By Cell Biology Kalyani ChivukulaCell the fundamental unit of life Capable of performing basic life processes, such as taking in nutrients, expelling waste, and reproducing that are characteristic of all living forms. Cell is a marvel of design and efficiency that carries out various biochemical reactions each minute and reproduce new cells that perpetuate life. A cell in a living organism is a complex living structure. It is the basic structural and functional unit of life.ORGANISMS Unicellular Multicellular (singlecelled) (manycelled) amoebaamoeba, Paramecium simple complex e.g. hydra e.g. plants, animalsAll living organisms are made up of cells. Organisms made up of a single cell are called unicellular (uni = one; cellular = cell) In unicellular organisms, a single cell constitutes a whole organism as in Amoeba, Chlamydomonas, Paramecium and bacteria. A singlecelled organism performs all the necessary functions that multicellular organisms perform. Organisms made of more than one cell are called multicellular (multi = many; cellular= cell) organisms. Plants, animals and fungi are multicellular; that is, they are composed of many cells working together. These cells group together in a single body of the multicellular organisms to perform different functions in it to form various body parts. Whether it is an entire bacterium or is just one of trillions in a human being, the cell is a marvel of design and efficiency. Similar functions in multicellular organisms are carried out by groups of specialised cells forming different tissues. Tissues, in turn, form organs.MULTICELLULAR ORGANISM ORGAN TISSUE CELLCell – e.g. blood cell, nerve cell, muscle cell etc. Tissue – e.g. epithelial tissue, connective tissue, nerve tissue, muscle tissue. Organ – e.g. heart, stomach, liver, lungs etc. Organism – Plant, animal.An organism with billions of cells begins its life as a single cell i.e., the fertilised egg. The fertilised egg cell multiplies in number and the number of cells increases as it develops. Thus cells divide to produce cells of their own kind. All cells thus come from preexisting cells.Vary in Size Cells vary considerably in size. The smallest cell, a bacterium known as a mycoplasma, measures 0.0001 mm (0.000004 in) in diameter. The largest cells are the nerve cells that run down a giraffe’s neck exceeding 3 m (9.7 ft) in length. Human cells also display a variety of sizes, from small red blood cells that measure 0.00076 mm (0.00003 in) to liver cells that may be ten times larger. Vary in Shape Cells occur in different shapes. Irregular Form – e.g. Amoeba (a protozoan) changes shape as it moves around. A white blood cell (WBC) in human blood is another example of a single cell which can change its shape. RodShaped – e.g. bacterium Escherichia coli. Paramecium, a protozoan, is slipper shaped. Plant cells typically resemble boxes or cubes. Shape of Human Cells The outermost layers of skin cells are flat. Muscle cells are long, thin and spindleshaped. Nerve cells (neurons) are elongated and branched with tentaclelike extensions. Red blood cells are spherical in shape.Basic components of a cell Cell Membrane CytoplasCell Cytoplasm (plasma membrane) NucleusA cell consists of molecules, nonliving structures (union of atoms). Proteins, nucleic acids, carbohydrates, and lipids, which include fats and oils, are the four major molecules that underlie cell structure These also participate in cell functions. For example, a tightly organized arrangement of lipids, proteins, and proteinsugar compounds forms the plasma membrane, or outer boundary, of certain cells. The organelles, membranebound compartments in cells, are built largely from proteins. Biochemical reactions in cells are guided by enzymes which are specialized proteins that speed up chemical reactions. The nucleic acid deoxyribonucleic acid (DNA) contains the hereditary information for cells, and another nucleic acid, ribonucleic acid(RNA), works with DNA to build the thousands of proteins the cell needs.Cell Membrane Also known as plasma membrane is selectively permeable allows only certain substances to pass through. The cell membrane gives shape to the cell. Plasma membrane is porous and allows the movement of substances or materials both inward and outward. It actively separates the cell’s contents from its surrounding fluids. The membrane also separates cells from one another. It separates the cell from the surrounding medium. Plasma membrane is composed of two layers of flexible lipid molecules. Plant cells are protected by an additional thick layer called the cell wall.Cytoplasm It is a jellylike semi fluid that fills the cell. It is composed of about 65 percent of water. The water provides a favourable environment for the thousands of biochemical reactions that take place in the cell. A billion molecules per cell are packed in cytoplasm which is a rich storehouse of enzymes and dissolved nutrients, such as sugars and amino acids. Various other components, or organelles, of cells are present in the cytoplasm. These are mitochondria, Golgi bodies, ribosome, etcNucleus It is an important component of the living cell, generally spherical in shape and located in the centre of the cell. It acts as the control centre of the activities of the cell. It contains numerous strands of DNA. The nucleus is surrounded by a doublelayered membrane called the nuclear membrane that protects the DNA from potentially damaging chemical reactions that occur in the cytoplasm. This membrane is also porous and allows the movement of materials between the cytoplasm and the inside of the nucleus. There is a smaller spherical body in the nucleus called the nucleolus. In addition, nucleus contains threadlike structures called chromosomes that carry genes. Gene is a unit of inheritance in living organisms. It controls the transfer of a hereditary characteristic from parents to offspring. The chromosomes can be seen only when the cell divides. The entire content of a living cell is known as protoplasm. It includes the cytoplasm and the nucleus. Protoplasm is called the living substance of the cell.Plant Cell Animal Cell Has a cell membrane Cell membrane protected by cell wall Coloured bodies called plastids are present Has a cell membrane Cell wall is absent Plastids are not present Green plastids having chlorophyll are called chloroplasts. Has a large central vacuole Chloroplast absent Has a number of small vacuolesCELL Prokaryotic (prenucleus) Eukaryotic(true nucleus) simple unicellular organism more complex organisms with such as a bacterium, cells that contain a nucleus, such as characterized by the absence plants and animals of a nucleus and other specialized cell structures