Origins of Cells: notes and practice questions
- This topic explores plausible hypotheses and evidence for the origin of life and the first cells on early Earth.
- Early Earth lacked free oxygen and ozone, with higher concentrations of carbon dioxide and methane, leading to spontaneous formation of carbon compounds.
- Cells are the smallest self-sustaining units of life; viruses are considered non-living.
- The spontaneous origin of cells required catalysis, self-replication of molecules, self-assembly, and compartmentalization.
- RNA is presumed to be the first genetic material due to its ability to replicate and act as a catalyst (ribozymes).
- Evidence for a last universal common ancestor (LUCA) includes the universal genetic code and shared genes across all organisms.
- The Miller-Urey experiment provides evidence for the pre-biotic formation of carbon compounds.
How it is examined
HL Paper 1A and Paper 2. Command terms lean towards outline, explain and evaluate, and this is one of the few places in the syllabus where evaluate is asked about a single named experiment. Expect 2 to 4 marks and answers that need both a strength and a limitation of Miller-Urey.
There is no SL content in A2.1. The whole subtopic is HL.
Do not set this topic for SL students at all.
Guiding questions
- What plausible hypothesis could account for the origin of life?
- What intermediate stages could there have been between non-living matter and the first living cells?
Linking questions
- For what reasons is heredity an essential feature of living things?
- What is needed for structures to be able to evolve by natural selection?
Practice questions
5 questions · 3 medium · 2 hardQuestion 1
MediumPaper 1A · calculator1 markThe 'RNA world' hypothesis proposes that RNA preceded DNA as the genetic material in early life. What property of RNA makes it a plausible candidate for the first self-replicating molecule?
A. RNA is more chemically stable than DNA, preventing mutations.
B. RNA can fold into complex structures that allow it to catalyse chemical reactions.
C. RNA is single-stranded, which allows for faster replication than double-stranded DNA.
D. The uracil base in RNA can be synthesized from simpler precursor molecules than thymine.
Consider what functions are essential for a molecule to be the basis of life. It needs to store information and also perform actions. Which molecule, DNA or RNA, has been shown to do both?
Question 2
HardPaper 2 · calculator15 marks(a) Outline why viruses are classified as obligate parasites rather than living cells.
(b) Explain the concept of convergent evolution in the context of viral origins.
(c) Discuss the mechanisms of rapid evolution in viruses and the consequences for treating viral diseases.
Think about what structures and metabolic processes viruses lack, and what they must obtain from a host cell in order to replicate.
Do all viruses share a single common ancestor? Consider how different lineages might independently develop similar traits to survive as intracellular parasites.
Consider the mutation rates of RNA viruses like HIV or influenza, how new variation is generated, and how this affects the long-term efficacy of vaccines and antiviral drugs.
Question 3
MediumPaper 2 · calculator5 marks(a) Explain the hypotheses and evidence for the spontaneous origin of the first cells on early Earth.
Think about the atmospheric conditions on early Earth, the Miller-Urey experiment, the unique properties of RNA, and the necessary steps such as compartmentalization.
Question 4
HardPaper 2 · calculator15 marks(a) Outline the processes and conditions required for the spontaneous origin of cells on early Earth.
(b) Describe the advantages of compartmentalization in eukaryotic cells.
(c) Explain how the structure of proteins allows them to perform diverse functions in cell membranes and metabolism.
Think about the atmosphere of early Earth and the steps needed to go from simple molecules to a fully functioning cell, including the Miller-Urey experiment.
Consider why it is beneficial for a cell to have membrane-bound organelles rather than having all its enzymes and substrates mixed together in the cytoplasm.
Link the sequence and properties of amino acids (like hydrophobic/hydrophilic R-groups) to the 3D shape of proteins, and then explain how this shape allows them to act as membrane transporters and metabolic enzymes.
Question 5
MediumPaper 2 · calculator5 marks(a) Outline two pieces of evidence that support the hypothesis that all living organisms evolved from a last universal common ancestor (LUCA).
(b) The spontaneous origin of cells required the development of a mechanism for inheritance. Explain why RNA is presumed to be the first genetic material.
Think about the fundamental molecules and processes that are identical in bacteria, plants, and animals.
Consider what two functions a molecule must perform to be the first genetic material before proteins existed.
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Where marks are lost
- Command terms are consistently under-read. Students answer describe when the question said explain, so they give an account with no reasons and cap at half marks. The reverse also happens on outline, where a student writes an essay for a 2-mark summary and runs out of time.
