IGCSE Biology Revision Notes: Core Concepts

Introduction to IGCSE Biology Revision

This document provides concise revision notes for key topics in Cambridge IGCSE Biology (CIE), designed to reinforce understanding and prepare for examinations. It includes essential definitions, processes, and self-assessment questions to test your knowledge.

1. Characteristics of Living Organisms

All living organisms share common characteristics, often remembered by the acronym MRS GREN:

  • Movement: An action by an organism or part of an organism causing a change of position or place.
  • Respiration: The chemical reactions in cells that break down nutrient molecules and release energy for metabolism.
  • Sensitivity: The ability to detect or sense changes in the environment (stimuli) and to make appropriate responses.
  • Growth: A permanent increase in size and dry mass by an increase in cell number or cell size or both.
  • Reproduction: The processes that make more of the same kind of organism.
  • Excretion: The removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements.
  • Nutrition: The taking in of materials for energy, growth and development.

Question 1: List three characteristics of living organisms beginning with the letter 'R'.

2. Cell Structure and Organisation

Animal Cell

  • Cell membrane: Partially permeable layer surrounding the cytoplasm, controlling what enters and leaves the cell.
  • Cytoplasm: Jelly-like substance where most chemical reactions happen.
  • Nucleus: Contains genetic material (DNA) and controls cell activities.
  • Mitochondria: Site of aerobic respiration, releasing energy.
  • Ribosomes: Site of protein synthesis.

Plant Cell

Includes all animal cell components plus:

  • Cell wall: Fully permeable outer layer made of cellulose, providing structural support and protection.
  • Chloroplasts: Contain chlorophyll, site of photosynthesis.
  • Permanent vacuole: Large central sac filled with cell sap, maintaining turgor pressure.

Question 2: Which cell component is responsible for controlling the cell's activities?

Question 3: Name two structures found in plant cells but not in animal cells.

3. Movement In and Out of Cells

Diffusion

  • Net movement of particles from a region of higher concentration to a region of lower concentration down a concentration gradient.
  • Does not require energy.
  • Examples: Oxygen into cells, carbon dioxide out of cells in lungs and tissues.

Osmosis

  • Net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
  • Does not require energy.
  • Crucial for water absorption by plant roots and maintaining cell turgor.

Active Transport

  • Movement of particles across a cell membrane against a concentration gradient (from lower to higher concentration).
  • Requires energy, usually from respiration.
  • Examples: Absorption of mineral ions by plant roots, glucose absorption in the small intestine.

Question 4: What type of movement requires energy to move substances against a concentration gradient?

Question 5: Describe the net movement of water in osmosis.

4. Biological Molecules

  • Carbohydrates: Energy source (e.g., glucose, starch, glycogen). Made of C, H, O.
  • Fats (Lipids): Energy storage, insulation, protection. Made of C, H, O.
  • Proteins: Growth and repair, enzymes, antibodies, hormones. Made of C, H, O, N (sometimes S). Building blocks are amino acids.
  • Water: Solvent, transport medium, involved in chemical reactions.

Question 6: Which biological molecule is primarily used for growth and repair?

5. Enzymes

  • Biological catalysts: speed up metabolic reactions without being used up.
  • Are proteins with specific 3D shapes.
  • Active site: Region where the substrate binds, forming an enzyme-substrate complex (lock and key mechanism).
  • Specificity: Each enzyme acts on a specific substrate.
  • Affected by temperature and pH.
  • Optimum temperature/pH: The conditions at which an enzyme is most active.
  • Denaturation: Irreversible change to the active site at extreme temperatures or pH, causing loss of function.

Question 7: What term describes the loss of an enzyme's function due to extreme temperature or pH?

Question 8: Explain why enzymes are said to be specific.

6. Plant Nutrition (Photosynthesis)

  • Process by which plants make their own food using light energy.
  • Word equation: Carbon dioxide + Water (light energy, chlorophyll) → Glucose + Oxygen
  • Chemical equation: 6CO2 + 6H2O (light energy, chlorophyll) → C6H12O6 + 6O2
  • Occurs in chloroplasts, which contain chlorophyll.
  • Factors affecting photosynthesis: light intensity, carbon dioxide concentration, temperature, water availability.
  • Glucose produced is used for respiration, converted to starch for storage, or cellulose for cell walls.

Question 9: Name the green pigment in plants that absorbs light energy for photosynthesis.

Question 10: Write the word equation for photosynthesis.

7. Human Nutrition

  • Diet: Requires a balanced intake of carbohydrates, fats, proteins, vitamins, minerals, fibre, and water.
  • Digestion: Mechanical (chewing, churning) and chemical (enzymes breaking down large insoluble molecules into small soluble ones).
  • Alimentary canal: Mouth, oesophagus, stomach, small intestine (duodenum, ileum), large intestine (colon, rectum), anus.
  • Accessory organs: Liver, pancreas, gallbladder.
  • Absorption: Small intestine (villi increase surface area).
  • Assimilation: Use of absorbed food molecules by cells.
  • Egestion: Removal of undigested food as faeces.

Question 11: Which part of the human digestive system is adapted for the absorption of digested food?

Question 12: State two functions of a balanced diet.

8. Transport in Humans (Circulatory System)

  • Heart: Pumps blood around the body (double circulatory system).
  • Blood vessels:
    • Arteries: Carry oxygenated blood away from the heart (except pulmonary artery), thick muscular walls, narrow lumen, high pressure.
    • Veins: Carry deoxygenated blood towards the heart (except pulmonary vein), thin walls, wide lumen, low pressure, contain valves to prevent backflow.
    • Capillaries: One cell thick walls, site of substance exchange between blood and tissues.
  • Blood components:
    • Red blood cells: Transport oxygen (contain haemoglobin), no nucleus.
    • White blood cells: Part of immune system (phagocytes, lymphocytes).
    • Platelets: Involved in blood clotting.
    • Plasma: Liquid part of blood, transports cells, nutrients, hormones, waste products.

Question 13: Which type of blood vessel has valves to prevent the backflow of blood?

Question 14: Name the component of blood responsible for transporting oxygen.

9. Respiration

Aerobic Respiration

  • Occurs in the presence of oxygen.
  • Releases a large amount of energy.
  • Word equation: Glucose + Oxygen → Carbon dioxide + Water + Energy
  • Chemical equation: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy
  • Primary site: Mitochondria.

Anaerobic Respiration

  • Occurs in the absence of sufficient oxygen.
  • Releases much less energy than aerobic respiration.
  • In animals: Glucose → Lactic acid + (small amount of) Energy
  • In yeast (fermentation): Glucose → Ethanol + Carbon dioxide + (small amount of) Energy
  • Lactic acid causes muscle fatigue.

Question 15: Where in the cell does aerobic respiration primarily occur?

Question 16: What is the main waste product of anaerobic respiration in humans?

10. Coordination and Response (Nervous System)

  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): Nerves connecting CNS to the rest of the body.
  • Reflex arc: Stimulus → Receptor → Sensory neuron → Relay neuron (CNS) → Motor neuron → Effector → Response.
  • Hormones: Chemical messengers produced by endocrine glands, transported in blood, acting on target organs.

Question 17: Name the two main parts of the Central Nervous System.

11. Reproduction

Asexual Reproduction

  • Involves only one parent.
  • Offspring are genetically identical (clones).
  • Examples: Budding in yeast, binary fission in bacteria, runners in strawberries.

Sexual Reproduction

  • Involves two parents.
  • Fusion of male and female gametes to form a zygote.
  • Offspring show genetic variation.
  • Examples: Humans, flowering plants.
  • Pollination: Transfer of pollen from an anther to a stigma.
  • Fertilisation: Fusion of male and female gamete nuclei.

Question 18: What is the key advantage of sexual reproduction over asexual reproduction?

12. Inheritance

  • Chromosome: Thread-like structure of DNA, carrying genetic information in the form of genes.
  • Gene: A length of DNA that codes for a specific protein, determining a characteristic.
  • Allele: Different forms of a gene (e.g., 'T' for tall, 't' for short).
  • Dominant allele: Expressed if present (e.g., TT, Tt).
  • Recessive allele: Only expressed if two copies are present (e.g., tt).
  • Genotype: Genetic makeup of an organism (e.g., TT, Tt, tt).
  • Phenotype: Observable characteristics of an organism (e.g., tall, short).
  • Homozygous: Having two identical alleles for a particular gene (e.g., TT, tt).
  • Heterozygous: Having two different alleles for a particular gene (e.g., Tt).

Question 19: What is the term for the observable characteristics of an organism?

Question 20: If an organism has two different alleles for a specific gene (e.g., Tt), what is its genotype described as?

Answer Key

  1. Respiration, Reproduction, Response/Responsiveness/Reactivity/Regulation (any three valid from MRS GREN starting with R).
  2. Nucleus.
  3. Cell wall, Chloroplasts, Permanent vacuole (any two).
  4. Active transport.
  5. Net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
  6. Proteins.
  7. Denaturation.
  8. Each enzyme has a specific active site shape that only fits a particular substrate molecule, like a lock and key.
  9. Chlorophyll.
  10. Carbon dioxide + Water (light energy, chlorophyll) → Glucose + Oxygen.
  11. Small intestine (specifically the ileum, or general small intestine is acceptable).
  12. Provide energy for daily activities; provide materials for growth and repair; maintain health; prevent deficiency diseases (any two).
  13. Veins.
  14. Red blood cells (or Haemoglobin).
  15. Mitochondria.
  16. Lactic acid.
  17. Brain and spinal cord.
  18. Genetic variation in offspring, increasing chances of survival in changing environments.
  19. Phenotype.
  20. Heterozygous.
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