Biology 101

2 February, 1998

Life on Planet Earth

Chapter 8: Harvesting Energy from Food:

Glycolysis & Cellular Respiration

A Brief Outline 1.Glucose Metabolism: An Overview
2. Glycolysis
3. Fermentation
4. Cellular Respiration
5. SuMMARY
6. Mitochondrial genetics

1. Glucose Metabolism: An Overview
Click HERE for a link to the University of Virginia page where this came from...

2. Glycolysis

Glucose must be activated by ATP before its energy can be harvested.



3. Fermentation

Some cells ferment pyruvic acid to lactic acid.

Other cells ferment pyruvic acid to alcohol.

note: the word "enzyme" actually means "in yeast" - we use (the enzymes from) yeast to ferment beer/wine and also to bake bread.

Cellular Respiration

Pyruvic acid is transported into the mitochondrial matrix.

Pyruvic acid is broken down by reactions in the mitochondrial matrix.

Energetic electrons are carried to electron transport systems in the inner mitochondrial membrane.

Chemiosmosis captures energy stored in a hydrogen ion gradient and produces ATP.

Mitochondria Overview - Chemiosmotic Gradient

The electron transport system cannot keep forcing protons into the intermembrane space forever. Since the inner membrane is impermeable to protons, they accumulate in the intermembrane space. This creates what is called a chemiosmotic gradient - simply put, a proton gradient. The gradient builds until the energy required to push any more protons into the space exceeds what is available from electron transport. At that point, electron transport must stop.

In reality, a number of processes act to dissipate the gradient, so electron transport never completely stops. Instead, the rate of electron transport is regulated by the rate at which the gradient is allowed to dissipate. The ability of the chemiosmotic gradient to limit electron transport is called respiratory control.

Of course, the primary purpose of mitochondria is to phosphorylate ADP. The energy needed to do that is stored as a gradient of protons. Presumably, if a proton were allowed to come back into the matrix there would be a release of energy. How can that energy be captured and exploited?

Embedded in the inner membrane among the structures of the electron transport system are structures called the ATP synthetase complex. The ATP synthetase complex consists of a proton channel and catalytic sites for the synthesis of ATP from ADP and phosphate. When ADP and phosphate are available, they bind the catalytic sites on the ATP synthetase. When this happens, the channel opens, and protons can come whooshing back in. The energy released is used to couple the phosphate to ADP, to make ATP.

The mechanism can be likened to a water wheel, where the flow of protons resembles a flow of water downhill, and the turning of the wheel is the turning of ADP toward phosphate to cause the bond to form.

Stimulation of electron transport by ADP

O.K. - one more thing. The built-up proton gradient was blocking electron transport, right? ...And the end result of electron transport is reduction of oxygen to water (oxygen consumption). ADP, by binding to ATP synthetase, provides an outlet for the protons that have accumulated in the intermembrane space. When they are allowed to enter the matrix, they leave space for more. That is, it becomes energetically favorable to force more protons into the intermembrane space.

So...Energy from food is channeled by enzymes into the mitochondria, in which it is channeled to the ETS. The ETS uses the energy to produce a chemiosmotic gradient, which is maintained at a constant level by electron transport. Electron transport is limited by the presence of the gradient. By binding to ATP synthetase, ADP permits protons to enter the matrix through a special channel, using the released energy to create a covalent bond between ADP and phosphate. By providing an outlet for the protons, activation of ATP synthetase by ADP leaves room for more, allowing electron transport to proceed.

This has been an oversimplified overview, designed to organize your thoughts so you may proceed with the details presented in the main path. Nearly everything presented here must be modified to provide a true picture of how mitochondria function, both in vivo and experimentally.

Created by D.R. Caprette (caprette@rice.edu), Rice University, 1 May 1996
Updated 9 June 1997

The above is from Rice University (click here for link to their web site).
http://ruf.rice.edu/~bioslabs/studies/mitochondria/mitoverview7.html

ATP is transported from the mitochondrial matrix and diffuses into the cell cytosol

Cellular metabolism influences the way entire organisms function.

hummingbird migration requires efficient energy storage and usage

human runners face similar challenges

An athelete training for the Olymipics

Metabolic Transformations - Why you can get fat by eating sugar (page 142)

5. Summary

Compartment	Function
Outer membrane	Fat metabolism (energy), transport
Intermembrane space	Proton reservoir ATPGTP, etc.
Inner membrane	Electron transport Symporters/antiporters ATPase
Matrix	Transcription/translation/replication TCA cycle

Chemiosmosis - the importance of proton gradient in driving transporters and in formation of ATP

Mitochondrial genetics

Mitochondria have their own DNA. The size ranges from a few hundred base-pairs (bp) to more than 2 million bp! For most animals (mammals certainly) the mtDNA is about 15,000 bp. Shown below is the mtDNA from a cat.

Mitochondrial DNA displays a high mutation rate

Heteroplasmy: ratio of mutant to normal mitochondria varies among individuals and among tissues

Several human diseases are due to mitochondria that aren't functioning properly (click here for a link)

Gene Linked to Hair Loss Is Identified
(article in the New York Times, 30-Jan-1998)

With No Other 'Dollys' Yet, Cloning Report Draws Critics
(article in the New York Times, 31-Jan-1998)

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Last modified on: 31 January, 2000 by Dave Ussery