L4. Origins: What is life?

Question 1

What is life?

Answer 1

• hard question to answer
• something that consumes and produces energy, reproduces, and is non-motile
• no single accepted definition

Question 2

Requirements to be “alive”

Answer 2

Most theories include:
- maintain internal homeostasis: cell must be able to maintain interior conditions (temperature, pH, concentrations of proteins, solutes) in the face of changing external conditions
- respond to external stimuli: cell has change a physical or chemical response to an environmental trigger
- consume and produce energy: must energy (Atp) and consume nutrients to carry out the chemical reactions that sustain life (metabolism).
- reproduce and have a form of heredity: must have a mechanism to give rise to offspring and transmit hereditary information. There is asexual and sexual forms.

Question 3

ATP

Answer 3

• Organisms get energy from breaking down organic compounds (heterotrophs) or light/chemical reactions (autotrophs)
• Energy is moved around the cell as adenosine triphosphate
• generated when protons accumulate on one side of a membrane establishing an electrochemical potential energy gradient. Movement of the ions back across the membrane is used to do work

Question 4

Necessary ingredients for life

Answer 4

• Liquid water
• Chemical building blocks, made up of six elements (oxygen, hydrogen, carbon, nitrogen, sulphur, and phosphorus), energy souce

Question 5

Chemical building blocks of life

Answer 5

All life is composed of mainly the four macromolecule building blocks: carbohydrates, lipids, proteins and nucleic acids

macromolecule: large biological molecules made up of smaller subunits

Question 6

Did life evolve once or multiple times?

Answer 6

Evidence suggests that all current life evolved from a single current ancestor (last universal common ancestor)

Life could have evolved before and gone extinct with no record in modern-day life for us to find

Question 7

Evidence for LUCA

Answer 7

All extant life is:
- carbon based
- has similar enzymes, with similar gene recipes
- enzymes for the most basic biological functions are the same across very different species
- passes hereditary information through DNA or RNA
- has amino acids and nucleic acids that are mirror copies of each other

Question 8

How did life first emerge?

Answer 8

Many different theories exist and field is constantly changing. All tend to focus on explaining the emergence of one of the major requirements for life. 3 common theories.

Question 9

Primordial soup theory

Answer 9

• first proposed by Darwin in 1871. Then worked on by Alexander Oparin and John Haldane
• based heavily on the idea that the early earth environment consisted of high concentrations of compounds and elements known to be abundant in life.
• experiments have shown it to work when oxygen is present

4 Steps
- 1. Chemically reducing atmosphere (low levels of oxygen)
- 2. This atmosphere when exposed to energy (sun or lightning) in various forms produced simple organic compounds (monomers)
- 3. These compounds accumulated in a “soup,” which may have been concentrated at various locations (ex. shorelines)
4. Concentration led to the formation of polymers which life eventually arose from.

Main issues:
- no mechanism for the generation of complex polymers from simple monomers
- no mechanism for the evolution of cells after formation of polymers
- need sustained energy for life to develop

Question 10

RNA world theory

Answer 10

• First proposed by Alexander Rich (1962)
• Believes that RNA formed first and eventually lead to DNA and protein formation. Warm little ponds cycling wet to dry, these conditions cause RNA to polymerize
• It is a powerful substance that can store replication information, can replicate, and can act like an enzyme.
• Could have provided heredity and catalyzed reactions before the evolution of DNA and proteins
• Experiments shows it works with conditions at the time and also some viruses contain RNA only and virus are considered to be an ancient form of life

Issues:
- how does a combination of DNA, RNA, and proteins lead to cell formation.
- how are membranes produced?
- attempts to create self-replicating RNA under plausible early-Earth conditions have failed. (RNA is not very stable in water)

Question 11

Hydrothermal deep sea vent theory

Answer 11

• proposed recently by Dr. Nick Lane in 2010
• involves the generation of electrochemical gradients and metal-containing enzymes as the first precursors to life
• hydrothermal vent: small cracks on the seafloor from which geo-thermally heated water issues found in mid-ocean ridges where seafloor is spreading(“black smokers”)
• hot water coming out of smoker has very alkaline water that mets slightly acidic oceanic water creating a proton gradient. The chimneys are porous and act as a primitive semi-permeable membrane. This proton gradient membrane would create energy and organic monomers and polymers would be formed naturally with RNA, DNA, and proteins following. Eventually all would combine in the membrane form life, with it then leaving the ocean
• experiments show could work

Issues:
- no clear mechanism yet for how biomolecules eventually developed

Question 12

What if some of those building blocks came from outer space?

Answer 12

Meteorite containing abundance of organic materials including amino acids and purines and pyrimidines

Question 13

Panspermia

Answer 13

Like is actually everywhere in the universe
- microbes which can survive the effects of space can become trapped in debris ejected into space after collisions between planets and small solar systems bodies that harbor life
- requires that organic molecules originate in space, that life originated from these molecules extra terrestrially, that this extraterrestrial life was transported to Earth