Evolution of Cells (HL) (HL IB Biology)

Cells as the smallest units of self-sustaining life

• Cells are considered to be the smallest units of life
• All cells have the following features in common:
  • They are enclosed by a plasma membrane, or cell surface membrane, which separate the cell contents from the outside
  • They store genetic information in DNA molecules which is expressed during protein synthesis

Features of all cells diagram

All cells have certain features in common

• Life can be defined by the following features:
  • The occurrence of metabolic reactions, e.g. respiration
  • The need for nutrition
  • The production of metabolic waste which must be excreted 
  • The ability to reproduce and to pass genetic information on to offspring
    • This allows for evolution by natural selection
  • The ability to receive and respond to stimuli from the external and internal environments
  • The ability to grow
• Viruses are considered to be non-living
  • They lack a cell structure and organelles and are therefore unable to perform most of the characteristics of life, e.g.
    • They do not carry out metabolic reactions
    • They do not require nutrition
  • They are also unable to replicate independently and must rely on the cellular components of the host cells that they infect

Explaining the spontaneous origin of cells

• Cells are considered to be complex structures that can only form from the division of pre-existing cells
• The question is, how did the first cells come into existence if there were no pre-existing cells to divide?
• We know that all cells:
  • Are surrounded by a partially permeable membrane
  • Contain genetic material that can be passed on to new cells
  • Are capable of metabolic processes to release energy which enables growth, maintenance and reproduction
• Assuming that the first cells did not arrive on Earth from somewhere else, they must have originated from the non-living components that made up the primordial atmosphere at the time
• This would have required the following steps:
  • Simple organic compounds needed to be synthesised from inorganic molecules
    • A possible mechanism for this process was demonstrated by Stanley Miller and Harold Urey
  • Simple organic compounds needed to be assembled into polymers
  • Some of these polymers needed to develop the ability to self-replicate
  • Membranes needed to surround the polymers, creating compartments with an internal chemistry different from the surroundings

Key stages involved in life arising from non-living components diagram

The key stages involved in life arising from non-living materials

Several theories exist that aim to explain the possible origin of cells:

Protocell-first theory

• This theory proposes that a cell-like compartment capable of some basic metabolic functions arose spontaneously
• These are called protocells
• Initially they would have lacked genetic material, but they would have been able to grow, as well as divide into daughter "cells"
• Eventually these protocells would have acquired genetic material (most likely RNA at first) as they evolved

Gene-first theory

• This theory starts with the spontaneous development of a nucleic acid (most likely RNA) that had the ability to replicate itself
• Evolution by natural selection would have resulted in genetic variants that could have developed a cell membrane and basic metabolic processes

Metabolism-first theory

• This theory proposes that life originated as a system of chemical reactions capable of sustaining itself
• The system would eventually evolve to form cells and genetic material
• This theory is favoured by many scientists as most life processes essential for the existence of cells require energy released by metabolic reactions

NOS: Hypotheses and theories in science should be testable; the problem of testing the theories on the origin of cells

• The origin of cells and how they evolved remain a highly debated topic in the scientific world
• One of the cornerstones of the scientific method is formulating hypotheses and theories that are testable
• In the case of the theories on the origin of cells, scientists must carry out experiments testing the underlying mechanisms upon which the theories are based
  • This would include replicating conditions that might have been present on early Earth in a laboratory
• This presents a problem however, since 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

The spontaneous formation of vesicles

• Membranes play an important role in cells because they separate the genetic material and biochemical processes inside the cell from its outside environment; this is known as compartmentalisation
  • Membrane formation would have been a crucial step in the origin of cells
• It is likely that the membranes of the first cells were composed of fatty acids because of the amphipathic nature of these molecules
  • Fatty acids are major components of lipids
• When a few lipid molecules are placed in water, they will naturally form a monolayer on the surface; the polar parts of the lipid will be in water, while the non-polar parts will stick out of the water surface
• If more lipid molecules are added, they form bilayers with the polar parts facing outward towards the watery environment while the non-polar parts will point towards each other
• These bilayers will spontaneously form microspheres, or small vesicles, which could possibly have formed the membranes of early cells

The formation of vesicles diagram

It is possible that the coalescence of fatty acids formed spherical lipid bilayers that surrounded the first cells

• These early membranes would have separated the internal chemistry of the cells from their outside environment
  • It is theorised that the fatty acids could have combined with glycerol during condensation reactions to form triglycerides as membranes evolved
  • Finally, these triglycerides could have undergone phosphorylation to form simple phospholipids which make up the main component of modern cell membranes
• Eukaryotic cells evolved to contain multiple internal compartments, allowing further division of activity within cells

RNA as a presumed first genetic material

• For early life to evolve, the following had to emerge:
  • A system capable of replicating itself
  • An ability to catalyse chemical reactions
• In modern cells, these functions are carried out by DNA and enzymes, but neither of these would have been present in the pre-biotic world
• Scientists believe that RNA may have performed both of these functions in early cells, since it can store genetic information as well as having enzymatic properties; this is known as the RNA world hypothesis
  • As life evolved, DNA took over the role of genetic storage molecule, while proteins (enzymes) became biological catalysts of chemical reactions
• Properties of RNA that provides evidence for this include
  • RNA can assemble sponateously from nucleotides
  • RNA is able to replicate itself
  • RNA can control the rate of chemical reactions; modern cells contain ribozymes that catalyse the formation of peptide bonds
• Evidence that RNA may have been around before DNA includes
  • Ribose can be formed from methanal, one of the main products of the Miller-Urey experiment
  • Deoxyribose in DNA is produced from ribose in an enzyme catalysed reaction
  • Ribozymes are able to join amino acids together to form proteins from an RNA template