Comprehensive IGCSE Biology Notes & Flashcards

Comprehensive vocabulary flashcards covering the human endocrine system, nervous coordination, the eye, plant tropisms, homeostasis, excretion, cardiovascular physiology, plant transport and transpiration, immunology, cellular respiration, photosynthesis, biological molecules, and human respiration.

Hormones

Chemical substances produced by endocrine glands and carried by the blood to alter the activity of one or more specific target organs, before being destroyed by the liver.

Endocrine Glands

Ductless glands that produce hormones and secrete them directly into the bloodstream.

Insulin

A hormone produced by the pancreas that lowers blood glucose levels by converting excess glucose into glycogen in the liver, increasing glucose respiration, and storing glucose as fat.

Glucagon

A hormone produced by the pancreas that raises blood glucose levels by converting stored glycogen into glucose, converting fats to glucose, and reducing glucose breakdown.

Accommodation

The adjustment in the shape and thickness of the eye lens by ciliary muscles and suspensory ligaments to focus light from objects at varying distances onto the retina.

Iris Reflex

An antagonistic muscular response controlling the size of the pupil: circular muscles contract and radial muscles relax in bright light to constrict the pupil; radial muscles contract and circular muscles relax in dim light to dilate the pupil.

Fovea

The central region of the retina with the highest concentration of cone cells, providing detailed, full-colour images when looking directly at an object.

Blind Spot

The region on the retina where the optic nerve leaves the eye, containing no photoreceptor cells (rods or cones) so no image can be formed.

Reflex Arc

The neural pathway followed by nerve impulses during an involuntary reflex action: stimulus $arrow$ receptor $arrow$ sensory neurone in dorsal root $arrow$ relay neurone in spinal cord grey matter $arrow$ motor neurone in ventral root $arrow$ effector $arrow$ response.

Synapse

A microscopic junction between two neurones across which electrical impulses are transmitted unidirectionally using chemical messengers called neurotransmitters.

Neurotransmitter

A chemical messenger stored in synaptic vesicles that diffuses across the synaptic cleft to bind with specific receptor proteins on the postsynaptic membrane.

Tropism

A directional growth movement of a plant in response to an external stimulus such as light or gravity.

Auxin

A plant growth hormone synthesized in shoot and root tips that regulates cell elongation; it diffuses to the shaded side of a shoot to stimulate elongation, but inhibits cell elongation in roots.

Clinostat

An apparatus with an electric motor that slowly rotates germinating seedlings horizontally to distribute the effect of gravity equally on all sides, serving as an experimental control in geotropism studies.

Homeostasis

The maintenance of a constant internal environment (including temperature, water, ions, glucose, and pH) in an organism despite external changes.

Negative Feedback

A homeostatic control mechanism in which a detected change away from the normal set point triggers a corrective response that reverses the change back to normal, whereupon the mechanism switches off.

Vasodilation

The widening of arterioles supplying skin surface capillaries, increasing blood flow through surface vessels to increase heat loss by radiation when body temperature rises.

Vasoconstriction

The narrowing of arterioles supplying skin surface capillaries, diverting blood through deeper shunt vessels to reduce heat loss by radiation in cold conditions.

Excretion

The removal from organisms of toxic waste products of metabolism and substances present in excess of requirements.

Nephron

The basic microscopic functional filtering unit of the kidney, consisting of a Bowman's capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct.

Ultrafiltration

The filtration of blood under high pressure within the glomerulus, forcing small molecules (water, glucose, amino acids, urea, and salts) through into the Bowman's capsule while retaining blood cells and large plasma proteins.

Selective Reabsorption

The uptake of useful substances from the glomerular filtrate back into the blood; glucose is reabsorbed at the proximal convoluted tubule by active transport, salts at the loop of Henle, and water at the distal convoluted tubule and collecting duct.

Osmoregulation

The homeostatic regulation of water and salt concentrations in blood and bodily fluids, controlled by the hypothalamus and pituitary release of antidiuretic hormone ($\text{ADH}$).

Mammalian Heart Structure

A four-chambered muscular pump composed of cardiac muscle, consisting of two thin-walled atria and two muscular ventricles, with a thicker left ventricular wall to pump blood at high pressure systemically.

Cardiac Cycle

The sequence of alternating contraction ($\text{systole}$) and relaxation ($\text{diastole}$) of the heart's atria and ventricles that pumps blood through the body.

Coronary Heart Disease (CHD)

A condition in which coronary arteries are narrowed or blocked by fatty deposits ($\text{plaque}$ containing cholesterol), restricting oxygen and glucose delivery to cardiac muscle and leading to angina or a myocardial infarction.

Artery

A thick-walled, muscular, and elastic blood vessel that carries oxygenated blood (except the pulmonary artery and umbilical artery) away from the heart under high pressure, having a narrow lumen and no valves.

Vein

A thin-walled blood vessel with a wide lumen and semilunar valves that carries deoxygenated blood (except the pulmonary vein and umbilical vein) toward the heart under low pressure.

Capillary

A microscopic, permeable blood vessel with walls only one endothelial cell thick, allowing rapid exchange of gases, nutrients, and metabolic wastes between blood and tissue fluid.

Double Circulatory System

A circulatory system in which blood passes through the heart twice during each complete circuit: once through the pulmonary circulation to the lungs and once through the systemic circulation to the body.

Transpiration

The evaporation of water vapour from the aerial surfaces of a plant, primarily through open stomata on the leaves.

Transpiration Stream

The continuous movement of water from the soil into root hairs by osmosis, up through the stem xylem vessels via cohesive forces, and out through the leaf stomata into the atmosphere.

Potometer

An experimental apparatus used to estimate the rate of transpiration by measuring the distance an air bubble moves in a capillary tube over a set period of time.

Translocation

The energy-requiring transport of sucrose and amino acids through the sieve tube elements of living phloem tissue from sources to sinks.

Blood Plasma

The pale-yellow liquid component of blood ($55\%$ by volume, containing $90\%$ water) that transports dissolved substances including $CO_2$, digested nutrients, urea, hormones, and heat energy.

Erythrocytes (Red Blood Cells)

Biconcave, anucleate blood cells formed in bone marrow that contain haemoglobin to bind and transport oxygen reversibly as oxyhaemoglobin throughout the body.

Phagocytosis

The active process by which phagocytes engulf pathogens using pseudopodia, enclosing them within a vacuole where lysosomal enzymes digest and destroy them.

Lymphocytes

White blood cells with large, round nuclei that mediate adaptive immunity by producing specific antibodies matching the surface antigens of invading pathogens.

Vaccine

A biological preparation of dead, weakened, or fragmented pathogens or antigens administered to stimulate primary antibody production and the formation of memory cells without causing disease.

Memory Cells

Long-lived lymphocytes formed during an initial immune response that remain in the blood to mount a faster and larger secondary antibody response upon re-infection.

Thrombocytes (Platelets)

Tiny, anucleate cell fragments produced in bone marrow that initiate blood clotting by releasing clotting factors to seal wounds, preventing haemorrhage and pathogen entry.

Fibrin

An insoluble, fibrous protein formed when thrombin converts soluble fibrinogen; it forms a mesh net that traps red blood cells and platelets to produce a blood clot.

Aerobic Respiration

The breakdown of glucose in the presence of oxygen within cells to yield carbon dioxide, water, and a high yield of $ATP$: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$.

Anaerobic Respiration

The cellular breakdown of glucose without oxygen; produces lactic acid in animals ($\text{Glucose} \rightarrow \text{Lactic acid}$) and ethanol plus carbon dioxide in yeast ($\text{Glucose} \rightarrow \text{Ethanol} + CO_2$).

Compensation Point

The specific light intensity at which the rate of photosynthesis exactly equals the rate of respiration in a plant, resulting in zero net gas exchange of $O_2$ and $CO_2$.

Hydrogen-Carbonate Indicator

A pH-sensitive indicator that changes colour according to dissolved carbon dioxide levels: red at normal atmospheric levels, purple at low $CO_2$ levels (photosynthesis > respiration), and yellow at high $CO_2$ levels (respiration > photosynthesis).

Basal Metabolic Rate (BMR)

The minimum daily amount of energy required by the body to sustain basic physiological life processes (such as breathing and cellular maintenance) at complete rest.

Calorimeter

An insulated piece of apparatus used to measure the energy released by burning a food sample, minimising heat loss to the surroundings to obtain accurate energy content values.

Photosynthesis

An endothermic process occurring in chloroplasts wherein plants manufacture carbohydrates from carbon dioxide and water using light energy absorbed by chlorophyll: $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$.

Limiting Factor

An environmental condition or resource present in such short supply that it directly restricts the rate of a physiological process, such as photosynthesis.

Inverse Square Law

The physical principle stating that light intensity is inversely proportional to the square of the distance from the source: $\text{Light intensity} \propto \frac{1}{\text{distance}^2}$.

Diffusion

The passive net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.

Osmosis

The passive net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

Active Transport

The movement of substances across a biological membrane from a region of lower concentration to higher concentration against a concentration gradient, requiring energy in the form of $ATP$.

Active Site

The specific region of an enzyme molecule with a unique complementary shape into which the substrate binds to form an enzyme-substrate complex.

Denaturation

The irreversible alteration of an enzyme's tertiary structure and active site shape caused by extreme temperature or pH, preventing substrate binding and catalysed reactions.

Monosaccharide

The simplest single-sugar unit of carbohydrates, sweet and water-soluble, classified as pentoses ($5C$) or hexoses ($6C$), including glucose, fructose, and galactose.

Peptide Bond

The covalent chemical bond linking adjacent amino acid monomers together in a polypeptide or protein chain.

Deamination

The biochemical process occurring in the liver where excess amino acids have their nitrogen-containing amino group removed and converted into urea for excretion.

Stem Cells

Undifferentiated biological cells capable of continuous cell division and differentiating into specialized cell types.

Meristem

Regions of unspecialized plant tissue located near root and shoot tips that retain the ability to divide by mitosis and produce new differentiated cells throughout the life of the plant.

Saprotrophic Nutrition

A mode of heterotrophic nutrition characteristic of fungi involving extracellular secretion of digestive enzymes onto dead organic matter, followed by absorption of soluble breakdown products.

Mycelium

The vegetative network of branch-like, thread-like feeding filaments called hyphae that make up the body of a multicellular fungus such as Mucor.

Virus

A microscopic, non-living, parasitic particle composed of a protein coat surrounding either DNA or RNA, capable of reproducing only within living host cells.

Human Respiratory System

The organ system responsible for gas exchange, comprising the nasal passages, pharynx, larynx, cartilage-reinforced trachea, bronchi, bronchioles, lungs, and alveoli surrounded by pleural membranes.

Alveoli

Microscopic, pouch-like air sacs at the ends of bronchioles that act as the primary gas exchange surface in humans; adapted with a one-cell thick wall, moist lining, dense capillary network, and huge total surface area.

Ventilation

The mechanical movement of air into and out of the lungs driven by pressure differences created by the antagonistic action of internal and external intercostal muscles and the diaphragm.

Emphysema

A chronic lung disease often caused by tobacco smoke where alveolar walls break down and fuse into enlarged, irregular air spaces, severely reducing the surface area available for gas exchange.

Palisade Mesophyll

A densely packed upper layer of columnar cells in the leaf containing numerous chloroplasts to maximise light absorption for photosynthesis.

Spongy Mesophyll

A lower layer of loosely arranged, irregular cells with large intercellular air spaces that facilitate rapid gas diffusion of carbon dioxide and oxygen within the leaf.