Cells generally begin life in a state called undifferentiated, which means that they are not yet adapted for any specific function. Once the cell is committed to a specific tissue, which determines what its function will be, the cell changes and differentiates so that it is better suited for its job. For example, nerve cells, or neurons, grow long extensions that reach out and connect to other neurons nearby so that the cell becomes part of a network of nervous tissue.
Red blood cells, or erythrocytes, have a nucleus when they are first made, but the nucleus is ejected as the cell matures, making room for the hemoglobin that fills a functioning red blood cell. White blood cells, by contrast, keep their nucleus, but in many white blood cells the nucleus becomes enlarged and lobed, increasing in size so that the cell can rapidly make proteins that help the organism fight off disease.
Cells in plants develop a large central vacuole, or water bag, as they differentiate. This helps the cell stay rigid, which assists the plant is standing upright. Plant cells exposed to light will usually develop chloroplasts, which do photosynthesis, whereas cells in the root will go a different route, converting the same basic organelle into amyloplasts, which store starch.