CELS191 Lecture 8: Cell Walls & Their Role in Regulating Plant Cell Shape

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Professor David Orlovich

  • Research Interests: Evolution of plants and fungi, Molecular ecology

  • Email: david.orlovich@otago.ac.nz

Lecture 8: Cell Walls & Their Role in Regulating Plant Cell Shape

Introduction to Plant Cell Walls

  • Annual production of plant cell walls is estimated to be 150–170 billion tons/year.

  • The energy in the cell wall is almost five times the global human energy use in 2022.

  • The plant cell wall could potentially be used as a carbon-neutral energy source and is a valuable resource.

Lecture 8 Objectives

  • Describe the structure and function of the primary plant cell wall and outline its synthesis.

  • Outline the structure of the vacuole and its role in maintaining cell shape.

  • Outline the structure and function of the secondary plant cell wall and plasmodesmata.

Focus: Eukarya Plant Cells

Animal vs. Plant Cells

Plant Cell Components

  • A Plant Cell = Cell Wall + The Protoplast

  • Key components include the cell wall, plasmodesmata, central vacuole, chloroplast, mitochondrion, Golgi apparatus, and nucleus.

Cell Wall Structure: Cellulose

  • Cellulose is the most abundant organic macromolecule on Earth.

  • It is a glucose polymer.

  • It is highly ordered with long, ribbon-like structures.

Cellulose Forms Microfibrils

  • Cellulose molecules form highly organised structures that are strong and a major component of both primary and secondary cell walls.

Cell Wall Structure: Phases

  • Phase 1: Microfibrils (Crystalline Phase) - Cellulose

  • Phase 2: Matrix (Non-crystalline Matrix) - Pectin and Hemicellulose polysaccharides

  • Extensin (a protein) network.

Cell Wall Structure: Matrix - Hemicellulose & Pectin

  • Hemicellulose: a heterogeneous group of polysaccharides. It contains 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 have gel-like properties.

Cell Wall Structure: The Protein Extensin

  • The extensibility (expansion) of cells can be controlled by extensin cross-linking.

  • Extensin cross-linking of pectin and cellulose dehydrates the cell wall, reduces extensibility, and increases strength.

  • Cell can expand in size when extensibility is present.

  • Cell cannot expand in size when extensibility is not present.

Synthesis of the Primary Cell Wall

Co-ordinated synthesis and delivery of:

  • A. Cellulose microfibrils synthesised at the plasma membrane.

  • B. Polysaccharides (pectin & hemicellulose) synthesised in the Golgi complex and are transported to the plasma membrane in vesicles.

  • C. Extensin (cell wall proteins) synthesised in the rough ER and transported via Golgi to plasma membrane in vesicles.

  • The vesicles fuse with the plasma membrane.

Exocytosis

  • Transports material out of the cell or delivers it to the cell surface.

  • Constitutive exocytosis releases extracellular matrix proteins.

Cytoskeleton

  • A network of microtubules, microfilaments (and intermediate filaments) that extend throughout the cytoplasm.

  • Many functions including maintaining the position of organelles, providing structural support.

Cytoskeleton & Cytoplasmic Streaming

Synthesis of the Primary Cell Wall: Microfibrils

  • The cellulose-producing rosettes move parallel to the cortical microtubules.

  • Phase 1: Microfibrils. Refer back to Slide 12 & image (A) on Slide 15

Synthesis of the Primary Cell Wall: Cellulose-Producing Rosettes

  • Cellulose-producing rosettes are protein complexes (enzymes) that span the plasma membrane.

The Primary Cell Wall & Middle Lamella

  • Primary cell wall components: cellulose, hemicellulose, pectin, and plasma membrane.

  • Middle lamella: pectin.

Revision: True or False?

  • Cellulose forms microfibrils that are highly ordered and provide strength to the cell wall (True).

  • Extensin cross-links polysaccharides this increases the extensibility of the cell wall (False).

  • Pectin and hemicellulose are transported to the cell surface by exocytosis (True).

Cell Wall Functions in Regulating Cell Shape

The cell wall:

  • influences cell morphology.

  • provides structural support.

  • prevents excessive water uptake.

Regulating Cell Shape: Cell Morphology

  • Orientation of the cellulose microfibrils influences cell morphology.

    • a) Randomly oriented: the cell will expand equally in all directions.

    • b) Right angles to the ultimate long axis of the cell: the cell will expand longitudinally along that axis.

Plant Cells Come in Many Shapes…

The Cell Wall: Provides Structural Support

  • The protoplast pushes against the cell wall. The cells become rigid, and this maintains the plant structure.

  • Wilting occurs when the protoplast is not pushing against the cell wall.

  • Water loss from cells reduces the protoplast volume and the protoplast does not press on the cell wall.

The Cell Wall: Prevents Excessive Water Uptake

  • As water enters the cell by osmosis, the protoplast expands and pushes against the cell wall (turgor pressure).

  • Pressure from the cell wall limits the volume of water that can be taken up.

  • Vacuoles are important in this process because they contain water and make up such a large portion of the protoplast.

Vacuoles: Structure

  • A vacuole is an organelle surrounded by a single membrane.

  • Typical mature plant cell has a single large vacuole.

  • It is highly selective, controlling much of what enters and leaves the vacuole.

  • Water moves in the vacuoles by osmosis (passive transport).

Osmosis

  • The diffusion of water across a selectively permeable membrane.

  • Movement occurs from a high water (low solute) concentration to a low water (high solute) concentration.

Vacuoles: Function in Regulation of Cell Shape

  • There is high concentrations of solutes in the vacuole.

  • This results in water uptake into the vacuole by osmosis.

  • The plant cell wall limits water uptake and prevents the cell bursting.

  • Plant cells build up a large internal pressure (turgor pressure) that contributes to plant structural support.

  • A happy plant cell is a turgid cell.

Revision: True or False?

  • Three functions of the cell wall are: influencing cell morphology, providing structural support, and preventing excessive water uptake (True).

  • Wilting occurs due to lack of water in the cell (True).

  • Vacuoles take up water by active transport to maintain turgid cells (False).

Water uptake without or with a cell wall

The Secondary Cell Wall

  • Not all plant cells have a secondary cell wall.

  • Produced only after cell growth has stopped.

  • Thicker and stronger than primary cell walls.

  • Provides more structural support than primary cell wall.

Secondary Cell Wall: Structure

  • Made up of multiple layers.

  • Microfibrils in each layer have different orientations.

  • This strengthens the secondary wall.

Secondary Cell Wall: Structure - Chemical Characteristics

  • More cellulose.

  • Less pectin.

  • Lignin.

Secondary Cell Wall Structure: Lignin

  • Lignin is the second most abundant organic macromolecule.

  • Lignin is a complex polymer.

  • Confers strength and rigidity to the secondary cell wall and acts to exclude water.

Secondary Cell Wall: Structure Support

  • The secondary cell wall provides structural support for:

    • specific cell types such as water transporting cells, and for the whole plant.

  • Cell walls of water transporting cells.

How do cells with a cell wall communicate?

Plasmodesmata: Cell Communication

  • Plasmodesmata are intercellular connections, that enable cell to cell communication.

  • The plasma membrane is continuous.

  • Small enough to prevent organelle movements – although endoplasmic reticulum is connected through plasmodesmata.

  • Allows the free exchange of small molecules.

Example of Intercellular Communication

  • This Sorghum plant is responding to fungal infection. The infected cell produces fungicide-containing inclusion bodies (IB), the neighboring cell secretes fungicide into the cell walls (black arrows).

Lecture 8 Summary

  • The primary plant cell wall gives structural strength to plant cells.

  • Cellulose microfibrils are linked by hemicelluloses; pectin provides a water-holding gel in between. Extensins crosslink cellulose and pectin to provide extra strength.

  • Cell walls synthesised in 3 coordinated steps (i) cellulose microfibrils made by cellulose synthase “rosettes” at the plasma membrane, (ii) hemicellulose and pectin synthesised by the Golgi bodies -> exocytosis, (iii) extensins started in the rER, then glycosylated by Golgi -> exocytosis.

  • Vacuoles are a single membrane-bound organelle that contain solutes. Vacuoles regulate osmotic pressure to give plants cells turgidity -> push against the cell wall.

  • Secondary plant cell walls contain lignin -> greater structural strength—especially important in water conducting cells.

  • Plant cells communicate via plasmodesmata, which are cytoplasmic connections with a desmotubule (from ER) in the centre. Allow movement of small molecules from cell-to-cell.

Objective-Based Questions

  • What are the main compositional differences between the plant primary and secondary cell walls?

  • If a plant is under mild drought stress, do the vacuoles shrink or swell?

  • What cellular structure could viruses use to travel from one plant cell to another?

  • Describe the structure and function of the primary plant cell wall and outline its synthesis.

    • The primary plant cell wall consists of cellulose microfibrils, hemicellulose, pectin, and extensin.

    • Cellulose microfibrils are synthesized at the plasma membrane, while polysaccharides (pectin & hemicellulose) are synthesized in the Golgi complex and transported to the plasma membrane in vesicles.

    • Extensin (cell wall proteins) are synthesized in the rough ER and transported via Golgi to the plasma membrane in vesicles.

    • These components provide structural strength, and the matrix allows for cell expansion.

  • Outline the structure of the vacuole and its role in maintaining cell shape.

    • A vacuole is an organelle surrounded by a single membrane, typically large in mature plant cells.

    • It contains high concentrations of solutes, facilitating water uptake by osmosis.

    • Vacuoles regulate osmotic pressure, providing turgidity to plant cells, which push against the cell wall to maintain cell shape.

  • Outline the structure and function of the secondary plant cell wall and plasmodesmata.

    • The secondary cell wall, present in some plant cells, is thicker and stronger than the primary cell wall.

    • It contains multiple layers with different microfibril orientations and includes more cellulose and lignin, and less pectin.

    • Lignin confers strength and rigidity.

    • Plasmodesmata are intercellular connections that enable cell-to-cell communication by allowing the free exchange of small molecules.