Cellular Respiration in Mitochondria

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  • 0:03 What Is Cellular Respiration?
  • 1:37 Krebs Cycle
  • 2:21 Electron Transport Chain
  • 4:19 Lesson Summary
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Lesson Transcript
Instructor: Dominic Corsini
How do cells actually get the energy needed to keep you alive? What does this energy look like? This lesson addresses these questions through an investigation of cellular respiration inside the mitochondria. A summary and quiz are included.

What Is Cellular Respiration?

All living things need energy to survive. Without energy, our cells cannot function and our bodies shut down. Life cannot exist without this constant supply of energy. But, where does it come from?

The energy your body uses to pump blood through your veins, inhale air into your lungs, and move your eyes across this page is derived from cellular respiration. Cellular respiration is a multi-step process that converts the chemical energy in food into usable cellular energy in the form of adenosine triphosphate, or ATP.

ATP is like the energy currency of cells. It is necessary for cellular function and sustaining your life. This ATP is produced primarily by an organelle called the mitochondria. Mitochondria are cellular organelles that synthesize ATP for our cells. Let's take a look at how cellular respiration works inside the mitochondria.

Remember that cellular respiration is aimed at creating ATP. Typically this process is broken into three phases, or steps. Those phases are glycolysis, the Krebs cycle, and the electron transport chain (or ETC for short). We'll only focus on these latter two stages because they occur inside the mitochondria.

  • The Krebs cycle uses pyruvic acid to create ATP and electron carrier molecules
  • The ETC uses these electron carrier molecules to synthesize large quantities of ATP

Let's look at each step in greater detail:

Krebs Cycle

We'll begin our investigation with the Krebs cycle. Pyruvic acid is created by breaking down your food. The Krebs cycle takes this pyruvic acid and changes it into citric acid by adding carbon and oxygen molecules. For this reason, the Krebs cycle is sometimes referred to as the citric acid cycle.

It then breaks the citric acid down into other forms. While these new forms aren't important, the by-products of the breakdown are. You see, the Krebs cycle creates the electron carrier molecules NADH and FADH2, along with ATP. Try not to get hung up on the chemistry. Instead, just remember that the Krebs cycle is creating a little energy in the form of ATP and electron carriers.

Electron Transport Chain

The electron transport chain is the final step in cellular respiration. It uses the electron carrier molecules NADH and FADH2 created during the Krebs cycle. This process occurs along the inner membrane of your mitochondria.

Inside your mitochondria, the cell's energy producing organelle, there is a membrane. That membrane is called the inner mitochondrial membrane and it's important. That's because when the NADH and FADH2 arrive near the membrane from deep inside the mitochondria, they have their hydrogen (H+) stripped away by carrier proteins. The hydrogen flows across the membrane and finds itself within the intermembrane space between the inner and outer membrane.

Then, once you get enough H+ molecules within the intermembrane space, they will start flowing back toward the membrane where they came from. However, they cannot exit the intermembrane space the same way they came in. Instead, they must flow through a protein called ATP synthase.

ATP synthase uses the hydrogen to create ATP. Thus, ATP is created by the ETC as H+ ions flow through the ATP synthase protein. Here's how that happens: remember that ATP stands for adenosine triphosphate. This molecule has three phosphates attached to it, hence the triphosphate name. Well, your cells also contain a molecule called adenosine diphosphate (ADP). This molecule contains two phosphates. ATP synthase uses energy from the H+ to bond another phosphate to ADP and turn it into ATP. Think of the H+ as providing the molecular glue needed to stick a phosphate onto ADP.

Once complete, the ETC will create approximately 34 ATP molecules for every 2 ATP created by the Krebs cycle. This makes the ETC your cells' primary source of ATP, but it could not function without electron carriers produced during the Krebs cycle.

Lesson Summary

Wow, take a breath, because that seems like a lot of technical information. So let's break it down, shall we?

Your body needs energy to survive, and this energy is synthesized during cellular respiration. Cellular respiration is a process that takes the energy from food and converts it into adenosine triphosphate, or ATP. ATP is the energy currency of cells and is produced inside the mitochondria.

Mitochondria are your cell's energy-producing organelles. They utilize two processes called the Krebs cycle and electron transport chain (ETC).

The Krebs cycle:

  • takes the pyruvic acid created by food digestion
  • adds a carbon and oxygen to create citric acid
  • breaks down citric acid into other forms, creating the byproducts of NADH and FADH2, which are electron carrier molecules

The ETC:

  • Strips the electron carriers of H+
  • H+ passes through into the intermembrane space
  • ATP synthase is a protein that takes the H+ from the intermembrane space and uses it to add a phospate to ADP to turn it into ATP

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