Defining Life and Its Origins
Why it Matters
In 1984, a meteorite landed on Earth and it carried evidence that life once lived on Mars
3.1-What is Life?
Both living and non living factors consist of cells that carry the same biochemical processes.
Their cells are the same
3.1a-seven characteristics shared by all life-forms
All life displays order, harnesses energy and utilizes it, reproduces, goes through homeostasis,
responds to stimuli, evolves and grows/develops
The best example of a biological system that is not of the norm is that of a virus: although they
can reproduce and evolve, they lack the cellular machinery and metabolism to use the genetic
information in their DNA/RNA to synthesize their own proteins.
To make proteins, they have to infect living cells and control their metabolism and translational
machinery to reproduce. Therefore, most scientists don’t consider viruses to be alive
3.1b-the characteristics of life are emergent
The 7 characteristics are emergent because each one emerges from simpler interactions. For
example, the ability to harness and utilize energy is not a property of molecules, proteins or
biological membranes in isolation; rather it is the combined effort of these components.
3.4b-the first cells relied on anaerobic metabolism
Earliest form of life are heterotrophs, organisms that obtain carbon from organic molecules
The early atmosphere had very little oxygen, meaning organisms had to undergo anaerobic
respiration and fermentative pathways to get energy from organic molecules
Autotrophs on the other hand obtain their carbon from inorganic compounds such as carbon
A majority of plants are autotrophs
The earliest type of photosynthesis was anoxygenic photosynthesis: compounds such as
hydrogen sulfide and ferrous iron are used as electron donors for the light reaction
3.4c-Oxygenic Photosynthesis Led to the Rise in Oxygen in the Atmosphere
Scientists look at banded rock as evidence of oxygen becoming prevalent starting 2.5 billion
years ago (the dissolved oxygen in the water reacted with the iron in the water forming iron
oxide which formed on rock formations) This oxygen came from cyanobacteria, which used water as an electron donor to release
electrons (which can be used for photosynthetic electron transport) as well as oxygen
Photosynthesis that relies on the oxidation of water to release oxygen is oxygenic
3.4e-All present day organisms are descended from a common ancestor
Based on RNA, all organisms can be categorized into one of three domains: Archae, Bacteria,
Archae and bacteria both lack nucleus and are referred to as prokaryotes
However, molecular evidence shows that archae are more related to Eukaryotes than to
All life forms on earth share common attributes, the most fundamental of which are the
following: cells are made of lipid molecules brought together to form a bilayer, a genetic system
is based on DNA, the system of information transfer-DNA to RNA to protein, a system of protein
assembly from a pool of amino acids by translation using mRNA and tRNA using ribosomes,
reliance on proteins as a structural and catalytic molecule, use of ATP as molecule of chemical
energy, breakdown of glucose by the metabolic pathway of glycolysis to generate ATP. (7
attributes in total). These common attributes show that life evolved from a single ancestor –
LUCA (last universal common ancestor)
3.5-the Eukaryotic Cell and the Rise of Multicellularity
Present day eukaryotic cells are distinguished from either the archae or bacteria in the following
ways: (1) the separation of DNA and cytoplasm by nuclear envelope and (2) the presence of a
mitochondria, chloroplasts, ER, Golgi complex, etc.
3.5a-the theory of endosymbiosis Suggests that mitochondria and chloroplasts evolved from ingested
Common features in all eukaryotic cells: mitochondria and chloroplasts. Evidence shows these
organelles are descended from prokaryotic cells, with mitochondria descending from aerobic
bacteria and chloroplasts descending from cyanobacteria.
Endosymbiosis suggests that prokaryotic ancestors of mitochondria and chloroplasts were
engulfed by larger prokaryotic cells, forming a beneficial relationship called symbiosis. Slowly
over time, the host cell and the symbionts became inseparable.
3.5b- several line of evidence support the theory of endosymbiosis
if chloroplasts and mitochondria did originate from prokaryotic cells, then they need to show
similarities to them - which they do
there are 6 pieces of evidence: morphology, reproduction, genetic information, transcription
and translation, electron transport and sequence analysis morphology: the general structures of chloroplast and mitochondria are similar to those of
reproduction: just like prokaryotic cells, mitochondria and chloroplasts cannot be synthesized.
Instead, they are made through binary fission
genetic information: like prokaryotes, the genetic information in chloroplasts and mitochondria
Transcription and translation: both chloroplasts and mitochondria contain complete
transcription and translation machi