The cell is the basic structural and functional unit of all living things. Cells contain specialised organelles, group into tissues, and build organs and organisms. Life is classified, from the binomial name up to the five great kingdoms, on the basis of cell type, body organisation and mode of nutrition.
- Robert Hooke first observed and named cells in 1665, looking at cork under a microscope. Anton van Leeuwenhoek first saw living cells.
- Cell theory (Schleiden and Schwann, extended by Virchow) states that all living organisms are made of cells, the cell is the basic unit of life, and all cells arise from pre-existing cells.
- Cells vary in size and shape (the longest are nerve cells; the smallest are bacteria; the largest single cell is the ostrich egg). Organisms may be unicellular (one cell, such as Amoeba and bacteria) or multicellular.
- Prokaryotic cells have no true membrane-bound nucleus and lack membrane-bound organelles; their genetic material lies free in the cytoplasm (bacteria and blue-green algae).
- Eukaryotic cells have a true nucleus enclosed by a membrane and possess membrane-bound organelles (plants, animals, fungi, protists).
- Cell membrane (plasma membrane): the selectively permeable outer boundary controlling what enters and leaves.
- Cell wall (in plants, fungi and bacteria): a rigid outer layer; in plants it is made of cellulose and gives shape and protection.
- Nucleus: the control centre, holding the genetic material (DNA in chromosomes); the nucleolus inside makes ribosomes.
- Mitochondria: the "powerhouse of the cell," the site of respiration that releases energy as ATP.
- Chloroplast (in green plant cells): contains chlorophyll and carries out photosynthesis.
- Ribosomes: the sites of protein synthesis.
- Endoplasmic reticulum: a transport and synthesis network (rough ER with ribosomes makes proteins; smooth ER makes lipids).
- Golgi apparatus: packages and dispatches materials, like a post office.
- Lysosomes: contain digestive enzymes; the "suicide bags" that clear worn-out parts.
- Vacuoles: storage sacs; plant cells have one large central vacuole that keeps the cell turgid.
- A plant cell additionally has a cellulose cell wall, chloroplasts, and a large central vacuole. An animal cell lacks these but typically has centrioles (involved in cell division). Both have a nucleus, mitochondria and the other common organelles.
- A tissue is a group of similar cells performing a common function.
- Plant tissues: meristematic (dividing, growth) tissue at root and shoot tips, and permanent tissues (simple tissues such as parenchyma, collenchyma and sclerenchyma; complex conducting tissues xylem and phloem).
- Animal tissues, four basic types: epithelial (covering and lining), connective (binding and support, including blood and bone), muscular (movement) and nervous (control and conduction of impulses).
- Cells form tissues, tissues form organs, organs form organ systems, and organ systems form the organism, in rising order of complexity.
- Binomial nomenclature (Carl Linnaeus): every species has a two-part Latin name (genus and species), for example Homo sapiens. This gives a universal, unambiguous name.
- The five-kingdom classification (R.H. Whittaker) groups all life into:
- Monera: prokaryotes (bacteria, blue-green algae).
- Protista: single-celled eukaryotes (Amoeba, Paramecium, algae).
- Fungi: heterotrophic, cell-walled decomposers (mushrooms, yeasts, moulds).
- Plantae: multicellular autotrophs (plants).
- Animalia: multicellular heterotrophs (animals).
- The hierarchy of classification runs kingdom, phylum (or division in plants), class, order, family, genus, species.
- Cell theory: all organisms are cellular; cells arise from cells (Schleiden, Schwann, Virchow); Hooke first saw cells in 1665.
- Prokaryote versus eukaryote: no true nucleus versus a membrane-bound nucleus.
- Mitochondria (powerhouse), chloroplast (photosynthesis), ribosome (protein synthesis), lysosome (suicide bag).
- Four animal tissues: epithelial, connective, muscular, nervous.
- Five kingdoms (Whittaker): Monera, Protista, Fungi, Plantae, Animalia; binomial nomenclature (Linnaeus).
Cell biology is the foundation of biotechnology and of the pathogens that cause disease, both relevant to public health in deployment and to the threat of biological agents (a CBRN, chemical-biological-radiological-nuclear, security concern). Understanding microbes (Monera) underlies field hygiene, water safety and the control of communicable disease in camps.
- The organelle known as the "powerhouse of the cell" because it is the site of respiration is the: (a) ribosome (b) mitochondrion (c) lysosome (d) Golgi apparatus. (Answer: b.) Authored practice, not a verbatim PYQ.
- Which scientist proposed the five-kingdom classification of living organisms? (Answer: R.H. Whittaker.) Authored practice, not a verbatim PYQ.