Krebs Cycle (Citric Acid Cycle) — Steps & Diagram

ATP Synthesis: Krebs Cycle

Cells need to make energy-rich molecules, like ATP and GTP, in order to carry out the processes necessary to stay alive.

The Krebs cycle (also called the citric acid cycle or TCA cycle) is just one part of ATP synthesis. The cycle is a set of reactions which produce a few ATP (or GTP) molecules and give off carbon dioxide. The reactions also produce a bunch of molecules of NADH and FADH$_2$. These are electron carriers that play a key role in the electron transport chain, where most ATP is made.

The chemical reactions that take place are pretty complex, but you can see an overview of the steps of the cycle in the diagram below.

Citric Acid Cycle

The second step in cellular respiration is called the citric acid cycle or the Krebs cycle.

The cycle requires oxygen, which means that it is an aerobic process. It takes place in the mitochondrial matrix. Recall that this is the liquid in the space between the folds of the inner membrane of the mitochondrion.

Each pyruvate made in glycolysis goes through the cycle. Since there are two pyruvate molecules made from each molecule of glucose, this means that the whole cycle runs two times.

In the cycle, each pyruvate from glycolysis is harvested for its electrons. In the process, for every two pyruvate molecules, two ATP are produced.

Let's talk for a second about this business of harvesting electrons. There are two electron carriers at work. They take electrons off molecules in the citric acid cycle and carry them to the next step of cellular respiration. Think of them as little electron suitcases.

The names of the carriers are NAD$^+$ and FADH. We know if they've got an electron with them because their names change slightly to NADH and FADH$_2$.

These guys will become important in the next step of cellular respiration.

For each turn of the cycle, three NAD$^+$ are converted into three NADH, and one FADH is converted into one FADH$_2$.

This means that for each glucose molecule we started with, we get a total of six NADH and two FADH$_2$.

The cycle also produces (per turn) two molecules of carbon dioxide ($CO_2$). You know the carbon dioxide you breathe out? This is where it comes from. Two acetyl groups each give up two carbon atoms, which are oxidized to create two molecules of $CO_2$.

Let's add all of this onto our picture of the citric acid cycle. Keep in mind, all of the values need to be doubled since the cycle turns twice per glucose molecule (our starting molecule).

Here's a summary for the entire cycle:

Key Vocabulary

1. Cellular respiration
2. Citric acid cycle
3. ATP
4. Mitochondria
5. Mitochondrial matrix
6. Aerobic

What Does the Krebs Cycle Produce?

The Krebs cycle, also known as the citric acid cycle, is the next step in cellular respiration. During this step, the pyruvate molecules are further broken down into carbon dioxide while their energy is harvested.

An enzyme known as coenzyme A grabs onto the acetic acid molecules and becomes acetyl-coenzyme A (acetyl-CoA). This coenzyme attaches the acetic acid molecule to another molecule in order to create citric acid.

This process kicks off when the pyruvic acid molecules pass from the cytoplasm, through the two membranes of the mitochondrion, and into the matrix of the mitochondrion. One carbon atom is then removed from the pyruvic acid and combined with two oxygen atoms to form carbon dioxide. The remaining two carbon atoms of the pyruvic acid rearrange themselves to form acetic acid (which you know as vinegar).

Energy Extraction

The next step of the Krebs cycle is energy extraction. The six-carbon citric acid is broken into another four-carbon molecule, releasing two more carbon dioxide molecules while NAD$^+$ grabs onto more electrons to become NADH. The electron carriers FAD and NAD$^+$ grab high-energy electrons a few different times during the cycle.

This NADH molecule is the same one needed to start the first step of the Krebs cycle. That's why it's called a cycle. If you look at the picture below, you'll see that ADP is turned into ATP once per cycle.

Each glucose molecule breaks down into two pyruvic acid molecules, so the Krebs cycle can be completed twice for each glucose present.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Glycolysis}}$ : A process that converts glucose molecules into usable energy

$\color{#8925AE}{\text{Cellular Respiration}}$ : A set of complex chemical reactions that provides a cell with energy

$\color{#8925AE}{\text{ATP}}$ : A molecule that stores energy in its bonds. The cell is able to break these bonds to release energy

$\color{#8925AE}{\text{Phosphorylation}}$ : Enzymes are able to break a bond between phosphate groups on ATP, turning it into ADP. This releases energy that the cell can use

$\color{#8925AE}{\text{Anaerobic}}$ : A reaction that needs oxygen to occur

$\color{#8925AE}{\text{Krebs Cycle}}$ : The first main part of cellular respiration where high energy electrons are moved to the electron transport chain

$\color{#8925AE}{\text{Electron Transport Chain}}$ : The final step of cellular respiration where 34 ATP molecules are made

(Video) The Citric Acid Cycle

by ndsuvirtualcell

This video gives a great visual explanation of the citric acid cycle and what is produced at each step.

(Video) Krebs!

by sciencemusicvideos

Learn about the Krebs cycle through song in this great video by Mr. W.