Outline of Cellular Respiration
Warning: this information is provided as a broad outline for the steps that take place in cellular
respiration. It should not be used as your only resource for studying respiration. Please consult
your notes and the book for more detail than what is provided here.
There are four main steps in cellular aerobic respiration:
1. Glycolysis
2. Intermediate Step: conversion of PyruvateAcetyl CoA
3. Kreb’s Cycle (aka--Citric Acid Cycle)
4. Oxidative Phosphorylation (Electron Transport Chain and Chemiosmosis)
Aerobic respiration starts with a glucose molecule (C6H12O6) and ends with lots of energy. The
inputs and outputs are listed for each of the four stages.
Glycolysis:
Glycolysis is the first step of aerobic or anaerobic cellular respiration. This process begins with
the input of a glucose molecule that enters an enzymatic pathway that will lead to the
production of 2 Pyruvate molecules.
A few things to note:
-Glucose itself is a stable molecule. It is made unstable by phosphorylation, in which ATP is
utilized.
-The third enzyme in the glycolytic pathway is Phosphofructokinase. This enzyme is important
because it is an allosteric enzyme that will allow for the regulation of respiration. Citrate and
ATP will act as non-competitive inhibitors. AMP will act as a non-competitive activator.
-Only Glyceraldehyde-3-Phosphate can continue to the end of the pathway to produce
pyruvate. For this reason, the Isomerase enzyme is critical in that it converts Dihydroxyacetone
Phosphate  G3P. These two compounds are structural isomers of each other.
-The inputs and outputs for glycolysis
Inputs:
2 ATP
1 glucose molecule C6H12O6
Outputs:
2 Pyruvate
2 NADH+H
4 ATP (only 2 net gain though)
Intermediate Phase:
In the intermediate phase, Pyruvate is converted to Acetyl CoA. This takes place because
Pyruvate cannot directly enter the Citric Acid Cycle, but Acetyl CoA is capable. There are three
steps in the conversion of Pyruvate to Acetyl CoA:
1) Pyruvate gives off a CO2
2) Pyruvate reduces(donates a hydrogen) NAD+  NADH
3) Acetyl CoA binds on
Things to note:
-Input and output
Inputs:
2 Pyruvates
Outputs:
2 Acetyl CoA
2 NADH+H
2 CO2
Citric Acid Cycle:
The Citric Acid Cycle is called a cycle because it is a cyclic pathway, meaning that the first
molecule in the process also acts as the final molecule. In the Kreb’s cycle, this molecule is
Oxaloacetate.
Things to note:
-The starting/final compound is Oxaloacetate
-The Kreb’s cycle will only turn with the addition of Acetly CoA(2C) to Oxaloacetate(4C) which
creates Citrate.
-Citrate acts a non-competitive inhibitor on the allosteric enzyme Phosphofructokinase.
-Substrate-level phosphorylation takes place once per turn. This happens when Succinyl CoA
releases its coenzyme A to form Succinate.
-Inputs and Outputs for the cycle
Inputs:
2 Acetyl CoA
Outputs(for 2 turns of the cycle):
6 NADH+H
2 FADH2
4 CO2
2 ATP
Oxidative Phosphorylation:
Oxidative phosphorylation takes place in two steps: 1)the Electron Transport Chain 2)
Chemiosmosis. The electron transport chain is a series of protein complexes and prosthetic
groups that are used in oxidation-reduction reactions. Electrons are dropped off onto the ETC
by the electron carriers NADH and FADH2. The proteins that receive each respective carrier
becomes reduced while the carrier is oxidized. The proteins and prosthetic groups then
undergo a series of redox reactions that move the electrons down the chain, creating energy as
this takes place. The energy from the ETC is used to pump H+ outside of the inner membrane,
creating a hydrogen ion chemical gradient. This means that the H+ on the outside of the inner
membrane is higher than the H+ concentration inside the mitochondrial matrix. This
electrochemical gradient is then used to power the protein/enzyme ATP Synthase, which will
ultimately give rise to the phosphorylation of ADP.
Things to note:
-Look at Fig. 9.14 in your book to understand the process by which ATP Synthase works
-Understand how the electrochemical gradient with hydrogen ions forms
-Know the difference between ETC and Chemiosmosis
-Oxidative Phosphorylation takes place on the Cristae of a mitochondria.
-Oxygen acts as the final electron acceptor in aerobic respiration. This is what forms water.
Electron Transport Chain:
Inputs:
10 NADH+H
2 FADH2
Outputs:
H2O(actually formed at the end of the ETC)
32-34 ATP
End result of cellular aerobic respiration is 36 to 38 ATP.
Anaerobic Respiration:
Anaerobic respiration is what takes place when there is an absence of oxygen. Alcohol
fermentation and lactic acid fermentation allow for the recycling of NAD+, so that the glycolysis
pathway can continue to produce a net gain of 2 ATP per glucose molecule.
Alcohol Fermentation: Pyruvate gives off CO2, and becomes 2-Acetaldehyde. This compound is
then reduced by NADH. The final product is Ethanol.
Lactic Acid Fermentation: Pyruvate is directly reduced to form Lactic acid as the final product.