A eukaryotic cell is a cell whose genetic material is enclosed inside a membrane-bound nucleus. Eukaryotic cells also contain other membrane-bound compartments, called organelles, that carry out specialized jobs inside the cell.
Animals, plants, fungi, and protists are made of eukaryotic cells. Although these organisms can differ enormously in size, structure, and lifestyle, their cells share the same basic organizational principle: different cellular processes are separated into specialized compartments.
This internal organization allows eukaryotic cells to perform complex tasks efficiently, from producing energy and building proteins to storing genetic information and responding to changes in their environment.
What makes a cell eukaryotic?
The defining feature of a eukaryotic cell is its nucleus. The nucleus is surrounded by a membrane and contains most of the cell’s DNA.
The word eukaryotic comes from Greek roots meaning “true nucleus.” This distinguishes eukaryotic cells from prokaryotic cells, such as bacteria and archaea, whose DNA is not enclosed within a membrane-bound nucleus.
Eukaryotic cells generally have several additional features that distinguish them from prokaryotic cells:
- A membrane-bound nucleus
- Membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus
- A complex internal cytoskeleton
- DNA organized into multiple linear chromosomes
- Ribosomes that are structurally different from those found in prokaryotic cells
Not every eukaryotic cell has every structure. For example, animal cells do not have cell walls or chloroplasts, while plant cells typically have both.
The basic structure of a eukaryotic cell
A eukaryotic cell can be thought of as an organized system in which different structures perform different functions. The plasma membrane forms the cell’s outer boundary, while the nucleus and other organelles occupy the interior.
Plasma membrane
The plasma membrane, also called the cell membrane, surrounds the cell. It is primarily made of a double layer of phospholipids containing proteins and other molecules.
Its most important job is controlling what enters and leaves the cell. Small molecules may cross the membrane directly, while other substances require membrane proteins or energy-dependent transport.
The membrane also helps cells communicate with their surroundings. Receptor proteins in the membrane can detect specific chemical signals and trigger changes inside the cell.
Nucleus
The nucleus stores most of a eukaryotic cell’s DNA. It is enclosed by the nuclear envelope, a double membrane containing nuclear pores that regulate movement between the nucleus and the surrounding cytoplasm.
DNA inside the nucleus is organized with proteins into chromatin. When a cell prepares to divide, the chromatin becomes more condensed into visible chromosomes.
The nucleus is also home to the nucleolus, a region where components of ribosomes are produced and assembled before being transported into the cytoplasm.
Cytoplasm
The cytoplasm consists of the material inside the plasma membrane but outside the nucleus. It includes the cytosol—the fluid portion of the cell—as well as organelles and the cytoskeleton.
Many metabolic reactions take place in the cytoplasm. It also provides the environment in which organelles and other cellular structures operate.
Ribosomes
Ribosomes are molecular machines that build proteins by reading information carried by messenger RNA.
Ribosomes are found throughout the cytoplasm, where they can produce proteins used within the cell, and on the surface of the rough endoplasmic reticulum, where they produce many proteins destined for secretion, membranes, or certain organelles.
Unlike most organelles, ribosomes are not surrounded by membranes.
The organelles and what they do
Eukaryotic cells contain specialized structures that divide cellular work into distinct compartments. This compartmentalization is one of their defining characteristics.
| Organelle or structure | Main function |
|---|---|
| Nucleus | Stores DNA and regulates gene expression |
| Nucleolus | Produces and assembles ribosomal components |
| Ribosomes | Build proteins |
| Rough endoplasmic reticulum | Produces and begins processing many proteins |
| Smooth endoplasmic reticulum | Produces lipids and participates in detoxification and calcium storage |
| Golgi apparatus | Modifies, sorts, and packages proteins and lipids |
| Mitochondria | Carry out most cellular aerobic respiration and produce ATP |
| Lysosomes | Break down cellular materials and macromolecules |
| Peroxisomes | Carry out specific oxidation reactions and help break down certain molecules |
| Cytoskeleton | Provides structural support, organization, movement, and intracellular transport |
| Chloroplasts | Carry out photosynthesis in plants and many algae |
| Vacuoles | Store substances and perform several specialized functions, especially in plant cells |
Endoplasmic reticulum
The endoplasmic reticulum (ER) is a network of membranes extending through the cytoplasm.
Rough ER is covered with ribosomes. It produces proteins that enter the secretory pathway, including many proteins that will be secreted from the cell or incorporated into cellular membranes. The ER also helps fold and process newly made proteins.
Smooth ER lacks ribosomes. It is involved in lipid production, certain forms of carbohydrate metabolism, detoxification, and storage and release of calcium ions in specialized cells.
Golgi apparatus
The Golgi apparatus receives proteins and lipids from the endoplasmic reticulum and modifies, sorts, and packages them.
It can be particularly important in cells that produce large quantities of substances for secretion. Materials leaving the Golgi are transported in membrane-bound vesicles to destinations elsewhere in the cell or outside it.
Mitochondria
Mitochondria are organelles that carry out many of the reactions involved in cellular respiration. They use energy stored in nutrients to help produce ATP, the cell’s primary readily usable energy currency.
Mitochondria have two surrounding membranes and contain their own DNA and ribosomes. Their evolutionary history is explained by the endosymbiotic theory, which proposes that mitochondria originated from bacteria that became permanent residents inside ancestral eukaryotic cells.
Lysosomes
Lysosomes contain enzymes that break down proteins, lipids, carbohydrates, and other biological materials.
They help digest materials taken into the cell and recycle components of worn-out or damaged cellular structures. Their acidic interior provides conditions that allow many digestive enzymes to function effectively.
Peroxisomes
Peroxisomes are small membrane-bound organelles involved in several oxidation reactions. They participate in the breakdown of certain fatty acids and help detoxify potentially harmful compounds.
These reactions can generate hydrogen peroxide, a reactive molecule. Peroxisomes contain enzymes such as catalase that help control and break down hydrogen peroxide.
The cytoskeleton gives the cell shape and organization
The cytoskeleton is an interconnected network of protein filaments that extends throughout the cell. It is not simply a rigid framework; it is constantly assembled, disassembled, and rearranged.
Three major components are microfilaments, intermediate filaments, and microtubules.
Microfilaments, made largely of actin, contribute to cell shape, movement, and changes in the cell membrane. Intermediate filaments provide mechanical strength. Microtubules help organize the interior of the cell, serve as tracks for intracellular transport, and play an essential role in chromosome separation during cell division.
Together, these structures allow cells to maintain their shape, move materials internally, change their form, and in some cases move through their environment.
Plant and animal cells are both eukaryotic
Plant and animal cells share the fundamental eukaryotic organization, but they are adapted to different ways of life.
Plant cells typically have a rigid cell wall outside the plasma membrane. They also contain chloroplasts, where photosynthesis occurs, and usually have a large central vacuole that contributes to storage and helps maintain internal pressure.
Animal cells lack a cell wall and chloroplasts. Their shapes are generally more flexible, and different cell types can develop highly specialized structures. Animal cells can contain small membrane-bound compartments involved in storage and digestion rather than the large central vacuole characteristic of many plant cells.
Fungi and many protists are also eukaryotic, although their cells have their own combinations of structures and adaptations.
How a eukaryotic cell uses its organelles together
The organelles of a eukaryotic cell do not work independently. They form interconnected systems.
For example, a cell producing a protein for secretion begins by using genetic information in the nucleus to produce messenger RNA. The messenger RNA travels to a ribosome associated with the rough ER, where the protein is synthesized and enters the ER. It may then be transported to the Golgi apparatus, modified and sorted there, and eventually delivered in a vesicle to the plasma membrane for secretion.
Energy production is similarly integrated with other cellular activities. Nutrients are processed through metabolic pathways, and mitochondria use products of those pathways to generate ATP. That ATP can then power processes such as active transport, movement, biosynthesis, and maintenance of cellular organization.
This coordination is possible partly because eukaryotic cells separate many biochemical reactions into specialized compartments.
How eukaryotic cells reproduce
Most eukaryotic cells reproduce through cell division. In somatic cell division, a process called mitosis distributes duplicated chromosomes between two daughter nuclei. The cell then divides its cytoplasm, producing two daughter cells that generally contain the same number of chromosomes as the original cell.
Before mitosis, the cell passes through stages of the cell cycle during which it grows, carries out its normal functions, duplicates its DNA, and prepares for division.
Sexual reproduction involves a specialized form of cell division called meiosis. Meiosis produces cells with half the usual chromosome number and contributes to genetic variation. In animals, for example, meiosis produces sperm or eggs.
Eukaryotic cells can become highly specialized
A multicellular organism can contain many cell types even though those cells generally contain the same basic genome.
The difference is largely a matter of gene expression: different cells activate different sets of genes. As a result, a muscle cell can develop structures suited to contraction, while a nerve cell develops structures suited to transmitting signals.
Specialization can involve changes in cell shape, organelle abundance, membrane proteins, metabolism, and the molecules a cell produces. The basic eukaryotic architecture provides a flexible foundation for this diversity.
Eukaryotic versus prokaryotic cells
The distinction between eukaryotic and prokaryotic cells is fundamental, but it should not be reduced simply to “cells with a nucleus versus cells without one.” Their internal organization differs in several ways.
Eukaryotic cells generally have a nucleus, numerous membrane-bound organelles, multiple linear chromosomes, and a more elaborate cytoskeleton. Prokaryotic cells lack a membrane-bound nucleus and generally have a simpler internal organization, although they are still structurally sophisticated and can perform many complex biochemical processes.
Both types of cells have a plasma membrane, DNA, ribosomes, and mechanisms for obtaining energy and building cellular components. In other words, the distinction is about cellular organization rather than whether one type is inherently “simple” and the other “complex.”
Why eukaryotic cell structure matters
The organization of a eukaryotic cell makes it possible to carry out many processes simultaneously while keeping incompatible reactions separated.
The nucleus protects and organizes genetic information. Organelles such as mitochondria specialize in energy-related processes, the ER and Golgi coordinate the production and distribution of many cellular materials, and the cytoskeleton organizes the cell and enables movement and transport. The plasma membrane connects the cell’s internal environment with the outside world.
Together, these structures allow eukaryotic cells to maintain a stable internal environment, respond to signals, grow, divide, specialize, and perform the diverse functions required for life.
