Structures Only in Animal Cells
CentriolesThe centriole is a small, barrel-shaped tube composed of protein located in the cytoplasm.
Barrels of fun:
The centriole's main function is to aid in cell division and in the spatial arrangement of structures within the cell. Less is known about the function of centrioles than many of the other organelles discussed in this section, but biologists are learning that these little protein tubes play a critical role in cellular reproduction and even cell growth. What’s more, centrioles are now known to be essential for the development of flagella and cilia. Cells with damaged or missing centrioles cannot form properly functioning flagella and cilia, a condition that can lead to disease and even death of the organism in which cells containing flagella and cilia are found.
Flagella and CiliaFlagella and cilia are extensions of the cell membrane that are lined with cytoskeleton and, in the case of flagella, mitochondria. Flagella are generally much longer than cilia, like whips, but there are often hundreds more cilia than flagella on a given cell. Flagella are primarily responsible for cell movement.
Here are some real-life flagella:
Electron micrograph image source
They function by spinning like a whip, allowing a cell to move through the environment. Sperm cells are an excellent example of animal cells that have flagella. In these cells, flagella spin rapidly to allow the sperm to move up the vaginal canal, into the uterus and into the egg.
Cilia, on the other hand, act more like short hairs moving back and forth across the outside of the cell.
A picture under the sea? Nope, just some cilia:
Scanning electron microgaph image source: Wikimedia Commons
Cilia generally move matter past a cell. The most common examples of ciliated cells are those that line the trachea, or wind pipe, of animals. Here, the cilia move mucus containing dirt and other inhaled particles up the windpipe and into the esophagus where they can be coughed up or swallowed.
The flagella and cilia in animal cells are not evolutionarily related to those found in bacterial cells; however, there are some strange little worms that cover their bodies with bacteria that look like flagella. Check them out here.