METABOLISM:
Metabolic processes all involve oxidation and reduction which are opposites.
For every oxidation there is also a reduction occurring somewhere else.
Oxidation
1. Add oxygen
2. Remove hydrogen
3. Remove electrons
Reduction
1. remove oxygen
2. Add hydrogen
3. Add electrons
Exchange of electrons always occurs!!!
Two major types of metabolic process (opposites) S P
Catabolism - oxidation, degradation, breaking of covalent bonds, release of energy
(Exergonic) . Energy is released to produce ATP from ADP by Phosphorylation.
Anabolism – reduction, biosynthesis of new macromolecules by forming covalent bonds,
requires energy input (Endergonic) . Energy is from ATP ADP (dephosphorylation).
ATP provided by catabolism therefore reactions coupled. Coupling Agent is ATP.
Catabolic reactions are theoretically Spontaneous because of Energy Gradient in
Substrate relative to end products. Anabolic reaction not.
Catabolic reactions are not spontaneous however, because energy in molecular motion
not sufficient to overcome covalent bond energy. Need an input of energy or the removal
of the energy difference. This energy is referred to as Activation Energy.
Regulation of catabolic reaction is accomplished by removing the activation Energy
using Enzymes.
Enzymes:
1.
2.
3.
4.
5.
Protein
Specific
Have an Active site to attach to substrate and reduce activation energy
are unchanged during the course of a reaction (thus can reuse)
Lower activation energies of reaction to make them Spontaneous
Catabolic Reactions (Energy Generating)
Respiration : 1. aerobic – requires molecular oxygen
2. anaerobic – requires a bound form of oxygen
Both of the above are the complete oxidation of glucose (remove all the hydrogen)
And thus produce the maximum amount of ATP
Fermentation: occurs in the presence or absence of oxygen because not required.
Incomplete oxidation of glucose ( not all hydrogen removed) and therefore very
little ATP produced.
Three biochemical processes are potentially involved with Respiration and fermentation
1. Glycolysis (removal of hydrogen with a coenzyme NADNADH)
2. Tricarboxylic Acid cycle {TCA, Citric Acid or Kreb’s cycles} (removal of hydroden
with NADNADH and FAD—FADH)
3. Oxidative phosphorylation (using cytochromes to transfer hydrogen from NADH and
FADH to a form of oxygen (molecular or bound).
Glycolysis:
Glucose  pyruvic acid + ATP + NADH
Three major steps:
1. Glucose Fructose Diphosphate Activation using ATP
2. Fructose Diphosphate Glyceraldehyde Phosphate splitting glucose with
Aldolase
3. Glyceraldehyde Phosphate  pyruvic acid + ATP + NADH oxidation by
removal of hydrogen – no oxygen required , and phosphorylation. No carbon
dioxide given off.
TCA cycle:
Pyruvic Acid  Carbon Dioxide + ATP + FADH + NADH
Oxidation by removal of hydrogen – no oxygen required, and phosphorylation.
By definition decarboxylation occurs. Carbon dioxide is given off in the TCA cycle
There is actually a transition step between Glycolysis and The TCA cycle proper.
Pyruvic Acid Acetic acid + Carbon dioxide + NADH
Oxidative Phosphorylation:
Oxidation of coenzymes and phosphorylation occurs.
NADH + FADH + Oxygen (molecular or bound) NAD + FAD + reduced Oxygen
+ATP
Hydrogen transferred from NADH and FADH to cytochromes
Cytochromes transfer hydrogen to a form of oxygen to reduce it.
Electrons flow through the cytochromes in a membrane in this process and Protons back
and forth across the membrane using ATPase which also produces ATP
For each NADH NAD
FADHFAD
3 ATP’s produced
2 ATP’s produced
All three processes described above occur in respiration
Only glycolysis in Fermentation
Fermentation is
Glucose acid/alcohol + ATP
It is therefore Gylcolysis plus one additional step to convert NADHNAD
That step is:Pyruvic Acid +NADH  acid/alcohol +NAD
The hydrogen is transferred from the NADH to the pyruvic acid to produce the
acid/alcohol
Glycolysis and the TCA cycle occur in the cytoplasm of Prokaryotes. Oxidative
Phosphorylation in the Cell membrane
Glycolysis occurs in the cytoplasm of Eukaryotes. The TCA cycle and Oxidative
phosphorylation in the mitochondria.
PHOTOSYNTHESIS: is the reduction of carbon dioxide to glucose using solar
radiation.
6CO2
+ 6H2O  C6H12O6 + 6O2
There are two major types of reaction involved in the above process
1. Light Dependant
2. Light Independent
In the light dependant reactions chlorophyll absorbs light as a source of energy. There are
two types of chlorophyll molecules and associated reactions termed photosystems I and
II.
Photosystem II occurs first.
On the absorption of a photon of light energy the chlorophyll becomes oxidized by the
loss of an electron. The electron passes through a cytochrome chain while protons move
back and forth across the membrane in which the cytochromes are located resulting in the
production of ATP. The electrons lost from chlorophyll on oxidation and the protons
moving across the membrane are derived from water when it is photolysed as follows:H20  O + 2H
2H  2H+ + 2eIn Photosystem I absorption of light also causes oxidation of chlorophyll by loss of an
electron which the enters a second cytochrome chain. The electron flows through the
chain just as in PSII however protons do not pass across the membrane and therefore
ATP is not produced. The electron lost from chlorophyll is replaced by electrons from the
cytochrome chain of PS II.
When electrons come out of the cytochrome chain associated with PSI they recombine
with protons from water and attach to NADP (a coenzyme) reducing it to NADPH.
In summary:
PSII generates ATP energy
PSI generates reducing power (H from water) in the form of NADPH
Light Independent Reactions: require
1. Carbon Dioxide from air
2. ATP from PS II
3. H in the form of NADPH from PSI
4. An enzyme Ribulose Biphosphate carboxylase (Rubisco)
5. Sugar – Ribulose biphosphate to attach Carbon dioxide to.
There are three major reactions:
1 Ribulose Biphosphate + carbon dioxide  Phosphoglyceraldehyde {PGA}
(catalyzed by Rubisco)
2. Some PGA is converted to Glucose by the reverse of glycolysis. This requires
ATP and H from NADPH (reduction) both generated in the Light dependant
reactions.
3. . Some PGA is used to regenerate Ribulose biphosphate. This also requires
ATP and H from NADPH (reduction) both generated in the Light dependant
Reactions. This particular series of reactions is referred to as the Calvin
Benson cycle.
Light dependant reactions occur in the thylakoid membranes and the light independent
reactions in the stroma of the chloroplasts.
Science, the Scientific Method and Biology
Science is the mechanism by which humans strive to understand the world around us.
The practice usually entails a combination of two distinct approaches: discovery driven
and hypothesis driven inquiry techniques or methods, to ultimately find the natural causes
for natural phenomena in our “world”. The scope is limited to the study of structures and
processes that are either directly or indirectly observable. The foundation of “discovery
science” is the observation and measurement of phenomena, that is verifiable, to draw
conclusions best describing a particular observation. This process is based on inductive
reasoning: that is the generalization or conclusion is based on summarization of many
concurrent observations. <Example cell theory: The observation of cells in all organisms
leads to the conclusion: all organisms are made of cells>
The aforementioned approach is the basis of the second approach to science: hypothesis
driven science which utilizes the Scientific Method, which succinctly can be described as
an organized rationale approach to problem solving. A key element of this approach is
the use of deductive reasoning: use of general observations to produce specific
conclusions <since all organisms are made of cells then any specific entity if an
organism must be composed of cells>.
The generalized procedure of the scientific method is to first make observations regarding
a particular phenomenon (either directly or by using the knowledge of others).
Subsequently, one can then make generalizations (hypotheses) to explain the
generalization. A hypothesis (alternate) is a tentative explanation of the phenomenon.
The hypothesis is the basis for organization of experimentation to see if results of
experimentation are in agreement or disagreement with the hypothesis. In this case
deductive reasoning is utilized. That is, we are extrapolating specific outcomes of
experimentation if the hypothesis (premise) is correct. A null hypothesis is the opposite
of the alternate hypothesis, that is the premise has no effect on the outcome of the
experiment. Ultimately after much experimentation the object is elevate the hypothesis (if
validates by experimentation) to a theory. This can only be accomplished if many
different types of experiments and observations support the hypothesis and there is no
valid contradictory information gathered during experimentation. In science theories are
considered to be the best explanation for particular phenomena. However, if during
experimentation evidence contradictory to the hypothesis or theories becomes available,
new hypotheses and ultimately new theories must be formulated. When generalizations
are strongly supported (theories) it is possible to make predictions about similar
phenomena. <Fig.1.18Essential Biology>
When asking questions using the scientific method experimentation has to be highly
organized and controlled to have any value. While there are no specific rules to
experimentation the guidelines should be followed:
First one should recognize there are two hypotheses:
The Null hypothesis, which says the experimental treatment, has no effect on the
outcome of the experiment and secondly the Alternate hypothesis, which says the
experimental treatment, has an effect on the out come of the experiment.
Since many things (known as variables) can affect the outcome of experimentation the
experiment should be designed such that there is potentially only a single explanation for
the results. The variable that is being tested (i.e. affects the outcome of the experiment –
the alternate hypothesis) is known as the independent variable (that factor manipulated
during the experiment). The outcome one measures as one varies the independent
variable is called the dependent variable. <Ex. Fertilizer and growth in plants>. Every
other factor or condition is referred to as a controlled variable and they must be
maintained constant during experimentation. During experimentation one usually runs
control and experimental procedures where is only difference is the experimental has a
quantifiable manipulation of the independent variable.
Remember experimentation requires validation, thus the results of a single experiment are
not valid. The experiment should be repeated (known as replication) several times and if
possible the results analyzed statistically.
Notice the hypotheses only include the dependent and independent variables.
The purpose of science is our attempt to understand the world around us, what is it, how
does it function and what are the controlling factors and may be ultimately gain some
control of it. As a result it must be guided by natural law and must be empirically
testable with conclusions usually being tentative.
Biology
Succinctly, Biology is the scientific study of Life. While life (or living things) is
intuitively obvious it is often difficult to concisely describe. Life can be determined or
defined by its characteristics. Despite life’s great diversity there are several generally
accepted characteristics that separate living and non-living things:
i)
ii)
iii)
it is a complex, highly and precisely organized structure consisting of organic
molecules
it is capable of growth and development
has self regulated metabolism where materials and energy are acquired from the
environment and converted to different forms
iv)
v)
vi)
vii)
viii)
is capable of movement
responds to stimuli
Has the ability to maintain its complex structure and internal environment by a
process called Homeostasis
is capable of reproduction whether simple or complicated using DNA genetic
information)
adapts to environmental changes which results in evolution
Life is not described as the sum of its parts. It arises as a result of a complex ordered
series of interactions among the parts known as Emergent Properties <explain>
In addition the characteristics or properties are dependent upon the following three major
factors:
i)
ii)
iii)
Evolution
Transmission of Information
Energy transformations
All of the above is the result of a phenomenon called the Central Dogma (DNA-> RNA> protein). All life forms are controlled by the same chain of commands. The information
that gives life its properties and capabilities are stored in genes which are parts of
chromosomes. The macromolecule that contains the information is Deoxyribonucleic
Acid. In order to utilize the information it must be converted to Ribonucleic acid.
Ribonucleic Acid is then utilized to produce the enzymes (protein) which control the
biochemical reactions resulting in the specific life forms attributes and capabilities.
Organization of Life
There are several methods or ways of looking at the organization of life, each of which
are outlined in the following text. The first and most common, is to look at life’s
organization from its basic sub units to its most complex. The smallest components of
life are electrons, protons and neutrons. (Of these electrons are probably the most
important as we shall see in chapter 2). These are the fundamental sub units of which the
next level of organization, atoms, are comprised. An atom is the smallest unit of an
element that still retains the properties of that element. It is the particular arrangement of
electrons, protons and neutrons that define the specific properties of a given element.
There are more than one hundred known elements varying in size and properties as a
result of their subatomic composition. However, 99% of biological material is comprised
of only six of these elements; carbon, hydrogen, oxygen, nitrogen, phosphorus and
sulfur. All of these elements are relatively small and chemical unstable. All unstable
atoms will naturally undergo chemical reactions to gain stability. These reactions require
the formation of some kind of bond between the atoms that result in some type of
association. The associated atoms form the next level of organization, molecules, where
molecules are a unit of two or more atoms (of the same or different elements) bonded
together. In biological systems hydrogen and oxygen bond to for water, which occupies
approximately 70% of the mass of life forms. While water is the dominant molecule there
are many other molecules of varying properties and sizes in biological systems. The next
level of organization requires stable molecules to become unstable such that they interact
by formation of bonds to produce specific macromolecules with definite properties. In
biological systems there are four major macromolecules with unique properties:
carbohydrates, proteins, nucleic acids and lipids. As we shall see later, the latter is not a
true macromolecule. To produce a basic life form (called a cell) there is a requirement to
associate macromolecules together in specific ratios and minimal quantities. A cell
may be defined as the smallest living unit that may live independently or as a part of an
organism. All cells have a membranous structure surrounding them to isolate them from
their external environment and thus impart organization. Robert Hooke coined the term
cell and was the first scientist to actually observe these structures as components of multicelled organisms. Anton Van Leeuvenhoek is credited with being the first person to
observe individual cells as single celled organisms. Notice all cells have the same
hierarchy of organization, known as biochemical unity. Approximately 170 years after
the first observation of cells, in the 1830’s, Schwan and Schlieden independently
proposed a fundamental concept called Cell Theory-all living things are composed of
cells and that all cells come from other cells. Cells have unique properties associated
with life, they have genetic information that enables them to make exact copies of
themselves, they are capable of biochemical reactions (Biosynthesis) for the formation of
new biological material (protoplasm) and are all capable of Energy transformations.
Biosynthesis and energy transformations in biochemistry are termed Anabolism and
Catabolism respectively.
Actually at the cell level there are two fundamentally different cell types, Eukaryote and
Prokaryote. There is a fundamental difference in their hierarchical organization. In
eukaryotic cells certain macromolecules and biochemical reactions are isolated and thus
separated from the rest of the cell in membrane bound structures called organelles. One
can thus organize life forms very simplistically into one of two groups based on cell type.
Organisms are either Prokaryotic or Eukaryotic. Prokaryotic organisms are always single
celled, while eukaryotic can either be single or multi celled.
In many multi-celled organisms and thus eukaryotic organisms, but not all, one finds
cells organized in masses called tissues. One can define tissues as a group of cells and
intercellular substances functioning together in a specialized way. One can then organize
one or more types of tissue into an interacting structural or functional unit known as an
organ. Two or more organs whose separate functions are integrated in the performance of
a specific task are termed an organ system. By specific aggregation of specialized
independent cells arranged in tissues, organs and organ systems one can create a highly
complex multi-celled organism that fundamentally has the same basic properties as a
very simple single celled organism (independent existence and the capability of
producing competitive offspring –Inclusive fitness).
Any organism prokaryotic or eukaryotic, single or multi –celled that has specific
properties is termed a species. Currently there are approximately ?????? known species in
the biological world. Many individuals of the same species occupying a given area are
known as a population. In the world around us, many different populations nearly always
coexist in some fashion in a given area. This level of organization in ecology is referred
to as a community. Any community and the physical and chemical environment in which
it exists is termed an Ecosystem. Many different types of ecosystems can be found on the
planet earth, each having its own communities due to variations in the physical and
chemical environment.
A biome usually contains many types of ecosystems that are determined by specific
climatic and vegetation conditions and thus characteristic organisms adapted to the
particular conditions. In essence a biome is a very large geographic ecosystem of which
there are eight major terrestrial currently recognized. Ultimately one can group the
biomes together. The regions of the earth’s surface (crust, waters and atmosphere) where
live forms are found, that is the biomes are recognized as the Biosphere. In one sense,
the biosphere may be considered one large complex living entity- the Gaia hypothesis.
Notice in this organizational scheme there is a similarity up to the cell level, variation or diversity only
occurs at the cell level and above.
Also associated with this organization is the idea of Emergent properties- that is the number of
properties exhibited at any given level is greater than the number of properties exhibited at the previous
level (i.e. the sub components 2+2 = <4)
Biosphere
(Those regions of the earth in which organisms are present)
Biome
(A combination of ecosystems in a given geographical or climatic region)
Ecosystem
( A community and its chemical and physical environment)
Community
(many population of different species living in conjunction)
Population
(Group of individuals of the same species)
Species
( an organism with specific properties/characteristics)
Organism/*Multicelled organism
(life form either * **single or *multicelled)
(macrobial organisms composed of specialized, independent cells organised into tissues,organs and organ
systems)
*Organ System
(two or more organs whose separate functions are integrated to perform a specific task)
*Organ
( one or more tissues interacting as a structural , functional unit)
*Tissue
( a group of cells functioning together in a specialized manner)
Eukaryotic*
Cell
**Prokaryotic (always single celled)
(smallest living unit capable of independent existance as an organism or part of an organism)
Organelle*
(membrane bound structure in a eukaryotic cell containing macromolecules with specific functions isolated
from the rest of the cell)
Macromolecule (4)
(large molecules with specific functions)
Molecule
(two or more atoms of the same or different elements bonded together)
Element
( aprticular combination of protons, neutrons, & electrons with specific properties)
Atom
(Smallest unitof an element that retains the properties of that element)
Sub Atomic Particle
(Electrons, protons, & neutrons-particles of which atom comprised)
We have previously inferred a second type of organization of the biological world based simply on cell
type (i.e. Prokaryotic versus eukayotic). It is also possible to organize life forms into the Microbial versus
the Macrobial World. This is frequently referred to as Lower versus Higher organism organization. The
terminologies higher and lower should not be used since they imply some kind of superiority and inferiority
among life forms based on their complexity. While the terms imply the classification is based on size
(small Vs large) and while relative sizes may be relevant the basis for organization is cell type, cell number
and cell organization primarily.
In the microbial world the following types of organisms are found:
i)
single celled prokaryotes
ii)
single celled eukaryotes
iii)
multicelled eukaryotes where all the cell are similar and independent
iv)
multicelled eukaryotes where the cells are dissimilar and independent
The single celled prokaryotes are generally small (usually not visible to the naked eye, whereas the multicelled eukaryotic variants can be small but are usually clearly visible and may easily be one meter in size.
Multi-cellularity in cases iii) and iv) does not technically include tissue, organ and organ system
development. In the macrobial world organisms are comprised of many eukaryotic cells. The cells by
definition are dissimilar however; they are dependent up on each other because they are organized into
tissues, organs and organ systems that aggregate to form the organism. This cell type and organization is
specific to macroorganisms and as a consequence usually dictates a large and more complicated organism
relative to microorganisms.
Figure:
Prokaryotic (always singled celled organisms)
Cell
Eukaryotic
Single celled organism
Multicelled organism i) similar independent cells
ii)dissimilar independent cells
iii)dissimilar dependant *
*macrobial world,
Systematics and Taxonomy
Systematics is the science of showing evolutionary phylogenetic relationships between organismsand its
classification. It employs Taxonomy ( the naming of organisms or groups of organisms to arrange
organisms) to reflect phylogeny.
Implicit in this organization is the naming of organisms using two names, Genus and species (Binomial
Nomenclature) as devised by Linnneaus. Organisms once named are then organized into a hierarchy of
broader taxonomic categories (Taxa) – family, order, class, phyla, and kingdom.
Initial classification schemes used motility and mode of nutrition to assign organisms into on of two
kingdoms – Animalia- motile, heterotrophs
Plantae - nonmotile, autotrophs
The discovery of “the microbial world” led to problems that Haekel addressed by creating a new kingdom,
the Protista
Further problems (the protista contained fundamentally different prokaryotic and eukaryotic cells) were
resolved by Whittaker. Using cell type and mode of nutrition (saprophitic), the protista were subdivided
into:Protista- eukayotes
Fungi- eukaryotes, saprophitic
Monera- prokaryotes
The macrobial world kingdoms Plantae and Animalia were retained
More recently this five kingdom classification scheme has been modified using molecular level attributes
of cells. In this classification scheme Woese includes a new taxonomic level above Kingdoms- Domain.
There are fundamental differences in the size of ribosomes (primarily responsible for translation of
proteins) between Eukaryotes (80S) and prokaryotes (70S). On analysis of a specific ribosomal RNA
component of the ribosomes Woese found all Eukaryotes to be similar and thus grouped all eukaryotic
kingdoms into a single domain – Eukaryae.
However prokaryotic cells had two different rRNA structures and thus he divided the monera into two
Domains – Archae and Eubacteria . Each domain possessing one kingdom Archaebacteria and
Eubacteria respectively. Thus in this classification there are 3 domains.
Eukaryae divided into 4 kingdoms , animalia, plantae, protista, and fungi.
Archae, kingdom Archaebacteria
Eubacteria, kingdom Eubacteria
Cell size and multi cellularity.
The size of an organism, the number of cells that make up the organism and the specific organization of the
cells as previously discussed, is not arbitrary. There are limits to the size of cells and thus these limits
will dictate the sizes of organisms and the number of cells and organization required to form a given
organism.
Generally, single cells and thus single celled organisms are no smaller than 1.0 (1/1000000M) in
diameter. One might ask why? The rationale is quite simple in reality. Remember cells have very specific
properties as a result of the four macromolecules of which they are comprised. Specific quantities of each
macromolecule are required to provide these properties that define life. Since macromolecules are
ultimately comprised of a given number of atoms and each atom has a specific size dictated by its
subatomic parts, one can theoretically calculate the minimum size of a cell simply by multiplication of
the sizes of the atoms by the number required to produce the minimum number of macromolecules
that result in the properties defining life. When this multiplication is performed the approximate size is
1.0. In this regard viruses are considered a-cellular because they are smaller than 1.0 and thus have
insufficient volume in which to place all the macromolecules required to define life. In fact most viruses
only have two of the four major macromolecules associated with them (nucleic acids and proteins).
The maximum size that a cell (and thus a single celled organism) can attain (approximately 100.) is
a function of the surface area to volume ratio. Prior to the discussion why? It is necessary to review
some simple geometry of three-dimensional shapes. The simplest case to review for conceptual purposes is
a cube. The surface area is simply calculated by multiplication of the length of a side by itself (L x L) to
obtain the surface area of a single side, this is then multiplied by six since there are six sides to a cube. If
one were calculating the surface area of a sphere (the approximate shape of most cells) the equivalent
calculation is 4  r2. To calculate the volume of a cube, the following formulation is used: L x L x L. Again
the equivalent calculation for a sphere for surface area is 4/3  r3..
The ratio of surface area to volume for a sphere thus is (6 x L x L) / (L x Lx L), or 6/L, whereas for a cube
the ratio is (4  r2) / (4/3  r3), or 3/ r. As can be seen in the following example, as one increase the value
of either L or r (as one does when the size of the object is increased) that while both the surface area and
volume increase, the volume increase proportionately more because the former is a square function and the
latter a cube function, thus decreasing the surface area to volume ratio. Consequently when the diameter of
a spherical cell is approximately 100 the surface area to volume ratio becomes limiting. Organisms larger
than this become multi-celled in order to overcome the limitation as we shall see. In addition the larger the
organism becomes the more complicated the organization of the cells that comprise the organism becomes.
There are exceptions to this general rule. If cells are elongated (the equivalent of cylinders) there is
technically no limitation to the absolute size (only diameter) because of different geometrical principles.
For cylinders the surface area is calculated in the following manner 2rL, where r is the radius of the
cylinder and L is its length. Volume is calculated according to the following formula,  r2 L. The ratio of
surface area :volume is thus 2rL /  r2 L. On solution the surface area : volume ratio is thus a function of
2/r. Consequently, cells can be of great length as long as they have a diameter no larger than 100., which
is generally the case. Exceptions to this rule require dead space with the cell to increase the ratio (as in the
case of plants).
The importance of the above is in relation to food uptake vs. food requirements. The volume represents the
food requirement of the cell. The larger the cell the more food required. One must recognize a volume
specific minimum requirement that needs to be provided to sustain life. Uptake through the surface area
provides this requirement. The more surface area the more uptake. However, there is an area specific
maximum uptake. Total minimal requirements can be determined by multiplication of the total volume of
the cell by the volume specific minimum requirement. Total uptake can be determined by multiplication of
the total surface area by the area specific maximum uptake. At approximately 100 cell diameter, the two
are equal, thus sufficient food can be acquired, however, above 100 cell diameter, uptake is less than that
required and the cell cannot sustain itself.
If organisms are larger than 100 then they are multicelled in order to overcome the surface area:volume
limitations. As they become very large there is a requirement for the evolution of differentiation of cells
into tissues organs etc. to take advantage of the surface area created (evolution of the microbial world).
History of Life
The processes leading to life are thought to have begun some 4 billion years ago when random chemical
reaction occurred that ultimately led to the formation of the four major macromolecules required for life.
The first single celled life form appeared as a result of these processes some 3.8 billion years ago and
presumably was a prokaryotic cell. Remember this is not necessarily true (urkaryote). The theory purported
to explain this is Spontaneous Generation. This first life form is now considered to be the common
ancestor to all organisms that exist today according to Buffon. Virchow proposed all organisms arose by
reproduction from this common ancestor. The initial organisms were metabolically relatively simple using
inorganic molecules as sources of nutrients ie Autotrophs.
It is thought that the autotrophic process of Photosynthesis evolved some 2 billion years ago. Initially this
was associated with prokaryotic organisms. The consequences of the evolution of this process are that
i organisms were now capable of migration from aquatic to terrestrial ecosystems
ii it enabled increased metabolic diversity- Heterotrophy (respiration & fermentation)
Between 1 amd 2 billion years ago Endosymbiosis occurred leading to single celled Eukaryotes. One
billion years ago the first multicelled Eukaryote evolved utlimately leading to the evolution of the
macrobial world somewhere between 1.billion years ago and now.
Tying it all together- Theory of Evolution: