Chapter 8: Cell: The Unit of Life (Cell: Structure and Function)

(a) is not correct: Robert Brown discovered the nucleus (the live cell was first seen by Leeuwenhoek).

(c) pre-existing cells (Omnis cellula-e cellula, Virchow).

(a)–(ii), (b)–(iii), (c)–(i).

(c).

A mesosome is a special membranous structure formed by infoldings (extensions) of the plasma membrane into the cell as vesicles, tubules and lamellae. Functions: (i) helps in cell wall formation; (ii) DNA replication and distribution to daughter cells; (iii) respiration; (iv) secretion; (v) increases the surface area of the membrane and its enzyme content.

Neutral solutes move by simple diffusion along the concentration gradient (higher to lower), without energy (passive transport); water moves by osmosis. Polar molecules cannot pass through the non-polar lipid bilayer in this way; they need carrier proteins of the membrane to facilitate their transport; and some ions or molecules are moved against the gradient by active transport using ATP (e.g. the Na⁺/K⁺ pump).

Mitochondria and chloroplasts (diagrams in the concept notes). Mitochondrion: sausage-shaped, 0.2–1.0 µm wide, 1.0–4.1 µm long; outer membrane smooth; inner membrane folded into cristae; inner compartment filled with matrix containing circular DNA, RNA and 70S ribosomes; divides by fission. Function: aerobic respiration, ATP production (power house). Chloroplast: lens-shaped, 5–10 µm × 2–4 µm; outer and less permeable inner membrane; stroma with thylakoids stacked as grana joined by stroma lamellae; chlorophyll in thylakoids; stroma has enzymes, circular DNA and 70S ribosomes. Function: photosynthesis (trapping light, making carbohydrates).

(i) Generally smaller, multiply rapidly; bacteria, blue-green algae, mycoplasma, PPLO. (ii) No well-defined nucleus: naked circular DNA (nucleoid); often plasmids. (iii) Cell wall present (except mycoplasma), with a three-layered envelope (glycocalyx, wall, membrane); Gram-positive or Gram-negative. (iv) No membrane-bound organelles except ribosomes (70S, on the membrane, forming polysomes). (v) Mesosomes; chromatophores in cyanobacteria. (vi) Flagella (filament, hook, basal body), pili and fimbriae. (vii) Inclusion bodies (phosphate, cyanophycean, glycogen granules) and gas vacuoles.

In multicellular organisms the basic functions are not done by every cell but by different groups of specialised cells. Similar cells with intercellular substance form tissues; tissues form organs (stomach, heart) and organs form organ systems (digestive, respiratory), each specialised for a particular function. Cells also differ in shape and size suited to their work (long nerve cells conduct impulses; RBCs carry oxygen). This sharing of work is division of labour; it makes the activities of millions of cells efficient and coordinated for the survival of the whole body.

(i) All organisms are made of one or more cells and their products (cell theory). (ii) A unicellular organism can live independently and perform all life functions within one cell; anything less than a complete cell cannot live independently. (iii) All metabolic reactions occur in cells; the cytoplasm is the arena of these activities. (iv) New cells arise only from pre-existing cells, and cells carry the hereditary material (DNA) to the next generation. Hence the cell is the fundamental structural and functional unit of life.

Minute pores in the nuclear envelope, formed where its two membranes fuse. They are passages through which RNA and protein molecules move in both directions between the nucleus and the cytoplasm.

Both are membrane-bound parts of the endomembrane system. Lysosomes are vesicles packaged by the Golgi, rich in hydrolytic enzymes active at acidic pH; their job is digestion of carbohydrates, proteins, lipids and nucleic acids. Vacuoles are spaces bounded by the tonoplast containing water, sap, excretory and other materials; they store materials, keep turgor (up to 90% of a plant cell), actively take ions into the sap, and in Amoeba the contractile vacuole does osmoregulation and excretion while food vacuoles hold engulfed food.

(Diagrams in the concept notes.) (i) Nucleus: a double nuclear envelope with a 10–50 nm perinuclear space; outer membrane continuous with the ER, bearing ribosomes; nuclear pores; inside, nucleoplasm (nuclear matrix) with chromatin (DNA, histones, non-histone proteins, RNA) and one or more nucleoli (non-membranous, rRNA synthesis). (ii) Centrosome: two cylindrical centrioles at right angles in pericentriolar material; each centriole has nine evenly spaced peripheral triplet fibrils of tubulin (adjacent triplets linked) and a central proteinaceous hub joined to the triplets by radial spokes: a cartwheel. It forms basal bodies of cilia and flagella and the spindle in animal cells.

Types of chromosomes by the position of the centromere