Comprehensive Study Notes for Cambridge O Level Biology (5090) Syllabus 2026-2028 Syllabus)
Cell Structure, Function, and Organization
The study of biology begins with the fundamental unit of life, the cell. Students are required to examine animal and plant cells using microscopy, utilizing locally available materials and temporary staining techniques such as methylene blue or iodine solution to enhance visibility of structures. Observations from these examinations should be documented through accurate diagrams. In an animal cell, key structures to identify on diagrams, photomicrographs, or electron micrographs include the ribosomes, mitochondria, nucleus, cytoplasm, and cell membrane. Plant cells contain these same structures but are further characterized by the presence of chloroplasts, a large sap vacuole, and a cellulose cell wall. Bacterial cells present a different structural profile, limited to ribosomes, circular deoxyribonucleic acid (DNA), plasmids, cytoplasm, a cell membrane, and a cell wall.
Cells can become specialized, adapting their structures to perform specific functions. This principle is foundational to understanding the hierarchy of biological organization, where cells form tissues, tissues form organs, organs form organ systems, and organ systems comprise an organism. In the context of microscopy and biological measurements, the relationship between the observed size of a specimen and its physical reality is expressed by the magnification formula: .
Classification Systems and Biodiversity
Organisms are classified into groups based on shared features to facilitate scientific study and identification. A species is defined specifically as a group of organisms capable of reproducing to produce fertile offspring. The binomial system of naming is an internationally recognized standard where each organism is given a scientific name consisting of two parts: the genus followed by the species. Identification of unknown organisms is achieved through the construction and use of dichotomous keys based on identifiable physical features.
All organisms are placed into one of five kingdoms: Animal, Plant, Fungus, Prokaryote, and Protoctist. Within the animal kingdom, classification further divides organisms into vertebrates (mammals, birds, reptiles, amphibians, and fish) and arthropods (myriapods, insects, arachnids, and crustaceans). The plant kingdom is categorized into ferns and flowering plants, which are further divided into monocotyledons and dicotyledons. Viruses represent a unique category of biological entities, characterized by a protein coat and genetic material, and are distinguished by the fact that they can only replicate inside living cells.
Movement Into and Out of Cells
Water serves as a vital solvent in organisms, playing a critical role in digestion, excretion, and transport. The movement of substances across cell membranes occurs through diffusion, osmosis, and active transport. Diffusion and osmosis are driven by the kinetic energy of the random movement of molecules and ions. Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, moving down a concentration gradient. Factors influencing the rate of diffusion include surface area, temperature, the steepness of the concentration gradient, and the distance over which diffusion must occur.
Osmosis is a specific form of diffusion involving the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. In plants, osmosis is essential for support, as the pressure of water inside cells (turgor pressure) presses outwards against the cell wall, keeping the tissue turgid. Immersing plant tissues in solutions of different concentrations leads to changes described as turgid, flaccid, or plasmolysis. Dialysis tubing is a common material used to investigate these osmotic properties experimentally.
Active transport is the movement of molecules or ions against a concentration gradient, from a region of lower concentration to a region of higher concentration. This process requires energy released during respiration and occurs through the cell membrane. A primary example of active transport is the uptake of ions by root hair cells in plants.
Biological Molecules and Enzyme Action
Biological molecules are composed of specific chemical elements: carbohydrates and lipids contain carbon, hydrogen, and oxygen; proteins contain carbon, hydrogen, oxygen, and nitrogen; and DNA contains carbon, hydrogen, oxygen, nitrogen, and phosphorus. Large biological molecules are polymers built from smaller subunits: starch, cellulose, and glycogen are made from glucose; proteins from amino acids; lipids from fatty acids and glycerol; and DNA from nucleotides.
Standardized chemical tests are used to identify these molecules. Starch is tested using iodine solution, while glucose and maltose are identified with Benedict’s solution. Proteins are detected via the biuret test, and lipids are identified using the ethanol emulsion test.
Enzymes are proteins that function as biological catalysts, substances that increase the rate of chemical reactions without being changed themselves. They are involved in all metabolic reactions. Enzyme action is explained by the "lock and key" hypothesis, where a specific substrate fits into the complementary shape of the enzyme's active site to form an enzyme-substrate complex, resulting in a product. The activity of enzymes is significantly affected by temperature and pH. Changes in these factors influence the kinetic energy of molecules and the frequency of effective collisions; however, extreme deviations can lead to denaturation, where the enzyme’s shape is permanently altered, and it can no longer fit the substrate.
Plant Nutrition and Leaf Anatomy
Photosynthesis is the process by which plants synthesize carbohydrates from raw materials (carbon dioxide and water) using light energy. Chlorophyll, a green pigment found in chloroplasts, transfers light energy into chemical energy to form glucose. The resulting carbohydrates are used for respiration, storage as starch, building cell walls (cellulose), or transport as sucrose. The process is summarized by the word equation: carbon dioxide + water → glucose + oxygen, and the balanced chemical equation: .
Factors that limit the rate of photosynthesis include light intensity, carbon dioxide concentration, and temperature. Leaves are adapted for this process by having a large surface area and being thin. Internal structures include the cuticle, upper and lower epidermis, stomata with guard cells, spongy and palisade mesophyll cells, air spaces, and vascular bundles containing xylem and phloem. Mineral nutrition is also essential: nitrate ions are required for protein production (via amino acids), and magnesium ions are necessary for the synthesis of chlorophyll.
Transport and Nutrition in Flowering Plants
Water and ions are taken up by root hair cells, whose structure is specialized for this function. Water travels through the root cortex into the xylem, which transports it through the stem to the mesophyll cells of the leaf. Xylem vessels are specialized for transport and support, featuring thick walls reinforced with lignin, no cell contents, and cells joined end-to-end without cross walls to form continuous tubes. Transpiration is the loss of water vapour from leaves; water evaporates from mesophyll surfaces into air spaces and diffuses out through stomata. This creates a transpiration pull that draws a column of water upwards. Environmental factors like wind speed, temperature, humidity, and light intensity affect the transpiration rate.
Translocation is the movement of sucrose and amino acids through the phloem from sources (production/release sites) to sinks (use/storage sites). In non-woody dicotyledonous plants, the positions of xylem, phloem, and cortex vary between the root and the stem.
Human Nutrition and the Digestive System
A balanced diet requires carbohydrates, lipids, proteins, vitamins (C and D), minerals (calcium and iron), fibre, and water. Deficiencies lead to specific diseases: scurvy (Vitamin C), rickets (Vitamin D or Calcium), and anaemia (Iron). Digestion involves both physical breakdown (increasing surface area) and chemical breakdown (enzymatic conversion of large molecules to small ones).
The digestive tract includes the mouth, salivary glands, oesophagus, stomach, small intestine (duodenum and ileum), pancreas, liver, gall bladder, and large intestine (colon, rectum, and anus). Specific enzymes facilitate chemical digestion: amylase breaks starch into maltose; maltase breaks maltose into glucose; proteases (pepsin and trypsin) break proteins into amino acids; and lipase breaks lipids into fatty acids and glycerol. The stomach uses hydrochloric acid to kill bacteria and provides the optimal pH for pepsin. Bile, produced in the liver and stored in the gall bladder, emulsifies fats to facilitate lipase action. Peristalsis moves food through the system via muscular contractions.
Absorption occurs primarily in the small intestine. Villi and microvilli increase the surface area for the movement of nutrients into the blood or lacteals via diffusion, osmosis, or active transport. Assimilation is the subsequent uptake and use of these nutrients by cells. The hepatic portal vein transports absorbed molecules from the ileum to the liver.
Human Gas Exchange and Respiration
Gas exchange surfaces in humans (the alveoli) are characterized by a large surface area, thin walls, and a rich supply of blood and air. The respiratory system includes the larynx, trachea, lungs, bronchi, bronchioles, and alveoli. Breathing is driven by volume and pressure changes in the thorax caused by the ribs, intercostal muscles, and the diaphragm. Goblet cells and ciliated cells protect the system by trapping and removing pathogens and particles via mucus.
Respiration is the chemical reaction in cells that releases energy from glucose. Aerobic respiration requires oxygen and releases a large amount of energy: . Anaerobic respiration releases less energy without oxygen. In humans, it produces lactic acid, leading to an "oxygen debt" (EPOC) that must be repaid after exercise through sustained high heart and breathing rates to process the lactic acid in the liver. In yeast, anaerobic respiration produces alcohol and carbon dioxide.
The Human Circulatory System
Humans possess a double circulation system where blood passes through the heart twice for every circuit: once at low pressure to the lungs and once at high pressure to the body. The heart consists of atria and ventricles separated by a septum, with atrioventricular and semilunar valves ensuring one-way flow. The left ventricle has thicker walls than the right to pump blood to the whole body. Coronary heart disease (CHD) involves the blockage of coronary arteries, influenced by factors like diet, smoking, and stress.
Blood vessels include arteries (thick walls, high pressure), veins (thinner walls, valves, low pressure), and capillaries (one-cell thick for exchange). Blood components include red blood cells (oxygen transport), white blood cells (phagocytes and lymphocytes for immunity), platelets (clotting via fibrinogen to fibrin conversion), and plasma (transport of nutrients, ions, and waste).
Disease, Immunity, and Excretion
Pathogens are disease-causing organisms. Transmissible diseases can spread through direct or indirect contact. The body defends itself with barriers like skin and stomach acid. Malaria is transmitted by mosquitoes (vectors), while cholera is a water-borne bacterial disease where a toxin causes massive water loss (diarrhoea) through osmotic imbalance. HIV is a virus that destroys lymphocytes, potentially leading to AIDS. Active immunity is gained through infection or vaccination, producing memory cells for long-term protection. Passive immunity is short-term and acquired from another individual (e.g., via breast milk). Antibiotics treat bacterial infections but are ineffective against viruses.
Excretion is the removal of toxic metabolic waste, primarily carbon dioxide (lungs) and urea (kidneys). Urea is produced in the liver through deamination of excess amino acids. The kidney filters blood in nephrons (consisting of the glomerulus and Bowman's capsule) to remove urea, excess salts, and water as urine while reabsorbing glucose and necessary water.
Coordination and Control
The nervous system consists of the Central Nervous System (CNS - brain and spinal cord) and the Peripheral Nervous System (PNS). Electrical impulses travel along sensory, relay, and motor neurones. Reflex arcs provide rapid, automatic responses. Synapses are junctions where neurotransmitters diffuse across a gap to bind with receptor proteins on the next neurone. The eye is a sense organ that focuses light using the cornea and lens onto the retina; its iris controls light entry (pupil reflex), and ciliary muscles allow for accommodation (viewing near/far objects).
Hormones are chemicals produced by glands (e.g., adrenal, pancreas, pituitary, testes, ovaries) and carried in the blood to target organs. Adrenaline prepares the body for action, while insulin and glucagon regulate blood glucose levels. Homeostasis maintains a constant internal environment via negative feedback, including temperature control (sweating, shivering, vasodilation/vasoconstriction) and blood glucose regulation.
Reproduction, Inheritance, and Selection
Plants respond to light (phototropism) and gravity (gravitropism) using the hormone auxin, which stimulates cell elongation. In genetics, chromosomes contain DNA and genes. Mitosis produces genetically identical cells for growth and repair, while meiosis produces genetically different haploid gametes. Asexual reproduction involves one parent and produces clones; sexual reproduction involves the fusion of haploid nuclei (fertilisation) to form a diploid zygote.
Flowers reproduce sexually through pollination (insect or wind). In humans, the male and female reproductive systems produce sperm and eggs. The menstrual cycle is regulated by hormones (FSH, LH, oestrogen, progesterone). The placenta facilitates the exchange of materials between mother and fetus.
Inheritance involves the transmission of genetic information. DNA is a double helix with base pairs (A-T, C-G). Monohybrid crosses can be predicted using Punnett squares. Mutations are random changes in DNA (e.g., sickle cell anaemia) or chromosome number (e.g., Down’s syndrome). Natural selection drives evolution as better-adapted individuals pass on alleles. Artificial selection is used by humans to breed plants and animals with desirable traits.