GCSE Biology Paper 2: The Full Summary

Contraception methods (hormonal or non-hormonal) reduce fertility and prevent pregnancy. Hormonal methods use estrogen (inhibits FSH, egg development) and progesterone (thickens cervical mucus). Fertility drugs containing FSH and LH stimulate ovulation but can lead to multiple pregnancies. IVF (in vitro fertilization) involves stimulating egg production, fertilizing eggs in a lab, and implanting embryos into the uterus. IVF has a low success rate, is emotionally/physically stressful, and can result in multiple births.

Homeostasis is the maintenance of a stable internal environment. This includes body temperature, blood glucose, and water content, regulated by nervous and hormonal responses. The nervous system, consisting of the central nervous system (brain and spinal cord), receptors, and effectors, coordinates responses. Neurons (sensory, motor, relay) transmit electrical impulses. Voluntary responses involve the brain, while reflex responses are rapid, automatic, and coordinated by relay neurons in the spinal cord, forming a reflex arc. Synapses are gaps between neurons where neurotransmitters diffuse across.

The endocrine system uses chemical messengers called hormones, transported by blood. Hormonal responses are slower but longer-lasting than nervous responses. Key glands include the pituitary, thyroid, adrenal, pancreas, ovaries, and testes. Adrenaline (adrenal gland) prepares the body for fight or flight, increasing heart rate and delivering oxygen/glucose. Thyroxine (thyroid gland) controls metabolic rate, growth, and development, regulated by negative feedback. Insulin and glucagon (pancreas) control blood glucose, also via negative feedback.

Diabetes occurs when blood glucose control fails. Type 1 diabetes (little to no insulin production) is inherited and managed with insulin injections. Type 2 diabetes (body cells resistant to insulin) is managed with diet and exercise, with obesity being a risk factor. Testosterone (testes) produces sperm and secondary sexual characteristics in men. Estrogen (ovaries) matures egg cells and causes breast development in women. The menstrual cycle involves estrogen, LH, FSH, and progesterone regulating the uterus lining and egg release.

Sexual reproduction involves two parents contributing gametes (sperm/egg, pollen/egg), leading to genetic variation in offspring. Asexual reproduction involves one parent, producing genetically identical offspring (clones), common in bacteria. Mitosis (asexual) produces two identical daughter cells for growth and repair. Meiosis (sexual) produces four distinct gametes with half the chromosomes.

DNA is a double helix polymer in chromosomes, coding for proteins. A gene is a DNA section coding for a specific protein. The genome is an organism's entire genetic material. The Human Genome Project helps find disease-causing genes, trace migration, and treat inherited diseases. Alleles are different forms of genes. Dominant alleles express themselves when present; recessive alleles only express when both are recessive. Homozygous (same alleles), heterozygous (different alleles). Genotype is the allele combination; phenotype is the expressed characteristic. Punnett squares predict offspring genotypes/phenotypes, including for inherited diseases like cystic fibrosis (recessive) and polydactyly (dominant). Sex determination (XX female, XY male) has a 50/50 chance.

Variation (differences among individuals) can be genetic (inherited DNA) or environmental. Mutations (random DNA changes) alter genes, potentially changing phenotype and driving evolution. Evolution is the gradual development of inherited characteristics in a population via natural selection. Organisms with beneficial phenotypes are more likely to survive, reproduce, and pass on genes, making those characteristics more common. Speciation occurs when populations become so different they can no longer interbreed. Extinction can result from environmental changes, predators, disease, competition, natural disasters, or human activity.

Charles Darwin's theory of natural selection was later supported by understanding DNA, genes, and antibiotic resistance. Antibiotic-resistant bacteria (e.g., MRSA) evolve as non-resistant bacteria are killed, leaving resistant ones to reproduce. Doctors should prescribe antibiotics judiciously, and patients should complete courses. Fossils (preserved remains/impressions) provide evidence of ancient organisms: mineral replacement, preservation in anoxic environments (amber, tar pits), and trace fossils (footprints). The Linnaean system classifies organisms into Kingdom, Phylum, Class, Order, Family, Genus, Species, with binomial naming (Genus species). Carl Woese's three-domain system includes Archaea, Bacteria, and Eukaryota.

Selective breeding involves choosing and breeding organisms with desirable traits (e.g., disease-resistant crops, more meat/milk, gentle pets, large flowers) over generations. This reduces the gene pool, leading to less variation, potential genetic defects, and increased susceptibility to new diseases. Genetic engineering involves modifying an organism's DNA by isolating and cutting out desired genes (e.g., insulin gene), inserting them into a vector (plasmid or virus), and introducing them into a new organism (e.g., bacteria) to produce modified products or organisms (e.g., GM crops). This has both advantages and disadvantages.

An ecosystem is the interaction of a community of living (biotic) organisms with non-living (abiotic) environmental factors. Biotic factors include predators, competition, pathogens, and food availability. Abiotic factors include moisture, light, temperature, CO2, wind, oxygen, soil pH, and mineral content. Organisms are interdependent, and stable ecosystems maintain constant population sizes. Organisms adapt to their environments: structural (body shape, color), behavioral (hibernation), and functional (camel's water conservation). Extremophiles adapt to harsh conditions.

Food chains show 'what eats what', starting with producers (plants/algae) that photosynthesize, followed by primary, secondary, and tertiary consumers. Biomass (energy) decreases at each trophic level. Predator-prey populations fluctuate in cycles within stable ecosystems. The water cycle involves evaporation (from oceans and plants via transpiration), condensation, and precipitation. The carbon cycle involves CO2 absorption by plants (photosynthesis), transfer through food chains, release by respiration (plants, animals, decomposers), and the formation of fossil fuels from dead organisms. Combustion of fossil fuels releases CO2.

Greenhouse gases (CO2, methane, water vapor) trap heat, warming the Earth. Increased greenhouse gas emissions from human activities lead to global warming, causing rising sea levels, flooding, and shifting ecosystems. Deforestation reduces CO2 absorption and increases CO2 release (decay/burning), also reducing biodiversity (variety of organisms). Peat bogs, rich in partially decayed plant material, store CO2 and serve as habitats; their destruction harms biodiversity and exacerbates global warming. Conservation efforts include breeding programs, habitat protection, recycling, reducing deforestation, and field margins to minimize environmental impact.