Linnaen System of Classification Originated in the 1700s and was based on physical characteristics, ex. if it make its own food, moves.
Linnaen genus:species scheme Still used, but two kingdoms (animals, plants) wasn't enough to explain fungi, etc.
Technology eventually advanced to where the contents of individual cells could be examined; two basic cell types were seen. Those with a "kernel" (=karyon=nucleus; eukaryotes).
Those without a "kernel" (prokaryotes).
Contents of a typical prokaryotic cell Contains a nucleoid, simple single chromosome, cytoplasmic membrane, and cell wall.
Nucleoid Area in a prokaryote where the chromosome is located.
Contents of a typical animal cell Nucleus surrounded by nuclear envelope, usually containing multiple chromosomes; mitochondria, lysosomes, endoplasmic reticulum, golgi apparatus, cytoplasmic membrane, and other.
Organelle Membrane-bound structures within cells.
Until ~1977, organisms were thought to fall into 2 "superkingdoms". Eukarotes and Prokarotes.
Physical/structural characteristics are useful for crude classifications, but to understand true evolutionary relationships among organisms (to make a true "tree of life" and find a common ancestor), it's necessary to look at... ... their genomes and biochemical systems.
Discovery of Archae ~1977, Carl Woese and others at Illinois compared the sequences in different species of molecules (small subunit ribosomal RNAs) which are an essential component of every cell's machinery for synthesizing proteins.
Why ribosomal RNAs? - evolutionarily ancient
- found in all organisms
- same function in all organisms
- highly conserved
"Prokaryotes" are actually two distinct groups of organisms: (Eu)bacteria: true bacteria like E. coli.
Archae: "ancient" prokaryotes, found in extreme environs resembling early Earth.
Based upon rRNA comparisons, it was found that Archae are more closely related to... Eukaroyes, as the archae rRNA sequences are more similar to those of eukaryotes than that of the bacteria.
Despite physically resembling bacteria (being prokaryotes)... ... in most molecular processes, archae are more similar to humans than bacteria.
Chromosome shapes for the 3 kindoms Bacteria - circular
Archae - circular
Eukarya - linear
Other genes have been analyzed using comparative methods; phylogenetic trees drawn from these data differ significantly. Not as simple as once thought; what do these studies suggest? The new tree/web of live includes Horizontal Gene Transfer.
Horizontal Gene Transfer Domains acquiring genes from other domains, esp. for small organisms. Messes up the tree of life.
Why must life exist as cells? Life requires a structural compartment separate from the external environment in which molecules can perform unique functions in this relatively constant internal environment.
Basic tenets of the cell theory. - cells are the fundamental units of life
- all organisms are composed of 1+ cells
- all cells come from preexisting cells
Why are most cells small? As size increases, surface area to volume ratio decreases; diffusion becomes inefficient, hard to move things around the cell.
Resolution The ability to identify the separation of two objects that are close together.
Microscopes Resolving power of light microscopes ~0.2 microns. Electron microscopy has resolution of ~0.5 nm; denser material absorbs more electrons and appears darker.
Plasma Membrane - allows cells to maintain a constant internal environment
- acts as a selectively permeable barrier
- is an interface for cells where information is received from adjacent cells and extracellular signals
- has molecules that are responsible for binding and adhering to adjacent cells
Composition of a bacteria cell Contains ribosomes, nucleoid (with chromosome), plasma membrane, cell wall (2 layers), and possibly a capsule (slimy protective layer to prevent dehydration and help it to stick to other cells; nonessential).
The four major macromolecules -