Lecture 14 Cell Walls 2025

Cell Walls: Composition, Biosynthesis, Application

Introduction

  • Plant Cell Walls

    • Essential for plant structure and function.

    • Impact on human applications such as biofuels and materials.

Learning Objectives

  1. Understand challenges associated with plant cell wall composition.

  2. Recognize significance for plants and human applications.

  3. Identify three cell wall types: primary, secondary, middle lamella.

    • Each with signature polymers and functions.

  4. Apply knowledge of Cellulose Synthase (CESA) to biological scenarios.

  5. Describe key experiments identifying CESA, including genetic and molecular phenotypes.

  6. General reaction performed by glycosyltransferases (structures not required).

  7. Importance of glycosyltransferases in cell function.

  8. Role of hemicellulose polymers in cell walls.

  9. Name an unknown hemicellulose based on nomenclature rules.

  10. Know the seven types of cross-links discussed in class.

  11. Distinguish between structural and enzymatic proteins in cell walls.

  12. Examples & roles of structural and enzymatic proteins in cell walls.

Types of Cell Walls

  • Middle Lamella: Glues neighboring cells; rich in pectin.

  • Primary Cell Wall: Thin, allows growth; low lignin, ~30% pectin.

  • Secondary Cell Wall: Formed when growth stops; high cellulose, contains lignin and lower pectin.

Secondary Cell Wall Polymers

  • Rods/Fibers: Carbohydrate polymers (cellulose).

  • Matrix Components:

    • Hemicelluloses: Various forms, important for providing structure.

    • Pectins: Gel-like properties, important in primary cell walls.

    • Lignin: Adds rigidity, important in defense.

    • Proteins: Contribute to structural integrity and function.

Importance and Relevance

  • Functionality: Maintains turgor pressure; provides structural support; defense against pests/infection.

  • Human Applications: Lignocellulosic biomass valuable for biofuels, development of bio-based materials, and adhesives.

Challenges in Study

  1. Structural Complexity: Difficult to analyze due to interconnected polymers.

  2. Intermeshed Polymers: Challenging to maintain native properties during study.

  3. Structural Diversity: Variation across plant species and developmental stages.

Glycosyltransferases

  • Enzymes responsible for forming glycosidic linkages, crucial in synthesizing polysaccharides in cell walls, especially cellulose.

Hemicelluloses and Pectins

  1. Hemicelluloses: Bind to cellulose, impart structural properties.

    • Types: Xylans (most abundant), Mannans, Glucans, Xyloglucans.

  2. Pectins: Gel-like polymers crucial in primary cell walls; enables cell adhesion and contributes to flexibility.

    • Types: Homogalacturonan, Rhamnogalacturonan, Substituted Galacturonans.

Role of Structural and Enzymatic Proteins in Cell Walls

  • Structural Proteins: E.g., HPRG, Glycine-rich proteins, Proline-rich proteins, AGP.

  • Enzymatic Proteins: E.g., Phenol oxidase, Xyloglucan endotransglycosylase, Pectin methylesterase, Polygalacturonase.

Cross-linking Cell Wall Components

  1. Cross-linking via H-bonds within cellulose microfibrils and with hemicelluloses.

  2. Pectin cross-links via ionic interactions (e.g. Ca2+).

  3. Lignin cross-links provide rigidity and defense.

Methods for Studying Cell Walls

  • Scientific Methods: Use of cotton as a model organism, various biochemical methods to elucidate CESA activity and function.

  • Freeze-Fracture Technique: Utilized in microscopy to study cellulose biosynthesis at the plasma membrane.

Summary of Plant Cell Walls

  • Complex structure: made from cellulose microfibrils interwoven with various polymers.

  • Important for vitality of plant cells, as well as applications in human industry.