1) CELL STRUCTURE AND FUNCTION
TERMS APPLIED TO CELLS
Heterotroph (other-feeder): an organism that obtains its energy from another organism. Animals, fungi, bacteria, and mant protistans are heterotrophs.
Autotroph (self-feeder): an organism that makes its own food, it converts energy from an inorganic source in one of two ways. Photosynthesis is the conversion of sunlight energy into C-C covalent bonds of a carbohydrate, the process by which the vast majority of autotrophs obtain their energy. Chemosynthesis is the capture of energy released by certain inorganic chemical reactions. This is common in certain groups of likely that chemosynthesis predates photosynthesis. At mid-ocean ridges, scientists have discovered black smokers, vents that release chemicals into the water. These chemical reactions could have powered early ecosystems prior to the development of an ozone layer that would have permitted life to occupy the shallower parts of the ocean. Evidence of the antiquity of photosynthesis includes: a) biochemical precursors to photosynthesis chemicals have been synthesized in experiments; and b) when placed in light, these chemicals undergo chemical reactions similar to some that occur in primitive photosynthetic bacteria.
Prokaryotes are among the most primitive forms of life on Earth. Remember that primitive does not necessarily equate to outdated and unworkable in an evolutionary sense, since primitive bacteria seem little changed, and thus may be viewed as well adapted, for over 3.5 Ga. Prokaryote (pro=before, karyo=nucleus): these organisms lack membrane-bound organelles, as seen in Figures 5 and 6. Some internal membrane organization is observable in a few prokaryotic autotrophs, such as the photosynthetic membranes associated with the photosynthetic chemicals in the photosynthetic bacterium Prochloron. (click here to view Prochloron and other cyanobacteria at the Tree of Life Page). A transmission electron micrograph of Prochloron is shown in Figure 5.
Figure 5 Prochloron, an extant prokaryote thought related to the ancestors of some eukarypote chloroplasts.
Figure 6. The main features of a generalized prokaryote 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.
The Cell Theory is one of the foundations of modern biology. Its major tenets are:
- All living things are composed of one or more cells;
- The chemical reactions of living cells take place within cells;
- All cells originate from pre-existing cells; and
- Cells contain hereditary information, which is passed from one generation to another.
Cell Membrane (also known as plasma membrane or plasmalemma) is surrounds all cells. It: 1) separates the inner parts of the cell from the outer environment; and 2) acts as a selectively permeable barrier to allow certain chemicals, namely water, to pass and others to not pass. In multicellular organisms certain chemicals on the membrane surface act in the recognition of self. Antigens are substances located on the outside of cells, viruses and in some cases other chemicals. Antibodies are chemicals (Y-shaped) produced by an animal in response to a specific antigen. This is the basis of immunity and vaccination.
Hereditary material (both DNA and RNA) is needed for a cell to be able to replicate and/or reproduce. Most organisms use DNA. Viruses and viroids sometimes employ RNA as their hereditary material. Retroviruses include HIV (Human Immunodefficiency Virus, the causative agent of AIDS) and Feline Leukemia Virus (the only retrovirus for which a successful vaccine has been developed). Viroids are naked pieces of RNA that lack cytoplasm, membranes, etc. They are parasites of some plants and also as possible glimpses of the functioning of pre-cellular life forms. Prokaryotic DNA is organized as a circular chromosome contained in an area known as a nucleoid. Eukaryotic DNA is organized in linear structures, the eukaryotic chromosomes, which are associations of DNA and histone proteins contained within a double membrane nuclear envelope, an area known as the cell nucleus.
Organelles are formed bodies within the cytoplasm that perform certain functions. Some organelles are surrounded by membranes, we call these membrane-bound organelles.
Ribosomes are the tiny structures where proteins synthesis occurs. They are not membrane-bound and occur in all cells, although there are differences between the size of subunits in eukaryotic and prokaryotic ribosomes.
The Cell Wall is a structure surrounding the plasma membrane. Prokaryote and eukaryote (if they have one) cell walls differ in their structure and chemical composition. Plant cells have cellulose in their cell walls, other organisims have different materials cpmprising their walls. Animals are distinct as a group in their lack of a cell wall.
Membrane-bound organelles occur only in eukaryotic cells. They will be discussed in detail later. Eukaryotic cells are generally larger than prokaryotic cells. Internal complexity is usually greater in eukaryotes, with their compartmentalized membrane-bound organelles, than in prokaryotes. Some prokaryotes, such as Anabaena azollae, and Prochloron, have internal membranes associated with photosynthetic pigments.
The Origins of Multicellularity
The oldest accepted prokaryote fossils date to 3.5 billion years; Eukaryotic fossils to between 750 million years and possibly as old as 1.2-1.5 billion years. Multicellular fossils, purportedly of animals, have been recovered from 750Ma rocks in various parts of the world. The first eukaryotes were undoubtedly Protistans, a group that is thought to have given rise to the other eukaryotic kingdoms. Multicellularity allows specialization of function, for example muscle fibers are specialized for contraction, neuron cells for transmission of nerve messages.
Microscopes are important tools for studying cellular structures. In this class we will use light microscopes for our laboratory observations. Your text will also show light photomicrographs (pictures taken with a light microscope) and electron micrographs (pictures taken with an electron microscope). There are many terms and concepts which will help you in maximizing your study of microscopy.
There are many different types of microscopes used in studying biology. These include the light microscopes (dissecting, compound brightfield, and compound phase-contrast), electron microscopes (transmission and scanning), and atomic force microscope.
The microscope is an important tool used by biologists to magnify small objects. There are several concepts fundamental to microscopy.
Magnification is the ratio of enlargement (or eduction) between the specimen and its image (either printed photograph or the virtual image seen through the eyepiece). To calculate magnification we multiply the power of each lens through which the light from the specimen passes, indicating that product as GGGX, where GGG is the product. For example: if the light passes through two,lenses (an objective lens and an ocular lens) we multiply the 10X ocular value by the value of the objective lens (say it is 4X): 10 X 4=40, or 40X magnification.
Resolution is the ability to distinguish between two objects (or points). The closer the two objects are, the easier it is to distinguish recognize the distance between them. What microscopes do is to bring small objects "closer" to the observer by increasing the magnification of the sample. Since the sample is the same distance from the viewer, a "virtual image" is formed as the light (or electron beam) passes through the magnifying lenses. Objects such as a human hair appear smooth (and feel smooth) when viewed with the unaided or naked) eye. However, put a hair under a microscope and it takes on a VERY different look!
Working distance is the distance between the specimen and the magnifying lens.
Depth of field is a measure of the amount of a specimen that can be in focus.
Magnification and resolution are terms used frequently in the study of cell biology, often without an accurate definition of their meanings. Magnification is a ratio of the enlargement (or reduction) of an image (drawing or photomicrograph), usually expressed as X1, X1/2, X430, X1000, etc. Resolution is the ability to distinguish between two points. Generally resolution increases with magnification, although there does come a point of diminishing returns where you increase magnification beyond added resolution gain.
Scientists employ the metric system to measure the size and volume of specimens. The basic unit of length is the meter (slightly over 1 yard). Prefixes are added to the "meter" to indicate multiple meters (kilometer) or fractional meters (millimeter). Below are the values of some of the prefixes used in the metric system.
kilo = one thousand of the basic unit
meter = basic unit of length
centi = one hundreth (1/100) of the basic unit
milli = one thousandth (1/1000) of the basic unit
micro = one millionth (1/1,000,000) of the basic unit
nano = one billionth (1/1,000,000,000) of the basic unit
The basic unit of length is the meter (m), and of volume it is the liter (l). The gram (g). Prefixes listed above can be applied to all of these basic units, abbreviated as km, kg, ml, mg, nm....etc. The Greek letter micron (µ) is applied to small measurements (thoud\sandths of a millimeter), producing the micrometer (symbolized as µm). Measurements in microscopy are usually expressed in the metric system. General units you will encounter in your continuing biology careers include micrometer (µm, 10-6m), nanometer (nm, 10-9m), and angstrom (Å, 10-10m).
Light microscopes were the first to be developed, and still the most commonly used ones. The best resolution of light microscopes (LM) is 0.2 µm. Magnification of LMs is generally limited by the properties of the glass used to make microscope lenses and the physical properties of their light sources. The generally accepted maximum magnifications in biological uses are between 1000X and 1250X. Calculation of LM magnification is done by multiplying objective value by eyepiece value.
To view relatively large objects at lower magnifications we utilize the dissecting microscope (shown in Figure 7). Common uses of this microscope include examination of prepared microscope slides at low magnification, dissection (hence the name) of flowers or animal organs, and examinations of the surface of objects such as pennies and five dollar bills. Magnification on the dissecting microscope is calculated by multiplying the ocular (or eyepiece) value (usually 10X) by the value of the objective lens (a variable between 0.7 and 3X). The value of the objective lens is selected using a dial on the body tube of the microscope.
Figure 7. Parts of a Nikon dissecting microscope. Image courtesy of Nikon Co.
The compound light microscope, shown in Figure 8, uses two ground glass lenses to form the image. The lenses in this microscope, however, are aligned with the light source and specimen so that the light passes through the specimen, rather than reflects off the surface (as in the dissecting microscope shown in Figure 7). The compound microscope provides greater magnification (and resoultion), but only thin specimens (or thin slices of a specimen) can be viewed with this type of microscope.
Figure 8. Parts of a Nikon compound microscope. Image courtesy of Nikon Co.
Electron microscopes, two examples of which are shown in Fgure 9, are more rarely encountered by beginning biology students. However, the images gathered from these microscopes reveal a greater structure of the cell, so some familiarity with the strengths and weaknesses of each type is useful. Instead of using light as an imaging source, a high energy beam of electrons (between five thousand and one billion electron volts) is focused through electromagnetic lenses (instead of glass lenses used in the light microscope). The increased resolution results from the shorter wavelength of the electron beam, increasing resolution in the transmission electron microscope (TEM) to a theoretical limit of 0.2 nm. The magnifications reached by TEMs are commonly over 100,000X, depending on the nature of the sample and the operating condition of the TEM. The other type of electron microscope is the scanning electron microscope (SEM). It uses a different method of electron capture and displays images on high resolution television monitors. The resolution and magnification of the SEM are less than that of the TEM although still orders of magnitude above the LM.
2) CELLULAR ORGANISATION
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.
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
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.
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).
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.
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.
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.
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.
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 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!)
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.
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
3) TRANSPORT IN AND OUT OF CELLS
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).
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 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.
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.
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.
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.
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.
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.
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.
4) CELL DIVISION: BINARY FISSION AND MITOSIS
Despite differences between prokaryotes and eukaryotes, there are several common features in their cell division processes. Replication of the DNA must occur. Segregation of the "original" and its "replica" follow. Cytokinesis ends the cell division process. Whether the cell was eukaryotic or prokaryotic, these basic events must occur.
Cytokinesis is the process where one cell splits off from its sister cell. It usually occurs after cell division. The Cell Cycle is the sequence of growth, DNA replication, growth and cell division that all cells go through. Beginning after cytokinesis, the daughter cells are quite small and low on ATP. They acquire ATP and increase in size during the G1 phase of Interphase. Most cells are observed in Interphase, the longest part of the cell cycle. After acquiring sufficient size and ATP, the cells then undergo DNA Synthesis (replication of the original DNA molecules, making identical copies, one "new molecule" eventually destined for each new cell) which occurs during the S phase. Since the formation of new DNA is an energy draining process, the cell undergoes a second growth and energy acquisition stage, the G2 phase. The energy acquired during G2 is used in cell division (in this case mitosis).
Regulation of the cell cycle is accomplished in several ways. Some cells divide rapidly (beans, for example take 19 hours for the complete cycle; red blood cells must divide at a rate of 2.5 million per second). Others, such as nerve cells, lose their capability to divide once they reach maturity. Some cells, such as liver cells, retain but do not normally utilize their capacity for division. Liver cells will divide if part of the liver is removed. The division continues until the liver reaches its former size.
Cancer cells are those which undergo a series of rapid divisions such that the daughter cells divide before they have reached "functional maturity". Environmental factors such as changes in temperature and pH, and declining nutrient levels lead to declining cell division rates. When cells stop dividing, they stop usually at a point late in the G1 phase, the R point (for restriction).
Prokaryotes are much simpler in their organization than are eukaryotes. There are a great many more organelles in eukaryotes, also more chromosomes. The usual method of prokaryote cell division is termed binary fission. The prokaryotic chromosome is a single DNA molecule that first replicates, then attaches each copy to a different part of the cell membrane. When the cell begins to pull apart, the replicate and original chromosomes are separated. Following cell splitting (cytokinesis), there are then two cells of identical genetic composition (except for the rare chance of a spontaneous mutation).
This animated GIF of binary fission is from: http://www.slic2.wsu.edu:82/hurlbert/micro101/pages/Chap2.html#two_bact_groups
The prokaryote chromosome is much easier to manipulate than the eukaryotic one. We thus know much more about the location of genes and their control in prokaryotes.
One consequence of this asexual method of reproduction is that all organisms in a colony are genetic equals. When treating a bacterial disease, a drug that kills one bacteria (of a specific type) will also kill all other members of that clone (colony) it comes in contact with.
Rod-Shaped Bacterium, E. coli, dividing by binary fission (TEM x92,750). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
Rod-Shaped Bacterium, hemorrhagic E. coli, strain 0157:H7 (division) (SEM x22,810). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
Due to their increased numbers of chromosomes, organelles and complexity, eukaryote cell division is more complicated, although the same processes of replication, segregation, and cytokinesis still occur.
Mitosis is the process of forming (generally) identical daughter cells by replicating and dividing the original chromosomes, in effect making a cellular xerox. Commonly the two processes of cell division are confused. Mitosis deals only with the segregation of the chromosomes and organelles into daughter cells.
Click here to view an animated GIF of mitosis from http://www.biology.uc.edu/vgenetic/mitosis/mitosis.htm.
Eukaryotic chromosomes occur in the cell in greater numbers than prokaryotic chromosomes. The condensed replicated chromosomes have several points of interest. The kinetochore is the point where microtubules of the spindle apparatus attach. Replicated chromosomes consist of two molecules of DNA (along with their associated histone proteins) known as chromatids. The area where both chromatids are in contact with each other is known as the centromere the kinetochores are on the outer sides of the centromere. Remember that chromosomes are condensed chromatin (DNA plus histone proteins).
Structure of a eukaryotic chromosome. 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.
During mitosis replicated chromosomes are positioned near the middle of the cytoplasm and then segregated so that each daughter cell receives a copy of the original DNA (if you start with 46 in the parent cell, you should end up with 46 chromosomes in each daughter cell). To do this cells utilize microtubules (referred to as the spindle apparatus) to "pull" chromosomes into each "cell". The microtubules have the 9+2 arrangement discussed earlier. Animal cells (except for a group of worms known as nematodes) have a centriole. Plants and most other eukaryotic organisms lack centrioles. Prokaryotes, of course, lack spindles and centrioles; the cell membrane assumes this function when it pulls the by-then replicated chromosomes apart during binary fission. Cells that contain centrioles also have a series of smaller microtubules, the aster, that extend from the centrioles to the cell membrane. The aster is thought to serve as a brace for the functioning of the spindle fibers.
Structure and main features of a spindle apparatus. 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 phases of mitosis are sometimes difficult to separate. Remember that the process is a dynamic one, not the static process displayed of necessity in a textbook.
Prophase is the first stage of mitosis proper. Chromatin condenses (remember that chromatin/DNA replicate during Interphase), the nuclear envelope dissolves, centrioles (if present) divide and migrate, kinetochores and kinetochore fibers form, and the spindle forms.
Pea Plant Nuclear DNA, from Vicea faba (TEM x105,000). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
Metaphase follows Prophase. The chromosomes (which at this point consist of chromatids held together by a centromere) migrate to the equator of the spindle, where the spindles attach to the kinetochore fibers.
Anaphase begins with the separation of the centromeres, and the pulling of chromosomes (we call them chromosomes after the centromeres are separated) to opposite poles of the spindle.
The events of Metaphase and Anaphase. 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.
Telophase is when the chromosomes reach the poles of their respective spindles, the nuclear envelope reforms, chromosomes uncoil into chromatin form, and the nucleolus (which had disappeared during Prophase) reform. Where there was one cell there are now two smaller cells each with exactly the same genetic information. These cells may then develop into different adult forms via the processes of development.
Cytokinesis is the process of splitting the daughter cells apart. Whereas mitosis is the division of the nucleus, cytokinesis is the splitting of the cytoplasm and allocation of the golgi, plastids and cytoplasm into each new cell.
- Access Excellence page on Mitosis
- Cell Division and the Cell Cycle (University of Alberta): Similar to this page, but with its own glossary and questions.
- Amoeba Proteus Mitosis Small photomicrographs of protistan mitosis.
- Cell Reproduction Notes from University of Georgia, plus some cool graphics of mitosis.
- Phases of Mitosis U Texas QuickTime® movies of mitosis.
- Animated Mitosis Yale University, a simplified series of cartoons about mitosis.
- Mitosis U Southern Mississippi, Grayscale drawings and photomicrographs of mitosis stages.
- Mitosis San Diego State U, shocked animation of the process. You will need the Shockwave plugin to view. If you don't have it, you can download it from them.
- McGill University Mitosis Page Quality site, with photos and downloadable animation and video.
- Comparison of Mitosis and Meiosis Whitman College, table summarizing each process.
- Whitefish Mitosis Review Cornell, photomicrographs of mitosis in whitefish. A nice review after lab! Part of a more extensive page of Cell Division Tutorials.
- Virtual Mitosis University of Cincinnati, Animated GIF and text about the stages of mitosis.
5) CELL DIVISION: MEIOSIS AND SEXUAL REPRODUCTION
Sexual reproduction occurs only in eukaryotes. During the formation of gametes, the number of chromosomes is reduced by half, and returned to the full amount when the two gametes fuse during fertilization.
Haploid and diploid are terms referring to the number of sets of chromosomes in a cell. Gregor Mendel determined his peas had two sets of alleles, one from each parent. Diploid organisms are those with two (di) sets. Human beings (except for their gametes), most animals and many plants are diploid. We abbreviate diploid as 2n. Ploidy is a term referring to the number of sets of chromosomes. Haploid organisms/cells have only one set of chromosomes, abbreviated as n. Organisms with more than two sets of chromosomes are termed polyploid. Chromosomes that carry the same genes are termed homologous chromosomes. The alleles on homologous chromosomes may differ, as in the case of heterozygous individuals. Organisms (normally) receive one set of homologous chromosomes from each parent.
Meiosis is a special type of nuclear division which segregates one copy of each homologous chromosome into each new "gamete". Mitosis maintains the cell's original ploidy level (for example, one diploid 2n cell producing two diploid 2n cells; one haploid n cell producing two haploid n cells; etc.). Meiosis, on the other hand, reduces the number of sets of chromosomes by half, so that when gametic recombination (fertilization) occurs the ploidy of the parents will be reestablished.
Most cells in the human body are produced by mitosis. These are the somatic (or vegetative) line cells. Cells that become gametes are referred to as germ line cells. The vast majority of cell divisions in the human body are mitotic, with meiosis being restricted to the gonads.
Life cycles are a diagrammatic representation of the events in the organism's development and reproduction. When interpreting life cycles, pay close attention to the ploidy level of particular parts of the cycle and where in the life cycle meiosis occurs. For example, animal life cycles have a dominant diploid phase, with the gametic (haploid) phase being a relative few cells. Most of the cells in your body are diploid, germ line diploid cells will undergo meiosis to produce gametes, with fertilization closely following meiosis.
Plant life cycles have two sequential phases that are termed alternation of generations. The sporophyte phase is "diploid", and is that part of the life cycle in which meiosis occurs. However, many plant species are thought to arise by polyploidy, and the use of "diploid" in the last sentence was meant to indicate that the greater number of chromosome sets occur in this phase. The gametophyte phase is "haploid", and is the part of the life cycle in which gametes are produced (by mitosis of haploid cells). In flowering plants (angiosperms) the multicelled visible plant (leaf, stem, etc.) is sporophyte, while pollen and ovaries contain the male and female gametophytes, respectively. Plant life cycles differ from animal ones by adding a phase (the haploid gametophyte) after meiosis and before the production of gametes.
Many protists and fungi have a haploid dominated life cycle. The dominant phase is haploid, while the diploid phase is only a few cells (often only the single celled zygote, as in Chlamydomonas ). Many protists reproduce by mitosis until their environment deteriorates, then they undergo sexual reproduction to produce a resting zygotic cyst.
Two successive nuclear divisions occur, Meiosis I (Reduction) and Meiosis II (Division). Meiosis produces 4 haploid cells. Mitosis produces 2 diploid cells. The old name for meiosis was reduction/ division. Meiosis I reduces the ploidy level from 2n to n (reduction) while Meiosis II divides the remaining set of chromosomes in a mitosis-like process (division). Most of the differences between the processes occur during Meiosis I.
The above image is from http://www.biology.uc.edu/vgenetic/meiosis/
Prophase I has a unique event -- the pairing (by an as yet undiscovered mechanism) of homologous chromosomes. Synapsis is the process of linking of the replicated homologous chromosomes. The resulting chromosome is termed a tetrad, being composed of two chromatids from each chromosome, forming a thick (4-strand) structure. Crossing-over may occur at this point. During crossing-over chromatids break and may be reattached to a different homologous chromosome.
The alleles on this tetrad:
A B C D E F G
A B C D E F G
a b c d e f g
a b c d e f g
will produce the following chromosomes if there is a crossing-over event between the 2nd and 3rd chromosomes from the top:
A B C D E F G
A B c d e f g
a b C D E F G
a b c d e f g
Thus, instead of producing only two types of chromosome (all capital or all lower case), four different chromosomes are produced. This doubles the variability of gamete genotypes. The occurrence of a crossing-over is indicated by a special structure, a chiasma (plural chiasmata) since the recombined inner alleles will align more with others of the same type (e.g. a with a, B with B). Near the end of Prophase I, the homologous chromosomes begin to separate slightly, although they remain attached at chiasmata.
Crossing-over between homologous chromosomes produces chromosomes with new associations of genes and alleles. 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.
Events of Prophase I (save for synapsis and crossing over) are similar to those in Prophase of mitosis: chromatin condenses into chromosomes, the nucleolus dissolves, nuclear membrane is disassembled, and the spindle apparatus forms.
Metaphase I is when tetrads line-up along the equator of the spindle. Spindle fibers attach to the centromere region of each homologous chromosome pair. Other metaphase events as in mitosis.
Anaphase I is when the tetrads separate, and are drawn to opposite poles by the spindle fibers. The centromeres in Anaphase I remain intact.
Events in prophase and metaphse I. 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.
Telophase I is similar to Telophase of mitosis, except that only one set of (replicated) chromosomes is in each "cell". Depending on species, new nuclear envelopes may or may not form. Some animal cells may have division of the centrioles during this phase.
During Prophase II, nuclear envelopes (if they formed during Telophase I) dissolve, and spindle fibers reform. All else is as in Prophase of mitosis. Indeed Meiosis II is very similar to mitosis.
Metaphase II is similar to mitosis, with spindles moving chromosomes into equatorial area and attaching to the opposite sides of the centromeres in the kinetochore region.
During Anaphase II, the centromeres split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell.
The events of Metaphase II and Anaphase II. 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.
Telophase II is identical to Telophase of mitosis. Cytokinesis separates the cells.
Mitosis maintains ploidy level, while meiosis reduces it. Meiosis may be considered a reduction phase followed by a slightly altered mitosis. Meiosis occurs in a relative few cells of a multicellular organism, while mitosis is more common.
Comparison of the events in Mitosis and Meiosis. 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.
Gametogenesis is the process of forming gametes (by definition haploid, n) from diploid cells of the germ line. Spermatogenesis is the process of forming sperm cells by meiosis (in animals, by mitosis in plants) in specialized organs known as gonads (in males these are termed testes). After division the cells undergo differentiation to become sperm cells. Oogenesis is the process of forming an ovum (egg) by meiosis (in animals, by mitosis in the gametophyte in plants) in specialized gonads known as ovaries. Whereas in spermatogenesis all 4 meiotic products develop into gametes, oogenesis places most of the cytoplasm into the large egg. The other cells, the polar bodies, do not develop. This all the cytoplasm and organelles go into the egg. Human males produce 200,000,000 sperm per day, while the female produces one egg (usually) each menstrual cycle.
Sperm production begins at puberty at continues throughout life, with several hundred million sperm being produced each day. Once sperm form they move into the epididymis, where they mature and are stored.
Human Sperm (SEM x5,785). This image is copyright Dennis Kunkel at www.DennisKunkel.com, used with permission.
The ovary contains many follicles composed of a developing egg surrounded by an outer layer of follicle cells. Each egg begins oogenesis as a primary oocyte. At birth each female carries a lifetime supply of developing oocytes, each of which is in Prophase I. A developing egg (secondary oocyte) is released each month from puberty until menopause, a total of 400-500 eggs.
Oogenesis. The above image is from http://www.grad.ttuhsc.edu/courses/histo/notes/female.html.
- Access Excellence page on Mitosis
- Cell Division and the Cell Cycle (University of Alberta): Similar to this page, but with its own glossary and questions.
- Amoeba Proteus Mitosis Small photomicrographs of protistan mitosis.
- Animated Meiosis Yale University, a simplified series of cartoons about meiosis.
- Meiosis Tutorial North Carolina State University, animations and 3-D graphics.
- McGill University Mitosis Page Quality site, with photos and downloadable animation and video.
- Virtual Meiosis University of Cincinnati, Animated GIF and text/images to explain meiosis.