Detailed Notes: 2 : Chapter 8 Cell: The Unit of Life

8.5 EUKARYOTIC CELLS

1. Classification of Eukaryotes:

   • Eukaryotes encompass a diverse range of organisms, including protists, plants, animals, and fungi.

2. Compartmentalization of Cytoplasm:

   • Eukaryotic cells exhibit extensive compartmentalization of cytoplasm due to the presence of membrane-bound organelles.

3. Nucleus:

   • Eukaryotic cells possess an organized nucleus surrounded by a nuclear envelope, which separates the genetic material from the cytoplasm.

4. Genetic Material:

   • The genetic material in eukaryotic cells is organized into chromosomes, consisting of DNA and associated proteins.

5. Structural Complexity:

   • Eukaryotic cells have a variety of complex locomotory and cytoskeletal structures, which are involved in cellular movement and support.

6. Differences Between Plant and Animal Cells:

   • Plant cells and animal cells exhibit differences in structure and composition.
   • Plant cells have cell walls, plastids (such as chloroplasts), and a large central vacuole, which are absent in animal cells.
   • Animal cells possess centrioles, which are typically absent in plant cells.

8.5.1 Cell Membrane

1. Composition of the Cell Membrane:

   • The cell membrane is mainly composed of lipids and proteins.
   • The major lipids are phospholipids arranged in a bilayer, with polar heads facing outward and hydrophobic tails facing inward.
   • Cholesterol is also present in the membrane, contributing to its stability.

2. Protein and Carbohydrate Components:

   • Cell membranes also contain proteins and carbohydrates.
   • The ratio of protein to lipid varies among different cell types.
   • Membrane proteins can be classified as integral (partially or totally buried in the membrane) or peripheral (lying on the surface of the membrane).

3. Fluid Mosaic Model:

   • Proposed by Singer and Nicolson in 1972, the fluid mosaic model describes the membrane as a quasi-fluid structure with proteins embedded within the lipid bilayer.
   • The fluidity of the membrane allows lateral movement of proteins, which is important for various cellular functions.

4. Functions of the Cell Membrane:

   • The cell membrane is involved in cell growth, formation of intercellular junctions, secretion, endocytosis, cell division, and other essential processes.
   • One of its crucial functions is the selective transport of molecules across it.

5. Selective Permeability:

   • The membrane is selectively permeable to molecules present on either side of it.
   • Passive transport allows molecules to move across the membrane without requiring energy, such as simple diffusion and osmosis.
   • For polar molecules, carrier proteins facilitate their transport across the nonpolar lipid bilayer.
   • Active transport, such as the Na+/K+ pump, requires energy (ATP) to transport ions or molecules against their concentration gradient.

8.5.2 Cell Wall

1. Composition of the Cell Wall:

   • The cell wall is a non-living rigid structure that forms an outer covering for the plasma membrane of fungi and plants.
   • It is composed of various substances depending on the organism. In algae, it is made of cellulose, galactans, mannans, and minerals like calcium carbonate. In plants, it consists of cellulose, hemicellulose, pectins, and proteins.

2. Functions of the Cell Wall:

   • Provides structural support and shape to the cell.
   • Protects the cell from mechanical damage and infection.
   • Facilitates cell-to-cell interactions.
   • Acts as a barrier to undesirable macromolecules.

3. Primary and Secondary Cell Wall:

   • In young plant cells, the primary wall is capable of growth. As the cell matures, the primary wall diminishes, and a secondary wall is formed on the inner side of the cell, towards the plasma membrane.

4. Middle Lamella:

   • The middle lamella is a layer mainly composed of calcium pectate.
   • It acts as a cementing material, holding or gluing neighboring cells together.

5. Plasmodesmata:

   • Plasmodesmata are channels that traverse the cell wall and middle lamellae.
   • They connect the cytoplasm of neighboring cells, allowing for communication and transport of substances between adjacent cells.

8.5.3 Endomembrane System

1. Definition and Components:

   • The endomembrane system refers to a network of membranous organelles within eukaryotic cells.
   • It includes the following components:
     • Endoplasmic reticulum (ER)
     • Golgi complex (Golgi apparatus)
     • Lysosomes
     • Vacuoles

2. Coordination of Functions:

   • Although each membranous organelle within the endomembrane system has distinct structures and functions, they work together to coordinate various cellular processes.
   • These processes may include protein synthesis, modification, packaging, transport, and degradation.

3. Exclusion of Other Organelles:

   • Mitochondria, chloroplasts, and peroxisomes are not considered part of the endomembrane system.
   • This is because their functions are not coordinated with the components of the endomembrane system.
   • Mitochondria are involved in energy production (cellular respiration), chloroplasts are involved in photosynthesis (found only in plant cells), and peroxisomes are involved in various metabolic processes, including the breakdown of fatty acids and detoxification of harmful substances.

 8.5.3.1 The Endoplasmic Reticulum (ER)

1. Structure of ER:

   • Electron microscopic studies reveal a network of tiny tubular structures scattered in the cytoplasm known as the endoplasmic reticulum (ER).
   • The ER divides the intracellular space into two distinct compartments: the luminal compartment (inside the ER) and the extra-luminal compartment (cytoplasm).

2. Types of ER:

   • ER can be classified into two main types based on its appearance and function:
     • Rough Endoplasmic Reticulum (RER): It has ribosomes attached to its outer surface, giving it a rough appearance. RER is involved in protein synthesis and secretion.
     • Smooth Endoplasmic Reticulum (SER): It appears smooth since it lacks ribosomes. SER is involved in lipid synthesis and various other metabolic processes.

3. Functions of RER:

   • RER is frequently observed in cells actively involved in protein synthesis and secretion.
   • It is extensive and continuous with the outer membrane of the nucleus.
   • RER plays a crucial role in the synthesis of proteins, particularly those destined for secretion or insertion into the cell membrane.

4. Functions of SER:

   • SER is the major site for the synthesis of lipids.
   • In animal cells, SER is involved in the synthesis of lipid-like steroidal hormones.

8.5.3.2 Golgi apparatus

1. Discovery and Structure:

   • Camillo Golgi first observed densely stained reticular structures near the nucleus in 1898, which were later named Golgi bodies after him.
   • The Golgi apparatus consists of many flat, disc-shaped sacs or cisternae with diameters ranging from 0.5µm to 1.0µm.
   • These cisternae are stacked parallel to each other and are concentrically arranged near the nucleus.
   • The Golgi complex has distinct convex cis or forming face and concave trans or maturing face.

2. Function:

   • The Golgi apparatus primarily functions in packaging materials to be delivered either to intracellular targets or secreted outside the cell.
   • Materials to be packaged, in the form of vesicles from the endoplasmic reticulum (ER), fuse with the cis face of the Golgi apparatus and move towards the maturing face.
   • This close association with the ER explains why the Golgi apparatus is often found near the endoplasmic reticulum.
   • Proteins synthesized by ribosomes on the endoplasmic reticulum are modified in the Golgi cisternae before being released from its trans face.
   • The Golgi apparatus is an important site for the formation of glycoproteins and glycolipids.

8.5.3.3 Lysosomes

1. Formation and Structure:

   • Lysosomes are membrane-bound vesicular structures formed by the process of packaging in the Golgi apparatus.
   • They are typically spherical organelles enclosed by a single membrane.

2. Enzyme Content:

   • Lysosomes contain a wide range of hydrolytic enzymes, collectively known as hydrolases.
   • These enzymes include lipases (for lipid digestion), proteases (for protein digestion), carbohydrases (for carbohydrate digestion), and nucleases (for nucleic acid digestion).

3. Optimal pH:

   • Lysosomal enzymes are optimally active at an acidic pH.
   • The acidic environment within lysosomes is maintained by proton pumps in the lysosomal membrane that actively transport protons (H⁺ ions) into the lysosome.

4. Function:

   • The primary function of lysosomes is intracellular digestion or autophagy.
   • They break down various biomolecules, including carbohydrates, proteins, lipids, and nucleic acids, into their constituent components.
   • Lysosomes play a crucial role in cellular waste disposal, recycling of cellular components, and regulation of cellular homeostasis.
   • They are also involved in the degradation of foreign substances engulfed by the cell through processes such as endocytosis and phagocytosis.

8.5.3.4 Vacuoles 

• Structure and Composition:

  • Vacuoles are membrane-bound organelles found in the cytoplasm of cells.
  • They contain water, sap, excretory products, and other materials that are not immediately useful for the cell.
  • The membrane surrounding the vacuole is called the tonoplast.

• Size and Volume:

  • In plant cells, vacuoles can occupy a significant portion of the cell volume, sometimes up to 90%.
  • Their large size and volume contribute to the rigidity and turgor pressure of plant cells.

• Functions:

  • Plant Vacuoles:
    • In plant cells, vacuoles play essential roles in maintaining cell structure, storage, and turgor pressure.
    • They store water, ions, sugars, pigments, and other substances.
    • The tonoplast facilitates the transport of ions and other materials into the vacuole, maintaining their concentration higher than in the cytoplasm.
  • Contractile Vacuoles in Amoeba:
    • In single-celled organisms like Amoeba, contractile vacuoles are important for osmoregulation and excretion.
    • They help regulate the water content of the cell by expelling excess water to maintain proper osmotic balance.
  • Food Vacuoles:
    • In many cells, particularly in protists, food vacuoles are formed by engulfing food particles through processes like phagocytosis.
    • These food vacuoles then fuse with lysosomes for digestion and nutrient absorption.