CIE IGCSE Biology 0610 Extended Syllabus Comprehensive Study Guide

Characteristics and Classification of Living Organisms

All living organisms are defined by seven fundamental characteristics. Movement is an action by an organism or part of an organism causing a change of position or place. Respiration describes the chemical reactions that break down nutrient molecules in living cells to release energy. Sensitivity refers to the ability to detect or sense changes in the environment, known as stimuli, and to make appropriate responses. Growth is defined as a permanent increase in size and dry mass by an increase in cell number, cell size, or both. Reproduction comprises the processes that make more of the same kind of organism. Excretion involves the removal from organisms of toxic materials, the waste products of metabolism, which include chemical reactions in cells like respiration, and substances in excess of requirements. Nutrition is the taking in of nutrients, which are organic substances and mineral ions containing raw materials or energy for growth and tissue repair; these are absorbed and assimilated by the organism.

Organisms are classified into groups based on the features they share, with classification systems aiming to reflect evolutionary relationships, which are changes in adaptive features of a population over time resulting from natural selection. A species consists of organisms that can reproduce successfully together. Traditionally, classification was based on morphology, the overall form and shape of bodies such as wings or legs, and anatomy, the detailed body structure determined by dissection. The binomial system is used for naming species, where the scientific name consists of two parts: the genus, which starts with a capital letter, and the species, which starts with a lower-case letter. When printed, these names are in italics, such as Homo sapiens. The taxonomic hierarchy follows the order of Kingdom, Phylum, Class, Order, Family, Genus, and Species, often remembered by the mnemonic King Philip Came Over For Good Spaghetti. As one moves from the Kingdom level toward the Species level, the similarity between organisms increases.

Modern classification also relies on DNA, the chemical from which chromosomes are made. Each DNA molecule consists of strings of smaller molecules containing four bases. Biologists compare the sequences of these bases in different species; the more similar the base sequence, the more closely related the species are. Organisms sharing a more recent ancestor have base sequences in DNA that are more similar than those sharing only a distant ancestor. This use of base sequences in DNA and amino acids in proteins provides a more accurate means of classification known as cladistics.

Living organisms are categorized into five kingdoms. Animals are multi-cellular ingestive heterotrophs that eat living organisms. Plants are multi-cellular photosynthetic autotrophic organisms possessing a cellulose cell wall. Fungi can be single-celled or multi-cellular heterotrophic organisms with cell walls not made of cellulose; they spread via spores in moist, dark, warm environments and act as saprotrophs, feeding off dead organisms, or parasites. Prokaryotes are single-celled organisms with no true nucleus. Protocists are single-celled organisms that do possess a nucleus.

Vertebrates can be remembered by the mnemonic MR FAB. Mammals have fur or hair on their skin, can live on land and in water, possess $4$ legs, use lungs to breathe, and give birth to live young. Reptiles have scales on their skin, usually have $4$ legs, use lungs to breathe, and lay hard eggs. Fish possess wet scales, utilize external fertilization to produce soft eggs, and use gills to breathe. Amphibians have smooth, moist skin, use external fertilization for soft eggs, can live on land and in water using both gills and lungs, and have $4$ legs. Birds have feathers on their bodies and scales on their legs, two legs and two wings, use lungs to breathe, and lay hard eggs.

Arthropods are invertebrates with legs, remembered by the mnemonic CAMI. Crustaceans, such as crabs, have an exoskeleton, one pair of compound eyes, two body segments consisting of a cephalothorax and abdomen, more than four pairs of legs, and two pairs of antennae sensitive to touch and chemicals. Arachnids, such as spiders, have two body segments, four pairs of legs, a pair of chelicerae to hold prey, two pedipalps for reproduction, and simple eyes. Myriapods, like centipedes, have a segmented body with additional segments formed, one pair of antennae, $70+$ pairs of legs with one or two pairs on each segment, a fused head and thorax, a segmented abdomen, and simple eyes. Insects, such as bees, have three body segments consisting of the head, thorax, and abdomen, three pairs of legs, one pair of antennae, one or two pairs of wings, and both compound and simple eyes.

Plants are classified into ferns and flowering plants. Ferns do not produce flowers; they are plants with roots, stems, and leaves called fronds and reproduce by spores. Flowering plants possess roots, stems, and leaves and reproduce sexually via flowers and seeds produced inside an ovary. They are divided into Monocotyledons, which have one cotyledon, parallel veins, fibrous roots, and floral parts in multiples of $3$, and Dicotyledons, which have two cotyledons, netlike veins, a taproot, and floral parts in multiples of $4$ or $5$.

Viruses and bacteria differ significantly in structure. Viruses are covered by a protein coat, whereas bacteria have a cell wall. Viruses lack a cell membrane and cytoplasm, while bacteria possess both. Viruses contain DNA or RNA with only a few genes, while bacteria contain DNA sufficient for several $100$ genes. Viruses are considered non-living unless within a host, while bacteria are living. To identify organisms, biologists use dichotomous keys, which provide a series of choices between two visible features, leading the user through a sequence until the organism is narrowed down to its genus and species.

Organization of the Organism

All living things are composed of cells. Typical cells contain several organelles: a cell membrane that is differentially or partially permeable to control the entry and exit of substances; cytoplasm where chemical reactions occur; a nucleus containing DNA to control the cell; mitochondria where aerobic respiration happens; and ribosomes, which make protein and float in the cytoplasm. Animal cells, such as liver cells, contain these parts. Plant cells, like palisade cells, contain additional structures: a vacuole to store food and water and maintain cell shape; a rigid cell wall to keep the cell's shape; and chloroplasts containing chlorophyll to absorb light energy for photosynthesis.

Levels of organization start with organelles, which are specialized parts of a cell with specific functions. A cell is the smallest part of a living structure that can operate as an independent unit. A tissue is a group of cells with similar structures working together for a shared function, such as muscle tissue. An organ is a structure made of a group of tissues working together for specific functions, like the heart. An organ system is a group of organs with related functions working together, such as the respiratory system. The magnification of a specimen is calculated using the formula: $Magnification = \frac{\text{size of drawing}}{\text{size of specimen}}$, or $M = \frac{I}{A}$.

Specialized cells are adapted for specific functions. Red blood cells transport oxygen and have a biconcave shape, no nucleus, flexibility, and contain haemoglobin. Muscle cells contract to bring structures closer and are long with many protein fibres in the cytoplasm. Ciliated cells use tiny hairs called cilia to move and push mucus. Root hair cells absorb minerals and water using an elongated shape to increase surface area. Xylem vessels transport water and support the plant; they have no cytoplasm, no cross walls to form a continuous tube, and are waterproofed with lignin. Palisade cells photosynthesize using a regular shape to fit many in a small space and containing many chloroplasts.

Movement In and Out of Cells

Diffusion is the movement of molecules from a region of high concentration to a region of low concentration down a concentration gradient, resulting in random movement until equilibrium. It is vital for moving oxygen, glucose, and carbon dioxide where they are needed for life. Factors that speed up diffusion include a larger concentration gradient, higher temperature, and a smaller surface area. Water is essential as a solvent in these processes.

Osmosis is the movement of water molecules from a region of high water potential to a region of low water potential through a partially permeable membrane. If the solute concentration is equal inside and outside a cell, there is no change. If the concentration is higher outside, the cell shrinks, a process called plasmolysis in plants or crenation in animals, which can lead to animal cells shriveling. If the concentration is lower outside, the cell swells. Plant cells become turgid as the vacuole fills, while animal cells may explode. In plants, a decrease in outside solute causes the cell to become flaccid, lose water, and pull the cell body away from the cell wall.

Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration against a concentration gradient, using energy released during respiration. This is necessary for organisms to optimize nutrient uptake, such as ion uptake by root hairs or glucose uptake by the epithelial cells of villi in the human digestive system.

Biological Molecules

Biological molecules are composed of specific elements. Carbohydrates and fats are made of carbon, hydrogen, and oxygen (CHO). Proteins consist of carbon, hydrogen, oxygen, nitrogen, and sometimes sulfur (CHONS). The basic monomers for these macromolecules are simple sugars for starch and glycogen, fatty acids and glycerol for fats and oils, and amino acids for proteins. Chemical tests identify these substances: the iodine test for starch turns blue-black; Benedict’s reagent heated for $2$ to $3$ minutes turns from blue to a brick-red precipitate for reducing sugars; Biuret reagent turns mauve for proteins; and the emulsion test using ethanol and distilled water produces a milky-white emulsion for fats. The decolorization of DCPIP confirms the presence of Vitamin C.

DNA consists of two strands coiled to form a double helix, making up chromosomes. Each strand contains chemicals called bases that form cross-links between the strands. These bases always pair specifically: $A$ with $T$ and $C$ with $G$.

Enzymes

Enzymes are proteins that function as biological catalysts, substances that speed up chemical reactions without being changed by them. They work by lowering the activation energy required for a reaction. The Lock and Key theory explains how substrates—the molecules before the reaction—bind to the enzyme's active site to form products. Catabolic reactions break molecules down, while anabolic reactions combine them. Enzymes have an optimum temperature, typically around $37\,^{\circ}C$ in animals, where they work fastest. If the temperature is too high, the enzyme denatures, losing its shape and ability to bind with substrates. If too low, molecules lack kinetic energy for collisions. Enzymes are also sensitive to pH, having an optimum pH; changes in pH can cause denaturation.

In nature, enzymes help seeds germinate by turning insoluble food stores into soluble ones. In industry, biological washing powders use lipases to remove grease and proteases to remove blood stains. In the food industry, isomerase converts glucose to sweeter fructose, and pectinase breaks down fruit cell walls to increase juice yield, lower viscosity, and reduce cloudiness.

Plant Nutrition

Photosynthesis is the process by which plants manufacture carbohydrates from raw materials using light energy: $6CO_2 + 6H_2O \xrightarrow{\text{light + chlorophyll}} C_6H_{12}O_6 + 6O_2$. Carbon dioxide diffuses through stomata, and water is taken up by roots. Chlorophyll traps light energy and converts it into chemical energy for carbohydrate formation and storage. Experiments can prove the requirements for photosynthesis. To show chlorophyll is needed, a variegated leaf is destarched for $48$ hours in darkness, exposed to light, boiled in water for $2$ minutes, warmed in ethanol to extract chlorophyll, and tested with iodine. Parts with chlorophyll turn blue-black. Light is proven necessary using a stencil; parts without light remain yellow/brown. Carbon dioxide's role is shown using sodium bicarbonate (NaHCO3) to produce CO2 in one jar and sodium hydroxide (NaOH) to absorb it in another; only the plant with CO2 produces starch.

Limiting factors for photosynthesis include light intensity, carbon dioxide concentration, and temperature. As light or CO2 Increases, the rate increases until another factor becomes limiting. Temperature increases the rate until the optimum of approximately $40\,^{\circ}C$, after which enzymes denature and the rate drops. Glasshouse systems optimize yield by burning paraffin to triple CO2 levels, using thermostatically controlled heaters, and providing high-intensity light with correct wavelengths (red and blue).

Leaf structure is adapted for photosynthesis. The waxy cuticle prevents water loss from the top. The transparent epidermis allows sunlight through. The palisade layer at the top contains many chloroplasts. The spongy mesophyll has irregular cells creating air spaces for gas exchange. The vascular bundle contains the xylem, which transports water and minerals through lignified cellulose walls, and the phloem, which transports nutrients. Stomata are holes that open during the day as guard cells swell and close at night to prevent water loss. Xylem is unidirectional, while phloem is bidirectional, using sieve elements for sugar transport and companion cells to provide energy for active transport ($ATP$ used to load and unload sucrose). Minerals are also required: Nitrogen is needed for protein synthesis (deficiency causes small plants and yellow leaves); Magnesium is needed for chlorophyll synthesis (deficiency causes yellow leaves). Nitrogen fertilizers supply nitrate, nitrite, or ammonium ions but can cause eutrophication if they leach into stagnant water.

Human Nutrition

A balanced diet provides all nutrients in correct proportions, varying by age, sex, and activity. Children under $12$ need calcium; teenagers require the highest calories; pregnant women need extra iron, calcium, and folic acid. Malnutrition results from an unbalanced diet, including overnutrition, undernutrition, or incorrect proportions. Effects include starvation, coronary heart disease (from excess saturated fat and cholesterol), constipation (lack of fiber/roughage), and obesity (excess fats/carbs leading to weight gain, stroke, or high blood pressure). Specific nutrients include: Carbohydrates for energy; Fats for energy, insulation, and hormones; Proteins for growth, enzymes, and antibodies; Vitamin C to protect cells and produce fibers (deficiency causes scurvy); Vitamin D for calcium absorption (deficiency causes rickets); Calcium for teeth and bones (deficiency causes rickets and poor clotting); Iron for hemoglobin (deficiency causes anaemia and fatigue); Fiber for peristalsis; and Water as a solvent.

The human alimentary canal facilitates ingestion (taking in food), digestion (breaking down molecules), and egestion (passing out faeces). The mouth performs mechanical digestion and mixes food with salivary amylase. The oesophagus uses peristalsis to move food to the stomach. The stomach uses pepsin and hydrochloric acid ($HCl$) to break down proteins and kill bacteria. The small intestine consists of the duodenum—where bile emulsifies fats and pancreatic enzymes (amylase, trypsin, lipase) digest nutrients—and the ileum, where maltase breaks down maltose to glucose and absorption occurs via villi and microvilli. The pancreas produces digestive juices. The liver produces bile, stores glucose as glycogen, and breaks down toxins. Bile is stored in the gall bladder. The large intestine consists of the colon for water reabsorption and the rectum for temporary faeces storage. Diarrhoea occurs when too little water is absorbed; cholera bacteria can cause this by secreting toxins that move chloride ions into the gut, drawing water out by osmosis. Treatment involves oral rehydration therapy.

Teeth perform mechanical digestion. Incisors are for cutting, canines for holding, and premolars/molars for crushing. A tooth consists of enamel (hardest tissue), dentine (calcium salts on collagen), a pulp cavity (nerves and blood vessels), and cement (anchoring). The neck lies between the crown and root. Tooth decay is prevented by low-sugar diets and regular brushing to remove plaque. Chemical digestion uses enzymes: Amylase breaks starch to maltose; Protease (pepsin and trypsin) breaks proteins to peptides then amino acids; Lipase breaks lipids to fatty acids and glycerol. Villi Increase surface area for absorption into capillaries (glucose/amino acids) and lacteals (fats). The small intestine absorbs $5-10\,dm^3$ and the colon absorbs $0.3-0.5\,dm^3$ of water daily.

Transport in Plants and Animals

In plants, root hair cells absorb water by osmosis and ions by active transport. Water travels through the cortex, across the endodermis, and into the xylem. Transpiration is the evaporation of water at mesophyll surfaces and loss through stomata. This creates a transpiration stream, drawing water up the stem due to tension and the cohesion of water molecules. Factors increasing transpiration include higher temperature, lower humidity, and higher light intensity. Translocation moves sucrose and amino acids in phloem from sources to sinks. In spring, sucrose moves from roots to leaves; in summer, it moves from leaves to roots.

The human circulatory system is a double circulation system where blood passes through the heart twice; once to the lungs for oxygenation and once to the rest of the body. Fish have a single circulation system with two heart chambers. The human heart contains the right atrium and ventricle (deoxygenated blood), left atrium and ventricle (oxygenated blood), septum, valves (tricuspid, bicuspid, pulmonary, aortic), and associated vessels (aorta, pulmonary artery/vein). The cardiac cycle moves from diastole (relaxation) to atrial systole and then ventricular systole. Heart activity is monitored by $ECG$, pulse rate, or valve sounds. Coronary Heart Disease ($CHD$) involves blocked coronary arteries and is linked to smoking, diet, and stress. Arteries carry high-pressure blood away from the heart and have thick elastic walls; veins carry low-pressure blood to the heart and have valves; capillaries are one cell thick for diffusion. Blood consists of red blood cells (oxygen transport), white blood cells (phagocytes for engulfing pathogens and lymphocytes for antibodies), platelets (clotting via fibrinogen to fibrin mesh), and plasma.

The immune system uses mechanical barriers (nose hair, skin), chemical barriers (mucus, stomach acid), and cells. Active immunity is gained via infection or vaccination using harmless pathogens to trigger memory cells. Passive immunity is short-term, such as a baby receiving antibodies via breast milk. Type $1$ diabetes is an autoimmune disease where the immune system destroys insulin-secreting cells.

Gas Exchange, Respiration, and Excretion

Gas exchange surfaces must be thin, large, moist, well-ventilated, and close to a blood supply. Inspired air contains $21\%$ oxygen and $0.04\%$ carbon dioxide, while expired air contains $18\%$ oxygen and $3\%$ carbon dioxide. Lung ventilation is driven by the diaphragm and intercostal muscles. During inspiration, external intercostals contract, the diaphragm moves up, volume increases, and pressure falls. Physical activity increases breathing rate and tidal volume, measured by a spirogram. Aerobic respiration releases large energy: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$. Anaerobic respiration occurs without oxygen. In muscles, it produces lactic acid ($C_6H_{12}O_6 \rightarrow 2C_3H_6O_3$), leading to an oxygen debt. In yeast, it produces ethanol and CO2 ($C_6H_{12}O_6 \rightarrow 2C_2H_5OH + CO_2$).

Excretion in humans is managed by the kidneys, which remove urea, excess water, and reabsorb glucose. Each kidney has a cortex (Bowman’s capsules), medulla (loops of Henle), and pelvis leading to the ureter. Ultrafiltration occurs in the glomerulus, forcing water, urea, salts, and glucose into the capsule. Selective reabsorption in the proximal tubule returns all glucose and some salts to the blood. Dialysis machines can clean blood as an alternative to kidney transplants. Transplants offer more freedom but carry rejection risks.

Coordination, Response, and Drugs

The nervous system consists of the Central Nervous System ($CNS$) and Peripheral Nervous System ($PNS$). Involuntary reflex actions protect the body, involving a receptor, sensory neurone, relay neurone (in the $CNS$), motor neurone, and effector. Synapses are gaps where impulses pass via neurotransmitters. The eye adjusts for distance (accommodation) via ciliary muscles and for light (pupil reflex) via circular and radial muscles. Rods are for low light and black-and-white; cones are for detail and color. Hormones like adrenaline prepare the body for action by increasing glucose and pulse rate. The pancreas regulates blood glucose using insulin (decreases glucose) and glucagon (increases glucose). Negative feedback loops maintain homeostasis, including thermoregulation where the hypothalamus controls sweating, shivering, vasodilation, and vasoconstriction. Plants show tropic responses like gravitropism and phototropism controlled by auxin.

Drugs modify chemical reactions. Antibiotics kill bacteria but not viruses; resistant strains like $MRSA$ must be managed by finishing treatments. Heroin is a depressant affecting synapses and causing addiction. Alcohol causes liver cirrhosis. Tobacco smoke contains nicotine (addictive stimulant), tar (cancer-causing irritant), and carbon monoxide (binds to haemoglobin). Emphysema reduces lung surface area. Anabolic steroids used in sports can cause kidney failure and mood swings.

Reproduction, Inheritance, and Environment

Asexual reproduction produces genetically identical offspring, as seen in binary fission of bacteria or spore formation in fungi. Sexual reproduction involves the fusion of haploid gamete nuclei to form a diploid zygote, creating genetic variation. In plants, pollination can be insect-assisted or wind-assisted. Human reproduction involves the male system (testes, sperm duct, prostate, urethra, penis) and female system (ovary, oviduct, uterus, cervix, vagina). The menstrual cycle is controlled by $FSH$, $LH$, oestrogen, and progesterone. Fertilization occurs in the oviduct. The fetus develops in the uterus, supported by the placenta and umbilical cord. Birth control methods include natural, chemical (pills), mechanical (condoms, $IUD$), and surgical (vasectomy) means. $HIV$ destroys lymphocytes, weakening the immune system.

Inheritance involves the transmission of genes, which are lengths of DNA coding for proteins. Mitosis is for growth and repair, yielding identical cells. Meiosis produces haploid gametes with genetic variation. Monohybrid inheritance uses terms like genotype, phenotype, homozygous, heterozygous, dominant, and recessive. Co-dominance exists in blood groups ($IA, IB, IO$). Variation can be continuous (height) or discontinuous (blood group). Sickle cell anaemia is a recessive disorder; carriers are resistant to malaria. Natural selection leads to evolution as the fittest survive and reproduce. Artificial selection is used by humans for economic value.

Energy enters ecosystems via the sun and flows through food chains (Producer $\rightarrow$ Primary Consumer $\rightarrow$ Secondary Consumer). Energy transfer is inefficient, with only about $10\%$ reaching the next level. The carbon, nitrogen, and water cycles recycle nutrients. Human populations grow exponentially due to better medicine and nutrition, but limiting factors like famine and disease control it. Humans influence ecosystems through habitat destruction, deforestation (causing soil erosion and flooding), and pollution (acid rain from $SO_2$, eutrophication from fertilizers, and global warming from $CO_2$ and methane). Conservation efforts aim for sustainable development, recycling, and protecting endangered species through seed banks and captive breeding.