Substrate-level Phosphorylation and Oxidative Phosphorylation

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  • 0:03 ATP
  • 0:53 Mitochondria
  • 1:25 Substrate-Level…
  • 2:53 Oxidative Phosphorylation
  • 4:03 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Your body is always using energy. This energy, in the form of ATP, is generated within your cells through substrate-level phosphorylation or oxidative phosphorylation. Learn about these processes and the role mitochondria play in making energy.


Life requires a lot of energy. It takes energy to move your muscles, make new cells, remove wastes, and perform countless other vital processes. So where does that energy come from? Well, it comes from the pancakes you had for breakfast and the salad you ate at lunch. But your cells don't directly run on pancakes and lettuce, instead they take energy from these food molecules and convert it into adenosine triphosphate, better known as ATP.

ATP is the molecule that supplies energy for metabolic processes within your cells. These energy compounds are built when a molecular unit of adenosine gets connected to three (tri) phosphate groups. The addition of phosphate groups is a process referred to as phosphorylation, and in this lesson, you will learn about two types of phosphorylation that your cells use to make energy.


To understand how ATP is made, we must look at mitochondria, which are the cellular bodies where ATP is produced. This important function has earned mitochondria the nickname, the 'powerhouses of the cell.'

Mitochondria consist of two membranes. The inner membrane is folded. This increases the surface area so more chemical reactions can take place across this membrane. The outer membrane surrounds the mitochondria and separates it from the cytoplasm, which is the gel-like intracellular substance it floats in.

Substrate-Level Phosphorylation

Inside the mitochondria is where most of your ATP are created, but some ATP can be made in the cytoplasm through a process called substrate-level phosphorylation. This is a process of forming ATP by the physical addition of a phosphate group to ADP. Substrate-level phosphorylation is a big name, but it's actually a straightforward process. With this type of phosphorylation you have an adenosine diphosphate (ADP), which is a unit of adenosine attached to two (di) phosphate groups. In order to turn it into an adenosine triphosphate, a phosphate group is taken from an intermediate compound, referred to as a substrate, and given to the ADP.

I mentioned that substrate-level phosphorylation takes place outside of the mitochondria, but it also takes place inside. When it takes place outside in the cytoplasm, it is occurring during glycolysis, which is the breakdown of glucose to produce pyruvate and energy.

Glycolysis does not require oxygen, but oxygen is an important part of ATP production inside the mitochondria. Substrate-level phosphorylation inside the mitochondria occurs during the Krebs cycle, also referred to as the citric acid cycle. This cycle is a sequence of chemical reactions in which molecules are broken down in the presence of oxygen to generate energy. The Krebs cycle takes place inside the folded inner membrane of the mitochondria, in an area known as the matrix.

Oxidative Phosphorylation

If we take a close look at the folded inner membrane, we find embedded structures that help out with oxidative phosphorylation, which is the process of forming ATP via the transfer of electrons. This process is not as straightforward as the other type of phosphorylation, but it's more productive, producing a good yield of ATP.

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