NCEA LEVEL 2 BIOLOGY — COMPLETE EXCELLENCE NOTES Year 12 | NZ Curriculum | Exam-focused | Excellence Level SECTION 1: CELL BIOLOGY & ORGANELLES Key
NCEA LEVEL 2 BIOLOGY — COMPLETE EXCELLENCE NOTES
Year 12 | NZ Curriculum | Exam-focused | Excellence Level
SECTION 1: CELL BIOLOGY & ORGANELLES
Key Organelles and Functions
Mitochondria
Site of aerobic cellular respiration
Produces ATP (energy) from glucose
Double membrane — outer membrane and inner folded membrane called cristae
The matrix (fluid inside) is where the Krebs cycle occurs
Contains its own DNA (evidence of ancient bacterial origin)
Cell Membrane (Plasma Membrane)
Controls what enters and exits the cell — selectively permeable
Made of a phospholipid bilayer with embedded proteins
Proteins act as channels, carriers, and receptors
Site of active transport, endocytosis, and exocytosis
EXCELLENCE LINK: The fluid mosaic model describes the membrane as flexible with proteins floating in the phospholipid bilayer
Chloroplast (plant cells only)
Site of photosynthesis
Contains thylakoids stacked into grana — site of light-dependent reactions
Contains stroma (fluid) — site of light-independent (Calvin cycle) reactions
Contains chlorophyll which absorbs light energy (mainly red and blue wavelengths)
Has its own DNA
Vacuole
Plant cells: large central vacuole filled with cell sap, maintains turgor pressure, keeps plant rigid
Animal cells: small, temporary vacuoles for storage or digestion
Ribosomes
Site of protein synthesis (translation)
Found free in cytoplasm or on rough endoplasmic reticulum
Made of rRNA and proteins
Small but critical — every protein in the body is made here
Cytoplasm
Gel-like fluid (cytosol) filling the cell
Site of glycolysis (first stage of respiration)
Suspends organelles and is the site of many metabolic reactions
Nucleus
Contains DNA organised into chromosomes
Surrounded by a double nuclear envelope with nuclear pores
Nuclear pores allow mRNA to exit and proteins to enter
Controls all cell activities by regulating gene expression
Rough Endoplasmic Reticulum (Rough ER)
Studded with ribosomes
Proteins made on ribosomes enter the rough ER for folding and processing
Transports proteins to the Golgi apparatus
Smooth Endoplasmic Reticulum (Smooth ER)
No ribosomes
Synthesises lipids and phospholipids
Detoxifies drugs and harmful substances
Golgi Apparatus
Receives proteins from rough ER
Modifies, packages, and labels proteins
Ships them to correct destination — secretion outside cell, lysosomes, or cell membrane
"Post office of the cell"
Lysosomes
Contain powerful digestive enzymes
Break down worn-out organelles, food particles, and pathogens
Important in phagocytosis — after a white blood cell engulfs a pathogen, lysosomes digest it
Plant Cell Structures NOT in Animal Cells
Cell wall (cellulose) — provides rigid structural support, fully permeable
Chloroplasts — site of photosynthesis
Large central vacuole — maintains turgor pressure
Three Regions of a Gene
Promoter region — where RNA polymerase binds to begin transcription
Coding region — contains the base sequence that codes for the protein, starts with start codon AUG
Terminator region — signals where transcription ends
SECTION 2: PHOTOSYNTHESIS
Equations
Word equation: Carbon dioxide + Water → Glucose + Oxygen (using light energy)
Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Stage 1: Light-Dependent Reactions
Where: Thylakoid membranes (in the grana of the chloroplast)
What happens:
Chlorophyll absorbs light energy
Water is split by photolysis: H₂O → 2H⁺ + ½O₂ + 2e⁻
Oxygen is released as a byproduct (this is where all our O₂ comes from)
Light energy is converted into chemical energy (ATP)
NADP⁺ is reduced to NADPH (picks up hydrogen ions)
ATP and NADPH pass to the light-independent stage
EXCELLENCE: The splitting of water (photolysis) is the source of the oxygen we breathe. Light energy is converted to chemical energy stored in ATP and NADPH — these are the energy carriers that power the Calvin cycle.
Stage 2: Light-Independent Reactions (Calvin Cycle)
Where: Stroma of the chloroplast
What happens:
CO₂ is fixed — combined with RuBP (a 5-carbon compound) by the enzyme RuBisCO
This forms an unstable 6-carbon compound that immediately splits into two 3-carbon molecules (GP)
ATP and NADPH from the light-dependent stage are used to convert GP into G3P (glyceraldehyde-3-phosphate)
G3P is used to make glucose
RuBP is regenerated to continue the cycle
EXCELLENCE: The Calvin cycle does NOT directly require light, but it stops if the light-dependent reactions stop because it depends on the ATP and NADPH they produce. This is why photosynthesis slows at night even though darkness doesn't directly affect the Calvin cycle enzymes.
Key Vocabulary
Stroma — fluid inside chloroplast, site of Calvin cycle
Thylakoid — flattened membrane sac inside chloroplast
Grana — stacks of thylakoids
NADPH — electron/hydrogen carrier produced in light-dependent stage
Stomata — pores on underside of leaf for gas exchange
Guard cells — control opening/closing of stomata
Photolysis — splitting of water using light energy
Factors Affecting Rate of Photosynthesis
Light intensity — more light = faster rate up to a saturation point; beyond that, another factor is limiting
CO₂ concentration — more CO₂ = faster rate up to a limit
Temperature — increases rate up to optimum (~25-35°C); above optimum, enzymes (including RuBisCO) denature
Water — needed as a reactant; water shortage causes stomata to close, reducing CO₂ entry
EXCELLENCE — Limiting Factors: At any given moment, the rate of photosynthesis is controlled by whichever factor is most limited. E.g., even with bright light, if CO₂ is low, the Calvin cycle slows. This is the concept of limiting factors.
SECTION 3: CELLULAR RESPIRATION
Overview
Cellular respiration releases energy (ATP) from glucose. Two types: Aerobic (with O₂) and Anaerobic (without O₂).
Aerobic Respiration
Word equation: Glucose + Oxygen → Carbon dioxide + Water + ATP
Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ~38 ATP
Stage 1: Glycolysis
Where: Cytoplasm
Glucose (6C) split into 2 pyruvate (3C) molecules
Net gain: 2 ATP
Produces 2 NADH
Does NOT require oxygen — can occur in aerobic or anaerobic conditions
Stage 2: Krebs Cycle (Citric Acid Cycle)
Where: Matrix of mitochondria
Pyruvate converted to Acetyl CoA (releasing CO₂) then enters cycle
Each turn produces: CO₂, ATP, NADH, FADH₂
Produces 2 ATP per glucose (1 per pyruvate)
CO₂ produced here is exhaled
Stage 3: Electron Transport Chain
Where: Inner mitochondrial membrane (cristae)
NADH and FADH₂ donate electrons down the chain
Energy released pumps H⁺ ions across the membrane, driving ATP synthase
~34 ATP produced
Oxygen is the final electron acceptor → combines with H⁺ to form water
This is where the MAJORITY of ATP is made
Total ATP: ~38 per glucose
EXCELLENCE: The electron transport chain uses the energy from NADH and FADH₂ to create a proton gradient across the inner mitochondrial membrane. ATP synthase uses this gradient to synthesise ATP — this is called chemiosmosis. Oxygen's role as the final electron acceptor is critical: without it, the chain stops and aerobic respiration halts.
Advantages of Aerobic Respiration
Produces ~38 ATP per glucose — very efficient
Complete breakdown of glucose
Byproducts (CO₂ and H₂O) are non-toxic
Disadvantages of Aerobic Respiration
Requires oxygen — useless in anaerobic conditions
Slower to initiate than anaerobic respiration
Anaerobic Respiration
In animals/humans: Glucose → Lactic acid + 2 ATP
Lactic acid builds up in muscles → fatigue, burning sensation
Lactic acid is broken down when oxygen becomes available again (oxygen debt)
In yeast/plants: Glucose → Ethanol + CO₂ + 2 ATP
Used in bread making (CO₂ causes dough to rise) and fermentation
Advantages of Anaerobic
No oxygen required
Fast — immediate ATP production
Disadvantages of Anaerobic
Only 2 ATP per glucose — very inefficient
Toxic byproducts (lactic acid, ethanol)
Not sustainable long-term
Comparing Photosynthesis and Aerobic Respiration
FeaturePhotosynthesisAerobic RespirationLocationChloroplastMitochondriaReactantsCO₂ + H₂OGlucose + O₂ProductsGlucose + O₂CO₂ + H₂OEnergyStores light energy as glucoseReleases energy as ATPOrganismsPlants, algaeAll living organisms
EXCELLENCE: These two processes are complementary — the products of one are the reactants of the other. In plants, both occur simultaneously. During the day, photosynthesis exceeds respiration (net gas exchange: CO₂ in, O₂ out). At night, only respiration occurs (CO₂ out, O₂ in).
SECTION 4: ENZYMES
What Are Enzymes?
Biological catalysts — speed up reactions without being used up
Made of protein — their shape is critical to their function
Have a specific active site that binds to a specific substrate
Lower activation energy of reactions
Two Models of Enzyme Action
Lock and Key Model
Active site is a fixed, rigid shape
Only a substrate with exactly the right shape fits
Simpler model but less accurate
Induced Fit Model (more accurate)
Active site is flexible and changes shape slightly when the substrate binds
Like a hand fitting into a glove — the glove molds around the hand
Better explains why some similar molecules can also bind (but less effectively)
EXCELLENCE: The induced fit model is the accepted modern model. It explains competitive inhibition more accurately — the inhibitor causes the active site to change shape, preventing normal substrate binding.
Factors Affecting Enzyme Activity
Temperature
Increasing temperature increases kinetic energy → more collisions between enzyme and substrate → faster rate
Above optimum temperature, the enzyme denatures — hydrogen bonds holding the 3D shape break
Active site changes shape permanently → enzyme no longer functional
Human enzymes: optimum ~37°C
pH
Each enzyme has an optimum pH (e.g., pepsin works best at pH 2, salivary amylase at pH 7)
Too high or too low pH disrupts hydrogen bonds → denaturation
Substrate Concentration
More substrate = more collisions with enzyme = faster rate
Once all active sites are occupied (enzyme saturation), rate plateaus
Enzyme Concentration
More enzymes = more active sites available = faster rate (if substrate is not limiting)
Inhibitors
Competitive inhibitors: similar shape to substrate, block active site, compete with substrate
Non-competitive inhibitors: bind to a different site (allosteric site), change the shape of the active site → substrate can no longer bind
Cofactors and Coenzymes
Cofactors: inorganic ions (e.g., Mg²⁺, Zn²⁺) needed for enzyme to function
Coenzymes: organic molecules that help enzymes work (e.g., NAD⁺, NADP⁺, Coenzyme A)
EXCELLENCE: Competitive inhibition is reversible — adding more substrate can outcompete the inhibitor. Non-competitive inhibition is often irreversible and changes the Vmax of the reaction. This distinction is important for Excellence answers.
SECTION 5: TRANSPORT ACROSS MEMBRANES
Diffusion
Movement of molecules from high to low concentration (down the concentration gradient)
Passive — no ATP required
Continues until equilibrium is reached
Rate affected by: concentration gradient, temperature, surface area, distance, molecule size
Active Transport
Movement of molecules AGAINST the concentration gradient (low → high)
Requires ATP and carrier proteins (protein pumps) in the membrane
Used when cells need to absorb substances already in high concentration inside
E.g., root hair cells absorbing mineral ions from soil
EXCELLENCE: Active transport allows cells to accumulate substances against their concentration gradient, which is essential for many biological processes. The sodium-potassium pump is a key example — it maintains the concentration difference needed for nerve impulse transmission.
Endocytosis and Exocytosis
Endocytosis — taking substances INTO the cell
Cell membrane folds inward, engulfing material into a vesicle
Phagocytosis — engulfing solid particles (e.g., white blood cells engulfing bacteria)
Pinocytosis — engulfing liquid droplets
Exocytosis — releasing substances OUT of the cell
Vesicles fuse with the cell membrane and release contents outside
Used for secreting hormones, enzymes, and neurotransmitters
NOTE on Osmosis: Osmosis is the diffusion of water across a selectively permeable membrane from high water potential (dilute) to low water potential (concentrated). While not a major exam focus, understanding it conceptually helps with transport questions.
SECTION 6: DNA STRUCTURE & REPLICATION
DNA Structure
Full name: Deoxyribonucleic acid
Double helix — two polynucleotide strands twisted together
Each nucleotide contains: deoxyribose sugar + phosphate group + nitrogenous base
Strands held together by hydrogen bonds between complementary bases
DNA is antiparallel — strands run in opposite directions (5'→3' and 3'→5')
The sugar-phosphate backbone forms the outside; bases face inward
Complementary Base Pairing
Adenine (A) pairs with Thymine (T) — 2 hydrogen bonds
Cytosine (C) pairs with Guanine (G) — 3 hydrogen bonds
This is the complementary base pairing rule — it is universal across all life
EXCELLENCE: The different numbers of hydrogen bonds matter — C-G bonds are stronger than A-T bonds. DNA with higher C-G content requires more energy to separate (higher melting point). This is why the complementary base pairing rule is not just about shape but also about bond strength.
DNA vs RNA
FeatureDNARNASugarDeoxyriboseRiboseBasesA, T, C, GA, U, C, G (Uracil replaces Thymine)StrandsDouble-strandedSingle-strandedLocationNucleus (mainly)Nucleus and cytoplasmStabilityVery stable, long-term storageLess stable, temporaryFunctionStores genetic informationCarries/reads instructions for protein synthesis
DNA Replication (Semi-Conservative)
Purpose: Before cell division, DNA must be copied so each daughter cell gets a full set.
Process:
Helicase unwinds and unzips the double helix (breaks hydrogen bonds)
Each original strand acts as a template
DNA polymerase reads each template strand (3'→5') and adds complementary nucleotides (building 5'→3')
DNA ligase joins Okazaki fragments on the lagging strand
Result: 2 identical DNA molecules, each with one original and one new strand
Semi-conservative means each new DNA molecule keeps one original strand — this was proven by the Meselson-Stahl experiment.
Leading strand — synthesised continuously (same direction as helicase)
Lagging strand — synthesised in fragments (Okazaki fragments) because DNA polymerase can only work 5'→3'
SECTION 7: GENE EXPRESSION — PROTEIN SYNTHESIS
The Central Dogma
DNA → mRNA → Protein
Transcription → Translation
Key Vocabulary
Gene — a section of DNA that codes for a specific protein
Codon — 3 bases on mRNA coding for one amino acid
Anticodon — 3 bases on tRNA, complementary to a codon
Triplet — 3 bases on a DNA strand (template for a codon)
Intron — non-coding section of DNA/pre-mRNA (removed during processing)
Exon — coding section of DNA/pre-mRNA (kept and expressed)
Example chain:
DNA triplet: CAT → mRNA codon: GUA → tRNA anticodon: CAU → amino acid: Valine
Why Is the RNA Step Essential? (Excellence — know all of these)
DNA is too large to fit through nuclear pores — mRNA is a smaller copy that can exit
mRNA can be edited — introns (non-coding sequences) are removed, exons joined
Once translated, mRNA is broken down — DNA remains protected and intact
Ribosomes are abundant in the cytoplasm — efficient translation can occur there
No space in the nucleus for the large number of ribosomes needed
Using RNA means one gene can produce many mRNA copies → many protein molecules simultaneously
Uracil in RNA (instead of thymine) allows enzymes to identify and degrade mRNA after use, protecting the original DNA
Stage 1: Transcription (in the nucleus)
RNA polymerase binds to the promoter region
DNA unwinds and unzips
RNA polymerase reads the template strand (3'→5') and builds complementary mRNA (5'→3')
Uracil (U) used instead of Thymine (T)
Transcription begins at start codon region and ends at terminator region
Pre-mRNA is produced
Stage 2: RNA Processing (in the nucleus)
Introns (non-coding) are removed by spliceosomes
Exons (coding) are spliced together
Mature mRNA exits through nuclear pore to the cytoplasm
Stage 3: Translation (at ribosomes in the cytoplasm)
mRNA attaches to a ribosome
Translation begins at start codon AUG (codes for methionine)
tRNA molecules carry specific amino acids to the ribosome
Each tRNA anticodon matches a complementary mRNA codon
Ribosome moves along mRNA, reading codons one at a time
Peptide bonds form between amino acids → growing polypeptide chain
Translation ends at a stop codon (UAA, UAG, or UGA)
Polypeptide folds into its 3D shape → functional protein
The Chain of Determination (Excellence Must-Know)
DNA triplet → mRNA codon → tRNA anticodon → Amino acid → Polypeptide sequence → Protein shape → Protein function
A change in ONE base in DNA can:
→ Change the mRNA codon
→ Change the amino acid incorporated
→ Change the polypeptide sequence
→ Change how the protein folds
→ Change the protein's shape
→ Change or destroy the protein's function
EXCELLENCE: This chain explains why mutations matter. A single base substitution might: have no effect (silent mutation — same amino acid), slightly change function (missense), or completely destroy function (nonsense — early stop codon). The further along the chain the change propagates, the more severe the effect.
Protein Shape and Function
Specific sequence of amino acids determines how the protein folds (primary → secondary → tertiary structure)
Shape determines function — enzymes, antibodies, hormones all depend on precise shape
Even ONE amino acid change can alter the entire protein's shape
Specific codons always produce the same amino acid in all organisms — the genetic code is universal
SECTION 8: MUTATIONS
What Is a Mutation?
A permanent change in the DNA base sequence
Can be spontaneous or caused by mutagens
Somatic mutation — in body cells, not inherited
Germline/gamete mutation — in sex cells, CAN be passed to offspring
Types of Gene Mutations
Substitution
One base replaced by another
E.g., original: ATG CAT → mutated: ATG AAT
May be silent (same amino acid), missense (different amino acid), or nonsense (stop codon created)
Does NOT cause a frameshift
Insertion
Extra base(s) added into the sequence
Causes a FRAMESHIFT — every codon after the insertion is shifted
Usually severely disrupts the protein
Deletion
Base(s) removed from the sequence
Also causes a FRAMESHIFT
Usually severely disrupts the protein
Effects of Mutations
TypeWhat happensEffect on proteinSilentDifferent codon, same amino acidNo changeMissenseDifferent codon, different amino acidPossibly altered shape/functionNonsenseCodon becomes a stop codonShortened, usually non-functional proteinFrameshiftAll codons after mutation shiftUsually completely non-functional protein
Mutagens
UV radiation — causes thymine dimers in DNA
X-rays and gamma rays — ionising radiation damages DNA
Chemical mutagens — e.g., chemicals in tobacco smoke
Some viruses — insert their DNA into host genome
Mutations and Evolution
Most mutations are neutral or harmful
Occasionally a mutation is beneficial in a particular environment
Beneficial mutations increase survival → organism reproduces more → mutation passed on
Over time, beneficial mutations accumulate in a population → evolution by natural selection
SECTION 9: GENETICS GLOSSARY
TermDefinitionGeneA section of DNA that codes for a specific proteinChromosomeStructure of tightly coiled DNA; humans have 23 pairs (46 total)AlleleAn alternative version of a gene (e.g., brown vs blue eye colour)GenotypeThe combination of alleles an individual hasPhenotypeThe physical expression of the alleles (what you observe)GameteSex cell (sperm or egg) — haploidHaploid (n)Cell with one set of chromosomes (e.g., gametes, n=23)Diploid (2n)Cell with two sets of chromosomes (e.g., body cells, 2n=46)ZygoteFirst diploid cell formed when sperm fuses with eggFertilisationFusion of sperm and eggHomozygousTwo identical alleles (BB or bb)HeterozygousTwo different alleles (Bb)DominantAllele expressed when one or two copies presentRecessiveAllele only expressed when two copies presentCodominanceBoth alleles fully expressed in heterozygote (e.g., AB blood group)
SECTION 10: MITOSIS
Purpose
Produce genetically IDENTICAL daughter cells
Growth and repair of tissues
1 diploid cell (2n) → 2 identical diploid cells (2n)
Stages (PMAT + C)
Interphase — DNA replicates; cell grows (technically before mitosis)
Prophase — chromosomes condense and become visible; spindle fibres form from centrioles
Metaphase — chromosomes line up at the cell equator (metaphase plate)
Anaphase — spindle fibres pull sister chromatids apart to opposite poles
Telophase — nuclear envelopes reform around each set of chromosomes; chromosomes decondense
Cytokinesis — cytoplasm divides → 2 identical daughter cells
EXCELLENCE: Mitosis maintains the diploid chromosome number. Every somatic (body) cell division uses mitosis to ensure each new cell has a complete, identical copy of the genome. Errors in mitosis (e.g., chromosomes not separating properly — non-disjunction) can lead to cancer or cell death.
SECTION 11: MEIOSIS
Purpose
Produce sex cells (gametes) — sperm and eggs
Produces 4 genetically DIFFERENT haploid cells (n) from 1 diploid cell (2n)
Called reduction division (2n → n)
Creates genetic variation
Meiosis vs Mitosis
FeatureMitosisMeiosisPurposeGrowth and repairProduce gametesDivisions12Cells produced24Genetic resultIdenticalGenetically uniquePloidy change2n → 2n2n → nCrossing overNoYes
Meiosis I — Reduction Division
Homologous chromosomes pair up (bivalents form)
Crossing over occurs (see below)
Homologous pairs separate → 2 haploid cells
Chromosome number halved here
Meiosis II — Similar to Mitosis
Sister chromatids pulled apart
Result: 4 haploid genetically unique cells
Homologous Chromosomes
Chromosome pairs — one from mum, one from dad
Same genes at same loci but may carry different alleles
Humans: 23 pairs
Crossing Over (EXCELLENCE TOPIC)
Occurs during Meiosis I (prophase I)
Homologous chromosomes pair up and physically exchange segments of DNA
The point of exchange is called a chiasma (plural: chiasmata)
Creates NEW combinations of alleles on chromosomes
Offspring have different combinations of alleles than either parent
This is a major source of genetic variation
Independent Assortment (EXCELLENCE TOPIC)
During Meiosis I, homologous pairs line up randomly at the equator
Which chromosome of each pair goes to which side is completely random
With 23 pairs in humans: 2²³ = over 8 million possible chromosome combinations
Creates enormous genetic variation even before fertilisation
Formation of Haploid Gametes
After meiosis: gametes have n = 23 chromosomes
Sperm (n=23) + egg (n=23) → zygote (2n=46)
Diploid number restored at fertilisation
SECTION 12: SOURCES OF GENETIC VARIATION
Three Main Sources
1. Crossing over
Exchange of DNA between homologous chromosomes during Meiosis I
Creates new combinations of alleles — chromosomes are reshuffled
Every crossover event produces chromosomes that have never existed before
2. Independent assortment
Random orientation of homologous pairs at Meiosis I
2²³ = 8+ million possible combinations in humans from this alone
3. Random fertilisation
Any sperm can fertilise any egg — completely random
8 million × 8 million = over 64 trillion genetically unique possible offspring
This is why siblings (other than identical twins) are never genetically identical
EXCELLENCE: These three mechanisms together mean that the probability of any two humans (other than identical twins) being genetically identical is essentially zero. This variation is the raw material for natural selection and evolution.
SECTION 13: GENE EXPRESSION & ENVIRONMENT
All Cells Have the Same DNA — Different Genes Are Expressed
Every cell in your body (liver, nerve, muscle) has the same DNA
Different cell types look and function differently because different genes are switched on or off
This is differential gene expression
How Gene Expression Is Controlled
Transcription factors — proteins that bind to promoter regions and activate or repress transcription
Methylation — methyl groups added to DNA silence genes (epigenetics)
Histone modification — DNA is wrapped around histone proteins; how tightly it's wrapped affects accessibility for transcription
Environmental Effects on Gene Expression
Temperature — Siamese cats: cooler extremities = darker fur because the pigment-producing enzyme only works at lower temperatures
Diet/nutrition — certain nutrients activate or suppress genes (e.g., folate affects gene methylation)
Light — plants regulate flowering genes based on day length
Stress — can alter gene expression patterns
Key Formula for Excellence
Phenotype = Genotype + Environment
Two organisms with identical genotypes can have different phenotypes in different environments
Identical twins have the same DNA but can differ in height, weight, disease risk
This proves that genes are not destiny — environment matters
EXCELLENCE: A common exam question asks you to explain why identical twins are not perfectly identical. The answer: same genotype, but environmental influences on gene expression mean different genes may be switched on or off, leading to different phenotypes. This is epigenetics.
SECTION 14: INHERITANCE PATTERNS
Monohybrid Inheritance
Inheritance of one gene with two alleles
Dominant allele (capital letter, e.g., B) — expressed with one or two copies
Recessive allele (lowercase, e.g., b) — only expressed with two copies
Punnett Square Example: Bb × Bb
BbBBBBbbBbbb
Genotype ratio: 1 BB : 2 Bb : 1 bb
Phenotype ratio: 3 dominant : 1 recessive
Codominance
Both alleles equally expressed in heterozygote
E.g., ABO blood groups — IA and IB codominant → blood group AB
Neither allele is dominant over the other
Sex-Linked Inheritance
Genes carried on the X chromosome
Males (XY) have only one X — a single recessive allele on X will be expressed (no second allele to mask it)
Females (XX) need two copies of recessive allele to show the trait
Males are more commonly affected by X-linked recessive conditions
Examples: colour blindness, haemophilia
SECTION 15: LEAF STRUCTURE & GAS EXCHANGE
Stomata — pores on underside of leaf; CO₂ enters, O₂ and water vapour exit
Guard cells — surround each stoma, control opening (open in light, close in drought)
Palisade mesophyll — tightly packed cells near top of leaf, packed with chloroplasts, main site of photosynthesis
Spongy mesophyll — loosely packed cells with air spaces, allows gas diffusion
Waxy cuticle — waterproof layer on top of leaf, reduces water loss
EXCELLENCE EXAM TIPS
What Separates Achieved from Excellence in NCEA Level 2 Biology
Achieved = correctly identify and describe biological concepts
Merit = explain HOW and WHY processes occur
Excellence = discuss, evaluate, or explain complex relationships and consequences; link multiple concepts together
Excellence Answer Strategies
Always explain the CONSEQUENCE of a change, not just what the change is
Use the full chain: e.g., mutation → changed codon → changed amino acid → changed protein shape → changed/lost function
Link topics together: e.g., how a mutation during meiosis creates variation, which natural selection then acts on
Use precise vocabulary — "denatures" not "breaks"; "complementary base pairing" not "matches up"
For enzyme questions: explain BOTH what happens to the active site AND the consequence for the reaction rate
For gene expression questions: always state WHERE each process occurs (nucleus vs cytoplasm)
Most Commonly Tested Excellence Topics
Full mutation chain — type of mutation → effect on protein → effect on organism
Why the RNA step is essential — need multiple reasons
Linking meiosis mechanisms to genetic variation
Explaining why crossing over and independent assortment increase variation
Phenotype = Genotype + Environment — using specific examples
Comparing aerobic and anaerobic respiration with advantages/disadvantages
Limiting factors in photosynthesis — explaining plateau on graphs
Enzyme inhibition — competitive vs non-competitive, and their different effects
NCEA Level 2 Biology — Complete Excellence Notes | Year 12 NZ | All content verified for accuracy and exam relevance