Energy from Biological Molecules (College Board AP® Biology) : Study Guide
Glycolysis
Glycolysis is the first stage of cellular respiration; it takes place in the cytosol of cells
During glycolysis glucose is:
phosphorylated
split into two 3-carbon molecules
The products of glycolysis include:
2 pyruvate molecules
a net gain of two molecules of ATP
ATP is synthesized from ADP + inorganic phosphate (Pi)
This is described as a 'net gain' because some ATP is also used during glycolysis
two molecules of NADH
NADH is synthesized from NAD+

Examiner Tips and Tricks
Memorization of the steps and intermediate products of glycolysis is not required.
Oxidation of pyruvate
Pyruvate from glycolysis is transported from the cytosol into the mitochondria, where oxidation occurs
Movement of pyruvate across the double membrane of the mitochondria occurs by active transport

Pyruvate is oxidized and acetyl coenzyme A (acetyl CoA) is formed; during this process:
NAD+ is converted to NADH
Carbon dioxide is released

Examiner Tips and Tricks
Remember that there are two pyruvate molecules produced per glucose molecule, so two pyruvate molecules will enter the mitochondrion and become oxidised during this stage of the process.
Krebs cycle
The Krebs cycle involves a series of enzyme-controlled reactions in the matrix of the mitochondria
Acetyl CoA, from the oxidation of pyruvate, enters the cyclic pathway, resulting in:
synthesis of ATP from ADP and inorganic phosphate
transfer of electrons to coenzymes NAD and FAD, producing NADH and FADH2
release of carbon dioxide from organic intermediates
The products of the Krebs cycle are then used as follows:
carbon dioxide is released as a waste product
NADH and FADH2 are transferred to the electron transport chain in the mitochondrial membrane

Examiner Tips and Tricks
The Krebs cycle is also known as the citric acid cycle.
Note that you are not expected to know details of the steps of the Krebs cycle or the names of the intermediate molecules
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