Table of Contents
Overview of Eukaryotic Cells
Eukaryotic cells (eucytes) are the cell type found in animals, plants, fungi, and protists. Their defining feature is a true, membrane-bound nucleus and a complex internal organization with many different membrane-bound organelles.
Compared with prokaryotic cells, eukaryotic cells are usually:
- Larger (typically 10–100 µm in diameter)
- More structurally complex
- Often part of multicellular organisms with specialized cell types
This chapter focuses on structures and features characteristic of eukaryotic cells and how they are organized.
Fundamental Features of Eukaryotic Cells
Compartmentalization
A hallmark of eukaryotic cells is compartmentalization: the interior of the cell is divided into distinct regions (organelles) by membranes. Each compartment provides a specialized environment for specific tasks, for example:
- DNA storage and processing in the nucleus
- Energy conversion in mitochondria
- Synthesis, modification, and sorting of molecules in the endomembrane system
This separation:
- Prevents interference between incompatible reactions
- Allows fine regulation of processes
- Increases efficiency by localizing enzymes and substrates
Cytosol and Cytoplasm
The cytoplasm includes:
- The cytosol (the aqueous solution inside the cell, outside organelles)
- Organelles (except the nucleus, which is often considered separately)
The cytosol:
- Contains ions, small molecules, and many enzymes
- Is the site of many metabolic reactions (e.g., parts of carbohydrate and protein metabolism)
- Provides a medium for the movement of molecules and organelles
The Nucleus: Defining Organelle of Eukaryotes
Nuclear Envelope and Nuclear Pores
The nucleus is surrounded by a double membrane, the nuclear envelope:
- Outer and inner membranes, with a space in between
- The outer membrane often continuous with the endoplasmic reticulum (ER)
Nuclear pores are large protein complexes that:
- Form regulated channels through the envelope
- Allow import of proteins (e.g., enzymes, structural proteins)
- Allow export of RNA molecules and ribosomal subunits
Transport through nuclear pores is selective and controlled, not just passive diffusion.
Chromatin and Chromosomes
Inside the nucleus, DNA is associated with proteins (chiefly histones) forming chromatin:
- Loosely packed chromatin (euchromatin): usually more active in gene expression
- Densely packed chromatin (heterochromatin): often less active or silent
Before cell division, chromatin condenses into visible chromosomes, but in this chapter we focus on their organization within the resting (interphase) nucleus:
- Each chromosome occupies a specific territory
- The arrangement influences which genes are accessible for reading (expression)
Nucleolus
The nucleolus is a dense region within the nucleus:
- Not separated by a membrane
- Site of rRNA synthesis and assembly of ribosomal subunits
- Size and number can change depending on ribosome production needs
The Endomembrane System
The endomembrane system is a network of membranes inside the cell that cooperate in the synthesis, modification, transport, and degradation of molecules. It includes:
- Endoplasmic reticulum (ER)
- Golgi apparatus
- Various vesicles (transport, secretory, lysosomes)
- The nuclear envelope
- In plant cells: the vacuole is closely integrated with this system
Endoplasmic Reticulum (ER)
The ER is a membranous network of tubules and flattened sacs (cisternae), continuous with the outer nuclear membrane.
Rough ER (rER)
Rough ER has ribosomes attached to its cytosolic surface:
- Main site of synthesis for proteins that will:
- Be secreted from the cell
- Be inserted into membranes
- Be sent to lysosomes or vacuoles
- Newly made proteins enter the ER lumen where they can:
- Fold
- Form disulfide bonds
- Undergo initial modifications (e.g., addition of carbohydrate groups)
Smooth ER (sER)
Smooth ER lacks attached ribosomes and is involved in:
- Lipid and steroid synthesis
- Detoxification of drugs and harmful substances (especially in liver cells)
- Storage and release of calcium ions (e.g., in muscle cells)
- Carbohydrate metabolism (e.g., in liver cells)
The ratio of rough to smooth ER depends on cell type and function.
Golgi Apparatus
The Golgi apparatus is made of stacked, flattened membrane sacs (cisternae) with a:
- Cis face (receiving side) facing the ER
- Trans face (shipping side) facing the plasma membrane or other destinations
Functions:
- Receives proteins and lipids from the ER in transport vesicles
- Modifies them (e.g., further glycosylation, trimming of sugar chains)
- Sorts and packages them into new vesicles
- For secretion (exocytosis)
- For delivery to plasma membrane, lysosomes, or vacuoles
It acts like a cellular “post office,” labeling and directing molecules.
Lysosomes (Mainly in Animal Cells)
Lysosomes are small membrane-bound organelles containing digestive enzymes active at acidic pH.
They:
- Break down macromolecules (proteins, nucleic acids, lipids, carbohydrates)
- Digest worn-out organelles (autophagy)
- Degrade material taken up from outside the cell via endocytosis or phagocytosis
Lysosomal enzymes are synthesized in the rough ER, modified in the Golgi, and targeted to lysosomes by specific molecular “tags.”
Vacuoles (Prominent in Plant and Fungal Cells)
Vacuoles are large membrane-bound compartments; in plant cells, the central vacuole is especially important:
- Surrounded by the tonoplast (vacuolar membrane)
- Filled with cell sap (water, ions, sugars, pigments, waste products)
Key functions:
- Maintaining turgor pressure for structural support
- Storage of nutrients (e.g., sugars), ions, and defensive compounds (e.g., toxic substances used against herbivores)
- Breakdown and recycling of cellular components (similar to lysosomal functions)
In animal cells, smaller vacuole-like vesicles can exist, but they are usually less dominant in volume.
Mitochondria: Energy Converters
Mitochondria are present in almost all eukaryotic cells and are often called the “powerhouses” of the cell.
Key structural features:
- Double membrane:
- Outer membrane: relatively smooth and permeable to small molecules
- Inner membrane: highly folded to form cristae, increasing surface area
- Matrix: internal compartment containing enzymes, mitochondrial DNA, and ribosomes
Main functions:
- Cellular respiration and ATP production by oxidative phosphorylation
- Involvement in regulation of cell death (apoptosis)
- Roles in certain biosynthetic pathways (e.g., some steps of amino acid and lipid metabolism)
- Buffering and signaling through calcium ions
Mitochondria contain their own DNA and ribosomes and divide independently, reflecting their evolutionary origin as once-free-living prokaryotes.
Chloroplasts and Other Plastids (in Plants and Some Protists)
Chloroplasts
Chloroplasts are the sites of photosynthesis in plants and some protists.
Key features:
- Double membrane envelope
- Internal membrane system forming flattened sacs called thylakoids
- Thylakoids stacked into grana
- Surrounded by stroma (fluid space containing enzymes, DNA, and ribosomes)
Functions:
- Capture light energy and convert it into chemical energy
- Fix carbon dioxide into organic molecules
Like mitochondria, chloroplasts:
- Have their own DNA and ribosomes
- Divide independently of the cell cycle
Other Plastids
Non-green plastids also occur in plant cells, for example:
- Chromoplasts: contain pigments other than chlorophyll (e.g., carotenoids in petals and fruits)
- Leucoplasts: non-pigmented, often storage plastids
- Amyloplasts: store starch
- Others store lipids or proteins
Plastids can differentiate from one form into another according to the cell’s needs (e.g., chloroplasts from leucoplasts upon light exposure).
The Cytoskeleton
The cytoskeleton is a dynamic network of protein filaments that:
- Provides internal support and shape
- Enables movement of the whole cell and of organelles within the cell
- Plays a critical role in cell division and intracellular transport
Three main filament systems are distinguished:
Microtubules
Microtubules are hollow tubes made of tubulin proteins.
Functions:
- Form tracks for movement of organelles and vesicles (with motor proteins like kinesin and dynein)
- Build the mitotic spindle for separating chromosomes during cell division
- Form the structural core of cilia and flagella (in a characteristic “9 + 2” arrangement)
They can rapidly assemble and disassemble, allowing flexible reorganization of the cell.
Microfilaments (Actin Filaments)
Microfilaments are thin filaments made of actin.
Functions:
- Support cell shape, especially in the cell cortex (region beneath plasma membrane)
- Participate in cell movement (e.g., amoeboid movement, formation of pseudopodia)
- In muscle cells, interact with myosin to generate contraction
- Involved in cytokinesis (division of the cytoplasm) in many animal cells
Intermediate Filaments
Intermediate filaments are rope-like fibers made of various proteins (depending on cell type).
Functions:
- Provide mechanical strength
- Help maintain cell shape
- Anchor organelles, including the nucleus
- Form structures such as the nuclear lamina (inner lining of the nuclear envelope)
Unlike microtubules and microfilaments, they are generally more stable and less dynamic.
Cell Surface Specializations and Motility Structures
The Plasma Membrane in Eukaryotes
The plasma membrane surrounds the eukaryotic cell and:
- Separates interior from environment
- Controls transport of substances
- Contains receptors for cell signaling
- Anchors cytoskeleton and sometimes extracellular matrix
Its basic structure is similar across all cells, but eukaryotic membranes often:
- Contain sterols (e.g., cholesterol) that modulate fluidity
- Display diverse proteins for communication and adhesion
- Bear carbohydrate chains on lipids and proteins (glycolipids, glycoproteins) important for recognition
Cell Walls in Eukaryotes
Some eukaryotes have an additional rigid structure outside the plasma membrane:
- Plant cells:
- Primary cell wall mainly composed of cellulose, hemicellulose, and pectins
- Provides mechanical support, determines cell shape, resists osmotic pressure
- Plasmodesmata (fine channels) connect neighboring plant cells, allowing exchange of substances
- Fungi:
- Cell walls usually contain chitin and other polysaccharides
- Many protists:
- May have cell walls or outer coverings (e.g., silica frustules in diatoms, pellicles in some flagellates)
Animal cells lack a cell wall, which allows more flexible shapes and diverse forms of movement but requires other forms of structural support (e.g., cytoskeleton, extracellular matrix).
Cilia and Flagella
Many eukaryotic cells possess motility structures on their surface:
- Cilia:
- Short, numerous projections
- Beat in coordinated waves
- Used for movement (e.g., single-celled protists) or for moving fluid over cell surfaces (e.g., ciliated cells in human airways)
- Flagella:
- Longer, usually fewer per cell
- Whip-like motion for propulsion (e.g., sperm cells, certain protists)
Internal structure:
- Both are built from microtubules in a typical “9 + 2” arrangement:
- Nine doublet microtubules around two central single microtubules
- Driven by dynein motor proteins, powered by ATP
These eukaryotic structures differ fundamentally from the simpler, rotating flagella of many prokaryotes.
Differences Between Major Eukaryotic Cell Types
Within eukaryotes, cells show structural differences related to lifestyle and function. As full details of kingdoms are treated elsewhere, only central contrasts are highlighted here.
Animal vs. Plant Cells (Key Structural Differences)
Typical plant cells usually have:
- A rigid cell wall containing cellulose
- A large central vacuole
- Chloroplasts (in green tissues) or other plastids
- A more fixed, box-like shape determined by the wall
Typical animal cells usually have:
- No cell wall (only plasma membrane)
- Many small vacuoles or vesicles instead of a single large central vacuole
- No chloroplasts or other plastids
- Often more variable and flexible shapes
- Commonly centrioles as part of the centrosome, organizing microtubules (centrioles are absent or unusual in most higher plant cells)
These structural differences reflect different modes of life: autotrophy with a rigid, supported body in plants, and more motile, flexible organization in animals.
Cell Specialization Within Multicellular Organisms
In multicellular eukaryotes, cells differentiate to perform specialized tasks:
- Nerve cells with long processes for signal conduction
- Muscle cells rich in contractile machinery
- Leaf mesophyll cells packed with chloroplasts
- Root hair cells elongated for water and nutrient uptake
Despite these differences, they share the core eukaryotic features described above: a nucleus, organelles, and cytoskeleton.
Summary of Defining Eukaryotic Features
Eukaryotic cells:
- Possess a true, membrane-bound nucleus containing linear chromosomes
- Are extensively compartmentalized into organelles (e.g., ER, Golgi, lysosomes, vacuoles)
- Contain energy-converting organelles (mitochondria, and in photosynthetic eukaryotes, chloroplasts)
- Have a complex cytoskeleton for support, transport, and movement
- Show diverse cell surface structures (cell walls in plants and fungi; extracellular matrix, cilia, and flagella in many groups)
- Allow high levels of internal specialization and, in many organisms, formation of complex tissues and organs
These features make eukaryotic cells versatile units capable of supporting a wide range of life strategies, from single-celled protists to complex multicellular plants, fungi, and animals.