A2.1 Origin of Cells

Conditions on early Earth

They were not able to support life, but are believed to have been instrumental in the origin of biological compounds that made life possible.

Higher atmospheric temperatures on early Earth

The atmosphere contained higher levels of carbon dioxide and methane than our present atmosphere. The greenhouse gases trap infrared radiation and prevent it from escaping into space, known as the greenhouse effect. The high levels of carbon dioxide and methane led to the surface temperatures of Earth being much higher than it is now

UV radiation on early Earth

The atmosphere lacked free oxygen, preventing ozone from forming. Ozone is formed when UV radiation from the sun interacts with oxygen. Ozone absorbs UV radiation and protects life on earth. Without ozone, UV radiation penetrated the surface of the earth, increasing mutations and DNA damage

Early Earth and the spontaneous formation of carbon compounds

Adding energy, such as heat or UV radiation to the mixture of gases present in the early atmosphere could have led to the formation of organic molecules (eg. amino acids, sugars, nucleotides, fatty acids). These organic molecules would have formed the building blocks of early cells. This was proposed by Oparin and Haldane as the primordial soup hypothesis. It is also possible that high levels of UV radiation catalysed the formation of larger polymers from these simpler molecules

The Miller-Urey experiment

Recreated the conditions thought to have existed on early Earth using a specific piece of apparatus. This allowed them to boil water to produce steam (recreating the early primordial soup evaporating in high temperatures), mix the steam with a mixture of gases (including methane, hydrogen, and ammonia to recreate the atmosphere), add electrical discharges to stimulate lightning, and cool the mixture (representing condensation of water in the atmosphere). After a week Miller and Urey analysed the condensed mixture and found traces of simple organic molecules

Evaluating the Miller-Urey experiment

They simulated the early atmosphere by including high levels of methane, however it is now believed that methane may have been in low supply in early Earth. They used electrical discharge as a source of energy rather than UV light

Cells as the smallest units of self-sustaining life

Cells are considered to be the smallest units of life. All cells are enclosed by a plasma membrane (or cell surface membrane) which separates the cell contents from the outside, and they store genetic information in DNA molecules

What features define life?

The occurrence of metabolic reactions, the need for nutrition, excretion, the ability to reproduce and pass genetic information to offspring, the ability to receive and respond to stimuli from external and internal environments, and the ability to grow (MRHGREN)

Why are viruses considered to be non-living?

They lack a cell structure and organelles, they do not carry out metabolic reactions or require nutrition. They are unable to replicate independently and rely on the cellular components of host cells that they infect

Explaining the spontaneous origin of cells

Cells must have originated from the non-living components that made up the primordial atmosphere in early Earth. This would have required the following steps: simple organic compounds needed to be synthesised from inorganic molecules (Miller-Urey)(catalysis), organic compounds needed to be assembled into polymers (self-assembly), some polymers needed to develop the ability to self replicate (self-replication), and membranes needed to form around the polymers, creating compartments (compartmentalisation)

Protocell-first theory

Proposes that a cell-like compartment capable of basic metabolic functions arose spontaneously, known as protocells. Initially they would have lacked genetic material, but they would have grown and divided into daughter cells. Eventually these protocells would have acquired genetic material as they evolved

Gene-first theory

There was spontaneous development of a nucleic acid which had the ability to replicate itself. Evolution by natural selection resulted in genetic variations that could have developed a cell membrane and metabolism

Metabolism-first theory

Life originated as a system of chemical reactions capable of sustaining itself. The system would eventually evolve to form cells and genetic material.

The spontaneous formation of vesicles

It is likely that the membranes of the first cells were composed of fatty acids because they contain both hydrophobic and hydrophilic parts in their structure. Fatty acids are a major component of lipids; when lipid molecules are placed in water, they naturally form a monolayer on the surface (polar parts in the water, non-polar parts sticking out). If more are added, they form bilayers (polar parts facing outwards, non-polar parts pointing inwards). These bilayers will spontaneously form vesicles

Compartmentalisation

The separation of genetic material and biochemical processes inside a cell from its outside environment

RNA as a presumed first genetic material

Scientists believe that RNA may have been able to replicate and catalyse chemical reactions in early cells, as RNA stores genetic information and has enzymatic properties

Properties of RNA that provide evidence for being the first genetic material

RNA can assemble spontaneously from nucleotides, it can replicate itself, it can control the rate of chemical reactions

Evidence that RNA may have been around before DNA

Ribose can be formed from methanal, one of the products of the Miller-Urey experiment. Deoxyribose in DNA is produced from ribose in an enzyme catalysed reaction

Last Universal Common Ancestor (LUCA)

All life on Earth is thought to have evolved from LUCA which existed about 4 billion years ago

Evidence for a common ancestry shared by all living organisms

Same biochemistry in all organisms, same DNA bases and genetic code, same amino acids forming proteins

Estimating the timescale for the origin of life

Carbon dating or radiometric dating can be used to date fossils or rocks in which the history of life on Earth are found. Older rocks are expected to have evidence of ancient forms of life. The average time for DNA mutations to occur can be estimated to determine the relative date when species branched from a common ancestor based on the number of mutations occurred between them

Evidence for the evolution of LUCA near hydrothermal vents

The conditions near hydrothermal vents provide opportunities for organisms to generate energy by chemosynthesis.