Cell biology is a branch of biology that studies the structure and function of the cell, the basic unit of life. Cell biology is concerned with the physiological properties, metabolic processes, signaling pathways, life cycle, chemical composition and interactions of the cell with their environment; this is done both on a microscopic and molecular level as it encompasses prokaryotic cells and eukaryotic cells. Knowing the components of cells and how cells work is fundamental to all biological sciences. Research in cell biology is related to genetics, molecular biology and cytochemistry. Cells, which were once invisible to the naked eye, were first seen in 17th century Europe with the invention of the compound microscope. Robert Hooke was the first person to term the building block of all living organisms as "cells" after looking at cork; the cell theory states. The theory states that both plants and animals are composed of cells, confirmed by plant scientist, Matthias Schleiden and animal scientist, Theodor Schwann in 1839.
19 years Rudolf Virchow contributed to the cell theory, arguing that all cells come from the division of preexisting cells. In recent years, there have been many studies. Scientists have struggled to decide. Viruses lack common characteristics of a living cell, such as membranes, cell organelles, the ability to reproduce by themselves. Viruses range from 0.005 to 0.03 micrometers in size. Modern day cell biology research looks at different ways to culture and manipulate cells outside of a living body to further research in human anatomy and physiology, to derive treatments and other medications, etc; the techniques by which cells are studied have evolved. Advancement in microscopic techniques and technology such as fluorescence microscopy, phase-contrast microscopy, dark field microscopy, confocal microscopy, transmission electron microscopy, etc. have allowed scientists to get a better idea of the structure of cells. There are two fundamental classifications of cells: eukaryotes; the major difference between the two is the absence of organelles.
Other factors such as size, the way in which they reproduce, the number of cells distinguish them from one another. Eukaryotic cells include animal, plant and protozoa cells which all have a nucleus enclosed by a membrane, with various shapes and sizes. Prokaryotic cells, lacking an enclosed nucleus, include bacteria and archaea. Prokaryotic cells are much smaller than eukaryotic cells, making prokaryotic cells the smallest form of life. Cytologists focus on eukaryotic cells whereas prokaryotic cells are the focus of microbiologists, but this is not always the case; the study of the cell is done on a molecular level. 75-85% of the cell's volume is due to water making it an indispensable solvent as a result of its polarity and structure. These molecules within the cell, which operate as substrates, provide a suitable environment for the cell to carry out metabolic reactions and signalling; the cell shape varies among the different types of organisms, are thus classified into two categories: eukaryotes and prokaryotes.
In the case of eukaryotic cells - which are made up of animal, plant and protozoa cells - the shapes are round and spherical or oval while for prokaryotic cells – which are composed of bacteria and archaea - the shapes are: spherical, rods and spirals. Cell biology focuses more on the study of eukaryotic cells, their signalling pathways, rather than on prokaryotes, covered under microbiology; the main constituents of the general molecular composition of the cell includes: proteins and lipids which are either free flowing or membrane bound, along with different internal compartments known as organelles. This environment of the cell is made up of hydrophilic and hydrophobic regions which allows for the exchange of the above-mentioned molecules and ions; the hydrophilic regions of the cell are on the inside and outside of the cell, while the hydrophobic regions are within the phospholipid bilayer of the cell membrane. The cell membrane consists of lipids and proteins which accounts for its hydrophobicity as a result of being non-polar substances.
Therefore, in order for these molecules to participate in reactions, within the cell, they need to be able to cross this membrane layer to get into the cell. They accomplish this process of gaining access to the cell via: osmotic pressure, concentration gradients, membrane channels. Inside of the cell are extensive internal sub-cellular membrane-bounded compartments called organelles. Cells contain specialized sub-cellular compartments including cell membrane, cytoplasm,mitochondria, ribosomes. See organelle; the growth process of the cell does not refer to the size of the cell, but instead the density of the number of cells present in the organism at a given time. Cell growth pertains to the increase in the number of cells present in an organism as it grows and develops. Cells are the foundation of all organisms, they are the fundamental unit of life; the growth and development of the cell are essential for the maintenance of the host, survival of the organisms. For this process the cell goes through the steps of
The ciliates are a group of protozoans characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different undulating pattern than flagella. Cilia occur in all members of the group and are variously used in swimming, attachment and sensation. Ciliates are an important group of protists, common anywhere there is water — in lakes, oceans and soils. About 3,500 species have been described, the potential number of extant species is estimated at 30,000. Included in this number are many ectosymbiotic and endosymbiotic species, as well as some obligate and opportunistic parasites. Ciliate species range in size from as little as 10 µm to as much as 4 mm in length, include some of the most morphologically complex protozoans. In most systems of taxonomy, "Ciliophora" is ranked as a phylum, under either the kingdom Protista or Protozoa. In some systems of classification, ciliated protozoa are placed within the class "Ciliata,".
In the taxonomic scheme proposed by the International Society of Protistologists, which eliminates formal rank designations such as "phylum" and "class", "Ciliophora" is an unranked taxon within Alveolata. Unlike most other eukaryotes, ciliates have two different sorts of nuclei: a tiny, diploid micronucleus, a large, polyploid macronucleus; the latter is generated from the micronucleus by amplification of the heavy editing. The micronucleus does not express its genes; the macronucleus provides the nuclear RNA for vegetative growth. Division of the macronucleus occurs by amitosis, the segregation of the chromosomes occurs by a process whose mechanism is unknown; this process is not perfect, after about 200 generations the cell shows signs of aging. Periodically the macronuclei must be regenerated from the micronuclei. In most, this occurs during conjugation. Here two cells line up, the micronuclei undergo meiosis, some of the haploid daughters are exchanged and fuse to form new micronuclei and macronuclei.
Food vacuoles are formed through phagocytosis and follow a particular path through the cell as their contents are digested and broken down by lysosomes so the substances the vacuole contains are small enough to diffuse through the membrane of the food vacuole into the cell. Anything left in the food vacuole by the time it reaches. Most ciliates have one or more prominent contractile vacuoles, which collect water and expel it from the cell to maintain osmotic pressure, or in some function to maintain ionic balance. In some genera, such as Paramecium, these have a distinctive star shape, with each point being a collecting tube. Cilia are arranged in rows called kineties. In some forms there are body polykinetids, for instance, among the spirotrichs where they form bristles called cirri. More body cilia are arranged in mono- and dikinetids, which include one and two kinetosomes, each of which may support a cilium; these are arranged into rows called kineties. The body and oral kinetids make up the infraciliature, an organization unique to the ciliates and important in their classification, include various fibrils and microtubules involved in coordinating the cilia.
The infraciliature is one of the main components of the cell cortex. Others are the alveoli, small vesicles under the cell membrane that are packed against it to form a pellicle maintaining the cell's shape, which varies from flexible and contractile to rigid. Numerous mitochondria and extrusomes are generally present; the presence of alveoli, the structure of the cilia, the form of mitosis and various other details indicate a close relationship between the ciliates and dinoflagellates. These superficially dissimilar groups make up the alveolates. Most ciliates are heterotrophs, feeding on smaller organisms, such as bacteria and algae, detritus swept into the oral groove by modified oral cilia; this includes a series of membranelles to the left of the mouth and a paroral membrane to its right, both of which arise from polykinetids, groups of many cilia together with associated structures. The food is moved by the cilia through the mouth pore into the gullet. Feeding techniques vary however; some ciliates are mouthless and feed by absorption, while others are predatory and feed on other protozoa and in particular on other ciliates.
Some ciliates parasitize animals, although only one species, Balantidium coli, is known to cause disease in humans. Ciliates reproduce asexually, by various kinds of fission. During fission, the micronucleus undergoes mitosis and the macronucleus elongates and undergoes amitosis; the cell divides in two, each new cell obtains a copy of the micronucleus and the macronucleus. The cell is divided transversally, with the anterior half of the ciliate forming one new organism, the posterior half forming another. However, other types of fission occur in some ciliate groups; these include budding.
A basal body is a protein structure found at the base of a eukaryotic undulipodium. It is formed from a centriole and several additional protein structures, is a modified centriole; the basal body serves as a nucleation site for the growth of the axoneme microtubules. Centrioles, from which basal bodies are derived, act as anchoring sites for proteins that in turn anchor microtubules, are known as the microtubule organizing center; these microtubules provide structure and facilitate movement of vesicles and organelles within many eukaryotic cells. Cilia and basal bodies form during the G1 phase of the cell cycle. Before the cell enters G1 phase, i.e. before the formation of the cilium, the mother centriole serves as a component of the centrosome. In cells that are destined to have only one primary cilium, the mother centriole differentiates into the basal body upon entry into G1 or quiescence. Thus, the basal body in such a cell is derived from the centriole; the basal body differs from the mother centriole in at least 2 aspects.
First, basal bodies have basal feet, which are anchored to cytoplasmic microtubules and are necessary for polarized alignment of the cilium. Second, basal bodies have pinwheel-shaped transition fibers that originate from the appendages of mother centriole. In multiciliated cells, however, in many cases basal bodies are not made from centrioles but are generated de novo from a special protein structure called the deuterosome. During cell cycle quiescence, basal bodies organize primary cilia and reside at the cell cortex in proximity to plasma membrane. On cell cycle entry, cilia resorb and the basal body migrates to the nucleus where it functions to organize centrosomes. Centrioles, basal bodies, cilia are important for mitosis, cell division, protein trafficking, signaling and sensation. Mutations in proteins that localize to basal bodies are associated with several human ciliary diseases, including Bardet–Biedl syndrome, orofaciodigital syndrome, Joubert syndrome, cone-rod dystrophy, Meckel syndrome, nephronophthisis.
Regulation of basal body production and spatial orientation is a function of the nucleotide-binding domain of γ-tubulin. Plants lack centrioles and only lower plants with motile sperm have basal bodies. Histology image: 21804loa – Histology Learning System at Boston University - "Ultrastructure of the Cell: ciliated epithelium and basal bodies"
A cilium is an organelle found on eukaryotic cells and are slender protuberances that project from the much larger cell body. There are two types of cilia: motile cilia and non-motile, or primary, which serve as sensory organelles. In eukaryotes, motile cilia and flagella together make up a group of organelles known as undulipodia. Eukaryotic cilia are structurally identical to eukaryotic flagella, although distinctions are sometimes made according to function and/or length. Biologists have various ideas about. Cilia can be divided into motile forms. In animals, primary cilia are found on nearly every cell. In comparison to motile cilia, non-motile cilia occur one per cell. In addition, examples of specialized primary cilia can be found in human sensory organs such as the eye and the nose: The outer segment of the rod photoreceptor cell in the human eye is connected to its cell body with a specialized non-motile cilium; the dendritic knob of the olfactory neuron, where the odorant receptors are located contains non-motile cilia.
Although the primary cilium was discovered in 1898, it was ignored for a century. Only has great progress been made in understanding the function of the primary cilium; until the 1990s, the prevailing view of the primary cilium was that it was a vestigial organelle without important function. Recent findings regarding its physiological roles in chemical sensation, signal transduction, control of cell growth, have led scientists to acknowledge its importance in cell function, with the discovery of its role in diseases not recognized to involve the dysgenesis and dysfunction of cilia, such as polycystic kidney disease, congenital heart disease, an emerging group of genetic ciliopathies, it is known that the cilium must be disassembled before mitosis can occur. However, the mechanisms that control this process are still unknown; the primary cilium is now known to play an important role in the function of many human organs. The current scientific understanding of primary cilia views them as "sensory cellular antennae that coordinate a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation.".
The primary non-motile cilia is divided into subdomains. The entire structure is enclosed by a plasma membrane continuous with the plasma membrane of the cell; the basal body, where the cilia originates, is located within the ciliary pocket. The cilium membrane and the basal body microtubules are connected by transition fibers. Vesicles carrying molecules for the cilia dock at the transition fibers; the transition fibers form a transition zone where entry and exit of molecules is regulated to and from the cilia. Molecules can move to the tip of the cilia with the aid of anterograde IFT particles and the kinesin-2 motor. Molecules can use retrograde IFT particles and the cytoplasmic dynein motor to move toward the basal body; some of the signaling with these cilia occur through ligand binding such as Hedgehog signaling. Other forms of signaling include G-coupled receptors including the somatostatin receptor 3 in neuronal cells. Larger eukaryotes, such as mammals, have motile cilia as well. Motile cilia are present on a cell's surface in large numbers and beat in coordinated waves.
In humans, for example, motile cilia are found in the lining of the trachea, where they sweep mucus and dirt out of the lungs. In female mammals, the beating of cilia in the Fallopian tubes moves the ovum from the ovary to the uterus; the functioning of motile cilia is dependent on the maintenance of optimal levels of fluid bathing the cilia. Epithelial sodium channels ENaC that are expressed along the entire length of cilia serve as sensors that regulate fluid level surrounding the cilia. Ciliates are microscopic organisms that possess motile cilia and use them for either locomotion or to move liquid over their surface. Inside cilia and flagella is a microtubule-based cytoskeleton called the axoneme; the axoneme of primary cilia has a ring of nine outer microtubule doublets, the axoneme of a motile cilium has two central microtubule singlets in addition to the nine outer doublets. The axonemal cytoskeleton acts as a scaffolding for various protein complexes and provides binding sites for molecular motor proteins such as kinesin II, that help carry proteins up and down the microtubules.
On the outside of cilia is a membrane like the plasma membrane, but compositionally distinct due to a blocking ring around the base, distinct in its population of receptors and other integral proteins. The ciliary rootlet is a cytoskeleton-like structure that originates from the basal body at the proximal end of a cilium, it extends proximally toward the cell nucleus. Rootlets are 80-100 nm in diameter and contain cross striae distributed at regular intervals of 55-70 nm. According to the Gene Ontology, the following proteins localize to the ciliary rootlet: amyloid precursor protein, rootletin and presenilins. Though they have been given different names, motile cilia and flagella have nearly identical structures and have the same purpose: motion; the movement of the appendage can be described as a wave. The wave tends to originate from the cilium base and can be described in terms of frequency and wave length; the beating motion is creat