L4 - What is Life? Flashcards
1
Q
What is life?
A
- Something that consumes and produces energy
- Something that is capable of reproducing or passing on its hereditary information
- Single-celled organisms (yeast) are a form of life
- Virus??? (has DNA/RNA. but needs a host to reproduce and cannot make its own energy)
2
Q
What is NASA’s definition of life?
A
- Life is a self-sustained chemical system capable of undergoing Darwinian evolution
- Under this definition viruses would be considered a form of life
3
Q
What are the requirements to be “alive”?
A
- Maintain internal homeostasis
- Respond to external stimuli
- Consume and produce energy
- Reproduce and have a form of heredity
4
Q
- Maintain internal homeostasis in the face of changing external conditions
A
- All life on earth likes a very narrow range of conditions
- Internal temperature range
- Internal pH
- Internal concentration of proteins & solutes
- Maintained by: cell membranes and the transport across these membranes
5
Q
- Respond to external stimuli (a change in the environment)
A
- Physical or chemical responses
6
Q
- Consume and produce energy
A
- Metabolism: sum total of the biochemical reactions occurring in an organism
- How do organisms get energy?
- Heterotrophs (must consume external food sources to get energy)
- Autotrophs (gets energy from light and converts it into energy/carbon compounds)
7
Q
What is ATP?
A
- Cellular energy currency
8
Q
How do cells generate ATP?
A
- Proton gradient across the cell membrane: protons accumulate on one side of a membrane, establishing an electrochemical potential energy gradient
- Movement of ions across the membrane generates energy in the form of ATP
9
Q
- Reproduction and Heredity
A
- Living things give rise to offspring and transmit genetic information
- Asexual reproduction: single organism creates clone of itself
- Most species on the planet reproduce asexually (ALL BACTERIA, many plants and some animals)
- Sexual reproduction: cells from new parents unite to form first cell of new organism
- Offspring is different from parent
- Plant can do both !!!
10
Q
How are organisms able to pass on hereditary information?
A
- Genes (sequence of DNA or RNA)
- A complex code made up of nucleotides
- Allows information to be recorded for billions and billions of years
11
Q
When did life first evolve on the planet?
A
- Life first evolved during the Archean - earliest undisputed evidence
12
Q
What are the necessary ingredients for life?
A
- Liquid water
- An energy source: UV light from the sun; electrical energy (lightning), chemical energy
- Chemical building blocks: made up of 6 elements - oxygen, carbon, hydrogen, nitrogen, sulphur and phosphorous
13
Q
What are the chemical building blocks of life?
A
- The 6 elements mentioned above compose 4 macromolecule building blocks
- Carbohydrates (sugars)
- Lipids (fatty acids)
- Proteins (amino acids)
- Nucleic acids (DNA/RNA)
14
Q
Did life evolve only once?
A
- Evidence from modern-day life suggests that current life evolved from a single common ancestor: “Last Universal Common Ancestor” (LUCA)
- Possible that life evolved multiple times, just no evidence to prove it
15
Q
What is the evidence for LUCA?
A
- All extant life…
- Is carbon-based
- Has similar enzymes (workhorses of cells), with similar gene recipes
- Enzymes for most basic biological functions are the SAME across very different species
- Passes hereditary information through DNA or RNA
- Amino acids and nucleic acids are found in two different conformations
- Has only L-alanine amino acids and D-alanine nucleic acids
16
Q
How did life first emerge?
A
- Primordial soup theory (oldest)
- RNA world theory (80s)
- Hydrothermal deep sea vent theory (2010s)
- ALL 3 THEORIES ASSUMED THAT LIFE EVOLVED IN A WORLD WHERE THERE WAS NO OXYGEN/COMPLETELY ANOXIC
17
Q
What is the primordial soup theory?
A
- First proposed by Darwin (1871)
- Later refined by Alexander Oparin and John Haldane
- Based on the idea that the early Earth environment consisted of high concentrations of compounds and elements known to be abundant in life
- Carbon, hydrogen, water vapour, ammonia
18
Q
What are the 4 steps of the primordial soup theory?
A
- Early Earth had a chemically reducing atmosphere
- very very low levels of oxygen and high levels of reducing gases (e.g., carbon monoxide)
- At this time oxygen was found on Earth, but it was almost all bound in minerals - This atmosphere, exposed to energy in various forms (lightning/UV light), produced simple organic compounds (monomers)
- These compounds accumulated in a “soup”, which may have been concentrated in various locations (shorelines, oceanic vents…)
- Highly concentrated “soups” led to the eventual formation of complex organic compounds (polymers)
- From these complex polymers, life eventually arose
- Early cells then broke down these organic molecules to generate ATP (heterotrophy)
19
Q
What is the Miller-Urey Experiment (1953)?
A
- Test for production of monomers under early primitive Earth conditions
- Anoxic conditions in the lab
- Project reducing gases
- Stimulate electrical discharge to represent lightning
20
Q
What were the results of the Miller-Urey Experiment (1953)?
A
- Found that 25 different amino acids (the basic building blocks of proteins) had been produced
- If free oxygen is present: no organic compounds are formed
21
Q
Why isn’t this theory accepted?
A
- No mechanism for the generation of complex polymers from simple monomers
- No mechanism for the evolution of cells from monomers and polymers
- Need sustained energy for life to develop (and not a one-time “spark”)
22
Q
What is the RNA World Theory (1962)?
A
- The primordial soup theory assumes that amino acids (monomers) formed first and would eventually connect somehow to form proteins (polymers)
- RNA world theory suggests that RNA formed first (first building block)
- The proliferation of RNA eventually led to DNA and protein formation
- RNA world theory is not mutually exclusive to the primordial soup theory
- Organic molecules that formed RNA could have come through interactions between the reducing atmosphere, lightning and UV??
23
Q
Why RNA World Theory?
A
- First proposed by Alexander Rich (1962)
- Upon the discovery that RNA could store genetic information like DNA AND ALSO CARRY OUT REACTIONS WITH RIBOZYMES
- Maybe RNA was the first macromalocule because an RNA can do all these things within a very very simple cell
- Least amount of support
24
Q
What is the evidence for the RNA World Theory?
A
- Experimental support: production of nucleic acid precursors and nucleotides in lab from simple commons common on early earth and UV light
- Observational support: some viruses (typically considered an ancient form of life) contain RNA only - but host?
25
Issues with the RNA world theory?
- How does a combination of DNA, RNA and proteins lead to cell protein?
- How are membranes produced?
- Attempts to create self-replicating RNA under plausible early Earth conditions have failed
- How would RNA viruses survive without a host
26
More recent support for RNA world theory
- RNA world: belief that it started in warm little ponds
- If you take nucleotides and put them under wet and dry cycles, they will naturally polymerize
- Supports the idea that if you have nucleotides in these early earth conditions, they will form, without life, what we know to be a long string of RNA
- Study argues that if there were enough wet and dry cycles at the time, then the RNA theory is possible
- Still a lot of questions to be answered
27
Deep Sea Hydrothermal Vent Theory
- Dr. Nick Lane (2010)
- First theory to seriously challenge the primordial soup theory
- Rather than having a focus on building blocks, it focuses on ENERGY (which the other two theories have no way of addressing )
- Involves the natural generation of electrochemical gradients and metal-containing enzymes as the first pre-cursors to life
- Allowed for the generation of (chemical) energy
28
What is a hydrothermal vent?
- Emerge at divergent plate boundaries forming mid-ocean ridges under water
- When you have these small cracks in the sea floor, magma and hot water and emerge and from this you can get black/white smokers
- These are in super deep places - no sunlight
- Life must find another form of energy
- Alkaline water from vents meets slightly acidic ocean water: formation of a natural proton gradient (similar to ATP)
- Vent fluids mixed with cold seawater end up solidifying the minerals in the fluids giving rise to very tall and porous chimney structures
- These structures have a primitive semi-permeable membrane
- Different proton concentrations on either side of the membrane
- Could get energy generated when the proton goes with the gradient
- Very early cell membrane that is completely abiotic
ONCE ENERGY IS NATURALLY AND RELIABLE GENERATED:
- Organic monomers and polymers would be formed naturally
- RNA, DNA and proteins would follow
- Eventually, all would combine and leave the vent and colonize the rest of the planet
29
What is the evidence for the deep see hydrothermal vent theory?
- Experimental: artificial hydrothermal vents have generated proton gradients under laboratory conditions
- Observational:
- Many of our enzymes require metals to function (metalo-enzymes): iron or sulphur as cofactors
- The fluids escaping these vents are also rich in these compounds
- Reconstruction of proteins from ancestral bacteria shows high heat stability (i.e., early proteins evolved/adapted to function in a very hot place)
30
What are the issues with the deep see hydrothermal vent theory?
- No clear mechanism yet for how biomolecules (amino acids) eventually developed
31
What is some of life's building blocks came from outer space?
- Tagish Lake Meteorite
- Contains and abundance of organic materials
- Age of meteorite is estimated to be about 4.55 billion years
- Found lots and lots of amino acids within the meteorite
32
Panspermia: what is LIFE came from outerspace?
- Life is actually everywhere in the universe
- Microbes that can survive the effects of space and space travel can become trapped in debris, would land on Earth and seed life on Earth
33
What does Panspermia require?
1. Organic molecules originated in space
2. Life originated from these molecules extra-terrestrially
3. This extraterrestrial life was transported to Earth (with everything it needed to survive DNA)
34
Why isn't Panspermia accepted?
- The temperatures needed to survive outerspace as well as to enter Earth's atmosphere and land on Earth are not temperatures are not temperatures that life as we know it can survive.
- The hottest life as we know that can survive on Earth is 121°C, and that's an extreme microbe
- Everything else will melt at 70-80 °C
