Lecture Cell Respiration - Part I

What do eukaryotic cells usually do to glucose in order to obtain energy?

An Overview

When glucose enters a cell, usually by active transport, it is split into 2 equal parts and each is eventually converted to a molecule called pyruvate. This process is called glycolysis and the cell nets 2 ATP molecules (and 1 NAPH + H+). This occurs in the cytoplasm and does not require oxygen (anaerobic).

The pyruvate molecules are transported into the mitochondria. Once in the matrix these 3-carbon molecules (pyruvates) are prepared for the next step when one of the three carbons is removed (decarboxylation) to become CO2 . The remaining 2-carbon molecule (acetyl group) is primed with Coenzyme A. This acetyl-CoA molecule is bonded to the resident 4-carbon molecule (oxaloacetic acid) to form citric acid. The Krebs or Citric acid cycle generates the following products through a series of steps including decarboxylation, phosphorylation etc. The main products for two turns of the cycle or one glucose molecule are:

  1. 2ATP
  2. 6 NADH + H+
  3. 2 FADH2
  4. 4 CO2

The third and final phase of aerobic respiration is also located in the mitochondria - this time the membranes play a vital role. Within the inner membrane locked in place are a group of molecules collectively called the electron transport chain with associated proton pumps. The NADH2+ and FADH2 molecules give up their electrons to the electron transport chain molecules which in turn use this energy to move protons into the outer compartment . This sets up a free energy potential which is tapped by the F0F1 complexes in the same mitochondrial membrane to make ATP from ADP and Phosphate.

Visit the Biomolecular Machines site for a comprehensive discussion of ATP synthase in F0F1 complexes. The ATP synthase molecule's two parts, a rotating stem and an mushroom-like cap stick out of the mitochondrial inner membrane. They act like an ATP generating motor fueled by the diffusion of Hydrogen ions moving down their gradient. Click here for animation of ATP synthase

An animation of ATP synthesis in mitochondria is provided by Thomas Terry at the University of Connecticut.

The final electron acceptor in the electron transport chain is oxygen. Some would argue that cells evolved this aerobic process in order to survive the damaging effects of free oxygen.

The last step in aerobic respiration is the reaction of oxygen gas and hydrogen to form water. This step is carried out by cytochrome c oxidase. Visit this link to find out more.

Summary of Respiration

In a eukaryotic cell, glycolysis occurs outside the mitochondria in the cytosol. The citric acid cycle and the electron transport chains are located inside the mitochondria. During glycolysis, each glucose molecule is broken down into two molecules of the compound pyruvate. The pyruvate is decarboxylated and combined with coenzymeA to cross the double membrane of the mitochondrion and enter the matrix, where the citric acid cycle decomposes it into 2 more carbon dioxide molecules. NADH formed by the citric acid cycle or glycolysis transfers electrons to electron transport chains, embedded in the membrane of the cristae (inner membrane). The electron transport chain transforms the chemical energy into a form that can be used to drive oxidative phosphorylation, which accounts for most (~32) of the ATP generated by cellular respiration. A smaller amount of ATP is formed directly during glycolysis and the citric acid cycle by substrate-level phosphorylation.

From Campbell, Biology. 4th Ed. pg 164

Links to Web Lectures and other Resources

Updated Nov. 10, 2005