4.2: Cell Size

Cell sizes vary widely among and within organisms. Bacterial cells range between 1-10 micrometers (μm)and are considerably smaller than most eukaryotic cells. The smallest bacteria are 0.1 μm in diameter—about a thousand times smaller than eukaryotic cells, which typically range from 10-100 μm.

Surface Area

Cells can take in nutrients and water via diffusion through the plasma membrane itself or through specific channels in the membrane. The area of the membrane surrounding the cells limits the exchange rate of these materials. Smaller cells tend to have a higher surface area-to-volume ratio than larger cells. When a sphere increases in size, the volume grows proportionally to the cube of its radius, while its surface area grows proportional to the square of its radius. Smaller cells have relatively more surface area compared to their volume than larger cells of the same shape. A larger surface area means more plasma membrane where materials can pass into and out of the cell. Substances also need to travel within cells. As a result, the diffusion rate may limit processes in large cells.

Adaptations

Prokaryotes are often small and divide before they face limitations due to cell size. Larger eukaryotic cells have organelles that facilitate intracellular transport and structural changes that help overcome limitations. Some cells that must exchange large amounts of substances with the environment develop long, thin protrusions that maximize the surface area to volume ratio. An example of such a structure is the root hair of plant cells that facilitate water intake and nutrients.

Cells come in a variety of shapes and sizes.

Prokaryotic cells, such as bacteria, have a diameter of a few micrometers, while eukaryotic cells vary in size due to structural specializations. Sperm cells are only about four micrometers, but neuronal cells may have long cytoplasmic extensions of several meters long.

Such variations highlight the vital relationship between volume and surface area. Picture a cell as a cube. When the length of a side increases, its surface area increases as six times its length squared, but the volume increases faster as the length cubed. So when a cell size increases, the surface area-to-volume ratio decreases.

A large surface area to volume ratio makes transporting materials quickly in and out of the cell easier.

For example, bacterial cells have a high surface area-to-volume ratio, allowing nutrients,  gasses, and waste to diffuse through the cell.

In larger animals, cells form specialized organs, such as the intestines, which are folded to maximize the surface area for food absorption.