Cell

Every animal, from a simple sponge to a mammal, is built from basic structural and functional units called cells. They are working units that carry out specific jobs. Understanding cells helps explain how animals feed, move, sense, defend, and survive.

What are animal cells?

An animal cell is a membrane-bound living unit that forms part of an animal’s body. It has an outer boundary (cell membrane), internal material (cytoplasm), and structures that help it function.

Note: This page focuses on animal cells only. Plant and bacterial cells exist, but they follow different rules.

Most animal cells have a common structural plan:

• A cell membrane that controls what enters and leaves
• Cytoplasm where many processes occur
• Internal structures that support energy use, transport, and maintenance
• In many cases, a nucleus holding genetic information

Functions

Not all cells are the same. Animal cells become specialized depending on their function.

• Some are built to contract (muscle cells)
• Some to send signals (nerve cells)
• Some to absorb or secrete substances
• Some to capture prey or defend the body

Their shape, structure, and internal design change based on the job.

Levels of Organization

Cells do not work alone. They organize into larger systems:

• Cells → Tissues → Organs → Body systems

For example:

• Muscle cells form muscle tissue
• Tissues combine to form organs like the heart
• Organs work together to move blood through the body

So, cells are not separate from anatomy. They are the foundation of it.

Specialized Cells

Some animals show extreme examples of cell specialization. For example, sponges have choanocytes, which help create water currents within their bodies and capture food. Similarly, cnidarians, such as jellyfish, possess cnidocytes or stinging cells that help them capture prey and defend themselves.