluid, so the
  proteins can move freely in two dimensions

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Electron Transport

• By moving a high-energy electron through the
  protein structure, different types of work can be
  done
• One major type of action that can occur is a
  proton can be moved from one side of the membrane
  to the other
• Remember, protons and electrons are attracted to
  each other!

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Proton Gradient

• If a larger amount of something is on one side of
  a membrane than the other, it is referred to as a
  concentration gradient
• With protons, this is specifically referred to as
  a proton gradient
• An electron transport chain can be used to build
  a proton gradient
• Why would you do this?

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Proton Gradient Use

• A proton gradient is a very useful tool thanks to
  this magic molecule ATP Synthase
• ATP Synthase is like a turnstile that will only
  turn when protons move down the gradient
• Also known as chemiosmosis
• As they turn the ATP synthase, the movement of
  the enzyme turns ADP and phosphate into ATP
• This is THE most efficient way to make ATP

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ATP Synthase in Action
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Photosynthesis

• Photosynthesis is made up of two stages
• Light-dependent (light) reactions
• Light-independent (dark) reactions
• The light reactions actually trap the energy in a
  photon (particle of light)
• The dark reactions dont actually have to take
  place in the dark, but they are driven by enzymes
  so they dont need light
• Cactuses do some of their reactions at night
  since its too hot during the day!

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Light-Dependent Reactions

• The light-dependent reactions begin when a photon
  hits a pigment molecule called chlorophyll
• Chlorophyll molecules are arranged into a
  structure called a photosystem, embedded in a
  chloroplast membrane
• The energy from the photon excites a photon into
  entering an electron transport chain

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Free Electrons?

• Once the chlorophylls electron has been stripped
  from its magnesium atom, it must reload
• The way it reloads an electron is by taking one
  from a water molecule
• This causes water molecules to turn into oxygen,
  which is a waste product, and hydrogen, which is
  used in the proton gradient
• Youre breathing in plant farts right now!

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Further Light-Dependent Reactions

• The complete photosynthesis chain has two
  photosystems
• At the conclusion of the light reactions the
  electron has released its light energy by pumping
  protons and is contained in a molecule called
  NADPH
• NADPH is an electron carrier, once it releases
  electrons it turns back into NADP and an H

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Light-Independent Reactions

• The light-independent reactions use the energy
  acquired in the light-dependent reactions to make
  energy-storing molecules
• ATP has a short shelf life, and will decay on its
  own
• NADPH are only a short-term solution, and having
  stray electrons in your cells is no bueno

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Acquiring Carbon

• Carbon for the sugar is acquired by a process
  called carbon fixation
• Another magic enzyme performs this difficult
  task, this time an enzyme called Ribulose
  1,5-bisphosphate carboxylase
• RUBISCO
• Carbon fixation is difficult even with rubisco,
  so most plants just produce a ton of it
• Estimated to be the most common enzyme on the
  planet Earth!

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Carbon Fixation

• Rubisco grabs onto a carbon dioxide and attaches
  it to a sugar molecule called RuBP (ribulose
  1,5-bisphosphate)
• This new 6-carbon sugar immediately splits into
  two 3-carbon sugars
• Some of these 3-carbon sugars are re-converted
  back to RuBP
• Extra 3-carbon sugars are used to make other
  sugars and other plant/bacteria structures

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Calvin-Benson Cycle

• The carbon fixation and enzymatic regeneration of
  RuBP is called the Calvin-Benson Cycle, or just
  Calvin Cycle (because no one cares about Andrew
  Benson)
• It is accomplished with enzymes, and with the ATP
  and NADPH produced during the light reactions
• We will NOT be memorizing any of that bull

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Acquiring Carbon Dioxide

• Carbon dioxide must be pulled in from the air
  (and oxygen must be allowed to escape)
• Plants have tiny windows called stomata that can
  open and close to prevent excess evaporation of
  water
• If O2 builds up it can block rubisco, so plants
  can suffocate on oxygen!
• Drought-resistant plants have chemical tricks to
  prevent O2 buildup

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Summary of Photosynthesis

• Light hits photosystem, energizing electron
• Electron goes through transport chain, building
  proton gradient
• Electron is trapped in NADPH ATP Synthase makes
  ATP using proton gradient
• Rubisco grabs CO2 and attaches it to RuBP, making
  two 3-carbon sugars
• Most 3-carbon sugars are remade into RuBP using
  ATP and NADPH extras become sugar

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See you in lab!

• WARNING The photosynthesis lab will fall AFTER
  the first exam
• Will be covered again on lab quiz and again on
  cumulative final exam!