CK12 Life Science

1. Glycolysis: the breakdown of glucose.


2. The citric acid cycle: the formation of electron carriers.


3. The electron transport chain: the formation of ATP.

In eukaryotic cells, the first phase takes place in the cytoplasm of the cell, while the other
phases are carried out in the mitochondria. This organelle is known as the “powerhouse” of
the cell because this is the organelle where the ATP that powers the cell is produced.

Glycolysis

The first step of cellular respiration is glycolysis. Duringglycolysis,the 6-carbon glucose
is practically ”cut in half,” broken down into two 3-carbon pyruvate molecules. Glycolysis
requires an initial energy-investment step, although in the end, glycolysis produces more
energy than was initially invested. Two ATP molecules are used to convert glucose into the
two 3-carbon pyruvate molecules. These 3-carbon molecules are then oxidized, which means
that they lose electrons, as electrons are transferred to the high energy electron acceptor
NAD+, producing the electron carrier NADH. This oxidation step helps produce 4 ATP
molecules from ADP. That means, taking into account the initial investment of 2 ATP
molecules, glycolysis has a net production of 2 ATP.

Table 4.1: An Overview of Glycolysis

Inputs Outputs

Glucose (6-carbon molecule) 2 pyruvate (3-carbon molecule)

2 NAD+ 2 NADH (electron carrier)

2 ATP (energy) 2 ADP

4 ADP 4 ATP (energy)

Afterglycolysis, thepyruvatecangodownseveraldifferentpaths. Ifthereisoxygenavailable,
the pyruvate moves inside the mitochondrion to produce more ATP during further break-
down stages. In the absence of oxygen, the fermentation process begins.

Inside the Mitochondria

If oxygen is available, the next step of cellular respiration is moving the pyruvate into the
mitochondria. The mitochondria have a double membrane. The inner membrane is known
as thecristae, and is folded to form many internal layers. Some steps of cellular respiration
occur in the cristae, while others take place in thematrix, the inner compartment of the
mitochondrion that is filled with enzymes in a gel-like fluid.

Within the mitochondria theKreb’s Cycleorcitric acid cycleoccurs. The citric acid