Electron Transport Chain

Biology Index

Where are we going with this? The information on this page should increase understanding related to this standard:  Model and understand aerobic respiration demonstrating the flow of matter and energy out of a cell and explain energy transfer systems. Also, compare aerobic respiration to alternative processes of glucose metabolism.

Article includes ideas, images, and content from Troy Smigielski (2021-10)

Electron Transport Chain

(Chains? So, no cycle and all that going around and around?)

So, how many names does this thing have?

At least three!

"The electron transport chain (ETC; respiratory chain) is a series of protein complexes that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples this electron transfer with the transfer of protons (H+ ions) across a membrane. The electron transport chain is built up of peptides, enzymes, and other molecules…" (Source, 2021-11).

Am I supposed to understand any of that! 

Let's try to get a handle on that. So…

The electron transport chain is the final step in cellular respiration. 

• It receives electron carriers from the Krebs cycle and uses them, along with oxygen (hence it is aerobic), to create ATP which is passed on to the cell to provide energy for cellular functioning.
• The process takes place in the inner membrane of the mitochondria. 
• It produces water as a byproduct.

Nice! Can we just stop now?

The whole process begins with glycolysis…

that sends pyruvate (which is broken down into acetyl CoA first) to the Krebs cycle.

The Krebs cycle creates and sends electrons to the electron transport chain. The electron carriers are NADH and FADH2.

The Krebs cycle is considered to be anaerobic. Why? 

It is part of a system that includes the electron transport system that, we will see later, needs oxygen. So, because without oxygen, the ETC cannot use the electrons from NADH and FADH2. If they can’t drop off electrons, they can’t become NAD+ and FAD. Without NAD+ and FAD, the Krebs cycle cannot function. Therefore, the Krebs cycle indirectly requires oxygen.

At the end of the Krebs cycle, the are sent to the electron transport chain to be converted into ATP.

The ETC is the final step of cellular respiration. We further discussed that it occurs in the mitochondria and that it is aerobic.

Okay… about those names…

It is also called oxidative phosphorylation.

Seriously! Stop!

“Oxidative” refers to oxidation, which is when a molecule loses an electron. “Phosphorylation” refers to the attachment of a phosphate to ADP, which makes ATP.

Now, recall that the overall, main purpose of cellular respiration is to create ATP!

Source 2021-11

In discussing glycolysis and the Krebs cycle, we previewed the role of the electron transport chain and learned that it is responsible for the bulk of ATP produced. 

How? 

The ETC is located in the inner membrane of the mitochondrion.

The electron transport chain will use electrons that are dropped off by NADH and FADH2 to create ATP. 

• Each NADH can produce 3 ATP.
• Each FADH2 can produce 2 ATP.

Steps of the Electron Transport Chain

• Electrons are brought to the ETC by NADH and FADH2.
• When electrons are dropped off, the Hydrogen that they are attached to is dropped off too. 
• Therefore, when electrons are dropped off, NADH becomes NAD+ and FADH2 becomes FAD.

NAD+ and FAD go back to glycolysis and Krebs Cycle to pick up more electrons and keep the process going.

As more H+ ions and electrons are dropped off, H+ ions start to build up in the intermembrane space. Naturally, they want to diffuse out, but they are charged. Therefore, they must exit through a transport protein via facilitated diffusion.

The transport protein that allows the H+ ions to leave is called ATP Synthase. When this happens, ATP Synthase spins which provides enough energy to create ATP. When this happens, ATP Synthase spins which provides enough energy to create ATP.

Meanwhile, the electrons travel down the ETC and release energy.

This energy is used to push more H+ ions out because the more H+ ions that go out, the more H+ ions must come back in through ATP Synthase. This forces your cell to produce more ATP.

At the end of the chain, the electrons are fresh out of energy. They are “accepted” by oxygen. 

In other words, oxygen is the final electron acceptor.

Some H+ ions attach to the electron and oxygen, which forms water.

The Totals

• About 36-38 total ATP are made in cellular respiration.
• Remember, 2 ATP were used in glycolysis.
• The net gain of ATP in cellular respiration is 34-36 ATP.
• Glycolysis: 2 ATP (anaerobic)
• Citric Acid Cycle + ETC: ~34 ATP (aerobic)

Despite the entirety of the system, some energy is left in the original glucose molecule. The energy in glucose that is not used to make energy is lost as heat, which helps keep our bodies at homeostasis.