Cells are the basic structural and functional units of living tissue, whether plant or animal. Cellular functions include such basic life processes as protein and lipid (fat) synthesis, cell division and replication, respiration, metabolism, and ion transport as well as providing structural support for tissues, protecting the body against disease or injury, and serving as selective barriers to the passage of various materials into and out of the cell. Cellular physiology is a term that is sometimes used to describe the study of cellular functions and processes.
All cells in the human body are eukaryotic, which means that they have a well-defined nucleus separated from the cytoplasm inside the cell by a nuclear membrane. Prokaryotic cells, which are found only in organisms belonging to the kingdom Monera, have their nuclear material either scattered throughout the cell or collected in a region resembling a nucleus but not defined by a nuclear membrane. Prokaryotic organisms include bacteria, viruses, and blue-green algae.
The reader may find it helpful to have a summary outline of the chemical composition and major structural components of human cells as background for understanding the various functions of a eukaryotic cell.
There are about 60 trillion cells in the body of a human adult. They contain the following components:
- Water. Water accounts for 60-90 percent of cellular content. It is an important part of many metabolic reactions because it can dissolve a wide variety of substances.
- Carbohydrates. Carbohydrates are compounds formed from carbon, oxygen, and hydrogen atoms. They account for 3 percent of the dry mass of most cells.
- Lipids (fatty or waxy substances). Lipids are composed primarily of carbon and hydrogen but cannot be dissolved in water. They represent about 40 percent of the dry mass of a typical cell and play a role in long-term energy storage.
- Proteins. Proteins are chains of amino acids held together by peptide bonds. They account for 50-60 percent of the dry mass of a cell. Proteins serve two major functions in a cell: structural support and speeding up chemical reactions. Proteins that serve as catalysts in chemical reactions within the cell are called enzymes.
- Ions. Ions are atoms or molecules that carry a positive or negative electrical charge. The most important ions in cellular functions are sodium, potassium, chloride, and calcium.
- Nucleic acids. These include DNA and RNA. Nucleic acids are necessary for cell reproduction and protein synthesis.
A typical cell can be divided into three structural areas: the plasma (or cell) membrane, the cytoplasm, and the nucleus.
PLASMA MEMBRANE. The plasma (or cell) membrane is the outer surface of the cell. It is composed primarily of proteins (about 60 percent of its structure) surrounded by a double layer (called a bilayer) of phospholipids (about 40 percent).
CYTOPLASM. Cytoplasm refers to the protoplasm in the cell that lies inside the plasma membrane but outside the nucleus. It is composed of a gel-like fluid called cytosol; organelles; and a cytoskeleton that gives the cell its basic shape.
- Cytosol. This fluid material, which is constantly changing in viscosity, is composed mostly of water and free-floating molecules of various chemicals.
- Cytoskeleton. The cytoskeleton of human cells consists of microfilaments (also called microtubules) that support the walls of the cell and help to move nutrients and other materials in and out of the cell. It is also thought that the cytoskeleton's maintenance of the cell's shape may be a factor in cell differentiation.
- Centrioles. Centrioles are paired cylindrical organelles located near the nucleus that lie at right angles to each other in a centrosome. Each centriole comprises nine tubes, each tube in turn composed of three tubules.
- Endoplasmic reticulum (ER). The endoplasmic reticulum is a network of tubes fused to the nuclear membrane surrounding the cell nucleus. The ER has two forms: rough (coated with ribosomes) and smooth.
- Golgi complex. The Golgi complex, which is sometimes called the Golgi apparatus or the Golgi body, is a membrane-like structure found near the nucleus that consists of a series of flattened sacs called cisternae (singular, cisterna).
- Mitochondria. Mitochondria (singular, mitochondrion) are oval-shaped organelles with a two-layered structure: a smooth outer membrane and an inner membrane with many small folds or ridges called cristae (singular, crista).
- Ribosomes. Ribosomes are tiny organelles found by the thousands in each cell. They may be stationary—embedded in rough endoplasmic reticulum—or scattered randomly throughout the cytoplasm. They may also be linked together in chains called polyribosomes.
- Lysosomes. Lysosomes are hollow spherical organelles enclosed by a membrane. They contain digestive enzymes and also function in destroying damaged cells—which is the reason for their nickname "suicide bags."
- Vacuoles. Vacuoles are small spaces or cavities within the cytoplasm that store water, various chemicals, and insoluble waste products.
- Cilia. Some human cells have hairlike projections called cilia, which help to move the cell itself or to move water or mucus across the outer surface of the cell. Longer hairlike projections are called flagella (singular, flagellum).
NUCLEUS. The nucleus is a spherical body within the cell, somewhat denser than the surrounding cytoplasm. There may be more than one nucleus in a human cell. The nucleus has three major components:
- Nuclear membrane. The nuclear membrane is a porous structure with two layers that surrounds the nucleus of a eukaryotic cell. It is also known as the nuclear envelope.
- Chromatin. Chromatin is an easily stained material in the nucleus that forms itself into long threadlike structures called chromosomes during the process of cell division.
- Nucleolus. The nucleolus is a small spherical body inside the nucleus that contains RNA used in the manufacture of proteins.
Barrier and transport functions
The plasma membrane surrounding each human cell functions as a barrier to keep bacteria, viruses, and other disease organisms out of the cell. It is a selective barrier, however, and allows nutrients to enter the cell and waste products to leave.
There are two ways in which molecules can be carried across the plasma membrane—passive and active transport. In passive transport, the cell does not need to expend any energy, as molecules always move by diffusion from regions of higher concentration to regions of lower concentration. Water, carbon dioxide, and oxygen can all move in and out of human cells by passive transport. The diffusion of water molecules through the semipermeable cell membrane is called osmosis.
In active transport, the cell must expend energy because it is moving molecules against the concentration gradient, from areas of low concentration to areas of higher concentration. Active transport is necessary because the lipid bilayer of the plasma membrane is impermeable to ions (charged molecules or atoms) and such small molecules as glucose. In active transport, proteins called carriers or transporters, which are embedded in the lipid bilayer, use the energy from adenosine triphosphate (ATP) to force ions or small molecules into or out of the cell against the concentration gradient.
A eukaryotic cell can move large amounts of material into itself by a process called endocytosis. In endocytosis, the plasma membrane invaginates (folds inward) taking inside itself a portion of extracellular fluid (ECF) containing dissolved or suspended nutrients. The plasma membrane then seals off the ingested fluid in a compartment called a vesicle.
Endocytosis includes two processes known as pinocytosis and phagocytosis. Pinocytosis, or "cell drinking," refers to the continuous taking-in of very small droplets of ECF. It occurs in almost all cells of the human body. Phagocytosis, by contrast, is carried out only occasionally, and only by specialized cells known as phagocytes. These cells can engulf large particles—usually bacteria or other microorganisms. Once inside the cell, the ingested particle is sealed off in a vacuole. Eventually the vacuole fuses with a lysosome, which contains a digestive enzyme that destroys the contents of the vacuole.
Exocytosis is the reverse of endocytosis; it is a process in which the waste products of digestion or toxins from destroyed bacteria are released into the extracellular fluid. The vesicle or vacuole containing the waste products moves toward the plasma membrane and fuses with it. This fusion releases the contents of the vesicle outside the cell. Exocytosis is also used to carry hormones or neurotransmitters produced inside the cell to the outside. In addition to transporting large molecules into the ECF, exocytosis also increases the total surface of the plasma membrane.
Another important function of human cells is communication and response to signals from their environment. Many of these signals are chemicals in the ECF secreted by distant glands (endocrine signals); by nearby cells (paracrine signals); or even by themselves (autocrine signals). The proteins that signal nearby cells or the cell itself are called cytokines. Cytokines include such substances as tumor necrosis factors and interferons. The chemical signal binds to protein receptors on the cell's surface that also serve to activate the cell's response to the signal.
Chemical signals may trigger either immediate or longer-term responses in receptor cells. Immediate responses include changes in the electrical charge across the cell's plasma membrane, changes in the cell's metabolism, or movement toward or away from the chemical stimulus (chemotaxis). Longerterm responses to chemical signals include changes in the genetic material inside the cell's nucleus.
Migration and movement
Cell migration refers to the capacity of cells to move within the body. In the early stages of embryo development, the rapidly dividing cells migrate to form different layers of tissue that eventually give rise to the brain and nervous system, the internal organs, the skin, and other specialized parts of the body. The pattern of cell migration is regulated by special proteins known as migration proteins. In the adult, cell migration is critical to the proper functioning of the immune system and wound healing. When a person cuts their finger, for example, white blood cells divide rapidly and move in the direction of the injury to engulf any bacteria that enter, while cells in the skin proliferate and migrate to repair the wound itself.
Cells migrate in a specific direction in response to a chemical signal picked up by receptor proteins in the plasma membrane. This orientation of a cell's movement toward or away from a chemical signal is called chemotaxis. Movement toward the chemical is called positive chemotaxis; movement away from it is negative chemotaxis.
Cell locomotion itself is a cyclical process. When the cell receives a chemical signal, it first forms a protrusion at its front. The protrusion then attaches itself to the surface or substrate over which the cell is moving. Next, the cell body contracts and pulls itself forward toward the protrusion. The attachments at the rear of the cell are then released and the cycle is repeated.
Some human cells have hairlike processes that extend outward from the plasma membrane called cilia (if there are many of them per cell) or flagella (if there is only one). Both cilia and flagella move mucus or other fluids over the surface of the cell. For example, the tissue that lines the airway is covered with cilia that move mucus upward toward the throat and mouth. The human sperm has a single flagellum that enables it to swim upward to fertilize the ovum.
Metabolism refers to the sum total of the physical and chemical processes that produce and maintain living cells (anabolism) and the breaking down of complex molecules to provide energy for the cells (catabolism). An important part of cellular metabolism is respiration, which is the exchange of oxygen and carbon dioxide between cells and the external environment. In cellular respiration, the cells break down molecules of glucose to form carbon dioxide and water. The energy released in this process is stored in the cells in the form of adenosine triphosphate, or ATP. Respiration has two phases: glycolysis, in which the glucose is broken down to pyruvic acid; and the complete oxidation of pyruvic acid to form water and carbon dioxide.
Glycolysis takes place in the cytosol of the cell. The pyruvic acid, however, is oxidized in the mitochondria through a process called chemiosmosis. Pairs of hydrogen ions (protons), which carry a positive charge, accumulate on one side of the inner membrane of the mitochondrion. The membrane becomes energized, which allows enzymes contained within it known as ATP synthases to make ATP from adenosine diphosphate (ADP) and a phosphate molecule.
Cell division and the cell cycle
When eukaryotic cells divide, each daughter cell must receive a complete set of genes, a pair of centrioles, some mitochondria, some ribosomes, and a portion of the endoplasmic reticulum. Division of the organelles is not difficult because there are many ribosomes and mitochondria in each cell. To divide the genetic material in the cell nucleus accurately, however, requires the cell to first duplicate each chromosome and then distribute one of each doubled chromosome to each daughter cell. The repeated process of doubling of the genetic material and then halving it during cell division is called the cell cycle. The cycle consists of four phases:
- G (which stands for gap)1: In this phase, the chromosomes are prepared for replication.
- S (which stands for synthesis). In the S phase, the chromosomes are doubled and form two identical sister chromatids.
- G (gap)2. In this phase, the cell prepares for mitosis.
- M (which stands for mitosis). Mitosis is a process with five phases during which one copy of each duplicated chromosome is distributed to each daughter cell.
The five phases of mitosis are:
- Prophase. In prophase, the nucleoli disappear from the nucleus and the chromatin condenses and forms chromosomes. The two centrosomes move to opposite ends of the cell and form the mitotic spindle.
- Prometaphase. The nuclear membrane (or envelope) disappears, which allows the spindle fibers to interact with the chromosomes. Protein structures called kinetochores appear at the centromere of each sister chromatid. The kinetochores attach to opposite poles of the spindle.
- Metaphase. In metaphase, all the paired structures in the cell are lined up midway between the spindle poles at the cell's equator. This position is called the metaphase plate.
- Anaphase. In anaphase, each of the sister chromatids moves toward one of the poles of the dividing cell. The movement is caused by motor proteins in the kinetochores and the shortening of microtubules in the kinetochores.
- Telophase. In telophase, the nuclear membrane reforms around the nucleus in each daughter cell, the spindle breaks down, and the chromatin begins to uncoil and disperse within the nucleus.
A special method of cell division occurs during maturation of sex cells (sperm and ova) known as meiosis. In meiosis, each daughter cell receives only half of the number of chromosomes found in the parent cell, as there is no S phase in meiosis.
Cells may die from either direct injury or from a process of self-destruction called apoptosis. Cells that are damaged by mechanical trauma or toxic chemicals swell because the plasma membrane can no longer control the movement of ions and water into the cell. The cell contents then leak out, causing inflammation in nearby cells.
In apoptosis, which is also called programmed cell death or PCD, the cell essentially commits suicide. Apoptosis is necessary to the body's health as part of normal functioning (as when the lining of a woman's uterus sloughs off during menstruation) or to destroy dangerous cells. Dangerous cells include cells infected with viruses, cells with damaged DNA, and cancer cells.
There are three possible pathways to apoptosis. The first is triggered by signals within the cell itself
Aneuploidy— A condition in which a cell no longer has the correct number of chromosomes because it has gained or lost entire chromosomes.
Apoptosis— The programmed death of a single cell, characterized by shrinkage and eventual disintegration. Apoptosis is sometimes called cell suicide or programmed cell death (PCD).
Centriole— Either of two cylindrical organelles located in the centrosomes of cells that play an important role in cell division.
Centromere— The constricted portion of a chromosome where the spindle fibers attach during cell division. The centromere may be located in the center of the chromosome or off-center, toward one end or the other.
Centrosome— A specialized area of cytoplasm near the nucleus of a cell that contains the two centrioles. It is sometimes called the centrosphere.
Chemotaxis— The orientation of a cell's movement either toward or away from a chemical stimulus.
Chromatin— The readily stainable portion of the cell nucleus that forms itself into chromosomes during the process of cell division.
Chromosome— One of a set of threadlike coiled structures composed of chromatin and a protein that form in the nucleus of a cell during cell division. There are 46 chromosomes in a normal human cell.
Cilia (singular, cilium)— Tiny hairlike processes extending from the surface of some specialized cells that help the cell to move or serve to move water or mucus over the cell's surface.
Cisternae (singular, cisterna)— Small closed sacs in the Golgi complex that serve as containers or reservoirs.
Codon— A set of three adjacent nucleotides on a single strand of DNA or RNA.
Cristae (singular, crista)— The small folds or ridges in the inner membrane of a mitochondrion.
Cytokine— A generic term for proteins that act as chemical signals among cells.
Cytology— The study of the origins, structures, and functions of cells. The term is also used to refer to the examination of cells obtained from the body for diagnostic purposes.
Cytoplasm— The protoplasm of a cell outside the nucleus. Most of the chemical processes in the cell take place in the cytoplasm.
Cytoskeleton— An internal reinforcement of the cytoplasm of a cell, consisting of various types of microfilaments.
Cytosol— The fluid portion of the cytoplasm in a cell.
Endocytosis— The process by which a cell takes in food particles from its environment by folding inward the part of its plasma membrane where the food particles are resting.
Endoplasmic reticulum— An organelle that consists of a network of tubular membranes within the cytoplasm. The endoplasmic reticulum may be either smooth (containing no ribosomes) or rough (bearing ribosomes on its surface). Rough endoplasmic reticulum is sometimes called granular.
Enzyme— A protein molecule that catalyzes the reactions of other chemical substances within a cell without being destroyed or altered by the reactions.
Eukaryotic— A type of cell containing a true nucleus (one that is bound by a nuclear membrane). All cells in the human body are eukaryotic.
Exocytosis— The expulsion from the cell of particles that are too large to pass through the plasma membrane by diffusion; the opposite of endocytosis.
Golgi complex— A complex structure within the cytoplasm consisting of several layers of flattened sacs that play a role in the formation of glycoproteins. The Golgi complex is also known as the Golgi apparatus or Golgi body. It is named for the Italian neurologist Camillo Golgi (1843–1926).
Kinetochore— A protein structure located beside the centromere on a chromosome. The spindle fibers attach to the kinetochore during the process of cell division.
Lysosome— An organelle that contains enzymes involved in the process of intracellular digestion. Lysosomes also help to destroy damaged cells during apoptosis; they are sometimes called "suicide bags" for this reason.
Meiosis— A special process of cell division that occurs during the maturation of sex cells, in which each daughter cell receives only half the number of chromosomes that are present in the organism's somatic (body) cells.
Mitochondrion (plural, mitochondria)— A small spherical or rod-shaped organelle responsible for energy generation and respiration.
Mitosis— The process of cell division by which the body grows and replaces damaged cells.
Nucleolus (plural, nucleoli)— A small rounded body present within the nucleus of most cells that contains RNA for protein manufacture. Some cells contain two or more nucleoli.
Nucleotide— Any of a group of molecules that are the building blocks of RNA or DNA when linked together.
Nucleus (plural, nuclei)— A spherical body within the cell, containing one or more nucleoli, and surrounded by a thin nuclear membrane.
Organelle— Any of the organized structures present within the cytoplasm of a cell, such as the mitochondria, Golgi complex, lysosomes, etc.
Osmosis— The diffusion of molecules of water (or other solvent) across a semipermeable membrane from a solution with a lower concentration of a solute (dissolved substance) to a solution with a higher concentration of the solute.
Physiology— The branch of science that deals with the functions and activities of living organisms and their parts.
Plasma membrane— The outer membrane of a cell that contains proteins and controls cellular traffic. It is also known as the cell membrane.
Prokaryotic— A type of cell that lacks a true nucleus, in contrast to eukaryotic cells.
Respiration— The exchange of carbon dioxide and oxygen between the environment and the cells of the body.
Ribosome— A small organelle that serves as the site of protein manufacture.
Transcription— The process in which messenger RNA is synthesized on a template of DNA.
Translation— The process in which messenger RNA specifies the order of amino acids lined up on a ribosome for protein synthesis.
Vacuole— Any small space or cavity formed within the protoplasm of a cell.
Viscosity— The resistance to flow of a fluid substance. The viscosity of cells is constantly changing.
related to damage to the mitochondria. A cascade of protein-dissolving enzymes is released that leads to digestion of the structural proteins in the cytoplasm and phagocytosis of the cell itself. A second pathway is external: death activators such as tumor necrosis factor or a chemical called lymphotoxin bind to receptors on the cell surface which in turn trigger the protein-dissolving cascade leading to phagocytosis of the cell. Nerve cells follow a third pathway to apoptosis. Their mitochondria contain a compound called apoptosisinducing factor or AIF. When the nerve cell self-destructs, the AIF is released from the mitochondria, migrates into the cell nucleus, and destroys the cell's DNA.
Transcription, translation, and protein synthesis
Still another important function of cells is the encoding of proteins through the mechanisms of transcription and translation. Transcription refers to the formation of messenger RNA (mRNA) from a template of DNA, whereas translation refers to the production of proteins by mRNA. In eukaryotic cells, transcription takes place within the cell nucleus, while translation occurs in the cytoplasm. In short, the cell's DNA determines the cell's functioning through controlling the process of protein synthesis through transcription and translation.
In transcription, there are about 50 different transcription factors that bind to promoter sites on the gene to be transcribed. An enzyme known as an RNA polymerase binds to the transcription factors. The transcription factors and the enzyme work together to open the double helix of the DNA. The RNA polymerase then moves down one strand of the DNA, the transcribed mRNA gradually elongating behind it. The double helix rewinds itself behind the moving polymerase. When transcription is complete, the polymerase releases the transcript and is itself released from the DNA.
In translation, the mRNA moves from the nucleus of the cell into the surrounding cytoplasm. Translation requires mRNA, another type of RNA called transfer RNA (tRNA), amino acids, and a ribosome located on rough endoplasmic reticulum. The molecules of tRNA are specific for one type of amino acid and for a specific triplet of nucleotides in the messenger RNA called a codon. There are 64 different codons; one (AUG) signals the start of translation and three (UAA, UAG, and UGA) are called stop codons because they signal the end of the translation process.
Translation has three stages or phases: initiation, elongation, and termination. In initiation, a subunit of the ribosome binds to a site on the messenger RNA preceding the start of the message. The ribosome moves "downstream" until it encounters the AUG start codon, where it is joined by a special initiator tRNA carrying the amino acid methionine. In elongation, the initiator tRNA is released and the ribosome moves one codon downstream. Another type of tRNA carrying an amino acid that fits the next codon arrives. The incoming amino acid is linked to the preceding amino acid by a peptide bond. The termination phase is reached when the ribosome reaches one of the three stop codons. A protein release factor then releases the newly synthesized protein from the ribosome.
Role in human health
Cellular functions are essential to normal human development and health at all ages. Cell biologists maintain that all human diseases can ultimately be traced to the failure of one or another cell mechanism or function.
Common diseases and disorders
Some examples of diseases resulting from failure of specific cell structures or functions are listed here:
- Apoptosis. P53 is a protein that stimulates apoptosis in cells with damaged DNA. A mutation in the gene that codes for p53 is the most common mutation leading to cancer. The mutation essentially allows cells with damaged DNA to continue to reproduce and eventually form a tumor rather than destroying themselves.
- Cell migration. Failure of cells to migrate to their proper locations in a developing embryo or to form appropriate connections with nearby cells may lead to miscarriage (loss of the pregnancy), epilepsy, mental retardation, and other abnormalities in brain development.
- Meiosis. It is estimated that 10-20 percent of all fertilized human ova contain chromosomal abnor-malities that occur during the first stage of meiosis. The most common form of chromosomal abnormality is the gain or loss of entire chromosomes, known as aneuploidy. Down's syndrome results from three copies (trisomy) of chromosome 21. Other disorders associated with aneuploidy are Edwards syndrome (trisomy 18), Patau syndrome (trisomy 13), Turner syndrome (loss of the second X chromosome in a female), Klinefelter syndrome (one or more extra X chromosomes in a male). In many cases aneuploidy results in miscarriage or death in infancy.
- Cilia. Inherited defects in the formation of the cilia lining the kidney tubules cause polycystic kidney disease.
- Mitochondria. Mutations in the cytochrome b gene in the mitochondria produce disorders of the brain and muscles, particularly a form of exercise intolerance. Other disorders related to mutations in the DNA of the mitochondria include a hereditary form of early visual loss, hearing loss, a progressive form of myoclonic epilepsy, and a stroke-like syndrome.
- Transcription. Faulty transcription can produce mRNA molecules that have premature stop codons. Translation then produces abbreviated versions of proteins that either fail to carry out their functions in the body or are actively harmful.
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American College of Medical Genetics (ACMG). 9650 Rockville Pike, Bethesda, MD 20814. (301) 634-7127. Fax: (301) 634-7275. 〈http://www.acmg.net〉.
American Society for Cell Biology (ASCB). 8120 Woodmont Avenue, Suite 750, Bethesda, MD 20814-2762. (301) 347-9300. Fax: (301) 347-9310. 〈http://www.ascb.org〉.
American Society of Human Genetics (ASHG). 9650 Rockville Pike, Bethesda, MD, 20814. (301) 634-7300 or (866) HUM-GENE. 〈http://genetics.faseb.org/genetics/ashg/ashgmenu.htm〉.
National Center for Biotechnology Information (NCBI), National Library of Medicine. Building 38A, Bethesda, MD 20894. (301) 496-2475. Fax: (301) 480-9241. 〈http://www.ncbi.nlm.nih.gov〉.
National Human Genome Research Institute (NHGRI). Building 31, Room 4B09, 31 Center Drive, MSC 2152, 9000 Rockville Pike, Bethesda, MD 20892. (301) 402-0911. Fax: (301) 402-2218. 〈http://www.genome.gov〉.
Cells Alive!〈http://www.cellsalive.com/toc.htm〉. This highly-recommended website contains movies, computer animations, and interactive features for students of cell biology, microbiology, and immunology.
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