INTRODUCTION TO CELLULAR RESPIRATION

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INTRODUCTION TO CELLULAR RESPIRATION

• The majority of organisms on earth use glucose as
  their main energy source. Through a series of
  redox reactions glucose is broken down and free
  energy is released
• Aerobic Cellular Respiration is the most often
  used method of converting glucose to free energy.
• Aerobic means that oxygen is used in the process.
  Respiration does not refer to the act of
  breathing or gas exchange in the lungs, but the
  20 or so reactions that take place to free up the
  energy in glucose.

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OXIDATION OF GLUCOSE

• Oxygen oxidizes the C-H in glucose (and fatty
  acids) in two ways
• If you look at the 12 H in glucose they are
  broken away to form water. The electrons in a
  C-H bond are equally shared, in water the
  electrons are drawn closer to oxygen, therefore H
  is oxidized (LEO). The electrons move from a
  less electronegative atom (H) to a more
  electronegative one (O).
• The same thing happens to the 6C glucose who are
  drawn into 6CO2.
• Overall the oxidation of glucose moves electrons
  from a higher free energy state to a lower free
  energy state, thereby decreasing potential energy.

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• Oxidation of glucose produces 2870 kJ/mol of
  glucose _at_ 25 degrees Celsius and 101.3 kPa (Lab
  conditions 3012 kJ/mol in a real cell).
• About 34 of this energy is trapped by the cell
  and used to fuel endergonic processes. The rest
  dissipates as heat or light.
• Figure 2 on page 92.
• Oxygen and glucose are stable molecules. They do
  not readily react with one another. Lots of
  activation energy is needed (flame in lab
  conditions).
• Enzymes catalyze each reaction step thereby
  reducing the activation energy and making it
  easier for the cell to undergo aerobic cellular
  respiration.

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AEROBES AND ANAEROBES

• Oxygen is not the only primary electron acceptor
  at the end of the respiration process, other
  molecules such as NO2, SO4, CO2, and Fe3 are
  used in some forms of bacteria to help undergo
  respiration (obligate anaerobes)
• Animals are obligate aerobes since they use
  oxygen as their final electron acceptor.
• Organisms that can tolerate the presence and
  absence of oxygen are called facultative aerobes
  (mostly bacteria).

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AEROBIC RESPIRATION

• In aerobic respiration there are three main
  goals
• break the bonds of glucose freeing the carbon to
  make CO2
• break the bonds of glucose freeing H to form
  water
• to trap as much free energy as possible in the
  form of ATP
• The entire process occurs in 4 main stages
  Glycolysis, Pyruvate Oxidation, Krebs Cycle and
  the Electron Transport Chain (Figure 1 on page
  94) and involves 2 types of phosphorylation
  substrate-level phosphorylation and oxidative
  phosphorylation.

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Two Types of Phosphorylation

• Substrate-Level Phosphorylation is the formation
  of ATP directly in an enzyme-catalyzed reaction.
  A phosphate containing compound transfers its
  phosphate group to ADP (forming ATP) directly on
  an enzyme.
• 4 molecules of ATP are formed this way in
  glycolysis and 2 in the Krebs cycle for every one
  glucose.
• Oxidative Phosphorylation is the indirect
  formation of ATP. It involves a series of redox
  reactions in which oxygen is the final electron
  acceptor. It is more complex than
  Substrate-Level Phosphorylation and therefore
  creates more ATP.
• The reduction of the electron carrying molecules
  NAD and FAD to NADH and FADH2 are energy
  harvesting reactions that will transfer the
  majority of their free energy to the creation of
  ATP.

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GLYCOLYSIS

• Name means sugar-splitting
• First 10 reactions of cellular respiration
• It occurs in the cytoplasm and is anaerobic.
• Each reaction is catalyzed by a specific enzyme.
• (The reactions are shown in figure 11 on page 98,
  as well as your handouts).
• Glycolysis produces 2.1 of the entire free
  energy of glucose in aerobic cellular
  respiration.
• Glycolysis is thought to have been the earliest
  form of energy metabolism.

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Steps in Glycolysis

• Steps 1-5 Two ATP are used (step 1 and step 3).
  This primes glucose for cleavage in steps 4 and
  5. (6 carbons)
• Step 4/5 Fructose 1, 6-biphosphate is split into
  dihydroxyacetone phosphate (DHAP) and
  glyceraldehyde 3-phosphate (G3P) and immediately
  the enzyme isomerase changes DHAP into G3P. (3
  carbons)
• Steps 6 through 10 happen twice (one for each
  molecule of G3P).
• In step 6, NAD is reduced to NADH H
• Step 7 Two ATP molecules are produced by
  substrate level phosphorylation. One for each
  1,3-bisphophoglycerate (BPG) processed.
• Step 10 Phosphoenolpyruvate (PEP) is converted
  into pyruvate this produces ATP by
  substrate-level phosphorlyation. (3 carbon)
• Net-reaction glucose 2ADP 2P 2NAD --gt 2
  pyruvate 2ATP 2NADH 2H