All life is built from the same molecular toolkit. This SL lesson covers A1.1 Water, A1.2 Nucleic acids and A2.2 Cell structure — the molecular and cellular basis of the unity shared by all organisms.
You will meet water's properties, the structure of DNA and RNA, cell theory, magnification and surface-area-to-volume ratio.
Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.
A1.1 Water
Water — the medium of life
Water is a polar molecule: oxygen pulls electrons more strongly, giving a slightly negative O and slightly positive H atoms. This lets water molecules form hydrogen bonds with each other.
Cohesion — molecules stick together, allowing continuous water columns to be pulled up xylem.
Adhesion — water sticks to other polar surfaces.
Solvent — dissolves ions and polar (hydrophilic) solutes; the medium for metabolism and transport.
Thermal — high specific heat and heat of vaporisation buffer temperature and cool by sweating.
Unity idea: the same water chemistry supports life from bacteria to whales — a shared property of all cells.
Quick check
Question
?Which property of water is a direct result of hydrogen bonding, allowing a continuous column to be pulled up the xylem of a tree?
A1.2 Nucleic acids
Nucleic acids — DNA and RNA
DNA and RNA are polymers of nucleotides. Each nucleotide is a pentose sugar + phosphate + a nitrogenous base.
DNA: deoxyribose sugar; bases A, T, G, C; a double helix of two antiparallel strands.
RNA: ribose sugar; uses uracil (U) instead of thymine; usually single-stranded.
Bases pair by hydrogen bonds: A–T and G–C (Chargaff's rule). This complementary base pairing lets DNA be copied accurately.
A1.2 Nucleic acids
Chargaff's base ratios
Because A pairs only with T and G only with C, in double-stranded DNA the amount of A equals T, and G equals C.
%A = %T · %G = %Cso %A + %T + %G + %C = 100%
If you know one base percentage you can work out the rest. For example, if adenine is 30%, then thymine is 30%, leaving 40% shared equally by G and C (20% each).
Calculate
Your turn — calculate
1In a double-stranded DNA sample, adenine makes up 28% of the bases. Using Chargaff base pairing, calculate the percentage that is guanine.
%
Hint: %A = %T = 28, so G + C = 100 − 56 = 44%, shared equally.
A2.2 Cell structure
Cell theory and cell types
Cell theory: all organisms are made of cells; cells are the basic unit of life; cells come from pre-existing cells.
Eukaryotic cells (animals, plants, fungi, protists) have a membrane-bound nucleus and organelles; ribosomes are 80S.
Prokaryotic cells (bacteria, archaea) have no nucleus — a single circular DNA loop in a nucleoid, and 70S ribosomes.
Unity and diversity: all cells share DNA, ribosomes, a plasma membrane and cytoplasm (unity), yet differ hugely in size and organisation (diversity).
Sort it
Sort each item into its group
Sort each feature by which cells have it.
All cells
Eukaryotes only
Prokaryotes only
A2.2 Cell structure
Microscopy and magnification
Cells are studied with microscopes. Magnification tells you how much bigger the image is than the real object; resolution is the smallest distance that can be distinguished.
magnification = image size ÷ real sizerearrange: real size = image size ÷ magnification
Keep units the same!
A cell 0.05 mm wide appears 20 mm wide in a photo: magnification = 20 ÷ 0.05 = ×400.
Calculate
Your turn — calculate
2A mitochondrion is 8 µm long. In an electron micrograph it measures 40 mm long. Convert to the same unit, then calculate the magnification.
?Almost all organisms use DNA, ribosomes and ATP. This shared molecular machinery is best explained by which idea?
A2.2 Cell structure
Surface-area-to-volume ratio
Exchange (diffusion, osmosis, heat loss) happens across a cell's surface, but the demand depends on its volume. As a cell grows, volume rises faster than surface area, so the SA:V ratio falls.
SA:V = surface area ÷ volumefor a cube: SA = 6 × side², V = side³
A high SA:V lets small cells exchange fast by diffusion alone. Large organisms evolve specialised exchange surfaces (lungs, gills, villi) to compensate.
Calculate
Your turn — calculate
3A cube-shaped cell has sides of 4 µm. Using SA = 6 × side² and V = side³, calculate the surface-area-to-volume ratio (SA ÷ V) as a single number.
: 1
Hint: SA = 6 × 4² = 96; V = 4³ = 64; ratio = 96 ÷ 64.
Quick check
Question
?In RNA, which base replaces the thymine found in DNA?
Match it
Match each item to its partner
Match each structural feature to what it provides.
Item
Partner
Quick check
Question
?Water is a versatile solvent for metabolism mainly because each molecule is...
Recap
The big ideas to take away
A1.1 Water: polar; hydrogen bonds give cohesion, solvent and thermal properties
A1.2 Nucleic acids: nucleotide polymers; DNA (A-T, G-C) vs RNA (uracil); Chargaff ratios