Photosynthesis

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Photosynthesis

• The Light and Dark Reactions
• (Chapter 10)
• http//www.wiley.com/college/boyer/0470003790/anim
  ations/photosynthesis/photosynthesis.htm

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Early Questions

• Question How can a plant grow from a seedling of
  a few grams to a tree of a several tons?
• Jan van Helmont conducts an experiment to find
  out (1600s)
• Measures the mass of a seed
• Measures the mass of a pot of soil
• Lets it grow for five years and measures the mass
  of the tree now at 75 kilograms
• Soil did not change in mass
• He assumes that the change in mass is due to the
  water.
• C.B. van Niel, Stanford
• O2 from H20 or C02?
• Melvin Calvin

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What is Photosynthesis?

• Definition of Photosynthesis
• In the green plants, the conversion of light
  energy from the sun into chemical energy stored
  in the bonds within carbohydrates-sugars and
  starches
• In other words Plants use sunlight to make sugar
  
• 30 to 40 chloroplasts per cell

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The Requirements of Photosynthesis

• Carbon Dioxide Water ? Glucose Oxygen
• 6CO2 6H2O Light? C6H12O6 6O2
• Sunlight is also required
• Autotrophs-use sunlight to make food
• Heterotrophs-obtain energy from food they eat
• Pigments are also required
• White light from the sun is actually a mixture of
  different color, or wavelengths of light
• Chlorophyll is very good at absorbing red and
  blue light (high and low ends of the spectrum).

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Light Spectrum
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Other Requirements of Photosynthesis

• Energy-Storing Compounds are also needed
• ATP (Adenosine triphosphate)
• The most important energy-storing compound
• Used by every living cell
• Made of the following
• A nucleotide called Adenine
• A sugar with 5 carbons in its ring called Ribose
• Three phosphate groups
• AMP P ADP P ATP
• Cellular uses of ATP
• Muscle contraction
• Protein synthesis
• Active transport
• The second part of photosynthesis
• NADP (nicotinamide adenine dinucleotide
  phosphate)
• High energy electron carrier/acceptor

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ATP
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The Light Reactions of Photosynthesis (Stage 1 of
2)

• The production of NADPH and ATP require sunlight
• The Light Reactions of Photosynthesis are the
  reactions that involve capturing the light energy
  and creating energy storing compounds.
• NADPH and ATP are not very stable and thus, dont
  hold the energy very long
• Mechanisms of the Light Reactions
• Take place in the photosynthetic membranes that
  contain chlorophyll
• These reactions can be divided into four main
  parts

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Light Reactions of Photosynthesis (Stage 1 of 2)

• Light Absorption
• Clusters of pigment molecules or Photosystems
  capture the light energy of the sun.
• There are two photosystems Photosystem I and
  Photosystem II
• Each photosystem contains hundreds of chlorophyll
  molecules and accessory pigments
• The accessory pigments are there to absorb the
  wavelengths in the spectrum that chlorophyll is
  not good at absorbing, thus, making it more
  efficient at capturing light energy.
• Energy from the sun excites molecules in the
  photosystems enough that an electron e- is torn
  away from its molecule, thus making it very
  energy rich.
• This electron is then passed along to the other
  pigment molecule until it reaches a special
  chlorophyll molecule that can actually process
  that energy.

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Light Reactions of Photosynthesis (Stage 1 of 2)

• Electron Transport
• The special molecule passes that electron off to
  special electron carriers in the photosynthetic
  membrane, called electron transport and the
  actual electron carriers, are the electron
  transport chain.
• At the end of this chain, high energy electrons
  are passed to NADP, where the plus signifies an
  atom without an electron, thus, an atom very
  eager to pick up a new electron.
• The high energy electron bonds to the NADP to
  form NADPH

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Light Reactions of Photosynthesis (Stage 1 of 2)

• Oxygen Production
• Light continues to strip electrons away from the
  chlorophyll
• These electrons are replaced by the breaking
  apart of 2 molecules of H2O
• 4 electrons are taken away from the 2 molecules
  of water leaving you with 4 H ions and O2 or
  oxygen gas

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Light Reactions of Photosynthesis (Stage 1 of 2)

• ATP Formation
• All the remaining H ions start to pool on the
  inside of the photosynthetic membrane.
• This pooling causes a distinct difference in
  charge between the inside and outside of the
  membrane.
• This difference in charge creates enough energy
  to help ATP synthetase (the enzyme that makes ATP
  from ADP and P) bond an extra phosphate to ADP in
  order to form ATP.

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Summary of The Light Reactions

• Use
• Water,
• ADP
• NADP
• Produce
• O2
• ATP
• NADPH

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The Dark Reactions of Photosynthesis The Calvin
Cycle or Light Independent Reactions (Stage 2 of
2)

• The reactions of photosynthesis that do not
  require sunlight but they do take place in light.
• This stage deals with the use of NADPH and ATP in
  the formation of glucose.
• Glucose is far more stable than NADPH and ATP.
• Glucose (a single molecule) can hold
  approximately 100 times more energy than a
  phosphate group in ATP does.

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The Dark Reactions of Photosynthesis (Stage 2 of
2)

• Mechanisms of the Dark Reactions (also known as
  the Calvin Cycle)
• CO2, or carbon dioxide is used in the Dark
  Reactions to form complex organic molecules
• 3 Reactions take place
• 1st Reaction A five carbon sugar C5 combines
  with CO2 to form two 3-Carbon sugars (2C3)
• 2nd and 3rd Reactions The energy stored in ATP
  and NADPH are used to convert the 3-carbon sugars
  into what is called PGAL or phosoglyceraldehyde.
  This is the building block for glucose
• Other intermediate compounds formed during the
  Calvin Cycle are used for
• Form sugars other than glucose
• Used to form amino acids
• Some converted to lipids

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Glycolysis, Respiration and Fermentation
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Glycolysis

• Breaking Down Glucose
• Glucose is a 6 carbon sugar (think hexagon!)
• If glucose completely breaks down in the presence
  of oxygen? CO2 and H2O are produced
• C6H12O6 6O2 ? 6CO2 6H2O
• This reaction gives off 3811 calories per gram of
  glucose (3.8 Kcal)
• Glycolysis takes place in the cytoplasm of the
  cell

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Mechanics of Glycolysis

• Glucose undergoes chemical reactions ? Split into
  two 3-carbon PGAL molecules (2 ATP used in the
  process)
• 2 PGAL molecules undergo chemical reactions ?
  transform them into 2 molecule of pyruvic acid, a
  3 carbon compound
• The energy is used to make
• 4 molecules of ADP
• 2 molecules of NADH (Similar to NADPH-electron
  storage)
• There are 4 ATP synthesized overall, but two are
  used initially ? NET GAIN of 2 ATP

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Respiration

• The breaking down of food molecules into energy
• Only takes place if oxygen is available (thus
  called an aerobic process)
• Takes place in the mitochondria of cells
• 1st reactions take place inside the enclosed
  inner membrane
• 2nd reactions take place in the actual membrane
• Utilizes the pyruvic acid generated in Glycolysis
  in order to make 34 additional ATP
• Consists of 2 sets of reactions

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The Krebs Cycle (a.k.a.- the citric acid cycle)

• There is no definitive end ? These processes are
  cyclical
• Mechanics of Krebs Cycle
• Pyruvic acid is broken down into CO2 and a
  2-carbon acetyl group (briefly bound to coenzyme
  A)
• Acetyl-coenzyme A complex passes the 2 carbons of
  the acetyl group in to the Krebs cycle where they
  join w/a 4-carbon compound to produce citric
  acid, a 6-carbon compound
• 9 reactions and 9 intermediates are found in the
  Krebs cycle
• CO2 released at 2 places (the carbon comes from
  glucose)
• High energy electrons are accepted at 4 places

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The Krebs Cycle (a.k.a.- the citric acid cycle)

• Summary of Events
• 2 carbon atoms added (from the breakdown of
  pyruvic acid)
• 2 carbon atoms removed (in two molecules of CO2
• 3 molecule of NAD converted to NADH
• 1 molecule of FAD converted to FADH2
• 1 molecule of GDP converted to GTP

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Electron transport in the mitochondria

• NADH and FADH2 are passed through transport
  enzymes (inner membrane)
• End of the chain
• Enzyme combines high energy electrons, hydrogen
  ions (H) and oxygen to form H2O, or water
• Oxygen is the final electron acceptor in this
  process

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ATP formation

• Similar to chloroplast
• Difference in charge generates the combining of
  ADP and P into ADP
• Energy Totals for Glucose Breakdown
• ATP Formation
• 6 ATP formed during Glycolysis
• 30 ATP formed during Respiration
• 36 ATP represents 37 of total energy from
  glucose
• More efficient than an automobile

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Obtaining Energy from food

• Lipids and Proteins can also be used as fuel
  sources
• They can be broken down into molecules that enter
  Glycolysis or respiration at different points
• Not as efficient at producing energy.
• Energy in Balance
• Photosynthesis makes that glucose, Glycolysis and
  Respiration break it down

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Fermentation

• Production of ATP in the absence of oxygen
  (anaerobic)
• Needs to convert NADH back to NAD
• Two types of Fermentation
• Lactic Acid Fermentation
• Pyruvic acid accumulates ? Glycolysis turns this
  into lactic acid
• Regenerates NAD so Glycolysis can continue
• Pyruvic acid NADH ? lactic acid NAD
• This is seen in activities that fatigue the
  muscles faster than oxygen can get to them to in
  order to produce all the ATP necessary
• Alcoholic Fermentation
• Pyruvic acid (3-carbon compound) is broken down
  into a 2-carbon compound and CO2
• Alcohol is a byproduct
• Pyruvic acid NADH ? alcohol CO2 NAD

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Glycolysis 1
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Glycolysis 2
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Carotene
Xanthophyll
Chlorophyll a
Chlorophyll b