GCSE Biology: Plant Defence Responses (Triple)

Plants produce chemicals primarily to defend against herbivores and pathogens, but these same compounds can benefit humans when properly understood and applied. Aspirin's primary ingredient, salicylic acid, is derived from willows. Similarly, senna (used as a laxative) comes from wild senna plants. Understanding plant chemistry reveals how natural defenses can translate into medicinal applications for human health.

Many common substances originated as plant defense chemicals. Aspirin was derived from salicylic acid in willow tree bark, used historically for pain relief. Red wine's color and tannins are defensive compounds. Nicotine, originally a plant defense chemical, became one of humanity's oldest successful insecticides. This pattern shows how humans have systematically exploited plant chemical defenses for practical applications.

Many accused witches were skilled herbalists providing medical care before modern physicians existed. Willow bark contains salicylic acid, the precursor to aspirin, demonstrating how natural compounds can be chemically modified for commercial pharmaceutical production. Digitalis, extracted from foxglove flowers, treats congestive heart failure by inhibiting the sodium-potassium ATPase pump, causing increased calcium levels and stronger heart contractions. This historical practice shows how traditional plant-based medicine evolved into modern pharmacology.

Original drugs were extracted from plants and microorganisms: aspirin from willow tree bark, digitalis from foxglove plant, penicillin from penicillum mold (discovered by Alexander Fleming). Modern synthetic drugs may still use plant extracts as bases but must be tested for toxicity, efficacy, and dosage before use.

Plants produce secondary metabolites (phytochemicals) that serve various functions including defense against pests and environmental stress, and humans have historically utilized these plant-derived chemicals for practical applications such as natural dyes, perfumes, medicines (including aspirin from willow bark, taxol from yew trees, and vinblastine from periwinkle), disinfectants, soaps, cosmetics, and pest control agents, demonstrating the important relationship between plant biodiversity and human health and wellbeing.

Non-specific body defences are the first line of immune protection that generally attack any foreign substance without discrimination. These include physical barriers like epithelial cells lining internal surfaces (digestive tract, lungs), chemical barriers such as tears, saliva, mucus, and stomach acid that trap or kill pathogens, and the inflammatory response involving mast cells releasing histamine to increase blood vessel permeability, allowing phagocytes to engulf pathogens via phagocytosis using lysosomal enzymes, while also releasing cytokines to attract additional immune cells to the site of infection.

The human body has four main non-specific defence systems that act as the first line of protection against pathogens: (1) The skin forms a physical barrier with dead outer cells and produces sebum to kill bacteria; (2) The nose contains hair and mucus that trap pathogens before they enter the respiratory system; (3) The trachea and bronchi are lined with cilia covered in mucus that sweep pathogens upward toward the throat; (4) The stomach contains hydrochloric acid that kills pathogens before they enter the digestive system. These barriers prevent pathogens from entering the body but cannot guarantee complete protection, which is why the immune system provides additional defense.

Non-specific defenses prevent pathogen entry: nose hairs and mucus trap pathogens, cilia move mucus away, stomach acid kills bacteria, skin acts as physical barrier. Specific defenses activate inside the body: phagocytes destroy pathogens, lymphocytes produce antibodies specific to detected antigens.

Non-specific defences are the body's first line of protection against pathogens, including physical barriers like the skin (made of dead keratinocytes filled with waterproof keratin) and mucous membranes with ciliated epithelium that trap and expel pathogens; these defences also include inflammatory responses triggered by mast cells releasing histamine, which causes vasodilation and increased permeability to allow immune cells to reach infection sites, along with additional mechanisms such as tears containing lysozyme enzymes, ear wax, expulsive reflexes like coughing and sneezing, and stomach acid that kill pathogens.

Non-specific defenses: nose hairs and mucus trap pathogens; cilia move mucus with trapped pathogens to nose/mouth; stomach acid kills bacteria; skin acts as physical barrier. Specific defenses involve white blood cells: phagocytes engulf pathogens; lymphocytes divide to form plasma cells producing antibodies specific to antigens on pathogens, and produce antitoxins to neutralize bacterial toxins.

Humans have selectively bred organisms for thousands of years by selecting parents with desired traits, breeding them, selecting offspring with desired traits, and repeating over generations. Examples: animals bred for meat/milk/wool yield and gentle nature; plants bred for disease resistance and ornamental flowers. Issues include reduced genetic variation, increased susceptibility to diseases, and potential for recessive disorders. Genetic engineering cuts genes from chromosomes and transfers them to different organisms. Applications include pest-resistant/cold-tolerant crop plants and bacteria producing human insulin. Concerns include genes spreading to wild populations, potential allergic reactions, lack of long-term safety studies, and ethical concerns about human modification.

Genetic engineering is used to improve crop plants by introducing desirable traits: (1) Increased nutritional value (e.g., Golden Rice with enhanced vitamin A); (2) Pest resistance (e.g., Bt cotton producing its own insecticide); (3) Herbicide tolerance; (4) Environmental stress tolerance (drought, salinity). The process involves: (1) isolating the gene of interest from a donor organism, (2) inserting it into a plasmid vector, (3) introducing the plasmid into bacterial cells, (4) using bacteria to transfer the plasmid into plant cells, and (5) selecting for cells that have successfully taken up the gene.

Crop improvement employs complementary strategies spanning traditional breeding to genetic engineering. Traditional methods like backcrossing transfer single resistance genes into elite varieties through repeated crosses, gradually replacing donor genetics while retaining desired traits. Hybrid vigor exploits heterosis to combine multiple resistance genes from different parents. Genetic engineering offers precise gene transfer using vectors like Agrobacterium, protoplast fusion, electroporation, and particle bombardment. Each approach has strengths: traditional breeding maintains genetic diversity while genetic engineering enables rapid introduction of complex traits. Together, these methods have transformed agriculture, with approximately 75% of U.S. commercial crops now genetically modified for improved resistance and productivity.

Selective breeding artificially mates organisms with desirable traits to produce offspring with required characteristics (e.g., high-yield cows, disease-resistant crops). Limitations include slow process, reduced gene pool, and risk of population elimination. Genetic engineering alters genes by inserting desired genes: isolate gene of interest, cut bacterial plasmid vector, join to form recombinant DNA, insert into bacteria (transformation), and multiply transformed bacteria. Applications include Golden Rice (vitamin A production) and tomatoes with extended shelf-life, representing modern biotechnology advances.

This section traces the evolution of genetic modification in agriculture from traditional selective breeding to modern genetic engineering. Agriculture has always relied on genetic modification, beginning with selective breeding that transformed wild plants over generations—for example, evolving cauliflower from wild mustard through selection for enhanced flower clusters, and modern corn from teosinte through generations of breeding for larger kernels. Modern genetic engineering accelerates this process dramatically, sometimes completing modifications in weeks rather than generations. The process involves identifying genes of interest, editing DNA, inserting modified DNA into crops, and growing plants with desired traits. Approximately 90% or more of U.S. corn, cotton, sugar beets, canola, and soybeans are genetically modified. Major modifications include insect resistance (crops producing their own narrow-spectrum insecticide) and herbicide resistance (crops surviving glyphosate applications that kill weeds). Examples include Roundup Ready corn, Golden Rice (increased vitamin A content), Flavor Saver tomatoes (resistance to spoilage), and Arctic apples (resistance to browning). Selectively bred animals in industrial meat production also represent modern GMOs, with chickens now growing three times faster than 1957 varieties.