1.5 The origin of cells
Essential idea: There is an unbroken chain of life from the first cells on Earth to all cells in organisms alive today.
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1.5.U.1 Cells can only be formed by division of pre-existing cells.
1.5.U.1 Cells can only be formed by division of pre-existing cells.
Be able to:
Discuss two implications of all cells being formed from preexisting cells.
Remember the Cell Theory?
all living things are composed of cells or cell parts
the cell is the smallest unit of life
cells only arise from pre-existing cells
Prokaryotic cells are formed during a process called binary fission.
Eukaryotic cells form new identical cells by the process called mitosis (genetically identical) and form sex cells through meiosis (haploid cells which not genetically identical to the parent cell and contain half the genetic material). Cell division forms the new cells from pre-existing cells replaced the concept of spontaneous generation, where cells were formed from inanimate matter.
The chemical processes that contributed to the initial formation of biological life required specific conditions to proceed. This included a reducing atmosphere and high temperatures (>100ºC) or electrical discharges to catalyse chemical reactions. These conditions do not commonly exist on modern Earth and hence living cells cannot arise independently by abiogenesis. Instead, cells can only be formed by the division of pre-existing cells (biogenesis)
1.5.U.2 The first cells must have arisen from non-living material.
1.5.U.2 The first cells must have arisen from non-living material.
Be able to:
Outline the four processes needed for the spontaneous origin of cells on Earth.
Outline the experiments of Miller and Urey into the origin of organic compounds.
Define polymerization, monomer and polymer.
Outline two properties of RNA that would have allowed it to play a role in the origin of life.
If we go back to how the very first living cells were created, we have to conclude they either originated from non-living material, came from somewhere else in the universe or were created by some other unknown entity
These are the hypothesized steps of how living cells possibly developed from non-living material over millions of years
Production of carbon compounds such amino acids and sugars. Miler and Urey’s experiment showed how this could happen by passing water vapour through Ammonia, methane and hydrogen (early earth atmosphere). They added electricity to simulate lightening discharge. They found they could create amino acids and carbon compounds
Assembly of carbon compounds into polymers might have occurred at the deep sea hydrothermal vents, which could have supplied the inorganic compounds such as iron sulphide and thermal energy for the assembly
Formation of membranes would be possible if phospholipids were some of the first polymers created. These phospholipids would naturally form vesicles allowing for a different environment to exist inside compared to the surrounding water
Development of a mechanism for inheritance would be needed in order for the organism to replicate and pass its DNA on to the next generation. Current organisms need enzymes to replicate DNA; however, enzymes are created by the genes on the DNA. A possible solution to this would be RNA being the first nucleic acid formed because it is self-replicating and can also act as a catalyst.
How did these complex structures develop?
How did these complex structures develop?
The first cells must have arisen from non-living material
Abiogenesis is a general term used to describe life emerging from non-living materials, such as simple organic compounds. It is impossible to recreate the conditions under which life emerged from non-life, but we predict that life arose in the following way:
Synthesis of simple organic molecules from inorganic molecules (carbon compounds and amino acids form from methane and ammonia)
In the 1950s Stanley Miller and Harold Urey of the University of California synthesised over 20 amino acids from inorganic compounds using a unique experimental apparatus.
origin_of_life.pdf1.5.U.3 The origin of eukaryotic cells can be explained by the endosymbiotic theory.
1.5.U.3 The origin of eukaryotic cells can be explained by the endosymbiotic theory.
Be able to:
State the endosymbiosis theory.
Outline the major events in the origin of eukaryotic cells.
Describe the evidence for the endosymbiotic theory
There is compelling evidence that mitochondria and chloroplasts were once primitive free-living bacterial cells.
Symbiosis occurs when two different species benefit from living and working together. When one organism actually lives inside the other it's called endosymbiosis. The endosymbiotic theory describes how a large host cell and the bacteria ingested through endocytosis, could easily become dependent on one another for survival, resulting in a permanent relationship. As long as the smaller mitochondria living inside the cytoplasm of the larger cell divided at the same rate, they could persist indefinitely inside those cells.
The smaller cell was provided food and protection by the larger cell and the smaller mitochondria would supply energy through aerobic respiration for the larger cell. Over millions of years of evolution, mitochondria and chloroplasts have become more specialized and today they cannot live outside the cell.
1.5.A.1 Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth.
1.5.A.1 Evidence from Pasteur’s experiments that spontaneous generation of cells and organisms does not now occur on Earth.
Be able to:
Define spontaneous generation.
Describe Pasteur’s experiments about spontaneous generation.
Explain why Pasteur’s experiments did not support the idea of spontaneous generation.
For centuries people accepted the "spontaneous generation" of life from inanimate matter. When this long-standing myth was finally dispelled in the mid-1800s, it became clear that all life must arise from pre-existing life — via a process of reproduction. If cells are the fundamental units of life, they too must have a reproductive mechanism that maintains the proper chromosome number in each cell. About a decade after the publication of Mendel's paper, scientists carefully documented the behavior of chromosomes during cell division (mitosis), using dyes to make them visible. First, each chromosome copies itself, and the duplicates line up at the "equator" of the cell. Then, duplicate copies of each chromosome are pulled toward opposite poles. Finally, the cell splits at the equator, producing two new cells with identical sets of chromosomes.
Biogenesis describes the principle that living things only arise from other living things by reproduction (not spontaneous generation)
"Omne vivum ex vivo” – All life (is) from life
The law of biogenesis is largely attributed to Louis Pasteur, who demonstrated that emergent bacterial growth in nutrient broths was due to contamination by pre-existing cells Broths were stored in vessels that contained long tubings (swan neck ducts) that did not allow external dust particles to pass. The broths were boiled to kill any micro-organisms present in the growth medium (sterilization). Growth only occurred in the broth if the flask was broken open, exposing the contents to contaminants from the outside. From this it was concluded that emergent bacterial growth came from external contaminants and did not spontaneously occur.
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