Photosynthesis and Cellular Respiration

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Photosynthesis and Cellular Respiration
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Photosynthesis

• Method of converting sun energy into chemical
  energy usable by cells
• Autotrophs self feeders, organisms capable of
  making their own food
• Photosynthesis takes place in specialized
  structures inside plant cells called chloroplasts
• Light absorbing pigment molecules (e.g.
  chlorophyll)

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Oxidation and Reduction

• Oxidation means that a reactant lose electrons
• Reduction means that a reactant gains electrons

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Overall Reaction

• 6CO2 6 H2O ? C6H12O6 6O2
• Carbohydrate made is glucose
• Water is split, resulting in the release of
  electrons and O2 as a byproduct

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Light-Dependent Reactions

• Overview
• Light energy is absorbed by electrons in the
  chlorophyll and boosts them to higher energy
  levels.
• Electrons are grabbed by other molecules
  (electron acceptors)
• The electrons fall to a lower energy state as
  they move from molecule to molecule, releasing
  energy that is harnessed to make ATP

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Energy Shuttling

• ATP cellular energy molecule with 3 phosphate
  groups bonded to it
• When the third phosphate group is removed, lots
  of energy is released!
• Other nucleotide based molecules move electrons
  and protons around within the cell
• NADP, NADPH
• NAD, NADP
• FAD, FADH2

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Light-dependent Reactions

• Photosystem light capturing unit, contains
  chlorophyll, the light capturing pigment
• Electron Transport Chain sequence of electron
  carrier molecules that shuttle electrons energy
  released is used to make ATP
• Light reactions yield ATP and NADPH used to fuel
  the reactions of the Calvin Cycle

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Step-by-Step

• Light energy excites electrons in Photosystem II
  and water molecules are split to provide
  additional electrons
• The excited electrons move along a sequence of
  electron carrier molecules in the thylakoid
  called the Electron Transport Chain (ETC)
• As they move, they lose energy which is used to
  move protons (H) into the thylakoid. This proton
  gradients lets ATP be made from ADP
• Electrons enter Photosystem I and are excited by
  more light energy
• The excited electrons move along another ETC
• This ETC moves the electrons close to the stroma
  where they combine with a proton and NADP to
  form NADPH
• Electrons from Photosystem II replace the ones
  used in Photosystem I. Water molecules are split
  to provide replacement electrons for Photosystem
  II.

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Calvin Cycle

• ATP and NADPH generated in light reactions are
  used to fuel the Calvin Cycle, reactions which
  take CO2 and break it apart, then reassemble the
  carbons into glucose.
• Carbon Fixation occurs
• Taking carbon from an inorganic molecule
  (atmospheric CO2) and making an organic molecule
  out of it (glucose)

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Step-By-Step

• CO2 diffuses into the stroma and combines each
  CO2 molecule with a five-carbon molecule (RuBP)
• The new six-carbon molecule is unstable and
  splits into two three-carbon molecules (3-PGA)
• Each three-carbon molecule is coverted into
  another three-carbon molecule (G3P). First, it
  receives a phosphate from ATP. Then it receives a
  proton from NADPH and releases a phosphate.
• One of the new three-carbon compounds leaves the
  cycle to make glucose.
• The remain three-carbon compounds are converted
  back into five carbon molecules (RuBP) through
  the addition of phosphates from ATP

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Harvesting Chemical Energy

• Plants and animals both use products of
  photosynthesis (glucose) for metabolic fuel
• Heterotrophs must take in energy from outside
  sources, cannot make their own e.g. animals
• When we take in glucose (or other carbs),
  proteins, and fats-these foods dont come to us
  the way our cells can use them

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Cellular Respiration Overview

• Transformation of chemical energy in food into
  chemical energy cells can use ATP
• These reactions proceed the same way in plants
  and animals. Process is called cellular
  respiration
• Overall Reaction
• C6H12O6 6O2 ? 6CO2 6H2O

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Cellular Respiration Overview

• Breakdown of glucose begins in the cytoplasm
• At this point life diverges into two forms and
  two pathways
• Anaerobic cellular respiration (aka fermentation)
• Aerobic cellular respiration

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Cellular Respiration Reactions

• Glycolysis
• Series of reactions which break the 6-carbon
  glucose molecule down into two 3-carbon molecules
  called pyruvate using ATP
• Process is ancient -all organisms from simple
  bacteria to humans perform it the same way
• Yields 2 ATP molecules for every one glucose
  molecule broken down
• Yields 2 NADH per glucose molecule

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Step-By-Step

• 2 ATP molecules attach to two phosphates to a
  glucose molecule, making a new six carbon
  compound
• The six-carbon compound is split into two
  three-carbon compounds (G3P)
• The two three-carbon compounds are oxidized and
  receive a phosphate and make a new three-carbon
  compound
• NAD is reduced to NADH
• The phosphate groups are removed, producing two
  molecules of pyruvic acid and 4 ATP are made

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Anaerobic Cellular Respiration

• Some organisms thrive in environments with little
  or no oxygen
• No oxygen used anaerobic
• Results in no more ATP, final steps in these
  pathways serve ONLY to regenerate NAD so it can
  return to pick up more electrons and hydrogens in
  glycolysis

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Lactic Acid Production

• After glycolysis, a hydrogen atom is transferred
  from NADH (oxidizing to NAD) and a free proton
  is added to pyruvic acid to form lactic acid
• NAD is used in glycolysis
• Fermentation can be used to produce cheese,
  yogurt, sour cream and more

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Alcoholic Fermentation

• After glycolysis, a CO2 molecule is removed from
  pyruvic acid, leaving a two-carbon compound
• Two hydrogen atoms, from NADH and a proton, are
  added to the two-carbon compound to form ethyl
  alcohol
• Alcoholic fermentation by yeast cells is the
  basis of the wine and beer industry
• Bread also rises due to CO2 production

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Aerobic Cellular Respiration

• Oxygen required aerobic
• 2 more sets of reactions which occur in a
  specialized structure within the cell called the
  mitochondria
• 1. Krebs Cycle
• 2. Electron Transport Chain

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Krebs Cycle

• Completes the breakdown of glucose
• Takes the pyruvate (3-carbons) and breaks it down
• The carbon and oxygen atoms end up in CO2 and H2O
• Hydrogens and electrons are stripped and loaded
  onto NAD and FAD to produce NADH and FADH2
• Production of only 2 more ATP but loads up the
  coenzymes with H and electrons which move to the
  3rd stage

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Step-By-Step

• A two-carbon compound (acetyl CoA) combines with
  a four-carbon compound (oxaloacetic acid) to make
  a six-carbon compound (citric acid)
• Citric acid releases a CO2 molecule and a
  hydrogen atom to form a five-carbon compound
• The hydrogen atom is transferred to NAD to make
  NADH
• The five-carbon compound releases a CO2 molecule
  and a hydrogen atom to form a four-carbon
  compound. NAD becomes NADH
• The four-carbon compound releases a hydrogen atom
  to form another four-carbon compound
• This hydrogen atom reduces FAD to FADH2
• The four-carbon compound releases another
  hydrogen atom to regenerate oxaloacetic acid and
  reduces NAD to NADH

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Krebs Cycle Outcome

• One glucose molecule is completely broken down
  after two turns of the Krebs Cycle
• Two turns produce four CO2 molecules, two ATP
  molecules and hydrogen molecules used to make six
  NADH and two FADH2
• CO2 diffuses as waste
• ATP is used for energy
• Add in the four NADH from glycolysis and
  conversion to pyruvic acid, and were ready for
  the next step!

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Electron Transport Chain

• Electron carriers loaded with electrons and
  protons from the Krebs cycle move to this
  chain-like a series of steps (staircase).
• As electrons drop down stairs, energy released to
  form a total of 32 ATP
• Oxygen waits at bottom of staircase, picks up
  electrons and protons and in doing so becomes
  water

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Step-By-Step

• Electrons in the hydrogen atoms from NADH and
  FADH2 are at a high energy
• NADH and FADH2 give up electrons to the ETC
• NADH donates them at the beginning
• FADH2 donates them midway
• The electrons move down the chain, loosing energy
• The energy creates a proton gradient and an
  electrical gradient from the positive charge
• ATP is generated from the gradients from ADP and
  phosphate
• Oxygen is the final acceptor of electrons that
  have passed down the chain. The protons,
  electrons, and oxygen combine to form water.

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Energy Tally

• 36 ATP for aerobic vs. 2 ATP for anaerobic
• Glycolysis 2 ATP
• Krebs 2 ATP
• Electron Transport 32 ATP
• 36 ATP
• Anaerobic organisms cant be too energetic but
  are important for global recycling of carbon

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