Understanding Cell Size and Efficiency

Consider this. A bacterial cell can be as small as 0.00001 meters. Even with our most advanced light microscopes, a bacterial cell appears as a small dot or squiggle. By contrast, the biggest cells are clearly visible to the naked eye. These frog cells, for example, are nearly one centimeter across. That means that a frog egg is 10,000 times bigger than a bacterial cell, yet they are made from the same exact subcellular components. In this video, we are going to see how this is possible and why cells can grow to such different sizes. Here is a quick outline of the topics we will be covering. First, we will take a look at the shape and size of cells and how it relates to the surface area to volume ratios. Then, we will review how to calculate surface area to volume ratios using simple geometry. We will see why surface area to volume ratios limit cell sizes. And finally, we will look at why the cell size is so important to obtaining nutrients and eliminating wastes. If you need to review specific parts of this section, Please feel free to skip forward to the times outlined here. The big picture of Section 2.3 is that the complex organization of living systems requires constant energy input and the exchange of macromolecules. To understand how cells accomplish these tasks, we must first look at the size and shape of cells. Surface area to volume ratios are incredibly important to cells, as they affect the ability of cells to obtain nutrients and gather energy from the environment. In fact, the smallest known cells are about 1 micrometer. These bacterial cells are similar in size to chloroplasts and mitochondria. Bacterial cells can get slightly bigger, up to about 100 micrometers. However, with their simple construction and lack of internal organelles, this is about the maximum size of a bacterial cell. To become larger, cells need an endomembrane system that helps efficiently distribute nutrients and excrete waste products. Eukaryotic cells can get up to 1 cm, though the large majority of eukaryotic cells are closer to 100 micrometers. In fact, if you look at the blood cells in an elephant and a mouse, you will find that they are the exact same size. That is because blood cells are maximally efficient at this particular size. Other cells may be bigger in the elephant, such as the nerve cells that must span much larger distances. But in general, cells need to reach a maximum size based on one factor, their surface area to volume ratio. Lets take a look at this. concept. All cells, from bacterial cells to the cells in your body, must exchange macromolecules, gases, and water with the outside environment. The cell membrane is composed of many phospholipid molecules. These phospholipid hold a high number of proteins which are responsible for importing or exporting substances to or from the cell. As a cell gets larger the volume of the cell increases much faster than the surface area. Therefore, the cell would need more membrane proteins in order to fulfill the needs of the internal cytosol. At a certain point, a cell would need more proteins than it can reasonably pack into the cell membrane. This is why cells have an upper limit. Likewise, as the cell gets smaller, It limits the number of phospholipid molecules that surround it, and therefore the number of proteins that can be held within the membrane. Plus, the internal volume of a cell must be able to house and replicate the DNA. Therefore, at a certain point, there is simply not enough room to complete the functions of life. This forms the lower limit of a cell size. This is why surface area to volume ratio of a cell is so important. It is an effective way to measure how efficiently a cell can exchange the substances it needs to survive. Let's take a break for a second and try to visualize all of the things that we're talking about. Here you can see a coffee bean, a grain of rice, and a sesame seed. If we zoom in a bit, we can see that the largest single-celled organisms are about the size of a grain of salt. A human egg cell is about half this size, and human skin cells are even smaller than that. Some of the smallest human cells are about half the size of a human egg. cells red blood cells are still much much larger than a bacterium most bacterium are about the size of the organelles within eukaryotic organisms but they are still much much larger than viruses from viruses it's still a long way to go before we get all the way down to a single carbon atom which is the basis of life on earth This amazing visual resource was created by the University of Utah and it can be found through the link in the video's description. Let's continue by learning how surface area to volume ratios affect the size of all of these different cell types. A surface area to volume ratio is exactly what it sounds like. It is the total surface area of a cell divided by the total volume of a cell. In order to calculate the surface area to volume ratio for a cell, first we have to to find an appropriate model for the cell. Most plant cells have a cuboid shape. If we pretend that this cell has a height of 2 millimeters and a width and depth of 1 millimeter each, we can easily calculate both the volume and the surface area. The volume is 2 while the surface area is 10. So the surface area to volume ratio is 10 divided by 2 which reduces to 5. If we consider a similar sized single-celled organism only with a spherical shape, let's see what we get. If the diameter of this cell is 2 millimeters, the radius is 1 millimeter. Therefore, the volume is about 4.2 millimeters cubed. The surface area is about 12.6 millimeters squared. Therefore, the surface area to volume ratio is 12.6 divided by 4.2. This reduces to 3. So this cell has a slightly smaller surface area to volume ratio simply because it is spherical in shape. This tells us not only does size matter, but shape can also increase or decrease the surface area to volume ratio. You can now pause the video and answer these questions. There is another quiz at the end of the video and you can find all the answers through the quick test prep link below. The limits imposed by surface area to volume ratio of a cell can be seen through several examples. Let's start with the largest cells in nature, eggs. At this end of the spectrum, the cell has much more volume than it has surface area. In fact, the size of cells in eggs rapidly reduces so the cell can become functional. Eggs only have one simple function that do not require exchanging many molecules with the environment. After only a few days, one cell becomes hundreds of cells. The cells get consecutively smaller with each division, becoming more efficient each time. Thus, by the time they hatch as a fully formed organism, the average cell size in an egg has decreased by orders of magnitude. By contrast, these bacteria are at the other end of the size spectrum. They are just large enough to house their DNA and carry out the basic functions of life. Compared to their volume, their surface area is much higher. giving them a very high surface area to volume ratio. In fact, if they were any smaller, these cells would not have the volume to allow DNA to replicate properly, and their lipid bilayer may not have enough room for all the proteins needed to transport macromolecules or reproduce the DNA. The best of us need to take a break from time to time. Studies have shown that a quick break while you're studying can help you retain more information. So go get some water, take a quick walk, or maybe go for a swing. When we come back we'll see why cell size is so important to organisms. While these surface area to volume constraints apply to single cells, there are many ways organisms can increase the efficiency of obtaining nutrients within a larger organism. Most common method of increasing efficiency is to create folds in tissues, cells, and cell membranes. In turn, this increases the surface area to volume ratio and allows the cells to be as efficient as possible. Let's consider a couple of examples. This structure is one of many alveoli found in your lungs. It is a small sac that is responsible for absorbing oxygen into the bloodstream. The alveoli also expel carbon dioxide from the bloodstream and into the lungs. Instead of one large cell at the end of each airway, these small sacs are lined with hundreds of tiny rectangular cells. If oxygen and carbon dioxide had to diffuse through only one large cell, it would take a lot of time and be much less efficient. By contrast, the many endothelial cells that create the alveoli can quickly transfer oxygen into the capillaries and carbon dioxide out of the body. Similar to the folding pattern seen in the alveoli, is the massive surface area increase seen in the small intestine. In fact, there are three levels of folding that happen to dramatically increase the surface area available to absorb important nutrients from your food. First, the small Intestine lining itself is folded, doubling the surface area of each fold. On top of these folds are more tiny projections, called villi, that also double the surface area available. Each villus is lined with blood vessels, allowing quick transfer of oxygen, nutrients, and waste products. Plus, each villus is covered in tiny rectangular cells. If we look closely at these cells, We can see that each cell has its own microvilli. These tiny extensions of the cell membrane massively increase the surface area available to take in nutrients. Altogether, the folds of this system increase the efficiency of the intestinal lining thousands of times. Similarly, The excretory system of humans also works by making convoluted folds in tissues, cells, and individual cell membranes that drastically increase their surface area. For example, let's take a look at the kidneys, the body's main excretory organ. The kidney is made up of many identical structures. Each medulla is packed with cells and has both arteries and veins, taking in blood from the heart and filtering out nitrogenous wastes and sending the blood back to the heart. These cells are folded into complex shapes that drastically increase their efficiency. Inside each medulla are the Malpighian tubules. These tubules are made of many smaller cells which have very different functions in different parts of the fold. Like the cells in each alveoli, Each of these cells is small, rectangular, and pretty flat, allowing for the maximum efficiency. In fact, a single kidney can filter half a cup of blood every minute. Since you have about 20 cups of blood in your body, that means your kidneys can filter all of the blood in your body every 40 minutes. While importing and exporting substances is the main reason why cells have surface area to volume ratio limits, there are many other functions that need cells of a certain size. For instance, certain organisms need to dissipate heat. They do so with many small cells on the skin and in the lungs that can transfer heat out of the body. By contrast, seals that swim in the frigid waters of the Arctic are covered in a layer of very large fat cells. These cells hold in heat, allowing them to live in water that would quickly give a human hypothermia. You can now pause the video a second time and answer this set of questions. You can find the answers for all the questions in this video through the quick test prep link found below. You can also explore the other resources we have for this section. They can help you study for the AP test, so check them out! If you enjoyed this video, please click the like button and subscribe to the Biology Dictionary channel. Please leave us comments if you have any questions about cell size or surface area to volume ratios. Good luck!

These frog cells, for example, are nearly one centimeter across. That means that a frog egg is 10,000 times bigger than a bacterial cell, yet they are made from the same exact subcellular components. In this video, we are going to see how this is possible and why cells can grow to such different sizes.

Here is a quick outline of the topics we will be covering. First, we will take a look at the shape and size of cells and how it relates to the surface area to volume ratios. Then, we will review how to calculate surface area to volume ratios using simple geometry. We will see why surface area to volume ratios limit cell sizes. And finally, we will look at why the cell size is so important to obtaining nutrients and eliminating wastes.

If you need to review specific parts of this section, Please feel free to skip forward to the times outlined here. The big picture of Section 2.3 is that the complex organization of living systems requires constant energy input and the exchange of macromolecules. To understand how cells accomplish these tasks, we must first look at the size and shape of cells. Surface area to volume ratios are incredibly important to cells, as they affect the ability of cells to obtain nutrients and gather energy from the environment. In fact, the smallest known cells are about 1 micrometer.

These bacterial cells are similar in size to chloroplasts and mitochondria. Bacterial cells can get slightly bigger, up to about 100 micrometers. However, with their simple construction and lack of internal organelles, this is about the maximum size of a bacterial cell.

To become larger, cells need an endomembrane system that helps efficiently distribute nutrients and excrete waste products. Eukaryotic cells can get up to 1 cm, though the large majority of eukaryotic cells are closer to 100 micrometers. In fact, if you look at the blood cells in an elephant and a mouse, you will find that they are the exact same size. That is because blood cells are maximally efficient at this particular size. Other cells may be bigger in the elephant, such as the nerve cells that must span much larger distances.

But in general, cells need to reach a maximum size based on one factor, their surface area to volume ratio. Lets take a look at this. concept.

All cells, from bacterial cells to the cells in your body, must exchange macromolecules, gases, and water with the outside environment. The cell membrane is composed of many phospholipid molecules. These phospholipid hold a high number of proteins which are responsible for importing or exporting substances to or from the cell.

As a cell gets larger the volume of the cell increases much faster than the surface area. Therefore, the cell would need more membrane proteins in order to fulfill the needs of the internal cytosol. At a certain point, a cell would need more proteins than it can reasonably pack into the cell membrane.

This is why cells have an upper limit. Likewise, as the cell gets smaller, It limits the number of phospholipid molecules that surround it, and therefore the number of proteins that can be held within the membrane. Plus, the internal volume of a cell must be able to house and replicate the DNA. Therefore, at a certain point, there is simply not enough room to complete the functions of life.

This forms the lower limit of a cell size. This is why surface area to volume ratio of a cell is so important. It is an effective way to measure how efficiently a cell can exchange the substances it needs to survive. Let's take a break for a second and try to visualize all of the things that we're talking about.

Here you can see a coffee bean, a grain of rice, and a sesame seed. If we zoom in a bit, we can see that the largest single-celled organisms are about the size of a grain of salt. A human egg cell is about half this size, and human skin cells are even smaller than that. Some of the smallest human cells are about half the size of a human egg. cells red blood cells are still much much larger than a bacterium most bacterium are about the size of the organelles within eukaryotic organisms but they are still much much larger than viruses from viruses it's still a long way to go before we get all the way down to a single carbon atom which is the basis of life on earth This amazing visual resource was created by the University of Utah and it can be found through the link in the video's description.

Let's continue by learning how surface area to volume ratios affect the size of all of these different cell types. A surface area to volume ratio is exactly what it sounds like. It is the total surface area of a cell divided by the total volume of a cell. In order to calculate the surface area to volume ratio for a cell, first we have to to find an appropriate model for the cell.

Most plant cells have a cuboid shape. If we pretend that this cell has a height of 2 millimeters and a width and depth of 1 millimeter each, we can easily calculate both the volume and the surface area. The volume is 2 while the surface area is 10. So the surface area to volume ratio is 10 divided by 2 which reduces to 5. If we consider a similar sized single-celled organism only with a spherical shape, let's see what we get. If the diameter of this cell is 2 millimeters, the radius is 1 millimeter.

Therefore, the volume is about 4.2 millimeters cubed. The surface area is about 12.6 millimeters squared. Therefore, the surface area to volume ratio is 12.6 divided by 4.2.

This reduces to 3. So this cell has a slightly smaller surface area to volume ratio simply because it is spherical in shape. This tells us not only does size matter, but shape can also increase or decrease the surface area to volume ratio. You can now pause the video and answer these questions.

There is another quiz at the end of the video and you can find all the answers through the quick test prep link below. The limits imposed by surface area to volume ratio of a cell can be seen through several examples. Let's start with the largest cells in nature, eggs. At this end of the spectrum, the cell has much more volume than it has surface area. In fact, the size of cells in eggs rapidly reduces so the cell can become functional.

Eggs only have one simple function that do not require exchanging many molecules with the environment. After only a few days, one cell becomes hundreds of cells. The cells get consecutively smaller with each division, becoming more efficient each time.

Thus, by the time they hatch as a fully formed organism, the average cell size in an egg has decreased by orders of magnitude. By contrast, these bacteria are at the other end of the size spectrum. They are just large enough to house their DNA and carry out the basic functions of life.

Compared to their volume, their surface area is much higher. giving them a very high surface area to volume ratio. In fact, if they were any smaller, these cells would not have the volume to allow DNA to replicate properly, and their lipid bilayer may not have enough room for all the proteins needed to transport macromolecules or reproduce the DNA.

The best of us need to take a break from time to time. Studies have shown that a quick break while you're studying can help you retain more information. So go get some water, take a quick walk, or maybe go for a swing.

When we come back we'll see why cell size is so important to organisms. While these surface area to volume constraints apply to single cells, there are many ways organisms can increase the efficiency of obtaining nutrients within a larger organism. Most common method of increasing efficiency is to create folds in tissues, cells, and cell membranes.

In turn, this increases the surface area to volume ratio and allows the cells to be as efficient as possible. Let's consider a couple of examples. This structure is one of many alveoli found in your lungs. It is a small sac that is responsible for absorbing oxygen into the bloodstream. The alveoli also expel carbon dioxide from the bloodstream and into the lungs.

Instead of one large cell at the end of each airway, these small sacs are lined with hundreds of tiny rectangular cells. If oxygen and carbon dioxide had to diffuse through only one large cell, it would take a lot of time and be much less efficient. By contrast, the many endothelial cells that create the alveoli can quickly transfer oxygen into the capillaries and carbon dioxide out of the body.

Similar to the folding pattern seen in the alveoli, is the massive surface area increase seen in the small intestine. In fact, there are three levels of folding that happen to dramatically increase the surface area available to absorb important nutrients from your food. First, the small Intestine lining itself is folded, doubling the surface area of each fold. On top of these folds are more tiny projections, called villi, that also double the surface area available. Each villus is lined with blood vessels, allowing quick transfer of oxygen, nutrients, and waste products.

Plus, each villus is covered in tiny rectangular cells. If we look closely at these cells, We can see that each cell has its own microvilli. These tiny extensions of the cell membrane massively increase the surface area available to take in nutrients.

Altogether, the folds of this system increase the efficiency of the intestinal lining thousands of times. Similarly, The excretory system of humans also works by making convoluted folds in tissues, cells, and individual cell membranes that drastically increase their surface area. For example, let's take a look at the kidneys, the body's main excretory organ.

The kidney is made up of many identical structures. Each medulla is packed with cells and has both arteries and veins, taking in blood from the heart and filtering out nitrogenous wastes and sending the blood back to the heart. These cells are folded into complex shapes that drastically increase their efficiency.

Inside each medulla are the Malpighian tubules. These tubules are made of many smaller cells which have very different functions in different parts of the fold. Like the cells in each alveoli, Each of these cells is small, rectangular, and pretty flat, allowing for the maximum efficiency.

In fact, a single kidney can filter half a cup of blood every minute. Since you have about 20 cups of blood in your body, that means your kidneys can filter all of the blood in your body every 40 minutes. While importing and exporting substances is the main reason why cells have surface area to volume ratio limits, there are many other functions that need cells of a certain size. For instance, certain organisms need to dissipate heat.

They do so with many small cells on the skin and in the lungs that can transfer heat out of the body. By contrast, seals that swim in the frigid waters of the Arctic are covered in a layer of very large fat cells. These cells hold in heat, allowing them to live in water that would quickly give a human hypothermia.

You can now pause the video a second time and answer this set of questions. You can find the answers for all the questions in this video through the quick test prep link found below. You can also explore the other resources we have for this section.

They can help you study for the AP test, so check them out! If you enjoyed this video, please click the like button and subscribe to the Biology Dictionary channel. Please leave us comments if you have any questions about cell size or surface area to volume ratios. Good luck!

Transcript for:Understanding Cell Size and Efficiency

Transcript for:
Understanding Cell Size and Efficiency