Cellular Respiration: Using Oxygen to Break Down Food for Energy

By Rene Fester Kratz

Autotrophs and heterotrophs do cellular respiration to break down food to transfer the energy from food to ATP. The cells of animals, plants, and many bacteria use oxygen to help with the energy transfer during cellular respiration; in these cells, the type of cellular respiration that occurs is aerobic respiration (aerobic means “with air”).

Three separate pathways combine to form the process of cellular respiration. The first two, glycolysis and the Krebs cycle, break down food molecules. The third pathway, oxidative phosphorylation, transfers the energy from the food molecules to ATP. Here are the basics of how cellular respiration works:

  • During glycolysis, which occurs in the cytoplasm of the cell, cells break glucose down into pyruvate, a three-carbon compound. After glycolysis, pyruvate is broken down into a two-carbon molecule called acetyl-coA.
  • After pyruvate is converted to acetyl-coA, cells use the Krebs cycle (which occurs in the matrix of the mitochondrion) to break down acetyl-coA into carbon dioxide.
  • During oxidative phosphorylation, which occurs in the inner membrane or cristae of the mitochondrion), cells transfer energy from the breakdown of food to ATP.
cellular respiration
An overview of cellular respiration.

Cellular respiration is different from plain ol’ respiration. Respiration, which is more commonly referred to as breathing, is the physical act of inhaling and exhaling. Cellular respiration is what happens inside cells when they use oxygen to transfer energy from food to ATP.

Cellular respiration is essential to the transfer of matter and energy through living systems. As living things use cellular respiration to transfer energy from food to ATP, some of the available energy in the food is transferred to the environment as heat, which travels through our atmosphere and goes back out to space. This energy transfer to heat occurs because cellular respiration isn’t 100 percent efficient at capturing the energy from food into ATP.

If you remember that the energy in food originally came from the Sun, you can put the pieces together and see that most of the energy needed by living systems comes from space, and eventually exits Earth and goes back into space.

Cellular respiration is also important in the movement of matter through living systems: As living things break down food molecules using cellular respiration, they release the atoms from the food molecules back out into the environment as carbon dioxide and water. Photosynthesis brings in carbon dioxide and water from the environment, and cellular respiration sends them out again, forming a circular matter pathway that scientists call the carbon cycle.

Breaking down food

After the large molecules in food are broken down into their smaller subunits during digestion, the small molecules can be further broken down to transfer their energy to ATP. During cellular respiration, enzymes slowly rearrange the atoms in food molecules. Each rearrangement produces a new molecule in the pathway and can also produce other useful molecules for the cell. Some reactions

  • Release energy that can be transferred to ATP: Cells quickly use this ATP for cellular work, such as building new molecules.
  • Oxidize food molecules and transfer electrons and energy to coenzymes: Oxidation is the process that removes electrons from molecules; reduction is the process that gives electrons to molecules. During cellular respiration, enzymes remove electrons from food molecules and then transfer the electrons to the coenzymes nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD). NAD+ and FAD receive the electrons as part of hydrogen (H) atoms, which change them to their reduced forms, NADH and FADH2. Next, NADH and FADH2 donate the electrons to the process of oxidative phosphorylation, which transfers energy to ATP.

NAD+ and FAD act like electron shuttle buses for the cell. The empty buses, NAD+ and FAD, drive up to oxidation reactions and collect electron passengers. When the electrons get on the bus, the driver puts up the H sign to show that the bus is full. Then the full buses, NADH and FADH2, drive over to reactions that need electrons and let the passengers off. The buses are now empty again, so they drive back to another oxidation reaction to collect new passengers. During cellular respiration, the electron shuttle buses drive a loop between the reactions of glycolysis and the Krebs cycle (where they pick up passengers) to the electron transport chain (where they drop off passengers).

  • Release carbon dioxide (CO2): Cells return CO2 to the environment as waste, which is great for the autotrophs that require CO2 to produce the food that heterotrophs eat. (See how it’s all connected?)

Different kinds of food molecules enter cellular respiration at different points in the pathway. Cells break down simple sugars, such as glucose, in the first pathway — glycolysis. Cells use the second pathway, the Krebs cycle, for breaking down fatty acids and amino acids.

Following is a summary of how different molecules break down in the first two pathways of cellular respiration:

  • During glycolysis, glucose breaks down into two molecules of pyruvate. The backbone of glucose has six carbon atoms, whereas the backbone of pyruvate has three carbon atoms. During glycolysis, energy transfers result in a net gain of two ATP and two molecules of the reduced form of the coenzyme NADH.
  • Pyruvate is converted to acetyl-coA, which has two carbon atoms in its backbone. One carbon atom from pyruvate is released from the cell as CO2. For every glucose molecule broken down by glycolysis and the Krebs cycle, six CO2 molecules leave the cell as waste. (The conversion of pyruvate to acetyl-coA produces two molecules of carbon dioxide, and the Krebs cycle produces four.)
  • During the Krebs cycle, acetyl-coA breaks down into carbon dioxide (CO2). The conversion of pyruvate to acetyl-coA produces two molecules of NADH. Energy transfers during the Krebs cycle produce an additional six molecules of NADH, two molecules of FADH2, and two molecules of ATP.

Transferring energy to ATP

In the inner membranes of the mitochondria in your cells, hundreds of little cellular machines are busily working to transfer energy from food molecules to ATP. The cellular machines are called electron transport chains, and they’re made of a team of proteins that sits in the membranes transferring energy and electrons throughout the machines.

The coenzymes NADH and FADH2 carry energy and electrons from glycolysis and the Krebs cycle to the electron transport chain. The coenzymes transfer the electrons to the proteins of the electron transport chain, which pass the electrons down the chain. Oxygen collects the electrons at the end of the chain. (If you didn’t have oxygen around at the end of the chain to collect the electrons, no energy transfer could occur.) When oxygen accepts the electrons, it also picks up protons (H+) and becomes water (H2O).

The proteins of the electron transport chain are like a bucket brigade that works by one person dumping a bucket full of water into the next person’s bucket. The buckets are the proteins, or electron carriers, and the water inside the buckets represents the electrons. The electrons get passed from protein to protein until they reach the end of the chain.

While electrons are transferred along the electron transport chain, the proteins use energy to move protons (H+) across the inner membranes of the mitochondria. They pile the protons up like water behind the “dam” of the inner membranes. These protons then flow back across the mitochondria’s membranes through a protein called ATP synthase that transforms the kinetic energy from the moving protons into chemical energy in ATP by capturing the energy in chemical bonds as it adds phosphate molecules to ADP.

The entire process of how ATP is made at the electron transport chain is called the chemiosmotic theory of oxidative phosphorylation.

chemiosmotic theory
The events happening inside mitochondria, as described by the chemiosmotic theory.

At the end of the entire process of cellular respiration, the energy transferred from glucose is stored in 36 to 38 molecules of ATP, which are available to be used for cellular work. (And boy do they get used quickly!)