Prokaryotes And The Evolution Of Metabolism
In this lecture we are concerned with the Archaean and prokaryote evolution.
Prokaryotes are composed of cells that contain:
- DNA in a circular loop
- Outer cell membrane surrounded by a cell wall that is not made of cellulose
- Cytoplasm containing ribosomes
- Flagellum composed of protein subunits
- The cells divide by constriction, not mitosis.
These cells lack:
- membrane-bound organelles
- chromosomes with histone proteins
- microtubules (and therefore, no mitosis or meiosis)
Some might suppose, on seeing such structurally simple organisms, that the
Archaean was a period of relatively little evolutionary change. This is not the case because the prokaryotes are biochemically very diverse and have
played a central role in the evolution of most of the important biochemical or metabolic
pathways used by life on the planet today.
Evolution of the Prokaryotes
Twenty years ago, the most primitive prokaryotes were thought to be cyanobacteria
(also called the blue-green algae). Why?
1. We have fossils dating to 2.8 billion years ago. Formed huge mats called
stromatolites - still seen in parts of Australia, Africa, and Florida.
2. Cyanobacteria are photosynthetic autotrophs. That is they can produce every
resource they need by converting light energy from the sun into chemical
energy by photosynthesis. This is a fairly passive process and so was seen as
"simplier" than engulfing food and so was thought to be more primitive.
More recently:
- 1. Fossils of non-photosynthetic organisms found that date to 3.8 bya.
- 2. Photosynthesizing organisms while certainly passive in appearance are
actually doing something that is biochemically very complex. How could
something as complex as photosynthesis evolve quickly when the intermediate
steps of photosynthesis give no advantage that could be favored by natural
selection ?
What could have happened?
1. The primitive ocean, because of outgassing and concentration of organic
compounds was a rich organic soup. The first organisms to arise would be the ones that
used this rich store of organic chemicals. They would have been heterotrophs.
anabolism catabolism
ATP is used to store energy and when it is broken down into ADP through the loss of
phosphate ions. ATP will form by naturally occuring chemical processes, and it may
have accumulated in the Earth's early oceans. The earliest forms of life would only
need to take in ATP from the environment and break it down into ADP to obtain energy.
However, as life forms multiplied, this free ATP would have been used up. At this
point in time, prokaryotes that were able to create ATP through simple chemical
pathways appeared and were favored by natural selection (in other words they need to
also be able to do anabolism). We have such prokaryotes still living today - they are
called the Archebacteria
Archebacteria
These are the only type of prokaryotes that could have survived in the world 3.8 billion
years ago. Archaea are similar to other prokaryotes in most aspects of cell structure and metabolism (they have DNA in a circular loop, a cell wall (although its chemical composition is unique), ribosomes, flagella may be present, lack membrane-bound organelles).
Individual archaeans range from 0.1 to over 15 µm in diameter, and some form aggregates or filaments up to 200 µm in length. They occur in various shapes, such as spherical, rod-shaped, spiral, lobed, or rectangular.
However, their genetic transcription and translation do not show the typical bacterial features, but are extremely similar to those of eukaryotes.
Several other characteristics also set the Archaea apart.
The most striking chemical differences between Archaea and other living things lie in their cell membrane.
Remember: The basic unit from which cell membranes are built is the phospholipid .
Like bacteria and eukaryotes, archaea possess glycerol based phospholipids. However, three features of the archaeal lipids are unusual:
- 1. the archaeal lipids have an L-glycerol while bacteria and eukaryotes have a D-glycerol.
- 2. Most bacteria and eukaryotes have membranes composed mainly of glycerol-ester lipids, whereas archaea have membranes composed of glycerol-ether lipids.
- 3. The hydrophobic side-chains in Eukaryotes and Bacteria are fatty acids while the chains in Archaea are made up of isoprenes. These isoprenes can be branched and joined in a greater variety of ways and this gives Archaea more variability in their membranes.
Like Bacteria, Archaea have a cell wall. In bacteria, however, the cell wall is made up of a material called peptidoglycan, while it is made of a variety of other materials.
Habitats
Archaea are usually harmless to other organisms and none are known to cause disease. Many archaeans are extremophiles (that is they live in extreme environments). Some live at very high temperatures, often above 100°C. Others are found in very cold habitats or in highly saline, acidic, or alkaline water. However, other archaeans are mesophiles, and have been found in environments like marshland,
sewage, and soil. Many methanogenic archaea are found in the digestive tracts of animals such as ruminants, termites, and humans.
Important to our story of evolution are one of there - the Methanogens
-
- anaerobes - intolerant to O2.
- Organisms able to generate energy from chemical reactions involving inorganic molecules that they take in from the environment.
Most archebacteria are extreme heterotrophs and would have depleted the organic
soup and caused an evolutionary crisis. We can call it the living world's first energy
crisis. The precursors of the organic compounds would still be naturally occurring.
Natural selection would favor those organisms which could produce their own
complex organic molecules from the naturally occurring precursors. This led to the
evolution of the Eubacteria (means true bacteria : as opposed to archebacteria) - Cell
structure a little more complex (cell walls contain murine, mesosomes-complex
membrane folds that are the site of many biochemical pathways) but more
importantly, these bacteria had more complex metabolic pathways that allowed
fermentation to produce energy.
Eubacteria
1. Fermenting bacteria
- Chemotrophs - In the absence of oxygen, these bacteria break glucose (which
has been absorbed from the environment) into two molecules of pyruvate. This
releases enough energy to produce two molecules of ATP. This process is
known as anaerobic glycolysis:
The bonanza of free glucose in the biotic soup could not last. If the earliest
heterotrophs continued to consume glucose molecules at a rate greater that they
could be produced by slow geochemical reactions, then eventually life would have
died out.
In this early famine, organisms with the ability to produce their own glucose would
have been favored by natural selection. Life on earth today depends primarily on
photosynthesis as the process of glucose production - a solution invented more than
2 billion years ago.
The earliest photosynthetic organisms lived in an anaerobic atmosphere.
2. Photosynthetic Bacteria
Trapping light energy and converting it to chemical energy is the process of
photosynthesis. Most organisms that do it use chlorophyll. Chlorophyll is a porphyrin
molecule.
Porphyrins are ring-shaped molecules that combine with metal ions and can be
formed from chemicals present on the early earth. Porphyrins include hemoglobin
(which contains iron), chlorophyll (which contains magnesium), and vitamin B12
(which contains cobalt). These molecules are able to act as catalysts and cause
electrons to be exchanges. Chlorophyll can capture light energy and store it by raising
the energy within the porphyrin ring. This energy then causes electrons to move
through the photosynthesis reaction resulting in formation of glucose.
Two types:
(a)The organisms that invented this process were the green sulfur bacteria
- In the presence of sunlight and chlorophyll:
- They removed electrons from H2S (hydrogen sulfide)-
- to create NADPH which can transform CO2 into glucose and water
(b) Cyanobacteria (blue-green algae)
A more abundant source of electrons than hydrogen sulfide, is water. But splitting
water to remove the necessary electrons requires abour 10 times more energy that it
does to split them from H2S.
Cyanobacteria accomplished this by evolving enzymes that lowered the energy
threshold and a more complex photosystem: Splitting water in cyanobacterial
photosynthesis generates oxygen as a waste product. Cyanobacteria created huge
mats (called stromatolites) and produced a great deal of oxygen. These mats can be
seen commonly in Archaean fossil rocks and, in a few places where they are
protected from animal predators, they still form today (for example in saline lagoons in
Australia:
About 2300 million years ago (2.3 bya) oxygen gas began accumulating in the
atmosphere. This created an enormous problem for existing life forms:
i. -- Oxidation
Oxygen (while extremely important to our metabolism) is extremely reactive and
can disrupt the delicate balance in a living cell.
ii. Block UV light - only biotic production of organic molecules can now occur in
natural environments
To anaerobic microorganisms, the arrival of oxygen was an unmitigated disaster.
Some (probably many) went extinct, some retreated into habitats without oxygen
(oblgate anaerobes) - that is the only place we find many archebacteria today.
This leads to a third type of eubacteria:
3. Facultative anaerobic bacteria
These anaerobic bacteria were the first to have evolved the ability to produce special
molecules that bind oxygen and neutralize it. These molecules, primarily porphyrins,
are still used today by microorganisms and their descendants to bind oxygen.
Once evolved the function of the molecule could have shifted --
- we find porphyrins in hemoglobin--
- Even bioluminescense (used in communication today by many
organisms) may have originally evolved in bacteria because it
generally involves reducing oxygen to a non-reactive form.
Finally, the ability to use the reactive properties of oxygen to generate ATP would have
appeared:
4. Aerobic Bacteria
In respiration, pyruvate is converted into Acetyl CoA. Acetyl CoA enters a series of
reactions (known as the krebs cycle) where it is broken down to give 30 molecules of
ATP, carbon dioxide, and water.
Summary (in the form of a cladogram):