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CELLULAR RESPIRATION
Metabolic Processes Strand
1. It is a redox reaction. C6H12O6 + 6O2 6CO2 + 6H2O + energy (ATP) - glucose is oxidized, oxygen is reduced - e- lose potential energy along the way - energy foods (carbohydrates / fats) are reservoirs
of e- associated with hydrogen and only the activation energy barrier holds back the “flood” of e- to a lower energy state
2. It involves an electron transport chain . Also involved are co-enzymes that accept H+ and e-.
NAD+ nicotinamide adenine dinucleotide FAD flavin adenine dinucleotide - electron transfer from NADH and FADH2 (reduced
forms) to oxygen, the terminal e- acceptor, is exergonic
- the electron “route” is :
food NADH / FADH2 ETC oxygen
3. Starts after the ingestion or creation / conversion of glucose, and is an accumulation of four metabolic stages :
• GLYCOLYSIS
glucose (6C) 2 pyruvate (3C) + ATP + NADH • PYRUVATE OXIDATION (OXIDATIVE DECARBOXYLATION)
pyruvate acetyl CoA + NADH + CO2 • KREBS CYCLE
acetyl CoA CO2 + ATP + NADH + FADH2 • ETC AND OXIDATIVE PHOSPHORYLATION
e- + H+ + O2 H2O + ATP
• the making of ATP in cellular respiration is called oxidative phosphorylation
• 90% of ATP generated by cell respiration is through this process
• there is a small amount of ATP formed directly in a few reactions of glycolysis and the Krebs Cycle by a mechanism called substrate-level phosphorylation
GLYCOLYSIS
OVERVIEW :
glucose (6C) 2 pyruvate (3C) + ATP + NADH
LOCATION :
cytosol of cytoplasm
- glycolysis is 10 steps, each catalyzed by a different specific enzyme found in the cytoplasm
- 2 phases : - energy investment phase (stages 1-5) • energy used to phosphorylate food molecules - energy payoff phase (stages 6-10) • ATP produced by substrate-level phosphorylation • NAD+ reduced to NADH, stored reducing power - no carbon is released during reactions - pathway not dependent on oxygen
PYRUVATE OXIDATION
OVERVIEW :
pyruvate acetyl CoA + NADH + CO2
LOCATION :
between cytoplasm and matrix of mitochondrion
- as a pyruvate molecule enters the matrix of a mitochondrion, it is converted to an acetyl coenzyme A molecule by a “multienzyme complex”
• 3 steps : 1. the enzyme pyruvate decarboxylase removes the carboxyl
group of pyruvate as a CO2 molecule, and it diffuses out of the cell
2. the remaining 2C fragment is oxidized to become acetate
and in the process, NAD+ is reduced to NADH 3. an available coenzyme A molecule attaches to acetate - acetyl coA is very unstable and reactive
THE KREBS CYCLE
OVERVIEW :
acetyl CoA CO2 + ATP + NADH + FADH2
LOCATION :
matrix of mitochondrion
the Krebs cycle is 8 steps, each catalyzed by a specific enzyme found in the matrix
- for each “turn” of the cycle : • 2 carbons in a reduced form (acetate) enter (actually, acetyl CoA enters but since it is so
unstable it quickly breaks down) • 2 carbons in an oxidized form (CO2) leave - as acetyl CoA enters, its unstable CoA bond is
broken and CoA leaves, ready to prime another 2 carbon fragment derived from pyruvate
ETC AND OXIDATIVE PHOSPHORYLATION
OVERVIEW :
e- + H+ + O2 H2O + ATP
LOCATION :
inner membrane of the mitochondrion
- so far only 4 ATP generated per glucose molecule
- NADH and FADH2 contain stored energy : they donate high-energy e- to the ETC to generate ATP (indirectly)
- the ETC is a collection of molecules embedded in the inner membrane of the mitochondrion
- most are proteins
- bound to these proteins are non-protein (prosthetic) groups which are needed by enzymes
- during e- transport, these prosthetic groups alternate between reduced / oxidized forms as they accept / donate e-
- the components of the ETC are arranged in order of increasing electronegativity
- the ETC involves 3 proton pumps :
• NADH dehydrogenase
• cytochrome b-c1 complex
• cytochrome oxidase complex
- between the pumps are 2 mobile e- carriers :
• ubiquinone (Q)
• cytochrome c
- the last component of the ETC, the cytochrome oxidase complex, passes the e- to oxygen, the terminal e- acceptor, which picks up a pair of H+ from the aqueous solution to form H2O
- for every 2 NADH molecules donated to the ETC, one O2
molecule is reduced to 2 H2O molecules - note : the ETC makes NO ATP directly; rather, it creates a
pH (proton) gradient - the ATP generator is the process of oxidative
phosphorylation • HOW IS THIS DONE? chemiosmosis
CHEMIOSMOSIS - chemiosmosis is an energy-coupling mechanism - the ETC in the mitochondrial inner membrane is
designed so that H+ are pumped from the matrix into the intermembrane space to create a pH gradient
● HOW? - certain members of the chain HAVE TO accept and
release H+ along with e- while others only transport e-
- these members pick up H+ from the matrix and move them into the intermembrane space
- a protein complex called an ATP synthase is built into the inner membrane which couples the passive diffusion of H+ back across the inner membrane to the matrix to the phosphorylation of ADP
● HOW?
- how the ATP synthase uses the pH gradient across the membrane to attach P to ADP is still relatively UNKOWN - H+ directly involved / induces shape change of
ATP Synthase?
PRODUCTION OF ATP WITHOUT OXYGEN
• aerobic : with oxygen
• anaerobic : without oxygen
- how do cells produce ATP in the absence of oxygen, the terminal e- acceptor?
- fermentation can generate ATP by substrate-level phosphorylation AS LONG AS there is a sufficient supply of the oxidizing agent NAD+ to accept e-
- under aerobic conditions, NAD+ is recycled from NADH when NADH’s e- are transferred to the ETC and end up joined with ½ O2 and 2 H+ as H2O
* important – cells possess a limited NAD+ supply, therefore it has to be recycled constatntly *
- under anaerobic conditions, NADH’s e- are transferred either to acetaldehyde or pyruvate in order to regenerate NAD+
Alcoholic Fermentation
- done by some bacteria and yeast in creating beer/wine/liquor, baked goods, pastries, soy sauce
Lactic Acid Fermentation
- done by some microorganisms but also animal muscle cells
Connection of Pathways
- all classes of nutrients may be used for energy
- most organisms possess metabolic pathways that, when necessary, metabolize proteins, lipids, and nucleic acids
- they are first digested into their component monomers, which the cell may reassemble into macromolecules for its own use
- otherwise, they may be metabolized for energy by feeding into various parts of glycolysis or Krebs cycle