The Origin of Cells

Origin of the Earth and Life

Scientific estimates place the origin of the Universe at between 10 and 20 billion years ago. The theory currently with the most acceptance is the Big Bang Theory, the idea that all matter in the Universe existed in a cosmic egg (smaller than the size of a modern hydrogen atom) that exploded, forming the Universe. Recent discoveries from the Space Telescope and other devices suggest this theory smay need some modification. Evidence for the Big Bang includes:

1) The Red Shift: when stars/galaxies are moving away from us the energy they emit is shifted to the red side of the visible-light spectrum. Those moving towards us are shifted to the violet side. This shift is an example of the Doppler effect. Similar effects are observed when listening to a train whistle-- it will sound higher (shorter wavelengths) approaching and lower (longer wavelengths) as it moves away. Likewise red wavelengths are longer than violet ones. Most galaxies appear to be moving away from ours. 2) Background radiation: two Bell Labs scientists discovered that in interstellar space there is a slight background radiation, thought to be the residual afterblast remnant of the Big Bang. Click here for additional information from sites dealing with the Big Bang, or here for a Powerpoint slideshow about the Big Bang.

Soon after the Big Bang the major forces (such as gravity, weak nuclear force, strong nuclear force, etc.) differentiated. While in the cosmic egg, scientists think that matter and energy as we understand them did not exist, but rather they formed soon after the bang. After 10 million to 1 billion years the universe became clumpy, with matter beginning to accumulate into solar systems. One of those solar systems, ours, began to form approximately 5 billion years ago, with a large "protostar" (that became our sun) in the center. The planets were in orbits some distance from the star, their increasing gravitational fields sweeping stray debris into larger and larger planetesimals that eventually formed planets.

The processes of radioactive decay and heat generated by the impact of planetesimals heated the Earth, which then began to differentiate into a "cooled" outer cooled crust (of silicon, oxygen and other relatively light elements) and increasingly hotter inner areas (composed of the heavier and denser elements such as iron and nickel). Impact (asteroid, comet, planetismals) and the beginnings of volcanism released water vapor, carbon dioxide, methane, ammonia and other gases into a developing atmosphere. Sometime "soon" after this, life on Earth began.

Where did life originate and how?

Extra-terrestrial: In 1969, a meteorite (left-over bits from the origin of the solar system) landed near Allende, Mexico. The Allende Meteorite (and others of its sort) have been analyzed and found to contain amino acids, the building blocks of proteins, one of the four organic molecule groups basic to all life. The idea of panspermia hypothesized that life originated out in space and came to Earth inside a meteorite. Recently, this idea has been revived as Cosmic Ancestry. The amino acids recovered from meteorites are in a group known as exotics: they do not occur in the chemical systems of living things. The ET theory is now not considered by most scientists to be correct, although the August 1996 discovery of the Martian meteorite and its possible fossils have revived thought of life elsewhere in the Solar System.

Supernatural: Since science is an attempt to measure and study the natural world, this theory is outside science (at least our current understanding of science). Science classes deal with science, and this idea is in the category of not-science. 

Organic Chemical Evolution: Until the mid-1800's scientists thought organic chemicals (those with a C-C skeleton) could only form by the actions of living things. A French scientist heated crystals of a mineral (a mineral is by definition inorganic), and discovered that they formed urea (an organic chemical) when they cooled. Russian scientist and academecian A.I. Oparin, in 1922, hypothesized that cellular life was preceeded by a period of chemical evolution. These chemicals, he argued, must have arisen spontaneously under conditions exisitng billions of years ago (and quite unlike current conditions).

Figure 1. Ingredients used in Miller's experiments, simple molecules thought at the time to have existed on the Earth billions of years ago. 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.

In 1950, then-graduate student Stanley Miller designed an experimental test for Oparin's hypothesis. Oparin's original hypothesis called for : 1) little or no free oxygen (oxygen not bonded to other elements); and 2) C H O and N in abundance. Studies of modern volcanic eruptions support inference of the existence of such an atmosphere. Miller discharged an electric spark into a mixture thought to resemble the primordial composition of the atmosphere. Miller's atmosphere contents are shown in Figure 1. From the water receptacle, designed to model an ancient ocean, Miller recovered amino acids. Subsequent modifications of the atmosphere have produced representatives or precursors of all four organic macromolecular classes. His experimental apparatus is shown in Figure 2. 

Figure 2. A diagrammatic representation of Miller's experimental 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 primordial Earth was a very different place than today, with greater amounts of energy, stronger storms, etc. The oceans were a "soup" of organic compounds that formed by inorganic processes (although this soup would not taste umm ummm good). Miller's (and subsequent) experiments have not proven life originated in this way, only that conditions thought to have existed over 3 billion years ago were such that the spontaneous (inorganic) formation of organic macromolecules could have taken place. The simple inorganic molecules that Miller placed into his apparatus, produced a variety of complex molecules, shown below in Figure 3.

Figure 3. Molecules recovered from Miller's and similar experiments. 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 interactions of these molecules would have increased as their concentrations increased. Reactions would have led to the building of larger, more complex molecules. A pre-cellular life would have began with the formation of nucleic acids. Chemicals made by these nucleic acids would have remained in proximity to the nucleic acids. Eventually the pre-cells would have been enclosed in a lipid-protein membrane, which would have resulted in the first cells.

Biochemically, living systems are separated from other chemical systems by three things.

1. The capacity for replication from one generation to another. Most organisms today use DNA as the hereditary material, although recent evidence (ribozymes) suggests that RNA may have been the first nucleic acid system to have formed. Nobel laureate Walter Gilbert refers to this as the RNA world. Recent studies suggest a molecular
2. The presence of enzymes and other complex molecules essential to the processes needed by living systems. Miller's experiment showed how these could possibly form.
3. A membrane that separates the internal chemicals from the external chemical environment. This also delimits the cell from not-cell areas. The work of Sidney W. Fox has produced proteinoid spheres, which while not cells, suggest a possible route from chemical to cellular life.

Fossil evidence supports the origins of life on Earth earlier than 3.5 billion years ago. The North Pole microfossils from Australia, illustrated in Figure 4, are complex enough that more primitive cells must have existed earlier. From rocks of the Ishua Super Group in Greenland come possibly the earliest cells, as much as 3.8 billion years old. The oldest known rocks on Earth are 3.96 billion years old and are from Arctic Canada. Thus, life appears to have begun soon after the cooling of the Earth and formation of the atmosphere and oceans. 

Figure 4. Microfossils from the Apex Chert, North Pole, Australia. These organisms are Archean in age, approximately 3.465 billion years old, and resemble filamentous cyanobacteria. Image from http://www.astrobiology.ucla.edu/ESS116/L15/1515%20Apex%20Chert.jpg.

These ancient fossils occur in marine rocks, such as limestones and sandstones, that formed in ancient oceans. The organisms living today that are most similar to ancient life forms are the archaebacteria. This group is today restricted to marginal environments. Recent discoveries of bacteria at mid-ocean ridges add yet another possible origin for life: at these mid-ocean ridges where heat and molten rock rise to the Earth's surface.

Archaea and Eubacteria are similar in size and shape. When we do recover "bacteria" as fossils those are the two features we will usually see: size and shape. How can we distinguish between the two groups: the use of molecular fossils that will point to either (but not both) groups. Such a chemical fossil has been found and its presence in the Ishua rocks of Greenland (3.8 billion years old) suggests that the archeans were present at that time.

Is there life on Mars, Venus, anywhere else?

The proximity of the Earth to the sun, the make-up of the Earth's crust (silicate mixtures, presence of water, etc.) and the size of the Earth suggest we may be unique in our own solar system, at least. Mars is smaller, farther from the sun, has a lower gravitational field (which would keep the atmosphere from escaping into space) and does show evidence of running water sometime in its past. If life did start on Mars, however, there appears to be no life (as we know it) today. Venus, the second planet, is closer to the sun, and appears similar to Earth in many respects. Carbon dioxide build-up has resulted in a "greenhouse planet" with strong storms and strongly acidic rain. Of all planets in the solar system, Venus is most likely to have some form of carbon-based life. The outer planets are as yet too poorly understood, although it seems unlikely that Jupiter or Saturn harbor life as we know it. Like Goldilocks would say "Venus is too hot, Mars is too cold, the Earth is just right!"

Mars: In August 1996, evidence of life on Mars (or at least the chemistry of life), was announced. Click here to view that article and related ones. The results of years of study are inconclusive at best. The purported bacteria are much smaller than any known bacteria on Earth, were not hollow, and most could possibly have been mineral in origin. However, many scientists consider that the chemistry of life appears to have been established on Mars. Search for martian life (or its remains) continues.

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. Image fom http://tidepool.st.usm.edu/pix/prochloron.gif.

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.

Components of Cells

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

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.

Figure 9. Electron microcsopes. The above (left) image of a transmission electron microscope is from http://nsm.fullerton.edu/~skarl/EM/Equipment/TEM.html. The above right image of a scanning electron microscope is from http://nsm.fullerton.edu/~skarl/EM/Equipment/SEM.html.