Eukaryotic Cell Structure, Endomembrane System, and Organelle Functions

Key Concepts in Eukaryotic Cell Biology

• Origins of the endomembrane system

• Organelles and their functions

• Ribosomes

• Cytoskeleton

• Extracellular matrix and cell walls

• Viruses

• Enzyme functions

Theories of Eukaryotic Origins

Outside-In Versus Inside-Out Theories

Eukaryotic cells are distinguished by the presence of a nuclear envelope and mitochondria. Two main theories explain the origin of these features: the outside-in and inside-out models.

• Outside-in theory: Proposes that infoldings of the plasma membrane formed the nuclear envelope and endomembrane system, eventually engulfing the mitochondrion.

• Inside-out theory: Suggests that the nucleus and endomembrane system originated from protrusions of the cell membrane, with mitochondria attaching to these protrusions.

Example: The figure illustrates both theories, showing the stepwise development of the nucleus and endomembrane system.

Endomembrane System

Overview and Function

The endomembrane system is a network of membranes within eukaryotic cells that synthesizes, processes, and transports proteins and lipids to various destinations, including organelles, the plasma membrane, or for secretion.

• Proteins are synthesized, packaged, and directed to their destinations in small, membrane-bound vesicles.

• Main components: endoplasmic reticulum (ER), Golgi apparatus, lysosomes, vesicles, and the plasma membrane.

Endoplasmic Reticulum (ER)

The ER is a continuous network of membranes in the cytoplasm, consisting of tubules and flattened sacs called cisternae. The internal space is known as the lumen.

• Rough ER (RER): Studded with ribosomes on the cytoplasmic side; synthesizes polypeptides that are either inserted into the membrane or transported into the lumen.

• Smooth ER (SER): Lacks ribosomes; involved in lipid and steroid synthesis, detoxification, and (in muscle cells) calcium storage.

Example: The rough ER is prominent in cells that secrete large amounts of protein, such as pancreatic cells.

Golgi Apparatus

The Golgi apparatus is a stack of flattened cisternae located near the ER. It functions as a processing and packaging center for proteins and lipids.

• Receives vesicles from the ER (transition vesicles).

• Modifies proteins (e.g., glycosylation) and sorts them for delivery to their final destinations.

• Packages materials into secretory vesicles for exocytosis.

Example: Glycoproteins are modified in the Golgi before being sent to the plasma membrane.

Vesicular Transport and Secretion

Processed materials are transported in vesicles that bud from the Golgi and fuse with the plasma membrane, releasing their contents outside the cell (exocytosis).

• The ER, Golgi, vesicles, and lysosomes together form the endomembrane system, responsible for intracellular trafficking.

Lysosomes

Lysosomes are single-membrane organelles containing hydrolytic enzymes (hydrolases) that digest biological macromolecules.

• Contain enzymes such as proteases, nucleases, glycosidases, lipases, and phosphatases.

• Hydrolases are sequestered to prevent damage to the cell.

• A carbohydrate coating on the inner membrane protects the lysosome from self-digestion.

Other Organelles: Peroxisomes and Vacuoles

Peroxisomes are similar in size and appearance to lysosomes but have distinct functions, such as breaking down hydrogen peroxide (H2O2) using catalase. Vacuoles are large vesicles with storage and structural roles, especially in plant cells.

• Peroxisomes are prominent in liver and kidney cells, where they detoxify harmful substances.

• Vacuoles maintain turgor pressure in plant cells.

Ribosomes

Structure and Function

Ribosomes are the sites of protein synthesis and are found in all cells. They differ in size and composition among bacteria, archaea, and eukaryotes.

• Composed of two subunits (large and small) with sedimentation coefficients measured in Svedberg units (S).

• Eukaryotic ribosomes: 80S (60S large + 40S small subunits).

• Bacterial/archaeal ribosomes: 70S (50S large + 30S small subunits).

• S values are not additive due to differences in shape and density.

Example: Ribosomes attached to the rough ER synthesize membrane-bound and secretory proteins.

Cytoskeleton

Components and Functions

The cytoskeleton is a dynamic, three-dimensional network of protein filaments that provides structural support, organization, and movement within the cytoplasm.

• Microtubules: Hollow tubes made of tubulin; involved in cell shape, transport, and division.

• Microfilaments: Thin filaments composed of actin; important for cell movement and muscle contraction.

• Intermediate filaments: Rope-like fibers made of various proteins (e.g., keratin); provide mechanical strength.

Extracellular Matrix and Cell Walls

Extracellular Structures

Most cells are surrounded by extracellular structures that provide support and protection.

• Extracellular matrix (ECM): Network of proteins and polysaccharides outside animal cells.

• Plant cell wall: Composed mainly of cellulose microfibrils in a matrix of polysaccharides and proteins.

• Bacterial cell wall: Made of peptidoglycan, a polymer of N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) cross-linked by peptides.

Example: The structure of the bacterial cell wall determines whether bacteria are Gram-positive or Gram-negative.

Activity: Organelle Membranes

• Double-membrane organelles: Nucleus, mitochondria, chloroplasts (in plants).

• Single-membrane organelles: Endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, vacuoles.