Origin of CellsPRESENTED BY IB SCREWED

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Origins of Life

There is an unbroken chain of life from the first cells on Earth to all

cells in organisms alive today.

We know this because cells can only be formed by division of pre-

existing cells.

It is estimated that life first emerged at least 3.8 billion years

ago, approximately 750 million years after Earth was formed

Can Life Arise Spontaneously?

It was hypothesised in the 1920s that organic

molecules (like amino acids) could arise under the

right conditions

Miller’s experiments in the 1950s supported the

hypothesis

With electricity (i.e. lightning), heat (from the sun),

water and atmospheric molecules like CO2 and NH3,

the organic molecules began to form over time.

From Organic Molecules to Cells

The first cells must have arisen from non-living material.

Miller’s experiments show that the materials to make proteins were

present.

The 64 codons in the genetic code have the

same meanings in nearly all organisms, but that

there are some minor variations that are likely to

have accrued since the common origin of life

on Earth.

The RNA World

In the 1980s, Altman and Cech discovered that RNA is capable of catalysing some

chemical reactions, including the polymerization of nucleotides. RNA is uniquely able

both to serve as a template for and to catalyse its own replication.

Consequently, RNA is generally believed to have been the initial genetic system, and

an early stage of chemical evolution is thought to have been based on self-

replicating RNA molecules—a period of evolution known as the RNA world.

Ordered interactions between RNA and amino acids

then evolved into the present-day genetic code, and

DNA eventually replaced RNA as the genetic material.

Cell Membrane

The first cell is assumed to have had a phospholipid bilayer membrane, enclosing a

cytoplasm and DNA.

The enclosure of self-replicating RNA and associated molecules in a phospholipid

membrane would thus have maintained them as a unit, capable of self-reproduction

and further evolution.

RNA-directed protein synthesis may already have evolved by this time, in which case the

first cell would have consisted of self-replicating RNA and its encoded proteins.

Metabolism

It is believed that cells initially used anaerobic

respiration (in the absence of oxygen) in a process like

glycolysis: the breakdown of glucose to lactic acid

Then two things must have happened:

Origin of photosynthesis

Aerobic respiration

Photosynthesis

Photosynthesis allows cells to use light and CO2 to make glucose for energy

The first photosynthetic bacteria, which evolved about3 billion years ago, probably used H2S to convert CO2 to organic molecules: a pathway still used by some bacteria today.

H2O as an electron donor and hydrogen for the conversion of CO2 to organic compounds evolved later and led to the oxygenation of Earth's atmosphere.

The use of H2O in photosynthetic reactions produces the by-product free O2; this mechanism is thought to have been responsible for making O2 abundant in Earth's atmosphere.

Aerobic Respiration

Once oxygen was abundant in the atmosphere, respiration began

to use oxygen: aerobic respiration.

O2 is a highly reactive molecule, and oxidative metabolism, utilizing

this reactivity, has provided a mechanism for generating energy

from organic molecules that is much more efficient than

anaerobic glycolysis.

Endosymbiotic Theory

Photosynthesis and respiration initially took place in the cytoplasm.

However, when we look at eukaryotic cells today (like our own), we

see that many have chloroplasts and mitochondria: organelles

where these reactions take place.

They also have a nuclear membrane encasing the DNA

The organelles are thought to have been acquired as a result of

the association of prokaryotic cells with the ancestor of eukaryotes.

Evidence for Endosymbiosis

The hypothesis that eukaryotic cells evolved from a symbiotic association of

prokaryotes—endosymbiosis—is particularly well supported by studies

of mitochondria and chloroplasts, which are thought to have evolved from

bacteria living in large cells.

Both mitochondria and chloroplasts are similar to bacteria in size, and like bacteria,

they reproduce by dividing in two.

Most importantly, both mitochondria and chloroplasts contain their own DNA,

which encodes some of their components.

What is Endosymbiosis?

Endosymbiotic theory suggests that chloroplasts,

mitochondria and perhaps even other organelles,

were prokaryotic cells which lived inside larger

cells.

Mitochondria and chloroplasts originated from the

endosymbiotic association of aerobic bacteria

and cyanobacteria, respectively, with the

ancestors of eukaryotes.

Pasteur’s Experiments

Pasteur’s experiments were evidence that spontaneous generation

of cells and organisms does not now occur on Earth.

In his day, people did not realise that bacterial infections were

transmitted from person to person, so hand washing and treatment

of food was not commonplace

Pasteur showed that bacterial growth on foods like milk, and the

infections in hospitals, were from pre-existing cells replicating and

being transported.

Thus, he developed the germ theory of disease

Pasteur’s Work

Using germ theory, Louis Pasteur was then able to

develop a cure for anthrax (in cattle) and rabies.

He developed the process of pasteurisation to

make milk safe to drink