Origins of cells (HL)

• Contents


  • Formation of Carbon Compounds (HL)
  • Evolution of Cells (HL)
  • Evidence for Evolution of Life (HL)
• Origin of Carbon Compounds


  Conditions on early Earth and the origin of carbon compounds
• Conditions on early Earth


  • Higher atmospheric temperatures
  • UV radiation
• The early atmosphere contained higher levels of carbon dioxide and methane than our present atmosphere
• Greenhouse effect


  Methane and carbon dioxide trap infrared radiation coming from the surface of the earth and prevent it from escaping into space, which in turn increases surface temperatures
• The higher levels of carbon dioxide and methane in the early atmosphere led to the surface temperatures of Earth being much higher than it is today
• The atmosphere of early Earth lacked free oxygen, which prevented ozone from forming
• Ozone


  Formed when ultraviolet (UV) radiation from the sun interacts with oxygen molecules, absorbs damaging UV radiation and protects life on Earth
• The lack of free oxygen, and therefore ozone, in the early atmosphere, led to UV radiation penetrating to the surface of Earth
• UV radiation cause DNA damage and increases the rate at which mutations occur; this can be harmful to living organisms
• These conditions on early Earth may have resulted in the spontaneous formation of carbon compounds by chemical processes that do not currently occur
• Formation of organic molecules

  1. Adding energy, such as heat or UV radiation, to the mixture of gases that would have been present in the early atmosphere

  2. Could have led to the formation of organic molecules such as amino acids, simple sugars, nucleotides and fatty acids
• Primordial soup hypothesis


  Proposed by scientists Alexander Oparin and JBS Haldane to explain the origin of biological molecules
• It is possible that the high levels of UV radiation on early Earth could then have catalysed the formation of larger polymers, such as proteins, complex sugars, mRNA and phospholipids, from these simpler molecules
• Heat and radiation could allow the synthesis of inorganic molecules into simple organic molecules which could then have assembled into polymers to provide the building blocks of life
• Miller-Urey experiment


  Scientists Miller and Urey recreated the conditions thought to have existed on Earth prior to life using a specific piece of apparatus
• Miller-Urey apparatus


  1. Boil water to produce steam

  2. Mix the steam with a mixture of gases (including methane, hydrogen and ammonia)

  3. Add electrical discharges to the gases to stimulate lightning

  4. Cool the mixture
• After a week Miller and Urey analysed the condensed mixture and found traces of simple organic molecules, including amino acids
• It is now believed that methane may have been in low supply in the atmosphere of early Earth
• Miller and Urey used an electrical discharge as a source of energy instead of UV light
• For the synthesis of organic molecules, carbon dioxide, nitrogen and water require nuclear and UV radiation along with electrical discharges
• In a watery environment amino acids tend to remain as monomers rather than joining to form proteins
• Miller and Urey were unable to generate nucleotides with their experiments
• Nucleotides have since been chemically synthesised using a different approach
• Cells: Units of Life

  Cells are considered to be the smallest units of self-sustaining life
• Features common to all cells


  • Enclosed by a plasma membrane
  • Store genetic information in DNA molecules
  • Express genetic information during protein synthesis
• Life


  Defined by the occurrence of metabolic reactions, need for nutrition, production of metabolic waste, ability to reproduce and pass on genetic information, ability to receive and respond to stimuli, and ability to grow
• Viruses are considered to be non-living as they lack a cell structure and organelles, and are unable to perform most characteristics of life
• The First Cells


  Explaining the spontaneous origin of cells
• Key stages involved in life arising from non-living components


  1. Simple organic compounds needed to be synthesised from inorganic molecules

  2. Simple organic compounds needed to be assembled into polymers

  3. Some polymers needed to develop the ability to self-replicate

  4. Membranes needed to surround the polymers, creating compartments with an internal chemistry different from the surroundings
• Theories on the origin of cells


  • Protocell-first theory
  • Gene-first theory
  • Metabolism-first theory
• The origin of cells and how they evolved remain a highly debated topic in the scientific world
• It is not possible to replicate the conditions on early Earth exactly as they might have been
• It is also impossible to know what the exact nature of the first cells were, since none of these early cells fossilised
• This makes it difficult to test the hypotheses that underpin the theories about the origin of life
• Membranes


  Play an important role in cells by separating the genetic material and biochemical processes inside the cell from its outside environment
• Testable theories


  Theories that can be experimentally tested
• Testing theories on the origin of cells


  1. Replicating conditions that might have been present on early Earth in a laboratory

  2. Impossible to exactly replicate conditions on early Earth

  3. Impossible to know exact nature of first cells since none fossilised
• This makes it difficult to test hypotheses that underpin theories about the origin of life
• Membranes


  Separate the genetic material and biochemical processes inside the cell from its outside environment; this is known as compartmentalisation