A2.1:  Origins of Cells

Guiding Questions:  
Guiding questions help students view the content of the syllabus through the conceptual lenses of both the themes and the levels of biological organization.

• What plausible hypothesis could account for the origin of life? 
• ​What intermediate stages could there have been between non-living matter and the first living cells?

Linking Questions:  
Linking questions strengthen students’ understanding by making connections between topics.  The ideal outcome of the linking questions is networked knowledge.

• For what reasons is heredity an essential feature of living things? 
• What is needed for structures to be able to evolve by natural selection?​​

AHL A2.1.1— Conditions on early Earth and the prebiotic formation of carbon compounds.

• Outline the conditions that are thought to have existed on prebiotic Earth, including atmosphere, temperature, UV radiation, volcanic activity and asteroid bombardment. 
• State that the conditions of prebiotic Earth may have caused a variety of carbon compounds to form spontaneously. ​

AHL A2.1.2— Cells as the smallest units of self-sustaining life.

• Discuss the challenges of defining matter as living or nonliving.
• Discuss the reasons why cells are considered to be living. 
• Discuss the reasons why viruses are considered to be non-living.

AHL A2.1.3— Challenge of explaining the spontaneous origin of cells.  

• Outline the intermediate stages needed for the evolution of the first cells on prebiotic Earth.
• Discuss limitations in testing hypotheses about the evolution of the first cells. 

AHL A2.1.4— Evidence for the origin of carbon compounds.

• Outline the methodology, results and conclusion that can be drawn from Miller and Urey’s experiments into the origin of biologically relevant carbon compounds.
• Discuss the benefits and limitations of the Miller-Urey apparatus as a model for a natural phenomena.

AHL A2.1.5— Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers.  ​

• Outline the cause and consequence of the spontaneous formation of membranes and vesicles by amphipathic molecules such as fatty acids and phospholipids on prebiotic Earth.

AHL A2.1.6— RNA as a presumed first genetic material.

• State that modern cells use DNA as the genetic material and enzyme proteins as catalysts of metabolism.
• List properties of RNA that suggest it was the first genetic material.
• Compare the genetic stability of RNA and DNA.
• Outline the ribosomal ribozyme as a type of RNA that is still used as a catalyst.

AHL A2.1.7— Evidence for a last universal common ancestor.

• Define LUCA.
• Discuss why the LUCA is not thought to be the first cell, but rather is thought to be the last common ancestor to all living cells.
• Explain the use of deductive reasoning to predict what genes were present in the LUCA cells.
• List characteristics of the LUCA.

AHL A2.1.8— Approaches used to estimate dates of the first living cells and the last universal common ancestor. 

• Compare the estimated dates for the evolution of the first cells and of the LUCA cells to the age of Earth.
• Describe stromatolites as the earliest direct evidence of fossilized life. 
• Outline the use of isotopes and the molecular clock for estimating dates of the first cells and of the LUCA cells. 

AHL ​A2.1.9— Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents. 

• Explain the use of deductive reasoning to predict what genes were present in LUCA cells. 
• Describe the conditions present at a white-smoker hydrothermal vent. 
• Explain how knowledge of the genes present in the LUCA cells can provide evidence that the cells lived in the vicinity of hydrothermal vents.