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Introduction to Cellular Organization

• Multicellular organisms consist of various cell types, not just one.
• Cells organize into tissues, which are essential for function.

Types of Tissues in Animals

• Four major tissues in animals:
• Nervous Tissue: Involved in receiving and transmitting nerve impulses.
• Muscle Tissue: Responsible for movement, including smooth muscle in guts.
• Epithelial Tissue: Covers body surfaces and lines cavities.
• Connective Tissue: Supports and protects other tissues.
• Tissues consist of cells and extracellular matrix (ECM).
• Cells secrete molecules that compose ECM, enabling connections between cells.

Plant vs. Animal Cells

• Plants and animals evolved multicellular structures independently due to differing needs:
• Animals: Mobile, requiring strong and adaptable tissue.
• Plants: Stationary, requiring rigid yet flexible tissue for support.

Plant Cell Walls

• Cell walls surround, protect, and define plant cell shapes.
• Considered a form of extracellular matrix.
• Plant cells can synthesize and secrete components affecting cell wall characteristics (thick/thin).
• Cell walls are tough but can be fragile due to sensitive cell membranes.
• Lacking intermediate filaments, plant cells depend on osmotic pressure.
• Loss of water leads to plant wilting.

Composition of Cell Walls

• Primary Cell Wall: Formed during initial cell growth; can expand as cells grow.
• Secondary Cell Wall: Formed when cells stop growing; results from thickening of primary wall or adding layers.
• Different plant cells can have unique secondary wall structures.

Observation of Cell Walls

• Cross sections show primary walls (roots) versus secondary walls (stems).
• Staining methods reveal components such as pectin (primary wall) and cellulose (secondary wall).

Strength and Structure of Cell Walls

• Extracellular matrices provide tensile strength via long fibers aligning with stress lines.
• Cellulose microfibrils resist compression and tension, formed from long unbranched chains of glucose.
• Orientation of cellulose microfibrils determines the growth direction of cells.
• Cells must overcome microfibril resistance to expand in specific directions.

Growth Dynamics

• Turgor pressure facilitates cell growth, influencing direction based on microfibril orientation.
• Enzyme complexes on the plasma membrane elongate microfibrils, creating new cellulose.
• While most ECM molecules are made intracellular, cellulose synthesis is unique.
• Microtubules guide cellulose synthesizing proteins, aligning with interior cell structure; microtubules control microfibril orientation.

Summary of Cell Wall Composition

• The mechanism of cellulose production illustrates a direct link between intracellular processes and extracellular components.
• Microtubules inside the cell shape how cellulose forms in the extracellular space, directly influencing plant growth and stability.