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Essay on Cell
Essay Contents:
- Essay on the Introduction to Cell
- Essay on the Characteristics of Cells
- Essay on the Basic Concepts of Cell
- Essay on the Types of Cell
1. Essay on the Introduction to Cell:
The cell is the basic unit of life. The life of all organisms including man starts from a single cell and the body is a consortium of individual cells of specific functions and features working in a coordinated manner. Each cell is an amazing world of several biochemical activities- it can assimilate nutrients, convert these nutrients into energy, carry out specialized functions, and reproduce as necessary.
Even more amazing is that each cell stores its own set of instructions for carrying out each of these activities. The largest known cell is an ostrich egg. There are millions of different types of cells and many organisms such as microscopic amoeba and bacteria are single celled.
Microscopes opened up new doors in the field of biology by allowing scientists to gaze into the cellular world. Credit for the first compound microscope is usually given to Zacharias Jansen, of Middleburg, Holland, around the year 1595. English scientist Robert Hooke, in 1665 first observed cells and coined the term “cell” in the slices of cork through a microscope. Actually, Hooke only observed cell walls because cork cells are dead and without cytoplasmic contents.
The term came from the Latin word cella which means “store room” or “small container”. At the same time, Anton van Leeuwenhoek (1632-1723) pioneered the invention of one of the best microscopes. Leeuwenhoek was the first to observe, draw, and describe a variety of living organisms, including bacteria gliding in saliva, one-celled organisms cavorting in pond water, and sperm swimming in semen.
Modern ideas about cells appeared in the 1800s, when improved light microscopes enabled scientists to observe more details of cells. The cell theory, or cell doctrine, states that all organisms are composed of similar units of organization, cells. The concept was formally articulated in 1839 by Schleiden and Schwann and has remained as the foundation of modern biology. It wasn’t until 1838 that Matthias Schleiden stated that all plant material was made up of cells.
The following year, Theodor Schwann came to the same conclusion about animals. Their findings are now known as the cell theory. German pathologist Rudolf Virchow (1821-1902) altered the thought of cellular biology with his powerful dictum, “Omnis cellulae e cellula”… “All cells only arise from pre-existing cells.” This was new information to most scientists and had not been understood before even for Schwann, who thought that new cells arose from particles in the fluid surrounding the cells. The idea predates other great paradigms of biology including Darwin’s theory of evolution (1859), Mendel’s laws of inheritance (1865), and the establishment of comparative biochemistry (1940).
In the following years, experiments conducted by Louis Pasteur provided the proof for Virchow’s proposal. Pasteur carried out experiments to determine how substances like milk and wine became curdled or fermented. Through carefully controlled environments, he proved that exposure to airborne particles caused the change. In other words, he proved that organisms did not arise spontaneously, but had to be provided by some means, in this case in the air.
Cell biologists have defined cells in many ways:
1. According to Robert Hooke (1635 – 1730) – “The cell is the structural and functional unit of the living matter and is capable of carrying on the processes of life independently.”
2. Loewy and Siekevitz defined cell “as a unit of biological activity delimited by a semipermeable membrane and capable, of self reproduction in a medium free of other living systems.”
3. “Cell is the unit of structure and function in plants and animals.”
4. “Cell is the Smallest unit of living organism capable of independent existence.”
5. “Cell is the smallest organized unit of living organism capable of prolonged independent existence and replacement of its products in a suitable environment.”
6. “Cell is the simplest integrated organization in living systems capable of prolonged independent survival.”
7. “The cell is considered as a definite unit of living substance made up of a mass of protoplasm called the cytoplasm, containing a nucleus and surrounded by the plasma membrane”.
Amoeba and Paramecium are the most primitive forms of unicellular organisms which are able to perform their activities within the walls of one cell. Human beings are multi-cellular animals. Human body is consisting of different types of cell. The functions of each cell are different. For e.g. kidney cells are different from nerve cells, Liver cells are different from blood cells and brain cells.
Though there are differences between the types of cells they all have some basic characteristics in common which indicate signs of life. Groups of similar cells are organised together to form tissues and organs, each of which has special function within the body.
Cells of multicellular organisms vary in shape and size. The shape of the cell can be elliptical spherical, rectangular, cylindrical, columnar, triangular, biconcave, biconvex etc. The shape of the cell is influenced by the environmental and physiological factors. The size of the cells varies but the cells of a particular tissue of a given species are nearly uniform in size. The number of cells varies from organism to organism.
2. Essay on the Characteristics of Cell:
Cells Possess the qualities of all living organisms, some of the characteristics of the cells are as follows:
1. Respiration:
The cell has the ability to use oxygen brought from the lungs by the blood. It also removes the gaseous waste product carbon dioxide to the blood for its functions and survival.
2. Ingestion and Assimilation:
Cell is a very active unit in which the nourishing food materials eaten by man are absorbed and assimilated. The cell is capable of absorbing fluids and dissolved substances directly through the cell membrane.
3. Growth and Repair:
Nutrition is essential for the growth and repair of cells. The food materials like amino acids are used by the cell to synthesize new protoplasm, by which the cell increases in size. In other words the growth and repair of worn out part of the cell can be possible by proteins. These constructive activities, growth and cell repair are known as anabolic function of the cell.
4. Metabolism:
The cell needs energy for its activities like glandular secretion, movement and nervous activity. Both anabolic and catabolic processes are going on in the cell which is known as metabolism.
5. Excretion:
The cell is able to discharge unused and waste materials through cell membrane. The waste materials resulting from the catabolic processes are eliminated from the cell into the interstitial fluid and then to the blood. Carbonic acid, the waste materials of the cells are removed from the body as carbon dioxide and other waste materials are eliminated in the form of urine by the kidney.
3. Essay on the Basic Concepts of Cell:
Now the basic concepts of cell theory can be summarized in the following statements:
i. The cell is the basic unit of life.
ii. All living organisms are composed of cells. They may be unicellular or multi- cellular.
iii. Cells arise from pre-existing cells.
iv. Over time and with the development of the electron microscope, the theory has continued to evolve. As more and more living material was observed at higher and higher magnifications, much more was learned about cells and the cell theory, leading to the modern cell theory. The modern cell theory includes the two basic components of the classic cell theory and it includes the following postulates.
v. The cell is structural and functional unit of all living things. When cells divide, the hereditary information they contain, as DNA, is passed from cell to cell.
vi. Energy flow (metabolism) occurs within cells.
vii. All cells have basically the same chemical composition.
viii. The activity of the organism is determined by the activity of the independent cells.
4. Essay on the Types of Cell:
a. Prokaryotic Cells:
Prokaryotes are cells that have no membrane-bound nucleus and instead of having chromosomal DNA, their genetic information is in a circular loop called a plasmid. Bacterial cells are very small, roughly the size of an animal mitochondrion (about 0.5-1 µm in diameter and 10 µm long). Small size of microorganisms is that the surface area/volume ratio of bacteria is exceedingly high compared to the same ratio for larger organisms of similar shape.
The shape of a bacterium is governed by its rigid cell wall. Typical bacterial cells are spherical (cocci; singular, coccus); straight rods (bacilli; singular, bacillus); or spiral that are helically curved (spirilla; singular, spirillum). Although most bacterial species have cells that are of a fairly constant and characteristics shape, some have cells that are pleomorphic, i.e., that can exhibit a variety of shapes.
A spherically shaped bacterium is known as a coccus, a term derived from the Greek kokkos, meaning, ‘berry’. Cocci tend to be quite small, being only 0.5-1 µm in diameter. They are usually round, may be oval, elongated or indented on one side. Those cocci that remain in pairs after reproducing are called diplococci. Cocci that remain in chains are called streptococci (Streptococcus pyogenes).
Another arrangement of cocci is the tetrad, consisting of four cocci forming a squire. A cube- like packet of eight cocci is called a sarcinia, Micrococcus luteus, a common inhabitant of the skin, is one example. Other cocci may divide randomly and form an irregular grape like cluster of cells called a staphylococcus, such as Staphylococcus aureus, cause of food poisoning and numerous skin infections.
Bacillus are rod-shaped bacteria and the cylindrical cell may be as long as 20 µm or as short as 0.5 µm. Certain bacilli are slender, such as those of Salmonella typhi that cause typhoid fever; the agent of anthrax (Bacillus anthracis), are rectangular with squired ends; others such as the diphtheria bacilli (Corynebacterium diphtheriae), are club shaped. Most rods occur singly, but some are arranged in to long chains called streptobacilli (strepto = chains).
The third major shape of bacteria is the spiral, which can take one of three forms. Certain spiral bacteria called vibrios are curved rods that resemble commas. Other spiral bacteria called spirilla (sing., spirillum) have a helical shape with a thick, rigid cell wall and flagella that assist movement. Those spiral- shaped bacteria known as spirochetes have a thin, flexible cell wall but no flagella in the traditional sence.
In addition to the bacillus, coccus and spiral shapes, other variations also exist. In the genus Caulobacter, there are appendaged bacteria; members of the genous Nocardia consist of branching filaments; and some Archaea have squire and cluster shapes.
Bacterial cells have a cell wall made up of peptidoglycan. Some bacteria have whip like locomotory structures attached to the cell wall called flagella. Some others also have pilli, short, finger like projections that help the bacteria to attach to tissues, one of the main features necessary for pathogenesis. Bacteria that cause pneumonia, for instance, attach to the tissues of the lung (Fig. 4.1).
Bacteria perform many important functions on earth. They serve as decomposers, agents of fermentation, and play an important role in our digestive system. Also, bacteria are involved in many nutrient cycles such as the nitrogen cycle, which restores nitrate into the soil for plants. Unlike eukaryotic cells that depend on oxygen for their metabolism, prokaryotic cells enjoy a diverse array of metabolic functions. For example, some bacteria use sulphur instead of oxygen in their metabolism. Even there are several anaerobic microbes which can survive in the absence of oxygen.
b. Eukaryotic Cells:
Eukaryotic cells (Eu = “true”, karyon = “nucleus”) comprise all life forms except kingdom Monera. Eukaryotes have a distinct membrane bound nucleus and organelles. An organelle is a small structure that performs a specific set of functions within the eukaryotic cell while in prokaryotes they lack distinct nucleus or organelles.
The basic structure of all cells is the same. All cells have an outer covering called a plasma membrane. The plasma membrane holds the cell together and permits the passage of substances into and out of the cell. With a few minor exceptions, plasma membranes are basically the same in prokaryotes and eukaryotes. The interior of both kinds of cells is called the cytoplasm. Within the cytoplasm of eukaryotes are embedded the cellular organelles.
Both types of cells contain small structures, called ribosomes, the site of synthesis of proteins. Ribosomes are not bounded by membranes and are not considered as organelles.
Now on the basis of presence of certain specialized organelles having specific functions like chloroplast, eukaryotic cells are of two types:
i. Plant cell and
ii. Animal cell (Fig. 4.2).
One of the primary differences between animal and plant cells is that, the plant cells have a cellulose cell wall. This protects the plant cell wall from osmotic shocks. A plant cell has to be able to accept large amounts of liquid through osmosis, without being destroyed. Animal cells do not have rigid cell wall (Fig.4.2). This allows animal cells to form and adopt various shapes. A type of animal cell called the phagocytic cell can even absorb other structures. This ability is not inherent in plant cells. Further, unlike animal cells, plant cells have chloroplasts containing chlorophyll by using them they perform the function of photosynthesis which is absent in animal cells.
Plant cells also contain a larger central vacuole (enclosed by a membrane) as compared to animal cells. Also, while animal cells depend on an analogous system of gap-junctions that allows communication between cells, the plant cells use linking pores in their cell wall to connect to each other and pass information. Plant cells are of three different types. The parenchyma cells help in storage, photosynthesis-support and other functions and collenchyma cells are only present during the time of maturity and have only a primary wall. The sclerenchyma cells help in mechanical support. There are about 210 distinct types of cells present in the human body.