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

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

At their root, organisms are non-equilibrium, homeostatic (i.e. steady state), adaptive, 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 now living on earth 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 the "last universal common ancestor" (luca) to all of today's organisms.

Organisms range from those consisting of a single-cell, known generically as 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 can cooperate or compete with one another. 

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

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

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 has been 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 live, and they either had to adapt or find an environment in which O₂ was not present. 

While a catastrophe for some, the appearance of O₂ was an opportunity for others ; 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 first transferred into a molecule of ribonucleic acid (RNA).   The synthesis of RNA is a process known as transcription.

RNA molecules have a number of roles in cells, one of which is to specify the synthesis of proteins - this process of protein synthesis is known as translation.   Translation is carried out using a macromolecular catalytic complex composed of RNAs and proteins, the ribosome. 

With each new cell formed, DNA is replicated and the "daughter" cell receives a copy - a copy is also retained by the mother cell.  DNA is not completely stable, and replication is not error free - changes in the DNA (mutations) occur.

With minor variations, all organisms use the same universal code that links DNA sequences to protein sequences. 

Both 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 is stored in DNA using a universal code
• DNA acts as the template for the synthesis of RNA (transcription).
• Some RNAs encode polypeptides; the synthesis of polypeptides (translation)  is catalyzed by ribosomes (large RNA-protein complexes).
• Cells are surrounded by lipid -protein membranes.
• Imported energy is used to maintain cellular structure and to build new cells. 
• Cells share many common chemical reactions
• All cells store chemical energy in molecules of 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 did 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 death.   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 genotypes that are associated with reproductively successful phenotypes.  These will become more common - the population will change, it will evolve.

On the other hand, a populations that fail to adapt rapidly enough to changes in their environment may disappear, they will 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?