A typical animal cell contains more than a dozen distinct structures, each with a specific job that keeps the cell alive, functional, and able to communicate with its neighbors. These cells are generally 10 to 20 micrometers in diameter, far too small to see without a microscope, yet they pack an impressive level of internal organization. Here’s what’s inside.
The Plasma Membrane
Every animal cell is enclosed by a plasma membrane, a thin, flexible boundary made primarily of phospholipids arranged in a double layer. The fatty acid tails of these phospholipids often contain one or more double bonds, which create kinks in their chains and prevent them from packing tightly together. This is what keeps the membrane soft and fluid rather than rigid.
Cholesterol molecules sit embedded among the phospholipids and act as a temperature buffer. At high temperatures, cholesterol stiffens the membrane slightly, reducing how freely molecules can pass through. At low temperatures, it does the opposite, preventing the membrane from becoming too rigid. Proteins embedded in or attached to the membrane handle tasks like signaling, transport, and cell-to-cell recognition. Unlike plant cells, animal cells have no cell wall outside this membrane, so the plasma membrane is the outermost boundary.
The Nucleus
The nucleus is the control center of the cell. Surrounded by a double-layered nuclear envelope dotted with pores, it houses the cell’s DNA in the form of chromatin, long threads of genetic material that condense into visible chromosomes during cell division. The nucleus determines both the structure and function of the cell by controlling which proteins get made and when.
Inside the nucleus sits the nucleolus, a dense region rich in RNA. This is where ribosomes, the cell’s protein-building machines, begin their assembly before being exported to the cytoplasm.
Mitochondria
Mitochondria are the cell’s power plants, generating most of the ATP that fuels nearly every cellular process. Each mitochondrion has two membranes. The outer membrane is smooth, while the inner membrane folds inward into structures called cristae. These folds dramatically increase the surface area available for energy production. In a liver cell, the inner membrane makes up roughly one-third of the cell’s total membrane area.
The energy-generating machinery sits embedded in that inner membrane. Electrons pass through a chain of protein complexes, and their movement pumps hydrogen ions across the membrane. Those ions then flow back through an enzyme called ATP synthase, which harnesses their movement to build ATP from simpler molecules. Cells with higher energy demands simply have more cristae: a heart muscle cell’s mitochondria contain about three times as many folds as a liver cell’s.
Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a sprawling network of membrane-enclosed channels that comes in two forms. Rough ER is studded with ribosomes on its outer surface and specializes in protein processing. Proteins destined for export or for use in membranes are threaded into the rough ER while still being built. Once inside, they fold into their correct three-dimensional shapes with help from specialized helper molecules, form critical structural bonds between amino acids, and receive initial sugar chain modifications.
Smooth ER lacks ribosomes and focuses on lipid production instead. It synthesizes the phospholipids that form cell membranes, along with cholesterol and other lipid-based molecules. Cells that produce steroid hormones, like those in the ovaries and testes, are packed with smooth ER to meet the demand for cholesterol-derived hormones.
The Golgi Apparatus
The Golgi apparatus works like a shipping and finishing department. Proteins arriving from the ER enter at the cis face (the receiving side, typically oriented toward the nucleus) and move through a series of flattened membrane compartments. Along the way, sugar chains attached to proteins are trimmed and rebuilt, giving each protein the molecular tags it needs to reach the right destination.
Finished products exit from the trans face (the shipping side), where they’re sorted and packaged into transport vesicles. Some head to the plasma membrane for secretion outside the cell, some get incorporated into the membrane itself, and others are routed to lysosomes.
Lysosomes
Lysosomes are membrane-bound compartments filled with digestive enzymes that break down proteins, lipids, carbohydrates, and nucleic acids. They act as the cell’s recycling centers, digesting worn-out organelles, foreign material brought in from outside the cell, and other cellular debris. The membrane surrounding each lysosome keeps these powerful enzymes safely contained so they don’t damage the rest of the cell. Lysosomes are one of the structures found in animal cells but generally absent from plant cells.
Peroxisomes
Peroxisomes handle a different kind of waste management. They carry out oxidation reactions that break down substances like fatty acids, amino acids, and uric acid. This fatty acid breakdown is a major source of metabolic energy. A byproduct of these reactions is hydrogen peroxide, which is toxic. Peroxisomes contain a dedicated enzyme, catalase, that immediately converts hydrogen peroxide into water or uses it to neutralize other compounds. In animal cells, fatty acid breakdown happens in both peroxisomes and mitochondria.
Ribosomes
Ribosomes are the structures that actually build proteins by reading genetic instructions copied from DNA. They’re assembled in the nucleolus and then exported to the cytoplasm, where they work either freely floating in the cell fluid or attached to the rough ER. Free ribosomes typically produce proteins used within the cytoplasm, while those on the rough ER make proteins destined for membranes or export.
The Cytoskeleton
Without a rigid cell wall, animal cells rely on an internal scaffolding called the cytoskeleton to maintain their shape and organize their contents. It consists of three types of protein filaments:
- Actin filaments (microfilaments) are the thinnest. They concentrate near the cell’s edges, drive changes in cell shape, and enable crawling movements.
- Intermediate filaments provide mechanical strength, acting like internal ropes that resist stretching and tearing forces.
- Microtubules are the largest. They serve as tracks for transporting organelles and vesicles across the cell and form the structural basis of the spindle that separates chromosomes during division.
Accessory proteins link these filaments to each other, to organelles, and to the plasma membrane, creating an integrated structural network that is also responsible for cell movement.
Centrosomes and Centrioles
Near the nucleus sits the centrosome, a structure unique to animal cells that serves as the main organizing center for microtubules. A typical centrosome contains a pair of centrioles, small cylindrical structures oriented at right angles to each other, surrounded by a cloud of proteins called pericentriolar material. This material includes the molecules that nucleate new microtubules, essentially acting as a launchpad for microtubule growth.
During cell division, the centrosome duplicates and the two copies move to opposite sides of the cell. They then recruit additional proteins and ramp up microtubule production to build the mitotic spindle, the apparatus that pulls chromosomes apart so each daughter cell gets a complete set of DNA.
The Extracellular Matrix
While not inside the cell, the extracellular matrix is a defining feature of animal tissues and worth understanding alongside cell structures. Since animal cells lack a cell wall, they instead secrete a meshwork of molecules that surrounds and supports them. Two main classes of molecules make up this matrix: long sugar-based chains called glycosaminoglycans (usually attached to proteins) and fibrous proteins including collagen, elastin, fibronectin, and laminin.
Collagen fibers strengthen and organize the matrix. Elastin gives tissues the ability to stretch and snap back. Fibronectin helps cells grip the matrix by binding simultaneously to collagen on one side and to receptors on the cell surface on the other, essentially anchoring cells in place. Specialized junction structures called desmosomes, found only in animal cells, act like spot welds that hold neighboring cells tightly together in tissues like skin.
How Animal Cells Differ From Plant Cells
Many of the structures above exist in plant cells too, but a few are distinctly animal. Animal cells have centrosomes, lysosomes, and desmosomes, while plant cells do not. In return, plant cells have a rigid cell wall, chloroplasts for photosynthesis, a large central vacuole for storage and structural support, and plasmodesmata (channels connecting neighboring cells). Animal cells may contain small vacuoles, but nothing comparable to the single large vacuole that dominates a plant cell’s interior. The absence of a cell wall is why animal cells are flexible and can take on a wide variety of shapes, from the flat discs of red blood cells to the long, branching extensions of nerve cells.