Eukaryotic Cells: Eukaryote Definition, Structure and Characteristics

What is a eukaryote? 

A eukaryote is any cell or organism that possesses a clearly defined nucleus. Eukaryotic cells form the foundation of complex, multicellular life, including apple trees, mushrooms, fish and humans. Their defining feature is the presence of membrane-bound organelles, particularly the nucleus.¹

These organelles compartmentalize cellular functions, allowing for greater specialization and efficiency. For instance, mitochondria generate energy by breaking down nutrients; lysosomes break down and recycle cellular components and the endoplasmic reticulum (ER) synthesizes lipids and proteins, aiding in their transport.

Eukaryotes are thought to have originated through a process known as endosymbiosis, in which an ancestral prokaryotic cell engulfed another, forming a mutually beneficial relationship. Over time, the engulfed cell evolved into modern organelles such as mitochondria and, in the case of photosynthetic eukaryotes, chloroplasts.²

However, new findings suggest that the origins of eukaryotic complexity may be more nuanced than the traditional endosymbiotic model alone can explain. Some prokaryotes, such as planctomycetes, exhibit phagocytosis-like behaviors, challenging the assumption that cellular engulfment was unique to early eukaryotes.³ This discovery implies that prokaryotes may have possessed more advanced cellular structures and functions than previously thought, potentially influencing how the first eukaryotic cells emerged.

Additionally, genomic analyses of modern eukaryotes indicate that their ancestors likely acquired genes from multiple sources, not just from the engulfed bacteria that became mitochondria. Lateral gene transfer between early microbial communities may have played a crucial role in shaping eukaryotic genomes, further complicating the evolutionary narrative.⁴

As researchers continue to refine models of eukaryogenesis, it is becoming increasingly clear that the transition from prokaryotic to eukaryotic life was not a singular event but a complex, dynamic process influenced by multiple evolutionary forces.

Structure of eukaryotic cells

Eukaryotic cells generally contain several membrane-bound organelles, including the nucleus, mitochondria, the ER, the Golgi apparatus and lysosomes (Figure 1). Plant cells also contain large vacuoles and chloroplasts.  Each organelle performs a specific function critical to the cell's survival (Table 1).

Table 1. The structure and function of key eukaryotic cell structures.

Figure 1. The structure of a eukaryotic cell. Credit: Technology Networks.

Characteristics of eukaryotic cells

Do eukaryotes have a cell wall and a cell membrane?

All eukaryotic cells have a cell membrane, also known as a plasma membrane. It consists of a phospholipid bilayer that contains various lipids, membrane proteins and sugars. These are essential for maintaining the integrity of the cell, regulating the passage of substances in and out, and facilitating communication with the external environment.⁵

While cell walls are less common in eukaryotes, they are still present in plants, fungi and some protists. These rigid structures provide mechanical support, maintain cell shape and protect against environmental stress.⁶

Do eukaryotic cells have a nucleus?

Of all the eukaryotic organelles, the nucleus is the most critical. The mere presence of a nucleus is considered one of the defining features of a eukaryotic cell.⁷ This structure is important because it houses the cell’s genetic material, encoding the instructions necessary for growth, development and function.

The nuclear envelope encloses the nucleus, a double membrane that regulates the exchange of molecules between the nucleus and the cytoplasm through nuclear pores.⁸ By compartmentalizing genetic material, the nucleus allows for controlled gene expression and efficient coordination of cellular activities. This regulation enables complex processes such as differentiation in multicellular organisms and adaptation to environmental changes, making the nucleus indispensable to eukaryotic life.

Do eukaryotes have organelles?

Eukaryotes have organelles that serve specific functions, such as energy production, photosynthesis and membrane construction. These specialized structures enable cells to carry out complex processes more efficiently, enabling multicellular life and advanced cellular functions.

Do eukaryotic cells have ribosomes?

Ribosomes are found in both eukaryotic and prokaryotic cells and are essential for protein synthesis.

Do eukaryotic cells have mitochondria?

Until recently, it was thought that all eukaryotic cells contained mitochondria. However, researchers have since identified some microbial eukaryotes that have lost these organelles over evolutionary time.⁹ This suggests that while mitochondria are fundamental to most eukaryotic cells, some lineages have adapted to survive without them.

Commonly known as the powerhouse of the cell, mitochondria play a vital role in energy production by generating ATP through oxidative phosphorylation, which fuels essential cellular functions.¹⁰

Beyond metabolism, they also contribute to apoptosis, calcium homeostasis and the synthesis of key biomolecules.¹¹ As a result, mitochondrial function is tightly linked to cellular health, and its dysfunction is implicated in numerous diseases. For example, mutations in mitochondrial DNA can disrupt oxidative phosphorylation, leading to reduced ATP production and increased reactive oxygen species (ROS). Excess ROS can cause cellular damage and have been implicated in several neurodegenerative diseases, including Parkinson’s and Alzheimer’s.¹²

Given their broad impact on health and disease, mitochondria are increasingly being explored as therapeutic targets. Strategies aimed at restoring mitochondrial function, such as mitochondrial transplantation, antioxidant therapies and drugs targeting mitochondrial biogenesis, hold promise for treating a variety of mitochondrial-related diseases.¹³

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Eukaryotic cell examples

Eukaryotic cells play essential roles in the natural world, supporting life across multiple domains and enabling complex biological functions. Below are some key examples of eukaryotic cells:

Plant cells

Plant cells are the building blocks of all plant life. They are characterized by a rigid cell wall made of cellulose, chloroplasts for photosynthesis and large central vacuoles that store water and nutrients (Figure 2). Examples of plant cells include leaf cells, root cells and xylem cells that help transport water throughout the plant.

Fungal cells

Fungi, such as molds, yeasts and mushrooms, are composed of eukaryotic cells. Unlike plant cells, fungal cells have cell walls made of chitin instead of cellulose. They lack chloroplasts, meaning they do not perform photosynthesis. Instead, fungi absorb nutrients from their environment. Examples include yeast cells, used in baking and fermentation and hyphal cells, which form the thread-like structures in fungi.

Animal cells

Animal cells lack cell walls but have a flexible plasma membrane. They come in various forms, including muscle cells for movement, nerve cells for transmitting signals and blood cells for transporting oxygen and nutrients throughout the body.

Protozoa

Protozoa are single-celled eukaryotic organisms that are often classified as protists. They are diverse in shape and function, with some acting as free-living organisms while others are parasitic. Many protozoa, such as amebae and paramecia, move using specialized structures like cilia, flagella or pseudopodia (Figure 2). Some, like plasmodia, cause diseases such as malaria in humans.

Figure 2. Examples of eukaryotic cells, including plant, animal, yeast and protist cells. Credit: Technology Networks.