International Baccalaureate Diploma (IB) Group 4: Sciences (Biology) Syllabus

Define the biological species concept.

A species is a group of organisms that can interbreed to produce fertile, viable offspring and are reproductively isolated from other such groups.

What is a dichotomous key?

An identification tool using a series of paired, contrasting statements; at each step the user chooses one option, progressively narrowing down to identify an organism.

Describe the fluid mosaic model of the cell membrane.

A phospholipid bilayer (fluid) with proteins, cholesterol, glycoproteins and glycolipids embedded or attached (mosaic); components can move laterally within the bilayer.

Why are phospholipids arranged as a bilayer in water?

They are amphipathic: hydrophilic phosphate heads face the aqueous environments inside and outside the cell, while hydrophobic fatty-acid tails point inward, away from water.

Contrast diffusion, osmosis and active transport.

Diffusion: net movement of particles down a concentration gradient (passive). Osmosis: movement of water across a partially permeable membrane from high to low water potential (passive). Active transport: movement against the gradient using carrier proteins and ATP.

What is the difference between endocytosis and exocytosis?

Endocytosis brings large materials into the cell by engulfing them in a vesicle from the membrane; exocytosis expels materials by fusing a vesicle with the membrane. Both require ATP.

What is facilitated diffusion?

Passive transport of substances down their concentration gradient through specific channel or carrier proteins, without using ATP.

Describe what happens to an animal cell and a plant cell placed in a hypotonic solution.

Water enters by osmosis; the animal cell swells and may lyse (burst); the plant cell becomes turgid as the cell wall resists bursting.

Why do single-celled organisms not need specialised gas exchange surfaces?

Their large surface area to volume ratio and short diffusion distances allow gases to diffuse directly across the cell membrane fast enough to meet their needs.

List four features of an efficient gas exchange surface.

Large surface area, thin (short diffusion distance), moist, and a steep concentration gradient maintained by good blood/air supply (ventilation).

Trace the pathway of air through the human respiratory system.

Nose/mouth, pharynx, larynx, trachea, bronchi, bronchioles, alveoli (where gas exchange occurs).

How are alveoli adapted for efficient gas exchange?

They provide a very large surface area, have walls one cell thick (short diffusion path), are moist, and are surrounded by dense capillaries that maintain a steep concentration gradient.

How do leaves carry out gas exchange in plants?

Gases diffuse through stomata (pores controlled by guard cells) into air spaces in the spongy mesophyll, then across moist cell surfaces.

What is the difference between structural, behavioural and physiological adaptations?

Structural: physical body features (e.g. thick fur). Behavioural: actions or responses (e.g. migration). Physiological: internal biochemical/process adaptations (e.g. producing concentrated urine).

Give two adaptations of a xerophyte (plant adapted to dry conditions).

Examples: thick waxy cuticle, sunken stomata, reduced/rolled leaves or spines, deep or widespread roots, and water-storage tissue, all reducing water loss.

What is countercurrent exchange and where is it used?

Two fluids flow in opposite directions to maintain a concentration gradient along the whole exchange surface, maximising transfer; e.g. water and blood flow in fish gills.

What is cell signalling?

The process by which cells communicate, typically a signal molecule (ligand) binding to a specific receptor, triggering a response inside the target cell.

Define homeostasis.

The maintenance of a stable internal environment within narrow limits despite changes in external conditions.

Explain negative feedback with an example.

A change in a variable triggers a response that counteracts the change, returning it toward the set point; e.g. when blood glucose rises, insulin is released to lower it.

How do insulin and glucagon regulate blood glucose?

Insulin (from beta cells) lowers blood glucose by promoting glucose uptake and conversion to glycogen; glucagon (from alpha cells) raises it by stimulating glycogen breakdown into glucose.

What is thermoregulation and name two mechanisms for cooling in humans.

Maintaining core body temperature; cooling mechanisms include vasodilation of skin blood vessels and sweating (evaporative cooling).

What is an enzyme and how does it speed up reactions?

A biological catalyst (usually a protein) that lowers the activation energy of a reaction without being consumed, by binding substrate at its active site.

Explain the induced-fit model of enzyme action.

The active site is not perfectly complementary at first; when the substrate binds, the active site changes shape slightly to fit closely, straining bonds and catalysing the reaction.

Describe the effect of temperature on enzyme activity.

Rate increases with temperature up to an optimum (more kinetic energy and collisions); beyond it, the enzyme denatures as its tertiary structure breaks, so activity drops sharply.

Distinguish competitive from non-competitive enzyme inhibition.

Competitive inhibitors bind the active site, competing with substrate (effect reduced by raising substrate concentration); non-competitive inhibitors bind elsewhere (allosteric site), changing the active site's shape regardless of substrate concentration.

Distinguish anabolism from catabolism.

Anabolism builds larger molecules from smaller ones, consuming energy (e.g. protein synthesis); catabolism breaks larger molecules into smaller ones, releasing energy (e.g. respiration).

Write the word equation for aerobic respiration.

Glucose + oxygen, yields carbon dioxide + water + energy (ATP).

Write the balanced equation for photosynthesis.

6CO2 + 6H2O, with light energy and chlorophyll, yields C6H12O6 + 6O2.

Define an ecosystem and distinguish it from a community and a population.

A population is all individuals of one species in an area; a community is all the populations of different species in an area; an ecosystem is the community plus its abiotic (non-living) environment, interacting together.

Distinguish a food chain from a food web, and define a trophic level.

A food chain shows a single linear feeding sequence; a food web shows interconnected chains. A trophic level is an organism's feeding position (e.g. producer, primary consumer, secondary consumer).

Why is only about 10% of energy transferred between trophic levels?

Most energy is lost as heat through respiration, in movement, in undigested/excreted material, and in uneaten parts, so roughly only 10% is passed on, limiting chain length.

Outline the key stages of the carbon cycle.

Carbon enters living matter via photosynthesis, passes along food chains by feeding, and returns to the atmosphere as CO2 through respiration, decomposition, and combustion of fossil fuels.

Distinguish mitosis from meiosis.

Mitosis produces two genetically identical diploid cells for growth and repair; meiosis produces four genetically varied haploid gametes through two divisions, halving the chromosome number.

Name the four stages of mitosis in order and what happens in each.

Prophase: chromosomes condense, spindle forms. Metaphase: chromosomes align at the equator. Anaphase: sister chromatids separate to opposite poles. Telophase: nuclear membranes reform around two sets of chromosomes.

How does meiosis generate genetic variation?

Through crossing over (exchange of alleles between homologous chromosomes in prophase I) and independent assortment (random orientation of homologous pairs at metaphase I).

Contrast asexual and sexual reproduction.

Asexual involves one parent and produces genetically identical clones (no gametes); sexual involves two parents and fusion of gametes, producing genetically varied offspring.

Define genotype, phenotype, allele, and homozygous.

Genotype: an organism's genetic makeup. Phenotype: its observable characteristics. Allele: an alternative version of a gene. Homozygous: having two identical alleles for a gene.

In a monohybrid cross between two heterozygotes (Tt x Tt), what is the expected phenotype ratio?

3:1 dominant to recessive (genotype ratio 1 TT : 2 Tt : 1 tt).

What is codominance, and give an example?

Both alleles are fully expressed in the heterozygote, neither masking the other; e.g. the AB blood group, where both A and B antigens are present.

How is sex determined in humans, and why are males more likely to show X-linked recessive conditions?

Females are XX, males are XY. Males have only one X chromosome, so a single recessive allele on it is expressed (no second X to mask it).

What is a gene mutation and how can it affect a protein?

A change in the DNA base sequence; it can alter the amino acid sequence of a protein, potentially changing its structure and function (e.g. sickle-cell anaemia).

State Darwin's theory of evolution by natural selection in four steps.

Variation exists in populations; more offspring are produced than can survive (overproduction/competition); individuals with advantageous traits survive and reproduce more (survival of the fittest); these traits are inherited, so the population changes over generations.

List four types of evidence for evolution.

The fossil record, comparative anatomy (homologous structures), molecular/biochemical similarities (DNA and proteins), and direct observation (e.g. antibiotic resistance, selective breeding).

Distinguish homologous from analogous structures.

Homologous structures share a common evolutionary origin but may differ in function (e.g. pentadactyl limbs), showing divergent evolution; analogous structures share function but not origin (e.g. bird and insect wings), showing convergent evolution.

What is biodiversity and why is it important?

The variety of life at genetic, species and ecosystem levels; it increases ecosystem stability and resilience, provides resources and services, and offers a genetic reservoir for future adaptation.

How does antibiotic resistance in bacteria illustrate natural selection?

Random mutation gives some bacteria resistance; when antibiotics are applied, susceptible bacteria die while resistant ones survive and reproduce, so the resistant allele spreads through the population.

Define independent, dependent, and controlled variables.

Independent: the variable deliberately changed by the experimenter. Dependent: the variable measured as the outcome. Controlled: variables kept constant to ensure a fair test.

What is the difference between accuracy and precision?

Accuracy is how close a measurement is to the true value; precision is how close repeated measurements are to each other (reproducibility), regardless of correctness.

Distinguish a random error from a systematic error.

Random errors cause unpredictable scatter around the true value and are reduced by repeating measurements; systematic errors shift all readings consistently in one direction (e.g. miscalibration) and are not reduced by repetition.

What is a null hypothesis?

A statement that there is no significant difference or relationship between the variables; statistical tests aim to reject or fail to reject it.

Why are repeats and a large sample size important in an experiment?

They reduce the effect of random error and anomalies, allow calculation of a reliable mean, and increase confidence that results are representative and statistically significant.

What does a standard deviation tell you about a data set?

It measures the spread of data around the mean; a small standard deviation indicates data clustered close to the mean (consistent), a large one indicates wide variability.

What is a controlled (negative) control and why is it used?

A treatment with the independent variable absent or at baseline; it shows what happens without the tested factor, providing a comparison to confirm the variable caused any observed effect.

What are the key stages of the scientific method in an investigation?

Make an observation, ask a question, form a testable hypothesis, design and conduct an experiment, collect and analyse data, draw a conclusion, and evaluate/communicate results.

What is the purpose of the IB Internal Assessment (individual scientific investigation)?

To let a student plan, carry out and evaluate an independent inquiry into a research question of personal interest, demonstrating experimental and analytical skills.

What is the aim of the IB Collaborative Sciences Project?

An interdisciplinary, collaborative activity in which students work together to address a real-world problem using scientific understanding from multiple disciplines, focusing on the role of science in a global context.