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

http://www.bozemanscience.com/science-videos/2012/5/6/cellular-respiration.html