Cellular respiration is the process by which cells use oxygen to break down glucose and release energy in the form of ATP. That ATP goes on to fuel cycles and metabolic processes in cells within the body.

People often confuse the process of respiration with breathing. Breathing is a physiological process in higher level organisms that is also known as ventilation. Respiration occurs at a cellular level.In photosynthesis, plants use CO2 in the presence of sunlight to make food in the form of carbohydrates. It occurs in the chloroplast.Cellular respiration occurs within and near the mitochondria of eukaryotic cells.

Ventilation

First, some basic, working definitions:

• When you take hydrogen ions or electrons away from a molecule, you “oxidize” that molecule.

• When you give hydrogen ions or electrons to a molecule, you “reduce” that molecule.

• When you give phosphate molecules to a molecule, you “phosphorylate” that molecule.

So, oxidative phosphorylation means the process that couples the removal of hydrogen ions from one molecule and giving phosphates to another molecule. How does this apply to mitochondria? Let’s analyze the processes that allow this to occur.

Cellular respiration is a process which generates energy in living things. It extracts stored energy from glucose to form ATP (from ADP and Pi). The energy stored in ATP can then be used to drive processes requiring energy, including biosynthesis, locomotion or transportation of molecules across cell membranes… etc.

The chemical equation describing this process is:

C6H12O6 + 6O2 6CO2 + 6H2O + Energy

Respiration in the presence of O2 is called aerobic respiration. Aerobic respiration is divided into three components:

• Glycolysis

• Krebs cycle

• Oxidative phosphorylation (electron transport chain)

This is the first step of respiration. It occurs in the cytoplasm, just outside the mitochondria. It is the decomposition (or lysis) of glucose into pyruvate (or pyruvic acid). The following is a summary of the steps:

In summary: Glycolysis takes 1 glucose and turns it into 2 pyruvate, 2 PGAL, 2 NADH, and a net of 2 ATP (it actually makes 4, but remember it used 2 to begin with!)

For those of you who are

“visual”, here is another flow map

that shows the basics of glycolysis,

and how one molecule of

glucose makes 2

pyruvates, and 2ATPs!

Facilitated diffusion

Remember when the PGAL was used to build glucose during the Calvin-Benson Cycle?

Remember, the addition of phosphate groups energizes these molecules.

What are the three parts of cellular respiration?Glycolysis; Krebs Cycle; Oxidative phosphorylation (electron transport chain)

How is respiration different than ventilation?Respiration occurs at the cellular level. Ventilation

is mechanical, such as breathing.

Where does glycolysis occur?Outside of the mitochondria, in the cytoplasm

The addition of two ATP during the first steps of glycolysis, changes the glucose, energizing it, enabling it to be split into…

2 PGAL

Why is there a net of 2 ATP, when 4 are produced by glycolysis?

2 ATPs are used at the beginning to split the glucose

• Space inside cristae is called the matrix, which contains DNA and ribosomes.

•Krebs cycle takes place in matrix . It takes 2 ATP to transport one important product of glycolysis (NADH) into the matrix.

• Mitochondria has an outer phospho-lipid membrane and folded inner membrane

• Folds are called cristae. The cristae are where the ETC occurs.

Now that the pyruvate has formed during glycolysis, we must move it inside the mitochondria for the rest of cellular respiration.

This cycle details what happens to the pyruvate end product of glycolysis. Although the Krebs cycle is described for 1 pyruvate, remember that glycolysis produces 2 pyruvate. Multiply any products of this cycle below by 2 to account for the products of a single glucose molecule. The Krebs cycle continues the process of respiration from within the mitochondria, whereas the pyruvate products from glycolysis were generated outside in the cytoplasm..

The CO2 produced by the Krebs cycle is the CO2 that animals exhale when they breathe.

• Pyruvate to acetyl CoA. In a step leading up to the Krebs cycle, pyruvate combines with coenzyme A. In that reaction 1 NADH and 1 CO2 are also produced. (Called oxidative decarboxylation)

• Krebs cycle: Produces 3 NADH, 1 FADH2, 1 ATP, CO2 and citric acid by combining acetyl CoA and oxaloacetic acid (OAA). This is sometime called the citric acid cycle as a result of the product.

• Citric Acid is regenerated to begin the cycle again, even without glycolysis if need be.

Again, for those of you who are “visual”, here is

yet another graphic

depicting the Krebs or the

Citric acid cycle.

ketoglutaric acid

succinic acid, fumaric acid, malic

acid

These are the molecules

which migrate out to

participate in the ETC

This is waste

Here’s the energy…and remember to multiply it by 2, because glycolysis has produced 2 pyruvates

Pyruvate from glycolysis combines with:

Where does the Krebs Cycle occur?Within the mitochondria, in the space inside the

cristae known as the matrix.

By what product name is the Krebs Cycle also called?Citric Acid

What products of the Krebs Cycle are exported for use in oxidative phosphorylation?

3 NADHs, and 1 FADH

What waste is produced by the Krebs Cycle?CO2

How much ATP is produced during the Citric Acid Cycle?2 ATP

Oxidative phosphorylation is the process of extracting ATP from NADH and FADH2.

• Electrons from these two energy-rich molecules (which were produced during the citric acid cycle) pass along a chain. As they move from one carrier protein to another along the chain, they lose energy.

• Cytochromes (carrier proteins) participate to move these electrons along. “Cytochrome C” is one carrier that is often compared amongst species to compare relative relatedness.

• Electron Transport Chain takes place in cristae

• The last electron acceptor in the chain is oxygen, thus the importance of O2 in aerobic respiration. When the Oxygen accepts the electrons, it joins with hydrogen to form water.

NADH & FADH2 deliver H+ ions to inter-membrane space of mitochondria, and electrons to the electron transport chain within the membrane. These electrons carry the energy needed to “pump” the ions through the membrane against the gradient. As the hydrogen ions (protons) accumulate in the intermembrane space, there is a charge differential that occurs. It is this charge differential that drives the phosphorylation of ADP into ATP at the very end of the process. This energy can be saved up, and is known as PMF, or proton motive force. Employing this force to phosphorylate ATP is called chemiosmosis.

Inter-membrane space

As H+ concentration gradient increases in inter-membrane space, they begin to diffuse back into matrix through a special membrane protein in the Cristae called ATP synthase. This enzyme uses pmf in chemiosmosis to generate ATP…and quite a bit! Up to 34 ATPs can be synthesized in the process.

Inter-membrane space

As electrons are passed through several membrane proteins, they lend their energy to the pumps which move the hydrogen ions through the protein channels. At the end of the electron transport system, protons, electrons, and oxygen, the terminal electron acceptor, combine to form water. Since oxygen is the final electron acceptor, the process is called aerobic respiration. . Without those oxygen atoms, NADH cannot donate its electrons, and NAD+ cannot be recycled in glycolysis, when it is used to help make ATP

Source Number Net ATP Produced

Glycolysis 2 ATP

Transport of NADH into Matrix.

-2 ATP

Krebs Cycle (ATP & GTP) 2 ATP

Electron Transport (NADH & FADH2)

34 ATP

NET TOTAL 36 ATP

GTP, or guanocine triphosphate is another high energy molecule that is produced during the Krebs Cycle. As an energy source, it is primarily used to drive protein synthesis.

What are the two primary electron donators in oxidative phosphorylation, or the ETC?

NADH, and FADH

When and where are these molecules produced?During the Krebs Cycle; in the matrix

Where does the ETC occur?Cristae

What is the final electron acceptor in aerobic respiration, and what is the product?

Oxygen; water

ATP Synthase carries hydrogen ions (protons) back through the membrane to the matrix, where ATP is phosphorylated. What energy or force is used to phosphorylate ADP into ATP?

PMF; chemiosmosis

How many potential ATPs can be phosphorylated through oxidative phosphorylation?

34

Notice that there are two possible pathways after glycolysis, depending on whether O2 is present or not.• Oxidative Decarboxylation

• Fermentation

What if oxygen is not present? Without it, there is no electron acceptor to accept the electrons at the end of the electron transport chain. Without this, the aerobic cell dies.

Anaerobic respiration can occur in some specialized cells, however. Once the cell has broken down glucose (glycolysis), if there is no oxygen present, fermentation will occur.

In plants and yeast (fungi), alcohol is produced as a product, and in animals and bacteria, lactate (lactic acid) is produced.

Lactic acid fermentation

•Occurs in human muscle tissue - causes pain under high exertion

•Allows recharging of NADH to NAD+ so glycolysis can continue

•Follows following formula:

Pyruvic Acid + NADH NAD+ + Lactic Acid

There is only one step in lactate fermentation. A pyruvate is converted to lactate, or lactic acid. In the process, NADH gives up its electrons to form NAD+. NAD+ can now be used for glycolysis. Net, 2 ATPs…far better than 0 ATP!

When O2 is available again, the lactate can be broken down and energy available in this storage molecule can be retrieved.

Alcoholic fermentation

• Is widely used commercially to produce alcoholic beverages

• Allows recharging of NADH to NAD+ so glycolysis can continue

• Follows following formula: Pyruvic Acid + NADH NAD+ + Alcohol + CO2

Although glycolysis does not require oxygen, it does require NAD+. Cells without oxygen available need to regenerate NAD+ from NADH so that in the absence of oxygen, at least some ATP can be made by glycolysis.

To regenerate NAD+ from NADH, the electrons from NADH are added to pyruvate to produce alcohol (plants, yeast) or lactate (animals, bacteria).

The total ATP yield of fermentation comes from glycolysis; 2 ATP molecules are produced per glucose.

For each pyruvate, 1 CO2, and 1 acetaldehyde are produced. The CO2 formed is the source of carbonation in fermented drinks like beer and champagne.

The important part of the second step, acetaldehyde to ethanol, is that the energy in NADH is used to drive the reaction, releasing NAD+. For each acetaldehyde, 1 ethanol is made and 1 NAD+ is produced. NAD+ can then be used by glycolysis. Net…2 ATP. Again, much better than 0 ATP!

The two possible pathways after glycolysis are…Oxidative decarboxylation or fermentation, depending upon whether oxygen is available as an electron acceptor.

The two types of fermentation are…alcohol; lactic acid

Animals and some bacteria employ _________ fermentation, while plants use _________ fermentation.

lactic acid; alcohol

The point of fermentation, whether alcohol, or lactic acid, is…ATP can be generated, although in moderate amounts.

Energy within NADH is used to generate this ATP, and the resulting molecule of NAD can be used to recharge the process of glycolysis. No oxygen is required! No Krebs Cycle, and no oxidative phosphorylation is required either!

The total ATPs generated from glucose during glycolysis is _____, and while not as effective as oxidative phosphorylation (34), it is still better than “0”...which is what you’d get aerobically.

2