IB Biology - Origin of Cells

Who proposed prebiotic Earth's atmosphere was significantly different than today?

Oparin and Haldane

Composition of prebiotic Earth's atmosphere?

Nitrogen, carbon dioxide and water vapor with smaller amounts of methane and hydrogen. There was no oxygen gas or ozone (first released by photosynthesis)

What did a lack of ozone result in?

High levels of ultraviolet light entering the atmosphere

What did high concentrations of carbon dioxide result in?

Higher temperatures due to the greenhouse effect (CO2 traps heat)

What did Oparin and Haldane propose?

They proposed that the energy available from ultraviolet light and higher temperatures (as well as lightning and volcanic activity) on the prebiotic Earth allowed the spontaneous generation of organic compounds

Cell Theory states..

1. Cells are the basic unit of life

2. All organisms are composed of at least one cell

3. All cells come from pre-existing cells

All living organisms...

1. Have at least one cell

2. Are capable of carrying out metabolism

3. Maintain homeostasis

4. Respond to stimuli

5. Are capable of reproduction

6. Grow and develop

7. Contain genetic information (in the form of DNA)

Why are viruses not alive?

Living organisms should possess all of the characteristics of life. Viruses do have genetic material (DNA or RNA), however they do not possess most of the characteristics of life:

- Viruses do not have cells which carry out metabolism and homeostasis. Viruses cannot respond to stimuli, and they do not grow.

- Viruses are not capable of reproducing themselves. They are replicated by host cells.

What was required for the development of the first cells?

1. Catalysis

2. Self-replication of molecules

3. Self-assembly

4. The emergence of compartmentalization

Why are claims about prebiotic Earth only theories and not facts?

It is impossible to know the exact conditions which existed on prebiotic Earth. There is some evidence, however scientists will never conclusively know if they are correct. Additionally, the first protocells did not leave any fossils so we do not have any evidence of their actual makeup

What was the Miller-Urey experiment?

Testing Haldane and Oparin's hypothesis, Miller and Urey attempted to simulate the conditions on a prebiotic Earth in a laboratory setting. Using gasses, water, energy, and heat, to simulate the atmosphere of prebiotic Earth, changes were observed to see if life could form.

Results of the Miller-Urey experiment

After one week the water became a brownish-black. Analysis of the water showed that many complex organic molecules, including amino acids, had been produced. The experiment showed that at least some of the organic molecules required for life could be spontaneously generated under certain conditions.

Strengths of the Miller-Urey experiment

1. Modeled prebiotic Earth and its atmosphere

2. Demonstrated that molecules such as amino acids can be generated spontaneously under certain conditions

3. The design of the experiment allow it to be replicated by other scientists

Limitations of the Miller-Urey experiment

1. There remains debate on the actual atmosphere of prebiotic Earth

2. The experiment did not produce all of the organic molecules required for life

3. The simulation could not account for all conditions on the prebiotic Earth

Cell Membrane

Requires the separation of from the external environment

Formed of phospholipid bilayers (Phospholipids are amphipathic with hydrophilic heads and hydrophobic fatty acid tails

What do fatty acids do in water?

Fatty acids spontaneously coalesce to form spherical bilayers when mixed with water; it has been hypothesized that the first genetic material was trapped within a phospholipid vesicle forming a protocell

Why is RNA hypothesized to be the first genetic material?

1. RNA can store genetic information

2. RNA is capable of self-replication

3. RNA can catalyze reactions

Ribozymes

RNA molecules in the ribosome catalyze the formation of peptide bonds during protein synthesis

Last Universal Common Ancestor (LUCA)

The most recent common ancestor for all organisms on Earth

What happened to other life forms existing at the same time as LUCA

These forms of life became extinct as LUCA and its descendants outcompeted them

Evidence for LUCA

There are no fossil remains for LUCA, however the fact that all organisms share the same genetic code strongly suggests that all organisms have evolved from LUCA. All living things share a number of genes which are assumed to have been inherited from LUCA

How long ago did LUCA appear?

4 billion years ago

How did scientists estimate the age of the First Living Cells?

The oldest fossils of cells are approximately 3.5 billion years old. The minerals and rocks around the earliest fossils have been dated using radioactive dating of uranium, which decays at a steady rate.

Molecular Clocks

Comparing the number of mutations in genes between species allows scientists to estimate when the two species diverged into separate species. Scientists have used this technique to estimate that LUCA existed approximately 4 billion years ago

Where did LUCA evolve?

Evidence suggests that LUCA evolved in the vicinity of hydrothermal vents. An analysis of genes common to all three domains of life has identified genes that are highly conserved. Some of these genes are involved in thermophilic metabolism, suggesting that LUCA may have evolve in a high temperature environment such as hydrothermal vents.

Why hydrothermal vents?

Hydrothermal vents are rich in organic compounds, such as methane, carbon dioxide, and hydrogen. Experiments similar to Miller and Urey's show that the building blocks of life, such as proteins and nucleotides can form from these chemicals