Unit 4: Molecules of the cell and cell division
Essential Biology notes · PTU syllabus (BMLS101-18)
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Unit summary
Cells are built from inorganic and organic molecules and multiply by division. This unit covers water, salts and mineral ions, carbohydrates, lipids, amino acids and proteins, nucleotides and nucleic acids, binary fission, mitosis and meiosis.
After this unit you can
- Explain the roles of water and mineral ions
- Describe carbohydrates, lipids, proteins and nucleic acids
- Describe binary fission and mitosis
- Describe meiosis and compare it with mitosis
PTU syllabus topics
- Inorganic and organic materials — water
- salts
- mineral ions
- carbohydrates
- lipids
- amino acids
- proteins
- nucleotides
- nucleic acids (DNA and RNA)
- binary fission
- mitosis and meiosis
Divisions
One
Two
Daughter cells
Two, identical, diploid
Four, different, haploid
Crossing over
No
Yes, in prophase I
Purpose
Growth and repair
Gamete formation
Topic 1
Inorganic molecules: water, salts and minerals
- Water (about 70% of cell mass): universal solvent, medium for reactions, temperature regulation, transport.
- Na⁺ and K⁺
- Nerve impulses, fluid balance
- Ca²⁺
- Bones, teeth, clotting, muscle contraction
- Fe²⁺
- Haemoglobin
- Mg²⁺
- Enzyme cofactor
- PO₄³⁻
- ATP, DNA, bones
- I⁻
- Thyroid hormones
Topic 2
Carbohydrates
Monosaccharides
Glucose, fructose, galactose, ribose
Immediate energy; building blocks
Disaccharides
Sucrose, lactose, maltose
Dietary sugars
Polysaccharides
Starch, glycogen, cellulose, chitin
Storage and structure
Topic 3
Lipids
- Insoluble in water; include triglycerides (glycerol + 3 fatty acids; energy storage), phospholipids (membranes), steroids (cholesterol, hormones) and waxes. Fatty acids are saturated (no double bonds) or unsaturated.
Topic 4
Amino acids and proteins
- Amino acids have an amino group, carboxyl group and side chain (R); 20 types; joined by peptide bonds.
- Primary
- Sequence of amino acids
- Secondary
- α-helix and β-sheet by hydrogen bonds
- Tertiary
- 3D folding
- Quaternary
- Several chains — haemoglobin
- Functions: enzymes, structure (collagen), transport (haemoglobin), defence (antibodies), hormones (insulin).
Topic 5
Nucleotides and nucleic acids
Sugar
Deoxyribose
Ribose
Bases
A, G, C, T
A, G, C, U
Strands
Double helix
Usually single
Function
Stores genetic information
Protein synthesis (mRNA, tRNA, rRNA)
- A nucleotide = nitrogenous base + pentose sugar + phosphate. ATP is a nucleotide energy carrier.
Topic 6
Binary fission
- Prokaryotes divide by binary fission: the circular DNA replicates, the cell elongates and splits into two identical cells — about every 20 minutes for E. coli in ideal conditions.
Topic 7
Mitosis
- 1
Interphase
DNA replicates (G1, S, G2)
- 2
Prophase
Chromosomes condense; spindle forms; nuclear membrane breaks
- 3
Metaphase
Chromosomes line up at the equator
- 4
Anaphase
Sister chromatids pulled to opposite poles
- 5
Telophase
Nuclear membranes reform
- 6
Cytokinesis
Cytoplasm divides — two identical diploid cells
Topic 8
Meiosis
- Two divisions producing four haploid cells (gametes). Prophase I includes pairing of homologous chromosomes (synapsis) and crossing over (exchange of segments at chiasmata) — sources of variation.
Divisions
One
Two
Daughter cells
Two, diploid, identical
Four, haploid, genetically different
Crossing over
No
Yes
Occurs in
Body cells — growth and repair
Germ cells — gamete formation
Key terms
- Peptide bond
- Link between amino acids
- Nucleotide
- Base, sugar and phosphate unit
- Binary fission
- Division of a prokaryote into two
- Crossing over
- Exchange of segments between homologous chromosomes
- Haploid
- Having one set of chromosomes (n)
Quick revision
- Water and mineral ions and their roles.
- Mono-, di-, polysaccharides; lipids; proteins and four levels of structure.
- DNA vs RNA; nucleotides; ATP.
- Binary fission; mitosis stages.
- Meiosis, crossing over; mitosis vs meiosis.
Important exam questions
Practice questions written to the PTU exam pattern for this unit's syllabus: short answers (Section A style) and long answers (Sections B and C style).
Short-answer questions
- Q1.State two roles of calcium.
- Q2.Name a storage polysaccharide in animals.
- Q3.What is a peptide bond?
- Q4.Give two differences between DNA and RNA.
- Q5.In which phase do chromatids separate?
- Q6.Why is meiosis important?
Long-answer questions
- Q1.Describe the biomolecules of the cell.
- Q2.Describe the stages of mitosis with diagrams.
- Q3.Describe meiosis and compare it with mitosis.
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