What exactly is cell diffusion?

What do elephants, humans, plants and insects all have in common? Cell diffusion! Diffusion is the exchange of materials entering or exiting across the cell membrane, where dissolved molecules move from areas of high concentration to low concentration. As cells grow larger, their surface area to volume ratio decreases. This means that while the cell (it’s volume) grows larger, its surface area does not increase to the same degree. This can make taking in nutrients, transporting electrons, and the elimination of waste, much more difficult to accomplish.

However, in eukaryotic cells (such as those in plants and animals), they are large, so they contain membrane-bound organelles to provide additional surface areas to allow for adequate transportation within the cell, such as:

• The endoplasmic reticulum
• The plasma membrane
• Cristae of the mitochondria.

How can this be demonstrated with agar cubes?

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The same result can be seen in our agar cubes. Initially, I had the hypothesis that the NaOH would diffuse into every cube fully, but at a faster rate than it had actually occurred. However, the NaOH that the cubes were submerged in, had to reach into each of the cube’s volumes quickly. In the time-lapse below, you are able to see that the NaOH will diffuse into each size of cube at the same rate, however with the given time period of ten minutes, the cube with the greatest reached proportion was the smallest (1cm³) cube. In the time that it took the NaOH to reach the center of the smallest cube, it was only able to reach 42.1% of the largest cube (3cm³). This means, that in order to maximize diffusion, the cube’s surface area to volume ratio must not be too small (for example: in the 3cm³ cube), otherwise diffusion will not be able to occur at fast enough rates to reach the entire volume of the cube.

The NaOH will dissolve through the outer surface of the cube, and then into the cube’s volume. As the NaOH was able to completely diffuse into the centre of the smallest cube, this renders it as well adapted to exchange substances with its environment through diffusion. If you exchanged NaOH with oxygen or nutrients, the process would be the same for a cell. However, while time, cell size, and the surface area to volume ratio were the only variables in our experiment, there are several other factors that can affect how materials diffuse into cells or tissues, such as: concentration, temperature, the type of material being diffused, and cell polarity (differences of shape, structure, and function in a cell).

As you could see with the agar cubes, when large cubes, unlike small cubes, are poorly adapted to exchange substances within its environment, as they are unable to reach its centre in a timely manner, due to the low surface area to volume ratio.

This could be seen by the pink NaOH only reaching 42.1% of the largest (3cm³) cube. It is the same result in cells, as oxygen won’t be able to reach its centre, and waste will not be able to exit through diffusion, ultimately causing a large portion to suffocate. Thus, if a cell grows to be too large, then the diffusion will not be able to compensate for the much larger volume to small surface area, meaning that cells will not grow to be too large. For example: if a cubes surface area increased by a factor of four, the volume would then increase by a factor of eight.

Knowing what we now know, if you were given the choice between three cubes with a surface area to volume ratio of: 3:1, 5:2, and 4:1, which would you choose based on maximized diffusion?

It would be the 4:1 cube! This is because, it has the highest ratio, just like the smallest (1cm³) cube in our experiment, which had a ratio of 6:1. Remember: the highest ratio means that the surface area of the cube / cell is going to be the closest in size to its volume, meaning that it can maximize diffusion and be properly adapted to its environment.

Now, you’re probably wondering how diffusion is used in humans or animals, since we are talking about cells, after all. In human exchange of gases, for example, there must first be enough surface area for the cell in order it to diffuse with its organelles, and the volume of the cell. In order to achieve more surface area, our bodies are use a few different techniques such as: dividing our cells so they become smaller, and also shape cells into longer thinner forms, instead of being round, and fat to further reduce cells volume, while simultaneously creating a larger surface area.

Cell division is the reason that cells cannot grow to be very big. One of its purposes is for the survival of the cell, to allow the cell to be able to continue its intake and disposal of waste, without having its volume becoming too big for its surface area. Its other functions include:

• Reproduction of an entire unicellular organism
• Growth and repair of tissues in multicellular animals
• Production of gametes for reproduction in multicellular animals

The cell is then able to exchange gases (by both entering and exiting it) without taking as long as it would have needed to, as the surface area to volume ratio is now larger. However, it is not only humans who strive to lower this ratio. Another example would be the size of ears that elephants have.

Much like humans, elephants also constantly maintain a high body temperature, however if this temperature grows too high, it can cause a reduction in enzymes function (due to a denaturing of the enzymes, which could lead to death). So, much like our largest agate cube (3cm³), elephants also have a relatively small surface area compared to their overall volume (a low surface area to volume ratio), which means that they will not diffuse heat out of their bodies well. So, elephants adapted to their large size (volume), by creating a large additional surface area (their ears, a tissue), to allow for better heat diffusion.

What is a multicellular organism?

While most cells are not able to grow past a certain size (due to cell division), some are still able to grow to a larger extent than others. So, in order to compensate for this large cell size, some organisms are multicellular. A multicellular organism differs from a unicellular organism, as it requires multiple cells to properly function, whereas a unicellular organism is able to conduct all required functions through only itself. Examples of a multicellular organisms are human beings, plants, animals, birds and insects.

Each of these multicellular organisms are composed of different cells joining together. These cells know when to join together, because once a singular cell becomes too large to be properly adapted to its environment, multiple cells will begin to join together. Eventually, the joined cells will begin communicating with each other and their genes will decide themselves when cell division should occur, rather than the environment leading this process. This growth brings multiple advantages to the organism. Firstly, it better protects your insides from the outside. It also allows for pieces of the joined cell to die, because they can be easily replaced, which ultimately allows you to live longer. It also allows different cells in the group, to begin to take on different roles (such as the outside cells using their flagella to move, while the inner one’s digest food).