Eukaryotic cells constitute animals, plants, fungi, and protists, and are defined by their complex internal architecture. They possess a true nucleus that houses genetic material, along with several other membrane-bound compartments known as organelles. This internal organization allows for specialized functions and coordinated activities within a single unit.
Measuring Eukaryotic Cell Dimensions
The typical dimensions of a eukaryotic cell fall within a specific microscopic range. The vast majority of these cells measure between 10 and 100 micrometers (µm) in diameter. A micrometer, sometimes called a micron, is the standard unit for measuring cell size and is one-millionth of a meter. This scale is far too small for the unaided human eye to distinguish individual cells.
Because of their minute size, specialized tools like light and electron microscopes are necessary to view and accurately measure eukaryotic cells. Even within the 10 to 100 µm range, there is significant variation; for instance, many animal cells cluster toward the lower end of this scale, while plant cells often occupy the higher end. This typical size range reflects a biological balance governed by physical and functional requirements.
Biological Constraints on Cell Size
The primary factor limiting the size of most eukaryotic cells is the surface area to volume (SA:V) ratio. As a cell increases in size, its volume—the interior space that requires nutrients and produces waste—grows at a much faster rate than its surface area—the outer membrane through which all exchange must occur. Mathematically, volume increases with the cube of the radius (\(r^3\)), while surface area increases only with the square of the radius (\(r^2\)).
If a cell were to grow too large, the relatively small surface area of its membrane would not be sufficient to take in enough materials like oxygen and glucose to supply the metabolic demands of its expansive internal volume. Similarly, the membrane surface would be too small to efficiently expel metabolic waste products, leading to a buildup of toxic substances. This imbalance would quickly compromise the cell’s ability to maintain homeostasis.
Eukaryotes manage this physical constraint through their internal complexity and compartmentalization. The presence of membrane-bound organelles, such as the endoplasmic reticulum and Golgi apparatus, creates a system of internal membranes that greatly increases the total working surface area for chemical reactions. Furthermore, these organelles organize cellular processes and facilitate internal transport, ensuring that materials are moved efficiently throughout the larger cell volume.
Size Extremes and Specialized Cells
While the 10 to 100 µm range is typical, the need for specialized functions in multicellular organisms has led to remarkable deviations in cell size and shape. These cells have evolved structural adaptations to bypass or manage the constraints of the SA:V ratio. A classic example is the human nerve cell, or neuron, which can extend for a meter or more from the spinal cord to the foot.
These extraordinarily long cells maintain a manageable SA:V ratio by being extremely thin along their extensive axonal length. Other cells specialize in volume; the unfertilized egg cell (ovum) is often the largest cell in an animal, such as the ostrich egg, which is the largest single cell known. This exceptional volume is necessary to store extensive food reserves and cytoplasmic resources required for the initial stages of embryonic development.
Specialized cells also include those that optimize surface area through shape modification. Root hair cells in plants, for instance, develop long, thin projections that dramatically increase the surface area available for absorbing water and mineral ions from the soil. Conversely, red blood cells are quite small and lack a nucleus entirely, which maximizes the internal space available for the hemoglobin protein needed to transport oxygen.
How Eukaryotes Compare to Prokaryotes
Placing the size of a eukaryotic cell into context requires comparing it to its simpler counterpart, the prokaryotic cell. Prokaryotes, which include bacteria and archaea, are significantly smaller, typically measuring only 0.1 to 5.0 µm in diameter. This means a typical eukaryotic cell is generally 10 to 100 times wider than a typical prokaryotic cell.
The fundamental difference in size is directly linked to structural complexity. Prokaryotic cells lack a nucleus and membrane-bound organelles, meaning their metabolic processes occur directly in the cytoplasm. This simple internal arrangement limits their size because they rely on rapid diffusion across a small distance to distribute materials. The eukaryotic cell’s ability to compartmentalize functions within organelles allows it to grow substantially larger while still maintaining efficient internal processes.