CELS191 Lecture 8: Cell Walls and Plant Cell Shape

Copyright Notice

• The lecture materials and resources are for educational purposes only.
• Unauthorized copying or distribution of materials is prohibited.

Professor Information

• Professor David Orlovich
• Research interests: Evolution of plants and fungi, Molecular ecology
• Contact: david.orlovich@otago.ac.nz
• Profile: Link

Lecture Overview

• CELS191 (2025) focuses on Cell Structure & Diversity.
• Lecture 8 Topic: Cell Walls & Their Role in Regulating Plant Cell Shape.

Significance of Plant Cell Walls

• Annual production estimated at 150-170 billion tons/year.
• Energy in the cell wall is 5 times the global human energy use in 2022.
• Potential questions include:
• Can cell wall be a carbon neutral energy source?
• Is it a valuable resource?

Learning Objectives
After this lecture, you should be able to:

• Describe structure and function of the primary plant cell wall and its synthesis.
• Outline the structure and role of the vacuole in maintaining cell shape.
• Describe the secondary plant cell wall structure and the function of plasmodesmata.

Comparative Cell Biology

• Focus on Eukarya: Understanding Plant vs. Animal Cells.
• Key components of plant cells include: Central Vacuole, Chloroplast, Mitochondrion, and Golgi Apparatus.

Plant Cell Wall Structure

• Cellulose

• Most abundant organic macromolecule on Earth.

• Comprised of glucose polymers arranged in long, ribbon-like structures.

• Forms microfibrils that provide strength to primary and secondary cell walls.

• Matrix Components

• Hemicellulose: Heterogeneous polysaccharides with rigid structure.

• Pectin: Negatively charged, gel-like polysaccharides that bind water.

• Extensins

• Proteins that control cell wall extensibility by cross-linking components.

• Influences cell wall strength and expansion.

Synthesis of Primary Cell Wall

• Phase 1: Cellulose microfibrils synthesized at plasma membrane.
• Phase 2: Polysaccharides (pectin & hemicellulose) synthesized in the Golgi and delivered to the plasma membrane via vesicles.
• Phase 3: Extensins synthesized in rough ER, transported via Golgi to the plasma membrane.

Cell Wall Functions

• Regulates cell shape and morphology:
• Orientation of cellulose affects expansion.
• Provides structural support:
• Prevents excessive water uptake.
• Wilting occurs when water is lost and protoplast shrinks.

Vacuoles

• Structure: Single large membrane-bound organelle in mature plant cells.
• Function:
• Regulates osmotic pressure, maintaining turgidity and structural support.
• Plays a critical role in preventing cell bursting when water enters by osmosis.

Secondary Cell Wall

• Not present in all plant cells, produced after growth stops.
• Thicker, stronger than the primary wall, providing enhanced support.
• Contains lignin, conferring additional strength and rigidity, particularly in water-transporting cells.

Plasmodesmata

• Structure enabling intercellular communication.
• Allows free exchange of small molecules between adjacent cells but prevents organelle movement.
• Important for coordinated responses, such as during pathogen attacks.

Summary of Key Concepts

• Primary cell wall provides strength through cellulose linked by hemicelluloses and pectin.
• Secondary cell wall offers additional structural strength, especially in specific cell types.
• Communication among plant cells is facilitated by plasmodesmata.

Revision Questions

• What are the compositional differences between primary and secondary cell walls?
• Do vacuoles shrink or swell when the plant is under mild drought stress?
• Through which cellular structure can viruses travel from one plant cell to another?

Visual Aids

• Figures illustrating plant cell structures, synthesis process, and examples of cell shapes and functions are referenced throughout the lecture.