CELL: THE UNIT OF LIFE

The cell is the fundamental structural and functional unit of all living organisms. A complete cell alone can exist independently and perform the essential functions of life. Organisms may be unicellular or multicellular; in multicellular organisms, cells show division of labour.

Antonie von Leeuwenhoek first saw and described a live cell, while Robert Brown discovered the nucleus. Cell size and shape vary with function: mycoplasma is among the smallest cells, the ostrich egg is the largest isolated single cell, and nerve cells are among the longest cells.

In 1838, Matthias Schleiden concluded that plants are made of cells; Theodore Schwann (1839) showed that animals are composed of cells and products of cells. Rudolf Virchow (1855) added that new cells arise by division of pre-existing cells (Omnis cellula-e cellula).

All cells have cytoplasm, a semi-fluid matrix that is the arena of cellular activities. Eukaryotic cells have a membrane-bound nucleus and membrane-bound organelles. Prokaryotic cells lack a membrane-bound nucleus and these organelles. Ribosomes are non-membrane-bound and occur in both cell types.

Prokaryotes include bacteria, blue-green algae/cyanobacteria, mycoplasma and PPLO. They are generally smaller and multiply more rapidly than eukaryotic cells. Bacterial forms include bacillus (rod), coccus (spherical), vibrio (comma-shaped) and spirillum (spiral).

They possess cytoplasm and usually a cell wall surrounding the cell membrane, except mycoplasma. Their genetic material is naked and not enveloped by a nuclear membrane. Besides genomic circular DNA, bacteria may have plasmids, which can confer characters such as antibiotic resistance. Ribosomes are the only organelles present.

The bacterial cell envelope is a three-layered protective unit: glycocalyx → cell wall → plasma membrane. Glycocalyx can be a loose slime layer or a thick, tough capsule. The cell wall gives shape and prevents bursting or collapse. Gram-positive bacteria take up Gram stain, whereas Gram-negative bacteria do not.

Prokaryotic ribosomes are 70S, made of 50S and 30S subunits. They are sites of protein synthesis. Several ribosomes can translate a single mRNA together as a polyribosome/polysome.

Eukaryotes include protists, fungi, plants and animals. Their cytoplasm is extensively compartmentalised by membrane-bound organelles. They have an organised nucleus with a nuclear envelope, chromosomes, cytoskeletal structures and complex locomotory structures.

The plasma membrane is mainly made of lipids and proteins. Phospholipids form a bilayer with polar heads facing the aqueous sides and hydrophobic tails inward. Cholesterol, proteins and carbohydrates also occur. Peripheral proteins lie on the surface, whereas integral proteins are partially or completely embedded in the membrane.

Singer and Nicolson's fluid mosaic model (1972) describes the quasi-fluid lipid bilayer that permits lateral movement of proteins. Membrane fluidity supports growth, intercellular junctions, secretion, endocytosis and cell division.

Plant and fungal cell walls are non-living, rigid coverings outside the plasma membrane. They provide shape and protection, aid cell-to-cell interaction and restrict undesirable macromolecules. In plants, the primary wall can grow; the secondary wall is deposited on its inner side as the cell matures. The middle lamella is mainly calcium pectate and cements neighbouring cells. Plasmodesmata connect the cytoplasm of adjacent cells.

The endomembrane system comprises ER, Golgi complex, lysosomes and vacuoles because their functions are coordinated. Mitochondria, chloroplasts and peroxisomes are not part of this system.

ER is a network of tubules in the cytoplasm and creates luminal and extra-luminal compartments. RER bears ribosomes and is prominent in protein-synthesising and secreting cells; it is continuous with the outer nuclear membrane. SER lacks ribosomes and is a major site of lipid synthesis, including steroidal hormones in animal cells.

The Golgi apparatus has stacks of flattened cisternae. Its convex cis/forming face receives vesicles from ER; its concave trans/maturing face releases processed material. It packages, modifies and sorts material, and forms glycoproteins and glycolipids.

Lysosomes are Golgi-packaged, membrane-bound vesicles with hydrolytic enzymes active at acidic pH. Their hydrolases digest carbohydrates, proteins, lipids and nucleic acids.

Vacuoles are membrane-bound spaces containing water, sap, excretory products and other materials. Their membrane is the tonoplast. Plant vacuoles may occupy up to 90% of cell volume. In Amoeba, contractile vacuoles support osmoregulation and excretion; food vacuoles form by engulfment.

Mitochondria are double-membrane organelles of aerobic respiration and ATP production. The outer membrane is smooth; the inner membrane forms cristae that increase surface area. The matrix contains circular DNA, RNA, 70S ribosomes and protein-synthesis components. Mitochondria divide by fission.

Ribosomes are non-membrane-bound granules of RNA and proteins, and are sites of protein synthesis. Eukaryotic cytoplasmic ribosomes are 80S (60S + 40S); prokaryotic ribosomes are 70S (50S + 30S). Mitochondria and chloroplasts contain 70S ribosomes. S is the Svedberg sedimentation coefficient.

The cytoskeleton consists of microtubules, microfilaments and intermediate filaments. It supports mechanical strength, motility and maintenance of cell shape.

Cilia are short and work like oars to move the cell or surrounding fluid; flagella are longer and move cells. Their axoneme usually follows the 9+2 arrangement: nine peripheral microtubule doublets and two central microtubules. Both arise from basal bodies. Bacterial flagella are structurally different.

A centrosome usually contains two perpendicular centrioles surrounded by pericentriolar material. Each centriole has nine peripheral tubulin triplets arranged like a cartwheel. Centrioles form basal bodies of cilia/flagella and spindle fibres in animal cell division.

The interphase nucleus contains chromatin, nucleoplasm/nuclear matrix and one or more nucleoli. A double nuclear envelope, separated by perinuclear space, has pores for bidirectional movement of RNA and proteins. The nucleolus is not membrane-bound and is the site of active rRNA synthesis.

Chromatin contains DNA, histones, non-histone proteins and RNA. During cell division it condenses into chromosomes. A chromosome has a primary constriction, the centromere, with kinetochores on its sides; the centromere holds two chromatids.

Microbodies are minute membrane-bound vesicles containing various enzymes, and occur in both plant and animal cells.

1. State the modern cell theory and name the scientists associated with it.
2. Differentiate prokaryotic and eukaryotic cells.
3. Describe bacterial cell envelope, mesosome, ribosomes and inclusion bodies.
4. Explain the fluid mosaic model and compare passive with active transport.
5. Why are ER, Golgi complex, lysosomes and vacuoles grouped as an endomembrane system?
6. Compare RER and SER; state the functions of Golgi complex and lysosomes.
7. Describe mitochondrial and chloroplast structure and their semiautonomous features.
8. Compare 70S and 80S ribosomes; explain 9+2 arrangement.
9. Describe nuclear envelope, nucleolus, chromatin and centromere-based chromosome types.