An animal cell is made up of a thin outer membrane, a control center called the nucleus, energy-producing mitochondria, and a network of internal structures that build proteins, transport materials, and break down waste. Water accounts for about 70% of a cell’s total mass, with proteins, fats, nucleic acids, and carbohydrates making up most of the rest. Here’s what each part does and how they work together.
The Plasma Membrane
Every animal cell is wrapped in a plasma membrane, a flexible boundary that holds the cell’s contents together and controls what moves in and out. Unlike plant cells, animal cells have no rigid cell wall, so this membrane is the only barrier between the cell’s interior and its surroundings.
The membrane is built from a double layer of fat molecules called phospholipids. The inner core of this bilayer repels water, which is what makes the membrane an effective barrier. Water-soluble molecules, including most biological compounds and ions, can’t simply pass through on their own. They need help from proteins embedded in the membrane, which act as selective gates, channels, and receptors. Some of these proteins sit on the membrane’s surface (peripheral proteins), while others span the entire thickness of the bilayer (integral proteins), handling tasks like transporting nutrients and recognizing signals from other cells.
Cholesterol is present in roughly equal amounts to the phospholipids and plays a stabilizing role. At high temperatures, cholesterol makes the membrane less fluid and less permeable. At low temperatures, it prevents the membrane from stiffening up. This keeps the membrane flexible across a range of conditions. Scientists describe this arrangement as the “fluid mosaic model,” proposed in 1972 by Jonathan Singer and Garth Nicolson, because the membrane behaves like a two-dimensional fluid with proteins floating within it.
The Nucleus
The nucleus is the largest organelle in the cell and serves as its command center. It stores the cell’s DNA and coordinates major activities including growth, metabolism, protein production, and cell division. A double-layered nuclear envelope surrounds the nucleus, with small pores that allow molecules to pass between the nucleus and the rest of the cell.
Inside the nucleus sits a dense structure called the nucleolus, which is the cell’s ribosome factory. Ribosomes are the tiny machines that build proteins, and the nucleolus produces their key components by transcribing and processing ribosomal RNA. The nucleolus also plays roles in regulating the cell cycle, responding to stress, and assembling other important molecular complexes. It’s present in nearly every type of animal cell and is typically the most prominent structure visible within the nucleus.
Mitochondria: The Cell’s Power Source
Mitochondria generate most of the cell’s energy in the form of ATP, the molecule cells use as fuel for nearly every process. Each mitochondrion has two membranes. The outer membrane is smooth, while the inner membrane folds inward to form ridges called cristae. These folds dramatically increase the surface area available for energy production, packing more of the necessary molecular machinery into a small space.
The inner membrane contains a chain of proteins that pass electrons from one to the next, releasing energy at each step. This energy is used to pump protons across the membrane, creating a gradient. Protons then flow back through a protein complex called ATP synthase, which harnesses that flow to produce ATP. It’s a bit like a hydroelectric dam: the buildup of protons on one side drives a turbine that generates energy. A single cell can contain hundreds or even thousands of mitochondria depending on how much energy it needs.
The Protein Production Line
Proteins are built and shipped through a connected system of organelles. The process starts at the rough endoplasmic reticulum (rough ER), a network of membrane-enclosed channels studded with ribosomes on its outer surface. These ribosomes read genetic instructions copied from DNA and assemble amino acids into protein chains. The rough ER also begins folding these new proteins into their correct shapes and adds initial chemical tags, like sugar groups, that help determine where each protein will end up.
The smooth endoplasmic reticulum, which lacks ribosomes, handles different work. It produces and stores lipids (fats) and steroids that the cell needs for building membranes and signaling.
Once a protein is properly folded, it gets packaged into small transport bubbles called vesicles that bud off from the edges of the rough ER. These vesicles carry their cargo to the Golgi apparatus, a stack of flattened membrane sacs that acts like a processing and shipping center. The Golgi further modifies proteins, sorts them, and sends them to their final destinations, whether that’s the plasma membrane, another organelle, or outside the cell entirely. Proteins that need to stay in the ER are sent back through a separate set of return vesicles.
Waste Breakdown and Detoxification
Cells generate waste and encounter toxic substances, and two organelles handle the cleanup. Lysosomes are membrane-bound sacs filled with digestive enzymes that break down worn-out cell parts, bacteria, and other debris. They function as the cell’s recycling center, dismantling large molecules into smaller components that can be reused. Lysosomes are one of the structures unique to animal cells; plant cells don’t have them.
Peroxisomes handle a different category of waste. They contain enzymes that use oxygen to break down fatty acids and neutralize toxic molecules. A key enzyme inside peroxisomes, catalase, converts hydrogen peroxide (a harmful byproduct of these reactions) into harmless water. Peroxisomes are especially active in liver and kidney cells, where they detoxify substances like alcohol, formaldehyde, and formic acid that enter the bloodstream. They also break down fatty acids by chopping them into two-carbon chunks, a process called beta oxidation, which feeds into the cell’s energy-producing pathways.
The Cytoskeleton
Animal cells hold their shape and move their internal parts using a network of protein fibers called the cytoskeleton. Three types of fibers work together, each with a different size and role.
- Microtubules are hollow tubes about 25 nanometers across, the largest of the three. They serve as structural supports and as tracks for transporting organelles and vesicles through the cell.
- Intermediate filaments are rope-like fibers about 8 to 10 nanometers in diameter. They provide mechanical strength and help stabilize the cell’s overall shape, acting like internal cables that resist stretching and compression.
- Microfilaments are the thinnest, at 4 to 6 nanometers, built from a protein called actin. They drive cell movement, maintain the distribution of membrane proteins, and control how organelles interact with the plasma membrane. They’re also essential for cell migration and for pinching a dividing cell into two.
Centrioles and Cell Division
Animal cells contain a structure called the centrosome, located near the nucleus, which plant cells lack. Each centrosome holds a pair of centrioles: barrel-shaped structures made of microtubule bundles arranged in a distinctive nine-fold pattern. The centrosome functions as the cell’s primary microtubule-organizing center.
During cell division, this role becomes critical. The centrosome duplicates, and the two copies migrate to opposite sides of the cell. From there, they sprout a web of microtubules called the mitotic spindle, which attaches to chromosomes and pulls them apart so each new daughter cell receives a complete copy of DNA. The spindle is also anchored to the cell’s outer edge by additional microtubules radiating from the centrosomes, helping orient the division correctly. Without centrosomes, cells can still divide in some cases, but the process is less organized and more error-prone.
How Animal Cells Differ From Plant Cells
Many organelles, like the nucleus, mitochondria, ER, and Golgi apparatus, are found in both animal and plant cells. The key differences come down to a few structures that only one type has. Animal cells have centrosomes and lysosomes, while plant cells do not. Plant cells, on the other hand, have a rigid cell wall outside the plasma membrane, chloroplasts for photosynthesis, and a large central vacuole for storing water and nutrients. These differences reflect the distinct lifestyles of animal and plant cells: animal cells prioritize flexibility and movement, while plant cells are built for structural rigidity and making their own food from sunlight.