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Understanding points

Early Earth conditions

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High CH₄ and CO₂ concentrations due to volcanic activities and meteorite bombardment → more greenhouse gasses → higher temperatures
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UV penetration to Earth’s surface due to lack of ozone layer and frequent lightning → triggered various chemical reactions
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Miller-Urey Experiment: mimicked early Earth conditions to show the synthesis of organic carbon compounds from inorganic gasses that represent the pre-biotic atmosphere
A2.1 Origins of cells

Understanding points

Cell theory

1.
All living things are composed of cells
2.
Cells can only be formed by division of pre-existing cells
3.
Cell is the basic unit of all “Living things”
A2.2 Cell structure

Understanding points

Virus

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Virus: non-living agents that infect host cells to reproduce inside them
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Small and fixed size
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Contains nucleic acid as genetic material
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DNA virus: transcribes their DNA directly
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RNA virus:
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Positive-sense RNA virus: uses their RNA genes directly as mRNA
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Negative-sense RNA virus: transcribe their RNA genes to make mRNA
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Retrovirus: synthesizes dsDNA from their RNA genes and transcribe the DNA to produce mRNA
A2.3 Viruses

Understanding points

Phospholipid bilayer

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Phospholipids have hydrophilic and hydrophobic regions
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Hydrophilic heads are attracted to water → face watery environment
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Hydrophobic tails are repelled by water → arrange themselves inwards
B2.1 Membranes and membrane transport

Understanding points

Advantage of nucleus

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Separation and control of transcription and translation
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*(AHL) double membrane
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Contains pores for RNA transport
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Easily dismantled and reconstructed during cell division
B2.2 Organelles and compartmentalization

Understanding points

Differentiation

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Involves the expression of some genes and not others in a cell’s genome
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All tissues have the same genes, but specialization involves the ‘turning on’, or expression, of particular genes, which define a specific function
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Specialised tissues can develop by cell differentiation in multicellular organisms
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Tissue: a group of specialised cells that perform the same function more efficiently

Stem cells

B2.3 Cell specialization

Understanding points

Receptors

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Signals can be chemical or electrical
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Ligand: a molecule that binds specifically to a site on a receptor
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Receptor: a molecule that binds with ligands to convey signals that affect cellular activity
C2.1 Chemical signaling

Understanding points

Neuron

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Cells of the nervous system
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The speed of nerve impulse transmission is increased by:
1.
Wider axon diameter: reduces resistance
2.
Myelination: *(AHL) enables saltatory conduction where action potentials only occur at Nodes of Ranvier
C2.2 Neural signaling

Understanding points

*(AHL) Cell Cycle

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Interphase → Mitosis [prophase → metaphase → anaphase → telophase] → Cytokinesis
D2.1 Cell and nuclear division

Understanding points

Gene expression

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Process of turning genotype into phenotype
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The pattern of gene expression determines that cell’s route of differentiation
D2.2 Gene expression

Understanding points

Solvation

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The process of dissolving
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Solvent: liquids that dissolve other substances
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Solute: dissolved substances in solutions
D2.3 Water potential