9-1 chem pathways

Both plant and animal cells carry out the final

stages of cellular respiration in the mitochondria.
One gram of the sugar glucose (C6H12O6), when

burned in the presence of oxygen, releases 3811

calories of heat energy.

A calorie is the amount of energy needed to raise

the temperature of 1 gram of water 1 degree

Celsius.Cells don't “burn” glucose. Instead, they gradually

release the energy from glucose and other food

compounds.

This process begins with a pathway called

glycolysis.

Glycolysis releases a small amount of energy.
If oxygen is present, glycolysis is followed by the

Krebs cycle and the electron transport chain.

Glycolysis, the Krebs cycle, and the electron

transport chain make up a process called cellular

respiration.
Cellular respiration is the process

that releases energy by breaking

down glucose and other food

molecules in the presence of oxygen.
The equation for cellular respiration is:

6O2 + C6H12O6 → 6CO2 + 6H2O + Energy

oxygen + glucose → carbon dioxide + water + Energy
Each of the three stages of cellular respiration

captures some of the chemical energy available in

food molecules and uses it to produce ATP.
Glycolysis takes place in the cytoplasm. The Krebs

cycle and electron transport take place in the

mitochondria.
Glycolysis is the process in which one

molecule of glucose is broken in half,

producing two molecules of pyruvic acid,

a 3-carbon compound.
At the beginning of glycolysis, the cell uses up 2

molecules of ATP to start the reaction.
When glycolysis is complete, 4 ATP molecules

have been produced.
This gives the cell a net gain of 2 ATP molecules.
One reaction of glycolysis removes 4 high-energy

electrons, passing them to an electron carrier

called NAD+.
Each NAD+ accepts a pair of high-energy electrons

and becomes an NADH molecule.
Glycolysis

The NADH molecule holds the electrons until they

can be transferred to other molecules.
The process of glycolysis is so fast that cells can

produce thousands of ATP molecules in a few

milliseconds.

Glycolysis does not require oxygen.
When oxygen is not present, glycolysis is followed

by a different pathway. The combined process of

this pathway and glycolysis is called fermentation.

Fermentation releases energy from food

molecules by producingATP in the absence of

oxygen.
During fermentation, cells convert NADH to NAD+

by passing high-energy electrons back to pyruvic

acid.

This action converts NADH back into NAD+, and

allows glycolysis to continue producing a steady

supply of ATP.

Fermentation does not require oxygen—it is an

anaerobic process.
Yeasts and a few other microorganisms use

alcoholic fermentation, forming ethyl alcohol and

carbon dioxide as wastes.

The equation for alcoholic fermentation after

glycolysis is:

pyruvic acid + NADH → alcohol + CO2 + NAD+
In many cells, pyruvic acid that accumulates as a

result of glycolysis can be converted to lactic acid.

This type of fermentation is called lactic acid

fermentation. It regenerates NAD+ so that

glycolysis can continue.
Lactic acid fermentation converts glucose into lactic

acid.
The equation for lactic acid fermentation after

glycolysis is:

pyruvic acid + NADH → lactic acid + NAD+