This section describes the different parts of a cell. The functions of each of these cell parts or organelles are also discussed. The cells of all living organisms share several structural characteristics together. Some of the many structural components of a cell (which co-ordinates the cells activities and their functions) are discussed in this section.
NUCLEUS: Nucleus is the control centre of the eukaryotic cell that contains deoxyribonucleic acid (DNA). DNA is the genetic material of the cell which directs cell division or replication in a cell. It is both the cells control centre and the storehouse of genetic information because it houses the genome of the organism. The genetic information in the DNA is used to synthesize specific protein molecules which are unique to each cell functions. The nucleus is the major characteristic that distinguishes a eukaryotic cell (which possesses it) from a prokaryotic cell (that lacks it). Nucleus therefore, is the most prominent organelle found in eukaryotic cells because all processes of a cell that has to do with DNA replication, RNA processing and transcription all takes place inside the nucleus of the cell. The nucleus of a cell is surrounded by a membrane that allows certain molecules to enter and leave it in order to keep the DNA well protected.
FLAGELLA: Flagella (singular flagellum) are long whip-like structures that extend from the body of certain cells or organisms. They are organs of motility, and thus help to propel microorganisms (eukaryotic and prokaryotic) from one point to another. However, flagella are most prominent in prokaryotic cells. Flagellum is also found in some archaeal organisms. Only motile bacteria possess this filamentous appendage that projects from the surface of microbes. There are actually five types of flagella that can be expressed by a microbial cell, and these flagella can occur in various forms or number on the surfaces of the cells. For example, bacterial cell with only one flagellum at either ends of the cell are generally called monotrichous bacterium. Amphitrichous bacteria are those with single flagellum at each end of the cell. Flagellum arranged as a tuft (i.e. in mass) and distributed over the cell surface of a bacterial cell is known as lophotrichous and peritrichous flagellum respectively. Helicotrichous flagella are those flagella which are inserted helically on the cell surface of the organism.
RIBOSOMES: Ribosomes are the protein synthesizing machinery of a cell. They are special structures found in both eukaryotic and prokaryotic cells. Ribosomes are primarily responsible for the synthesis of proteins. Ribosomes are composed mainly of ribonucleic acids (RNAs) and proteins. The three types of RNA found in the ribosome are ribosomal RNA (rRNA), messenger RNA (mRNA), and transfer RNA (tRNA); and it is upon these structures that new proteins of a cell are manufactured. Different types of ribosomes are found in eukaryotic and prokaryotic cells. Svedberg unit or sedimentation coefficient which is usually designated with the symbol ‘S’ is the symbol or unit in which the sedimentation coefficient of a macromolecule or particle (in this case RNA) is determined or measured in an ultracentrifugation machine. For example, 70S is the sedimentation coefficient of prokaryotic cells while 80S is the sedimentation coefficient of eukaryotic cells. It is noteworthy that each ribosome of a cell contains two subunits (a larger one and a smaller one) each of which are composed of proteins and RNA.
VACUOLES: Vacuoles are non-rigid; membrane bound fluid-filled sac or organelles that function as storage space of cells. They also take part in the elimination of water and the digestion of food materials in the cell. The types of vacuoles that can be found in a cell may include food vacuole, contractile vacuole and gas vacuole; and these perform various functions in the cell.
NUCLEOLUS: Nucleolus (plural nucleoli) is a specialized structure in the cell nucleus of most eukaryotic organisms formed from regions of chromosomes. It is usually the site of rRNA synthesis and the assembly of ribosomal subunits as well.
NUCLEOID: Nucleoid is the part of a prokaryotic cell that contains the genetic material (DNA) of the organism. It is the bacterial chromosome, and can be referred to as the nuclear area of a prokaryotic cell. Nucleoid is a clear region consisting of genetic materials in a single circular DNA of a bacterial cell. In eukaryotic cells, their chromosomes are packaged or contained in the nucleus but this is not the case for prokaryotes that lack a membrane-bound nucleus. Thus, the chromosomes of prokaryotic organisms are located inside some irregular structures known as the nucleoid. Nucleoid on the other hand, is the sum total of the DNA that make up the chromosome of prokaryotic cells. It can also be called the nuclear body or nuclear region of prokaryotic cells. Prokaryotic cells (e.g., bacteria) also contain small circular DNA molecules that are extra-chromosomal and are capable of autonomous replication. This small circular DNA molecule found in bacteria cells is called plasmids. Plasmids are important tools in molecular biology techniques e.g. gene cloning procedures.
CHROMOSOMES: Chromosomes are structures that are found in the nucleus of a cell, and that contains genes (the units of heredity) and chromatin (a nucleoprotein compound containing genomic DNA and other nucleic materials). They are found in both prokaryotic and eukaryotic cells. The chromosomes of prokaryotic cells are usually circular in form while those of eukaryotic cells are linear; and each contains vital genetic information or genes that are required for the replication of cells.
PLASMA MEMBRANE: Plasma membrane is a selectively permeable structure that surrounds the contents of the cell and regulates the movement of materials in and out of the cell. It is present in both eukaryotic and prokaryotic cells, and it is the site or location of many metabolic activities of the cell. Plasma membrane allows only ions and certain molecules to pass through it. It also helps to maintain the structural integrity (shape) of the cell. Plasma membrane is usually the main barrier between the internal and external environments of a cell.
MITOCHONDRION: Mitochondrion (plural: mitochondria) is a membrane-bound organelle that is mainly responsible for cellular respiration or energy generation in eukaryotic cells. It is the power house of a cell because of its biological role in energy generation within the cell. It is the organelle that is primarily responsible for the production of energy in eukaryotic cells. Mitochondria are machineries of aerobic respiration and electron transport phosphorylation (i.e. oxidative phosphorylation) in eukaryotic cells. They are the site for energy generation from food and oxygen molecules in a cell. The energy in this case refers to adenosine triphosphate (ATP), the energy currency of the cell (prokaryotic and eukaryotic cells inclusive). Mitochondria are lacking in prokaryotic cells (e.g., bacteria) and viruses. They are only found in eukaryotic cells including fungi, protozoa, helminthes and animal cells. Oxidative phosphorylation occurs in the plasma or cell membrane of prokaryotic cells since they lack mitochondria. Since viruses are intracellular parasites and live within their host cells whose cellular and metabolic machinery they overwhelm, viruses derive their own energy or ATP from that of their host cell.
CHLOROPLAST: Chloroplast is the organelle that contains the green-pigment chlorophyll, and which serve as the photosynthetic machinery of phototrophic cells and photosynthetic organisms. Examples of photosynthetic cells include plant cells, algae and some protists. Chloroplasts contain their own small genetic material, and they help photosynthetic cells to carry out photosynthesis – the process by which green plants and other photosynthetic cells manufacture their own food and energy in the presence of sunlight and carbon (iv) oxide (CO2). The chloroplasts are thus the site for CO2 fixation and energy generation for all eukaryotic cells that are photosynthetic in nature.
CELL WALL: Cell wall is the rigid coating or cover of cells that is exterior to the cytoplasmic membrane, and which helps to prevent the cells from lysis by controlling the inflow and outflow of materials between the cell and its surrounding environment. The cell wall is present in both eukaryotic and prokaryotic cells and it is solely responsible for the shape of the organism or cell. However, the structure and chemical composition of the cell wall varies between prokaryotic and eukaryotic cells even though they perform similar functions in each of these organisms. Generally, cell walls provide both regulatory and protective functions to the cell.
FIMBRIA: Fimbria (singular: fimbriae) are hair-like structures which are uniformly found on the outside of a prokaryotic cell. Their main function in prokaryotic cell is to promote the attachment or adherence of a bacterial cell to another. Fimbria are smaller than flagella, and they contain attachment factors (known as adhesins) which enables bacterial cell to attach firmly to another cell.
PILI: Pili (singular pilus) are structures which help to transfer genetic material (DNA) from one bacterial cell to another. They are also known as sex pili, and they are formed between two bacterial cells when they undergo conjugation as channels via which chromosome segments are easily transferred and exchanged between cells.
GOLGI APPARATUS: Golgi apparatus is the site for transfer, storage and packaging of macromolecules from endoplasmic reticulum for secretion to other organelles of the cell. They are found in animal cells.
ENDOPLASMIC RETICULUM: Endoplasmic reticulum (ER) is a structural network in the cytoplasm of a cell. They connect with the nuclear envelop of the cell. There are basically two types of ER viz: rough endoplasmic reticulum and smooth endoplasmic reticulum. Rough ER is the site for protein synthesis synthesized by ribosomes on its outer surface while the smooth ER is the site for the synthesis of lipid. ER is found in animal cells.
CHROMATIN: Chromatin is the chromosomal DNA component of a cell that is usually associated with histone proteins in a dispersed form. They are found in animal cells.
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