Plant Cell Walls: Structure and Function

• The annual cell wall production of plant cells is an estimated 150–170 billion tons per year.
• The energy stored in plant cell walls is almost five times the global human energy use in 2022.

Lecture 8 Objectives

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

Tree of Life and Plant Cells

• The focus is Eukarya Plant Cells.

Plant Cell Components

• A plant cell consists of the cell wall and the protoplast.
• Key structures include the nucleus, Golgi apparatus, central vacuole, chloroplast, mitochondrion, cell wall, and plasmodesmata.

Cellulose: Core Component of Plant Walls

• Cellulose is the most abundant organic macromolecule, it is a glucose polymer in a highly ordered, long, ribbon-like structure.

Cellulose Microfibrils

• Highly organized cellulose structures are strong
• They are a major component of primary and secondary cell walls.

Cell Wall Phases

• Phase 1: Microfibrils (Crystalline Phase) - Consists of cellulose.
• Phase 2: Matrix (Non-crystalline Matrix) - Contains pectin and hemicellulose polysaccharides.
• Extensin, a protein, is also part of the cell wall network.

Matrix: Hemicellulose and Pectin

• Hemicellulose: A heterogeneous group of polysaccharides with a long chain of one type of sugar and short side chains, forming a rigid structure.
• Pectin: Branched, negatively charged polysaccharides that bind water and provide gel-like properties.

Protein: Extensin

• Cell extensibility is controlled by extensin cross-linking.
• Extensin cross-linking of pectin and cellulose dehydrates the cell wall, reducing extensibility and increases strength.

Synthesis of Primary Cell Wall

• Coordinated synthesis and delivery happen via:
• Cellulose microfibrils synthesized at the plasma membrane.
• Polysaccharides (pectin and hemicellulose) are synthesized in the Golgi complex and transported to the plasma membrane in vesicles.
• Extensin (cell wall proteins) is synthesized in the rough ER, transported via Golgi to the plasma membrane in vesicles, which then fuse with the plasma membrane.

Exocytosis

• Exocytosis transports material out of the cell or delivers it to the cell surface.
• Constitutive exocytosis releases extracellular matrix proteins.

Cytoskeleton

• The cytoskeleton consists of a network of microtubules and microfilaments extending throughout the cytoplasm, and maintains the position of organelles.

Cellulose Production Summary

• Cellulose-producing rosettes move parallel to the cortical microtubules with cellulose microfibrils are made by cellulose synthase "rosettes" at the plasma membrane.
• Cellulose-producing rosettes are protein complexes (enzymes) that span the plasma membrane.

Primary Cell Wall Makeup

• In the primary cell wall and middle lamella, cellulose is a compound.

Cell Wall Functions

• The cell wall influences cell morphology, provides structural support, and prevents excessive water uptake.

Regulating Cell Shape

• Orientation of cellulose microfibrils influences cell morphology.
• Randomly oriented cells expand equally in all directions.
• Cells with microfibrils at right angles to the long axis expand longitudinally.

Structural Support

• The protoplast pushes against the cell wall
• This makes the cells become rigid to maintain the plant structure.
• Wilting occurs when the protoplast does not push against the cell wall.
• Water loss reduces protoplast volume and lessens pressure on the cell wall.

Preventing Water Uptake

• As water enters via osmosis, the protoplast expands and pushes against the cell wall, creating "turgor pressure".
• The cell wall limits the volume of water that can be absorbed.
• Vacuoles contain water and comprise a large portion of the protoplast.

Vacuoles

• A vacuole is a single-membrane organelle, which is selective in controlling what enters and leaves.
• Water moves in by osmosis (passive transport).
• Mature plant cells typically have a single large vacuole.

Osmosis

• Water diffuses across a selectively permeable membrane.
• Movement occurs from high water (low solute) concentration to low water (high solute) concentration.

Regulation of Cell Shape

• High concentrations of solutes are in the vacuole.
• This leads to water uptake via osmosis.
• The plant cell wall prevents the cell from bursting.
• Plant cells have high internal pressure (turgor pressure), which contributes to structural support.

Secondary Cell Walls Overview

• Not all plant cells have a secondary cell wall.
• Secondary cell walls are produced after cell growth stops.
• They are thicker and stronger than primary cell walls
• They provide more structural support.

Secondary Cell Walls Traits

• Secondary cell walls are made up of multiple layers and microfibrils in each layer oriented differently.
• Secondary cell walls typically consist of more cellulose and less pectin than primary walls.
• They also include lignin.

Lignin

• Lignin is the second most abundant organic macromolecule and a complex polymer.
• Lignin strengthens the secondary cell wall, giving rigidity and excluding water.

Communication

• Cells with a cell wall can communicate via plasmodesmata.

Plasmodesmata Communication

• Plasmodesmata are intercellular connections that enable cell-to-cell communication.
• The plasma membrane is continuous through plasmodesmata, which are small enough to prevent organelle movements
• However, the endoplasmic reticulum is connected through plasmodesmata.
• Plasmodesmata allow the free exchange of small molecules.

Lecture 8 Summary

• The primary plant cell wall gives structural strength to plant cells.
• Cellulose microfibrils are linked by hemicelluloses, and pectin provides a water-holding gel in between
• Extensins crosslink cellulose and pectin, adding strength.
• Cell walls are synthesized in three coordinated steps: cellulose microfibrils are made by cellulose synthase "rosettes" at the plasma membrane, hemicellulose and pectin are synthesized by the Golgi bodies (through exocytosis), and extensins are started in the rER and glycosylated by Golgi (also through exocytosis).
• Vacuoles are single membrane-bound organelles that contain solutes and regulate osmotic pressure, giving plant cells turgidity - pushing against the cell wall.
• Secondary plant cell walls contain lignin for greater structural strength, especially important in water-conducting cells.
• Plant cells communicate via plasmodesmata with a desmotubule (from ER) in the center, allowing movement of small molecules from cell-to-cell.