Supported HL focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: diversity of organisms from A.3.1 and DNA sequences from A.1.2. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
7.2
Classification and cladistics 分类学
English
What would explain this observation?
Birds were once classed far from reptiles, yet their DNA and skull bones place them next to crocodiles. Which evidence should decide the grouping?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
Classification places every species in the nested hierarchy: domain, kingdom, phylum, class, order, family, genus, species, named by the binomial system 双名法 (genus + species, as in Homo sapiens). Cladistics 支序分类 groups organisms by shared derived characters 共有衍征 — features that appeared in a common ancestor — drawn as a cladogram 分支图 whose nodes mark those characters.
Molecular evidence such as DNA sequence differences usually decides between competing trees. Rotating two branches at a node changes nothing: the branching pattern, not the left-right order, records the relationships.
classification: Placing organisms into nested groups by shared features; binomial system: The two-part naming of species by genus and species; shared derived character: A feature inherited from the group's most recent common ancestor; cladogram: A branching diagram of relationships built from shared derived characters.
Choose evidence that can test it
Build the tree from a character matrix first, then ask whether a NEW dataset (for example a DNA sequence) reproduces the same groupings; independent agreement is the strongest support.
A single visible feature can mislead through convergent evolution — distantly related animals evolve similar shapes for similar lives — so one trait alone never defines a clade.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: two aligned DNA sequences differ at 8 of 50 sites. Percentage difference = differences / sites × 100 = 8/50 × 100 = 16%. A third species differing at only 5 of 50 sites (10%) shares a more recent ancestor with the second sequence than the first does.
Example:
Another alignment shows 6 differences in 40 sites. Find the percentage difference using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: 6/40 × 100 = 15%. The percentage is an observed comparison, not a calibrated molecular clock — dating a split needs rate assumptions.)
Check the conclusion and its limits
A cladogram is a hypothesis of relationships, revisable when better evidence arrives. The observed percentage difference ignores multiple changes at the same site, so it underestimates true divergence for distant pairs.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
A modern species at one branch tip is the ancestor of the species at another tip. This claim is false: every tip is a modern descendant; the nodes represent shared ancestors that no longer exist. Birds are descended from archosaur ancestors — they are not descended from today's crocodiles.
Key:
Classification and cladistics: nested groups from domain to species, named binomially; cladograms group by shared derived characters, with molecular differences as decisive evidence. Count differences as a percentage, and read the branching — never the tip order — as the relationship.
Supported HL focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: read the stated quantities and units, use arithmetic and the model conditions below. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
8.2
Adaptation 适应: a function in an environment
English
What would explain this observation?
Leaves in dry environments often have features that reduce water loss. The value of a feature depends on environmental conditions and trade-offs.
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
An adaptation is a heritable feature associated with improved survival or reproduction in particular conditions. A thick cuticle can reduce evaporation; stomatal control affects gas exchange and water loss. Acclimatization 适应过程 is an individual response during life and is different from evolutionary adaptation.
adaptation: A heritable feature benefiting survival or reproduction in a context; acclimatization: An individual adjustment to changed conditions during life.
Choose evidence that can test it
Explain both a benefit and a cost. Reducing stomatal opening conserves water but can limit carbon dioxide entry. A feature cannot be judged as universally best without its ecological context.
Compare replicated observations across a measured environmental gradient. Account for relatedness, leaf area and age. Use local permitted plant observations rather than removing protected species.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: water loss is 12 g from 0.20 square metres of leaf area over 2 h. Area-time-normalized rate = 12/(0.20 × 2) = 30 g/(m² h). Comparisons require the same humidity and temperature.
Example:
Water loss is 8 g from 0.20 m² over 2 h. Calculate rate per area per hour. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Check the conclusion and its limits
An individual organism does not evolve a new inherited trait because it needs it. A plausible story about function is a hypothesis requiring evidence.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
Every useful individual response is evidence of a newly evolved inherited trait. This claim is false: An individual organism does not evolve a new inherited trait because it needs it. A plausible story about function is a hypothesis requiring evidence.
Key:
Adaptation: a function in an environment: Explain both a benefit and a cost. Reducing stomatal opening conserves water but can limit carbon dioxide entry. A feature cannot be judged as universally best without its ecological context.
ข้อมูล已知: สถานที่ A มีจำนวน 8, 1 และ 1; สถานที่ B มี 4, 3 และ 3. ทั้งสองแห่งมี species richness เท่ากับ 3 และ total abundance เท่ากับ 10. Species ที่พบมากที่สุดคิดเป็น 80% ใน A และ 40% ใน B. B มีความสม่ำเสมอ (evenness) สูงกว่าภายใต้จำนวนเหล่านี้
ตัวอย่าง:
การสำรวจบันทึก 6, 5, 3 และ 2 ตัวในสปีชีส์สี่ชนิด จงหา species richness ใช้ลำดับขั้นตอนเดียวกัน: ค่า Known → โมเดล →ความสัมพันธ์ → แทนค่า → หน่วยและการตีความ
Supported HL/SL core focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: amino-acid structure and condensation reactions from B.1.1 (carbohydrates and lipids), and microscopy units from B.1. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
11.2
Proteins 蛋白质
English
What would explain this observation?
An egg white turns from clear runny liquid to firm white solid when heated — and cooling never turns it back. What happened to its proteins?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
Proteins are chains of amino acids joined by peptide bonds 肽键 in condensation reactions; each bond forms with the loss of one water molecule. The chain order is the primary structure 一级结构, which folds into local helices and sheets (secondary), then into a working three-dimensional shape (tertiary), sometimes with several chains together (quaternary).
The twenty R groups differ in charge and polarity, so the sequence decides the fold. Heating or extreme pH breaks the weak bonds holding the fold — denaturation 变性 — while the peptide bonds survive; the protein unravels and loses its function, which is why the egg does not run again on cooling.
peptide bond: The covalent link between two amino acids, formed with the loss of water; primary structure: The sequence of amino acids in a protein chain; denaturation: The loss of a protein's working shape when its weak structure bonds break.
Choose evidence that can test it
The Biuret test identifies peptide bonds: it turns from blue to purple with protein but not with free amino acids. Use it to check whether a sample contains peptide linkages.
Compare an enzyme's activity before and after heating, keeping substrate and pH constant: a lasting loss of activity after cooling supports permanent denaturation, not just a reversible pause.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: a peptide chain contains 35 amino-acid residues joined in one unbranched chain. Each link between two neighbours is one peptide bond, so bonds = residues − 1 = 35 − 1 = 34. Each bond formed by condensation releases one water molecule, so 34 water molecules were released when this chain formed.
Example:
A different chain contains 120 amino-acid residues. Find the number of peptide bonds and the number of water molecules released during its formation, using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: bonds = 120 − 1 = 119; waters released = 119. Hydrolysis back to single amino acids would need 119 water molecules.)
Check the conclusion and its limits
The bond count assumes a single unbranched chain; real proteins may be branched or cross-linked by disulfide bridges, which are not peptide bonds. Denaturation is usually irreversible in food but some small proteins can refold under careful laboratory conditions.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
Heating denatures a protein by breaking its peptide bonds. This claim is false: denaturation breaks the weaker hydrogen bonds and interactions that hold the fold; the peptide bonds of the primary structure survive, which is why the amino-acid sequence is unchanged in a cooked egg.
Key:
Proteins: amino acids join by peptide bonds through condensation, releasing one water per bond (n residues → n−1 bonds). The sequence folds through secondary and tertiary structure into the working shape; denaturation destroys the fold, not the chain.
ซาร์โคเมียร์ (Sarcomere): หน่วยหดตัวซ้ำกันระหว่างเส้น Z ทั้งสอง; ครอสบริดจ์ (Cross-bridge): ปฏิสัมพันธ์ระหว่างไมโอซินและแอคตินในช่วงรอบการหดตัว
เลือกหลักฐานที่นำมาทดสอบได้
ระหว่างการหดตัว ความทับซ้อนจะเพิ่มขึ้น ความยาวของ A-band คงที่ประมาณเดิม ในขณะที่ I bands และ H zone เล็กลง การเคลื่อนไหวที่ข้อต่อขึ้นอยู่กับ tendon insertion, lever geometry และกิจกรรมของกล้ามเนื้อตรงข้ามด้วย
Supported HL/SL core focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: transpiration from B.2.3 onwards and natural selection from D.4.1. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
18.2
Adaptation to environment 适应
English
What would explain this observation?
A cactus survives months without rain while a water lily lives with its leaves flat on the water surface. Both are plants, yet their bodies are nearly opposite. Why?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
An adaptation is an inherited feature that improves survival or reproduction in a particular environment. Adaptations are structural (rolled leaves, sunken stomata in a xerophyte 旱生植物), physiological (concentrated urine in a desert mammal) or behavioural (a lizard basking to warm up).
A useful adaptation explanation links feature → effect → advantage: rolled leaves trap moist air, so less water is lost by transpiration, so the plant survives drought. Surface area to volume ratio 表面积体积比 (SA:V) explains many adaptations: a smaller body has a larger SA:V and loses heat or water faster.
adaptation: An inherited feature improving survival or reproduction in a given environment; xerophyte: A plant adapted to dry conditions; surface area to volume ratio: Surface area divided by volume, controlling exchange rates.
Choose evidence that can test it
Compare water loss from a xerophyte leaf and a mesophyte leaf under the same warm, dry, moving air: the adaptation model predicts less loss from the xerophyte. Keep leaf size, temperature and humidity constant, or the comparison proves nothing.
Measure the SA:V of model cubes or of organisms and compare with their habitats; a claim about surface area must come from measured dimensions, not from looking at the picture.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: cube side s = 3 cm. Surface area = 6s² = 6 × 9 = 54 cm²; volume = s³ = 27 cm³. SA:V = 54/27 = 2 per cm. For a 1 cm cube: SA:V = 6/1 = 6 per cm. Tripling the side cut the ratio to one third — larger bodies have less surface per unit volume, so they lose less heat per unit of tissue.
Example:
A desert mammal model has body surface 400 cm² and volume 2,000 cm³. Find its SA:V and compare it with a small desert mouse at 2 per cm, using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: SA:V = 400/2,000 = 0.2 per cm — one tenth of the mouse's, so the larger animal loses proportionally less water and heat through its surface.)
Check the conclusion and its limits
A plausible story about a feature's use is a hypothesis until evidence supports it; some features are side effects of other traits, not adaptations. The cube model assumes a uniform shape — real organisms extend surface where exchange is needed (lungs, gills, root hairs), breaking the simple scaling.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
A desert plant adapts during one hot summer by growing thicker cuticle because it needs to. This claim is false: adaptations are INHERITED features shaped by selection over generations; an individual's one-season response is acclimatization, not adaptation, and need never directs the change.
Key:
Adaptation to environment: inherited features link to survival through a feature → effect → advantage chain, testable by controlled comparison. SA:V = 6/s for a cube, and it falls as size rises — exchange surfaces and body plans follow from it.
การหายใจแบบใช้ออกซิเจนใช้-floatingออกซิเจนและปล่อยคาร์บอนไดออกไซด์และน้ำจากสารตั้งต้นทางอินทรีย์ พลังงานที่ปล่อยออกมาสามารถสนับสนุนการสร้าง ATP การไฮโดรไลซิสของ ATP สามารถเชื่อมโยงกับกระบวนการต่างๆ เช่น การขนส่งแบบแอคทีฟและการหดตัวของกล้ามเนื้อ
กระบวนการแบบไม่ใช้ออกซิเจนช่วยให้ผลิต ATP ได้เมื่อปริมาณ-floatingออกซิเจนไม่สามารถรองรับอัตราการหายใจแบบใช้ออกซิเจนที่ต้องการได้ แต่จะให้ ATP น้อยกว่าต่อกลูโคส ในมนุษย์แลคเตตอาจสะสมขึ้น; ยีสต์สามารถผลิตเอทานอลและคาร์บอนไดออกไซด์ได้
การหายใจ, ATP และการถ่ายโอนพลังงาน: กระบวนการแบบไม่ใช้ออกซิเจนช่วยให้ผลิต ATP ได้เมื่อปริมาณ-floatingออกซิเจนไม่สามารถรองรับอัตราการหายใจแบบใช้ออกซิเจนที่ต้องการได้ แต่จะให้ ATP น้อยกว่าต่อกลูโคส ในมนุษย์แลคเตตอาจสะสมขึ้น; ยีสต์สามารถผลิตเอทานอลและคาร์บอนไดออกไซด์ได้
Supported HL focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: cells and membranes from B.2 and enzymes from B.1. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
26.2
Defence against disease
English
What would explain this observation?
Most people catch chickenpox once and never again, while a cold can return every year. What does the immune system remember, and why does it fail against the cold?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
Pathogens carry antigens 抗原 — marker molecules the body reads as foreign. Phagocytes engulf and digest pathogens, while lymphocytes make antibodies 抗体, proteins shaped to bind one specific antigen. The first response is slow because the matching lymphocytes must be selected and multiplied.
After the first infection, memory cells 记忆细胞 remain. On re-exposure they produce a faster, stronger secondary response, often destroying the pathogen before symptoms appear. Vaccination 疫苗接种 creates memory cells without the disease. When enough of a population is immune, the pathogen cannot spread — herd immunity 群体免疫.
antigen: A molecule recognised as foreign by the immune system; antibody: A protein that binds one specific antigen; memory cell: A long-lived lymphocyte that enables a fast secondary response; vaccination: Giving antigen material to build memory without disease; herd immunity: Population protection when too few susceptible hosts remain for spread.
Choose evidence that can test it
Track antibody concentration after a first and a second exposure to the same antigen: the model predicts a faster and larger second peak. A cold virus returns because its antigens CHANGE (new strains), so the memory cells no longer match — the exception that tests the rule.
Compare outbreak sizes in populations with different vaccination coverage; above the threshold, outbreaks should fail to grow.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: a pathogen has a basic reproduction number R₀ = 4, so one case infects four others in a fully susceptible population. The herd-immunity threshold is 1 − 1/R₀ = 1 − 1/4 = 0.75. At least 75% of the population must be immune for each case to infect fewer than one person, so the outbreak dies out.
Example:
A different pathogen has R₀ = 2.5. Find the herd-immunity threshold as a percentage using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: 1 − 1/2.5 = 1 − 0.4 = 0.6 = 60%. A less contagious pathogen needs less coverage.)
Check the conclusion and its limits
The threshold model assumes uniform mixing and equal susceptibility; real populations cluster, so actual protection can differ. Vaccine effectiveness is never 100%, so the needed coverage is higher than the arithmetic minimum.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
An antibody can bind equally well to any pathogen. This claim is false: each antibody is specific to the shape of ONE antigen — which is why a new cold strain escapes last year's antibodies, and why each vaccine targets chosen antigens.
Key:
Defence against disease: phagocytes engulf; lymphocytes make antigen-specific antibodies; memory cells give a faster, stronger second response. Vaccination builds memory safely, and herd immunity at 1 − 1/R₀ protects the whole population.
หลังการจำลอง DNA แต่ละเกลียวคู่ลูกจะประกอบด้วยสายเดิมหนึ่งสายและสายที่สังเคราะห์ใหม่หนึ่งสาย การคัดลอกโมเลกุลแตกต่างจากการแสดงออกของข้อมูลในโมเลกุลนั้น
DNA replication เป็นแบบกึ่งอนุรักษ์ (semi-conservative) การจับคู่เบสแบบ complementarity ช่วยคัดลอกลำดับเบส: อะดีนีนจับกับไทมิน และไซโตซีนจับกับ구아ניน สายทั้งสองมี orientation ตรงข้าม Helicase แยกสายคู่ออก ในขณะที่ DNA polymerase สร้าง DNA ใหม่โดยใช้ template และ nucleotides ที่มีอยู่
การจำลองแบบกึ่งอนุรักษ์: การคัดลอก DNA ที่แต่ละเกลียวคู่ลูกคงไว้ซึ่งสาย parental หนึ่งสาย; DNA polymerase: เอ็นไซม์ที่สังเคราะห์ DNA โดยใช้แม่แบบ
เลือกหลักฐานที่นำมาทดสอบได้
DNA ใหม่สังเคราะห์ในทิศทาง 5′ ถึง 3′ ในโมเดลвил์ที่ลดทอนมา สายใหม่สามารถสังเคราะห์ต่อเนื่องได้ mientras สายอีกสายถูกผลิตเป็น Fragment ที่ถูกเชื่อมต่อกัน แยกกลไกโมเลกุลเหล่านี้กับการแยกโครโมโซมในภายหลังของการแบ่งเซลล์ ความผิดพลาดในการคัดลอกอาจเกิดขึ้นแม้จะมีกระบวนการแก้ไขความผิดพลาดและการซ่อมแซม
Supported HL/SL core focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: DNA structure and base pairing from A.1.2, and enzymes from B.1. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
30.2
Protein synthesis 蛋白质合成
English
What would explain this observation?
A gene hundreds of bases long produces a much shorter protein, and a one-base mutation can destroy the whole product. How does the cell read a base sequence into an amino-acid sequence?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
In transcription 转录, the enzyme RNA polymerase builds an mRNA strand complementary to one DNA template strand. In translation 翻译, a ribosome reads the mRNA in groups of three bases — codons 密码子 — and joins the matching amino acids into a polypeptide.
Each codon specifies one amino acid through the genetic code, which is degenerate: several codons can give the same amino acid. A stop codon 终止密码子 ends the chain and codes for nothing. Because the reading frame runs in threes, inserting or deleting one base shifts every later codon — a frameshift.
transcription: Copying a gene's DNA sequence into a complementary mRNA; translation: Reading mRNA codons at a ribosome to build a polypeptide; codon: A group of three mRNA bases specifying one amino acid; stop codon: A codon that ends the chain without coding for an amino acid.
Choose evidence that can test it
The base sequence of a gene predicts the amino-acid sequence of its protein through the code table; compare a known gene and its protein to check. Radioactive-labelling experiments placed amino acids in the order the mRNA specified — evidence for the codon reading, not a guess.
Mutations that change a codon to a stop codon truncate the protein exactly at that point, which can be checked against the sequence.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: a coding mRNA segment has 63 bases, including one stop codon. Codons = bases / 3 = 63 / 3 = 21. The stop codon codes for no amino acid, so the peptide has 21 − 1 = 20 amino acids.
Example:
A coding segment of 90 bases includes one stop codon. Find the peptide length using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: codons = 90/3 = 30; peptide = 30 − 1 = 29 amino acids. Bases are not amino acids — divide by three first.)
Check the conclusion and its limits
This counting model covers the coding sequence only: real genes also carry promoters, untranslated regions and, in eukaryotes, introns that are removed before translation. A gene's total base count is therefore not a peptide-length calculation.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
Each base in the coding sequence codes for one amino acid. This claim is false: amino acids are coded by triplets — three bases per codon — and the stop codon adds none, so a 63-base segment gives 20 amino acids, not 63.
Key:
Protein synthesis: transcription copies the gene into mRNA; translation reads codons in threes at the ribosome. Codons = bases ÷ 3, and the stop codon codes for no amino acid; a one-base insertion or deletion shifts the whole reading frame.
ที่ทราบ: ลำดับรหัส AUG CCU GGA UAA อ่านเป็นโคดอนสามตัว ลบฐานที่สี่ (C ตัวแรก) ลำดับจะกลายเป็น AUG CCG GAU AA โคดอนแรกยังคงสมบูรณ์ แต่โคดอนที่เหลือทั้งหมดถูกเลื่อนกรอบอ่าน และโคดอนหยุดถูกทำลาย — ribosome จะอ่านต่อเข้าสู่โคดอนที่ผิด การแทนที่ C ด้วย A แทนจะได้ AUG CAU GGA UAA: มีเพียงโคдонที่สองที่เปลี่ยนเท่านั้น
ตัวอย่าง:
ลำดับรหัสจำนวน 30 base สัมผัสกับการสูญเสียหนึ่ง base ใกล้จุดเริ่มต้น จำนวนโคดอนใดบ้างที่จะถูกอ่านผิดพลาด โดยใช้ลำดับเดียวกัน: ปริมาณที่ทราบ → โมเดล → ความสัมพันธ์ → การแทนค่า → หน่วยและตีความ? (คำตอบ: 30/3 = 10 โคดอน; การสูญเสียหนึ่ง base Near จุดเริ่มต้นจะเลื่อนทุก 10 ในขณะที่การแทนที่อาจเปลี่ยนได้สูงสุดเพียงหนึ่ง)
การจำลอง DNA เพิ่มจำนวนโครโมโซมทันทีภายใต้ข้อยกเว้น centromere ข้ออ้างนี้ผิด: การจำลอง DNA เพิ่มปริมาณ DNA โดยไม่เพิ่มจำนวนโครโมโซมทันที Immediately Meiosis ไม่ได้สร้างเซลล์ไดพลอยด์ identical สองเซลล์เพียงอย่างเดียว
จุดสำคัญ:
การแบ่งเซลล์: นับนิวเคลียสและโครโมโซมอย่างละเอียด: นับโครโมโซดตามจำนวน centromere ภายใต้ข้อยกเว้นที่กำหนด ปริมาณ DNA และจำนวนโครโมโซดเป็นปริมาณที่แตกต่างกัน Crossing over และการแยกกลุ่มอิสระมีส่วนสร้างความหลากหลายใน meiosis
แยกแยะโครโมโซมโฮโมโลกัสจากโครเมทิดพี่น้อง และจำนวนโครโมโซมออกจากปริมาณ DNA การจำลองแบบก่อนไมโอซิสคัดลอก DNA โดยไม่เพิ่มจำนวนชุดโครโมโซม การข้ามสาย (crossing over) และการจัดเรียงอิสระ (independent assortment) ช่วยสร้างความหลากหลายทางพันธุกรรม ในขณะที่การปฏิสนธิแบบสุ่มเปลี่ยนแปลงองค์ประกอบเพิ่มเติม กระบวนการเหล่านี้ไม่ได้ทำให้ลูกหลานแต่ละตัวมีความแตกต่างทางพันธุกรรมในทุกสถานการณ์ที่เป็นไปได้
ใช้สิ่งมีชีวิตต้นแบบที่มีป้ายกำกับและระบุจำนวนโครโมโซมที่ชัดเจน ติดตามชุดโครโมโซมผ่านการจำลองแบบ ไมโอซิส I ไมโอซิส II และการปฏิสนธิ ใช้ข้อมูลสิ่งมีชีวิตต้นแบบที่ไม่ระบุตัวตนแทนข้อมูลครอบครัวหรือสุขภาพการเจริญพันธุ์ส่วนบุคคล เปรียบเทียบรอบชีวิตของสิ่งมีชีวิตอย่างระมัดระวัง โครงสร้างการสืบพันธุ์ของพืชมีดอกและสัตว์จำเป็นต้องมีการสอนเพิ่มเติมเฉพาะด้าน
ในการผสมข้ามแบบโมโนไฮบริดอย่างง่าย Aa × Aa เซลล์สืบพันธุ์จะนำ A หรือ a มา เมื่อรวมเซลล์สืบพันธุ์ที่เป็นอิสระกันจะได้ AA, Aa, Aa และ aa ความน่าจะเป็นที่คาดการณ์ไว้อธิบายความเป็นไปได้ในการปฏิสนธิจำนวนมาก ไม่ใช่ลำดับตายตัวของลูก
การถ่ายทอดลักษณะ, ความหลากหลาย และความน่าจะเป็น: ในการผสมข้ามแบบโมโนไฮบริดอย่างง่าย Aa × Aa เซลล์สืบพันธุ์จะนำ A หรือ a มา เมื่อรวมเซลล์สืบพันธุ์ที่เป็นอิสระกันจะได้ AA, Aa, Aa และ aa ความน่าจะเป็นที่คาดการณ์ไว้อธิบายความเป็นไปได้ในการปฏิสนธิจำนวนมาก ไม่ใช่ลำดับตายตัวของลูก
Supported HL/SL core focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: hormones and cell signalling from D.3.1–D.3.2, and percentages from mathematics. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
37.2
Homeostasis 稳态
English
What would explain this observation?
After a large meal your blood glucose jumps, yet within an hour it returns to near where it started — and after a night without food it still does not collapse. What keeps it steady?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
Homeostasis is the maintenance of a nearly constant internal environment around a set point. It runs on negative feedback 负反馈: a detector notices the change, a control centre compares it with the set point, and an effector produces a response that OPPOSES the change.
For blood glucose: when it rises, the pancreas releases insulin 胰岛素, which makes liver and muscle cells take up glucose and store it as glycogen, so glucose falls. When glucose falls too low, glucagon 胰高血糖素 triggers the liver to convert glycogen back to glucose, so it rises. Each hormone switches off as the level returns to the set point.
homeostasis: Maintaining a nearly constant internal environment around a set point; negative feedback: A control loop whose response opposes the original change; insulin: The hormone that lowers blood glucose by promoting uptake and storage; glucagon: The hormone that raises blood glucose by releasing stored glucose.
Choose evidence that can test it
Measure blood glucose at intervals after a meal: the model predicts a rise, a peak, and a return towards the set point as insulin acts — and the same record shows insulin rising before glucose falls, the cause arriving before the correction.
Compare a person whose pancreas cannot make insulin: glucose stays high after a meal, direct evidence that insulin is the link between the detector and the response.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: a person's blood glucose rises from a set point of 5.0 to 7.0 mmol/L after a meal. Change = final − initial = 7.0 − 5.0 = 2.0 mmol/L. Percentage deviation = 2.0/5.0 × 100 = 40% above the set point. Negative feedback acts until the value returns close to 5.0 mmol/L.
Example:
During a fast, blood glucose falls from 5.0 to 4.2 mmol/L. Find the percentage deviation from the set point and name the responding hormone, using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: change = 4.2 − 5.0 = −0.8 mmol/L; deviation = −0.8/5.0 × 100 = −16%; glucagon responds to raise it.)
Check the conclusion and its limits
Homeostasis is NEAR-constant, not fixed: the value oscillates around the set point because each response takes time. This model covers blood glucose; temperature, water and carbon dioxide each have their own detectors and effectors with the same logic.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
Negative feedback makes the measured value negative. This claim is false: "negative" means the response OPPOSES the change — a rise triggers a fall and a fall triggers a rise. The measured value stays positive and close to the set point.
Key:
Homeostasis: detector → control centre → effector loops keep internal conditions near a set point by negative feedback. Insulin lowers raised blood glucose; glucagon raises lowered blood glucose; the loop switches off as balance returns.
Supported HL/SL core focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: variation and inheritance from D.1 and evolution evidence from A.4.1. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
38.2
Natural selection 自然选择
English
What would explain this observation?
After several dry summers, most plants in a population have deeper roots than their grandparents' generation did. Nobody chose this — how did the population change?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
Natural selection needs three ingredients: inherited variation 变异 exists in the population; a selection pressure 选择压力 makes some variants survive and reproduce better than others; and the successful variants' alleles become commoner in the next generation. Over generations the population's traits shift.
Selection does not create mutations and no individual evolves: random mutation supplies the variation BEFORE the pressure acts, and it is the population's allele frequency 等位基因频率 that changes, not any one organism.
natural selection: The process by which heritable variants that fit the environment leave more offspring, changing allele frequencies over generations; variation: Inherited differences among individuals of a population; selection pressure: An environmental factor that affects survival or reproduction; allele frequency: The proportion of one allele among all copies of that gene in a population.
Choose evidence that can test it
The model predicts the direction of change: after repeated droughts, measure the deep-root allele's frequency in stored seed collections or dated samples — it should rise. If it does not, the explanation fails.
A fair test compares the SAME population across time or matched populations across a boundary, so that migration or sampling differences cannot fake the shift.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: before the droughts, 200 gene copies include 90 copies of the deep-root allele; after several generations, 120 of 200. Allele frequency = copies of the allele / total copies. Before: 90/200 = 45%. After: 120/200 = 60%. The frequency rose by 15 percentage points — evidence of selection acting on the population.
Example:
In a new generation, 140 of 200 copies carry the deep-root allele. Find the frequency and the change from the previous generation using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: 140/200 = 70%; change = 70% − 60% = +10 percentage points. Frequencies of two alleles at one gene sum to 100%.)
Check the conclusion and its limits
A frequency shift shows association between the trait and survival; proving the mechanism needs the variation to be heritable and the survival difference measured. Genetic drift in small populations can shift frequencies with no selection at all.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
An organism develops a useful adaptation during its lifetime because it needs one. This claim is false: mutation is random with respect to need — the deep-root alleles existed before the drought. Selection only changes which variants leave more offspring; individuals do not evolve.
Key:
Natural selection: inherited variation + a selection pressure + differential reproduction shifts allele frequencies over generations. Track the shift with allele counts divided by total copies, and remember that the population evolves — never the individual.
Supported HL/SL core focus. First assessment 2025; current subject brief acquired; full Biology guide not acquired. Remaining guide, assessment and practical requirements retain their recorded holds.
Prerequisites: populations and ecosystems from D.4.1 and energy flow from earlier ecology. Each lesson develops its own method before independent transfer.
These are original or explicitly fictional teaching examples, not actual measurements or completed assessed learner investigations.
39.2
Sustainability and change 可持续性
English
What would explain this observation?
A fishery that harvests the same tonnage every year can thrive for decades, while a neighbouring one collapses in five. Both fish the same sea — what separates them?
Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
Populations cannot grow without limit. The logistic model 逻辑斯蒂模型 shows growth slowing as resources run short, levelling at the environment's carrying capacity 环境容纳量 — the largest population the habitat can support. Food, space, disease and predation supply the limits.
A renewable resource 可再生资源 is sustainable only when harvest stays at or below regrowth. Change is constant in ecosystems: climate shifts, species arrive, and human activity alters habitats, so management must track stocks and adjust.
logistic model: Growth that levels at the carrying capacity as resources become limiting; carrying capacity: The largest population an environment can support; renewable resource: A resource that regenerates, such as a fish stock or a forest.
Choose evidence that can test it
Compare the stock before and after each harvest season: a sustainable fishery holds the stock near its level, while decline year on year means harvest exceeds regrowth. Distinguish a falling stock from a falling CATCH, which can hide the decline until late.
Track the same population over several years rather than once; a single count cannot show a trend.
Work from known quantities
State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
Known: a fish stock is 1,000 tonnes and regrows 100 tonnes per year. A harvest of 150 tonnes per year gives net change = regrowth − harvest = 100 − 150 = −50 tonnes per year. After two years the stock is 1,000 − 2 × 50 = 900 tonnes — a sustainable harvest at this regrowth is at most 100 tonnes per year.
Example:
After better management the regrowth is 120 tonnes per year and the harvest quota is set at 80 tonnes per year. Find the yearly change in the stock and the stock after three years from 900 tonnes, using the same sequence: known quantities → model → relation → substitution → unit and interpretation. (Answer: net change = 120 − 80 = +40 tonnes per year; after three years, 900 + 3 × 40 = 1,020 tonnes — the stock recovers.)
Check the conclusion and its limits
The balance model assumes a constant regrowth rate; in reality regrowth depends on the stock size itself, collapsing fastest when the stock is already small. One year's figures cannot establish a trend — management needs the whole time series and honest catch reporting.
Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Warn:
A renewable resource cannot be used up. This claim is false: renewable means it CAN regenerate, not that it always outpaces use — a stock harvested faster than its regrowth shrinks every year and can collapse entirely, as overfished cod stocks did.
Key:
Sustainability and change: populations level at the carrying capacity, and renewable resources stay only if harvest ≤ regrowth. Track stocks over time and set harvests from measured regrowth, because ecosystems never stand still.
One local weather observation establishes a global climate trend. This claim is false: The greenhouse effect is not the same process as ozone depletion. A carbon footprint estimate depends on its system boundary.