Characteristics of Cells
The cell is the basic unit common to all living organisms. Two general types of cells exist: prokaryotic and eukaryotic. The main differences between the two groups are the presence or absence of a nuclear membrane, structure enclosing the cell’s genetic information, and the use of functional organelles.
Prokaryotes
Prokaryotic cells have a plasma membrane that separates the internal features from the environment, regulates the flow of materials in and out of the cell, and carries out various metabolic processes. The membrane contains both lipids and proteins and is selectively permeable. The phospholipids of the plasma membrane form a lipid bilayer where the hydrophilic ends lie on the outer surfaces and the hydrophobic fatty acid chains extend into the interior of the membrane. The majority of the membranous proteins are hydrophobic and integrated in the phospholipid matrix. The general structure of a biological membrane is shown in Figure 1.
The nucleoid is a region within the cell that contains the genetic information of the cell. There is no membrane enclosing this material. Some prokaryotes contain additional genetic information in a circular molecule called a plasmid. The remaining material within the cell is called the cytoplasm. Ribosomes are spherical structures within the cytoplasm that help to coordinate the synthesis of proteins. The cytosol is the fluid portion of the cytoplasm that excludes the ribosomes, membranous structures, and organelles.
Some prokaryotic cells have a cell wall that provides additional support and determines the cell shape. The structure of the cell wall is often used to distinguish between two groups of bacteria, gram-positive and gram-negative. Such distinctions can help to further classify a particular bacteria and assess the potential influence it may have on humans and the environment. Hans Christian Gram developed a bacterial staining procedure (Gram Stain) in 1884 that created two different responses between the groups. The gram-positive cells possess a single, thick peptidoglycan layer outside of the plasma membrane and stain purple. The gram-negative cells have a thin peptidoglycan layer outside the plasma membrane that is surrounded by an outer membrane. This complex cell wall structure stains red. Some prokaryotes also have an external polysaccharide or polypeptide coating called a capsule.
Cellular movement can occur through appendages called flagella and short threadlike structures extending from the cell surface, called pili. Pili also allow cells to attach to other cells. Some cells produce intracellular spores called endospores that are resistant to harsh environmental conditions. These spores are capable of germinating to give life to new cells.
Although the structure of the prokaryotic cell is simple, the functions carried out by the cell are fairly complex. The genetic information and biochemicals produced allow the cell to metabolize, regulate growth, and reproduce.
Prokaryotic cells are characteristic of the Eubacteria and Archaebacteria kingdoms. The Eubacteria can be separated into three general groups: gram-positive bacteria, gram-negative bacteria, and mycoplasma. Mycoplasma do not have cell walls and aside from their clinical importance, often contaminate mammalian cell cultures. Actinomycetes structurally resemble molds but because they lack a nuclear membrane they are considered to be a part of the Eubacteria kingdom. They are capable of producing amylolytic and cellulolytic enzymes and antibiotics that have importance in the bioprocessing industry. Cyanobacteria, also a part of the eubacteria, contain chlorophyll and are capable of fixing carbon dioxide and converting it to simple sugars.
Archaebacteria live in habitats that are notable for their extreme environmental conditions. Regions of high salinty are often populated with halobacteria. These archaebacteria are also found in low oxygen environments as well as high temperature and low or high pH regions. Thermoacidophiles are microorganisms that can survive in high temperature and highly acidic conditions. Methanogens belong to this group and are capable of producing methane. Three main characteristics that separate the Eubacteria from the Archaebacteria are: 1) the Archaebacteria do not have peptidoglycan in their cellular makeup 2) the nucletide sequences in ribosmoal RNA differ and 3) the type of lipids in the plasma membrane also differ significantly.
Eukaryotes
Eukaryotic cells are structurally more complex than the prokaryotic cells and can be found in animals, plants, fungi, and protists (algae, protozoa). The internal makeup of an eukaryotic cell includes many membranous structures that regulate all the cellular functions. Such structures are called organelles. The structure of the eukaryotic plasma membrane is very similar to that of the prokaryotic membrane and the organelles within the membrane are suspended within the cytoplasm. Sterols are often found in eukaryotic cell walls to strengthen the structure. The cell walls also show variation in the type of proteins, carbohydrates and other molecules that make up their composition. Plant cell walls are generally made up of cellulose and pectin for added strength. Animal cells do not have cell walls making them more fragile and shear-sensitive.
The nucleus contains the genetic information of cell and is surrounded by two porous membranes called the nuclear envelope. The nucleolus lies within the nucleus and functions in ribosome construction. The various compounds within the nucleus are suspended in a fluid called nucleoplasm. Ribosomes provide a site for protein synthesis to occur. The ribosome structure consists of two subunits, one large and one small. They can be found throughout the cytoplasm, attached to the Endoplasmic reticulum and in energy-processing organelles such as the mitochondria.
The mitochondria carry out cellular respiration by converting energy from food into a form that is useful for the cell, adenosine triphosphate (ATP). The outer membrane of the mitochondria is smooth and unfolded. The inner membrane folds throughout the structure forming shelf-like structures called cristae. The region enclosed by the inner membrane is called the mitochondrial matrix. The mitochondria contain their own DNA and ribosomes for the production of proteins needed to complete their functions. Some plants and protists have chloroplasts. The chloroplasts serve as sites for photosynthesis, where light energy is converted into chemical bond energy. Chloroplasts have two unfolded membranes and within this double layer are stacks of closely packed circular sacks called thylakoids. Thylakoids have a membrane and contain lipids, proteins, and chlorophyll. The thylakoids are suspended in a fluid called the stroma. Like the mitochondria, chloroplasts have their own DNA for the synthesis of necessary proteins.
The endoplasmic reticulum (ER) is a complex membrane system that branches throughout the cytoplasm. The region of this network of tubes and sacs closest to the nucleus has ribosomes attached to the outer surface. These regions are involved in protein synthesis and considered the rough ER. Other parts of the ER do not have ribosomes and function in modifying proteins and lipid synthesis. This portion is considered the smooth ER.
The golgi apparatus appears like flattened sacs lying together that extend from the rough ER to the cell surface. The main function of the golgi apparatus is to export and retain proteins for cellular processes. Proteins are delivered to other parts of the cell as well as outside of the cell. Lysosomes are membrane bound structures that contain and transport digestive enzymes. They serve as sites for food breakdown and digestion of foreign objects. Vacuoles are membranous organelles that contain many dissolved substances. In some plant cells they contain various pigments. They are involved in food digestion, waste storage, and osmotic regulation. Vacuoles often make up a large portion of the cell’s volume, as much as 90% in plant cells.
The cytoskeleton is a set of fibers that give eukaryotic cells shape and texture. Microfilaments are found within the cytosol and drive cellular movement. They allow cells to contract and change shape. Intermediate filaments are fibrous proteins with a ropelike organization that stabilize cell structure and make it less sensitive to tension. Microtubules are long hollow cylinders made up of a protein called tubulin. They are capable of changing length rapidly. This characteristic is applied during the moving of chromosomes. They help to control the arrangement of cell wall components and provide added stability and tracks for the movement of vesicles throughout the cell.
Like the prokaryotes, some eukaryotes have flagella and cilia. These appendages push and pull the cell and promote the movement of fluids over the cellular surface. Flagella are longer appendages that appear singly or in pairs. Cilia are short extensions from the cell that occur in large numbers.
Fungi are heterotrophic organisms that live as saprobes (organisms that obtain their energy from organic matter through enzymatic action), parasities, or mutualists. They can be classified into two different groups, yeasts and molds. Yeasts have a spherical, cylindrical, or oval shape and appear much larger than bacteria. They can reproduce asexually through budding or fission. In budding a second cell buds off of the parent cell. Fission is accomplished by one cell dividing into two once it reaches a certain size. Yeasts can also reproduce sexually by forming a zygote (diploid cell or a cell with two sets of chromosome) through the combination of two haploid cells (cells with one set of chromosomes). Molds are generally multicellular and their cells are organized into individual filaments called hyphae. The entire body of branching hyphae is referred to as mycelium. Figure 2 shows the mycelia structure of fungal molds. Spores are often formed and they can be developed sexually or asexually. Asexual spores are released from aerial structures and are called conidia. Sexual spores are resistive to harsh environmental conditions and are able to form new organisms. Molds can be separated into four classes based on their method of sexual reproduction.
Yeasts are often used in the production of alcohol, such as beer and wine. A common yeast used in the beverage industry is Saccharomyces cerevisiae. Molds are also used to make citric acid (Aspergillus niger) and other organic acids, antibiotics such as penicilin (Penicillium chrysogenum), enzymes like cellulase (Trichoderma reesei), and many other products. Plant and animal cells are also used to produce various metabolic byproducts such as many of the biopharmaceuticals on the market today.
Algae are generally unicellular and photosynthetic where they have chloroplasts. Many contain chlorophyll, however other pigments are common. Some contain silica or calcium carbonate within their cell walls and are often used during filtration processes in waste water treatment. Others produce gelling agents such as agars and alginic acid.
Protozoa are unicellular organisms that lack cell walls. They are motile and the majority of them ingest food by endocytosis. The various species are classified by motion (i.e. flagella, cilia, amoeboid motion). Many can cause diseases like malaria (Plasmodium spp.) and dysentery, yet they do have potential in helping to remove bacteria from waste water treatment processes.
Viruses
Viruses are not cells, do not consist of cells and do not metabolize energy. They are much smaller than most bacteria and are obligate parasites of other cells. They contain genetic material, as deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), within a protein coat called a capsid. Generally, the genetic material sits within a hexagonal head that is supported by a tail and tail fibers. All types of cells can be infected. Viruses that attack bacterial cells are called bacteriophages. The structure of a bacterial virus, a bacteriophage, is provided in Figure 3. During the infection of a bacterial cell, a bacteriophage is able to reproduce itself through two cycles, the lytic cycle and the lysogenic cycle. The lytic cycle involves the following steps:
The lysogenic cycle involves:
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