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Cell shape and size depend on their specific function. Red blood cells are disc-shaped to flow smoothly through narrow blood vessels, while smooth muscle cells are spindle-shaped to increase surface area for cell-cell contact. Sperm cells measure about four micrometers, whereas neuronal cells extend several meters with long cytoplasmic projections. This structural diversity enables cells to perform specialized roles efficiently.

In unicellular organisms like bacteria or yeast, a single cell performs all required life functions independently. Multicellular organisms, such as humans with over a hundred trillion cells, distribute functions among specialized cells that work together. Individual cells in multicellular organisms develop specific roles, such as macrophages engulfing pathogens or endocrine cells producing hormones like adrenaline.

Cell specialization occurs when undifferentiated cells differentiate into specific cell types with distinct structures and functions. During development, early cells receive signals from surrounding cells and express different genes to produce specific proteins. This process creates about 200 human cell types with varied shapes, including squamous, cuboidal, and columnar cells, each adapted for particular roles in tissues and organs.

The human body contains diverse specialized cells adapted for specific functions. Squamous skin cells are flat and tightly packed to provide protective barriers, living briefly before replacement. Nerve cells are star-shaped with processes extending up to a meter in length, persisting throughout an organism's lifetime. Macrophages engulf pathogens, while endocrine cells produce hormones, demonstrating how cellular structure directly supports specialized function.

Different specialized cell types form tissues that work together to perform complex organism functions. Multicellular organisms develop from a single fertilized egg into trillions of cells organized into specialized tissues. These tissues combine to create organs and systems. Cell diversity enables the formation of functional structures like protective skin barriers and nervous systems that coordinate body activities.

Cells control protein production by selectively expressing different genes based on internal conditions and external signals from surrounding cells. Different cell types express different genes, meaning they use their genes to make certain proteins but not others. This gene expression regulation allows specialized cells to develop unique structures and functions appropriate to their roles in tissues and organs.

Cell structure directly determines function. The disc shape of red blood cells enables smooth flow through narrow vessels, while spindle-shaped smooth muscle cells increase surface area for enhanced cell-cell contact. Long neuronal extensions allow nerve cells to transmit signals across distances. Squamous skin cells form flat protective barriers when packed tightly. This structure-function relationship demonstrates how cellular diversity supports specialized roles.