Cell Organelles & Functions

A goat grazing in Ilorin, a mango tree in your compound and the yeast that makes bread dough rise all have one thing in common: each is built from cells. The cell is the smallest unit that can carry out every activity of life on its own. Inside it, tiny structures called organelles divide the work the way departments divide the work of a hospital. In this lesson you will list the characteristics that separate living things from non-living things, name the main organelles of a cell and state the precise function of each, and arrange the levels of organisation from a single cell up to a whole organism. These are the building blocks of every later topic in UTME Biology.

Cell membrane (plasma membrane): a thin, flexible, partially permeable layer around the cytoplasm. It controls which substances enter and leave the cell. Nucleus: bounded by a nuclear membrane and containing chromosomes made of DNA. It controls all the activities of the cell and carries hereditary information. The nucleolus inside it makes ribosomes. Cytoplasm: the jelly-like fluid in which the organelles sit and in which many chemical reactions take place. Mitochondria: rod-shaped organelles with a folded inner membrane. They are the site of aerobic respiration, where glucose is oxidised to release energy as ATP, so they are often called the powerhouse of the cell. Ribosomes: tiny granules, free or on the rough endoplasmic reticulum, where proteins are made. Endoplasmic reticulum: a network of membranes that transports materials through the cell; the rough type carries ribosomes and the smooth type makes lipids. Golgi apparatus (Golgi body): packages proteins and other products into vesicles for secretion; it is large in gland cells. Lysosomes: small sacs of digestive enzymes that break down worn-out organelles and engulfed food or bacteria. Centrioles (animal cells): help to form the spindle during cell division. Plant cells also have chloroplasts, a cellulose cell wall and a large permanent vacuole; these are compared in the next lesson.

Some organisms, such as Amoeba, Paramecium and Chlamydomonas, are unicellular: one cell carries out every life process. Others are colonial, like Volvox, where many similar cells live together with little division of labour. Most plants and animals are multicellular, with cells specialised for particular jobs. Specialisation brings division of labour: a nerve cell is long to carry impulses, a red blood cell has no nucleus to make room for haemoglobin, and a root hair cell has a long extension to absorb water. Similar cells working together form a tissue, tissues form organs and organs form systems. The price of specialisation is dependence: a muscle cell cannot survive on its own outside the body.