Pre-reading_Cellulose___The_cell_wall_

Plant Cells Overview

Plant cells share several characteristics with animal cells but also possess unique organelles that enable them to perform specific functions related to photosynthesis and structural integrity.

Chloroplasts

• Function: Chloroplasts are specialized organelles found predominantly in the cells of leaves and other green parts of the plant. They contain chlorophyll, which absorbs sunlight and facilitates the conversion of light energy into chemical energy during photosynthesis, producing glucose and oxygen as byproducts.

• Structure: Chloroplasts have a double membrane, with an inner membrane system of thylakoids that are stacked to form grana, where light-dependent reactions take place.

Cell Wall

• Function: Every plant cell is enclosed within a rigid cell wall that provides structural support, protection from mechanical stress, and helps maintain the shape of the cell.

• Composition: The primary component of the cell wall is cellulose, a carbohydrate that provides durability and strength. The cell wall is porous, allowing for the exchange of water, nutrients, and signaling molecules.

Vacuoles

• Structure: Plant cells typically contain one large permanent vacuole that can occupy up to 90% of the cell's volume.

• Function: Vacuoles serve multiple roles including storage of water, ions, and nutrients; maintaining turgor pressure which helps keep the plant upright; and acting as a waste disposal system for harmful materials.

Cell Wall Structure

Incomplete Sealing

• Plasmodesmata: These microscopic channels connect the cytoplasm of adjacent plant cells, facilitating communication and transport of materials such as nutrients and hormones.

Cell Wall Composition and Structure

• Cellulose: Composed of strong polysaccharide chains of beta glucose molecules linked by beta 1–4 glycosidic bonds to form microfibrils. These microfibrils contribute to the rigidity and resistance to osmotic pressure.

• Arrangement: Cellulose molecules are organized in parallel rows due to the monomer inversion that occurs during synthesis, leading to strong hydrogen bonding between chains.

• Flexibility: The presence of a glue-like carbohydrate (hemicellulose) enables microfibrils to remain flexible, essential for plant growth and adaptation to external stresses.

Turgidity and Flaccidity

• Turgidity: The pressure from the vacuole against the cell wall maintains turgidity, which supports the plant structure.

• Flaccidity: When a plant cell loses water, it becomes flaccid, causing the whole plant to wilt, indicating the importance of water for maintaining cell structure.

Primary and Secondary Cell Walls

Primary Cell Wall

• Characteristics: Initially formed during cell division, composed of cellulose and pectins, it is flexible to allow cell growth.

Secondary Cell Wall

• Formation: Developed as the plant matures, involves the deposition of additional layers of cellulose microfibrils oriented at various angles, often reinforced with lignin, providing added strength and rigidity.

Middle Lamella

• Function: A layer rich in pectin that acts as a cementing substance, binding adjacent cells together and enabling mechanical stability.

Microfibrils and Pits

Microfibrils in Secondary Thickening

• Structure: Interconnected by pectin, these microfibrils create a robust structure, increasing overall integrity against external pressures.

Function of Pits

• Communication: Pits allow intercellular communication; cytoplasmic threads can transport signaling substances, creating continuity known as the symplast.

• Transport: In xylem vessels, pits facilitate the lateral flow of water and dissolved minerals, essential for plant hydration and nutrient transport.

Lignin in Secondary Thickening

• Role: Lignin is a complex organic polymer that reinforces the secondary cell wall. It provides additional rigidity, making the walls water-resistant and significantly enhancing structural support crucial for the function of xylem, which transports water upward from roots to leaves.

• Exam Importance: Understanding the structure of lignin and its contribution to overall plant biology is often a focal point in examinations, highlighting its ecological and physiological significance.