The Properties of Life from a Biological Perspective

What is Life Made Of? §

• Cell Theory postulates three axioms regarding life.
  • All organisms are composed of one or more cells.
  • The cell is the basic unit of strcuture and organization in organisms
  • Cells arise from pre-existing cells.
• Cells are characterized by the following properties.
  • Highly organized and complex. This is despite the fact that such processes arise stochastically (see Levinthal’s Paradox )
  • Contains a genetic program and the means to use it. This genetic program governs all cellular processes, and it can mutate to induce variations.
  • Capable of reproduction. This is via cell division.
  • Acquires and Utilizes Energy. The primary energy source for all things is the sun.
  • Carries out a variety of chemical reactions (i.e., metabolism)
  • Engages in mechanical and dynamical processes
  • Responds to stimuli
  • Has self-regulation mechanisms. These mechanisms protect the cell from dangerous fluctuations in internal composition or behavior.
  • Can Evolve. All organisms arose from a single common ancestral cell (see LUCA)

Why is Water Important For Life? §

1. It is highly asymmetric, with atoms on one end and atoms on the other end.
2. It is highly polarized.
3. Its constituent atoms can form hydrogen bonds
4. Water can dissolve many substances, more than any other solvent.
5. It is the fluid matrix around which the insoluble fabric of the cell is constructed. It can also be used for transporting materials.
6. Itis involved in many chemical processes (i.e., hydrolysis)
7. It protects the cell from excessive heat, cold or radiation.
8. It can interact with many chemical groups (in particular hydrophobic and hydrophilic molecules)
9. It is amphoteric (i.e., both an acid and a base).

Endosymbiont Theory §

• One theory on the origins of life is that Eukaryotic cells came from prokaryotic cells.
• It is also thought that a prokaryotic cell consumed another prokaryote which resisted phagocytosis. This ingested prokaryote then became integrated into the cell as the mitochondria and evolved alongside the eukaryote. The same happened for chloroplasts
• This has been shown through the following:
  • It is now known that some bacteria can form complex membrane structures.
  • Sequencing of ribosomal RNA (i.e., 165 rRNA in prokaryotes and 185 rRNA for Eukaryotes) show a match between chloroplasts and cyanobacteria.

Links §

• Karp Ch. 1.1 - 1.2 - reading on Cell Theory and the properties of cells.
• Karp p. 37 - the importance of water in life.
• Karp p. 56 - Endosymbiont theory.