A2.1 Origin of Cells [IB Biology HL]

Active recall notes made following this video by Sirius Revision https://youtu.be/QBfI2OTR8E0?si=v-XJ3gWJ7iSAZCSF

What do we call Earth before life?

Pre-biotic Earth

Describe the atmosphere on pre-biotic Earth.

There was no atmosphere, very little oxygen, lots of methane and some CO₂, making it very hot with lots of lightning.

What key layer did pre-biotic Earth lack?

An ozone layer

What was pre-biotic Earth perfect for the spontaneous generation of?

complex carbon compounds

Why is Earth not in the pre-biotic stage anymore?

Life evolved and changed Earth’s nature, eliminating pre-biotic conditions.

What is the smallest unit of self-sustaining life?

a cell

Define self-sustaining.

Using energy to maintain a highly ordered state that can be passed on to offspring.

What is evidence that cells are living?

They use energy to maintain order, reproduce new cells, and can survive outside the body.

Why aren’t organelles self-sustaining?

Organelles can’t perform all the functions needed for life on their own.

Why are Pasteur’s experiments important?

They falsified spontaneous generation.

Define spontaneous generation.

The idea that living things can arise from non-living matter.

Where do new cells come from?

Only from existing cells.

What were the requirements for the first cell to form?

Catalysis, self-assembly of polymers, compartmentalization, and self-replicating molecules.

Define catalysis.

Some control over which reactions occur in a system.

Give an example of self-assembly of polymers.

Amino acids forming proteins or nucleotides forming DNA.

Define compartmentalization.

The ability of a cell to separate itself from its external environment.

What was the aim of the Miller-Urey experiment?

To test how biomolecules could form under pre-biotic Earth conditions.

Describe the Miller-Urey apparatus.

Water vapor was added to a chamber with ammonia, methane, and hydrogen, then exposed to energy (lightning/volcanic heat). Condensed products were collected and analyzed.

What were the results of the Miller-Urey experiment?

Amino acids were formed.

What was notable about Miller-Urey’s findings?

They showed that biomolecules could form from non-living conditions, supporting pre-biotic chemistry.

What are advantages of the Miller-Urey experiment?

It simulated pre-biotic Earth and supported hypotheses of carbon compounds forming before life.

What are disadvantages of the Miller-Urey experiment?

We can’t be sure what pre-biotic Earth’s conditions were exactly, proteins were never formed and different results arise when the apparatus is glass vs teflon

Define vesicles.

Structures surrounded by membranes.

Why were vesicles important?

They allowed cells to compartmentalize from their surroundings.

Define primordial soup.

A solution rich in organic compounds in early oceans, thought to be the origin of life.

Where were phospholipids found?

In the primordial soup.

Outline the structure of phospholipids.

A hydrophilic circular head and a hydrophobic rectangular tail.

What do phospholipids spontaneously form?

Bilayers.

Why do phospholipids form bilayers spontaneously?

The hydrophobic tails avoid water, so they cluster together while heads face outward toward water.

What are two advantages of a phospholipid bilayer for early cells?

They formed spontaneously for compartmentalization and were partially permeable.

Define partially permeable.

Selective about what enters and exits the cell.

What is presumed to have been the first genetic material?

RNA.

What is evidence that RNA was the first genetic material?

It is self-replicating, catalytic, exists in some viruses, and mutates quickly.

Why is viral RNA evidence for RNA being the first genetic material?

Viral RNA performs similar functions that early self-replicating molecules would have needed.

Why does RNA mutate at a high rate?

It lacks a complementary strand for proofreading during replication.

Why are a dog, tree, and horse not as different as they seem?

They share a common ancestor.

Define LUCA.

Last Universal Common Ancestor.

Why do scientists believe LUCA existed?

The universality of the genetic code—DNA works the same in all organisms.

Why are only LUCA’s descendants alive today?

Other early life forms likely died due to competition from LUCA and its descendants.

Define carbon dating.

Using carbon isotopes with known half-lives to calculate the age of a sample.

What’s a key requirement for carbon dating?

Fossils.

What’s a limitation of fossils in carbon dating?

Fossils undergo changes over time, so the fossil may be older than the life form it represents.

When did LUCA emerge based on fossils?

Around 4.1 billion years ago.

How can genetics estimate when we last shared a common ancestor?

By comparing shared genes and genetic differences to calculate divergence time.

When did LUCA exist based on genomic information?

~ 4.5 billion years ago

What will all descendants of LUCA have in common?

Shared genes.

What’s interesting about shared genes among all life?

Some genes code for proteins that help organisms live in extreme environments (iron or hydrogen-rich).

What does shared genes for living in extreme conditions suggest about LUCA’s environment?

LUCA likely evolved in environments with those extreme conditions.

Define white smokers.

Hydrothermal vents on the ocean floor that emit white, mineral-rich plumes.

Why are white smokers relevant to LUCA?

They contain minerals and heat that could have enabled spontaneous carbon-based compound formation.

What is a ribozyme

an RNA molecule capable of acting as an enzyme.

What role do ribozymes play?

they are still used to catalyze the formation of peptide bonds in protein synthesis.