surface area to volume ratio in biology?

Surface area to volume ratio decreases whenever volume increases, which means the larger an object, the smaller its surface area to volume ratio. For 2 objects of the same volume, however, the more elongated the object, the larger the surface area to volume ratio. That means larger cells will have smaller surface to volume ratios than smaller cells, and elongated cells (such as some nerve cells) will have more surface area than spherical cells of the same volume.

Since the rate of diffusion is proportional to surface area, the larger cells will have slower diffusion rates than smaller cells and spherical cells will have slower diffusion rates than elongated cells. Because of that, it is much harder to get stuff, such as water and oxygen, into and out of a large cell. As a result, there is a practical limit to how big a cell can get since a large cell will have trouble getting enough oxygen into it by diffusion. As a result, large multicellular organisms are made of numerous small cells rather than a few large cells. The larger single-celled organisms tend to be cylindrical or elongated in shape as opposed to spherical because there is more surface area on a cylinder than a sphere of the same volume.

A large surface to volume area means that a small amount of living matter has a large surface through which nutrients, oxygen and wastes can diffuse. A small surface to volume ratio means that a large amount of living matter has a small surface through which nutrients, oxygen and wastes can diffuse, which means that such a cell will have more difficulty taking in sufficient nutrients and oxygen, and eliminating wastes. And the larger a cell is, the smaller is its surface to volume ratio.

Picture a cell a being cube-shaped (even though that is not usually the case).

If the edge of the cube measures 1, then the surface of the cube will be 6 and the volume will be 1, a ratio of 6:1

If the edge of the cube measures 2, then the surface of the cube will be 24 and the volume will be 8, a ratio of 3:1

If the edge of the cube measures 3, then the surface of the cube will be 54 and the volume will be 27, a ratio of 2:1

And so on …

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Consider a cube of potato 10cm by 10cm by 10 cm. In salt water, water will move from the cube into the water. This cube will have a volume of 1000 cubic cm. Its surface area (the area exposed to the environment) will be 10*10*6=600 square cm. Each square cm exposed to the environment can interact with the environment. If we could increase the surface area (SA) exposed without increasing volume then more SA would interact and water would leave the potato at a greater rate. Cut the potato cube in half. The volume remains the same but suddenly two new surfaces are exposed to the environment. 800 square cm. The SA:V has increased because more surface area is exposed. Water diffusing from the potato to salt water would occur at a much greater rate than before. Cut the cube into quarters and still more SA is exposed now 1000 square cm are exposed to the environment with no change to the volume. The diffusion of water (osmosis) to the salt water would jump again. This is true for cells as well. they interact with their environment- the larger they are the lower the rate of diffusion. Because diffusion is a “free” transport mechanism cells like to use it. If a cell becomes too large it does not allow diffusion to occur adequately. I hope this helps!