Biology, Eighth Edition (Raven)

Chapter 7: How Cells Harvest Energy

Electron Transport System and ATP Synthesis

How is the energy contained in food converted into energy used in cells? The energy in food is stored in the chemical bonds within the molecules. During glycolysis and the Krebs cycle food molecules are broken down. The energy is transferred to other molecules, the coenzymes NAD+ and FADH when they are reduced to NADH and FADH2. This energy is then transferred to the electron transport chain in the form of electrons. These electrons move from molecule to molecule in the electron transport chain. Each movement is a reduction-oxidation reaction in which the molecules move to a lower energy state. The energy released is used to move protons (hydrogen ions) across the membrane. The movement of protons increases potential energy in the form of an electrochemical gradient. Thus the chemical bond energy has been converted into potential energy. The enzyme ATP synthase uses the potential energy to form ATP.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	Arrange the following electron acceptors in the proper order in which they participate in electron transport. 1 = Cytochrome c 2 = Oxygen 3 = Cytochrome c oxidase

		A)	1, 2, 3
		B)	1, 3, 2
		C)	2, 3, 1
		D)	3, 1, 2
		E)	3, 2, 1

2.	The electrons that are transferred through the electron transport system initially belonged to

		A)	NADH.
		B)	FADH₂.
		C)	ATP.
		D)	oxygen.
		E)	NADH and FADH₂.

3.	The movement of protons through ATP synthase occurs from the

		A)	matrix to the intermembrane space.
		B)	matrix to the cytoplasm.
		C)	intermembrane space to the matrix.
		D)	intermembrane space to the cytoplasm.
		E)	cytoplasm to the intermembrane space.

4.	When oxygen accepts electrons, water is produced as a byproduct.

		A)	True
		B)	False

5.	The formation of ATP as a result of the activity of the electron transport system is termed substrate-level phosphorylation.

		A)	True
		B)	False