The bodies of all animals and plants are made up of tiny microscopic units called cells. These make up the structure of the organism and are responsible for carrying out the various processes characteristic of the organism. Some organisms are composed of only one cell and are referred to as unicellular and include Amoeba, Paramecium and bacteria. Others are composed of many cells and are said to be multicellular and this mainly includes higher plants and animals. Cells were first described in 1665 by an English Biologist, Robert Hooke after he invented the simple microscope. Thereafter he used it to examine thin slices of cork, a substance that comes from the back of a tree. He noted that the material was composed of tiny boxes which he called cells. Hooke observed that other living material was also composed of cells. This work laid the foundation of the cell theory which postulates that all living things are made up of cells (Mazzarello, 1999).
Therefore to get a clear understanding of the physiology and morphology of living things it is important to study the cell. The cell is comprised of internal structures known as cell organelles which have different functions to sustain the proper functioning of the cell. However the distribution of these organelles clearly distinguishes plant and animal cells. Each organelle is enclosed by a cell membrane that is simply a phospholipid bilayer which contains peripheral and integral proteins. This cell membrane is semi permeable and selectively limits the passage of material and only allows the movement of nutrients, water and waste material via the processes of diffusion, osmosis and active transport. Knowledge of this movement has been exploited the by pharmaceutical company in the invention of medicine that can transverse the cell membrane via diffusion or by utilizing ion transporters via the process of active transport. In addition to the cell membrane, plants cells contain the cell wall which is mainly made of cellulose. This gives the plant cell turgidity and a definite shape unlike animal cells which have an amorphous shape since they only contain a cell membrane.
Other organelles include the mitochondria, centrioles, golgi apparatus, lysosomes, vacuoles, ribosomes, smooth and rough endoplasmic reticulum (NIGMS, 2005).The mitochondria are sausage shaped organelles that are responsible for cellular respiration to yield energy for the cell. Any malfunctioning of the mitochondria will ultimately lead to the death of the organism. Tissues and specialized cells that require a lot of energy are densely packed with mitochondria e.g. muscles and sperms. Centrioles are only found in animal cells and take part in cell division and formation of cilia and flagella. Cilia helps in the propulsion of ova at the fallopian tube and trapping of foreign material in the ears and respiratory tract. The golgi apparatus are membranous structures that package and secrete substances. They are numerous in the endocrine glands that secrete hormones. The lysosomes take par in autolysis and autophagy i.e. destruction of the cell and worn out organelles respectively. This is very essential for homeostasis and turn over of material in the body. Cavities of the smooth endoplasmic reticulum are used for lipid transport and break down of foreign material such as medicine. The rough endoplasmic reticulum anchors ribosomes and its cavities transport the newly formed proteins from the ribosomes. Chloroplasts are only found in plants and contain chlorophyll, a pigment used during photosynthesis.
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The nucleus is centrally placed in animal cells but occurs at the periphery in plant cells. It controls all the cellular activities and contains DNA that is responsible for heredity. Genetic engineers and Molecular Biologists have manipulated the nucleus in cloning using bacterial plasmids. The gene of choice is extracted from the cellular DNA and incorporated in bacterial plasmids and cloned to produce large quantities of the particular protein. This has been applied in the manufacture of synthetic hormones such as insulin. Genetic engineering has been utilized in the development of plants that give higher yields and are drought resistant (Massey, 2010). Therefore it is only by understanding the cell that great breakthrough can be made in the Pharmaceutical and Agricultural fields.