Cellular Respiration: an Overview

Cellular respiration is nothing more than an energy conversion process.
It converts energy in the form of glucose into energy in the form of ATP.
The process, when done at its greatest efficiency, requires oxygen.
The products are carbon dioxide and water.

Glucose	+	Oxygen	→	Carbon dioxide	+	Water	+	Energy
C₆H₁₂O₆	+	6 O₂	→	6 CO₂	+	6 H₂O	+	ATP^4-

Cellular respiration is analogous to money conversion.
When you travel to France, you arrive in Paris and your dollars are useless: they need to be converted into Euros.
- You must go to a bank and exchange your money.
- After the conversion, the amount of money you have is still the same; it is just in a new form that is useful to you in France.
In living systems energy is generally transferred between cells in a chemical form called glucose, a monosaccharide carbohydrate.
- This is the way cells like to exchange energy between themselves.
- However, once in the cell, glucose is useless, just like dollars in France.
- So the cell converts the glucose into something it can use—ATP—via cellular respiration.

A Drama in Three Parts

Cellular respiration is a process that occurs in individual cells, and can be thought of as a play that has three acts:

Figure 9.6, page 152, Campbell's Biology, 5th Edition

Cytoplasm (cytosol)
- Metabolic pathway: glycolysis
- Carbon processing: conversion of 1 glucose (a 6-carbon molecule) into 2 pyruvates (3-carbon molecules)
Mitochondria
- Metabolic pathways: Krebs cycle, electron transport chain and oxidative phosphorylation
- Carbon processing: conversion of 2 pyruvates (3-carbon molecules) each into 3 carbon dioxides (1-carbon molecules)

Word derivation:
- Glyco: glucose
- Lysis: to split
- Glycolysis: “to split glucose”
Glucose enters the cytoplasm of the cell, and glycolysis occurs here.
Glycolysis begins with 1 glucose (6-carbon molecule) and ends with 2 pyruvates (3-carbon molecules).
- It is a ten-step process (you do not need to know the details).
- It occurs in two phases:
  1. Energy investment phase
  2. Energy payoff phase (twice for each molecule of glucose)

In this first phase of glycolysis, glucose (a 6-carbon sugar) is split in half and becomes two 3-carbon molecules.
This phase requires an energy investment of 2 ATPs for each molecule of glucose.
Glyceraldehyde phosphate is the 3-carbon molecule that continues into the next phase.
The numbers, as in “glucose 6-phosphate,” refer to the number of the carbon to which the phosphate group is bound.
The 6-carbon sugars are shown here as simplified linear structures to indicate the location of the phosphate groups. In reality are ringed structures.

Via Mr. Hammack's mad graphics-creation skills

In this second phase of glycolysis, the 3-carbon sugars are modified in a series of steps such that 2 ATP molecules are produced.
One NADH molecule is also produced.
The final molecule in the pathway is pyruvate, a 3-carbon sugar.
This phase of glycolysis must occur twice for every one molecule of glucose (a 6-carbon sugar).

Via Mr. Hammack's mad graphics-creation skills

At the end of glycolysis, all the energy that was originally in glucose is now in three different kinds of molecules: ATP, NADH, and pyruvate.
The total energy count at the end of the process is:
- 2 ATP
- 2 NADH
- 2 pyruvate