Line 1: As you may recall, a non-living rigid structure called the cell wall forms an outer covering for the plasma membrane of fungi and plants.
Explanation: This line reminds you that plant and fungal cells have a cell wall external to the plasma membrane (cell membrane). This cell wall is a rigid, non-living structure.
Enrichment: Animal cells lack a cell wall and rely solely on the plasma membrane for protection and structure.
Line 2: Cell wall not only gives shape to the cell and protects the cell from mechanical damage and infection, it also helps in cell-to-cell interaction and provides a barrier to undesirable macromolecules.
Explanation: This line elaborates on the multiple functions of the cell wall:
Shape and Support: The cell wall provides a defined shape for the cell and helps it maintain internal pressure.
Protection: The rigid structure protects the cell from physical damage like squeezing or bursting. It also acts as a barrier against pathogens (disease-causing organisms) and harmful molecules.
Cell-to-Cell Interaction: The cell wall facilitates communication and adhesion between neighboring plant cells.
Selective Permeability: The cell wall acts as a filter, allowing essential molecules to enter while restricting the entry of unwanted large molecules.
Line 3: Algae have cell wall, made of cellulose, galactans, mannans and minerals like calcium carbonate, while in other plants it consists of cellulose, hemicellulose, pectins and proteins.
Explanation: This line highlights the variation in cell wall composition.
Algae: Their cell walls can contain cellulose (a complex sugar), galactans and mannans (sugars), and minerals like calcium carbonate (chalk).
Other Plants: The primary component is still cellulose, but the cell wall is reinforced with hemicellulose (another sugar), pectins (complex carbohydrates), and proteins.
Line 4-5: The cell wall of a young plant cell, the primary wall is capable of growth, which gradually diminishes as the cell matures and the secondary wall is formed on the inner (towards membrane) side of the cell.
Explanation: This section describes the two main types of plant cell walls:
Primary Wall: This is the first wall laid down in a young cell. It is flexible and allows for cell growth and expansion.
Secondary Wall: As the cell matures, a thicker and more rigid secondary wall may be deposited on the inner side of the primary wall. This strengthens and supports the cell.
Line 6: The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together.
Explanation: This line introduces the middle lamella, a thin layer rich in calcium pectate (a sticky carbohydrate) that lies between the primary walls of adjacent plant cells.
Enrichment: The middle lamella acts like a glue, cementing neighboring cells together and forming a continuous tissue.
Line 7: The cell wall and middle lamellae may be traversed by plasmodesmata which connect the cytoplasm of neighbouring cells.
Explanation: This line describes the presence of plasmodesmata, which are tiny channels that pierce through the cell wall and middle lamella, connecting the cytoplasm of neighboring plant cells.
Enrichment: Plasmodesmata allow for communication and exchange of materials between plant cells, even though they have separate cell walls.
Line 8: While each of the membranous organelles is distinct in terms of its structure and function, many of these are considered together as an endomembrane system because their functions are coordinated.
Explanation: This line introduces the concept of the endomembrane system, a network of interconnected membranous organelles within the cell.
Enrichment: Although each organelle within this system has a specific function, they work together in a coordinated manner to manage various cellular processes.
Line 9: The endomembrane system includes endoplasmic reticulum (ER), golgi complex, lysosomes and vacuoles.
Explanation: This line lists the major components of the endomembrane system:
Endoplasmic Reticulum (ER): A network of interconnected membranes involved in protein synthesis, lipid synthesis, and modification of molecules.
Golgi Complex: A processing and packaging station for molecules synthesized by the ER.
Lysosomes: Sac-like organelles containing digestive enzymes that break down waste materials and foreign invaders within the cell.
Vacuoles: Storage compartments for various materials, including water, nutrients, and waste products.
Line 10: Since the functions of the mitochondria, chloroplasts and peroxisomes are not coordinated with the above components, these these are not considered as part of the endomembrane system.
Explanation: This line clarifies that mitochondria, chloroplasts, and peroxisomes are not considered part of the endomembrane system because their functions are not directly interrelated with the processes carried out by the ER, Golgi complex, lysosomes, and vacuoles.
Line 11: 8.5.3.1 The Endoplasmic Reticulum (ER)
Explanation: This line marks the beginning of a section specifically focused on the Endoplasmic Reticulum (ER).
Line 12: Electron microscopic studies of eukaryotic cells reveal the presence of a network or reticulum of tiny tubular structures scattered in the cytoplasm that is called the endoplasmic reticulum (ER) (Figure 8.5).
Explanation: This line introduces the ER and its structure. Electron microscopes provide high-resolution images, revealing the ER as a network of interconnected tubules within the cytoplasm.
Enrichment: You might have come across the term “reticulum” before, referring to a net-like structure. The ER’s interconnected tubules resemble a net spread throughout the cytoplasm.
Line 13: Hence, ER divides the intracellular space into two distinct compartments, i.e., luminal (inside ER) and extra luminal (cytoplasm) compartments.
Explanation: This line highlights how the ER creates two separate compartments within the cell:
Luminal compartment: The space enclosed within the ER network.
Extra-luminal compartment: The remaining cytoplasm outside the ER.
Line 14: The ER often shows ribosomes attached to their outer surface. The endoplasmic reticulum bearing ribosomes on their surface is called rough endoplasmic reticulum (RER).
Explanation: This line describes the rough endoplasmic reticulum (RER). Ribosomes are cellular machines for protein synthesis. When ribosomes are attached to the outer surface of the ER, it’s called rough ER.
Enrichment: The ribosomes give the rough ER a rough appearance under an electron microscope, hence the name.
Line 15: In the absence of ribosomes they appear smooth and are called smooth endoplasmic reticulum (SER).
Explanation: This line introduces the smooth endoplasmic reticulum (SER). When ribosomes are not present on the outer surface, the ER appears smooth and is called smooth ER.
Line 16: RER is frequently observed in the cells actively involved in protein synthesis and secretion.
Explanation: This line emphasizes the association of rough ER with protein synthesis and secretion. Cells that produce large amounts of proteins often have abundant rough ER.
Enrichment: The ribosomes on the rough ER are responsible for synthesizing proteins, which can then be modified, packaged, and exported from the cell.
Line 17: They are extensive and continuous with the outer membrane of the nucleus.
Explanation: This line describes the extensive network of rough ER and its continuity with the outer nuclear membrane.
Enrichment: This continuity allows for efficient transfer of RNA (ribonucleic acid), essential for protein synthesis, from the nucleus to the ribosomes on the rough ER.
Line 18: The smooth endoplasmic reticulum is the major site for synthesis of lipid. In animal cells lipid-like steroidal hormones are synthesised in SER.
Explanation: This line highlights the function of smooth ER in lipid (fat) synthesis. Additionally, in animal cells, the smooth ER is the site for the production of steroid hormones.
Enrichment: Lipids play various roles in cells, including energy storage and cell membrane structure. Steroid hormones are signaling molecules involved in regulating various physiological processes.
