Heme Group: Hemoglobin & Definition

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  • 0:00 What is Heme?
  • 2:00 How Does Hemoglobin Work?
  • 2:45 Malformation of Hemoglobin
  • 3:25 Lesson Summary
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Lesson Transcript
Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

You've probably heard that something described as 'blood red.' But why, exactly, is blood red? The answer to this questions lies with a molecule called hemoglobin. In this lesson, we'll explore what hemoglobin is, how it works, and the role it plays in making horror movies so visually appealing.

What is Heme?

The primary job of an animal's circulatory system is to transport oxygen to the cells for aerobic respiration, and to carry carbon dioxide away. Red blood cells are the primary players in oxygen hauling, but they wouldn't be able to do their job if it weren't for a special molecule called heme. This molecule is the most important component in a protein called hemoglobin.

If you listen closely to conversation in a doctor's office or watch a medical drama on television, you'll notice a few terms that start with 'heme' being used - hematocrit, hemostat, and hemophilia, to name a few. All of those terms have something to do with blood. That's because 'heme' is the Greek word for blood.

A medical dictionary might define heme as something like: 'a prosthetic group containing an iron molecule centered in a porphyrin ring.' Let's translate that into simpler terms while looking at a line drawing of the heme molecule:

Line drawing of the heme molecule.
Line Drawing of the Heme Molecule

The heme molecule is a special chemical compound that is required for a protein to function (this defines a prosthetic group or co-factor). It contains an iron molecule, represented by the Fe in the molecule. The iron molecule is found the middle of a porphyrin ring (the clover-shaped molecule around the iron molecule.)

The most important part of heme, as far as we are concerned, is the iron atom. This is because iron binds oxygen in the form of O2, and is the critical component of the hemoglobin protein.

Hemoglobin is a protein that's made of four polypeptide chains. Each of these chains contains a heme group. Therefore, every hemoglobin molecule has the ability to bind four molecules of oxygen. Look at the following image of one molecule of hemoglobin. Each of the four protein chains is a different color.

One molecule of hemoglobin. Each of the four protein chains is a different color.
Space filling model of the hemoglobin molecule

Hemoglobin is the primary protein found in red blood cells, or erythrocytes. Each erythrocyte contains hundreds of thousands of hemoglobin molecules.

How Does Hemoglobin Work?

As red blood cells pass through the capillaries in the lungs, they are exposed to a high concentration of oxygen. This oxygen moves by diffusion into the cells, and binds to the iron atoms in the hemoglobin. We now call the molecule oxyhemoglobin. Oxyhemoglobin is bright red in color because the iron atom is in an oxidized state.

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