Topic 5: Eukaryotes

Cell Exteriors

The exterior of eukaryotic cells consists of structures and components outside the plasma membrane. These exteriors play crucial roles in protection, communication, and structural support. The composition and complexity of cell exteriors vary among different eukaryotic groups, such as diatoms, fungi, and animals.

Exteriors of Major Eukaryotic Cells

Overview and Evolutionary Order

Eukaryotic cell exteriors have evolved to suit the needs of different organisms. The main types include:

• Diatoms: Possess a silica-based cell wall (frustule), providing rigidity and protection.

• Fungi: Have cell walls composed primarily of chitin, similar to the exoskeletons of arthropods.

• Animals: Feature an extracellular matrix (ECM) rather than a rigid cell wall.

The plasma membrane serves as the boundary between the cell's interior and exterior. Components outside the plasma membrane must be exported and are typically complex arrangements of polymers.

Fungal Cell Wall

Structure and Composition

The fungal cell wall is a highly complex structure that provides protection and shape to the cell. It consists of multiple layers and various biomolecules:

• Chitin: A polysaccharide that forms the main structural framework, also found in arthropod exoskeletons.

• β-glucans: Polysaccharides that provide additional strength and flexibility.

• Mannoproteins: Proteins with attached mannose sugars, contributing to wall integrity and cell interactions.

Two distinct layers can be identified:

• Outer wall: Responsible for hydrophobicity and adhesiveness.

• Inner wall: Provides a flexible, viscoelastic frame.

Example: The cell wall of Candida albicans demonstrates this complexity, with a network of chitin, β-glucans, and mannoproteins.

Animal Cell Exterior: Extracellular Matrix (ECM)

Structure and Main Components

The extracellular matrix (ECM) is a dynamic meshwork outside animal cells, providing structural support, regulating cell behavior, and facilitating communication. Its composition varies by cell type and tissue.

• Fibrous proteins: Collagen (most abundant), elastin.

• Proteoglycans: Protein-polysaccharide complexes, including chondroitin, heparan, and keratan.

• Polysaccharides: Hyaluronan and others.

• Fibronectin: Attaches ECM components to integrins in the plasma membrane.

Some ECMs are highly specialized, depending on tissue function.

Main Proteins and Other Components of Animal ECM

Collagen in the ECM

Structure and Function

Collagen is the most abundant protein in mammals and is essential for ECM strength. Its structure is a triple helix, and its assembly involves several steps:

• Collagen is synthesized as procollagen in the endoplasmic reticulum (ER).

• Procollagen is secreted and processed into mature collagen fibers.

• Peptide chains are cross-linked by hydroxyproline residues, a process dependent on Vitamin C.

Lack of collagen leads to diseases such as scurvy, due to impaired connective tissue formation.

Collagen Synthesis Equation

The formation of hydroxyproline in collagen requires Vitamin C:

Comparison of Cell Exteriors

Animal vs. Fungal Cell Exteriors

Summary

• Eukaryotic cell exteriors are diverse and adapted to organismal needs.

• Fungal cell walls are complex, with chitin and other polysaccharides.

• Animal cells rely on the ECM for support and communication, with collagen as a key protein.

• Comparing cell exteriors highlights differences in structure, composition, and function among eukaryotes.