32 Steps in Cellular Respiration

Key Idea: There are four major steps to cellular respiration. The majority of ATP is produced in the electron transport chain. Enzymes catalyse each reaction of cellular respiration.

` ATP is produced during cellular respiration by both substrate level phosphorylation and oxidative phosphorylation. ` Substrate level phosphorylation of ADP to ATP occurs during glycolysis and the Krebs cycle. An enzyme transfers a phosphate group directly from a substrate to ADP to form ATP. ` Oxidative phosphorylation occurs at the end of the electron transport chain. Electrons are transferred (by NADH and FADH2) from electron donors (e.g. glucose) to the electron transport chain and finally to electron acceptors (e.g. oxygen) in redox reactions. The energy released is used to produce ATP from ADP.

Glycolysis

` An enzyme strips two electrons from glucose to produce two pyruvate molecules, each of which can enter the Krebs cycle. ` The electrons are transferred to NAD+ to form the coenzyme NADH. NADH carries hydrogens to the electron transport chain. ` Glycolysis uses 2 ATP but produces 4 ATP molecules. This net 2 ATP is made by transferring a phosphate directly from a substrate to ADP (substrate level phosphorylation).

2 ATP Glycolysis Glucose

2 Pyruvate 2NADH

Link reaction

` The link reaction removes CO2 from pyruvate and adds coenzyme A, producing the 2C molecule acetyl coenzyme A, which enters the

Krebs cycle. NADH is also produced for use in the electron transport chain.

Krebs cycle

` Acetyl coenzyme A is attached to a 4C intermediate and coenzyme A is released. ` The intermediate is remade in a cyclic series of enzyme-controlled reactions that remove carbons and produce more NADH and FADH2 for the electron transport chain. ` The Krebs cycle turns twice per glucose molecule (once per pyruvate molecule). ` 1 ATP is made by substrate level phosphorylation and 2 CO2 are produced per turn.

2CO2

Acetyl coenzyme A

2 ATP

Krebs cycle

4CO2 2NADH

6NADH

2FADH2

Electron transport chain (ETC)

` Electrons carried by NADH and FADH2 are passed to a series of electron carrier enzymes embedded in the inner membrane of the mitochondria. ` The energy from the electrons is used to pump

H+ ions across the inner membrane from the matrix into the intermembrane space. These flow back to the matrix via the membrane-bound enzyme ATP synthase, which uses their energy to produce 34 ATP per glucose molecule. ` The electrons are coupled to H+ and oxygen at the end of the ETC to form water. ` The process is called oxidative phosphorylation.

NADH

NAD+ Electron transport chain

H+ H+ H+

e–

FADH2 FAD+

H2O

2H+ +1/2O2 H+

H+

73

34 ATP (via ETC)

(b) How many ATP are produced this way during cellular respiration (per molecule of glucose)?

2. (a) What is oxidative phosphorylation?

(b) How many ATP are produced this way during cellular respiration (per molecule of glucose)?

3. Which parts of cellular respiration produce CO2?

4. What is the purpose of NADH and FADH2 in cellular respiration?

5. How are glycolysis and the Krebs cycle linked to the electron transport chain?

6. Describe how ATP is produced in the electron transport chain and explain the importance of the proton gradient:

7. Use the space below to draw a summary of cellular respiration, including the location, inputs, and outputs of each stage (glycolysis, the link reaction, Krebs cycle, and the electron transport chain):