IGCSE Biology (9-1) Comprehensive Notes
Nature and Variety of Living Things (Topic 1a)
Core idea: Understanding what it means to be alive and how living things are categorized and described.
1a – Characteristics of living organisms
Living organisms share a set of objective characteristics that distinguish them from non-living things.
Key list of processes (MRS C GREN):
Movement: The ability to change position or place. Animals move their whole body; plants move parts (e.g., leaves turning towards light).
Respiration: Chemical processes that release energy (ATP) from nutrients (e.g., glucose). This fundamental process occurs in all living cells.
Sensitivity: The ability to detect and respond to changes in surroundings (stimuli).
Control of internal conditions (Homeostasis): Maintaining a stable internal environment (e.g., temperature, water balance) despite external changes.
Growth: A permanent increase in size and dry mass, involving an increase in cell number, cell size, or both.
Reproduction: Producing offspring to continue the species, ensuring its survival. This can be sexual or asexual.
Excretion: The removal of toxic waste products of metabolism (e.g., carbon dioxide, urea) from the body.
Nutrition: The process of taking in and using nutrients for energy, growth, and repair. This can be autotrophic (making own food, like plants) or heterotrophic (consuming other organisms, like animals).
These eight processes are the core criteria used to judge whether something is alive.
1a – Exam-style jellyfish passage (example of living characteristics)
An example using jellyfish demonstrates characteristic evidence:
Movement: Jellyfish use rhythmic contractions of their bell to move through water.
Nutrition: They feed on small organisms, using tentacles to capture prey and ingest it.
Reproduction: They release gametes into the water for sexual reproduction, and some can also clone themselves (asexual reproduction).
Respiration: They take up oxygen from the surrounding water (often via diffusion) and release carbon dioxide.
Control/Sensitivity: They possess a nerve net for coordinating responses to stimuli and controlling body movements.
Excretion: Waste products are typically removed by diffusion into the surrounding water.
1b – Variety of Living Things (Topic 1b)
All living things can be classified into groups based on shared characteristics. The five-kingdom system is a simple form: Animals, Plants, Fungi, Protoctists, Bacteria.
Viruses are non-living and are classified separately from the five kingdoms.
1b – Common features of eukaryotes and prokaryotes
Eukaryotes: Organisms (e.g., plants, animals, fungi, protoctists) whose cells possess a true nucleus (containing genetic material) and other membrane-bound organelles.
Prokaryotes: Organisms (e.g., bacteria) whose cells do not have a true nucleus or other membrane-bound organelles; their genetic material is free in the cytoplasm.
1b – Pathogens
A pathogen is any organism that causes disease. Pathogens can be fungi, bacteria, protoctists, or viruses. They typically cause disease by reproducing rapidly and producing toxins or damaging host cells.
1b – Pathogens (examples)
Fungi: e.g., athlete’s foot (skin infection).
Bacteria: e.g., cholera (causes severe diarrhea), Pneumococcus (causes pneumonia).
Protoctists: e.g., Plasmodium (causes malaria, spread by mosquitoes).
Viruses: e.g., influenza (flu), HIV (causes AIDS), tobacco mosaic virus (TMV) (affects plants).
1b – Viruses (key features)
Viruses are not made of cells and do not carry out normal living processes on their own; they are obligate intracellular parasites, meaning they must infect a host cell to reproduce.
They are much smaller than bacteria.
All viruses are parasites that harm their host by hijacking host cell machinery to reproduce.
They are composed of genetic material (DNA or RNA) enclosed in a protein coat (capsid).
Natural viruses can cause disease in all kinds of organisms (animals, plants, bacteria).
1b – Taxonomy and examples
Plants:
Multicellular organisms.
Contain chloroplasts and carry out photosynthesis (autotrophic nutrition).
Possess rigid cellulose cell walls.
Store carbohydrates as starch or sucrose.
Animals:
Multicellular organisms.
Are heterotrophic (feed on other organisms).
Capable of complex movement and nervous coordination.
Do not have cell walls or chloroplasts.
Store carbohydrates as glycogen.
Fungi:
Typically multicellular (e.g., mushrooms) or unicellular (e.g., yeast).
Possess cell walls with chitin.
Often grow as a network of thread-like structures called hyphae, forming a mycelium.
Exhibit saprobic nutrition (secrete enzymes externally onto food, then absorb digested nutrients).
Store carbohydrates as glycogen.
Protoctists:
Mostly single-celled eukaryotic organisms, though some are multicellular (e.g., some algae).
Very diverse; some are plant-like (e.g., Chlorella, photosynthesize) and some are animal-like (e.g., Amoeba, ingest food particles).
Bacteria:
Single-celled prokaryotic organisms.
No true nucleus; genetic material is a circular chromosome in the cytoplasm, may also have smaller circular plasmids.
Possess a cell wall with peptidoglycan.
Many have flagella for movement.
Varied metabolisms; some photosynthesize, some are chemosynthetic, many are decomposers or pathogens.
Level of Organisation (Topic 2a)
2a – Levels of organisation (building a multicellular organism)
Multicellular organisms exhibit a hierarchical organization, from simplest to most complex:
Organelles: Specialized structures within cells that perform specific functions (e.g., mitochondria, chloroplasts).
Cells: The fundamental building blocks of all living things; the smallest unit of life. Organelles inside cells perform functions that enable the cell to work.
Tissues: Groups of similar cells working together to perform a specific function (e.g., muscle tissue for contraction, epithelial tissue for lining surfaces).
Organs: Groups of different tissues working together to perform a specific, more complex function (e.g., brain, heart, liver, kidney, skin in animals; leaf and stem in plants).
Organ Systems: Multiple organs working together to perform major bodily functions (e.g., digestive system composed of stomach, intestines, liver, etc.).
Organism: A complete living being, made up of various organ systems working in coordination.
Cell Structure (Topic 2b)
2b – General cell structure basics
All living organisms are made of cells; they can be unicellular (single-celled) or multicellular. Multicellular organisms have many different cell types, which are specialized for particular functions.
Within cells are organelles; each organelle has a distinct task.
2b – Plant vs Animal cells (generalised features)
Generalised animal cell features include: Cell membrane, Nucleus, Cytoplasm, Ribosome, Mitochondria.
Generalised plant cell features include: All of the above, plus a rigid Cell wall, Chloroplasts, and a large permanent central Vacuole.
2b – Detailed Cell Structures and Functions:
Nucleus:
Contains the cell's genetic information on chromosomes, which are made of DNA (deoxyribonucleic acid).
Genes (segments of DNA) determine the substances the cell makes (e.g., proteins).
The nucleus controls all cell activities by regulating gene expression and protein synthesis.
Cell membrane (also known as plasma membrane):
A semi-permeable boundary surrounding the cytoplasm of all cells.
Regulates the entry and exit of substances into and out of the cell, maintaining its internal environment.
Cytoplasm:
The jelly-like fluid filling the cell, comprising a watery solution (cytosol) and various organelles.
It is the primary site of most metabolic reactions within the cell.
Mitochondria:
Often called the "powerhouses" of the cell.
The site of aerobic respiration, where glucose is broken down in the presence of oxygen to release energy (ATP) for all cell activities.
Cells with high energy demands (e.g., muscle cells, sperm cells, liver cells) have many mitochondria.
Ribosomes:
Very small organelles, often found free in the cytoplasm or attached to endoplasmic reticulum.
Primary function is to synthesize proteins (protein synthesis), following instructions coded by genes in the nucleus.
Chloroplasts:
Found only in plant cells and some protoctists.
Contain the green pigment chlorophyll, which absorbs light energy.
The site of photosynthesis, converting light energy into chemical energy (glucose).
Cell Wall:
(Plant cells): A rigid outer layer made primarily of cellulose. Provides structural support, shape, and protection to the plant cell, preventing excessive water uptake.
(Fungi cells): Composed of chitin.
(Bacteria cells): Composed of peptidoglycan.
Central Vacuole:
(Plant cells): A large, permanent central vacuole filled with cell sap (a solution of dissolved sugars, mineral salts, and pigments).
Stores substances and helps maintain turgor pressure against the cell wall, providing structural rigidity to the plant.
(Animal cells): May have small, temporary vacuoles for various purposes, but not a large central one.
2b – Specialised cells and stem cells
Cell differentiation: The process by which a less specialized cell (e.g., a stem cell) becomes a more specialized cell type, adopting a particular structure and function (e.g., muscle cell, nerve cell).
Specialised cells: Cells that have undergone differentiation to perform specific functions. Examples include:
Red Blood Cell: Biconcave shape, no nucleus, packed with haemoglobin for efficient oxygen transport.
Guard Cell (in plants): Regulates the opening and closing of stomata (pores on leaf surface) for gas exchange and water loss.
Nerve Cell (Neuron): Long axon for transmitting electrical impulses rapidly over distances.
Root Hair Cell (in plants): Long, thin extension (hair) for increasing surface area for efficient water and mineral absorption from the soil.
Stem cells: Undifferentiated cells with the remarkable potential to differentiate into various specialized cell types.
Sources of stem cells:
Early embryos (embryonic stem cells): Pluripotent, meaning they can differentiate into almost any cell type.
Adult tissues (adult stem cells): Multipotent, meaning they can differentiate into a limited range of cell types (e.g., bone marrow stem cells can produce different blood cells).
Usefulness in medicine/research:
Potential to replace or repair damaged tissues (e.g., treating spinal cord injuries, retinal damage, heart muscle damage, diabetes).
Study of disease mechanisms and drug testing.
Ethical issues (primarily concerning embryonic stem cells):
Debate over the moral status of embryos and the destruction of embryos for research.
Potential for contamination of implanted cells with viruses.
Concerns about the risk of cancer if implanted stem cells misbehave and divide uncontrollably.
Ongoing search for alternative sources of stem cells (e.g., induced pluripotent stem cells, iPSCs).
Biological Molecules (Topic 2c)
2c – What are biological molecules?
Carbohydrates, Lipids, and Proteins are the main macromolecules essential for life. They are built from simpler repeating units.
Elements involved:
Carbohydrates: Carbon, Hydrogen, Oxygen (often in a ratio of Cx(H2O)yCx(H2O)y).
Lipids: Carbon, Hydrogen, Oxygen (with a lower proportion of oxygen than carbohydrates).
Proteins: Carbon, Hydrogen, Oxygen, Nitrogen (and sometimes Sulfur).
2c – Carbohydrates
Composed of carbon, hydrogen, and oxygen.
Main function is for energy supply (released during respiration).
Are polymers built from simpler sugar units (monosaccharides).
Major dietary forms and storage molecules:
Starch: A large, insoluble polysaccharide storage carbohydrate stored in plants (e.g., potatoes, grains). It is a polymer of glucose.
Glycogen: A branched polysaccharide storage carbohydrate stored in animals (primarily liver and muscles). Also a polymer of glucose.
The digestive system breaks down large carbohydrate molecules (polysaccharides like starch) into smaller, soluble simple sugars (monosaccharides like glucose) for absorption into the bloodstream.
Practical tests (testing for presence):
Starch test with iodine solution: Initial color yellow/brown; a positive result is a distinct blue/black color.
Glucose test (reducing sugars) with Benedict’s solution (requires heating): Initial blue solution; positive results range from green to orange to a brick-red precipitate as glucose concentration increases.
2c – Lipids (fats and oils)
Also composed of carbon, hydrogen, and oxygen, but with a lower proportion of oxygen compared to carbohydrates.
Functions: Excellent for long-term energy storage (more energy per gram than carbohydrates), insulation (body heat), protection of vital organs, and as a major component of cell membranes.
Structure: A typical lipid molecule (a triglyceride) is formed from one molecule of glycerol bonded to three fatty acids.
Health note: Excess intake of saturated fats and cholesterol is strongly linked to an increased risk of heart disease and circulatory problems.
Practical test: Emulsion test – mixing a food sample with ethanol to extract lipids, then adding water to the ethanol solution. A positive result is the formation of a distinct cloudy white emulsion.
2c – Proteins
Complex macromolecules essential for virtually all cell functions.
Made from building blocks called amino acids; there are 20 different common amino acids used to construct a vast array of proteins.
Proteins are long chains of amino acids that fold into specific 3D shapes, which are crucial for their function (e.g., globular or fibrous).
Essential for growth and repair of tissues, forming enzymes, hormones, antibodies, and structural components (e.g., collagen, keratin).
Dietary sources: Plant sources include beans, peas, nuts, lentils; animal sources include meat, fish, cheese, eggs.
Practical test: Biuret test – adding Biuret reagent (containing copper sulfate and sodium hydroxide). Initial color blue; a positive result is a distinct purple/lilac color.
2c – Enzymes
Enzymes are proteins that function as highly specific biological catalysts.
They speed up the rate of metabolic reactions in living organisms by lowering the activation energy without being consumed or permanently changed in the process.
Each enzyme has a specific active site, a region with a unique 3D shape, that precisely binds to a complementary molecule called the substrate (often described by the lock and key mechanism).
When the substrate binds to the active site, an enzyme–substrate complex forms. The enzyme then facilitates the conversion of the substrate into products, which are then released. The enzyme is then free to catalyze another reaction.
Enzyme structure and activity are coded by genes (segments of DNA).
2c – Factors affecting enzyme activity
Temperature:
Enzymes have an optimum temperature at which they show maximum activity. For most human enzymes, this is around body temperature (370˘0b0C370˘0b0C).
Below the optimum, enzyme activity is reduced as molecules have less kinetic energy, leading to fewer collisions between enzyme and substrate.
Above the optimum, the enzyme's 3D structure, particularly the active site, begins to permanently change (or denature). Denaturation causes the active site to lose its specific shape, preventing the substrate from binding and rendering the enzyme inactive.
pH:
Each enzyme also has an optimum pH at which it functions most efficiently (e.g., pepsin in the stomach has a low optimum pH, while trypsin in the small intestine has a higher optimum pH).
Significant deviations from the optimum pH (either too acidic or too alkaline) can alter the shape of the enzyme's active site, leading to denaturation and a loss of enzyme activity.
Coordination and Response (Topic 2j) – Phototropism (Plant Response)
Topic 2j – Flowering plants: Responding to change
Phototropism: The growth response of a plant to light.
Shoots typically grow toward light (positive phototropism) to maximize light absorption for photosynthesis. This tropism is mediated by plant hormones (auxins).
This is an example of how plants coordinate growth in response to environmental stimuli (light).
Roots typically grow away from light (negative phototropism) or towards gravity (positive gravitropism) to find water and nutrients.
Quick Reference: Key Terms and Concepts
MRS C GREN: Acronym for the eight characteristics of life: Movement, Respiration, Sensitivity, Control, Growth, Reproduction, Excretion, Nutrition.
Eukaryotic vs Prokaryotic: Cells with a true nucleus (eukaryotic) vs. cells lacking a true nucleus (prokaryotic, e.g., bacteria).
Five Kingdoms: Plants, Animals, Fungi, Protoctists, Bacteria. Viruses are non-living.
Pathogens: Organisms causing disease; examples: fungi (athlete’s foot), bacteria (cholera), protoctists (Plasmodium/malaria), viruses (influenza, TMV, HIV).
Plant cell features: Chloroplasts, cellulose cell wall, large central vacuole, mitochondria, nucleus.
Animal cell features: No chloroplasts, no cell wall, smaller/temporary vacuoles; mitochondria, nucleus, cell membrane, cytoplasm, ribosomes.
Enzymes: Biological catalysts (proteins); active site specificity (lock and key); form enzyme–substrate complex; affected by temperature and pH (denaturation). Optimum for humans: 370˘0b0C370˘0b0C.
Food tests:
Starch: Iodine (blue/black).
Glucose: Benedict’s (brick red precipitate with heat).
Protein: Biuret (purple/lilac).
Lipids: Emulsion test with ethanol/water (cloudy white emulsion).
Stem cells: Undifferentiated cells; embryonic (pluripotent) vs adult (multipotent); potential medical uses (tissue repair) vs ethical concerns (embryo use, cancer risk).
Phototropism: Plant growth toward light (shoots) for photosynthesis.
Connections to Foundational Principles
Cell theory: All living organisms are composed of cells; cells arise from existing cells; cells are the basic unit of life.
Levels of organization: Cells form tissues; tissues form organs; organs form organ systems; organ systems form the organism.
Homeostasis: Internal conditions are regulated to maintain stable functioning.
Genetics: Nucleus stores DNA; genes code for proteins; enzymes are proteins coded by genes.
Evolution and classification: Diversity of living things is organized into kingdoms; viruses as non-living or separate category illustrate the scope and limits of biological classification.
Practical and Ethical Implications
Stem cell research: Potential to treat diseases, but ethical issues around embryo use and consent; ongoing debates about alternative sources and regulation.
Diet and health: Fats and cholesterol link to heart disease; importance of balancing lipid intake and understanding fat types.
Biochemical testing in food science and nutrition: Practical skills for detecting macronutrients; relevance to diet, health, and food technology.
Important Equations and Notation (LaTeX)
Body temperature reference (humans): 370˘0b0C370˘0b0C