Biology is a rapidly maturing science, with its roots in the work of early philosophers, beginning approximately 2600 years ago.

After much observation, experimentation, hypothesizing, testing, and revising, we now know the broad outlines of how living things work and how the remarkable diversity of organisms evolved.

At their root, organisms are non-equilibrium, homeostatic (i.e. steady state), self-replicating systems of macromolecules , built using information stored in nucleic acids.

The simplest organisms consist of a single cell.   We can think of cells as the atoms of life, the smallest living units.   

The Cell Theory of Life holds that all cells are derived from pre-existing cells. 

Based on the fossil record, and molecular similarities, it appears that all organisms are derived from a common ancestor that lived between 3.5 to 3.8 x 10⁹ years ago.

Since then, cells (and organisms) have been formed from pre-existing cells (and organisms). 

There is an unbroken chain of life from that "last common ancestor" to all of today's organisms.

Today there are organisms that range is size from less than 0.000001 meters (1 µm) in size to those that are 30 meters long, a range of over 10⁷ fold.   

Organisms range from those consisting of a single-cell, generally known generically microbes, to multicellular plants and animals, which can contain over 10¹³ distinct cells. 

Organisms can live independently or in communities.  For example, microbes often live together in biofilms.  Within a biofilm, organisms cooperate and compete with one another. 

The simplest form of multicellular organisms are colonies of related cells, held together in a loose aggregate. 

• What does it mean to be a non-equilibrium system?
• Why can't cells make their own energy? 
• What are the advantages and disadvantages of being unicellular versus multicellular? 
• Why isn't the "Cell Theory" not a fact?

Ecosystems:   Organisms interact with one another and their physical environment to form ecosystems.   

These interactions take many forms, including predator-prey, host-pathogen, and mutual dependence. 

In an ecosystem, organisms have to deal with the impacts of other organisms on their physical environment. 

The most dramatic life-based environmental impact to date was the generation of molecular oxygen (O₂) as a waste product of photosynthesis. 

O₂ is highly reactive.  Its accumulation transformed the conditions under which most organisms lived, and they either had to adapt or or find an environment in which O₂ was not present. 

While a catastrophe for some, the appearance of O₂ was also an opportunity for other organisms; it made possible the emergence of large, active multicellular organisms, such as ourselves. 

• What are the differences between a multicellular organism and an ecosystem?
• Are ecosystems equilibrium or non-equilibrium systems? 

Common features of cells (top) 

All cell share a large number of common features, inherited from their predecessors. 
 

In analogy with computers, all cells use a version of the same basic operating system.

They store genetic information in molecules of deoxyribonucleic acid (DNA), to use this information it must be transferred into ribonucleic acid (RNA).

With each new cell formed, DNA is replicated, and the daughter cells receives a copy. 

DNA is not completely stable, and replication is not error free - changes in DNA (mutations) occur.

The synthesis of RNA, a process known as transcription. 

RNA molecular have a number of roles in the cell, one of which is to specify the synthesis of proteins; this process is known as translation.

With minor variations, all organisms use the same universal code link DNA sequence to protein sequence. 

Translation is carried out using a macromolecular catalytic complex composed of RNAs and proteins, the ribosome. 

Transcription and translation require energy to occur.  This energy is captured from the external world.   Cells store energy for immediate use as adenosine triphosphate (ATP).

Many of the chemical reactions used to capture energy, to build and disassemble macromolecules (e.g. proteins and nucleic acids) are common to all cells.  Cells share a common central metabolism.

Features of cells

• Information stored in DNA using universal code
• DNA acts as the template for synthesis of RNA (transcription).
• Some RNAs encode proteins.
• Synthesis of proteins (translation) catalyzed by ribosomes (RNA-protein complex).
• Cell is surrounded by a lipid -protein membrane
• Imported energy used to maintain cellular structure and build new cells. 
• Common metabolic reactions
• Common short term energy storage – ATP

Cells have a boundary layer, a plasma membrane, that separates their insides (cytoplasm) from the external world. This membrane is composed of a lipid bilayer and associated proteins. 

• How does the Cell Theory explain the common features of cells and organisms?
• Would the Cell Theory have to be abandoned or drastically altered if an organism were discovered that this not share the features of life described above? 

The Theory of Evolution. 

Because DNA is not a completely stable molecule, and because errors can occur during its replication, changes in the DNA (mutations) occur and are passed on to daughter cells.   

Mutations can have a range of effects on the organism that inherits them, from little or none to lethality.   Over time, different organisms will have different genotypes (DNA molecules) and different phenotypes (different behavior, characteristics, etc).

Because of their phenotypic differences, some organisms reproduce more successfully than others.  

Which organisms reproduce most successfully will be determined in part by interactions with their environment. 

Over generations, the differential reproduction of individuals will lead to those genotypes, that are associated with reproductively successful phenotypes, to become more common - the population will evolve.

On the other hand, a populations that fail to adapt rapidly enough to changes in their environment may become extinct. 

Populations can also divide and adapt to different environments, a process that over time leads to new types or species of organisms.

• Attempt to devise a system in which a population of organisms does not accumulate genotypic and phenotypic difference over time. 
• If the environment were constant, would extinction occur?