Animals and plants are complex forms of life made up of eukaryotic cells, meaning they contain a nucleus and other membrane-bound compartments. While they share common components, such as mitochondria and ribosomes, their distinct lifestyles led to the evolution of unique cellular structures. Animal cells are typically flexible, mobile, and capable of engulfing food, requiring specialized structures not found in the more rigid, stationary plant cells.
Centrioles and the Organization of Cell Division
The process of cell division (mitosis) is managed differently in animal cells due to the presence of centrioles. Centrioles are small, cylindrical structures usually found in pairs within the centrosome, a region near the nucleus. The centrosome acts as the main microtubule-organizing center (MTOC) in animal cells.
Centrioles consist of nine triplets of microtubules arranged in a ring (a “9+0” arrangement). During cell division, the centrosome duplicates, and the two centers migrate to opposite sides of the cell. They organize the formation of spindle fibers, which pull the duplicated chromosomes apart into the two new daughter cells. Higher plant cells lack centrioles entirely and organize their mitotic spindle using a different system.
Lysosomes and Intracellular Digestion
Animal cells rely on specialized, membrane-bound organelles called lysosomes for waste management and digestion. Lysosomes contain hydrolytic enzymes that function best in an acidic environment. These enzymes break down complex organic molecules, including proteins, lipids, nucleic acids, and worn-out cellular components.
Lysosomes defend the cell by engulfing and digesting foreign invaders (phagocytosis). They also perform autophagy, the self-digestion of old or damaged cell parts, allowing the cell to recycle molecular building blocks. Plant cells do not have dedicated lysosomes. Instead, they concentrate digestive enzymes within a single, large central vacuole, which also handles storage and maintaining internal pressure.
Specialized Structures for Movement and Sensing
Many animal cells possess specialized external appendages for motility and environmental sensing that are absent in higher plants: cilia and flagella. These hair-like projections extend from the cell surface and are built from an internal core of microtubules called the axoneme.
Flagella are long and few in number, using a whip-like motion to propel the entire cell (e.g., sperm cells). Cilia are shorter and often cover the cell surface in large numbers, moving fluids or material across the cell’s surface, such as the mucus lining the respiratory tract.
The core structure of motile cilia and flagella is characterized by a “9+2” arrangement of microtubules. Furthermore, many animal cells possess a single, non-motile primary cilium that acts as a sensory antenna, relaying signals from the environment. These structures are anchored by a basal body.