HUMAN GENETICS

Table of Contents

The human karyotype | Human chromosomal abnormalities

Human allelic disorders (recessive) | Human allelic disorders (dominant)

Sex-linked traits | Diagnosis of human genetic diseases | Radioactive probes

Links

The human karyotype | Back to Top

There are 44 autosomes and 2 sex chromosomes in the human genome, for a total of 46. Karyotypes are pictures of homologous chromosomes lined up together during Metaphase I of meiosis. The chromosome micrographs are then arranged by size and pasted onto a sheet.

Click here for a larger picture. This picture is from The Primate Cytogenetics Network at ( http://www.selu.com/~bio/cyto/karyotypes/Hominidae/Hominidae.html).

Human chromosomal abnormalities | Back to Top

A common abnormality is caused by nondisjunction, the failure of replicated chromosomes to segregate during Anaphase II. A gamete lacking a chromosome cannot produce a viable embryo. Occasionally a gamete with n+1 chromosomes can produce a viable embryo.

In humans, nondisjunction is most often associated with the 21st chromosome, producing a disease known as Down's syndrome (also referred to as trisomy 21). Sufferers of Down's syndrome suffer mild to severe mental retardation, short stocky body type, large tongue leading to speech difficulties, and (in those who survive into middle-age), a propensity to develop Alzheimer's Disease. Ninety-five percent of Down's cases result from nondisjunction of chromosome 21. Occasional cases result from a translocation in the chromosomes of one parent. Remember that a translocation occurs when one chromosome (or a fragment) is transferred to a non-homologous chromosome. The incidence of Down's Syndrome increases with age of the mother, although 25% of the cases result from an extra chromosome from the father. Click here to view a drawing (from Bioweb) of a karyotype of Down's syndrome.

Sex-chromosome abnormalities may also be caused by nondisjunction of one or more sex chromosomes. Any combination (up to XXXXY) produces maleness. Males with more than one X are usually underdeveloped and sterile. XXX and XO women are known, although in most cases they are sterile. What meiotic difficulties might a person with Down's syndrome or extra sex-chromosomes face?

Human sex chromosome abnormalities. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Kleinfelter's syndrome (click here to view a karyotype from Bioweb) How does this differ from the normal karyotype?

Turner's syndrome (click here to view a karyotype from Bioweb) How does this differ from the normal karyotype?

Chromosome deletions may also be associated with other syndromes such as Wilm's tumor.

Prenatal detection of chromosomal abnormalities is accomplished chiefly by amniocentesis. A thin needle is inserted into the amniotic fluid surrounding the fetus (a term applied to an unborn baby after the first trimester). Cells are withdrawn have been sloughed off by the fetus, yet they are still fetal cells and can be used to determine the state of the fetal chromosomes, such as Down's Syndrome and the sex of the baby after a karyotype has been made.

Human Allelic Disorders (Recessive) | Back to Top

The first Mendelian trait in humans was described in 1905 (brachydactly) by Dr. Farabee (no relation to your author). Now more than 3500 human genetic traits are known.

Albinism, the lack of pigmentation in skin, hair, and eyes, is also a Mendelian human trait. Homozygous recessive (aa) individuals make no pigments, and so have face, hair, and eyes that are white to yellow. For heterozygous parents with normal pigmentation (Aa), two different types of gametes may be produced: A or a. From such a cross 1/4 of the children could be albinos. The brown pigment melanin cannot be made by albinos. Several mutations may cause albinism: 1) the lack of one or another enzyme along the melanin-producing pathway; or 2) the inability of the enzyme to enter the pigment cells and convert the amino acid tyrosine into melanin.

Phenylketonuria (PKU) is recessively inherited disorder whose sufferers lack the ability to synthesize an enzyme to convert the amino acid phenylalanine into tyrosine Individuals homozygous recessive for this allele have a buildup of phenylalanine and abnormal breakdown products in the urine and blood. The breakdown products can be harmful to developing nervous systems and lead to mental retardation. 1 in 15,000 infants suffers from this problem. PKU homozygotes are now routinely tested for in most states. If you look closely at a product containing Nutra-sweet artificial sweetener, you will see a warning to PKU sufferers since phenylalanine is one of the amino acids in the sweetener. PKU sufferers are placed on a diet low in phenylalanine, enough for metabolic needs but not enough to cause the buildup of harmful intermediates.

Tay-Sachs Disease is an autosomal recessive resulting in degeneration of the nervous system. Symptoms manifest after birth. Children homozygous recessive for this allele rarely survive past five years of age. Sufferers lack the ability to make the enzyme N-acetyl-hexosaminidase, which breaks down the GM2 ganglioside lipid. This lipid accumulates in lysosomes in brain cells, eventually killing the brain cells. Although rare in the general population (1 in 300,000 births), it was (until recently) higher (1 in 3600 births) among Jews of eastern central European descent. One in 28 American Jews is thought to be a carrier, since 90% of the American Jewish population emigrated from those areas in Europe. Most Tay-Sachs babies born in the US are born to non-Jewish parents, who did not undergo testing programs that most US Jewish prospective parents had.

Sickle-cell anemia is an autosomal recessive we have discussed in other sections. Nine-percent of US blacks are heterozygous, while 0.2% are homozygous recessive. The recessive allele causes a single amino acid substitution in the beta chains of hemoglobin. When oxygen concentration is low, sickling of cells occurs. Heterozygotes make enough "good beta-chain hemoglobin" that they do not suffer as long as oxygen concentrations remain high, such as at sea-level.

Human Allelic Disorders (Dominant) | Back to Top

Autosomal dominants are rare, although they are (by definition) more commonly expressed.

Achondroplastic dwarfism occurs, even though sufferers have reduced fertility.

Huntington's disease (also referred to as Woody Guthrie's disease, after the folk singer who died in the 1960s) is an autosomal dominant resulting in progressive destruction of brain cells. If a parent has the disease, 50% of the children will have it (unless that parent was homozygous dominant, in which case all children would have the disease). The disease usually does not manifest until after age 30, although some instances of early onset phenomenon are reported among individuals in their twenties.

Polydactly is the presence of a sixth digit. In modern times the extra finger has been cut off at birth and individuals do not know they carry this trait. One of the wives of Henry VIII had an extra finger. In certain southern families the trait is also more common. The extra digit is rarely functional and definitely causes problems buying gloves, let alone fitting them on during a murder trial.

Sex-linked Traits | Back to Top

Color blindness afflicts 8% of males and 0.04 % of human females. Color perception depends on three genes, each producing chemicals sensitive to different parts of the visible light spectrum. Red and green detecting genes are on the X-chromosome, while the blue detection is on an autosome.

Hemophilia is a group of diseases in which blood does not clot normally. Factors in blood are involved in clotting. Hemophiliacs lacking the normal Factor VIII are said to have Hemophilia A, the most common form. Normal Factor VIII can be supplied at a high dollar and health risk cost, although the development of biotechnologically engineered Factor VIII produced by bacteria lessens the health risk. England's Queen Victoria was a carrier for this disease. The allele was passed to two of her daughters and one son. Since royal families in Europe commonly intermarried, the allele spread, and may have contributed to the downfall of the Russian monarchy (Czar Nicholas' son Alexei suffered from hemophilia A inherited from his mother who carried Victoria's genetic secret).

Inheritance of a human sex-linked trait. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Muscular dystrophy is a term encompassing a variety of muscle wasting diseases. The most common type, Duchenne Muscular Dystrophy (DMD), affects cardiac and skeletal muscle, as well as some mental functions. DMD is an X-linked recessive occurring in 1 in 3500 newborns. Most sufferers die before their 20th birthday. In 1987, Louis Kunkel claimed to have isolated a protein, dystrophin, present in normal individuals (about 0.002 % of their muscle protein) but absent in two individuals with DMD. The lack of dystrophin is accompanied with a condition of muscle hardening known as fibrosis, which restricts blood supply to the muscle which then die. The gene technologies discussed in an earlier chapter have been employed to sequence and clone the dystrophin gene, which is the largest known human gene (some 2-3 million base pairs), with 60 exons and many large introns.

Diagnosis of Human Genetic Diseases | Back to Top

Restriction enzymes, such as Hpa I were used in a study on sickle-cell anemia. The probe hybridized in normal hemoglobin with two fragments 7000 or 7600 nucleotides long. Sickle-cell hemoglobin had hybridization with a 13,000 nucleotide single sequence. A similar result has been obtained from amniocentesis studies, providing a tool to screen fetus and adult for sickle-cell. The markers where hybridization occurred are referred to as RFLPs (restriction-fragment-length polymorphisms). The longer fragment in sickle-cell individuals is interpreted as evidence of a mutation in the recognition sequence. Two nucleotide sequences close together on the same DNA molecule tend to stay together. In the sickle-cell DNA the beta-chain hemoglobin gene has become linked with another gene that somehow alters the recognition sequence at which Hpa I hybridizes. Heterozygotes will have both long and short fragments, while a single type (short or long) will occur in homozygous dominant and recessive, respectively.

Huntington's disease was studied by James F. Gusella and his research team, who used RFLPs to identify a marker. Testing a large library of human DNA fragments, Gusella et al. found the needle in the haystack. The enzyme used was Hind III. Four fragments have been identified in an American family that has members suffering from the disease. The presence of fragment A has been identified in individuals who suffer from (or will suffer from) Huntington's. Pattern A occurs in 60 percent of the population, as well as the Huntington's sufferers. A Venezuelan family of 3000 members is descended from a German sailor who had Huntington's. This family had a strong correlation between Fragment C and the disease. Pattern C is much less common among the general population in this country. Many individuals do not wish to know if they will develop this disease; Woody Guthrie's children have chosen not to be tested.

Cystic fibrosis (CF) has also been studied with RFLP technology. CF is the most common genetic disease in Caucasians.

Radioactive probes | Back to Top

Hemophiliacs suffer from defective Factor VIII, which can be detected in fetuses 20 weeks old. A more accurate test, which can also be administered earlier during pregnancy, involves the use of a radioactive probe (36 nucleotide RNA fragment) which hybridizes restriction fragments. The gene for hemophilia is 186,000 base pairs, and has 26 exons separated by 25 introns. Mutations in the gene can be detected by RFLPs. This technology has also been used to detect the single base-pair difference between normal and mutated beta-chains, a screen for sickle-cell anemia. A DNA probe has been developed that hybridizes with the gene for dystrophin. The previous screening for Duchenne Muscular Dystrophy was a sex screen, with option to abort a male. The new technique allows differentiation between the healthy and diseased male fetus, so parents have more information with which to make an informed choice (if they chose). The hybridization only occurs if the normal dystrophin gene is present, no hybridization occurs in the DMD sufferer.

Links | Back to Top

• A Gene Map of the Human Genome As of mid 1996. Click on a chromosome number and view an enlarged map of that chromosome, continue clicking for details about that chromosome's genes and their sequences.
• Primate Cytogenetics Network Karyotypes of a number of primates.
• Gene Cards (Weizmann Institute) Search a database for information about a number of human genetic disorders.
• Chromosome Abnormality Databases
• Human Karyotypes Exhibiting Various Abnormalities (from Bioweb) Drawings of a number of human karyotypes.
• Human Cytogenetics The human karyotype and ideograms of each chromosome. The chromosome ideograms are NOT connected to the web as they are at other sites.
• X-Chromosome Mapping Project (Max Planck Institut)
• Unigene A searchable database of human genetic transcripts.
• Genetics Education Center (University of Kansas)
• ECCLES INSTITUTE OF HUMAN GENETICS A collection of local and other links involving human genetics.
• THE DOPAMINE D2 RECEPTOR AS A CANDIDATE GENE FOR ALCOHOLISM An honors thesis on the possible alcoholism gene, by Karen Johnson.
• An Introduction to Skin Cancer
• Breast Cancer Case Study (the Virtual Health Care Team)
• Sickle Cell Anemia Information Center
• Tay Sachs Disease March of Dimes Public Information Fact Sheet
• Turner's Syndrome

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Text ©1992, 1994, 1997, 2000, 2001, by M.J. Farabee, all rights reserved. Use for educational purposes is encouraged.

Back to Table of Contents |

Email: mj.farabee@emcmail.maricopa.edu

Last modified: Tuesday May 18 2010

The URL of this page is: www2.estrellamountain.edu/faculty/farabee/biobk/BioBookhumgen.html

REACTIONS AND ENZYMES

Table of Contents

Endergonic and exergonic | Oxidation/Reduction | Catabolism and Anabolism

Enzymes: Organic Catalysts | Learning Objectives | Links

Endergonic and exergonic | Back to Top

Energy releasing processes, ones that "generate" energy, are termed exergonic reactions. Reactions that require energy to initiate the reaction are known as endergonic reactions. All natural processes tend to proceed in such a direction that the disorder or randomness of the universe increases (the second law of thermodynamics).

Time-energy graphs of an exergonic reaction (top) and endergonic reaction (bottom). Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Oxidation/Reduction | Back to Top

Biochemical reactions in living organisms are essentially energy transfers. Often they occur together, "linked", in what are referred to as oxidation/reduction reactions. Reduction is the gain of an electron. Sometimes we also have H ions along for the ride, so reduction also becomes the gain of H. Oxidation is the loss of an electron (or hydrogen). In oxidation/reduction reactions, one chemical is oxidized, and its electrons are passed (like a hot potato) to another (reduced, then) chemical. Such coupled reactions are referred to as redox reactions. The metabolic processes glycolysis, Kreb's Cycle, and Electron Transport Phosphorylation involve the transfer of electrons (at varying energy states) by redox reactions.

Passage of electrons from compound A to compound B. When A loses its electrons it is oxidized; when B gains the electrons it is reduced. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Oxidation/reduction via an intermediary (energy carrier) compound, in this case NAD⁺. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Catabolism and Anabolism | Back to Top

Anabolism is the total series of chemical reactions involved in synthesis of organic compounds. Autotrophs must be able to manufacture (synthesize) all the organic compounds they need. Heterotrophs can obtain some of their compounds in their diet (along with their energy). For example humans can synthesize 12 of the 20 amino acids, we must obtain the other 8 in our diet. Catabolism is the series of chemical reactions that breakdown larger molecules. Energy is released this way, some of it can be utilized for anabolism. Products of catabolism can be reassembled by anabolic processes into new anabolic molecules.

Enzymes: Organic Catalysts | Back to Top

Enzymes allow many chemical reactions to occur within the homeostasis constraints of a living system. Enzymes function as organic catalysts. A catalyst is a chemical involved in, but not changed by, a chemical reaction. Many enzymes function by lowering the activation energy of reactions. By bringing the reactants closer together, chemical bonds may be weakened and reactions will proceed faster than without the catalyst.

The use of enzymes can lower the activation energy of a reaction (E_a). Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Enzymes can act rapidly, as in the case of carbonic anhydrase (enzymes typically end in the -ase suffix), which causes the chemicals to react 107 times faster than without the enzyme present. Carbonic anhydrase speeds up the transfer of carbon dioxide from cells to the blood. There are over 2000 known enzymes, each of which is involved with one specific chemical reaction. Enzymes are substrate specific. The enzyme peptidase (which breaks peptide bonds in proteins) will not work on starch (which is broken down by human-produced amylase in the mouth).

Enzymes are proteins. The functioning of the enzyme is determined by the shape of the protein. The arrangement of molecules on the enzyme produces an area known as the active site within which the specific substrate(s) will "fit". It recognizes, confines and orients the substrate in a particular direction.

Space filling model of an enzyme working on glucose. Note the shape change in the enzyme (indicated by the red arrows) after glucose has fit into the binding or active site. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The induced fit hypothesis suggests that the binding of the substrate to the enzyme alters the structure of the enzyme, placing some strain on the substrate and further facilitating the reaction. Cofactors are nonproteins essential for enzyme activity. Ions such as K⁺ and Ca⁺² are cofactors. Coenzymes are nonprotein organic molecules bound to enzymes near the active site. NAD (nicotinamide adenine dinucleotide).

A cartoonish view of the formation of an enzyme-substrate complex. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Enzymatic pathways form as a result of the common occurrence of a series of dependent chemical reactions. In one example, the end product depends on the successful completion of five reactions, each mediated by a specific enzyme. The enzymes in a series can be located adjacent to each other (in an organelle or in the membrane of an organelle), thus speeding the reaction process. Also, intermediate products tend not to accumulate, making the process more efficient. By removing intermediates (and by inference end products) from the reactive pathway, equilibrium (the tendency of reactions to reverse when concentrations of the products build up to a certain level) effects are minimized, since equilibrium is not attained, and so the reactions will proceed in the "preferred" direction.

Negative feedback and a metabolic pathway. The production of the end product (G) in sufficient quantity to fill the square feedback slot in the enzyme will turn off this pathway between step C and D. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Temperature: Increases in temperature will speed up the rate of nonenzyme mediated reactions, and so temperature increase speeds up enzyme mediated reactions, but only to a point. When heated too much, enzymes (since they are proteins dependent on their shape) become denatured. When the temperature drops, the enzyme regains its shape. Thermolabile enzymes, such as those responsible for the color distribution in Siamese cats and color camouflage of the Arctic fox, work better (or work at all) at lower temperatures.

Concentration of substrate and product also control the rate of reaction, providing a biofeedback mechanism.

Activation, as in the case of chymotrypsin, protects a cell from the hazards or damage the enzyme might cause.

Changes in pH will also denature the enzyme by changing the shape of the enzyme. Enzymes are also adapted to operate at a specific pH or pH range.

Plot of enzyme activity as a function of pH for several enzymes. Note that each enzyme has a range of pH at which it is active as well as an optimal pH at which it is most active. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Allosteric Interactions may allow an enzyme to be temporarily inactivated. Binding of an allosteric effector changes the shape of the enzyme, inactivating it while the effector is still bound. Such a mechanism is commonly employed in feedback inhibition. Often one of the products, either an end or near-end product act as an allosteric effector, blocking or shunting the pathway.

Action of an allosteric inhibitor as a negative control on the action of an enzyme. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Competitive Inhibition works by the competition of the regulatory compound and substrate for the binding site. If enough regulatory compound molecules bind to enough enzymes, the pathway is shut down or at least slowed down. PABA, a chemical essential to a bacteria that infects animals, resembles a drug, sulfanilamide, that competes with PABA, shutting down an essential bacterial (but not animal) pathway.

Top: general diagram showing competitor in the active site normally occupied by the natural substrate; Bottom: specific case of succinate dehydrogenase and its natural substrate (succinate) and competitors (oxalate et al.). Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Noncompetitive Inhibition occurs when the inhibitory chemical, which does not have to resemble the substrate, binds to the enzyme other than at the active site. Lead binds to SH groups in this fashion. Irreversible Inhibition occurs when the chemical either permanently binds to or massively denatures the enzyme so that the tertiary structure cannot be restored. Nerve gas permanently blocks pathways involved in nerve message transmission, resulting in death. Penicillin, the first of the "wonder drug" antibiotics, permanently blocks the pathways certain bacteria use to assemble their cell wall components.

Learning Objectives | Back to Top

• Reactions that show a net loss in energy are said to be exergonic; reactions that show a net gain in energy are said to be endergonic. Describe an example of each type of chemical reaction from everyday life.
• What is meant by a reversible reaction? How might this be significant to living systems?
• What is the function of metabolic pathways in cellular chemistry? Want more? Try Metabolic Pathways of Biochemistry.
• What are enzymes? Explain their importance.
• Explain what happens when enzymes react with substrates.

Links | Back to Top

• Biology Project (U of A) Energy and Enzymes Problem Set
• The G6PD Deficiency Homepage 400 Million folks have this problem, and it is enzymatic!
• Enzyme Inhibition and Regulation (3/1/96) From WSU's chemistry site.
• MIT Hypertextbook Enzyme Chapter
• Enzyme Reaction Tutorial (U.C. Davis)
• EC Enzyme Search the EC enzyme databse, includes links to OMIM (Online Mendelain Inheritance in Man) and SWISSPROT (Swiss Protein Database).
• Interactive Cytochrome Oxidase You will need the Chime plugin (available at this site), but it will be well worth it. View either of the subunits of cytochrome oxidase as well as related molecules. You can check buttons on the left frame to display selected portions of the molecule, zoom in, and zoom out.
• Metabolic Pathways of Biochemistry Check out the metabolic pathway of your choice in 2-D or 3-D (with the Chime plugin) models. is provided than in most general biology textbooks, but the point is driven home...a chemical reaction in one of these pathways needs its own enzyme.

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Text ©1992, 1994, 1997, 1999, 2000, 2001, by M.J. Farabee. Use for educational purposes is encouraged.

Back to Table of Contents | Go To ATP AND BIOLOGICAL ENERGY

Email: mj.farabee@emcmail.maricopa.edu

Last modified: Tuesday May 18 2010

The URL of this page is: www2.estrellamountain.edu/faculty/farabee/biobk/BioBookEnzym.html

TRANSPORT IN AND OUT OF CELLS

Table of Contents

Water and Solute Movement | The Cell Membrane | Cells and Diffusion | Active and Passive Transport | Carrier-assisted Transport

Types of transport molecules | Vesicle-mediated transport | Learning Objectives | Terms | Links | References

Water and Solute Movement | Back to Top

Cell membranes act as barriers to most, but not all, molecules. Development of a cell membrane that could allow some materials to pass while constraining the movement of other molecules was a major step in the evolution of the cell. Cell membranes are differentially (or semi-) permeable barriers separating the inner cellular environment from the outer cellular (or external) environment.

Water potential is the tendency of water to move from an area of higher concentration to one of lower concentration. Energy exists in two forms: potential and kinetic. Water molecules move according to differences in potential energy between where they are and where they are going. Gravity and pressure are two enabling forces for this movement. These forces also operate in the hydrologic (water) cycle. Remember in the hydrologic cycle that water runs downhill (likewise it falls from the sky, to get into the sky it must be acted on by the sun and evaporated, thus needing energy input to power the cycle).

The hydrologic cycle. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Diffusion is the net movement of a substance (liquid or gas) from an area of higher concentration to one of lower concentration. You are on a large (10 ft x 10 ft x10 ft) elevator. An obnoxious individual with a lit cigar gets on at the third floor with the cigar still burning. You are also unfortunate enough to be in a very tall building and the person says "Hey we're both going to the 62nd floor!" Disliking smoke you move to the farthest corner you can. Eventually you are unable to escape the smoke! An example of diffusion in action. Nearer the source the concentration of a given substance increases. You probably experience this in class when someone arrives freshly doused in perfume or cologne, especially the cheap stuff.

Since the molecules of any substance (solid, liquid, or gas) are in motion when that substance is above absolute zero (0 degrees Kelvin or -273 degrees C), energy is available for movement of the molecules from a higher potential state to a lower potential state, just as in the case of the water discussed above. The majority of the molecules move from higher to lower concentration, although there will be some that move from low to high. The overall (or net) movement is thus from high to low concentration. Eventually, if no energy is input into the system the molecules will reach a state of equilibrium where they will be distributed equally throughout the system.

The Cell Membrane | Back to Top

The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell. Electron microscopic examinations of cell membranes have led to the development of the lipid bilayer model (also referred to as the fluid-mosaic model). The most common molecule in the model is the phospholipid, which has a polar (hydrophilic) head and two nonpolar (hydrophobic) tails. These phospholipids are aligned tail to tail so the nonpolar areas form a hydrophobic region between the hydrophilic heads on the inner and outer surfaces of the membrane. This layering is termed a bilayer since an electron microscopic technique known as freeze-fracturing is able to split the bilayer.

Diagram of a phospholipid bilayer. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Phospholipids and glycolipids are important structural components of cell membranes. Phospholipids are modified so that a phosphate group (PO₄^-) replaces one of the three fatty acids normally found on a lipid. The addition of this group makes a polar "head" and two nonpolar "tails".

Structure of a phospholipid, space-filling model (left) and chain model (right). Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Diagram of a cell membrane. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Cell Membranes from Opposing Neurons (TEM x436,740). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Cholesterol is another important component of cell membranes embedded in the hydrophobic areas of the inner (tail-tail) region. Most bacterial cell membranes do not contain cholesterol.

Proteins are suspended in the inner layer, although the more hydrophilic areas of these proteins "stick out" into the cells interior as well as the outside of the cell. These integral proteins are sometimes known as gateway proteins. Proteins also function in cellular recognition, as binding sites for substances to be brought into the cell, through channels that will allow materials into the cell via a passive transport mechanism, and as gates that open and close to facilitate active transport of large molecules.

The outer surface of the membrane will tend to be rich in glycolipids, which have their hydrophobic tails embedded in the hydrophobic region of the membrane and their heads exposed outside the cell. These, along with carbohydrates attached to the integral proteins, are thought to function in the recognition of self. Multicellular organisms may have some mechanism to allow recognition of those cells that belong to the organism and those that are foreign. Many, but not all, animals have an immune system that serves this sentry function. When a cell does not display the chemical markers that say "Made in Mike", an immune system response may be triggered. This is the basis for immunity, allergies, and autoimmune diseases. Organ transplant recipients must have this response suppressed so the new organ will not be attacked by the immune system, which would cause rejection of the new organ. Allergies are in a sense an over reaction by the immune system. Autoimmune diseases, such as rheumatoid arthritis and systemic lupus erythmatosis, happen when for an as yet unknown reason, the immune system begins to attack certain cells and tissues in the body.

Cells and Diffusion | Back to Top

Water, carbon dioxide, and oxygen are among the few simple molecules that can cross the cell membrane by diffusion (or a type of diffusion known as osmosis ). Diffusion is one principle method of movement of substances within cells, as well as the method for essential small molecules to cross the cell membrane. Gas exchange in gills and lungs operates by this process. Carbon dioxide is produced by all cells as a result of cellular metabolic processes. Since the source is inside the cell, the concentration gradient is constantly being replenished/re-elevated, thus the net flow of CO₂ is out of the cell. Metabolic processes in animals and plants usually require oxygen, which is in lower concentration inside the cell, thus the net flow of oxygen is into the cell.

Osmosis is the diffusion of water across a semi-permeable (or differentially permeable or selectively permeable) membrane. The cell membrane, along with such things as dialysis tubing and cellulose acetate sausage casing, is such a membrane. The presence of a solute decreases the water potential of a substance. Thus there is more water per unit of volume in a glass of fresh-water than there is in an equivalent volume of sea-water. In a cell, which has so many organelles and other large molecules, the water flow is generally into the cell.

Animated image/movie illustrating osmosis (water is the red dots) and the selective permeability of a membrane (blue dashed line). Image from the Internet. Click on image to view movie.

Hypertonic solutions are those in which more solute (and hence lower water potential) is present. Hypotonic solutions are those with less solute (again read as higher water potential). Isotonic solutions have equal (iso-) concentrations of substances. Water potentials are thus equal, although there will still be equal amounts of water movement in and out of the cell, the net flow is zero.

Water relations and cell shape in blood cells. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Water relations in a plant cell. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

One of the major functions of blood in animals is the maintain an isotonic internal environment. This eliminates the problems associated with water loss or excess water gain in or out of cells. Again we return to homeostasis. Paramecium and other single-celled freshwater organisms have difficulty since they are usually hypertonic relative to their outside environment. Thus water will tend to flow across the cell membrane, swelling the cell and eventually bursting it. Not good for any cell! The contractile vacuole is the Paramecium's response to this problem. The pumping of water out of the cell by this method requires energy since the water is moving against the concentration gradient. Since ciliates (and many freshwater protozoans) are hypotonic, removal of water crossing the cell membrane by osmosis is a significant problem. One commonly employed mechanism is a contractile vacuole. Water is collected into the central ring of the vacuole and actively transported from the cell.

The functioning of a contractile vacuole in Paramecium. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Active and Passive Transport | Back to Top

Passive transport requires no energy from the cell. Examples include the diffusion of oxygen and carbon dioxide, osmosis of water, and facilitated diffusion.

Types of passive transport. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Active transport requires the cell to spend energy, usually in the form of ATP. Examples include transport of large molecules (non-lipid soluble) and the sodium-potassium pump.

Types of active transport. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Carrier-assisted Transport | Back to Top

The transport proteins integrated into the cell membrane are often highly selective about the chemicals they allow to cross. Some of these proteins can move materials across the membrane only when assisted by the concentration gradient, a type of carrier-assisted transport known as facilitated diffusion. Both diffusion and facilitated diffusion are driven by the potential energy differences of a concentration gradient. Glucose enters most cells by facilitated diffusion. There seem to be a limiting number of glucose-transporting proteins. The rapid breakdown of glucose in the cell (a process known as glycolysis) maintains the concentration gradient. When the external concentration of glucose increases, however, the glucose transport does not exceed a certain rate, suggesting the limitation on transport.

In the case of active transport, the proteins are having to move against the concentration gradient. For example the sodium-potassium pump in nerve cells. Na⁺ is maintained at low concentrations inside the cell and K⁺ is at higher concentrations. The reverse is the case on the outside of the cell. When a nerve message is propagated, the ions pass across the membrane, thus sending the message. After the message has passed, the ions must be actively transported back to their "starting positions" across the membrane. This is analogous to setting up 100 dominoes and then tipping over the first one. To reset them you must pick each one up, again at an energy cost. Up to one-third of the ATP used by a resting animal is used to reset the Na-K pump.

Types of transport molecules | Back to Top

Uniport transports one solute at a time. Symport transports the solute and a cotransported solute at the same time in the same direction. Antiport transports the solute in (or out) and the co-transported solute the opposite direction. One goes in the other goes out or vice-versa.

Vesicle-mediated transport | Back to Top

Vesicles and vacuoles that fuse with the cell membrane may be utilized to release or transport chemicals out of the cell or to allow them to enter a cell. Exocytosis is the term applied when transport is out of the cell.

This GIF animation is from http://www.stanford.edu/group/Urchin/GIFS/exocyt.gif. Note the vesicle on the left, and how it fuses with the cell membrane on the right, expelling the vesicle's contents to the outside of the cell.

Endocytosis is the case when a molecule causes the cell membrane to bulge inward, forming a vesicle. Phagocytosis is the type of endocytosis where an entire cell is engulfed. Pinocytosis is when the external fluid is engulfed. Receptor-mediated endocytosis occurs when the material to be transported binds to certain specific molecules in the membrane. Examples include the transport of insulin and cholesterol into animal cells.

Endocytosis and exocytosis. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Learning Objectives | Back to Top

• Materials are exchanged between the cytoplasm and external cell environment across the plasma membrane by several different processes, some require energy, some do not..
• Describe the general structure of a phospholipid molecule and what makes it suitable as a major component of cell membranes.
• Explain the behavior of a great number of phospholipid molecules in water.
• Describe the most recent version of the fluid mosaic model of membrane structure.
• Molecules moving to regions where they are less concentrated are moving down their concentration gradient.
• Random movement of like molecules or ions down a concentration gradient is called simple diffusion.
• When salt is dissolved in water, which is the solute and which is the solvent?
• Explain osmosis in terms of a differentially permeable membrane.
• Define tonicity and be able to use the terms isotonic, hypertonic, and hypotonic.
• When water moves into a plant cell by osmosis, the internal turgor pressure developed pushes on the wall. What does this do to your understanding of a neglected houseplant?

Terms | Back to Top

Active transport	ATP	diffusion	endocytosis	exocytosis
fluid-mosaic model	glycolipids	glycolysis	homeostasis	hydrophilic
hydrophobic	hypertonic	hypotonic	immune system	isotonic
osmosis	passive transport	phagocytosis	phosphate group	phospholipid
sodium-potassium pump	vacuoles	vesicles

Links | Back to Top

• MIT Hypertextbook: Membrane Transport Mechanisms
• MIT Hypertextbook: Membranes
• Equilibrium Dialysis Experiment Try out osmosis in action! Warning: high numbers content, not for the numerically challenged :)

References | Back to Top

Singer, S. J., and Nicolson, G.L. 1972. The fluid mosaic model of the structure of cell membranes. Science 175, 720-731.

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Text ©1992, 1994, 1997, 1998, 2000, 2001, 2007, by M.J. Farabee, all rights reserved. Use for educational; purposes is encouraged.

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Email: mj.farabee@emcmail.maricopa.edu

Last modified: Tuesday May 18 2010

The URL of this page is: www2.estrellamountain.edu/faculty/farabee/biobk/BioBooktransp.html

CELLS II: CELLULAR ORGANIZATION

Table of Contents

Cell Size and Shape | The Cell Membrane | The Cell Wall | The Nucleus | Cytoplasm | Vacuoles and Vesicles | Ribosomes | Endoplasmic Reticulum | Golgi Apparatus and Dictyosomes | Lysosomes | Mitochondria | Plastids | Cell Movement | Learning Objectives | Terms | Review Questions | Links | References

Life exhibits varying degrees of organization. Atoms are organized into molecules, molecules into organelles, and organelles into cells, and so on. According to the Cell Theory, all living things are composed of one or more cells, and the functions of a multicellular organism are a consequence of the types of cells it has. Cells fall into two broad groups: prokaryotes and eukaryotes. Prokaryotic cells are smaller (as a general rule) and lack much of the internal compartmentalization and complexity of eukaryotic cells. No matter which type of cell we are considering, all cells have certain features in common, such as a cell membrane, DNA and RNA, cytoplasm, and ribosomes. Eukaryotic cells have a great variety of organelles and structures.

Cell Size and Shape | Back to Top

The shapes of cells are quite varied with some, such as neurons, being longer than they are wide and others, such as parenchyma (a common type of plant cell) and erythrocytes (red blood cells) being equidimensional. Some cells are encased in a rigid wall, which constrains their shape, while others have a flexible cell membrane (and no rigid cell wall).

The size of cells is also related to their functions. Eggs (or to use the latin word, ova) are very large, often being the largest cells an organism produces. The large size of many eggs is related to the process of development that occurs after the egg is fertilized, when the contents of the egg (now termed a zygote) are used in a rapid series of cellular divisions, each requiring tremendous amounts of energy that is available in the zygote cells. Later in life the energy must be acquired, but at first a sort of inheritance/trust fund of energy is used.

Cells range in size from small bacteria to large, unfertilized eggs laid by birds and dinosaurs. The realtive size ranges of biological things is shown in Figure 1. In science we use the metric system for measuring. Here are some measurements and convesrions that will aid your understanding of biology.

1 meter = 100 cm = 1,000 mm = 1,000,000 µm = 1,000,000,000 nm

1 centimenter (cm) = 1/100 meter = 10 mm

1 millimeter (mm) = 1/1000 meter = 1/10 cm

1 micrometer (µm) = 1/1,000,000 meter = 1/10,000 cm

1 nanometer (nm) = 1/1,000,000,000 meter = 1/10,000,000 cm

Figure 1. Sizes of viruses, cells, and organisms. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

The Cell Membrane | Back to Top

The cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing the majority of organically produced chemicals inside the cell. Electron microscopic examinations of cell membranes have led to the development of the lipid bilayer model (also referred to as the fluid-mosaic model). The most common molecule in the model is the phospholipid, which has a polar (hydrophilic) head and two nonpolar (hydrophobic) tails. These phospholipids are aligned tail to tail so the nonpolar areas form a hydrophobic region between the hydrophilic heads on the inner and outer surfaces of the membrane. This layering is termed a bilayer since an electron microscopic technique known as freeze-fracturing is able to split the bilayer, shown in Figure 2.

Figure 2. Cell Membranes from Opposing Neurons (TEM x436,740). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Cholesterol is another important component of cell membranes embedded in the hydrophobic areas of the inner (tail-tail) region. Most bacterial cell membranes do not contain cholesterol. Cholesterol aids in the flexibility of a cell membrane.

Proteins, shown in Figure 2, are suspended in the inner layer, although the more hydrophilic areas of these proteins "stick out" into the cells interior as well as outside the cell. These proteins function as gateways that will allow certain molecules to cross into and out of the cell by moving through open areas of the protein channel. These integral proteins are sometimes known as gateway proteins. The outer surface of the membrane will tend to be rich in glycolipids, which have their hydrophobic tails embedded in the hydrophobic region of the membrane and their heads exposed outside the cell. These, along with carbohydrates attached to the integral proteins, are thought to function in the recognition of self, a sort of cellular identification system.

The contents (both chemical and organelles) of the cell are termed protoplasm, and are further subdivided into cytoplasm (all of the protoplasm except the contents of the nucleus) and nucleoplasm (all of the material, plasma and DNA etc., within the nucleus).

The Cell Wall | Back to Top

Not all living things have cell walls, most notably animals and many of the more animal-like protistans. Bacteria have cell walls containing the chemical peptidoglycan. Plant cells, shown in Figures 3 and 4, have a variety of chemicals incorporated in their cell walls. Cellulose, a nondigestible (to humans anyway) polysaccharide is the most common chemical in the plant primary cell wall. Some plant cells also have lignin and other chemicals embedded in their secondary walls.

The cell wall is located outside the plasma membrane. Plasmodesmata are connections through which cells communicate chemically with each other through their thick walls. Fungi and many protists have cell walls although they do not contain cellulose, rather a variety of chemicals (chitin for fungi).

Animal cells, shown in Figure 5, lack a cell wall, and must instead rely on their cell membrane to maintain the integrity of the cell. Many protistans also lack cell walls, using variously modified cell membranes o act as a boundary to the inside of the cell.

Figure 3. Structure of a typical plant cell. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 4. Lily Parenchyma Cell (cross-section) (TEM x7,210). Note the large nucleus and nucleolus in the center of the cell, mitochondria and plastids in the cytoplasm. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Figure 5. Liver Cell (TEM x9,400). This image is copyright Dennis Kunkel. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

The nucleus | Back to Top

The nucleus, shown in Figures 6 and 7, occurs only in eukaryotic cells. It is the location for most of the nucleic acids a cell makes, such as DNA and RNA. Danish biologist Joachim Hammerling carried out an important experiment in 1943. His work (click here for a diagram) showed the role of the nucleus in controlling the shape and features of the cell. Deoxyribonucleic acid, DNA, is the physical carrier of inheritance and with the exception of plastid DNA (cpDNA and mDNA, found in the chloroplast and mitochondrion respectively) all DNA is restricted to the nucleus. Ribonucleic acid, RNA, is formed in the nucleus using the DNA base sequence as a template. RNA moves out into the cytoplasm where it functions in the assembly of proteins. The nucleolus is an area of the nucleus (usually two nucleoli per nucleus) where ribosomes are constructed.

Figure 6. Structure of the nucleus. Note the chromatin, uncoiled DNA that occupies the space within the nuclear envelope. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 7. Liver cell nucleus and nucleolus (TEM x20,740). Cytoplasm, mitochondria, endoplasmic reticulum, and ribosomes also shown.This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

The nuclear envelope, shown in Figure 8, is a double-membrane structure. Numerous pores occur in the envelope, allowing RNA and other chemicals to pass, but the DNA not to pass.

Figure 8. Structure of the nuclear envelope and nuclear pores. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 9. Nucleus with Nuclear Pores (TEM x73,200). The cytoplasm also contains numerous ribosomes. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Cytoplasm | Back to Top

The cytoplasm was defined earlier as the material between the plasma membrane (cell membrane) and the nuclear envelope. Fibrous proteins that occur in the cytoplasm, referred to as the cytoskeleton maintain the shape of the cell as well as anchoring organelles, moving the cell and controlling internal movement of structures. Elements that comprose the cytoskeleton are shown in Figure 10. Microtubules function in cell division and serve as a "temporary scaffolding" for other organelles. Actin filaments are thin threads that function in cell division and cell motility. Intermediate filaments are between the size of the microtubules and the actin filaments.

Figure 10. Actin and tubulin components of the cytoskeleton. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Vacuoles and vesicles | Back to Top

Vacuoles are single-membrane organelles that are essentially part of the outside that is located within the cell. The single membrane is known in plant cells as a tonoplast. Many organisms will use vacuoles as storage areas. Vesicles are much smaller than vacuoles and function in transporting materials both within and to the outside of the cell.

Ribosomes | Back to Top

Ribosomes are the sites of protein synthesis. They are not membrane-bound and thus occur in both prokaryotes and eukaryotes. Eukaryotic ribosomes are slightly larger than prokaryotic ones. Structurally, the ribosome consists of a small and larger subunit, as shown in Figure 11. . Biochemically, the ribosome consists of ribosomal RNA (rRNA) and some 50 structural proteins. Often ribosomes cluster on the endoplasmic reticulum, in which case they resemble a series of factories adjoining a railroad line. Figure 12 illustrates the many ribosomes attached to the endoplasmic reticulum. Click here for Ribosomes (More than you ever wanted to know about ribosomes!)

Figure 11. Structure of the ribosome. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 12. Ribosomes and Polyribosomes - liver cell (TEM x173,400). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Endoplasmic reticulum | Back to Top

Endoplasmic reticulum, shown in Figure 13 and 14, is a mesh of interconnected membranes that serve a function involving protein synthesis and transport. Rough endoplasmic reticulum (Rough ER) is so-named because of its rough appearance due to the numerous ribosomes that occur along the ER. Rough ER connects to the nuclear envelope through which the messenger RNA (mRNA) that is the blueprint for proteins travels to the ribosomes. Smooth ER; lacks the ribosomes characteristic of Rough ER and is thought to be involved in transport and a variety of other functions.

Figure 13. The endoplasmic reticulum. Rough endoplasmic reticulum is on the left, smooth endoplasmic reticulum is on the right. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 14. Rough Endoplasmic Reticulum with Ribosomes (TEM x61,560). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Golgi Apparatus and Dictyosomes | Back to Top

Golgi Complexes, shown in Figure 15 and 16, are flattened stacks of membrane-bound sacs. Italian biologist Camillo Golgi discovered these structures in the late 1890s, although their precise role in the cell was not deciphered until the mid-1900s . Golgi function as a packaging plant, modifying vesicles produced by the rough endoplasmic reticulum. New membrane material is assembled in various cisternae (layers) of the golgi.

Figure 15. Structure of the Golgi apparatus and its functioning in vesicle-mediated transport. Images from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 16. Golgi Apparatus in a plant parenchyma cell from Sauromatum guttatum (TEM x145,700). Note the numerous vesicles near the Golgi. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Lysosomes | Back to Top

Lysosomes, shown in Figure 17, are relatively large vesicles formed by the Golgi. They contain hydrolytic enzymes that could destroy the cell. Lysosome contents function in the extracellular breakdown of materials.

Figure 17. Role of the Golgi in forming lysosomes. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Mitochondria | Back to Top

Mitochondria contain their own DNA (termed mDNA) and are thought to represent bacteria-like organisms incorporated into eukaryotic cells over 700 million years ago (perhaps even as far back as 1.5 billion years ago). They function as the sites of energy release (following glycolysis in the cytoplasm) and ATP formation (by chemiosmosis). The mitochondrion has been termed the powerhouse of the cell. Mitochondria are bounded by two membranes. The inner membrane folds into a series of cristae, which are the surfaces on which adenosine triphosphate (ATP) is generated. The matrix is the area of the mitochondrion surrounded by the inner mitochondrial membrane. Ribosomes and mitochondrial DNA are found in the matrix. The significance of these features will be discussed below. The structure of mitochondria is shown in Figure 18 and 19.

Figure 18. Structure of a mitochondrion. Note the various infoldings of the mitochondrial inner membrane that produce the cristae. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 19. Muscle Cell Mitochondrion (TEM x190,920). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Mitochondria and endosymbiosis

During the 1980s, Lynn Margulis proposed the theory of endosymbiosis to explain the origin of mitochondria and chloroplasts from permanent resident prokaryotes. According to this idea, a larger prokaryote (or perhaps early eukaryote) engulfed or surrounded a smaller prokaryote some 1.5 billion to 700 million years ago. Steps in this sequence are illustrated in Figure 20.

Figure 20. The basic events in endosymbiosis. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Instead of digesting the smaller organisms the large one and the smaller one entered into a type of symbiosis known as mutualism, wherein both organisms benefit and neither is harmed. The larger organism gained excess ATP provided by the "protomitochondrion" and excess sugar provided by the "protochloroplast", while providing a stable environment and the raw materials the endosymbionts required. This is so strong that now eukaryotic cells cannot survive without mitochondria (likewise photosynthetic eukaryotes cannot survive without chloroplasts), and the endosymbionts can not survive outside their hosts. Nearly all eukaryotes have mitochondria. Mitochondrial division is remarkably similar to the prokaryotic methods that will be studied later in this course. A summary of the theory is available by clicking here.

Plastids | Back to Top

Plastids are also membrane-bound organelles that only occur in plants and photosynthetic eukaryotes. Leucoplasts, also known as amyloplasts (and shown in Figure 21) store starch, as well as sometimes protein or oils. Chromoplasts store pigments associated with the bright colors of flowers and/or fruits.

Figure 21. Starch grains ina fresh-cut potato tuber. Image from http://images.botany.org/set-13/13-008v.jpg.

Chloroplasts, illustrated in Figures 22 and 23, are the sites of photosynthesis in eukaryotes. They contain chlorophyll, the green pigment necessary for photosynthesis to occur, and associated accessory pigments (carotenes and xanthophylls) in photosystems embedded in membranous sacs, thylakoids (collectively a stack of thylakoids are a granum [plural = grana]) floating in a fluid termed the stroma. Chloroplasts contain many different types of accessory pigments, depending on the taxonomic group of the organism being observed.

Figure 22. Structure of the chloroplast. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Figure 23. Chloroplast from red alga (Griffthsia spp.). x5,755--(Based on an image size of 1 inch in the narrow dimension). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Chloroplasts and endosymbiosis

Like mitochondria, chloroplasts have their own DNA, termed cpDNA. Chloroplasts of Green Algae (Protista) and Plants (descendants of some of the Green Algae) are thought to have originated by endosymbiosis of a prokaryotic alga similar to living Prochloron (the sole genus present in the Prochlorobacteria, shown in Figure 24). Chloroplasts of Red Algae (Protista) are very similar biochemically to cyanobacteria (also known as blue-green bacteria [algae to chronologically enhanced folks like myself :)]). Endosymbiosis is also invoked for this similarity, perhaps indicating more than one endosymbiotic event occurred.

Figure 24. Prochloron, a photosynthetic bacteria, reveals the presence of numerous thylakoids in the transmission electron micrograph on the left. Prochloron occurs in long filaments, as shown by the light micrograph on the right below. Image from http://www.cas.muohio.edu/~wilsonkg/bot191/mouseth/m19p32.jpg.

Cell Movement | Back to Top

Cell movement; is both internal, referred to as cytoplasmic streaming, and external, referred to as motility. Internal movements of organelles are governed by actin filaments and other components of the cytoskeleton. These filaments make an area in which organelles such as chloroplasts can move. Internal movement is known as cytoplasmic streaming. External movement of cells is determined by special organelles for locomotion.

The cytoskeleton is a network of connected filaments and tubules. It extends from the nucleus to the plasma membrane. Electron microscopic studies showed the presence of an organized cytoplasm. Immunofluorescence microscopy identifies protein fibers as a major part of this cellular feature. The cytoskeleton components maintain cell shape and allow the cell and its organelles to move.

Actin filaments, shown in Figure 25, are long, thin fibers approximately seven nm in diameter. These filaments occur in bundles or meshlike networks. These filaments are polar, meaning there are differences between the ends of the strand. An actin filament consists of two chains of globular actin monomers twisted to form a helix. Actin filaments play a structural role, forming a dense complex web just under the plasma membrane. Actin filaments in microvilli of intestinal cells act to shorten the cell and thus to pull it out of the intestinal lumen. Likewise, the filaments can extend the cell into intestine when food is to be absorbed. In plant cells, actin filaments form tracts along which chloroplasts circulate.

Actin filaments move by interacting with myosin, The myosin combines with and splits ATP, thus binding to actin and changing the configuration to pull the actin filament forward. Similar action accounts for pinching off cells during cell division and for amoeboid movement.

Figure 25. Skeletal muscle fiber with exposed intracellular actin myosin filaments. The muscle fiber was cut perpendicular to its length to expose the intracellular actin myosin filaments. SEM X220. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Intermediate filaments are between eight and eleven nm in diameter. They are between actin filaments and microtubules in size. The intermediate fibers are rope-like assemblies of fibrous polypeptides. Some of them support the nuclear envelope, while others support the plasma membrane, form cell-to-cell junctions.

Microtubules are small hollow cylinders (25 nm in diameter and from 200 nm-25 µm in length). These microtubules are composed of a globular protein tubulin. Assembly brings the two types of tubulin (alpha and beta) together as dimers, which arrange themselves in rows.

In animal cells and most protists, a structure known as a centrosome occurs. The centrosome contains two centrioles lying at right angles to each other. Centrioles are short cylinders with a 9 + 0 pattern of microtubule triplets. Centrioles serve as basal bodies for cilia and flagella. Plant and fungal cells have a structure equivalent to a centrosome, although it does not contain centrioles.

Cilia are short, usually numerous, hairlike projections that can move in an undulating fashion (e.g., the protzoan Paramecium, the cells lining the human upper respiratory tract). Flagella are longer, usually fewer in number, projections that move in whip-like fashion (e.g., sperm cells). Cilia and flagella are similar except for length, cilia being much shorter. They both have the characteristic 9 + 2 arrangement of microtubules shown in figures 26.

Figure 26. Cilia from an epithelial cell in cross section (TEM x199,500). Note the 9 + 2 arrangement of cilia. This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.

Cilia and flagella move when the microtubules slide past one another. Both oif these locomotion structures have a basal body at base with thesame arrangement of microtubule triples as centrioles. Cilia and flagella grow by the addition of tubulin dimers to their tips.

Flagella work as whips pulling (as in Chlamydomonas or Halosphaera) or pushing (dinoflagellates, a group of single-celled Protista) the organism through the water. Cilia work like oars on a viking longship (Paramecium has 17,000 such oars covering its outer surface). The movement of these structures is shown in Figure 27.

Figure 27. Movement of cilia and flagella. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Not all cells use cilia or flagella for movement. Some, such as Amoeba, Chaos (Pelomyxa) and human leukocytes (white blood cells), employ pseudopodia to move the cell. Unlike cilia and flagella, pseudopodia are not structures, but rather are associated with actin near the moving edge of the cell. The formation of a pseudopod is shown in Figure 28.

Figure 28. Formation and functioning of a pseudopod by an amoeboid cell. Image from Purves et al., Life: The Science of Biology, 4th Edition, by Sinauer Associates (www.sinauer.com) and WH Freeman (www.whfreeman.com), used with permission.

Learning Objectives | Back to Top

• Give the function and cellular location of the following basic eukaryotic organelles and structures: cell membrane, nucleus, endoplasmic reticulum, Golgi bodies, lysosomes, mitochondria, ribosomes, chloroplasts, vacuoles, and cell walls.
• A micrometer is one-millionth of a meter long. A nanometer is one-billionth of a meter long. How many micrometers tall are you?
• Describe the function of the nuclear envelope and nucleolus.
• Describe the details of the structure of the chloroplast, the site of photosynthesis.
• Mature, living plant cells often have a large, fluid-filled central vacuole that can store amino acids, sugars, ions, and toxic wastes. Animal cells generally lack large vacuoles. How do animal cells perform these functions?
• Microtubules, microfilaments, and intermediate filaments are all main components of the cytoskeleton.
• Flagella and cilia propel eukaryotic cells through their environment; the microtubule organization in these organelles is a 9+2 array.

Terms | Back to Top

actin	carotenes	cellulose	cell walls	chemiosmosis	chitin
chlorophyll	cristae	cyanobacteria	cytoplasm	cytoskeleton	dinoflagellates
endoplasmic reticulum	erythrocytes	eukaryotic	fluid-mosaic	Golgi complexes	grana
Green Algae	hydrophilic	hydrophobic	leukocytes	lysosomes	microtubules
mitochondria	mutualism	neurons	nucleus	nucleolus	ova
parenchyma	phospholipid	photosystems	plasmodesmata	plastid	pseudopodia
Red Algae	ribosomal RNA	ribosomes	stroma	symbiosis	thylakoids
vacuoles	zygote

Review Questions | Back to Top

1. There are ____ micrometers (µm) in one millimeter (mm). a) 1; b) 10; c) 100; d) 1000; e) 1/1000
2. Human cells have a size range between ___ and ___ micrometers (µm). a) 10-100; b) 1-10; c) 100-1000; d) 1/10-1/1000
3. Chloroplasts and bacteria are ___ in size. a) similar; b) at different ends of the size range; c) exactly the same; d) none of these.
4. The plasma membrane does all of these except ______. a) contains the hereditary material; b) acts as a boundary or border for the cytoplasm; c) regulates passage of material in and out of the cell; d) functions in the recognition of self
5. Which of these materials is not a major component of the plasma membrane? a) phospholipids; b) glycoproteins; c) proteins; d) DNA
6. Cells walls are found in members of these kingdoms, except for ___, which all lack cell walls. a) plants; b) animals; c) bacteria; d) fungi
7. The polysaccharide ___ is a major component of plan cell walls. a) chitin; b) peptidoglycan; c) cellulose; d) mannitol; e) cholesterol
8. Plant cells have ___ and ___, which are not present in animal cells. a) mitochondria, chloroplasts; b) cell membranes, cell walls; c) chloroplasts, nucleus; d) chloroplasts, cell wall
9. The ___ is the membrane enclosed structure in eukaryotic cells that contains the DNA of the cell. a) mitochondrion; b) chloroplast; c) nucleolus; d) nucleus
10. Ribosomes are constructed in the ___. a) endoplasmic reticulum; b) nucleoid; c) nucleolus; d) nuclear pore
11. Rough endoplasmic reticulum is the area in a cell where ___ are synthesized. a) polysaccharides; b) proteins; c) lipids; d) DNA
12. The smooth endoplasmic reticulum is the area in a cell where ___ are synthesized. a) polysaccharides; b) proteins; c) lipids; d) DNA
13. The mitochondrion functions in ____. a) lipid storage; b) protein synthesis; c) photosynthesis; d) DNA replication; e) ATP synthesis
14. The thin extensions of the inner mitochondrial membrane are known as _____. a) cristae; b) matrix; c) thylakoids; d) stroma
15. The chloroplast functions in ____. a) lipid storage; b) protein synthesis; c) photosynthesis; d) DNA replication; e) ATP synthesis
16. Which of these cellular organelles have their own DNA? a) chloroplast; b) nucleus; c) mitochondrion; d) all of these
17. The theory of ___ was proposed to explain the possible origin of chloroplasts and mitochondria. a) evolution; b) endosymbiosis; c) endocytosis; d) cells
18. Long, whiplike microfibrils that facilitate movement by cells are known as ___. a) cilia; b) flagella; c) leather; d) pseudopodia

Links | Back to Top

• The March of Heredity This page from Access Excellence details the role of the nucleus in heredity. Link is http://www.accessexcellence.org/AB/BC/March_of_Heredity.html
• Protist Image Data: Pictures and resources about Protista.
• Cell Biology Lab Manual: Lab protocols and links pertaining to cell biology. A nice place to look for new things to do in labs.
• MIT Hypertextbook Chapter on Cell Biology: Excellent site with illustrations and additional details to complement the above material.
• Virtual Plant Cell: Zoom in on a virtual plant cell. An excellent first step.
• WWW Cell Biology Course: An excellent site employing image maps and details about cell biology.
• Dictionary of Cell Biology: A searchable dictionary pertinent to this topic.
• Cells Alive! Very interesting site with new features each month.
• Ribosomes A text with links to illustrations. More than you ever wanted to know about ribosomes!
• Charlie and Bobby Jo's Excellent Adventure Look out, honey, I shrunk the lab! Humorous journey through a cell. (Dr. Sata, University of Texas).
• The Cell Nucleus This easily navigated series of well illustrated pages presents an impressive amount of information about the various functions carried out by the nucleus.
• The Cytoskeleton This site provides additional details about the components of the cytoskeleton.

References | Back to Top

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Text ©1992, 1994, 1997, 1998, 1999, 2000, 2001, 2007, by M.J. Farabee, all rights reserved. Use for educational purposes is heartily encouraged.

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Email: mj.farabee@emcmail.maricopa.edu

Last modified: Tuesday May 18 2010

The URL of this page is: www2.estrellamountain.edu/faculty/farabee/biobk/BioBookCELL2.html