Edexcel International GCSE (9-1) Biology: Comprehensive Notes (Units 1–6)
UNIT 1: ORGANISMS AND LIFE PROCESSES
All living organisms are composed of cells. Basic life processes operate within cells; cells are the building blocks of life.
The eight life processes (in most organisms): nutrition, respiration, excretion, response to stimuli, movement, control of internal conditions (homeostasis), reproduction, growth and development.
CELL STRUCTURE AND ORGANELLES
Common features in all cells: cytoplasm, nucleus, cell membrane, mitochondria, ribosomes.
Plant-specific structures: cell wall (made of cellulose), permanent/large vacuole, chloroplasts (contain chlorophyll).
Animal cells lack a cell wall and chloroplasts; bacteria have simpler cells not covered here.
Nucleus: contains chromosomes (genes) that control cellular activities; DNA remains in nucleus; instructions for making proteins travel to cytoplasm to be assembled by ribosomes.
Cytoplasm: jelly-like interior where most metabolic reactions occur; enzymes catalyse reactions.
Cell membrane: selectively permeable; controls movement of substances into and out of the cell; acts as a boundary.
Mitochondria: site of respiration, energy release; more mitochondria where energy demand is high.
Ribosomes: site of protein synthesis.
Plant cell specifics: chloroplasts for photosynthesis, wall structure provides shape/support, large central vacuole for storage, supports turgor.
Key POINT: nearly all cells contain cytoplasm, a nucleus, a cell membrane and mitochondria; plant cells have a cell wall, a permanent vacuole, and chloroplasts if they photosynthesize.
ENZYMES: CONTROLLING REACTIONS IN THE CELL
Enzymes are biological catalysts that speed up reactions without being used up.
Substrate binds to the enzyme’s active site; the substrate fits like a key in a lock (lock-and-key model).
Enzymes are highly specific, catalysing only one type of reaction.
Enzyme reactions depend on genes in the nucleus that code for enzymes; genes → proteins (enzymes) → catalyse reactions.
Many reactions in cells are metabolic reactions; metabolism is the sum of all metabolic reactions in the cell.
Some enzymes act outside cells (extracellular) in digestion; most act inside cells (intracellular).
Secretion: release of fluids/substances from a cell or tissue.
KEY POINT: metabolism relies on enzymes; the activity of enzymes is crucial for life processes.
ENZYME ACTION IN DETAIL
The molecule that an enzyme acts on is called the substrate; the enzyme has an active site where the substrate binds.
The binding lowers the activation energy required for the reaction, increasing rate.
Reactions can build larger molecules from smaller ones (biosynthetic pathways) or break them down.
Substrate fits the active site like a lock and key; products are released and enzyme can catalyse more reactions.
Figure reference: enzyme–substrate complex forms, products released.
FACTORS AFFECTING ENZYMES
Rate of enzyme-catalysed reactions can be increased by higher enzyme or substrate concentrations.
Temperature and pH affect enzyme activity.
OPTIMUM TEMPERATURE: the temperature at which an enzyme works best (e.g., human enzymes around 37°C).
Denaturation: above the optimum temperature, proteins (including enzymes) can be denatured; active site shape changes, substrate no longer fits.
pH: most enzymes work best at neutral pH (~7); extremes reduce activity by altering enzyme structure.
Some enzymes have unusual optimum pH (e.g., pepsin in the stomach with optimum ~pH 2).
KEY POINT: enzyme activity depends on temperature and pH, with optimum conditions for each enzyme.
ENERGY AND RESPIRATION
Cells require energy; respiration releases energy by oxidising glucose to form ATP (adenosine triphosphate).
ATP transfers energy to cellular processes (muscle contraction, active transport, building molecules, cell division).
The overall respiration equation (aerobic respiration):
\ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O (+ energy)}
The energy in ATP is released when a phosphate group is removed (ATP -> ADP + Pi + energy); ATP is regenerated from ADP + Pi during respiration.
ATP is often described as the energy currency of the cell.
ANAEROBIC RESPIRATION:
Occurs when oxygen is limited; glucose is not fully oxidised, so less energy is released.
In yeast: glucose → ethanol + carbon dioxide (+ energy).
In muscle: when oxygen is scarce, glucose → lactate (+ energy).
Oxygen debt describes the extra oxygen required after exercise to fully metabolise lactate.
The difference between aerobic and anaerobic respiration is the amount of energy released and the end products.
ATP – ENERGY CURRENCY (DETAILED)
ATP is formed by adding a phosphate to ADP: \ce{ADP + Pi -> ATP}
When energy is needed: \ce{ATP -> ADP + Pi + energy}
ENERGY FROM FOODS
Foods provide chemical energy that cells can use; energy density depends on macronutrient composition.
Energy values (typical):
Carbohydrates: about
Lipids: about
Protein: about
Food energy is often displayed in kilojoules (kJ) or kilocalories (kcal).
Energy content of common foods varies (examples include margarine, bread, meat, vegetables, sugar, etc.).
The energy content of a food can be calculated from its macronutrient content.
CARBOHYDRATES AND ENERGY STORAGE
Carbohydrates are the body's main fuel; glucose is the immediate energy source used in respiration.
Monosaccharides (e.g., glucose, fructose) are single sugars; disaccharides (e.g., sucrose, lactose); polysaccharides (e.g., starch, glycogen, cellulose).
Starch is a polymer of glucose used for storage in plants; glycogen stores energy in liver and muscle; cellulose is a plant structural polysaccharide (not digestible by humans).
Dietary fibre (cellulose) aids gut movement.
Lipids are fats and oils; triglycerides are glycerol bound to three fatty acids; fats are energy-dense and provide insulation and protection.
Cholesterol is essential but excessive amounts can contribute to heart disease.
PROTEINS
Proteins make up about 18% of body mass; essential for growth, tissue repair, and enzymes.
Foods rich in protein include meat, fish, cheese, eggs; plant sources include beans, nuts.
Proteins are polymers of amino acids (20 different amino acids).
Each protein has a unique sequence of amino acids; the sequence determines 3D shape and function.
Some deficiency diseases are related to protein and amino-acid intake; essential amino acids must be obtained from the diet.
Figure references illustrate chains of amino acids and protein structures.
MINERALS AND VITAMINS
Minerals/ minerals ions (e.g., calcium, iron, sodium, potassium, magnesium) are essential in small amounts.
Calcium supports bones/teeth; iron is a key component of haemoglobin; iodine is needed for thyroid hormone production (examples provided in class notes).
Vitamins (A, B group, C, D) have specific roles:
Vitamin A: retina and vision; body surface maintenance
Vitamin C: connective tissue, wound healing (scurvy when deficient)
Vitamin D: calcium uptake; bone growth (rickets if deficient)
Daily amounts vary; deficiency diseases described (e.g., scurvy, rickets, beri-beri).
A table summarizes vitamins, daily amounts, roles, and food sources.
DIGESTION AND ABSORPTION
The digestive system breaks down carbohydrates (starch to glucose), proteins to amino acids, lipids to fatty acids and glycerol via enzymes (amylases, proteases, lipases).
The alimentary canal includes mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine, pancreas, and liver/gall bladder involvement.
Bile produced by liver and stored in gall bladder; bile emulsifies lipids and neutralises stomach acid.
Peristalsis moves food along the gut; absorption occurs mainly in the small intestine via villi.
Tests exist to identify presence of nutrients in foods ( starch, glucose, protein, lipids ).
Practical activities illustrate digestion and energy content of foods.
EXTENSION IDEAS AND PRACTICALS
Essential amino acids must be supplied via the diet; vegetarians can obtain them through a varied plant-based diet.
Vitamin deficiencies and their health consequences discussed with everyday examples.
Practical activities for food testing (starch with iodine; glucose with Benedict’s reagent; protein with Biuret test; lipids with ethanol and water emulsion).
KEY NUMERICAL/FORMULA NOTES
Aerobic respiration equation: \ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O (+ energy)}
ATP cycle: \ce{ATP -> ADP + Pi + energy} and \ce{ADP + Pi -> ATP}
Energy densities (per gram):
CONNECTIONS AND CONTEXT
Enzymes connect genetics (gene expression) to metabolic processes in the cytoplasm.
The balance of energy intake and expenditure is essential for growth, tissue repair and metabolic function.
A balanced diet supports energy needs, growth, and health; vitamin and mineral sufficiency prevents deficiency diseases.
END OF UNIT 1 CORE IDEAS
UNIT 2: ANIMAL PHYSIOLOGY
INTRODUCTION: physiology studies how organisms function; focus here on the human body including respiration, digestion, circulation, coordination, and homeostasis.
BREATHING AND GAS EXCHANGE
STRUCTURE OF THE GAS EXCHANGE SYSTEM
Thorax encased by ribs; diaphragm separates thorax from abdomen.
Trachea (windpipe) => two bronchi => bronchioles => alveoli (air sacs).
Ribs connected by intercostal muscles (external and internal) and a flexible diaphragm.
The trachea and bronchi have cartilage rings to keep airways open; complete rings in bronchi, C-shaped rings in trachea.
The pleural membranes surround the lungs; pleural cavity contains pleural fluid for lubrication; airtight seal in the thorax.
KEEPING THE AIRWAYS CLEAN
Mucus-secreting cells trap dirt and bacteria; cilia sweep mucus towards the mouth.
Smoking damages cilia, leading to increased infection risk and bronchitis.
VENTILATION OF THE LUNGS
Ventilation requires volume changes in the thorax to create pressure differences.
Inhalation: external intercostals contract and diaphragm moves down, increasing thorax volume and decreasing thoracic pressure; air in.
Exhalation: internal intercostals contract and diaphragm relaxes, decreasing thorax volume; air out. Lungs’ elasticity also helps exhalation.
The process is driven by pressure differences, not by lungs forcing air in.
GAS EXCHANGE IN THE ALVEOLI
Air in the lungs contains oxygen; blood in surrounding capillaries carries deoxygenated blood.
Alveolar walls and capillary walls are thin (less than 1/1000 mm) to allow diffusion of gases.
Alveoli provide a large surface area (~60 m^2 in total) and are covered with capillaries to enable efficient gas exchange.
Oxygen diffuses from alveoli into blood; carbon dioxide diffuses from blood into alveoli to be exhaled.
Gas exchange is aided by the moist lining of the alveoli where gases dissolve before diffusion.
EXAMPLES OF PRACTICALS AND HEALTH TOPICS
Practical: comparing carbon dioxide content in inhaled vs exhaled air using limewater or bicarbonate indicator.
Exercise affects breathing rate; immediate post-exercise breathing rate remains elevated before returning to resting rate.
Smoking effects: reduced oxygen transport due to carbon monoxide forming carboxyhaemoglobin; long-term smoking linked to bronchitis, emphysema, COPD, and lung cancer risk (statistical data included).
EMPHYSEMA AND COPD
Emphysema: destruction of alveolar walls; reduced surface area; poor gas exchange; often requires supplemental oxygen.
CARBON MONOXIDE IN SMOKE
CO binds tightly to haemoglobin, reducing blood’s oxygen-carrying capacity, contributing to heart disease.
SMOKING STATISTICS AND PUBLIC HEALTH
Global smoking prevalence and health impacts; quitting can substantially reduce risk over time.
FOOD AND DIGESTION (OVERVIEW FOR CONTEXT)
The digestive system breaks down carbohydrates, proteins and lipids; absorption occurs in the small intestine via villi.
Enzymes (amylase, proteases, lipases) catalyse digestion; bile emulsifies fats; peristalsis moves food along the gut.
Nutrients are absorbed into the bloodstream to be used by tissues; some vitamins/minerals require specific roles for health (e.g., calcium for bones; iron for haemoglobin).
RESPIRATION AND ENERGY IN THE BODY
Oxygen is required for aerobic respiration; energy from glucose is used to form ATP and for cellular processes; energy release also generates heat to maintain body temperature in mammals and birds.
Aerobic respiration yields more energy than anaerobic respiration; glucose oxidation proceeds through a sequence of enzyme-catalysed steps in mitochondria.
Some cells can respire anaerobically if oxygen is limited (e.g., yeast, muscle cells under intense activity); lactate may accumulate and is cleared by subsequent aerobic respiration.
LOOKING AHEAD AND PRACTICALS
Many activities and experiments illustrate respiration, gas exchange, diffusion, and the effects of exercise and smoking on the respiratory system.
Chapter questions strengthen understanding of gas exchange, lung mechanics, and related physiology.
KEY FORMULAS AND FACTS FOR UNIT 2
Oxygen and carbon dioxide exchange depends on diffusion gradients across alveolar walls and capillary walls.
Ventilation is driven by pressure differences created by volume changes in the thorax.
The diffusion barrier in alveoli is extremely thin, enabling rapid gas diffusion.
UNIT 3: PLANT PHYSIOLOGY
PLANTS AND FOOD
Plants are multicellular and contain chloroplasts; they photosynthesize to produce glucose and other compounds.
Cellulose cell walls give plants a fixed shape; storage carbohydrates include starch (in plants) and glycogen (in animals).
Glucose is a building block; starch and glycogen are polymeric forms; cellulose forms plant cell walls.
Dietary fibre (cellulose) helps gut movement and reduces constipation.
Monosaccharides (glucose, fructose); disaccharides (sucrose, lactose); polymers (starch, cellulose, glycogen).
TRANSPORT IN PLANTS
Plants use xylem and phloem for transport; water and mineral ions move through xylem; sugars move through phloem.
Transpiration drives water movement; root uptake happens via osmosis and active transport depending on solute gradients.
CHEMICAL COORDINATION IN PLANTS
Plant hormones (like auxins) regulate growth responses; coordination includes responses to light, gravity, and other stimuli.
REPRODUCTION IN PLANTS
Sexual and asexual reproduction; seeds and fruit; pollination strategies; life cycles.
EXTENSION IDEAS (LOOK AHEAD)
Plant physiology extends into topics such as plant responses to stimuli, hormonal control, and nutrient transport.
UNIT 4: ECOLOGY AND THE ENVIRONMENT
ECOSYSTEMS AND ENVIRONMENTAL INFLUENCES
Ecosystems consist of communities of organisms and their physical environment; energy flow and nutrient cycling.
Human influences on the environment include pollution, land-use change, climate change, and conservation issues.
UNIT 5: VARIATION AND SELECTION
CHROMOSOMES, GENES AND DNA
Chromosomes carry genes; DNA structure stores genetic information; genes govern protein production.
CELL DIVISION
Mitosis leads to two identical daughter cells; the process is essential for growth and tissue repair; at fertilisation, zygote divides repeatedly.
GENES AND INHERITANCE
Inheritance patterns; dominant and recessive alleles; genetic variation arises from meiosis and fertilisation.
NATURAL SELECTION AND EVOLUTION
Variation among individuals; differential survival and reproduction lead to changes in allele frequencies over generations.
SELECTIVE BREEDING
Human-directed selection to produce desirable traits in plants and animals.
UNIT 6: MICROORGANISMS AND GENETIC MODIFICATION
USING MICROORGANISMS
Bacteria and fungi decompose dead material; some microbes are used in food production and biotechnology.
GENETIC MODIFICATION
Techniques to modify organisms’ genetic material for research and applications.
APPENDICES AND STUDY HELP
APPENDIX A: A GUIDE TO EXAM QUESTIONS ON EXPERIMENTAL SKILLS
APPENDIX B: COMMAND WORDS
GLOSSARY and INDEX for quick reference
ASSESSMENT OVERVIEW (EDUCATIONAL CONTEXT)
PAPER 1 (4BI1/1B and 4SD0/1B): 61.1%, 110 marks, 2 hours; externally assessed; firsts in June 2019; topics cover a broad range of biology concepts.
PAPER 2 (4BI1/2B): 38.9%, 70 marks, 1 hour 15 minutes; externally assessed; firsts in June 2019.
If studying Biology, students take both Papers 1 and 2; if studying Science Double Award, Paper 1 is taken along with Paper 1 for Physics and Chemistry.
ASSESSMENT OBJECTIVES AND WEIGHTINGS:
AO1: Knowledge and understanding of biology (38–42%)
AO2: Application, analysis and evaluation (38–42%)
AO3: Experimental skills, analysis and evaluation of data and methods (19–21%)
PRACTICAL AND SKILLS NOTES
Experimental skills may involve problem solving in practical contexts, planning investigations, data analysis, evaluating methods, identifying variables, and communicating findings with appropriate graphs and calculations.
Calculators allowed; no QWERTY keyboards; no calculators that retrieve text/formulas.
KEY WORDS TO REMEMBER
Diffusion, osmosis, active transport, metabolism, respiration, enzymes, substrate, active site, optimum temperature, denaturation, coenzymes, respiration equation, energy, ATP, ADP, phosphorylation, skeletal/respiratory/muscle systems, xylem/phloem, transpiration, alveoli, diffusion gradient, capillaries, haemoglobin, carboxyhaemoglobin, emulsification, peristalsis, villi, microvilli, enzyme specificity, signal transduction, homeostasis, thermoregulation, feedback loops, selective permeability, endoplasmic reticulum (rough), mitochondria, chloroplasts, nucleus, ribosomes, vacuole, Golgi apparatus.
LA TEX IN NOTES
Aerobic respiration: \ce{C6H12O6 + 6O2 -> 6CO2 + 6H2O + energy}
ATP cycle: ;
Energy densities:
Note: This set of notes is a compact yet comprehensive synthesis of the transcripted content across Units 1–2 (with references to Units 3–6 topics as indicated by the contents), focusing on key concepts, definitions, processes, and standard numerical values/equations used in Edexcel International GCSE Biology (9–1). It preserves the structure of the original source through top-level headings and detailed bullet points, and includes essential LaTeX-formatted equations for clarity in study and revision.