8.2: Aerobic Cellular Respiration

Learning Objectives

• Identify the reactants and products of aerobic cellular respiration.
• Explain each step of aerobic cellular respiration and where in the cell it occurs.

Not only do plants produce sugars through photosynthesis, but they also break down these sugars to generate usable energy in the form of ATP through aerobic cellular respiration. Glucose begins its breakdown outside of the mitochondria in a metabolic pathway called glycolysis. However, the majority of the reactions that produce ATP happen within the mitochondria (in eukaryotic cells; Figure \(\PageIndex{1}\)). During these reactions, electron carriers are created and oxygen pulls the electrons through an electron transport chain to create ATP, which powers cellular activity. The oxygen you breathe in combines with electrons to form water, which you breathe out. The carbon dioxide you breathe out comes from the carbon in glucose, which your body metabolized.

Figure \(\PageIndex{1}\): The mitochondria are the energy-conversion factories of the cell. A mitochondrion is composed of two separate lipid bilayer membranes. The folds of the inner membrane are called cristae, and the space between membranes is the intermembrane space. The matrix is in the center. Along the inner membrane are various molecules that work together to produce ATP, the cell’s major energy currency. Image (relabeled) and caption (modified) by OpenStax (CC-BY). Access for free at openstax.org.

Here is a net reaction for cellular respiration:

\[\ce{C_6H_{12}O_6 + 6O_2\rightarrow6CO_2 + 6H_2O + ATP} \nonumber\]

glucose + oxygen \(\ce{\rightarrow}\) carbon dioxide + water + energy

Step 1: Glycolysis

When glucose is transported into the cytoplasm of cells, it is broken down into two molecules of pyruvate (Figure \(\PageIndex{2}\)). This process is called glycolysis (glyco- for glucose and -lysis, meaning to break apart). Glycolysis involves the coordinated action of many different enzymes. As these enzymes start to break the glucose molecule apart, an initial input of energy is required. This initial energy is donated by molecules of ATP.

Figure \(\PageIndex{2}\): In glycolysis, glucose (represented by a ring of six carbons) is converted to fructose-1,6-bisphosphate (labeled fructose diphosphate). This consumes 2 ATP, releasing 2 ADP. Fructose-1,6-bisphosphate is broken into two glyceraldehyde-3-phosphate molecules. Each is represented by a chain of three carbons attached to a phosphate (P_i). The phosphate is removed from each of the two glyceraldehyde-3-phosphate molecules, producing 2 pyruvates, 2 NADH, and 4 ATP. Overall, glycolysis consumes 2 ATP, but it then generates 4 ATP and 2 NADH. Image by OpenStax (CC-BY). Access for free at openstax.org.

Though two molecules of ATP are used to get glycolysis going, four more molecules of ATP are produced during the reaction, resulting in the net production of two ATP per molecule of glucose. In addition to ATP, two molecules of nicotinamide adenine dinucleotide (NAD⁺) are reduced to form NADH (Figure \(\PageIndex{3}\)). When NAD⁺ is reduced to NADH, two high energy electrons derived from breaking the bonds of glucose are added to it. One of those negatively charged electrons is balanced by the positive charge (+) on NAD⁺. The other is balanced by adding a proton (H⁺) to the molecule.

Figure \(\PageIndex{3}\): When NAD⁺ is reduced to NADH, it gains one proton (H⁺) and two electrons (e^-). The reverse reaction (oxidation) can also occur. The chemical structure of NAD⁺ is a modified version of two nucleotides attached together. It is represented by ADP (adenosine diphosphate) attached to ribose (rib, a five-carbon sugar), attached to a ring of carbon and nitrogen.

Step 2: Pyruvate oxidation

If oxygen is present, aerobic cellular respiration can continue. The two molecules of pyruvate are transported into the matrix of the mitochondrion. During transport, each pyruvate is converted into a 2-carbon molecule called acetyl-\(\ce{CoA}\). The other carbon atom from each pyruvate molecule exits the cell as \(\ce{CO2}\). The electrons from this broken bond are captured by another molecule of NAD⁺, reducing it to NADH. Because two molecules of pyruvate are produced from each glucose molecule during glycolysis, two acetyl CoA molecules are produced (one from each pyruvate) during pyruvate oxidation (Figure \(\PageIndex{4}\)).

Step 3: The Citric Acid (Krebs) Cycle

The two acetyl-\(\ce{CoA}\) molecules enter a cycle which, much like glycolysis, involves the action of many different enzymes to release energy and transport it in energy-carrying molecules, including 2 ATP, 6 NADH, and 2 \(\ce{FADH2}\), another electron carrier (Figure \(\PageIndex{4}\)). This cycle takes place within the matrix of the mitochondrion.

Figure \(\PageIndex{4}\): Combined view of pyruvate oxidation and the citric acid cycle. During pyruvate oxidation, pyurvate/pyruvic acid (three carbons) is converted to acetyl CoA (two carbons). In this process, one carbon dioxide molecule is released and one molecule of NAD⁺ is reduced to NADH. In the citric acid cycle, acetyl CoA breaks into two carbon dioxide molecules. This process releases 2 NADH, 1 FADH₂, and 1 ATP per pyruvate, and double that per glucose. Image by OpenStax (CC-BY). Access for free at openstax.org.

Step 4: Oxidative Phosphorylation

This stage of cellular respiration has two steps. During the electron transport chain, our electron carriers power a series of proton pumps that move \(\ce{H+}\) ions from the mitochondrial matrix to the space between the inner and outer mitochondrial membranes. During chemiosmosis, an enzyme called ATP synthase allows the protons to flow back into the mitochondrial matrix, using the physical flow of the protons to turn ADP into ATP.

The Electron Transport Chain

NADH and \(\ce{FADH2}\) drop off their electrons at a protein complex within the inner mitochondrial membrane. This effectively “turns on” this protein complex, which pumps a \(\ce{H+}\) from the mitochondrial matrix to the intermembrane space. The electrons are then passed down a line of protein complexes, much like a current of electricity, powering these complexes to each pump a \(\ce{H+}\) from the matrix into the intermembrane space. This is appropriately named the electron transport chain (Figure \(\PageIndex{5}\)).

At the end of the electron transport chain, the low energy electrons need to be picked up to make space for more electrons. An oxygen atom picks up two electrons and, to balance the charge, two \(\ce{H+}\) from the matrix, forming a water molecule (\(\ce{H2O}\)). In cellular respiration, oxygen is the terminal electron acceptor, because it picks up the electrons at the end (the terminus) of the electron transport chain. This job is so important that, as you saw above, if oxygen is not present, this part of cellular respiration will not occur.

Figure \(\PageIndex{5}\): This view of the mitochondrion shows the outer membrane and inner membrane, which borders the matrix. The intermembrane space is between the outer and inner membranes. Pyruvate oxidation (not shown) and the citric acid cycle occur in the matrix. The citric acid cycle (along with glycolysis and pyruvate oxidation) supplies NADH to the electron transport chain of oxidative phosphorylation. (FADH₂ also provides electrons to the electron transport chain, but it is not shown here.) Electrons from NADH are passed from complex I to coenzyme Q to complex III to cytochrome c to complex IV to gaseous oxygen, which is reduced to form water. The three complexes, which are embedded in the inner membrane pump protons into the intermembrane space. Protons move down their proton gradient through ATP synthase, generating ATP.

Chemiosmosis

Why are the protein complexes pumping \(\ce{H+}\) into the intermembrane space? The intermembrane space is relatively small. As more \(\ce{H+}\) are added to this area, the intermembrane space becomes increasingly positively charged, while the matrix becomes increasingly negatively charged. This is similar to how a battery stores energy--by creating an electrochemical gradient. The positive charges repel each other and would “prefer” to be balanced across both sides of the membrane. However, they cannot directly pass through the membrane. Even though they are small, \(\ce{H+}\) ions carry a full charge, making them too polar to pass through the nonpolar tails of the phospholipid bilayer that composes the mitochondrial membranes.

An enzyme called ATP synthase allows the \(\ce{H+}\) to move back into the matrix. This enzyme is structured much like a waterwheel or turbine -- the flow of protons through the enzyme physically rotates it, converting the potential energy stored in the electrochemical gradient into kinetic energy (movement)! This kinetic energy is used to force another phosphate group onto ADP, converting the kinetic energy back into chemical energy, which is stored in the bonds of ATP

Attributions

Curated and authored by Melissa Ha using the following sources:

• Kammy Algiers (CC-BY-NC)
• 13.3: Cellular Respiration in Botany Lab Manual by Maria Morrow (CC-BY)