Erythrocytes: Hemoglobin - Online Tutor, Practice Problems & Exam Prep

Hemoglobin (Hb) is a crucial protein in red blood cells, primarily responsible for transporting oxygen (O₂) and carbon dioxide (CO₂). Each hemoglobin molecule consists of four subunits, each containing a heme group with an iron atom that binds O₂ reversibly, allowing for efficient gas exchange in the lungs and tissues. While hemoglobin plays a significant role in O₂ transport, most CO₂ is carried by plasma. The color of blood varies with oxygen content, appearing bright red when oxygenated and darker red when deoxygenated.

Function of Hemoglobin

In this video, we're going to talk about the function of hemoglobin. So hemoglobin can be abbreviated as HB, and it is a protein that is found inside of red blood cells or found inside of erythrocytes. Functionally, its role is to transport gases, specifically, oxygen gas or O₂ and carbon dioxide gas or CO₂. It's really important to note that inside of every red blood cell, there are tons and tons of hemoglobin protein molecules. In fact, it's estimated that just 1 single red blood cell contains about 250,000,000 hemoglobin protein molecules, making up about 97% of each red blood cell's mass. That is a ton of hemoglobin, so much so that we can pretty much think of red blood cells, or erythrocytes, as bags packed with tons of hemoglobin molecules. The reason that hemoglobin is able to effectively transport oxygen and carbon dioxide gas is because hemoglobin is able to bind them reversibly. The reversible binding is incredibly important, and what it means is that not only are the hemoglobin molecules able to bind to oxygen and carbon dioxide, but they're also able to release the oxygen and carbon dioxide appropriately in the right place at the right time. The appropriate release of oxygen and carbon dioxide is just as critically important as the initial binding of them.

Let's take a look at our image down below where we can start to piece some things together. Notice that this image is a bit of a silly cartoon that's designed to help you better understand the basic function of hemoglobin. Notice that we're representing hemoglobin with these buses that you can see throughout the image. Again, every single red blood cell would have about 250,000,000 of these hemoglobin buses inside of them. Notice on the left-hand side, we have the lung lounge, which is supposed to represent our lungs, which allows us to inhale oxygen gas and exhale carbon dioxide gas into the environment. And then on the right, what we have is the tissue tower, which represents the tissues that are found throughout our body that need to be able to receive oxygen gas from the blood. Back to the lung lounge, again, the lungs allow us to inhale oxygen gas. Notice that we're showing you the oxygen gas molecules right here in the image. In the lungs is where the hemoglobin molecules can pick up or bind to those oxygen gas molecules. We can say that O₂ pickup or oxygen gas pickup occurs in the lungs. Notice again the hemoglobin molecule is being shown as this bus, and what you'll notice is that this is a 4-seater bus that is binding to 4 oxygen gas molecules. This has to do with the structure of the hemoglobin molecule, which we'll talk more about in our next lesson video. But for now, you should think of the hemoglobin molecule as a 4-seater bus because each hemoglobin molecule can bind to 4 oxygen gas molecules, and so the hemoglobin is able to effectively transport oxygen because it can reversibly bind to the oxygen. It can bind to oxygen and pick it up in the lungs, but when the blood is pumped to the tissues, the conditions change in the tissues so that the hemoglobin will actually release or drop off the oxygen. We have O₂ drop off in the tissue tower. The oxygen that is dropped off to the tissues can then be utilized by those tissues to drive their metabolism. Through the metabolism of the tissues, they will also create waste products, such as carbon dioxide gas. Notice that we have carbon dioxide gas being shown below here. After the hemoglobin has dropped off the oxygen gas, it can then pick up the carbon dioxide gas waste product. Notice that we have CO₂ pickup in the tissue tower. You can see here that the CO₂ molecules are bound to the hemoglobin. But notice that the CO₂ molecules are bound in a different position than what we saw the oxygen gas molecules bound up above. The reason for this, again, has to do with the structure of the hemoglobin and the nature of how it goes about binding to oxygen versus how it goes about binding to carbon dioxide. We'll talk more about this in our next lesson video. But for now, what you should note is that the CO₂ molecules will bind to hemoglobin but at a different position than it will bind to oxygen gas. But regardless, the hemoglobin can still effectively transport the carbon dioxide gas because, again, it can bind to them in the tissues, but when it gets to the lungs, it will encounter a different set of conditions that will allow it to drop off and release the carbon dioxide gas. Notice we have CO₂ drop off in the lungs, and that allows the lungs to exhale the carbon dioxide. And then when we inhale again, we can inhale more oxygen gas so that this entire cycle can repeat over and over again. Something very important that you should know is that although hemoglobin does transport both oxygen and carbon dioxide gas, hemoglobin actually plays a larger role in transporting oxygen gas and a smaller role in transporting carbon dioxide gas. The reason for that is because hemoglobin only transports a small fraction of the total amount of carbon dioxide gas. Most of the carbon dioxide gas is actually transported by the blood's plasma, not by hemoglobin. This is an idea that we'll get to talk more about as we move forward.

The last thing that I'll leave you off with is notice we're using this reddish color to represent the oxygenated blood where hemoglobin is bound to oxygen. And notice we're using this bluish color to represent the deoxygenated blood where hemoglobin is bound to carbon dioxide instead of oxygen. These colors are pretty consistently used moving forward in our video lessons, but also in sources outside of our video lessons, like textbooks, for example. The reason these colors are used is only to help visually distinguish the two types of blood and to help enhance student learning. However, it's important to note that blood is never actually a blue color. When hemoglobin is bound to oxygen, it is going to make the blood a bright red color. But when hemoglobin is bound to carbon dioxide, it's not actually a blue color. Instead, it's a dark red color. Again, blood is always going to be red, but the shading of that red will vary depending on the oxygen content of the blood. Brighter red when it has high oxygen content and darker red when it has low oxygen content. That being said, this year concludes our video lesson on the function of hemoglobin, and moving forward, we'll get to learn more and talk about the structure of hemoglobin. So I'll see you all in our next video.

Erythrocytes: Hemoglobin Example 1

So here we have an example problem that's asking, which of the following is most likely to occur if hemoglobin started binding to oxygen irreversibly, and we've got these four potential answer options down below. And so first, we need to recall from our previous lesson video that hemoglobin actually binds to oxygen reversibly, not irreversibly, and so this problem is really just an if scenario. Recall that the reversible binding means that hemoglobin can not only bind to oxygen but can also release oxygen appropriately. And so what this means is the reversible binding allows hemoglobin to bind and release oxygen appropriately in the right place at the right time. Ultimately, this is what allows hemoglobin to serve as an efficient transporter of oxygen and deliver oxygen to the tissues. Now irreversible binding would mean that hemoglobin would bind to oxygen but would not release the oxygen. If it didn't release the oxygen, it would not be able to deliver the oxygen to the tissues, a

Why is it important for erythrocytes to have a high surface area to volume ratio?

• A. Increased cell flexibility.
• B. Allows for more efficient gas exchange.
• C. Allows each RBC to carry more hemoglobin.
• D. Allows RBCs to ft through small gaps.

Structure of Hemoglobin

This video, we're going to talk about the structure of hemoglobin.

And so we already know from our last lesson video that hemoglobin is a molecule found inside of red blood cells or inside of erythrocytes, and it's found in very large numbers. Functionally, we already know hemoglobin is important for the transport of oxygen and carbon dioxide gases. Although recall, hemoglobin plays a larger role in the transport of oxygen gas since most carbon dioxide gas is transported by the blood's plasma, not by hemoglobin. Now, structurally, hemoglobin, which again can be abbreviated as HB, is a 4 subunit protein, which means that it has 4 separate protein chains that come together to form the complete hemoglobin molecule, and this 4 subunit protein is called globin. Now this 4 subunit protein globin specifically transports oxygen gas using what are known as heme groups. And so we'll be able to see this down below in our image. But, really, this is where hemoglobin gets its name from, from the presence of the heme groups and the presence of this 4 subunit protein globin.

Now what's really important to note is that each of the 4 subunits in the hemoglobin molecule is going to have its own heme group, And so there are 4 heme groups, one for each of the 4 subunits of the hemoglobin molecule. And each heme group has its own central iron atom, or its own central Fe²⁺ atom, and each iron atom is capable of reversibly binding just one oxygen gas molecule. And so when hemoglobin binds to oxygen gas in this way, we refer to it as oxygen-rich hemoglobin. And so once again, because there are 4 subunits in one hemoglobin molecule, each with their own heme group and each capable of binding 1 oxygen gas molecule, what this means is that each hemoglobin molecule can actually carry up to a maximum of 4 oxygen gas molecules at once. Again, since each oxygen each subunit can carry 1 oxygen gas molecule. And so we can actually indicate this with this chemical formula that you see right here, where again the HB is the abbreviation for hemoglobin, the O₂ is the chemical formula for oxygen gas, and then the 4 subscript here is indicating that there are 4 oxygen gas molecules bound to this one hemoglobin, which has 4 subunits. And so this detail here is very important to keep in mind that each hemoglobin can bind up to a maximum of 4 oxygen gas molecules.

Now again, we know hemoglobin can also bind and transport carbon dioxide gas, And so it turns out that hemoglobin can also bind up to 4 carbon dioxide gas molecules at once. And when hemoglobin is bound to carbon dioxide gas, we specifically refer to it as deoxygenated hemoglobin or carbaminohemoglobin, But what's really important to note is that hemoglobin will bind oxygen and carbon dioxide via different mechanisms. Once again, hemoglobin will bind to oxygen gas using heme groups. But the heme groups do not bind to carbon dioxide gas. Instead, the carbon dioxide gas is going to be bound via amino groups, not the heme group. So let's take a look at our image down below to get a better understanding. And notice on the left hand side, we're showing you the same exact buses that we showed you in our last lesson video, which represent hemoglobin. And notice that this red bus at the top is bound to 4 oxygen gas molecules, and so the red bus represents oxygenated hemoglobin. And the blue bus down below, notice, is bound to 4 carbon dioxide gas molecules, and so the blue bus represents deoxygenated hemoglobin or carbaminohemoglobin.

Now one thing to notice is that the deoxygenated hemoglobin is binding to the carbon dioxide gas at a different position than the oxygenated hemoglobin binds to the oxygen gas. And so this is supposed to represent that hemoglobin will bind to oxygen and carbon dioxide via different mechanisms.

Now on the right, we're focusing in on hemoglobin structure, and we know once again that hemoglobin is a 4 subunit protein. In fact, hemoglobin has 2 subunits, shown here in blue, which are alpha subunits, and the alpha can actually be written out as alpha or symbolized with the Greek letter alpha, and then it has these 2 beta subunits, and again, it can be written out as beta or symbolized with the Greek letter beta. And so the 2 alpha subunits are identical to each other, and the 2 beta subunits are identical to each other. But, of course, the alpha and beta subunits are different from one another. But regardless, all 4 of these subunits, which you'll notice, has a heme group that is present. And so notice on the right, we are zooming in to the structure of the heme group. And so it does have a pretty complex structure. And, in most cases, you wouldn't be expected to have to memorize the structure. But this structure here is the heme group, so we can go ahead and label it as so. And what you'll notice is that right in the center of the heme group, is going to be that iron atom, the Fe²⁺ atom. And so this iron atom is what is capable of reversibly binding oxygen gas. And so although it's not being shown, again, it's important to know that the iron interacts with the oxygen gas for binding. And so again, because each of these subunits has a heme group, that allows each one of these subunits to reversibly bind 1 oxygen, and that means that this entire hemoglobin molecule can bind a maximum of 4 oxygen gas molecules as we discussed.

And so this here concludes our brief lesson on the structure of hemoglobin, and as we move forward, we'll be able to apply these concepts and learn more. So I'll see you in our next video.

Erythrocytes: Hemoglobin Example 2

So here we have an example problem that's asking if each erythrocyte, or red blood cell, can carry 250,000,000 Hemoglobin molecules, how many molecules of oxygen gas can each erythrocyte carry at a time? And we've got these four potential answer options down below. To solve this problem, we'll need to do just a little bit of math. Again, the problem is telling us that there are 250,000,000 hemoglobin molecules, which we can abbreviate as HB, for every one erythrocyte, or for every one red blood cell, which we can abbreviate as RBC. We need to recall from our previous lesson videos that for every one hemoglobin molecule, it can carry up to a maximum of 4 oxygen gas molecules. So really, what we need to do is multiply these two ratios together. This becomes a very simple multiplication problem. You'll notice that the units of hemoglobin are going to cancel each other out, so we can cross those off. What we're left with are units of oxygen and units of red blood cells. We can go ahead and have our ratio here, and add in those units of oxygen, and again, red blood cell on bottom. All we need to do is multiply our numbers straight across in these ratios. So 250,000,000×4 is actually 1,000,000,000. And so we can put 1,000,000,000 on top, and of course, 1×1 is going to be 1, so we can put 1 down below. This is going to be our answer over here that there are going to be 1,000,000,000 oxygen gas molecules for every one red blood cell, and that's exactly what the problem is asking us. So notice that answer option d over here says 1,000,000,000, and that is going to be the correct answer for this problem. Theoretically, if every red blood cell carried 250,000,000 hemoglobin molecules, then theoretically, each erythrocyte could carry up to a maximum of 1,000,000,000 oxygen gas molecules. So this concludes this problem, and I'll see you all in our next video.

How many heme groups are required to synthesize 4 hemoglobin molecules?

• A. 4
• B. 8
• C. 16
• D. 32

Cynthia lives in Miami, a city at low altitude. She goes on a month-long trip to the Andes Mountain range (at high altitude) where the air pressure is lower and therefore there is less oxygen in each breath she takes. Upon returning to Miami, how might a sample of her blood differ from a sample taken before she left for her trip?

• A. Each of her blood cells would carry fewer hemoglobin molecules.
• B. Each of her hemoglobin molecules would carry 5-6 oxygen molecules.
• C. Her hematocrit will have increased (more erythrocytes per μL).
• D. There would likely be no change.

Anemia is a blood disorder where the $O_2$-carrying capacity of blood is too low to support the body's tissues. Which of the following is not a possible cause of anemia?

• A. Iron deficiency.
• B. Blood loss.
• C. Deformed hemoglobin.
• D. Excess erythrocytes.

Here’s what students ask on this topic:

What is the primary function of hemoglobin in red blood cells?

The primary function of hemoglobin (Hb) in red blood cells is to transport gases, specifically oxygen (O₂) and carbon dioxide (CO₂). Hemoglobin binds to oxygen in the lungs and releases it in the tissues, facilitating cellular respiration. It also picks up CO₂ from the tissues and releases it in the lungs for exhalation. This reversible binding is crucial for efficient gas exchange and maintaining homeostasis in the body.

How many oxygen molecules can one hemoglobin molecule bind?

One hemoglobin molecule can bind up to four oxygen (O₂) molecules. This is because hemoglobin is a tetramer, consisting of four subunits, each with its own heme group containing an iron atom (Fe²⁺). Each iron atom can reversibly bind one oxygen molecule, allowing a single hemoglobin molecule to carry a maximum of four oxygen molecules at once.

What is the difference between oxyhemoglobin and deoxyhemoglobin?

Oxyhemoglobin refers to hemoglobin that is bound to oxygen (O₂) molecules, typically found in oxygen-rich environments like the lungs. It appears bright red in color. Deoxyhemoglobin, on the other hand, is hemoglobin that is not bound to oxygen but may be bound to carbon dioxide (CO₂) instead. It appears darker red and is found in oxygen-poor environments like the tissues. The binding sites for O₂ and CO₂ on hemoglobin are different, with O₂ binding to the heme groups and CO₂ binding to amino groups.

Why does hemoglobin play a larger role in oxygen transport compared to carbon dioxide transport?

Hemoglobin plays a larger role in oxygen (O₂) transport because it is specifically designed to bind and release oxygen efficiently. Each hemoglobin molecule can carry up to four oxygen molecules, making it highly effective for oxygen transport. In contrast, only a small fraction of carbon dioxide (CO₂) is transported by hemoglobin. Most CO₂ is carried in the blood plasma as bicarbonate ions (HCO₃^-), which is a more efficient method for CO₂ transport in the body.

What is the structure of hemoglobin and how does it facilitate its function?

Hemoglobin is a tetrameric protein composed of four subunits: two alpha (α) and two beta (β) chains. Each subunit contains a heme group with a central iron atom (Fe²⁺) that can reversibly bind one oxygen (O₂) molecule. This structure allows each hemoglobin molecule to carry up to four oxygen molecules. The reversible binding of oxygen and carbon dioxide to different sites on the hemoglobin molecule facilitates efficient gas exchange in the lungs and tissues, supporting cellular respiration and maintaining homeostasis.