Author: Admin

D3.1.2 HUMAN REPRODUCTIVE SYSTEMS

📌Definition Table

Term	Definition
Testes	Male gonads producing sperm and testosterone.
Ovaries	Female gonads producing eggs, estrogen, and progesterone.
Uterus	Muscular organ where embryo implants and develops.
Menstrual cycle	Regular hormonal cycle preparing the uterus for pregnancy.
Gametogenesis	Formation of gametes (spermatogenesis in males, oogenesis in females).

📌Introduction

The human reproductive system enables gamete production, fertilisation, and development of offspring. It consists of specialised structures in males and females, regulated by hormones to ensure timing and coordination. Understanding these systems is vital for medicine, fertility management, and reproductive health

📌 Male Reproductive System

Testes produce sperm in seminiferous tubules and hormones in interstitial cells.
Epididymis stores and matures sperm.
Vas deferens transports sperm during ejaculation.
Prostate gland and seminal vesicles produce fluids for semen, nourishing sperm.
Penis delivers sperm into the female reproductive tract.

🧠 Examiner Tip: Always specify the role of accessory glands (prostate, seminal vesicles) when describing semen composition.

📌 Female Reproductive System

Ovaries produce eggs and hormones (estrogen, progesterone).
Fallopian tubes transport eggs and are the site of fertilisation.
Uterus supports embryo implantation and development.
Endometrium thickens in preparation for implantation, sheds during menstruation.
Vagina receives sperm and serves as the birth canal.

🧬 IA Tips & Guidance: Possible IA – effect of hormones or contraceptives on menstrual cycle modeling, linking human physiology with statistical data.

📌 Hormonal Regulation of Reproduction

In males: FSH stimulates sperm production; LH stimulates testosterone release.
In females: FSH and LH regulate follicle growth and ovulation.
Estrogen builds the endometrium; progesterone maintains it.
Hormone feedback loops regulate menstrual cycles.
Contraceptives exploit hormone manipulation to prevent ovulation.

🌐 EE Focus: An EE could investigate the effect of endocrine disruptors on reproductive health or compare gametogenesis processes across species.

📌 Reproductive Health and Technology

Infertility treatments: IVF, artificial insemination, hormonal therapies.
Contraceptives: hormonal pills, barriers, IUDs, surgical sterilisation.
Diseases: STIs (HIV, syphilis, chlamydia) affect reproductive systems.
Hormone imbalances cause infertility or menstrual irregularities.
Modern technology advances reproductive choice and health.

❤️ CAS Link: Students could design awareness workshops on reproductive health, contraception, or STI prevention, linking science with public health.

🌍 Real-World Connection: Knowledge of reproductive systems underpins medicine, public health, and biotechnology. Treatments for infertility, contraception methods, and education programs rely on physiology. Reproductive technology also raises ethical debates about accessibility, choice, and societal impacts.

📌 Integration with Development and Health

Reproductive health links to population growth and demographics.
Technology provides solutions for infertility but raises ethical challenges.
Comparative anatomy reveals evolutionary adaptations in reproduction.
Hormonal cycles illustrate system-wide physiological regulation.
Understanding reproduction informs medicine and society.

🔍 TOK Perspective: Discussions of reproduction involve both scientific facts and cultural values. TOK reflection: To what extent does culture shape how scientific knowledge about reproduction is taught, accepted, or applied?

📝 Paper 2: Be ready to label male and female reproductive systems, describe hormonal control of gametogenesis, and explain fertility treatments. Data analysis may involve hormone levels across menstrual cycles.

September 26, 2025

D3.1.1 ASEXUAL AND SEXUAL REPRODUCTION

📌Definition Table

Term	Definition
Asexual reproduction	Production of offspring from a single parent without gametes, producing clones.
Sexual reproduction	Production of offspring by fusion of male and female gametes, leading to genetic variation.
Gamete	Haploid reproductive cell (sperm or egg) involved in sexual reproduction.
Fertilisation	Fusion of male and female gametes to form a diploid zygote.
Genetic variation	Differences in DNA sequences among individuals, arising from meiosis, recombination, and fertilisation.

📌Introduction

Reproduction is a fundamental process ensuring continuity of life. Organisms reproduce either asexually, producing genetically identical offspring, or sexually, producing genetically varied offspring. Both strategies have evolutionary advantages: asexual reproduction ensures rapid population increase in stable environments, while sexual reproduction enhances genetic diversity, promoting adaptation

📌 Comparison of Asexual and Sexual Reproduction

Asexual reproduction is rapid, requires no mate, and maintains successful genotypes.
Methods include binary fission (bacteria), budding (yeast, hydra), and vegetative propagation (plants).
Sexual reproduction requires specialized gametes and fertilisation but generates variation.
Meiosis introduces genetic diversity through crossing over and independent assortment.
Sexual reproduction increases resilience to environmental change and disease.

🧠 Examiner Tip: Always contrast the advantages and disadvantages of both reproductive strategies, especially in terms of speed vs variation.

📌 Examples in Nature

Bacteria reproduce asexually via binary fission.
Fungi and algae can switch between asexual and sexual modes depending on conditions.
Plants reproduce asexually via runners, bulbs, or tubers, and sexually via flowers and seeds.
Many animals, such as starfish, can regenerate lost parts (asexual) but also reproduce sexually.
Alternation between asexual and sexual phases increases survival chances.

🧬 IA Tips & Guidance: An IA could involve investigating propagation in plants (cuttings vs seeds), linking experimental design to differences in reproduction modes.

📌 Significance of Sexual Reproduction

Generates diversity for natural selection to act upon.
Provides adaptability against environmental stress, parasites, or pathogens.
Costs include energy investment in finding mates and slower population growth.
Asexual reproduction dominates in stable environments, while sexual reproduction prevails in changing ones.
Evolutionary theory (Red Queen hypothesis) highlights the advantage of sexual variation in resisting parasites.

🌐 EE Focus: An EE could explore alternation of generations in plants or the evolutionary significance of sexual vs asexual reproduction in specific species.

📌 Role of Reproduction in Evolutionary Adaptation

Mutations plus recombination generate new allele combinations.
Asexual species rely mainly on mutation for change.
Sexual reproduction accelerates evolutionary responses.
Hybridisation between species contributes to biodiversity.
Genetic variation underpins speciation and adaptation.

❤️ CAS Link: Students could create awareness campaigns on cloning in plants or artificial reproductive technologies, linking classroom concepts to food security and ethics.

🌍 Real-World Connection: Reproductive strategies are central to agriculture, conservation, and medicine. Asexual propagation enables cloning of desirable crops, while sexual reproduction underpins selective breeding. In medicine, assisted reproductive technologies like IVF mimic sexual reproduction, offering solutions for infertility.

📌 Reproductive Strategies in Humans and Other Species

Humans rely exclusively on sexual reproduction through gamete fusion.
Some organisms use parthenogenesis (development from unfertilised eggs).
Many species can switch modes depending on environmental stress.
Comparison reveals how reproduction adapts to survival needs.
Human use of reproductive technology reflects natural principles.

🔍 TOK Perspective: Our categorisation of reproduction as “asexual” or “sexual” simplifies complex realities. TOK reflection: To what extent do human classifications of natural processes shape what we consider knowledge in biology?

📝 Paper 2: Be prepared to compare asexual and sexual reproduction with examples, describe their advantages and disadvantages, and explain their evolutionary significance.

September 26, 2025

D2.3.3 WATER MOVEMENT IN PLANTS

📌Definition Table

Term	Definition
Apoplast pathway	Movement of water through cell walls and intercellular spaces.
Symplast pathway	Movement of water through cytoplasm via plasmodesmata.
Transmembrane pathway	Movement of water across cell membranes repeatedly via aquaporins.
Cohesion-tension theory	Explains upward water transport in xylem via cohesion and transpiration pull.
Transpiration	Loss of water vapour from leaves through stomata.
Root pressure	Osmotic pressure in roots that pushes water upward in xylem.

📌Introduction

Water transport in plants occurs from roots to leaves through integrated pathways, driven by water potential gradients. Root absorption, xylem transport, and transpiration are coordinated by physical and biological processes. Cohesion, adhesion, and tension explain how water columns remain continuous despite being pulled upwards against gravity

📌 Pathways of Water Movement

Apoplast pathway: fast movement through cell walls until Casparian strip blocks entry.
Symplast pathway: water flows cell-to-cell via plasmodesmata.
Transmembrane pathway: aquaporins regulate selective water movement.
Casparian strip ensures all water entering xylem crosses a plasma membrane.
Multiple pathways allow redundancy and regulation.

🧠 Examiner Tip: Always mention the Casparian strip as a checkpoint — this ensures selective uptake of ions and prevents harmful substances from freely entering the xylem.

📌 Driving Forces of Transport

Root pressure: minor contributor, generated by osmotic uptake in roots.
Cohesion-tension theory: transpiration pull creates negative pressure in xylem.
Cohesion between water molecules maintains continuous column.
Adhesion to xylem walls prevents collapse under tension.
Negative pressure gradient drives water upwards passively.

🧬 IA Tips & Guidance: A simple IA could measure transpiration using a potometer under varying light, humidity, or wind conditions to show environmental effects on water movement.

📌 Transpiration Stream

Evaporation from mesophyll cells creates negative pressure.
Pulls water from xylem into leaf tissue.
Supplies minerals dissolved in water.
Helps cool plant by evaporative cooling.
Rate regulated by stomatal opening and cuticle thickness.

🌐 EE Focus: An EE could compare transpiration rates in sun vs shade plants, linking water transport efficiency to ecological adaptations.

📌 Regulation and Adaptations

Stomata regulate water loss vs CO₂ uptake.
Xerophytes: thick cuticles, sunken stomata, CAM photosynthesis reduce water loss.
Hydrophytes: large air spaces aid gas exchange and buoyancy.
Mesophytes: balanced features for moderate environments.
Shows diversity of structural/physiological adaptations.

❤️ CAS Link: Students could build potometers for school/community gardens to demonstrate practical plant physiology.

🌍 Real-World Connection: Understanding water transport is critical for agriculture under climate change. Crop breeding now focuses on water-use efficiency to maintain yields in drought.

📌 Xylem Structure and Function

Lignified walls provide support under negative pressure.
Vessel elements and tracheids adapted for efficient water transport.
Pits allow lateral movement between xylem elements.
Dead cells reduce resistance by removing organelles.
Structure-function correlation illustrates plant engineering for water movement.

🔍 TOK Perspective: The cohesion-tension theory is widely accepted, but direct observation is impossible. TOK issue: To what extent should models be trusted when they cannot be directly proven, only inferred?

📝 Paper 2: Questions may ask to compare apoplast/symplast pathways, interpret potometer data, or explain cohesion-tension using diagrams.

September 26, 2025

D2.3.2 PRESSURE POTENTIAL AND CELL TURGOR

📌Definition Table

Term	Definition
Pressure potential (Ψp)	Hydrostatic pressure exerted by cell contents against cell wall.
Turgor pressure	Internal pressure that keeps plant cells rigid and tissues upright.
Incipient plasmolysis	Point where Ψp = 0 and plasma membrane detaches from cell wall.
Wall pressure	Force exerted by cell wall back on the protoplast, balancing turgor.
Flaccid cell	Cell with no net pressure, soft and limp.
Wilting	Loss of turgor in plant tissues due to water deficit.

📌Introduction

Pressure potential is the second key component of water potential, arising from the physical pressure inside plant cells. Together with solute potential, it determines the direction of water flow. Turgor pressure is vital for maintaining cell rigidity, supporting plant structure, and driving growth through cell expansion

📌 Role of Pressure Potential (Ψp)

Positive pressure builds up as water enters a cell in hypotonic solution.
Provides counterforce to Ψs, preventing excessive water entry.
Keeps plant cells turgid → structural support without skeleton.
Enables growth by cell expansion, as pressure stretches the wall.
Loss of Ψp leads to wilting, reduced photosynthesis, and metabolic slowdown.

🧠 Examiner Tip: Many students forget Ψp can be positive or zero, but never negative in living cells. Ψs is always negative.

📌 Turgor Pressure in Plant Physiology

Guard cells: turgor changes open/close stomata to regulate gas exchange.
Leaf movements: turgor-driven changes in pulvini cells control nyctinasty (sleep movements).
Growth zones: elongation requires local wall loosening under turgor force.
Provides resistance to wilting in drought-tolerant plants.
Turgor collapse is reversible if water is restored before permanent plasmolysis.

🧬 IA Tips & Guidance: Students can use a pressure probe to measure Ψp in plant cells or compare wilting rates under different humidity/light conditions.

📌 Interaction of Ψs and Ψp

Water potential equation Ψ = Ψs + Ψp explains water status.
At full turgor: Ψp balances Ψs, preventing further uptake.
At incipient plasmolysis: Ψp = 0, water potential determined solely by Ψs.
Explains osmotic adjustment in drought-stressed plants.
Allows quantification of water relations in tissues.

🌐 EE Focus: An EE could analyse adaptations in desert plants that maintain Ψp, such as osmotic adjustment by accumulating solutes.

📌 Agricultural and Ecological Relevance

Irrigation practices must prevent Ψp collapse through soil water deficits.
Wilting reduces crop yield; recovery depends on restoring Ψp quickly.
High turgor required for fruit enlargement and storage root swelling.
Stomatal regulation under water stress balances Ψp with photosynthetic needs.
Ecological distribution of species often linked to ability to maintain Ψp.

❤️ CAS Link: Students could create an awareness project on watering schedules and plant wilting for school/community gardens.

🌍 Real-World Connection: Pressure potential is crucial in post-harvest biology; loss of turgor explains wilting of vegetables and cut flowers. Solutions in floriculture aim to restore Ψp to prolong freshness.

📌 Cell Wall–Turgor Interplay

Cell wall provides resistance to overexpansion.
Dynamic remodeling allows controlled growth.
Mutants with weak walls show irregular swelling under turgor.
Wall–turgor interactions key in tropic responses.
Balance ensures structural integrity of plants.

🔍 TOK Perspective: The concept of Ψp is measured indirectly; TOK issue: To what extent do models of “invisible forces” like turgor pressure help us understand plant physiology, despite not being directly observable?

📝 Paper 2: Could involve data on wilting recovery, pressure probe readings, or interpreting graphs of Ψs vs Ψp relationships.

September 26, 2025

D2.3.1 OSMOSIS AND SOLUTE POTENTIAL

📌Definition Table

Term	Definition
Osmosis	Passive movement of water molecules across a selectively permeable membrane from higher water potential to lower water potential.
Water potential (Ψ)	Measure of the potential energy of water per unit volume; determines direction of water movement.
Solute potential (Ψs)	Effect of solute concentration on water potential; always negative relative to pure water.
Hypertonic	Solution with lower water potential than the cell, causing water to leave and cell to shrink.
Hypotonic	Solution with higher water potential than the cell, causing water to enter and cell to swell.
Isotonic	Solution with equal water potential, no net water movement.

📌Introduction

Osmosis is fundamental to maintaining plant cell homeostasis and driving water uptake from soil. The water potential equation Ψ = Ψs + Ψp explains how solute potential and pressure potential together regulate osmosis. Solute potential decreases water potential, while pressure potential counteracts it by exerting force on the cell wall. Understanding osmosis underpins all water transport processes in plants, from root absorption to transpiration streams in leaves

📌 Principles of Osmosis

Water moves down its potential gradient — from regions of higher Ψ to lower Ψ.
Solute concentration decreases Ψ, making solutions more negative.
Plant cell membranes are selectively permeable, allowing water but not large solutes.
Cells in hypertonic solutions lose water and plasmolyse; in hypotonic, they gain water and become turgid.
Osmosis is critical for nutrient uptake and maintaining cell shape.

🧠 Examiner Tip: Don’t confuse osmosis with diffusion. Diffusion involves solute particles, whereas osmosis is strictly about water movement across membranes.

📌 Solute Potential (Ψs)

Calculated using: Ψs = -iCRT (ionisation constant × concentration × gas constant × temperature).
Always negative since solutes lower free water concentration.
Higher solute concentrations → more negative Ψs → stronger pull for water.
Explains why fertiliser salts or seawater can dehydrate plants.
Experimental measurement involves observing plasmolysis in tissues (e.g., onion cells).

🧬 IA Tips & Guidance: Design an experiment using plant tissue (potato, onion) in different sucrose solutions to measure Ψs. Graph percentage mass change vs. solute concentration to estimate water potential.

📌 Cellular Responses to Osmosis

Plasmolysis: cytoplasm shrinks away from cell wall in hypertonic solution.
Turgidity: cell swells in hypotonic solution but wall prevents bursting.
Incipient plasmolysis: point at which plasma membrane just detaches from wall; used to estimate Ψs.
Osmosis regulates guard cell turgor → stomatal opening/closing.
Critical in seed germination, where water uptake reactivates metabolism.

🌐 EE Focus: An EE could explore how osmotic stress (e.g., salinity) influences Ψs and plant survival, connecting to ecological distribution of species.

📌 Applications of Osmosis

Agricultural irrigation must balance soil salinity to prevent water loss from roots.
Food preservation uses hypertonic solutions (salt, sugar) to inhibit microbes.
Osmotic gradients exploited in desalination and biotechnological water purification.
Medical relevance: IV fluids must be isotonic to avoid damaging blood cells.
Environmental stress tolerance in plants often linked to osmotic adjustments.

❤️ CAS Link: Students could demonstrate osmosis with simple potato experiments in school/community, showing real-life effects of solute concentration on water uptake.

🌍 Real-World Connection: Drought and soil salinity crises highlight the importance of understanding osmotic regulation in crops, linking plant physiology to food security.

📌 Integration into Plant Systems

Osmosis drives initial root water uptake before bulk flow takes over.
Interacts with pressure potential in cells to generate turgor.
Works with transpiration pull to maintain continuous water column.
Explains wilt recovery when water is restored.
Foundation for understanding water transport models.

🔍 TOK Perspective: Models of osmosis simplify membranes as perfect barriers, but in reality aquaporins and active transporters complicate water movement. TOK issue: How reliable are simplified models in representing complex systems?

📝 Paper 2: Expect data-based questions on mass change in tissues, plasmolysis diagrams, or calculating Ψs using sucrose concentrations.

September 26, 2025

D2.2.3 ENVIRONMENTAL INFLUENCES ON GENE EXPRESSION

📌Definition Table

Term	Definition
Phenotypic plasticity	The ability of an organism to change phenotype in response to environment.
Inducible genes	Genes expressed only under specific environmental conditions.
Heat shock proteins	Stress-induced proteins that protect cells from damage.
Hormonal regulation	Control of gene expression via external or internal chemical signals.
Signal transduction	Process of converting environmental signals into cellular responses.
Epigenetic-environmental link	Environmental cues that alter gene expression via epigenetic changes.

📌Introduction

Environmental conditions profoundly influence gene expression, shaping phenotypes without altering DNA sequence. Organisms respond to temperature, nutrition, toxins, stress, and social cues by switching genes on or off. These responses enable survival in fluctuating environments but also link lifestyle to disease risk

📌 Nutritional Influences

Nutrient availability regulates metabolic genes.
Low glucose triggers expression of alternative energy pathway genes.
Vitamins act as cofactors influencing gene regulation.
Maternal nutrition during pregnancy affects offspring gene expression epigenetically.
Malnutrition alters immune system gene activity.

🧠 Examiner Tip: Always link environmental influence to specific gene-level mechanisms (e.g., methylation, transcription factors), not just “environment changes phenotype.”

📌 Temperature and Stress

Heat shock proteins induced at high temperatures protect proteins.
Cold stress activates antifreeze protein genes in some fish.
Drought stress upregulates water-conservation genes in plants.
Stress hormones (cortisol, adrenaline) regulate gene expression in target tissues.
Chronic stress alters immune and brain gene expression, linked to disease.

🧬 IA Tips & Guidance: Students could study plant gene expression indirectly by testing germination or growth under different stress conditions (e.g., light vs dark, drought vs normal).

📌 Toxins and Pollutants

Tobacco smoke induces mutations and alters methylation.
Heavy metals trigger detoxification gene expression.
Air pollutants alter immune and respiratory genes.
Endocrine disruptors mimic hormones, misregulating gene expression.
Toxins often leave epigenetic “fingerprints.”

🌐 EE Focus: An EE could analyse how pollutants influence epigenetic marks, contributing to diseases like cancer.

📌 Social and Behavioral Influences

Social stress in animals alters brain gene expression.
In humans, childhood trauma influences long-term epigenetic patterns.
Physical activity induces expression of metabolic and mitochondrial genes.
Learning and memory rely on activity-dependent gene regulation.
Social interactions can therefore leave molecular signatures.

❤️ CAS Link: A CAS project could involve raising awareness about how lifestyle (diet, exercise, stress) affects gene expression and long-term health.

🌍 Real-World Connection: Epigenetic effects of famine (e.g., Dutch Hunger Winter) show how environment shapes health across generations. Personalized medicine now considers gene–environment interactions.

📌 Long-Term Adaptations

Environmentally induced changes can persist across generations via epigenetic inheritance.
Provides a mechanism for rapid adaptation to changing environments.
Explains phenotypic plasticity in plants and animals.
Supports evolutionary theory beyond pure DNA sequence variation.
Human populations show adaptations to diet (e.g., lactose tolerance).

🔍 TOK Perspective: Environmental impacts challenge gene determinism. TOK issue: Does focusing on “genes as destiny” oversimplify the interactive reality of gene–environment dynamics?

📝 Paper 2: Questions may involve explaining gene–environment interactions, interpreting case studies of nutrition/stress effects, or analysing data on inducible genes.

September 26, 2025

D2.2.2 EPIGENETIC REGULATION

📌Definition Table

Term	Definition
Epigenetics	Heritable changes in gene expression not caused by changes in DNA sequence.
DNA methylation	Addition of methyl groups to cytosine bases, usually silencing genes.
Histone modification	Chemical changes to histone proteins that alter chromatin structure and gene expression.
Chromatin	Complex of DNA and proteins that packages genetic material.
Euchromatin	Loosely packed, transcriptionally active DNA.
Heterochromatin	Densely packed, transcriptionally inactive DNA.

📌Introduction

Epigenetics provides an extra layer of gene regulation beyond DNA sequence. Chemical modifications to DNA and histones affect chromatin accessibility, determining whether genes are expressed or silenced. These changes can be inherited but are also reversible, making them critical in development, cell differentiation, and disease

📌 DNA Methylation

Methyl groups added to cytosine bases (CpG islands).
Methylation condenses chromatin, blocking transcription factor binding.
Common in silencing repetitive DNA and transposable elements.
Abnormal methylation patterns associated with cancer (silencing tumour suppressor genes).
Methylation is maintained through cell division.

🧠 Examiner Tip: Epigenetics ≠ mutation. It’s about chemical modifications that affect expression without altering the base sequence.

📌 Histone Modification

Histone tails undergo acetylation, methylation, phosphorylation, ubiquitination.
Acetylation → loosens chromatin, increases transcription.
Deacetylation → tightens chromatin, reduces transcription.
Combinations of modifications form a “histone code.”
Enzymes (HATs, HDACs) regulate these modifications.

🧬 IA Tips & Guidance: Research projects can analyse how environmental stress (e.g., salinity in plants) affects histone acetylation patterns using published datasets.

📌 Chromatin Remodeling

Epigenetic changes alter balance between euchromatin and heterochromatin.
Euchromatin is transcriptionally active, heterochromatin is silent.
Remodeling complexes move or restructure nucleosomes.
This allows access for transcription factors when needed.
Essential in developmental gene activation/repression.

🌐 EE Focus: An EE could investigate epigenetic regulation in model organisms — e.g., how nutrition affects methylation in bees (workers vs queens).

📌 Epigenetics in Development

Epigenetic programming directs stem cells into different lineages.
X-chromosome inactivation in females equalises gene dosage.
Imprinting silences one parental allele in certain genes.
Epigenetic memory maintains cell identity across divisions.
Reprogramming occurs during gamete formation and early development.

❤️ CAS Link: Students could create a science communication project explaining epigenetics to the public with the metaphor of “switches and dimmers on genes.”

🌍 Real-World Connection: Epigenetic drugs (HDAC inhibitors, DNMT inhibitors) are used in cancer therapy. Epigenetics explains why identical twins can differ in disease susceptibility.

📌 Inheritance and Reversibility

Epigenetic marks can be passed to daughter cells.
Some marks escape erasure in gametes, leading to transgenerational effects.
Diet, toxins, and stress can all induce reversible epigenetic changes.
Shows environment–gene interactions at a molecular level.
Offers therapeutic potential for modifying gene expression.

🔍 TOK Perspective: Epigenetics blurs nature vs nurture. TOK issue: To what extent can science separate genetic destiny from environmental influence?

📝 Paper 2: Questions may involve explaining methylation, comparing euchromatin vs heterochromatin, or interpreting epigenetic modification data.

September 26, 2025

D2.2.1 TRANSCRIPTIONAL AND POST-TRANSCRIPTIONAL CONTROL

📌Definition Table

Term	Definition
Transcription factors	Proteins that bind DNA to regulate transcription by promoting or blocking RNA polymerase binding.
Promoter	DNA sequence where RNA polymerase and transcription factors initiate transcription.
Enhancer	DNA sequence that increases transcription levels when bound by activator proteins.
Silencer	DNA sequence that represses transcription when bound by repressor proteins.
Post-transcriptional control	Regulation of gene expression after transcription, e.g., RNA splicing, capping, polyadenylation.
Alternative splicing	Process in which different exons are combined to produce multiple mRNA variants from one gene.

📌Introduction

Gene expression in eukaryotes is highly regulated to ensure proteins are produced at the right time, in the right place, and in the correct amounts. Transcriptional control governs whether a gene is transcribed, while post-transcriptional control fine-tunes mRNA after it is produced. These mechanisms underpin cellular differentiation, development, and responses to environmental signals

📌 Transcriptional Regulation

Promoters: core sequences where RNA polymerase II binds to begin transcription.
Transcription factors:
- Activators recruit RNA polymerase and stabilise binding.
- Repressors block polymerase or recruit proteins that modify chromatin.
Enhancers and silencers: act at a distance, looping DNA to contact promoters.
Specific combinations of transcription factors give each cell type its identity.
Malfunctions in transcriptional regulation can lead to cancer or developmental disorders.

🧠 Examiner Tip: Don’t just write “transcription factors control transcription.” Name specific mechanisms — e.g., activators at enhancers, repressors at silencers — to gain full credit.

📌 RNA Processing

5′ capping: methylated guanine added, protecting mRNA and aiding ribosome binding.
Poly-A tail: adenine chain added to 3′ end, stabilising transcript and regulating nuclear export.
RNA splicing: introns removed, exons joined.
Alternative splicing: allows one gene to code for multiple proteins, increasing proteome diversity.
mRNA editing can change specific nucleotides, altering protein sequences.

🧬 IA Tips & Guidance: An IA could compare RNA extraction and gel electrophoresis of differently treated cells to show mRNA processing differences.

📌 Nuclear Export and Stability

Processed mRNA must pass through nuclear pores.
Export depends on proper capping, tailing, and splicing.
Cytoplasmic stability of mRNA influences how much protein is made.
Small RNAs (miRNAs, siRNAs) can degrade specific mRNAs or block translation.
This regulation ensures quick responses to signals.

🌐 EE Focus: An EE could investigate how alternative splicing produces protein diversity, e.g., in the immune system (antibody variation) or nervous system.

📌 Translation Readiness

Only properly processed mRNA is translated efficiently.
Proteins binding untranslated regions (UTRs) regulate ribosome access.
Iron metabolism regulated by proteins binding ferritin mRNA UTRs.
This adds another checkpoint for control.
Demonstrates tight integration of transcription and translation regulation.

❤️ CAS Link: Students could create a workshop or animation to teach juniors how “one gene can make many proteins” through splicing and control mechanisms.

🌍 Real-World Connection: Viruses hijack transcriptional/post-transcriptional machinery. HIV, for example, requires alternative splicing to produce multiple viral proteins.

📌 Integration with Cell Function

Gene regulation explains cell differentiation in multicellular organisms.
Errors in splicing can cause diseases like β-thalassemia (faulty haemoglobin production).
Regulatory mechanisms explain tissue-specific gene expression.
Understanding control helps develop therapies (antisense RNA, RNAi).
Highlights complexity of genome usage beyond simple “one gene = one protein.”

🔍 TOK Perspective: Models show neat gene-to-protein pathways, but reality involves layers of regulation. TOK issue: How do simplified models both help and hinder understanding of complex biology?

📝 Paper 2: Expect diagram questions on splicing, or data analysis on transcription factor binding. Could also involve experimental design interpreting RNA gels.

September 26, 2025

D2.1.3 IMPORTANCE OF CELL DIVISION IN GROWTH AND REPRODUCTION

📌Definition Table

Term	Definition
Growth	Increase in size and mass of organisms, requiring cell proliferation.
Reproduction	Biological process of producing offspring; may be sexual or asexual.
Differentiation	Process by which cells become specialised for particular functions.
Regeneration	Replacement or repair of damaged tissue through cell division.
Asexual reproduction	Production of offspring genetically identical to parent via mitosis.
Sexual reproduction	Production of genetically diverse offspring via meiosis and fertilisation.

📌Introduction

Cell division underpins growth, development, reproduction, and tissue maintenance in all organisms. Mitosis drives somatic growth and repair, while meiosis ensures genetic diversity in sexual reproduction. Together, these processes sustain life, enable adaptation, and explain fundamental biological patterns.

📌 Role in Growth and Development

Growth results from repeated mitotic divisions.
Embryonic development depends on rapid cycles of cell division.
Differentiation follows division, producing specialised tissues.
Cell division continues through adolescence to maintain growth.
Controlled by genetic and hormonal signals.

🧠 Examiner Tip: Don’t just say “cell division causes growth” — always mention mitosis specifically.

📌 Role in Asexual Reproduction

Mitosis produces identical clones of parent cells.
Common in unicellular organisms (binary fission in bacteria, mitosis in protists).
Also occurs in multicellular organisms (vegetative propagation in plants, budding in yeast).
Advantages: rapid, efficient, no mate required.
Disadvantage: lack of genetic diversity reduces adaptability.

🧬 IA Tips & Guidance: Students could observe budding in yeast or regeneration in planaria as examples of mitosis in reproduction.

📌 Role in Sexual Reproduction

Meiosis produces haploid gametes.
Fertilisation restores diploid chromosome number.
Crossing over and independent assortment → variation.
Variation enhances survival in changing environments.
Errors in division impact fertility and offspring viability.

🌐 EE Focus: An EE could explore the evolutionary significance of meiosis in generating variation vs the efficiency of asexual reproduction.

📌 Repair and Regeneration

Mitosis replaces damaged or dead cells in tissues (e.g., skin, gut lining).
Regeneration capacity varies: high in salamanders, low in mammals.
Stem cells enable replacement and repair.
Controlled cell division prevents scarring or overgrowth.
Imbalances lead to cancer or degenerative diseases.

❤️ CAS Link: Students could volunteer in health awareness programs on tissue regeneration and healing.

🌍 Real-World Connection: Regenerative medicine (stem cell therapy, tissue engineering) relies on controlled cell division.

📌 Evolutionary and Ecological Importance

Asexual reproduction enables rapid colonisation of habitats.
Sexual reproduction drives adaptation via variation and natural selection.
Cell division supports population dynamics by enabling reproduction.
Errors in meiosis can reduce fitness but also drive evolution.
Division processes explain diversity of life strategies.

🔍 TOK Perspective: Reproduction is often simplified into “asexual = identical, sexual = diverse.” TOK issue: Are such binary categories too simplistic, given cases like parthenogenesis or horizontal gene transfer?

📝 Paper 2: Questions may involve explaining roles of mitosis and meiosis in growth and reproduction, evaluating advantages of sexual vs asexual reproduction, or analysing micrographs.

September 26, 2025

D2.1.2 MITOSIS AND MEIOSIS

📌Definition Table

Term	Definition
Mitosis	Nuclear division producing two genetically identical daughter cells.
Meiosis	Nuclear division producing four genetically diverse haploid cells.
Chromatid	One half of a duplicated chromosome.
Homologous chromosomes	Chromosome pairs (one maternal, one paternal) with same loci.
Crossing over	Exchange of genetic material between homologous chromosomes during prophase I of meiosis.
Independent assortment	Random distribution of homologous chromosomes to gametes.

📌Introduction

Mitosis and meiosis are distinct processes of nuclear division with vital roles in growth, repair, and reproduction. Mitosis ensures identical daughter cells for somatic functions, while meiosis produces genetic variation in gametes for sexual reproduction. Both follow structured stages but differ fundamentally in outcomes and evolutionary importance.

📌 Mitosis Stages

Prophase: chromosomes condense, spindle forms, nuclear envelope breaks down.
Metaphase: chromosomes align at equator.
Anaphase: sister chromatids separate, pulled to poles.
Telophase: nuclear membranes reform, chromosomes decondense.
Cytokinesis divides cytoplasm → two identical cells.

🧠 Examiner Tip: Label diagrams carefully; metaphase and anaphase are frequently confused.

📌 Meiosis I

Prophase I: homologous chromosomes pair (synapsis); crossing over occurs.
Metaphase I: homologous pairs align randomly (independent assortment).
Anaphase I: homologues separate, but sister chromatids remain together.
Telophase I: cells divide into haploid nuclei.
Results in genetic diversity through recombination and assortment.

🧬 IA Tips & Guidance: Microscopic examination of meiosis in plant anthers (e.g., lily) can illustrate crossing over.

📌 Meiosis II

Resembles mitosis but starts with haploid cells.
Prophase II: spindles form again.
Metaphase II: chromosomes align singly.
Anaphase II: sister chromatids finally separate.
Telophase II: results in four haploid daughter cells.
Provides further diversification of gametes.

🌐 EE Focus: An EE could investigate how meiosis underpins Mendelian genetics, testing segregation and assortment principles.

📌 Comparison of Mitosis and Meiosis

Mitosis: 1 division → 2 diploid identical cells.
Meiosis: 2 divisions → 4 haploid genetically varied cells.
Mitosis: growth and repair; meiosis: reproduction.
Crossing over and independent assortment unique to meiosis.
Errors in meiosis → chromosomal disorders (e.g., Down syndrome).

❤️ CAS Link: Students could design peer teaching sessions using role-play to act out mitosis vs meiosis stages.

🌍 Real-World Connection: Fertility treatments and genetic counselling rely on understanding meiosis. Cancer research depends on mitotic control.

📌 Errors and Their Consequences

Non-disjunction: failure of chromosomes to separate → aneuploidy.
Can cause conditions such as trisomy 21 (Down syndrome).
Mitotic errors can cause tumour growth.
Checkpoint failures during meiosis lead to infertility.
Genetic diversity depends on accurate meiotic processes.

🔍 TOK Perspective: Mitosis and meiosis are taught as distinct “sets of stages.” TOK issue: Do these rigid models oversimplify what is, in reality, a fluid continuum of molecular events?

📝 Paper 2: Questions may require comparing mitosis vs meiosis, explaining crossing over, or interpreting chromosome diagrams.

September 26, 2025