Sigam o Carbono -2

Four types of organic macromolecules

in living systems.

Most of the molecules in the living systems are water (H2O) and large organic macromolecules:

• Carbohydrates

• Lipids

• Proteins

• Nucleic Acids

Carbohydrates (sugars, starches)

• Representatives:Glucose, Fructose• Many hydroxyl groups (-OH)• Soluble in water• Form Polysaccharides• Good energy source• Structural support for organisms (cellulose- the main constituent of wood)

Glucose

FructoseTable sugar

Glucose polymerization

In starch molecule (potato) there can be 100s thousands of glucose units

H2O

Linked by dehydration reactionPolysaccharides

Carbohydrates are important as a source of energy for life

• Respiration

CH2O + O2 → CO2 + H2O + Energy

In reality:

C6H12O6 + 6O2 → 6CO2 + 6H2O +Energy

• Fermentation

C6H12O6 → 2CO2 + 2C2H6O + Energy

But there are ways to get energy without carbohydrates.

• Methanogenesis

CO2 + 4 H2 → CH4 + 2H2O + Energy

• Sulfate reduction

4H2 + SO42- → S2- + 4H2O + Energy

Lipids (fats and oils)

• Representatives: fatty acids and cholesterol

• Poorly soluble

• Good (concentrated)

energy source

• Flexible

(cell membrane material)

• Some lipids have hydrophilic (love water) head and hydrophobic tail

• In solution these lipids can form monolayers, bilayers and bilayer vesicles spontaneously – pre-cells.

Lipids are important for the formation of the cell membrane

Monolayer

BilayerBilayer vesicle

Micells

Membrane Lipids are very complex

Cell Membrane Lipids

ORIGINS OF LIFE?

Two Approaches to the origin of life

“Top-down strategy” is to

look at the present day biology

and extrapolate (project) back towards

the simplest living entities.

“Bottom-up strategy” is to make

the complex building blocks of life

(organic macromolecules) and put

them together.

• Cells are very diverse in size, shape and complexity but there are 2 basic cell types

• Prokaryotes are much simpler and smaller than eukaryotes

• Eukaryotes have a cell nucleus (to protect DNA) while prokaryotes do not

• Prokaryotes are single-celled only• Eukaryotes may be either single-celled or

multicellular

Top-down strategy: How would the most ancient cell look like?

Prokaryotes

Eukaryotic cell

Eukaryotes vs. Prokaryotes

• Who outnumbers and outweighs the other?

• Who is more dependent on the other?

Eukaryotes look much more complex then prokaryotes.Do we see the difference in the DNA structure? Yes

• Is the genome (order of nucleotides) of Eukaryotes and Prokaryotes completely different? No

• Some parts of the genome change more easily than others in the course of evolution

• A segment of the DNA responsible for coding of ribosomal parts (16S RNA, 1500 nucleotides) is very well conserved.

• A part of the gene for 16S RNA (~1500 nucleotides) for different organisms.

• Corresponding segments of nucleotide sequence from an archaean (Methanococcus jannaschii), a eubacterium (Escherichia coli) and a eucaryote (Homo sapiens) are aligned in parallel.

• Sites where the nucleotides are identical between species are indicated by a vertical line.

• Genetic information conserved since the beginnings of life.

Top-down strategy:The Tree of Life

• By looking at the changes in 16S RNA we can identify three domains of life Bacteria, Archaea, and Eucaryotes

Prokaryotes

• “Bottom-up” strategy to the origin of life is to make complex building blocks of life and try to put them together.

• But first we need to answer how was it possible to make any organic molecules at all without biology?

Bottom-up strategy

EARTH TODAY

Abiotic synthetic reactions on the early Earth

Prebiotic soup

Prebiotic polymers

The origin of lifeRNA world

Protein/DNAworld

(modern biochemistry)

Organics from space

Bada & Lazcano (Science, 2002)

Problems with abiotic organic synthesis.

• Almost all organic carbon which we observe today is produced biologically (photosynthesis):

CO2 + H2O CH2O + O2

• Carbon which comes out of volcanoes is in a form of CO2

• CO2 gas mixture does not produce

organic molecules on its own inorganicorganic

Terrestrial vs. Extraterrestrial

• Terrestrial origin – organic synthesis occurred somewhere in the Earth environment

• Extraterrestrial origin – organic material was synthesized in space and was brought to Earth somehow

Urey-Miller Experiment1) At some point scientists believed that the ancient atmosphere was rich in CH4 and NH3

2) But! Just mixing CH4, NH3, H2O, H2 would not produce any organic material

3) Miller showed that a spark discharge (lightning) would produce organic molecules up to 10-15% of the initial CH4 by mass

The Miller-Urey-Experiment

FIRST EXPERIMENTAL FORMATION OF BIOLOGICALLYRELEVANT MOLECULES UNDER PREBIOTIC CONDIDTIONS

Formation of organic molecules in the gas phase

• UV can be used for organic synthesis as well

• The key is to produce carbon radicals

• CH4 + h CH3 + H

• CH3 + CH3 C2H6 + M

• C2H6 + h C2H

• C2nH2 + C2H C2n+2H2 + H

• Polymerization is extremely sensitive to O abundance

Titan’s Organic Haze Layer

Haze is thought toform from photolysis(and charged particleirradiation) of CH4

(Picture fromVoyager 2)

Advantages of Organic synthesis in the ancient atmosphere

• UM-like experiments produced many types of organic molecules which are used in proteins

• We would expect lightning and UV radiation in the prebiotic atmosphere

• No need to deliver organic material to Earth – it would be already here

Difficulties for the organic synthesis via UM experiment

• It is hard to justify large amounts of NH3 and CH4 in the prebiotic atmosphere

• In the CO2-rich atmosphere organic production by spark discharge is not very efficient

• If CH4/CO2 < 0.1 essentially no organic production

Trainer et al., 2005; 2006)

Hydrothermal vents

• Fischer-Tropsch synthesis. Under high temperatures and pressures CO and H2 can form hydrocarbons

• (2n+1)H2 + nCO → CnH(2n+2) + nH2O

where 'n' is a positive integer

Advantages of Organic synthesis in the Hydrothermal Vents

• Hydrothermal vents were likely to be present in the prebiotic environment

• Organic synthesis requires only CO2, H2O and silicate rocks.

• Serpentinization:

• Spinel polymerization:

OlivineSerpentine Magnetite (spinel group)

Difficulties of the organic synthesis via Hydrothermal Vents

• No clean catalysts in nature. Original Fischer-Tropsch reaction goes fast in the presence of iron and cobalt

• Only very simple organics can be generated. No amino-acids, no PAHs etc.

• Since both atmosphere and hydrothermal vents have problems producing organics we need to look somewhere else.

Space!

Hale-BoppHale-Bopp MurchinsonMurchinson

Gaseous Pillars – Eagle NebulaGaseous Pillars – Eagle Nebula Key hole NebulaKey hole Nebula

TitanTitan