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5: Cellular Structure - Biology

5: Cellular Structure - Biology


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5: Cellular Structure

Cell: Structure and Functions (With Diagram)

Let us make an in-depth study of the structure and functions of cell. After reading this article you will learn about: 1. Comparison of Prokaryotic Cells and Eukaryotic Cells and 2. Structure and Components of a Human Cell.

Cell is a compartment where all the activities of life takes place. There are two basic types of cells in nature, viz., prokaryotic cells and eukaryotic cells.

Comparison of Prokaryotic Cells and Eukaryotic Cells:

Prokaryotic Cells:

1. Prokaryotes are the simplest cells without a nucleus and cell organelles.

2. Prokaryotic cells are the smallest cells (1-10 μm).

3. Unicellular and earliest to evolve (

4 billion years ago), still available.

5. These cells reproduce asexually.

6. They include bacteria and archaea.

7. Some species are highly evolved pathogens e.g., Borrelia burgdorferi.

Eukaryotic Cells:

1. Eukaryotes are sophisticated cells with a well defined nucleus and cell organelles.

2. The cells are comparatively larger in size (10-100 μm).

3. Unicellular to multicellular in nature and evolved

4. The cell membrane is semipermeable and flexible.

5. These cells reproduce both asexually and sexually.

6. Include the animals, plants and fungi.

7. Size ranges from tiny yeasts to giant sequoias, dinosaurs, etc.

Structure and Components of a Human Cell:

A group of cells forms tissue, various tissues forms an organ and different organs make up the body.

The structure and components of a human cell are given below:

Thin layer of protein and fat that surrounds the cell is the cell membrane. It is semipermeable, allowing some substances to pass into the cell and blocking others.

Jelly-like material present outside the nucleus in which the cell organelles are located. It is the site of protein synthesis and many metabolic events. The cytoplasm contains many enzymes for general metabolism. It contains fibre of the cytoskeletal system, which organizes cytoplasmic structure.

Mitochondria:

Spherical to rod-shaped organelles with a double membrane. The inner membrane is in-folded many times, forming a series of projections (called cristae). The mitochondrion is known as the power house of the cell as it generates ATP (adenosine triphosphate), the energy currency of the cell.

Small organelles composed of RNA-rich cytoplasmic granules that are sites of protein synthesis. Ribosome size is measured in Svedberg (S) units derived from sedimentation in ultracentrifuge (used before electron microscopes were available).

In prokaryotes the ribosomes are made of 30S and 50S subunits, assemble into 70S ribosome whereas in eukaryotes the ribosomes are made of 40S and 60S subunits, assemble into 80S ribosome. In bacteria they occupy 25% of cell volume and use 90% of cell energy. Less in many specialized eukaryotic cells but still are the dominant activity of almost all the cells.

It is a spherical body containing many organelles, including the nucleolus. It controls many of the functions of the cell (by controlling protein synthesis) and contains DNA (in chromosomes). The nucleus is surrounded by the nuclear membrane. It is the locus of DNA/RNA synthesis and protein assembly. It contains chromatin i.e., DNA-protein complexes. Chromatin can condense into chromosomes during cell division.

The nuclear membrane is a double layered structure surrounding the nucleus containing many nuclear pores. These pores allow different materials to move in and out of nucleus. The pores have octagonal ‘doors’ made of protein which open and close on either side depending on specific signals. Pore diameter is about 10 nanometers (10 x 10 -9 m), smaller than the diameter of a complete ribosome. They can open up to as much as 26 nm in response to certain signals. Some signals allow motion in but not out, other signals control reverse transport.

The nucleolus is present within the nucleus. Some cells have more than one nucleolus. It is the assembly plant for ribosomes. Ribosomal proteins are made in cytoplasm and transported back into the nucleus. Ribosomal RNA is made in the nucleus. These two elements are integrated inside nucleolus to create ribosomal subunits. These are then exported out of nucleus through nuclear pores.

A small body located near the nucleus, also called the ‘microtubule organizing centre’. It has a dense center and radiating tubules. The centrosomes are where microtubules are made. During cell division (mitosis), the centrosome divides and the two parts move to opposite sides of the dividing cell. The centriole is the dense centre of the centrosome.

Endoplasmic Reticulum:

Rough endoplasmic reticulum (rough ER):

A vast system of interconnected, membranous, in-folded and convoluted sacks that are located in the cell’s cytoplasm (the ER is continuous with the outer nuclear membrane). Rough ER is covered with ribosomes that give it a rough appearance. Rough ER transports materials through the cell.

It synthesizes proteins in sacks called cisternae for export or movement to different cell organelles like the Golgi body, or inserted into the cell membrane but not to cytoplasm. The transport proteins designated for export carry a peptide signal at growing end, causing growing protein to move to ER (docking), insert peptide into membrane, translocate growing polypeptide chain across ER membrane.

Smooth endoplasmic reticulum (smooth ER):

A vast system of interconnected, membranous, in-folded and convoluted tubes that are located in the cell’s cytoplasm (the ER is continuous with the outer nuclear membrane). The space within the ER is called the ER lumen. Smooth ER transports materials through the cell. It contains enzymes which produces and digests lipids (fats) and membrane proteins smooth ER buds off from rough ER, moving the newly-made proteins and lipids to the Golgi body, lysosomes and membranes. It detoxifies drugs and poisons (in liver).

A flattened, layered, sac-like organelle that looks like a stack of pancakes. It is also called the Golgi apparatus or Golgi complex. It is located near the nucleus. It produces the membranes that surround the lysosomes. The Golgi body packages proteins and carbohydrates into membrane-bound vesicles for export from the cell.

Functions as intracellular ‘post office’ for sorting new proteins made on rER. Vesicles containing protein pinch off from ER, fuse with cis face of Golgi. Inside Golgi, oligosaccharide chains on proteins are modified. Vesicles pinch off from trans face of Golgi, carry proteins to several possible destinations: export (out of cell), lysosomes, peroxisomes, cell membrane, etc.

These are round organelles surrounded by a membrane where the digestion of cell nutrients takes place due to presence of the digestive enzymes. They contain —40 hydrolytic enzymes such as lipases, proteases, nucleases, etc. which break down organic polymers of all types. Lysosomes continuously break down old proteins, foreign materials, and many wastes.

They also bring about phagocytosis, a process in which foreign materials are brought into the cell and ‘chewed up’. Sometimes lysosomes open up in cell itself causing death of the cell termed as apoptosis, hence are called suicide bags of the cell.

Fluid-filled, membrane-surrounded cavities inside a cell. The vacuole fills with food being digested and waste material that is on its way out of the cell. There are specialized vacuoles which function to store fat as fat droplets (TAG).

These are single membrane oval or spherical cellular organelles. They are also called as micro bodies. They contain catalase enzyme. Peroxisomes are involved in the oxidation of long chain fatty acids and synthesis of plasmalogens and glycolipids.

It provides internal fibrous structure to cells because cell is not ‘just a bag in a bubble’, it contains lots of internal fibres or internal ‘skeleton’. It is not rigid like bone, instead it is capable of being assembled, allows cell movement, cell division, internal motion of the organelles and is broken down in minutes. The cytoskeletal system is composed of microtubules and microfilament.

The microtubules have the largest diameter among the fibres found in the cytoplasm of all eukaryotes. It involves many structures: Cilia, flagella, spindle fibres that polymerize from centrioles during mitosis/meiosis. They are made of the protein called tubulin and polymerizes into hollow tubules of 25 nm diameter.

They are organelles of locomotion. Both of them contain 9 double rings of microtubules, 2 central microtubules, two motor proteins, i.e., motor protein 1-dynein and motor protein 2-kinesin, which allow motion along microtubules.

Another kind of fiber found in cytoplasm of most eukaryotes. Involved in muscle contraction, cell support, pinching off of daughter cells after mitosis.

Extracellular matrix (ECM):

Animal cells do not have cell walls, but have ECM, i.e., a meshwork of macromolecules outside plasma membrane. It consists mainly of glycoproteins (proteins with oligosaccharide chains), especially collagen. Some cells are attached directly to ECM by bonding to collagen or fibronectin.

Intracellular junctions:

In multicellular organisms, adjacent cells are held together by several types of specialized junctions:

1. Tight junctions:

Specialized ‘belts’ that bind two cells tightly to each other, prevent fluid from leaking into intracellular space.

Intercellular ‘rivets’ that create tight bonds between cells, but allow fluids to pass through intracellular spaces.

Formed by two connecting protein rings embedded in cell membrane of adjacent cells. Allows passage of water, small solutes, but not proteins, nucleic acids.


Cell Structure and Function

Research in cell structure and function seeks to determine mechanisms by which parts of cells undergo change and interact with one another in carrying out basic cellular functions. The field encompasses cell morphology, physiology, biochemistry, and molecular biology. The goals are to understand the organization and activities of cells at all levels,from the behavior of entire cells and cell organelles to relationships between their component molecules. A wide range of approaches is employed, including light and electron microscopy, electrophysiology, molecular genetics, and biochemical analysis. A number of rapidly developing areas are under investigation by members of the Cell and Molecular Biology group.

Mechanisms of cell motility, including movement of whole cells and of their parts, constitute an active area of investigation in cell biology. In recent years, there has been considerable progress in the structure and chemistry of microtubules, microfilaments, and membrane proteins. Research in these areas at the University of Virginia includes studies on the mechanism of chromosome movement during mitosis, of organelle tranport along microtubules and actin filaments, and on the behavior of flagella during cell motility.

Another important area of cell biology concerns the control of cell function by external signals. A major aspect of research at the University of Virginia relates to the function of cells involved in the circulatory systems, particularly the vascular smooth muscle and endothelial cells of the circulation. Techniques ranging from video microscopy to gene cloning are used to investigate the factors that control the circulation and growth of relevant cell types.

A source of information that is very important in determining the proper function, arrangement, and development of cells is provided by the extracellular matrix. This network of proteins and other macromolecules helps organize and inform cells of their locations, and provides guideposts for their migrations through an organism during embryogenesis and for wound healing. Several labs at the University of Virginia explore the extracellular matrix, its composition, and its effect on the development of cells and tissues.

Research into basic aspects of cell function address myriad topics, ranging from the mechanism of storage and secretion of proteins destined to be exported from the cell to the nature of sites that initiate chromosome replication. The diversity of experience and equipment available makes collaborative projects possible and facilitates interdisciplinary approaches to current projects.


Contents

Cells were first seen in 17th century Europe with the invention of the compound microscope. In 1665, Robert Hooke termed the building block of all living organisms as "cells" after looking at a piece of cork and observing a cell-like structure, [3] [4] however, the cells were dead and gave no indication to the actual overall components of a cell. A few years later, in 1674, Anton Van Leeuwenhoek was the first to analyze live cells in his examination of algae. All of this preceded the cell theory which states that all living things are made up of cells and that cells are the functional and structural unit of organisms. This was ultimately concluded by plant scientist, Matthias Schleiden [4] and animal scientist Theodor Schwann in 1838, who viewed live cells in plant and animal tissue, respectively. [5] 19 years later, Rudolf Virchow further contributed to the cell theory, adding that all cells come from the division of pre-existing cells. [5] Although widely accepted, there have been many studies that question the validity of the cell theory. Viruses, for example, lack common characteristics of a living cell, such as membranes, cell organelles, and the ability to reproduce by themselves. [6] Scientists have struggled to decide whether viruses are alive or not and whether they are in agreement with the cell theory.

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, and to derive medications. The techniques by which cells are studied have evolved. Due to advancements in microscopy, techniques and technology have allowed for scientists to hold a better understanding of the structure and function of cells. Many techniques commonly used to study cell biology are listed below: [7]

    : Utilizes rapidly growing cells on media which allows for a large amount of a specific cell type and an efficient way to study cells. [8] : Fluorescent markers such as GFP, are used to label a specific component of the cell. Afterwards, a certain light wavelength is used to excite the fluorescent marker which can then be visualized. [8] : Uses the optical aspect of light to represent the solid, liquid, and gas phase changes as brightness differences. [8] : Combines fluorescence microscopy with imaging by focusing light and snap shooting instances to form a 3-D image. [8] : Involves metal staining and the passing of electrons through the cells, which will be deflected upon interaction with metal. This ultimately forms an image of the components being studied. [8] : The cells are placed in the machine which uses a beam to scatter the cells based on different aspects and can therefore separate them based on size and content. Cells may also be tagged with GFP-florescence and can be separated that way as well. [9] : This process requires breaking up the cell using high temperature or sonification followed by centrifugation to separate the parts of the cell allowing for them to be studied separately. [8]

There are two fundamental classifications of cells: prokaryotic and eukaryotic. Prokaryotic cells are distinguished from eukaryotic cells by the absence of a cell nucleus or other membrane bound organelle. [10] Prokaryotic cells are much smaller than eukaryotic cells, making them the smallest form of life. [11] Prokaryotic cells include Bacteria and Archaea, and lack an enclosed cell nucleus. They both reproduce through binary fission. Bacteria, the most prominent type, have several different shapes which include mainly spherical, and rod-shaped. Bacteria can be classed as either gram positive or gram negative depending on the cell wall composition. Bacterial structural features include a flagellum that helps the cell to move, [12] ribosomes for the translation of RNA to protein, [12] and a nucleoid that holds all the genetic material in a circular structure. [12] There are many process that occur in prokaryotic cells that allow them to survive. For instance, in a process termed conjugation, fertility factor allows the bacteria to possess a pilus which allows it to transmit DNA to another bacteria which lacks the F factor, permitting the transmittance of resistance allowing it to survive in certain environments. [13]

Eukaryotic cells can either be unicellular or multicellular [12] and include animal, plant, fungi, and protozoa cells which all contain organelles with various shapes and sizes. [14]

Structure of eukaryotic cells Edit

Eukaryotic cells are composed of the following organelles:

    : This functions as the genome and genetic information storage for the cell, containing all the DNA organized in the form of chromosomes. It is surrounded by a nuclear envelope, which includes nuclear pores allowing for transportation of proteins between the inside and outside of the nucleus. [15] This is also the site for replication of DNA as well as transcription of DNA to RNA. Afterwards, the RNA is modified and transported out to the cytosol to be translated to protein. : This structure is within the nucleus, usually dense and spherical in shape. It is the site of ribosomal RNA (rRNA) synthesis, which is needed for ribosomal assembly. : This functions to synthesize, store, and secrete proteins to the golgi apparatus. [16] : This functions for the production of energy or ATP within the cell. Specifically, this is the place where the Krebs cycle or TCA cycle for the production of NADH and FADH occurs. Afterwards, these products are used within the electron transport chain (ETC) and oxidative phosphorylation for the final production of ATP. [17] : This functions to further process, package, and secrete the proteins to their destination. The proteins contain a signal sequence which allows the golgi apparatus to recognize and direct it to the correct place. [18] : The lysosome functions to degrade material brought in from the outside of the cell or old organelles. This contains many acid hydrolases, proteases, nucleases, and lipases, which breakdown the various molecules. Autophagy is the process of degradation through lysosomes which occurs when a vesicle buds off from the ER and engulfs the material, then, attaches and fuses with the lysosome to allow the material to be degraded. [19] : Functions to translate RNA to protein. : This functions to anchor organelles within the cells and make up the structure and stability of the cell. : The cell membrane can be described as a phospholipid bilayer and is also consisted of lipids and proteins. [12] Because the inside of the bilayer is hydrophobic and in order for molecules to participate in reactions within the cell, they need to be able to cross this membrane layer to get into cell via osmotic pressure, diffusion, concentration gradients, and membrane channels. [20] : Function to produce spindle fibers which are used to separate chromosomes during cell division.

Eukaryotic cells may also be composed of the following molecular components:

    : This makes up chromosomes and is a mixture of DNA with various proteins. : They help to propel substances and can also be used for sensory purposes. [21]

Cell metabolism Edit

Cell metabolism is necessary for the production of energy for the cell and therefore its survival and includes many pathways. For cellular respiration, once glucose is available, glycolysis occurs within the cytosol of the cell to produce pyruvate. Pyruvate undergoes decarboxylation using the multi-enzyme complex to form acetyl coA which can readily be used in the TCA cycle to produce NADH and FADH2. These products are involved in the electron transport chain to ultimately form a proton gradient across the inner mitochondrial membrane. This gradient can then drive the production of ATP and H2O during oxidative phosphorylation. [22] Metabolism in plant cells includes photosynthesis which is simply the exact opposite of respiration as it ultimately produces molecules of glucose.

Cell signaling Edit

Cell signaling is important for cell regulation and for cells to process information from the environment and respond accordingly. Signaling can occur through direct cell contact or endocrine, paracrine, and autocrine signaling. Direct cell-cell contact is when a receptor on a cell binds a molecule that is attached to the membrane of another cell. Endocrine signaling occurs through molecules secreted into the bloodstream. Paracrine signaling uses molecules diffusing between two cells to communicate. Autocrine is a cell sending a signal to itself by secreting a molecule that binds to a receptor on its surface. Forms of communication can be through:

    : Can be of different types such as voltage or ligand gated ion channels. The allow for the outflow and inflow of molecules and ions. (GPCR): Is widely recognized to contain 7 transmembrane domains. The ligand binds on the extracellular domain and once the ligand binds, this signals a guanine exchange factor to convert GDP to GTP and activate the G-α subunit. G-α can target other proteins such as adenyl cyclase or phospholipase C, which ultimately produce secondary messengers such as cAMP, Ip3, DAG, and calcium. These secondary messengers function to amplify signals and can target ion channels or other enzymes. One example for amplification of a signal is cAMP binding to and activating PKA by removing the regulatory subunits and releasing the catalytic subunit. The catalytic subunit has a nuclear localization sequence which prompts it to go into the nucleus and phosphorylate other proteins to either repress or activate gene activity. [22] : Bind growth factors, further promoting the tyrosine on the intracellular portion of the protein to cross phosphorylate. The phosphorylated tyrosine becomes a landing pad for proteins containing an SH2 domain allowing for the activation of Ras and the involvement of the MAP kinase pathway. [23]

Cell cycle Edit

The growth process of the cell does not refer to the size of the cell, but 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 as the organism gets larger so does the number of cells present. Cells are the foundation of all organisms and are the fundamental unit of life. The growth and development of cells are essential for the maintenance of the host and survival of the organism. For this process, the cell goes through the steps of the cell cycle and development which involves cell growth, DNA replication, cell division, regeneration, and cell death. The cell cycle is divided into four distinct phases: G1, S, G2, and M. The G phase – which is the cell growth phase – makes up approximately 95% of the cycle. The proliferation of cells is instigated by progenitors. All cells start out in an identical form and can essentially become any type of cells. Cell signaling such as induction can influence nearby cells to differentiate determinate the type of cell it will become. Moreover, this allows cells of the same type to aggregate and form tissues, then organs, and ultimately systems. The G1, G2, and S phase (DNA replication, damage and repair) are considered to be the interphase portion of the cycle, while the M phase (mitosis) is the cell division portion of the cycle. Mitosis is composed of many stages which include, prophase, metaphase, anaphase, telophase, and cytokinesis, respectively. The ultimate result of mitosis is the formation of two identical daughter cells.

The cell cycle is regulated by a series of signaling factors and complexes such as cyclins, cyclin-dependent kinase, and p53. When the cell has completed its growth process and if it is found to be damaged or altered, it undergoes cell death, either by apoptosis or necrosis, to eliminate the threat it can cause to the organism's survival. [24]

Cell mortality, cell lineage immortality Edit

The ancestry of each present day cell presumably traces back, in an unbroken lineage for over 3 billion years to the origin of life. It is not actually cells that are immortal but multi-generational cell lineages. [25] The immortality of a cell lineage depends on the maintenance of cell division potential. This potential may be lost in any particular lineage because of cell damage, terminal differentiation as occurs in nerve cells, or programmed cell death (apoptosis) during development. Maintenance of cell division potential over successive generations depends on the avoidance and the accurate repair of cellular damage, particularly DNA damage. In sexual organisms, continuity of the germline depends on the effectiveness of processes for avoiding DNA damage and repairing those DNA damages that do occur. Sexual processes in eukaryotes, as well as in prokaryotes, provide an opportunity for effective repair of DNA damages in the germ line by homologous recombination. [25] [26]

The scientific branch that studies and diagnoses diseases on the cellular level is called cytopathology. Cytopathology is generally used on samples of free cells or tissue fragments, in contrast to the pathology branch of histopathology, which studies whole tissues. Cytopathology is commonly used to investigate diseases involving a wide range of body sites, often to aid in the diagnosis of cancer but also in the diagnosis of some infectious diseases and other inflammatory conditions. For example, a common application of cytopathology is the Pap smear, a screening test used to detect cervical cancer, and precancerous cervical lesions that may lead to cervical cancer.


Contents

Cells are of two types: eukaryotic, which contain a nucleus, and prokaryotic, which do not. Prokaryotes are single-celled organisms, while eukaryotes can be either single-celled or multicellular.

Prokaryotic cells

Prokaryotes include bacteria and archaea, two of the three domains of life. Prokaryotic cells were the first form of life on Earth, characterized by having vital biological processes including cell signaling. They are simpler and smaller than eukaryotic cells, and lack a nucleus, and other membrane-bound organelles. The DNA of a prokaryotic cell consists of a single circular chromosome that is in direct contact with the cytoplasm. The nuclear region in the cytoplasm is called the nucleoid. Most prokaryotes are the smallest of all organisms ranging from 0.5 to 2.0 μm in diameter. [13]

A prokaryotic cell has three regions:

  • Enclosing the cell is the cell envelope – generally consisting of a plasma membrane covered by a cell wall which, for some bacteria, may be further covered by a third layer called a capsule. Though most prokaryotes have both a cell membrane and a cell wall, there are exceptions such as Mycoplasma (bacteria) and Thermoplasma (archaea) which only possess the cell membrane layer. The envelope gives rigidity to the cell and separates the interior of the cell from its environment, serving as a protective filter. The cell wall consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from expanding and bursting (cytolysis) from osmotic pressure due to a hypotonic environment. Some eukaryotic cells (plant cells and fungal cells) also have a cell wall.
  • Inside the cell is the cytoplasmic region that contains the genome (DNA), ribosomes and various sorts of inclusions. [4] The genetic material is freely found in the cytoplasm. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Linear bacterial plasmids have been identified in several species of spirochete bacteria, including members of the genus Borrelia notably Borrelia burgdorferi, which causes Lyme disease. [14] Though not forming a nucleus, the DNA is condensed in a nucleoid. Plasmids encode additional genes, such as antibiotic resistance genes.
  • On the outside, flagella and pili project from the cell's surface. These are structures (not present in all prokaryotes) made of proteins that facilitate movement and communication between cells.

Eukaryotic cells

Plants, animals, fungi, slime moulds, protozoa, and algae are all eukaryotic. These cells are about fifteen times wider than a typical prokaryote and can be as much as a thousand times greater in volume. The main distinguishing feature of eukaryotes as compared to prokaryotes is compartmentalization: the presence of membrane-bound organelles (compartments) in which specific activities take place. Most important among these is a cell nucleus, [4] an organelle that houses the cell's DNA. This nucleus gives the eukaryote its name, which means "true kernel (nucleus)". Other differences include:

  • The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
  • The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are associated with histone proteins. All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. [4] Some eukaryotic organelles such as mitochondria also contain some DNA.
  • Many eukaryotic cells are ciliated with primary cilia. Primary cilia play important roles in chemosensation, mechanosensation, and thermosensation. Each cilium may thus be "viewed as a sensory cellular antennae that coordinates a large number of cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation." [15]
  • Motile eukaryotes can move using motile cilia or flagella. Motile cells are absent in conifers and flowering plants. [16] Eukaryotic flagella are more complex than those of prokaryotes. [17]

All cells, whether prokaryotic or eukaryotic, have a membrane that envelops the cell, regulates what moves in and out (selectively permeable), and maintains the electric potential of the cell. Inside the membrane, the cytoplasm takes up most of the cell's volume. All cells (except red blood cells which lack a cell nucleus and most organelles to accommodate maximum space for hemoglobin) possess DNA, the hereditary material of genes, and RNA, containing the information necessary to build various proteins such as enzymes, the cell's primary machinery. There are also other kinds of biomolecules in cells. This article lists these primary cellular components, then briefly describes their function.

Membrane

The cell membrane, or plasma membrane, is a biological membrane that surrounds the cytoplasm of a cell. In animals, the plasma membrane is the outer boundary of the cell, while in plants and prokaryotes it is usually covered by a cell wall. This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from a double layer of phospholipids, which are amphiphilic (partly hydrophobic and partly hydrophilic). Hence, the layer is called a phospholipid bilayer, or sometimes a fluid mosaic membrane. Embedded within this membrane is a macromolecular structure called the porosome the universal secretory portal in cells and a variety of protein molecules that act as channels and pumps that move different molecules into and out of the cell. [4] The membrane is semi-permeable, and selectively permeable, in that it can either let a substance (molecule or ion) pass through freely, pass through to a limited extent or not pass through at all. Cell surface membranes also contain receptor proteins that allow cells to detect external signaling molecules such as hormones.

Cytoskeleton

The cytoskeleton acts to organize and maintain the cell's shape anchors organelles in place helps during endocytosis, the uptake of external materials by a cell, and cytokinesis, the separation of daughter cells after cell division and moves parts of the cell in processes of growth and mobility. The eukaryotic cytoskeleton is composed of microtubules, intermediate filaments and microfilaments. In the cytoskeleton of a neuron the intermediate filaments are known as neurofilaments. There are a great number of proteins associated with them, each controlling a cell's structure by directing, bundling, and aligning filaments. [4] The prokaryotic cytoskeleton is less well-studied but is involved in the maintenance of cell shape, polarity and cytokinesis. [19] The subunit protein of microfilaments is a small, monomeric protein called actin. The subunit of microtubules is a dimeric molecule called tubulin. Intermediate filaments are heteropolymers whose subunits vary among the cell types in different tissues. But some of the subunit protein of intermediate filaments include vimentin, desmin, lamin (lamins A, B and C), keratin (multiple acidic and basic keratins), neurofilament proteins (NF–L, NF–M).

Genetic material

Two different kinds of genetic material exist: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Cells use DNA for their long-term information storage. The biological information contained in an organism is encoded in its DNA sequence. [4] RNA is used for information transport (e.g., mRNA) and enzymatic functions (e.g., ribosomal RNA). Transfer RNA (tRNA) molecules are used to add amino acids during protein translation.

Prokaryotic genetic material is organized in a simple circular bacterial chromosome in the nucleoid region of the cytoplasm. Eukaryotic genetic material is divided into different, [4] linear molecules called chromosomes inside a discrete nucleus, usually with additional genetic material in some organelles like mitochondria and chloroplasts (see endosymbiotic theory).

A human cell has genetic material contained in the cell nucleus (the nuclear genome) and in the mitochondria (the mitochondrial genome). In humans the nuclear genome is divided into 46 linear DNA molecules called chromosomes, including 22 homologous chromosome pairs and a pair of sex chromosomes. The mitochondrial genome is a circular DNA molecule distinct from the nuclear DNA. Although the mitochondrial DNA is very small compared to nuclear chromosomes, [4] it codes for 13 proteins involved in mitochondrial energy production and specific tRNAs.

Foreign genetic material (most commonly DNA) can also be artificially introduced into the cell by a process called transfection. This can be transient, if the DNA is not inserted into the cell's genome, or stable, if it is. Certain viruses also insert their genetic material into the genome.

Organelles

Organelles are parts of the cell which are adapted and/or specialized for carrying out one or more vital functions, analogous to the organs of the human body (such as the heart, lung, and kidney, with each organ performing a different function). [4] Both eukaryotic and prokaryotic cells have organelles, but prokaryotic organelles are generally simpler and are not membrane-bound.

There are several types of organelles in a cell. Some (such as the nucleus and golgi apparatus) are typically solitary, while others (such as mitochondria, chloroplasts, peroxisomes and lysosomes) can be numerous (hundreds to thousands). The cytosol is the gelatinous fluid that fills the cell and surrounds the organelles.

Eukaryotic

  • Cell nucleus: A cell's information center, the cell nucleus is the most conspicuous organelle found in a eukaryotic cell. It houses the cell's chromosomes, and is the place where almost all DNA replication and RNA synthesis (transcription) occur. The nucleus is spherical and separated from the cytoplasm by a double membrane called the nuclear envelope. The nuclear envelope isolates and protects a cell's DNA from various molecules that could accidentally damage its structure or interfere with its processing. During processing, DNA is transcribed, or copied into a special RNA, called messenger RNA (mRNA). This mRNA is then transported out of the nucleus, where it is translated into a specific protein molecule. The nucleolus is a specialized region within the nucleus where ribosome subunits are assembled. In prokaryotes, DNA processing takes place in the cytoplasm. [4]
  • Mitochondria and chloroplasts: generate energy for the cell. Mitochondria are self-replicating organelles that occur in various numbers, shapes, and sizes in the cytoplasm of all eukaryotic cells. [4]Respiration occurs in the cell mitochondria, which generate the cell's energy by oxidative phosphorylation, using oxygen to release energy stored in cellular nutrients (typically pertaining to glucose) to generate ATP. Mitochondria multiply by binary fission, like prokaryotes. Chloroplasts can only be found in plants and algae, and they capture the sun's energy to make carbohydrates through photosynthesis.
  • Endoplasmic reticulum: The endoplasmic reticulum (ER) is a transport network for molecules targeted for certain modifications and specific destinations, as compared to molecules that float freely in the cytoplasm. The ER has two forms: the rough ER, which has ribosomes on its surface that secrete proteins into the ER, and the smooth ER, which lacks ribosomes. [4] The smooth ER plays a role in calcium sequestration and release.
  • Golgi apparatus: The primary function of the Golgi apparatus is to process and package the macromolecules such as proteins and lipids that are synthesized by the cell.
  • Lysosomes and peroxisomes: Lysosomes contain digestive enzymes (acid hydrolases). They digest excess or worn-out organelles, food particles, and engulfed viruses or bacteria. Peroxisomes have enzymes that rid the cell of toxic peroxides. The cell could not house these destructive enzymes if they were not contained in a membrane-bound system. [4]
  • Centrosome: the cytoskeleton organiser: The centrosome produces the microtubules of a cell – a key component of the cytoskeleton. It directs the transport through the ER and the Golgi apparatus. Centrosomes are composed of two centrioles, which separate during cell division and help in the formation of the mitotic spindle. A single centrosome is present in the animal cells. They are also found in some fungi and algae cells.
  • Vacuoles: Vacuoles sequester waste products and in plant cells store water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba, have contractile vacuoles, which can pump water out of the cell if there is too much water. The vacuoles of plant cells and fungal cells are usually larger than those of animal cells.

Eukaryotic and prokaryotic

  • Ribosomes: The ribosome is a large complex of RNA and protein molecules. [4] They each consist of two subunits, and act as an assembly line where RNA from the nucleus is used to synthesise proteins from amino acids. Ribosomes can be found either floating freely or bound to a membrane (the rough endoplasmatic reticulum in eukaryotes, or the cell membrane in prokaryotes). [20]

Many cells also have structures which exist wholly or partially outside the cell membrane. These structures are notable because they are not protected from the external environment by the semipermeable cell membrane. In order to assemble these structures, their components must be carried across the cell membrane by export processes.

Cell wall

Many types of prokaryotic and eukaryotic cells have a cell wall. The cell wall acts to protect the cell mechanically and chemically from its environment, and is an additional layer of protection to the cell membrane. Different types of cell have cell walls made up of different materials plant cell walls are primarily made up of cellulose, fungi cell walls are made up of chitin and bacteria cell walls are made up of peptidoglycan.

Prokaryotic

Capsule

A gelatinous capsule is present in some bacteria outside the cell membrane and cell wall. The capsule may be polysaccharide as in pneumococci, meningococci or polypeptide as Bacillus anthracis or hyaluronic acid as in streptococci. Capsules are not marked by normal staining protocols and can be detected by India ink or methyl blue which allows for higher contrast between the cells for observation. [21] : 87

Flagella

Flagella are organelles for cellular mobility. The bacterial flagellum stretches from cytoplasm through the cell membrane(s) and extrudes through the cell wall. They are long and thick thread-like appendages, protein in nature. A different type of flagellum is found in archaea and a different type is found in eukaryotes.

Fimbriae

A fimbria (plural fimbriae also known as a pilus, plural pili) is a short, thin, hair-like filament found on the surface of bacteria. Fimbriae are formed of a protein called pilin (antigenic) and are responsible for the attachment of bacteria to specific receptors on human cells (cell adhesion). There are special types of pili involved in bacterial conjugation.

Replication

Cell division involves a single cell (called a mother cell) dividing into two daughter cells. This leads to growth in multicellular organisms (the growth of tissue) and to procreation (vegetative reproduction) in unicellular organisms. Prokaryotic cells divide by binary fission, while eukaryotic cells usually undergo a process of nuclear division, called mitosis, followed by division of the cell, called cytokinesis. A diploid cell may also undergo meiosis to produce haploid cells, usually four. Haploid cells serve as gametes in multicellular organisms, fusing to form new diploid cells.

DNA replication, or the process of duplicating a cell's genome, [4] always happens when a cell divides through mitosis or binary fission. This occurs during the S phase of the cell cycle.

In meiosis, the DNA is replicated only once, while the cell divides twice. DNA replication only occurs before meiosis I. DNA replication does not occur when the cells divide the second time, in meiosis II. [22] Replication, like all cellular activities, requires specialized proteins for carrying out the job. [4]

DNA repair

In general, cells of all organisms contain enzyme systems that scan their DNA for damages and carry out repair processes when damages are detected. [23] Diverse repair processes have evolved in organisms ranging from bacteria to humans. The widespread prevalence of these repair processes indicates the importance of maintaining cellular DNA in an undamaged state in order to avoid cell death or errors of replication due to damages that could lead to mutation. E. coli bacteria are a well-studied example of a cellular organism with diverse well-defined DNA repair processes. These include: (1) nucleotide excision repair, (2) DNA mismatch repair, (3) non-homologous end joining of double-strand breaks, (4) recombinational repair and (5) light-dependent repair (photoreactivation).

Growth and metabolism

Between successive cell divisions, cells grow through the functioning of cellular metabolism. Cell metabolism is the process by which individual cells process nutrient molecules. Metabolism has two distinct divisions: catabolism, in which the cell breaks down complex molecules to produce energy and reducing power, and anabolism, in which the cell uses energy and reducing power to construct complex molecules and perform other biological functions. Complex sugars consumed by the organism can be broken down into simpler sugar molecules called monosaccharides such as glucose. Once inside the cell, glucose is broken down to make adenosine triphosphate (ATP), [4] a molecule that possesses readily available energy, through two different pathways.

Protein synthesis

Cells are capable of synthesizing new proteins, which are essential for the modulation and maintenance of cellular activities. This process involves the formation of new protein molecules from amino acid building blocks based on information encoded in DNA/RNA. Protein synthesis generally consists of two major steps: transcription and translation.

Transcription is the process where genetic information in DNA is used to produce a complementary RNA strand. This RNA strand is then processed to give messenger RNA (mRNA), which is free to migrate through the cell. mRNA molecules bind to protein-RNA complexes called ribosomes located in the cytosol, where they are translated into polypeptide sequences. The ribosome mediates the formation of a polypeptide sequence based on the mRNA sequence. The mRNA sequence directly relates to the polypeptide sequence by binding to transfer RNA (tRNA) adapter molecules in binding pockets within the ribosome. The new polypeptide then folds into a functional three-dimensional protein molecule.

Motility

Unicellular organisms can move in order to find food or escape predators. Common mechanisms of motion include flagella and cilia.

In multicellular organisms, cells can move during processes such as wound healing, the immune response and cancer metastasis. For example, in wound healing in animals, white blood cells move to the wound site to kill the microorganisms that cause infection. Cell motility involves many receptors, crosslinking, bundling, binding, adhesion, motor and other proteins. [24] The process is divided into three steps – protrusion of the leading edge of the cell, adhesion of the leading edge and de-adhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each step is driven by physical forces generated by unique segments of the cytoskeleton. [25] [26]

Navigation, control and communication

In August 2020, scientists described one way cells – in particular cells of a slime mold and mouse pancreatic cancer–derived cells – are able to navigate efficiently through a body and identify the best routes through complex mazes: generating gradients after breaking down diffused chemoattractants which enable them to sense upcoming maze junctions before reaching them, including around corners. [27] [28] [29]


Structure of Mitochondrial DNA (With Diagram) | Cell Biology

Mitochondrial DNA is a double stranded circular molecule, which is inherited from the mother in all multi-cellular organisms, though some recent evidence suggests that in rare instances mitochondria may also be inherited via a paternal route. Typically, a sperm carries mitochondria in its tail as an energy source for its long journey to the egg. When the sperm attaches to the egg during fertilization, the tail falls off. Consequently, the only mitochondria the new organism usually gets are from the egg its mother provided. There are about 2 to 10 transcripts of the mt-DNA in each mitochondrion. Compared to chromosomes, it is relatively smaller, and contains the genes in a limited number.

The size of mitochondrial genomes varies greatly among different organisms, with the largest found among plants, including that of the plant Arabidopsis, with a genome of 200 kbp in size and 57 protein-encoding genes. The smallest mtDNA genomes include that of the protist Plasmodium falciparum, which has a genome of only 6 kbp and just 2 protein- encoding genomes. Humans and other animals have a mitochondrial genome size of 17 kbp and 13 protein genes.

Mitochondrial DNA consists of 5-10 rings of DNA and appears to carry 16,569 base pairs with 37 genes (13 proteins, 22 t-RNAs and two r-RNA) which are concerned with the pro­duction of proteins involved in respiration. Out of the 37 genes, 13 are responsible for mak­ing enzymes, involved in oxidative phosphorylation, a process that uses oxygen and sugar to produce adenosine tri-phosphate (Fig. 4.56). The other 14 genes are responsible for making molecules, called transfer RNA (t-RNA) and ribosomal RNA (r-RNA). In some metazoans, there are about 100 – 10,000 separate copies of mt-DNA present in each cell.

Unlike nuclear DNA, mitochondrial DNA doesn’t get shuffled every generation, so it is presumed to change at a slower rate, which is useful for the study of human evolution. Mito­chondrial DNA is also used in forensic science as a tool for identifying corpses or body parts and has been implicated in a number of genetic diseases, such as Alzheimer’s disease and diabetes. Changes in mt-DNA can cause maternally inherited diseases, which leads to faster aging process and genetic disorders.


One Name, Many Types

There are many types of cells. In biology class, you will usually work with plant-like cells and animal-like cells. We say "animal-like" because an animal type of cell could be anything from a tiny microorganism to a nerve cell in your brain. Biology classes often take out a microscope and look at single-celled microbes from pond water. You might see hydra, amoebas, or euglena.

Plant cells are easier to identify because they have a protective structure called a cell wall made of cellulose. Plants have the wall animals do not. Plants also have organelles such as the green chloroplast or large, water-filled vacuoles. Chloroplasts are the key structure in the process of photosynthesis.

Cells are unique to each type of organism. If you look at very simple organisms, you will discover cells that have no defined nucleus (prokaryotes) and other cells that have hundreds of nuclei (multinucleated).

Humans have hundreds of different cell types. You have red blood cells that are used to carry oxygen (O2) through the body and other cells specific to your heart muscle. Even though cells can be very different, they are basically compartments surrounded by some type of membrane.