Complete Biology for Cambridge IGCSE Notes
Characteristics and Classification of Living Organisms
Biology is the study of life. Living organisms are distinguished from non-living things by a set of processes known by the acronym RINGER: Respiration (releasing energy from nutrients), Irritability (sensitivity to stimuli), Nutrition (taking in materials for energy/growth), Growth (permanent increase in size/mass), Excretion (removal of metabolic waste), and Reproduction (making more of the same kind). Life is defined as a set of processes resulting from the organization of matter requiring energy expenditure.
Organisms are classified into five Kingdoms: Prokaryotes, Protoctistans, Plants, Fungi, and Animals. This nested hierarchy follows the sequence: Kingdom, Phylum, Class, Order, Family, Genus, and Species. The binomial system (Carolus Linnaeus) assigns each species a two-part Latin name (Genus species). Modern classification utilizes morphology, anatomy, and increasingly, DNA/protein sequences to determine evolutionary relationships.
Biological Components: Cells and Molecules
The cell is the basic unit of life. All cells share a cell membrane, cytoplasm, and a nucleus containing DNA. Plant cells are distinguished by a cellulose cell wall, chloroplasts for photosynthesis, and a large permanent vacuole. Multicellular organisms show a high degree of organization: specialized cells form tissues, tissues form organs, and organs form organ systems (e.g., the digestive system).
Living things are composed of organic molecules containing carbon and hydrogen. The four main groups are: Carbohydrates (energy source, e.g., glucose, starch), Lipids (energy storage/membranes), Proteins (enzymes/structure, made of amino acids), and Nucleic Acids (DNA/RNA for genetic code). Metabolism includes anabolic (building) and catabolic (breaking down) reactions, all controlled by enzymes acting as biological catalysts.
Mechanisms of Movement and Plant Nutrition
Substances move in/out of cells via Diffusion (passive movement down a concentration gradient), Osmosis (diffusion of water across a partially permeable membrane), or Active Transport (movement against a gradient requiring energy and carrier proteins). Diffusion efficiency is limited by the surface area to volume ratio.
Plants are autotrophs that manufacture food via photosynthesis: . This occurs in chloroplasts. Leaves are specialized for gas exchange via stomata and light absorption via the palisade mesophyll. Factors like light intensity, $CO_2$ concentration, and temperature act as limiting factors. Plants also require minerals like Nitrate (for proteins) and Magnesium (for chlorophyll).
Animal Nutrition and Transport
Balanced human diets require carbohydrates, lipids, proteins, vitamins (C, D), minerals (Calcium, Iron), water, and fiber. Digestion is the breakdown of large, insoluble molecules into small, soluble ones through mechanical (teeth) and chemical (enzymes) means. Amylase, protease, and lipase are key enzymes. Villi in the small intestine provide a large surface area for absorption into the blood/lymph.
The circulatory system is a mass flow system consisting of a heart (pump), blood (medium), and vessels (arteries, veins, capillaries). Humans have a double circulation: one circuit to the lungs (pulmonary) and one to the body (systemic). Blood components include red cells (oxygen transport), white cells (defense), platelets (clotting), and plasma (nutrient/waste transport). Coronary heart disease involves blockage of the heart's own blood supply.
Respiration, Excretion, and Homeostasis
Respiration releases energy () from food. Aerobic respiration requires oxygen; anaerobic respiration occurs without oxygen but is less efficient and produces lactic acid (in humans) or ethanol (in yeast). Gas exchange in humans occurs in the alveoli of the lungs, ventilated by the diaphragm and intercostal muscles.
Excretion is the removal of toxic metabolic waste (e.g., via lungs, Urea via kidneys). The kidney nephrons filter blood and selectively reabsorb water and glucose to maintain osmoregulation. Homeostasis is the maintenance of a constant internal environment via negative feedback, regulating temperature, blood glucose (insulin/glucagon), and water potential.
Coordination, Drugs, and Environment
Coordination involves the nervous system (rapid electrical impulses) and the endocrine system (slower chemical hormones). Reflex arcs provide rapid automatic responses. Sensory organs, like the eye, transduce environmental stimuli into nerve impulses. Drugs like alcohol (depressant) or nicotine (stimulant) modify these pathways. Plants show growth responses called tropisms (phototropism/gravitropism) controlled by the hormone auxin.
Ecology focuses on interactions within ecosystems. Energy flows one-way through food chains/webs, while nutrients (Carbon, Nitrogen, Water) are recycled. Human impacts include deforestation, pollution (acid rain/global warming), and eutrophication (nutrient runoff killing aquatic life). Conservation aims to protect biodiversity and manage resources sustainably.