Chapter 5: Capturing and Releasing EnergyPart 3

A Global Connection The first cells on Earth didn’t use sunlight for energy. Instead, they used molecules such as methane and

hydrogen sulfide (gases that were abundant in early Earth’s toxic atmosphere) for energy.

About 3.2 billion years ago, the first photosynthetic cells evolved.

These photoautotrophs were able able to utilize an unlimited supply of energy (the sun) and were hugely successful.

Oxygen gas from the abundant photosynthetic cells accumulated in the water and in the atmosphere.

A Global Connection However, this oxygen was toxic to most living cells at that

time. Since oxygen gas reacts easily with metals such as those

used by cells for enzyme cofactors and free radicals, which damage biological molecules, form during this process, oxygen is dangerous to life.

Early cells had no way to detoxify these free radicals so most living cells at that time quickly died out.

The only ones who were able to survive were those that were able to find oxygen-free (or anaerobic) environments such as deep water or mud.

A Global Connection Due to the selection pressures that an oxygen-rich

atmosphere put on the living cells at the time, new metabolic pathways that detoxified oxygen free radicals evolved.

Cells that possessed these pathways were able to survive in the presence of oxygen and were the first aerobic organisms.

Aerobic respiration is one of these pathways that does more than just allow the cell to tolerate oxygen; it allows the cell to USE oxygen to break down the bonds of carbohydrates that are produced by photosynthesis to release their energy.

A Global Connection Aerobic respiration evolved to enable cells to live in the

oxygen-rich environment created by photosynthesis. Furthermore, aerobic respiration also produces as its by-

products the molecules that photosynthetic organisms require to perform photosynthesis.

Was this a coincidence?? NO!!! With this connection, the cycling of carbon, hydrogen, and

oxygen through living things came full circle.

A Global Connection

Extracting Energy From Carbohydrates The energy stored in the bonds of carbohydrates must be

transferred to ATP so that it can be used by cells for the energy-requiring reactions of life.

There are a few different pathways that can be used to break the bonds of carbohydrates to release the energy that drives the synthesis of ATP, but aerobic respiration is by far the most common.

Aerobic Respiration There are three stage of aerobic respiration. The first stage, called glycolysis, cuts one 6-carbon glucose

molecule into two 3-carbon molecules called pyruvate. Glycolysis occurs in the cytoplasm of the cell. During glycolysis, two ATP are used to form four ATP, so there is

an overall gain of 2 ATP from this anaerobic process. Also produced during glycolysis are 2 NADH molecules. NADH is formed when coenzyme NAD+ combines with

H+/electrons. Thus NADH is a coenzyme carrier of high energy electrons whose

energy can be released and stored as ATP when they are passed along an electron transport chain later on during aerobic respiration.

Glycolysis

Aerobic Respiration The second stage of aerobic respiration occurs in the matrix

of the mitochondrion.

Aerobic Respiration The second stage includes both Acetyl-CoA formation and the Kreb’s cycle. When pyruvate (a 3-carbon molecule) from glycolysis enters the matrix of the

mitochondrion, a carbon is cut off of the pyruvate, resulting in the production of carbon dioxide and an intermediate called Acetyl-CoA (a 2-carbon molecule).

Oxygen combines with carbon from the pyruvate to form the carbon dioxide. The Kreb’s cycle then adds the 2-carbon Acetyl-CoA molecule to a 4-carbon

molecule, forming a 6-carbon molecule that will then be broken down into carbon dioxide molecules.

The Kreb’s cycle, like the Calvin-Benson cycle of photosynthesis, is a cyclic pathway.

In the end, two molecules of pyruvate (two 3-carbon molecules) are converted to 6 molecules of carbon dioxide (six 1-carbon molecules).

Also produced during this second stage of aerobic respiration are 2 ATP molecules, 6 NADH, and 2 FADH2 molecules.

FADH2 is another coenzyme carrier of high energy electrons (like NADH) that will enter the electron transport chain, where its electrons will have their energy released and stored in the form of ATP.

Acetyl-CoA Formation and the Kreb’s Cycle

Aerobic Respiration At this point, there is a total net gain of 4 ATP (2 from glycolysis

and 2 from the Kreb’s cycle), 8 NADH (2 from glycolysis and 6 from the Kreb’s cycle), and 2 FADH2 (both from the Kreb’s cycle).

As you can see, very little of the energy from the bonds of glucose has actually been stored as ATP.

Most of the energy is still carried in the high energy electrons of NADH and FADH2 and is waiting to be released.

Aerobic Respiration The third and final stage of aerobic respiration, in which ATP

is produced as a result of hydrogen ions moving through ATP synthase in an electron transport chain (called electron transfer phosphorylation -sound familiar??) occurs on the cristae of the mitochondria.

Aerobic Respiration As electrons from NADH and FADH2 pass through the electron transfer

chain embedded in the cristae, they give up their energy a little at a time.

This energy is then used to pump H+ from the matrix into the intermembrane compartment.

As H+ are pumped across the cristae, an H+ gradient forms (there are more H+ in the intermembrane compartment than in the matrix).

This then causes the hydrogen ions to want to diffuse back into the matrix (to move from higher to lower concentration).

In order to cross the cristae, the H+ must move through ATP synthases (transport proteins in the cristae).

As the H+ move through the ATP synthase, Pi is added to ADP forming ATP.

By this process, 32 ATP can be produced from one glucose molecule, resulting in a total of 36 ATP being produced as a result of aerobic respiration.

Electron Transport Chain: also called Oxidative Phosphorylation

Aerobic Respiration At the end of the electron transport chain, oxygen accepts

the energy depleted electrons/H+, forming water. Therefore, oxygen is the final electron acceptor of this

pathway; this electron transport pathway cannot occur without oxygen.

Aerobic Respiration

Energy Totals from Aerobic Respiration

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HOMEWORK 1. How are photosynthesis and aerobic respiration similar

and how are they different? Give examples of specific processes that happen in both or are unique to each.

2. Which stage of respiration does not actually require oxygen in order to proceed?

3. How much energy (ATP) could be produced if no oxygen were available for the oxygen requiring stage of respiration?