RESPIRATION

Mitochondria
We might think of plants as largely photosynthetic, generating their energy directly from sunlight. Non-green parts of the plant get their energy from respiration and even the green parts do this at night. Few plants respire as fast as animals; exceptions are plants in the Araceae where the spadix generates heat through respiration in order to attract pollinators.

There is no part of the bacterial cell set aside for respiration. In all eukaryotes most of the energy release in respiration occurs in special structures, mitochondria (singular mitochondrion). These are about as large as bacteria (5 um long) and have their own DNA and protein synthesis machinery. Current thinking is that mitochondria originated from bacteria that were captured by the ancestors of eukaryotic cells.

Electron transport
The key to respiration (and photosynthesis) lies in the strange phenomenon of electron transport. When we looked at membranes we saw how the breakdown of ATP could be used to push ions and molecules up a concentration gradient across a membrane. In respiration and photosynthesis ion movement down a concentration gradient drives ATP formation. The rest of the process is chemistry that generates the ion gradient.

Respiratory pathways
Respiration occurs in three phases, glycolysis, the TCA cycle and electron transport or energy conversion. The usual input is glucose although in some cells fats may be a starting point. Glycolysis occurs in the cytosol (the space between organelles), more or less in solution. It begins with the consumption of ATP to produce glucose-phosphate and all of the intermediates are phosphorylated down to the last step in the pathway. Leaving out the detail:

1. a six carbon sugar, glucose, gets split into two three carbon sugars
2. Each of these gets oxidized by NAD to make a sugar acid (PGA) and NADH
3. After further rearrangement and loss of H2O we get two molecules of a three carbon acid (pyruvate) and a net recovery of two molecules of ATP above those consumed in the pathway

To see this in action:

Up to this point we have produced no CO2 and no O2 has been taken up. CO2 production occurs in three steps (pyruvate has 3 C's) and at 5 points NAD or a related compound is reduced to NADH. This all occurs in the mitochondrion:

1. pyruvate is converted to acetyl Coenzyme A (2 carbons)
2. Acetyl CoA (2 carbons) enters the TCA cycle (tricarboxylic acid, Kreb's or citric acid cycle), combines with a 4 carbon acid (oxaloacetate) to make a 6 carbon acid (citrate).
3. The cycle regenerates the 4 carbon acid giving off 2 CO2 and with 4 further reductive steps.

To see this in action:

From glucose we have 6 CO2 and 10 NADH (2 from glycolysis and 8 from the TCA cycle), 2 FADH2. and 2 ATP. There has been no oxygen uptake and although the NADH can be used for some purposes we have not got much of the energy out.

Energy production and oxygen uptake occur through the final phase of respiration, the electron transport chain, and oxidative phosphorylation.

NADH is oxidized by a series of proteins called cytochromes in the inner membrane of the mitochondrion. H+ and e- are passing down the chain until they combine with O to make H2O.

The H+ is taken from the inside of the mitochondrion and pushed into the space between the inner and outer membranes

it finds its way back through an ATPase - instead of using ATP to move ions across a membrane, ions returning back across the membrane produce ATP.

To see this in action: 

One NADH inside the mitochondrion yields 3 ATP. External NADH and FADH2 each give 2 ATP. With the 2 ATP's from glycolysis the total yield is 36. 6 of these came directly or indirectly from glycolysis and 30 from pyruvate onwards. So more than 80% of the energy recovered as ATP comes from the TCA cycle.

Of the 686 kcal released in oxidizing glucose (C6H12O6) to CO2, 263 kcal/mole or 38% is retained in ATP. This may not sound too good, but remember that engines typically work at les than 25% efficiency. 38% is the maximum energy efficiency and organisms may function below this.

Notice that O2 is absolutely necessary for good energy yield. In the absence of O2 electron transport stops and ATP cannot be made. The TCA cycle shuts down and the products of glycolysis (pyruvate and NADH) start to pile up. Cells have a limited supply of NAD so when this all gets reduced to NADH something must be done with it if metabolism is to continue at all.

This is a real problem because fungi and other microrganisms quite often grow in low oxygen (hypoxic) or no oxygen (anoxic or anaerobic) environments. It also applies to higher plants; roots growing in flooded or compacted soils are deprived of oxygen.

In anaerobic conditions the cells cannot recycle the NADH to make ATP. The other product of glycolysis, pyruvate is decarboxylated to acetaldehyde then this is reduced by NADH to alcohol.

In beer and wine production we try to make sure that the yeast follows this pathway from sugar to alcohol. For higher plants fermentation is a response to one kind of environmental stress. It is an inefficient way of utilizing food reserves since only 2 molecules of ATP are produced for every glucose. This is about 5% of the yield in complete oxidation through the TCA cycle, but may provide enough energy enough to stay alive until conditions improve.

Copyright © Michael Knee
The Ohio State University
All rights reserved.