Eukaryotic Cells: Structure, Function, and Organelle Systems

Eukaryotic Cells: Structure, Function, Evolution

Major Differences Between Eukaryotic Cells and Bacteria/Archaea

Eukaryotic cells differ fundamentally from prokaryotic cells (bacteria and archaea) in several structural and functional aspects.

• Larger size: Eukaryotic cells are generally larger than prokaryotic cells.

• Distinct nucleus: Eukaryotes possess a nucleus surrounded by a nuclear envelope, whereas prokaryotes lack a true nucleus.

• Membrane-bound organelles: Eukaryotic cells contain specialized organelles (e.g., mitochondria, Golgi apparatus) enclosed by membranes.

• Endomembrane system: Eukaryotes have a complex system of internal membranes.

Example: Animal and plant cells are eukaryotic, while Escherichia coli is a prokaryote.

Basic Structure of Animal and Plant Cells & Organelle Functions

Both animal and plant cells share many organelles, but plant cells have additional structures such as a cell wall and chloroplasts.

• Nucleus: Stores genetic material and coordinates cell activities.

• Endoplasmic Reticulum (ER): Synthesizes proteins (RER) and lipids (SER).

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Lysosomes: Digestive organelles (mainly in animal cells).

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Mitochondria: Produce ATP via cellular respiration.

• Chloroplasts: Site of photosynthesis (plant cells only).

• Vacuoles: Storage and structural support (large central vacuole in plant cells).

• Plasma membrane: Controls entry and exit of substances.

• Cell wall: Provides structural support (plant cells only).

Example: The presence of chloroplasts distinguishes plant cells from animal cells.

Subcellular Compartments and Organelles

Definition and Examples

Organelles are specialized structures within cells, each with distinct functions and compositions.

• Nucleus

• Mitochondria

• Golgi apparatus

• Centrosomes

Major Eukaryotic Cell Structures

• Nuclear membrane

• Endoplasmic Reticulum (SER, RER)

• Golgi Apparatus

• Lysosomes

• Vesicles

• Peroxisomes

• Mitochondria

• Chloroplasts (plants only)

• Cytoskeleton

• Plasma membrane and cell wall

Endomembrane System

Components of the Endomembrane System

The endomembrane system is a network of membranes within eukaryotic cells that work together in the synthesis, modification, and transport of cellular materials.

• Nuclear membrane

• Endoplasmic Reticulum (SER, RER)

• Golgi Apparatus

• Vesicles

• Lysosomes

• Vacuoles

Functions of the Endomembrane System

• Synthesis, modification, transport, and secretion of proteins

• Synthesis of lipids and detoxification of toxins

• Transportation and breakdown of large biomolecules

• Membranes are interconnected or can fuse with each other

Endomembrane System Evolution

It is hypothesized that endomembrane organelles evolved from the plasma membrane through invagination and specialization.

Nucleus: Structure and Function

Nuclear Genome and Envelope

The nucleus stores hereditary material and is surrounded by a double membrane (nuclear envelope) with nuclear pores for transport.

• Double membrane: Two lipid bilayers

• Nuclear lamina: Network of intermediate filaments (lamins) providing structural support

Chromatin Organization

• Euchromatin: Loosely packed, transcriptionally active DNA (lighter regions)

• Heterochromatin: Densely packed, transcriptionally inactive DNA (darker regions)

Nucleolus Function

• Site of rRNA synthesis and ribosome assembly

• Non-membrane bound structure inside the nucleus

Nuclear Transport

• Nuclear pore complexes (NPCs): Control movement of molecules in and out of the nucleus

• Nuclear Localization Signals (NLS) and Nuclear Export Signals (NES): Direct proteins to and from the nucleus

Protein Synthesis and Secretion

Transcription and Translation

After transcription of DNA to mRNA, the mRNA is translated into protein by ribosomes.

Endoplasmic Reticulum (ER)

• Rough ER (RER): Studded with ribosomes; involved in protein synthesis and export

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids and detoxifies toxins

Protein Targeting to the ER

• Step 1: Ribosome synthesizes ER signal sequence as part of the protein chain

• Step 2: ER signal sequence binds to signal recognition particle (SRP)

• Step 3: Ribosome + signal sequence + SRP move to ER membrane and bind to SRP receptor

• Step 4: SRP is released; protein synthesis continues through a translocon channel

• Step 5: The growing protein is fed into ER lumen and the ER signal sequence is ultimately removed

Vesicle Transport Between Organelles

• Vesicles: Membrane-enclosed structures that transport proteins and other molecules between organelles

• Connect ER with Golgi, Golgi with plasma membrane, and with lysosomes/peroxisomes

Golgi Apparatus

• Cisternae: Flattened sacs where proteins are further modified

• Proteins enter at the cis face and exit at the trans face

• Modification includes adding sugars/lipids, removing amino acids

• Products sorted to plasma membrane or other endomembrane system parts

Tracking Protein Movement

• Pulse-chase experiment: Cells are exposed to radioactive amino acids (pulse), then to non-radioactive ones (chase) to track protein movement

Endocytosis

• Vesicles can also form by endocytosis, bringing molecules into the cell from the exterior

Smooth ER Functions

• No associated ribosomes

• Synthesizes lipids for cell membranes

• Detoxifies drugs, poisons, and stores calcium ions

Lysosomes

• Present only in animal cells

• Contain enzymes for digestion (autophagy, endocytosis, phagocytosis)

• Low pH maintained by proton pumps

Summary Table: Major Eukaryotic Organelles and Functions

Organelle	Main Function	Present in
Nucleus	Stores genetic material, coordinates cell activities	All eukaryotes
Rough ER	Protein synthesis and export	All eukaryotes
Smooth ER	Lipid synthesis, detoxification	All eukaryotes
Golgi Apparatus	Protein modification and sorting	All eukaryotes
Lysosomes	Digestion and recycling	Animal cells
Peroxisomes	Breakdown of fatty acids, detoxification	All eukaryotes
Mitochondria	ATP production	All eukaryotes
Chloroplasts	Photosynthesis	Plant cells
Vacuole	Storage, structural support	Plant cells

Key Equations and Concepts

• Central Dogma of Molecular Biology:

$\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$

• Pulse-Chase Experiment: Used to track the movement of proteins through cellular compartments.

Additional info: Some explanations and examples have been expanded for clarity and completeness.