25 terms
Gas exchange
All gas exchange surfaces share the same core features: they are thin, moist, and large in surface area relative to volume. How each organism achieves this depends on its body plan and habitat. **Single-celled organisms** (e.g. Amoeba) are so small that their surface area to volume ratio (SA:V ratio — the proportion of outer surface relative to body size) is naturally high. Gases diffuse directly
Organisms exchange substances with their environment
Gas exchange
Insects and drought-resistant plants both need to let gases in and out for respiration and photosynthesis. Every adaptation that helps gas exchange also risks letting water escape, so each organism uses structures that balance both needs.
Organisms exchange substances with their environment
Gas exchange
The human gas exchange system moves air from your mouth and nose down into your lungs. It branches from one large tube into millions of tiny air sacs where oxygen enters your blood.
Organisms exchange substances with their environment
Gas exchange
Alveoli are tiny air sacs in the lungs. Their walls have special features that allow oxygen and carbon dioxide to diffuse rapidly between air and blood.
Organisms exchange substances with their environment
Gas exchange
Ventilation moves fresh air into the lungs and stale air out. This keeps oxygen levels high and carbon dioxide levels low at the alveolar surface, so diffusion into the blood continues rapidly.
Organisms exchange substances with their environment
Gas exchange
Breathing moves air by changing pressure inside the thoracic cavity (the airtight space in your chest that contains the lungs). Boyle's Law explains the link: when volume increases, pressure falls; when volume decreases, pressure rises. During inspiration (breathing in): 1. The diaphragm — a dome-shaped sheet of muscle below the lungs — contracts and flattens downward. 2. The external intercostal
Organisms exchange substances with their environment
Gas exchange
Every living organism needs to move oxygen in and carbon dioxide out, and the structures that make this possible are shaped by the same core problem: maximising the rate of diffusion across an exchange surface while minimising harmful water loss. This subtopic compares how organisms as different as insects, fish, flowering plants, and humans have each evolved specialised gas exchange systems — from the tracheal system (a network of air-filled tubes) in insects to the alveoli (tiny air sacs) deep in the human lung — revealing the structural trade-offs each solution involves. Understanding these adaptations also underpins how ventilation (the active movement of air or water over an exchange surface) maintains the concentration gradients that drive diffusion, which connects directly to mass transport and whole-body physiology.
Organisms exchange substances with their environment
Gene expression is controlled by a number of features
Not every gene in a cell is active at the same time — which genes are switched on or off is tightly controlled, and mutations (spontaneous, heritable changes to DNA base sequences) can disrupt this control in ways that range from harmless to life-threatening. Much of the genome consists of non-coding DNA — sequences that do not code for proteins but play a crucial role in regulating when and how much a gene is expressed. Understanding these control mechanisms explains how genetically identical cells can become specialised, and why some mutations cause disease while others have no effect at all.
The control of gene expression
Gene technologies allow the study and alteration of gene function
Modern gene technologies give scientists the tools to read, manipulate, and alter the genetic information inside cells — opening up both medical and agricultural applications that were impossible just decades ago. Techniques such as automated sequencing (using machines to rapidly decode the order of bases in DNA) and genome projects have revealed how genes are organised and regulated across entire organisms. Understanding these technologies matters because they allow scientists to investigate what individual genes actually do, correct faults caused by mutations (changes to the base sequence of DNA), and deliberately engineer new traits — building directly on the gene expression principles covered in earlier subtopics.
The control of gene expression
Genetic diversity and adaptation
Genetic diversity measures how many different versions of genes exist across all individuals in a population. More different versions means higher genetic diversity.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Natural selection needs variation to work on. Genetic diversity — the range of different alleles in a population — supplies that variation, giving selection something to act on.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Natural selection follows a clear causal chain: 1. A random mutation — a change in the DNA base sequence — produces a new allele (a new version of a gene). 2. By chance, that allele gives some individuals an advantage in their current environment. For example, a mutation in a bacterium might alter the active site of an enzyme that an antibiotic normally targets, making the antibiotic ineffective.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Directional selection happens when one extreme version of a trait gives organisms a survival advantage. Over generations, that extreme trait becomes more common in the population.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Stabilising selection is a type of natural selection that favours average individuals. It removes extreme traits from a population, keeping most individuals close to the middle of the range.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Natural selection acts as a filter over many generations. Individuals with traits that suit their environment survive and reproduce more, so those traits become more common in the species.
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Adaptations are features that help an organism survive and reproduce in its environment. They fall into three types: structural (body shape or parts), chemical/functional (internal processes), or behavioural (what the organism does).
Genetic information, variation and relationships between organisms
Genetic diversity and adaptation
Genetic diversity — the total number of different alleles, meaning different versions of genes, present in a population — is the raw material that makes natural selection possible. When an allele gives an organism a survival or reproductive advantage in its environment, that allele is passed on more frequently until it becomes more common across generations. This process drives two distinct patterns: directional selection, where one extreme trait is favoured (such as antibiotic resistance in bacteria), and stabilising selection, where intermediate traits are favoured (such as average birth weight in humans). Understanding these mechanisms explains how populations gradually accumulate adaptations — anatomical, physiological or behavioural features that suit them to their environment.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
A gene mutation is a permanent change to the sequence of DNA bases in a chromosome. This change alters the genetic instructions carried by that gene.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
During DNA replication, copying errors can occur by accident. These errors — called mutations — change the sequence of bases in the DNA, and they happen without any external cause.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
The genetic code is degenerate — multiple codons can code for the same amino acid. So swapping one DNA base for another sometimes makes no difference to the protein produced.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Some chemicals and types of radiation damage DNA. They are called mutagenic agents, and they make gene mutations happen more often than normal.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Sometimes chromosomes fail to separate properly during meiosis. This error, called non-disjunction, produces sex cells with the wrong number of chromosomes.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Meiosis is a type of cell division that makes sex cells. Each sex cell it produces carries a unique combination of genetic information.
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Meiosis starts with one diploid parent cell — a cell containing two full sets of chromosomes (46 in humans). It produces four haploid daughter cells, each with only one set (23 in humans). These daughter cells become gametes: sperm or eggs. The process involves two sequential nuclear divisions: 1. Before division begins, DNA replicates so each chromosome consists of two identical sister chromati
Genetic information, variation and relationships between organisms
Genetic diversity can arise as a result of mutation or during meiosis
Genetic diversity — the range of different alleles (versions of genes) present within a population — can arise through two main routes: gene mutations, which are changes to the DNA base sequence that may alter the protein an organism produces, and meiosis, the type of cell division that produces sex cells. During meiosis, processes called independent segregation (the random sorting of chromosome pairs) and crossing over (the exchange of DNA segments between paired chromosomes) shuffle alleles into new combinations, ensuring offspring are genetically unique. Understanding these mechanisms explains where heritable variation comes from — the raw material that natural selection acts on.
Genetic information, variation and relationships between organisms