1.5 The origin of cells

The origin of cells

- Cells can only be formed by division of pre-existing cells
- The first cells must have arisen from non -living material
- Evidence from Pasteur's experiments that spontaneous generation of cells and organism does not now occur on earth

Louis Pasteur's experiment

- Performed a series of experiments to disprove the theory of spontaneous generation
( The idea that cell can appear from nothing)
- Set out to investigate whether microbes could grow in sterile broth
- Used swan-neck flaks that would not allow anything into the flask

Experiment 1

- Sterile broth
- Broken flask to allow air in \= microbial growth

Experiment 2

- Sterile broth
- Unbroken flask but tilted to allow contact with the U-bend of the flask \= microbial growth

Experiment 3

- Sterile broth
- Unbroken flask \= no microbial growth

Key processes for spontaneous origins of life

- Non living synthesis of organic molecules
e.g. amino acids & sugars
- Assembly of these non-organic molecules into polymers
- Formation of polymers that can self-replicate
(Enabling inheritance)
- Formation of membrane to package the organic molecules

Miller and Urey's experiment

1. Water was boiled to vapour to reflect the high temperature common to the earth's original conditions
2. The vapour was mixed with a variety of gases (including H2, CH4, NH3) to create a reducing atmosphere (no O2)
3. This mixture was then exposed to an electrical discharge (simulating lightning)
4. The mixture was then allowed to cool (concentrating components) and left for a period of 1 week
5. After this time, the condensed mixture was analysed and found to contain traces of simple organic molecules
(Amino acids & complex carbohydrates)

Endosymbiotic theory

- Explains the origin of eukaryotic cells
- A cell which lives within another cell with mutual benefit
- Eukaryotic cells are believed to have evolved from early prokaryotes that were engulfed by phagocytosis
- Theory states that organelles (e.g. mitochondria and chloroplasts) originated as symbioses between single-celled organisms
- Mitochondria/ chloroplasts have similar sizes to prokaryotes

MAD DR

Membranes - Double membrane bounds
Antibiotics - Susceptibility
Division - Mode of replication

DNA - Presence and structural composition
Ribosomes - Size

Membranes

- Some organelles have double membranes
- Inner membrane is their own
- Outer membrane from Endocytosis
- Inner membrane has proteins similar to prokaryotes

Antibiotics

- Susceptible to antibiotics
- E.g. Chloramphenicol
- Indicates that organelles may have bacterial origins (prokaryotes)

Division

- Reproduction occurs via fission-like process
- Can only be produced by division of pre-existing mitochondria and chloroplasts

DNA

- Has own DNA which is naked and circular (prokaryotes)
- Reproduces separately from host cell

Ribosomes

- Have ribosomes which are 70s in size
- Identical to prokaryotic ribosomes
- Manufacture proteins

Development of Mitochondria

- An aerobic prokaryote enters a larger and anaerobic prokaryote
( Possibly as prey or parasite, by Endocytosis)
- Survives digestion to become a valuable endosymbiant
( A cell which lives within another cell with mutual benefit)
- The aerobic prokaryote provides a rich source of ATP to it's host
( Enables it to out-compete other anaerobic prokaryotes)
- As the host cell grows and divides, so does the aerobic prokaryotes
(Subsequent generations automatically contain aerobic prokaryotes)
- Aerobic prokaryotes evolves and is assimilated to become a mitochondrion