Chapter 8: Cell: The Unit of Life | Summary

Cells that have a membrane-bound nucleus are called eukaryotic, while those lacking it are prokaryotic. In both types, a semi-fluid cytoplasm occupies the cell's volume and serves as the main arena of cellular activities. Eukaryotic cells possess membrane-bound organelles such as the endoplasmic reticulum, Golgi complex, lysosomes, mitochondria, and vacuoles, whereas prokaryotic cells lack these.

Ribosomes are non-membrane-bound organelles found in all cells. Cells differ greatly in size, shape, and function — from the 0.3 µm Mycoplasma to the large ostrich egg. Plant cells differ from animal cells in possessing a cell wall, plastids, and a large central vacuole, whereas animal cells have centrioles absent in most plant cells.

Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO. They are generally smaller and divide more rapidly than eukaryotic cells. The four basic bacterial shapes are bacillus (rod), coccus (spherical), vibrio (comma-shaped), and spirillum (spiral). All prokaryotes have a cell wall surrounding the cell membrane, except Mycoplasma. Their genetic material is a single, naked circular DNA not enclosed by a nuclear membrane; many bacteria also carry smaller circular DNA called plasmids that may confer antibiotic resistance.

The prokaryotic cell envelope consists of three layers: glycocalyx (outermost), cell wall, and plasma membrane. Glycocalyx may be a loose slime layer or a tough capsule. Mesosomes are infoldings of the plasma membrane that assist in cell wall formation, DNA replication, respiration, and secretion. Flagella provide motility; pili and fimbriae are surface structures that aid attachment. Prokaryotic ribosomes are 70S (50S + 30S subunits) and are the site of protein synthesis. Inclusion bodies store reserve materials in the cytoplasm.

The plasma membrane is mainly composed of phospholipids arranged in a bilayer, with polar heads facing outward and hydrophobic tails facing inward. It also contains cholesterol, proteins (integral and peripheral), and carbohydrates. Singer and Nicolson (1972) proposed the fluid mosaic model, in which lipids give the membrane quasi-fluid properties allowing lateral movement of proteins.

The membrane is selectively permeable. Passive transport (simple diffusion, osmosis) moves molecules along their concentration gradient without energy. Polar molecules require carrier proteins for facilitated diffusion. Active transport moves molecules against their concentration gradient using ATP, as in the Na+/K+ pump. The fluid nature of the membrane is essential for cell growth, endocytosis, secretion, and cell division.

The cell wall is a non-living, rigid structure outside the plasma membrane in fungi and plants. It provides shape, mechanical protection, and a barrier against undesirable macromolecules. In algae it contains cellulose, galactans, and mannans; in other plants it contains cellulose, hemicellulose, pectins, and proteins. The middle lamella (calcium pectate) glues neighbouring cells together, and plasmodesmata traverse the cell wall to connect adjacent cytoplasms.

The endomembrane system comprises the endoplasmic reticulum (ER), Golgi complex, lysosomes, and vacuoles. The rough ER (RER) bears ribosomes and is involved in protein synthesis and secretion; smooth ER (SER) synthesises lipids and steroidal hormones. The Golgi apparatus packages and modifies proteins into vesicles, and is the site of glycoprotein and glycolipid formation. Lysosomes contain hydrolytic enzymes active at acidic pH and digest macromolecules. Vacuoles store water, sap, and excretory products; in plants they can occupy up to 90% of cell volume.

Mitochondria are double membrane-bound organelles, typically sausage-shaped (0.2–1.0 µm diameter). The inner membrane folds into cristae that increase surface area for enzyme activity. The matrix contains circular DNA, RNA, 70S ribosomes, and enzymes for protein synthesis. Mitochondria are the sites of aerobic respiration and ATP production — hence called the powerhouses of the cell. They divide by fission.

Plastids are double membrane-bound organelles found in all plant cells and euglenoids. Chloroplasts contain chlorophyll and carotenoid pigments for photosynthesis; their stroma contains enzymes for carbohydrate synthesis, circular DNA, and 70S ribosomes. Thylakoids stacked into grana are the sites of light reactions. Chromoplasts contain carotenoid pigments giving yellow/orange/red colours. Leucoplasts are colourless storage plastids: amyloplasts store starch, elaioplasts store oils, and aleuroplasts store proteins.

Ribosomes are non-membrane-bound granular structures composed of RNA and proteins, first observed by George Palade (1953). Eukaryotic ribosomes are 80S (60S + 40S subunits); prokaryotic ribosomes are 70S (50S + 30S subunits). Multiple ribosomes attached to a single mRNA form a polysome. The cytoskeleton is a network of microtubules, microfilaments, and intermediate filaments providing mechanical support, motility, and shape maintenance.

Cilia and flagella are hair-like outgrowths of the cell membrane. Their axoneme has a 9+2 arrangement of microtubules (nine peripheral doublets and one central pair). They emerge from basal bodies derived from centrioles. The centrosome contains two perpendicular centrioles, each made of nine peripheral triplets of tubulin. Centrioles form the basal body of cilia/flagella and the spindle apparatus during cell division in animal cells.

The nucleus was first described by Robert Brown (1831). It is enclosed by a nuclear envelope consisting of two parallel membranes with a 10–50 nm perinuclear space. Nuclear pores formed by the fusion of the two membranes allow bidirectional movement of RNA and proteins between the nucleus and cytoplasm. The outer nuclear membrane is continuous with the ER and bears ribosomes.

The nucleoplasm contains chromatin (DNA + histones + non-histone proteins + RNA) and nucleoli. Nucleoli are the sites of ribosomal RNA synthesis and are not membrane-bound. During cell division, chromatin condenses into chromosomes. Each chromosome has a centromere flanked by kinetochores. Based on centromere position, chromosomes are classified as metacentric, sub-metacentric, acrocentric, or telocentric. Secondary constrictions on some chromosomes produce satellite fragments.

Chapter 8 establishes the cell as the irreducible structural and functional unit of all life. From the historical development of cell theory by Schleiden, Schwann, and Virchow to the detailed molecular architecture of organelles revealed by electron microscopy, the chapter builds a comprehensive picture of how both prokaryotic and eukaryotic cells are organised. Key themes include the division of labour among organelles, the endomembrane system as a coordinated secretory pathway, the semi-autonomous nature of mitochondria and chloroplasts, and the nucleus as the genetic and regulatory centre of the eukaryotic cell.

For CBSE board examinations, this chapter is a high-yield source of questions spanning all difficulty levels. Students must be thorough with the fluid mosaic model, differences between prokaryotic and eukaryotic cells, structure and function of all major organelles, the 9+2 microtubule arrangement in cilia/flagella, chromosome classification based on centromere position, and the components of the endomembrane system. Diagrams of the nucleus, mitochondrion, chloroplast, and centrosome are frequently asked, and comparisons between RER/SER, plant/animal cells, and 70S/80S ribosomes are standard board-exam staples.