Cellular Respiration in Yeast

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  • 0:02 Definition of Yeast &…
  • 0:59 Aerobic Cellular Respiration
  • 3:26 Anaerobic Cellular Respiration
  • 4:15 Lesson Summary
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Lesson Transcript
Instructor: Amanda Robb
In this lesson, we'll learn about cellular respiration in yeast. Here, we'll cover what yeast is and the two ways that it uses cellular respiration to make energy.

Definition of Yeast & Cellular Respiration

Picture a calm Sunday in the kitchen. The smell of freshly baked bread rises from the oven. By mixing yeast, flour, sugar, and water, you were able to turn these simple ingredients into a loaf of bread. If you've ever made bread you know it takes a long time. After mixing the dough, it needs time to rise. Almost like magic, your bread doubles or even triples in size over an hour or two. How does this happen?

The key ingredient for making fluffy bread is yeast. Yeast is a single-celled fungus, which is alive and must make its own energy to survive. The yeast in your bread uses a process called cellular respiration, where glucose is converted to ATP and carbon dioxide. The carbon dioxide is what causes the bread to rise. The yeast produces this gas and the bread puffs up, incorporating the gas in between the flour. Today, we'll learn about this process, which requires oxygen, called aerobic respiration, and another process of yeast called anaerobic respiration.

Aerobic Cellular Respiration

Aerobic cellular respiration is cellular respiration that requires oxygen. There are three main steps to this process: glycolysis, the citric acid cycle, and oxidative phosphorylation. Here, we will look at each step in more detail.

Step 1: Glycolysis

During the first step of cellular respiration, glucose, a simple sugar, enters the cell. The yeast uses the glucose and creates two ATP, two NADH (a molecule that carries electrons to be used later), and two pyruvate needed for the next step.

Step 2: The Citric Acid Cycle

In the citric acid cycle, the pyruvate are converted to a molecule called acetyl-CoA in the mitochondria, the powerhouse of the cell. This molecule is regenerated during this step, hence the cycle part of the name. During this conversion, each pyruvate releases one carbon dioxide molecule. Next, acetyl-CoA is converted to several other molecules before being regenerated. During these reactions, two GTP, which are later converted to ATP, are made. This step also makes six NADH, the most of any step in cellular respiration, and one FADH2, a similar molecule. At this stage, two additional carbon dioxide are released.

Step 3: Oxidative Phosphorylation

Oxidative phosphorylation is the big energy payoff for yeast. This step occurs at the inner membrane of the mitochondria. All of the NADH and FADH2 created come to the membrane to drop off their electrons. They deliver them to proteins in the electron transport chain. Each protein likes each electron more than the one before it, so the electrons keep getting passed down the chain. At the end of the chain is oxygen, the ultimate electron acceptor. Oxygen takes two electrons and two hydrogen ions from the mitochondria and turns them into water.

You might still be wondering how energy is made. The secret is the electron transport chain. As the electrons are passed down, they use the energy to pump hydrogen ions into the intermembrane space between the inner and outer mitochondrial membrane. These hydrogen ions build up like water behind a dam. Like a dam, there is only one way for them to flow - down their concentration gradient to release the stored energy. The hydrogen ions flow through a protein called ATP synthase, which spins like a turbine to create ATP. Aerobic respiration is the most efficient and releases the most energy, 36 to 38 ATP per glucose molecule.

Anaerobic Respiration

Anaerobic respiration has to take place without oxygen. Yeast is special in that it can do both aerobic and anaerobic respiration; most organisms can only do one or the other. For example, humans are obligate aerobes, meaning we must have oxygen to make energy.

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