Specialised Cells in Human Biology Essay Example for Free

Specialised Cells in Human Biology Essay â€Å"The cells in your body are tiny-a row of about forty would fit into 1mm. But although they are too small to see with the naked eye their complexity is remarkable. A single cell can be thought of as an organised chemical system, separated from its disorganised surroundings by a membrane† (Boyle et al 1999) First here is a brief summary of major eukaryotic organelles and their structures. The nucleus is the largest and most prominent organelle in the cell. It is usually spherical and about 10um in diameter. It contains the DNA and chromosomes and carries information that allows the cell to divide and carry out its cellular processes. Almost all eukaryote cells have a nucleus. The nucleolus is located in the nucleus it is 1-2 um. Its function is to manufacture ribosomes. The ribosomes are found either free in the cytoplasm or attached to the rough endoplasmic reticulum R. E. R, They are 20nm in size and their function is to synthesise the protein in the cell. The cell surface membrane controls the entry and exit of substances in and out of the cell. It is partially permeable. It separates’ the cells contents from the external environment. It is a double layer of phospholipid molecules around 7-10nm thick. Cytoplasm is a jelly like substance that fills the cell it is around 80% water. This is where cell activities occur and most chemical reactions take place. Organelles are suspended in it. The R. E. R. occurs throughout the cytoplasm, it has an extensive membrane network. Its function is to isolate and transport newly synthesised proteins. The mitochondrion is numerous in cytoplasm and there are usually up to 1000per cell. Its function is aerobic respiration and synthesises most of the A. T. P. for the cell. The smooth endoplasmic reticulum S. E. R. is found in small patches in the cytoplasm, they vary in size and their function is to synthesise lipids and steroids. The muscle cells (see figure 1) that are found in the muscles that move the bones at joints in the skeleton. These cells allow for movement like walking or running. The skeletal muscle is made up of specialised cells also referred to as muscle fibres. These cells are long and thin, these elongated cells are tubular in appearance. The cells contain several nuclei for the cell to contract and relax the muscle; the cell needs many nuclei as the nucleus contains the DNA the genetic information that carries the instructions for making each protein i. e. the first step in making proteins takes place in the nuclei. The actual building of the proteins takes place in the sarcoplasm otherwise known as the cytoplasm. There is lots of mitochondria in the cell this because it is needed foe aerobic respiration to create the A. T. P. energy that is needed for the muscle to contract and relax. It is surrounded by the cell membrane or the sarcolemma, filled with sarcoplasm. The sarcoplasm contains many thread like fibres known as myofibrils. These run along the length if the muscle fibres parallel. The sarcoplasmic retilum that is around each myofibril consists of a network of tubes that contain calcium ions these play a major role in bringing about muscle movement. The neurone is the functional unit of the nervous system. They form connections with other neurones. They can carry electrical impulses in one direction; receiving and transmitting information. The thicker the insulation the faster the electrical impulses travel. They are found in the brain or the spinal cord. They are long and thin in fact they are the longest cell in the body. They vary in shape and size depending on their position and function. The neurones running down your leg can be over one metre long. Although all neurones have a similar basic structure, see figure 2 which shows the structure of a typical motor neurone. It transmits signals to muscle fibres. The cell body contains cytoplasm, a large nucleus which is found at the end of the cell and other organelles. Coming off the cell body are as many as 200 thread like dendrites that make the area of the cell body larger, these allow many connections to be made to other neurones and one long limb called an axon. The dendrites bring impulses into the cell body while the axon brings impulses away to connect with other neurones or with effectors such as glands or muscles. A significant feature of a neurone is that their cell bodies contain nissi granules; these work to be a maintenance system that monitors the cell. They are also the site of protein synthesis in the cell. The axoplasm or the cytoplasm extends throughout the cell into the dendrites, synoptic bulbs, cell body and axon. Materials reach different parts of the neurone by axoplasmic transport. Most human cells are microscopic. The largest cell in the human body is the egg cell or the ova. This is just visible with the naked eye. It needs to be bigger than other cells as it needs the space to store food reserves. It is large in diameter at 0. 2mm and has a large cytoplasm this is packed with food reserves which allow the cell to divide before it implants in to the nucleus. It has a large nucleus (see figure 3) this is because it contains all the genetic material of the cell including all of the DNA, 23 chromosomes and all of the information necessary to make a complete human being in just nine months. .The egg cell is surrounded by several layers of cells and the complete unit is called a follicle. The production of egg cells oogenesis takes place within the ovaries of the developing female fetus. At birth a girl already has two million. These fuse with the male gametes called spermatozoa or sperm in a process known as fertilisation. The resulting cell the zygote develops inside the uterus nourished by the placenta into a new individual. The sperm (see figure 4) has a large nucleus; this is because it contains digestive enzymes it also contains the 21 chromosomes that must be delivered to the egg in order for the complete 42 chromosomes to be in the zygote, it also contains chemicals to penetrate the egg and carries genetic information. The sperm has many mitochondria this is because it needs a lot of energy for its long swim to the egg. It has a long tail called a flagellum that is a modified cilium that it uses for swimming to the egg cell. It is one of 50-200 million sperm that attempt the journey to the egg cell but only one can make it and fertilise the egg cell. When the sperm reaches the egg cell; digestive enzymes that are found on the tip of the sperm head are released when the bag splits releasing the enzymes which digest a pathway through any remaining follicle cells and the zona pellucida. Fertilisation has begun.