Eukaryotic Cells: Structure, Function, and Organelle Systems

Eukaryotic Cells: Structure, Function, Evolution

Major Differences Between Eukaryotic Cells and Bacteria/Archaea

Eukaryotic cells are distinguished from prokaryotic cells (bacteria and archaea) by several key structural and functional features.

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

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

• Membrane-bound organelles: Eukaryotic cells contain organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus, which are absent in prokaryotes.

• 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

General Structure and Function of Organelles

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

• Nucleus: Contains genetic material (DNA) and controls cellular activities.

• Endoplasmic Reticulum (ER): Network for protein and lipid synthesis; includes rough (RER) and smooth (SER) types.

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

• Vesicles: Small membrane-bound sacs for transport.

• Mitochondria: Site of cellular respiration and energy production.

• Lysosomes: Contain digestive enzymes (mainly in animal cells).

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

• Plasma Membrane: Controls entry and exit of substances.

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

Example: Plant cells have chloroplasts and a cell wall, while animal cells do not.

Major Eukaryotic Cell Structures

Key Organelles and Their Functions

• Nuclear membrane

• Endoplasmic Reticulum (SER, RER)

• Golgi Apparatus

• Lysosomes

• Vesicles

• Mitochondria

• Chloroplasts (plants only)

• Cytoskeleton

• Plasma membrane and cell wall

Endomembrane System

Definition and Components

The endomembrane system is a group of interconnected organelles that work together to modify, package, and transport lipids and proteins.

• 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 the endomembrane system evolved from the plasma membrane through invagination and specialization.

Nucleus and Nuclear Genome

Structure and Function

The nucleus is the control center of the cell, housing the genetic material and separating it from the cytoplasm.

• Double membrane: The nuclear envelope consists of two lipid bilayers.

• Nuclear lamina: Provides structural support (made of intermediate filaments called lamins).

Nuclear DNA Organization

• Chromatin: DNA exists as chromatin, which can be euchromatin (loosely packed, active genes) or heterochromatin (densely packed, inactive genes).

Nucleolus

• Site of rRNA synthesis: The nucleolus is a non-membrane bound structure where ribosomal RNA is synthesized and ribosome assembly begins.

Nuclear Transport

• Nuclear pore complexes (NPCs): Embedded in the nuclear envelope, they regulate movement of molecules in and out of the nucleus.

Nuclear Localization and Export Signals

• NLS: Nuclear Localization Signal, required for import into the nucleus.

• NES: Nuclear Export Signal, required for export out of the nucleus.

Endoplasmic Reticulum (ER)

Structure and Functions

The ER is a network of membranes involved in protein and lipid synthesis.

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

• Smooth ER (SER): Lacks ribosomes; synthesizes lipids, detoxifies drugs, and stores calcium.

Protein Synthesis and 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.

Golgi Apparatus

Structure and Function

The Golgi apparatus consists of flattened sacs called cisternae and is responsible for modifying, sorting, and packaging proteins and lipids.

• Proteins from the RER enter at the cis face and leave via the trans face.

• Modification includes adding sugars or lipids, removing amino acids.

• Products are sorted to plasma membrane or other endomembrane system parts.

Protein Transport and Vesicles

• Proteins exported from ER via vesicles.

• Vesicles fuse with Golgi, plasma membrane, lysosomes, or peroxisomes.

• Vesicles are membrane-enclosed structures that deliver cargo between organelles.

Tracking Protein Movement

Pulse-Chase Experiments

Pulse-chase experiments use radioactive labeling to track protein movement through the cell.

• Pulse: Cells exposed to radioactive amino acids for a short time.

• Chase: Cells washed and exposed to non-radioactive amino acids; movement of labeled proteins is tracked.

Endocytosis and Lysosomes

Endocytosis

Endocytosis is the process by which cells take in materials by engulfing them in vesicles.

• Vesicles form from the plasma membrane and bring molecules into the cell.

Lysosomes

Lysosomes are membrane-bound organelles containing digestive enzymes, present only in animal cells.

• Low pH maintained by proton pumps.

• Digest macromolecules and recycle cellular material.

• Enzymes originate from the Golgi apparatus.

Smooth ER Functions

• No associated ribosomes.

• Synthesizes lipids and detoxifies toxins.

• Stores calcium for cell signaling.

Summary Table: Major Eukaryotic Organelles and Functions

Organelle	Main Function	Present in
Nucleus	Genetic material storage, transcription	All eukaryotes
Rough ER	Protein synthesis and export	All eukaryotes
Smooth ER	Lipid synthesis, detoxification	All eukaryotes
Golgi Apparatus	Protein and lipid modification, sorting	All eukaryotes
Lysosomes	Digestion and recycling	Animal cells
Chloroplasts	Photosynthesis	Plant cells
Mitochondria	ATP production	All eukaryotes
Vesicles	Transport	All eukaryotes

Key Equations and Concepts

• Central Dogma of Molecular Biology:

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

• Pulse-Chase Labeling: Used to track protein movement and modification.

Additional info: Some explanations and examples have been expanded for clarity and completeness, including the summary table and the central dogma equation.