HL Origin of Cells

Cells – A 2.1 HL Origin of cells Guiding Questions “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?” Syllabus Objectives A2.1.1 AHL Conditions on early Earth and the prebiotic formation of carbon compounds Include the lack of free oxygen and therefore ozone, higher concentrations of carbon dioxide and methane, resulting in higher temperatures and ultraviolet light penetration. The conditions may have caused a variety of carbon compounds to form spontaneously by chemical processes that do not now occur. A2.1.2 AHL Cells as the smallest units of selfsustaining life Discuss the differences between something that is living and something that is non-living. Include reasons that viruses are considered to be non-living. A2.1.3 AHL Challenge of explaining the spontaneous origin of cells Cells are highly complex structures that can currently only be produced by division of pre-existing cells. Students should be aware that catalysis, self-replication of molecules, self-assembly & the emergence of compartmentalization were necessary requirements for the evolution of the first cells. A2.1.4 AHL Evidence for the origin of carbon compounds Evaluate the Miller–Urey experiment. A2.1.5 AHL Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers Formation of a membrane-bound compartment is needed to allow internal chemistry to become different from that outside the compartment. A2.1.6 AHL RNA as a presumed first genetic material RNA can be replicated and has some catalytic activity so it may have acted initially as both the genetic material and the enzymes of the earliest cells. Ribozymes in the ribosome are still used to catalyse peptide bond formation during protein synthesis. A2.1.7 AHL Evidence for a last universal common ancestor Include the universal genetic code, several hundred types of genes. Include the likelihood of other forms of life having evolved but becoming extinct due to competition from the last universal common ancestor (LUCA) and descendants of LUCA. A2.1.8 AHL Approaches used to estimate dates of the first living cells and the last universal common ancestor Students should develop an appreciation of the immense length of time over which life has been evolving on Earth. A2.1.9 AHL Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents Include fossilized evidence of life from ancient seafloor hydrothermal vent precipitates and evidence of conserved sequences from genomic analysis. 1|Page Conditions on early Earth and the pre-biotic formation of carbon compounds: Sun formed about 4500 million years ago and Earth formed soon afterwards as gravity caused gas and dust to come together. Temperatures were higher and the composition of gases in the atmosphere was very different from now. Look at the Earth’s prebiotic and modern atmosphere. Explain how it was different for: Oxygen (O2): Carbon dioxide (CO2) Methane (CH4) The temperature and UV radiation of the Earth was also considerably higher on Earth, which is a consequence of the lack of ozone layer 4 billion years ago. What is the ozone layer composed of? What is the function of the ozone layer? Why was the ozone layer not present at that time – how was it generated? 2|Page The oxygen content of Earth’s atmosphere has considerably changed over the last 4.5 billion of years. The concentrations was at zero, and has now stabilized at ca. 20%. Read the article on the rise and fall of oxygen by following this link https://theconversation.com/the-riseand-fall-of-oxygen-18954 and answer the questions below: The concentration of oxygen in the early Earth’s atmosphere was practically zero. Why? What caused the sudden rise of O2 in the atmosphere at ca. 2.4 billion years ago? What specifically is referred to as the “Great Oxidation event”? What consequences did the rise in oxygen levels have on the Earth’s climate? At around 450 million years ago the Earth’s atmospheric oxygen concentration was even higher than nowadays. What was the implication of this? 3|Page Cells as the smallest units of self-sustaining life: It is difficult to define what life is – typically the MRHGREN characteristics of life are used to describe what is required to maintain life, not life itself. A key difference between living and non-living thing is that living things use energy to keep themselves in a highly ordered state. Cells are self-sustaining with all its components, while the individual components on its own would not be considered to be alive. A virus is a very powerful pathogen. Is it alive or not alive? Challenge of explaining the spontaneous origin of cells: Throughout the 1700 and 1800’s the idea of abiogenesis and spontaneous generation of life (rats from sweaty grain, maggots from meat, etc) persisted – until Louis Pasteur (and others) disproved the idea in 1864. By preventing entry of airborne particles to a nutrient broth, he stopped growth of the culture. Leaving it open allowed microbes to grow. It seems obvious to us but was a huge step forward for cell biology. 4|Page In 1668, Francesco Redi challenged the idea that maggots arose sponatenously from rotting meat. How did Francesco Redi demonstrate that life does not simply spontaneously arise: Therefore, cells come only from other cells through cell division (mitosis or meiosis). But where did the cells that those cells come from? And the ones before those. And before those? Cells must have a common ancestor. Ergo, all living things evolved from a common ancestor and are related. The two conflicting theories keep scientists busy, and although there is no evidence to support the theory on spontaneous generation, it is possible for life to have arisen over a series of intermediate steps. 5|Page Cells could have arisen from non-living materials. How can structures as complex as the cell have arisen by natural means from non-living material? The process of abiogenesis is theorized to have occurred over four key stages: 1. There was non-living synthesis of simple organic molecules (from primordial inorganic molecules) 2. These simple organic molecules became assembled into more complex polymers 3. These molecules became packaged into membranes with an internal chemistry different from their surroundings (protobionts) 4. Certain polymers formed the capacity to self-replicate (enabling inheritance) Evidence for the origin of carbon compounds: Stanley Miller and Harold Urey`s experiment simulated the atmosphere at the early Earth and managed to create amino acids and carbon compounds needed for life from a mixture of methane, ammonia and hydrogen. Watch the video (https://youtu.be/NNijmxsKGbc) and describe the Miller & Urey experiment : Spontaneous formation of vesicles : If phospholipids were among the first carbon compounds, they would naturally assemble into bilayers. These bilayers readily form vesicles, resembling the plasma membrane of a small cell. This allows compartmentalization. 6|Page RNA as a presumed first genetic material: The polymers formed the capacity to self-replicate. DNA replication requires the action of enzymes. However, for enzymes to be made, genes are needed. The solution to this conundrum may have been an earlier phase in evolution when RNA was the genetic material. This theory is further supported because of RNAs ability to act as a catalyst because it can form complex 3D structures of polypeptides. For a long time it was believed that only enzymes are complex enough to act as catalysts in reactions. How can act RNA as a catalyst (ribozyme)? 7|Page Evidence for a last universal common ancestor: Explain why all organisms can be traced back to a common ancestor (Last Common Universal Ancestor, LUCA): Would it be possible that other forms of life with a different genetic code have evolved? Estimating dates of the first living cells and LUCA Research has shown that in Western Australia 3.5 billion-year-old stromatolites (rock mounds built up by cyanobacteria) established the oxygen content of the Earth’s atmosphere. The rocks were isotope dated and it showed remains of life. No rocks older than 3.5-4 bio years exist (destruction by tectonic movement, heat or pressure), so only isotope ratios (13C/12C) can be used for the dating of the first living cells using the hardest fragments that can exist. 8|Page Evidence for the evolution of LUCA near hydrothermal vents: A possible origin of complex carbon compounds is around deepsea vents. Hydrothermal vents (black smokers) are found at fissures in the oceanic crust, gushing hot water which often carries reduced inorganic chemicals such as iron sulphide, hydrogen, methane, or ammonia. These chemicals are used as sources of energy for the assembly of simple carbon compounds into polymers. Researchers have identified 355 genes coding for protein families that occur widely among bacteria and archea bacteria – the organisms which are believed to have originated first in the history of life. These genes were inherited from an early common ancestor – LUCA. Many of the genes are needed for anaerobic metabolism and carbon dioxide fixation – conditions preferably found around hydrothermal vents. Read more about this here: https://www.nytimes.com/2016/07/26/science/lastuniversal-ancestor.html 9|Page