4.2
Cell Size
Cells vary widely in size and shape, and these differences are closely linked to their function and physical constraints
Prokaryotic cells, such as bacteria, usually measure only a few micrometers in diameter. Eukaryotic cells are often larger and vary greatly in size due to structural specializations.
For example, the head of a human sperm cell is about four to five micrometers in length. On the other hand, some neurons have axons that extend up to a meter within the human body.
Such variations in size highlight the vital relationship between a cell's surface area and its volume, which affects the efficiency with which materials move in and out of the cell.
Imagine a cell as a cube. When the length of a side increases, the surface area increases to six times the length squared, while the volume increases by the length cubed.
As cell size increases, the surface area-to-volume ratio decreases.
A higher surface area-to-volume ratio makes it easier for materials to move in and out of the cell.
Bacterial cells are small and have a high surface area-to-volume ratio. This high ratio allows nutrients, gases, and waste products to diffuse efficiently across the cell membrane.
Because large cells have a low surface area-to-volume ratio, this limits how quickly they can exchange materials relative to their needs. This also limits how large a single cell can become. Multicellular organisms overcome this limitation by using many small cells that work together rather than a single large cell.
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 vary widely in size and shape, and these differences are closely linked to their function and physical constraints
Prokaryotic cells, such as bacteria, usually measure only a few micrometers in diameter. Eukaryotic cells are often larger and vary greatly in size due to structural specializations.
For example, the head of a human sperm cell is about four to five micrometers in length. On the other hand, some neurons have axons that extend up to a meter within the human body.
Such variations in size highlight the vital relationship between a cell's surface area and its volume, which affects the efficiency with which materials move in and out of the cell.
Imagine a cell as a cube. When the length of a side increases, the surface area increases to six times the length squared, while the volume increases by the length cubed.
As cell size increases, the surface area-to-volume ratio decreases.
A higher surface area-to-volume ratio makes it easier for materials to move in and out of the cell.
Bacterial cells are small and have a high surface area-to-volume ratio. This high ratio allows nutrients, gases, and waste products to diffuse efficiently across the cell membrane.
Because large cells have a low surface area-to-volume ratio, this limits how quickly they can exchange materials relative to their needs. This also limits how large a single cell can become. Multicellular organisms overcome this limitation by using many small cells that work together rather than a single large cell.
From Chapter 4:
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4.2 : Cell Size
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4.1 : What are Cells?
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4.3 : Prokaryotic Cells
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4.4 : Eukaryotic Compartmentalizations
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4.13 : Plasmodesmata
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