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Class notes BIOL 102 (BIOL102)
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Biology and the Tree of Life:What Does it Mean to Say Something is Alive?
An organism is a life-form, a living entity made up of one or more cells. Although definitions differ, most
biologists agree that organisms share a suite of five fundamental characteristics.
The five tenets of life:
1. Organisms are made up of one or more cells
2. Organisms grow and replicate
3. Organisms are the products of evolution and continue to evolve
4. Organisms possess genetic information units called genes. They also respond to information
from the environment and adjust their internal systems
5. Organisms must acquire and use energy
These tenets come from the theory of evolution, cell theory, and chromosome theory of inheritance and
ask the questions what are organisms made of, where do they come from, and how is hereditary
information transmitted from one generation to the next.
All Organisms are Made of Cells:
Advances in microscopy have made it possible to examine how diverse and complex cells are. We now
know that cells have individual compartments and organelles and differ quite greatly from one another.
The realization that all life is made of cells was fundamentally important for biology, but it didn’t show
the whole picture. To do that, scientists needed to understand how cells come to be.
Where Do Cells Come From?
German Scientist Rudolph Virchow proposed that all cells come from pre-existing cells. The complete
cell theory builds on this; all organisms are made from cells, and all cells come from pre-existing cells.
Two Hypotheses:
This theory was in opposition with spontaneous generation theory. Scientists of the time believed that
life occurred randomly when the right conditions were available. Cell-from cell theory instead said that
all life must come from existing life. This theory then became a testable hypothesis. The hypothesis was
proven when a scientist exposed one nutrient broth to open air and another to sterile air. The open air
flask quickly grew bacteria colonies while the sterile flask remained sterile.
Life Replicates through Cell Division:
Most cells are able to reproduce by dividing. New cells arise when pre-existing cells split.
SEE PHGY 170 NOTES FOR DETAILED INFORMATION ON CELLS AND LIFE
Artificial Selection:
Natural selection is the process through which evolution occurs. Traits that create fitter lifeforms are
replicated and passed down, becoming more and more prominent in a group. Artificial selection is due
to human intervention. Humans have been artificially selecting lifeforms with traits beneficial to the
survival of humans in crops and animals for thousands of years. This sometimes accumulates as
detrimental traits to the species as human will overrides natural selection.
, Water and Carbon – The Chemical Basis of Life:
A classic experiment by Louis Pasteur tested the idea that organisms arise from nonliving materials. The
results of his work helped build a consensus that spontaneous generation does not occur. But for life to
exist, it must have once come from something nonliving. The question of where did life come from has
developed into the leading theory of chemical evolution. This theory states that inputs of energy led to
the formation of increasingly complex carbon-containing substances, until eventually it created a
complex that could replicate itself. At this point, the switch was made from chemical to biological
evolution. As natural selection took over, eventually a descendant of the original molecule gained a
metabolism and a membrane, fulfilling the five tenets of life.
Atoms, Ions, and Molecules – the Building Blocks of Evolution:
Hydrogen, Carbon, Nitrogen, and Oxygen make up to 96% of all living matter. Scientists postulate that
most critical reactions that took place to create life were done in an aqueous environment.
Properties of Water and the Early Ocean:
Water is essential to life. ~75% of the volume in a typical cell is water. Water is an incredible solvent that
allows many different chemicals to be dissolved and absorbed. This is due to water’s polarity. Oxygen
found in H2O has a partial negative charge while Hydrogen has a partial positive. This gives water very
unique properties. The polarity of water allows it to pull apart compounds into positive and negative
solutes. The water molecules surround the two solutes and pull them away from each other. This allows
almost any polar or ionic compound to dissolve in water.
Model Systems for Investigating Chemical Evolution:
There are two primary models that describe how chemical evolution may have arisen on earth:
1. Prebiotic Soup Model: Molecules were synthesized in the atmosphere or arrived via meteorites.
These would have then condensed in rain and fallen into the ocean, creating a “organic soup”
that would have allowed for greater molecules to emerge.
2. Surface metabolism Model: Dissolved gases came into contact with minerals lining the walls of
deep-sea vents and formed more complex organic molecules.
Since it is impossible to know for sure how and where chemical evolution occurred, the next best thing
is to re-create conditions in a lab to see the results. Micro-biome experiments were done to determine if
the prebiotic soup theory held up. Miller first tried the experiment, using CH4 and NH3 in a simulated
environment of water and lightning. The results of this experiment were larger, more complex
compounds including amino acids. The results of this experiment were later confirmed by others using
conditions and chemicals more similar to what would have been found on earth at that time.
Molecules of Life:
Protein Structure and Function:
In water, amino acids ionize. The amino group acts as a base while the carboxyl group acts as an acid.
This allows the amino to stay in solution and interact with many different solutes.
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