The Cell, Tissues and Classification of Living Things

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Discovery and cell theory

Robert Hooke (1665) saw box-like compartments in a thin slice of cork under his microscope and named them cells. Anton van Leeuwenhoek (1674), with his improved microscope, first saw free-living cells in pond water, and a few years later he also saw bacteria. Robert Brown discovered the nucleus in 1831, and Purkinje coined the term protoplasm in 1839. Matthias Schleiden (1838) and Theodor Schwann (1839) stated the cell theory, that all plants and animals are made of cells and the cell is the basic unit of life; Rudolf Virchow (1855) added that all cells arise from pre-existing cells. The ostrich egg is the largest single cell, the nerve cell the longest cell in the human body and Mycoplasma (PPLO) the smallest known cell.

Prokaryotes, eukaryotes and the cell membrane

Prokaryotic cells (bacteria and blue-green algae) have no nuclear membrane, their DNA lying in a region called the nucleoid, and no membrane-bound organelles. Eukaryotic cells (protists, fungi, plants and animals) have a true nucleus and membrane-bound organelles. The plasma membrane, made of lipids and proteins, is selectively permeable. Water crosses it by osmosis from a dilute to a concentrated solution, which is why raisins swell in water and why salt and sugar preserve pickles and jams; a plant cell in strong salt solution loses water and its contents shrink away from the wall (plasmolysis).

Cell organelles

The nucleus is the control centre and holds chromosomes made of DNA and protein. Mitochondria are the powerhouses where aerobic respiration releases energy as ATP; like chloroplasts, they have their own DNA and ribosomes. Plastids occur only in plant cells: chloroplasts carry chlorophyll for photosynthesis, chromoplasts colour flowers and fruits, and leucoplasts store starch, oil and protein. Ribosomes, which have no membrane, make proteins; rough endoplasmic reticulum bears ribosomes, while smooth endoplasmic reticulum makes lipids and, in liver cells, detoxifies drugs and poisons. The Golgi apparatus, named after Camillo Golgi, packages and dispatches materials and forms lysosomes, the suicide bags whose enzymes can digest the cell itself. Vacuoles store sap and wastes, and centrioles help animal cells divide.

Plant and animal cells

A plant cell has a rigid cell wall of cellulose, plastids and a large central vacuole that keeps it turgid, and usually no centrioles; an animal cell has no cell wall or plastids, has small vacuoles and has centrioles. Plants store food as starch and animals as glycogen. The cell wall of fungi is made of chitin. Mature red blood cells of mammals have no nucleus.

Cell division

Mitosis takes place in body cells for growth and repair: one division gives two daughter cells with the same chromosome number as the parent, through prophase, metaphase, anaphase and telophase. Meiosis takes place in the reproductive organs to form gametes: two divisions give four cells, each with half the chromosome number, and crossing over during meiosis creates variation. Human body cells have 46 chromosomes (23 pairs) and gametes 23, so fertilisation restores the number to 46. Uncontrolled cell division causes cancer.

Tissues

A tissue is a group of similar cells doing one job. In plants, meristems divide: apical meristem at root and shoot tips adds length, lateral meristem (cambium) adds girth, and intercalary meristem at the base of leaves lets grass regrow after grazing. Permanent plant tissues include parenchyma (storage), collenchyma (flexibility), sclerenchyma (hard, dead cells, as in coconut husk and jute), xylem (carries water and minerals) and phloem (carries food). Animal tissues are epithelial (covering and lining), connective (bone, cartilage, fat, blood, which is a fluid connective tissue, tendons joining muscle to bone and ligaments joining bone to bone), muscular (striated voluntary, smooth involuntary, and cardiac, which is striated yet involuntary) and nervous.

The five kingdoms

R. H. Whittaker (1969) replaced the old two-kingdom system of plants and animals with five kingdoms, based on cell type, body organisation and mode of nutrition. Monera are prokaryotes (bacteria, cyanobacteria, Mycoplasma); Protista are unicellular eukaryotes (Amoeba, Paramecium, Euglena, Plasmodium); Fungi are heterotrophs, mostly saprophytes, with chitin walls (yeast, mushroom, Penicillium, bread mould); Plantae are multicellular autotrophs, from algae (Spirogyra) and bryophytes (mosses, the amphibians of the plant kingdom) to pteridophytes (ferns), gymnosperms with naked seeds (pine, cycas, deodar) and angiosperms with seeds inside fruits; Animalia are multicellular heterotrophs without cell walls, from sponges to mammals. Lichens, a partnership of an alga and a fungus, are sensitive indicators of air pollution by sulphur dioxide.

Names, viruses and bacteria

Carolus Linnaeus, the father of taxonomy, popularised binomial nomenclature: a genus name with a capital letter followed by a species name in small letters, printed in italics, as Homo sapiens (human), Panthera tigris (tiger), Panthera leo (lion), Mangifera indica (mango) and Pavo cristatus (peacock). The ranks are kingdom, phylum, class, order, family, genus and species. Viruses fit none of the five kingdoms: they are DNA or RNA in a protein coat, can be crystallised, and reproduce only inside living cells; Dmitri Ivanovsky (1892) first detected one, the tobacco mosaic virus, and antibiotics do not act on viruses. Bacteria may be cocci (round), bacilli (rods), spirilla (spirals) or vibrio (comma-shaped); Lactobacillus sets milk into curd and Rhizobium fixes nitrogen in the root nodules of legumes.

Key facts

Hooke named the cell (1665, cork); Leeuwenhoek saw living cells (1674); Brown found the nucleus (1831); Schleiden (1838) and Schwann (1839) gave the cell theory; Virchow (1855): all cells come from cells.
Prokaryotes (bacteria, blue-green algae) have no nuclear membrane; eukaryotes have a true nucleus and membrane-bound organelles.
Mitochondria: powerhouse (ATP, own DNA); ribosome: protein synthesis; lysosome: suicide bag; Golgi: packaging; chloroplast: photosynthesis; smooth ER: detoxification.
Plant cell: cellulose wall, plastids, large vacuole, no centrioles; animal cell: no wall, small vacuoles, centrioles; fungal wall: chitin.
Mitosis: body cells, 2 identical cells, growth and repair; meiosis: gametes, 4 cells with half the chromosomes (46 becomes 23 in humans).
Tendon joins muscle to bone, ligament joins bone to bone; blood is a fluid connective tissue; cardiac muscle is striated but involuntary.
Whittaker (1969): Monera, Protista, Fungi, Plantae, Animalia; bryophytes are the amphibians of the plant kingdom; gymnosperms have naked seeds.
Binomial names: Homo sapiens (human), Panthera tigris (tiger), Panthera leo (lion), Mangifera indica (mango), Pavo cristatus (peacock), Oryza sativa (rice).
Viruses: nucleic acid in a protein coat, multiply only inside living cells, belong to none of the five kingdoms; tobacco mosaic virus found by Ivanovsky (1892); antibiotics do not act on viruses.
Bacterial shapes: cocci, bacilli, spirilla, vibrio; Lactobacillus sets curd; Rhizobium fixes nitrogen in legume root nodules.

Quick revision

Hooke 1665 named cell; Leeuwenhoek 1674; Brown nucleus 1831; Schleiden and Schwann cell theory; Virchow 1855; prokaryotes lack nuclear membrane; mitochondria powerhouse; lysosome suicide bag; ribosome makes protein; plant cell wall cellulose, fungal wall chitin; mitosis 2 identical cells, meiosis 4 cells with half chromosomes; 46 chromosomes; tendon muscle-bone, ligament bone-bone; Whittaker 1969 five kingdoms; bryophytes amphibians of plants; Homo sapiens; virus TMV Ivanovsky 1892.

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