IGCSE Biology Revision Notes (AQA)

Cell Biology

Animal and Plant Cells

  • Animal Cells: Cell membrane, cytoplasm, nucleus, mitochondria, ribosomes.
  • Plant Cells: All components of animal cells + cell wall, large permanent vacuole, chloroplasts.
  • Specialised Cells: E.g., sperm (for reproduction), egg (for reproduction), root hair (for water/mineral absorption), palisade (for photosynthesis).

Microscopy

Magnification = Image size / Actual size

Microscopes allow us to view cells and subcellular structures. Electron microscopes have higher magnification and resolution than light microscopes.

Organisation

Levels of Organisation

  • Cells: Basic unit of life.
  • Tissues: Group of similar cells working together (e.g., muscle tissue).
  • Organs: Group of different tissues working together (e.g., stomach).
  • Organ Systems: Group of organs working together (e.g., digestive system).
  • Organism: A complete living being.

Digestive System

Pathway: Mouth → Oesophagus → Stomach → Small Intestine → Large Intestine → Anus

  • Enzymes: Biological catalysts that speed up chemical reactions.
  • Amylase: Digests carbohydrates into simple sugars (in mouth, small intestine).
  • Protease: Digests proteins into amino acids (in stomach, small intestine).
  • Lipase: Digests fats into fatty acids and glycerol (in small intestine).

Bioenergetics

Photosynthesis

The process by which plants make their own food using light energy.

Word Equation: Carbon dioxide + Water --(light energy)--> Glucose + Oxygen

Balanced Symbol Equation: 6CO2 + 6H2O --(light energy)--> C6H12O6 + 6O2

  • Occurs in chloroplasts, which contain the pigment chlorophyll.
  • Limiting Factors: Light intensity, carbon dioxide concentration, temperature.

Respiration

The process of releasing energy from glucose.

Aerobic Respiration

Occurs with oxygen, releases a large amount of energy.

Word Equation: Glucose + Oxygen → Carbon dioxide + Water (Energy)

Balanced Symbol Equation: C6H12O6 + 6O2 → 6CO2 + 6H2O (Energy)

  • Primarily occurs in the mitochondria.

Anaerobic Respiration

Occurs without oxygen, releases less energy.

  • In Animals: Glucose → Lactic acid (Energy) - leads to oxygen debt.
  • In Yeast (Fermentation): Glucose → Ethanol + Carbon dioxide (Energy) - used in brewing and bread-making.

Homeostasis and Response

Homeostasis

The maintenance of a constant internal environment (e.g., body temperature, blood glucose levels, water levels).

Nervous System

  • Central Nervous System (CNS): Brain and spinal cord.
  • Peripheral Nervous System (PNS): Nerves connecting the CNS to the rest of the body.
  • Reflex Arc: Stimulus → Receptor → Sensory neuron → Relay neuron (in CNS) → Motor neuron → Effector (muscle/gland) → Response.

Hormonal Control

  • Hormones: Chemical messengers transported in the blood.
  • Insulin: Lowers blood glucose levels by promoting glucose uptake by cells and conversion to glycogen in the liver and muscles.
  • Glucagon: Raises blood glucose levels by promoting conversion of glycogen to glucose in the liver.
  • Adrenaline: Prepares the body for 'fight or flight' (increases heart rate, breathing rate, blood flow to muscles).

Inheritance, Variation and Evolution

DNA and Genetics

  • Chromosome: Long DNA molecule containing many genes.
  • Gene: A segment of DNA that codes for a specific protein, determining a characteristic.
  • Allele: Different versions of a gene (e.g., for eye colour, blue or brown).
  • Dominant Allele: Expressed even if only one copy is present (e.g., 'B' for brown eyes).
  • Recessive Allele: Only expressed if two copies are present (e.g., 'b' for blue eyes).
  • Homozygous: Having two identical alleles for a gene (BB or bb).
  • Heterozygous: Having two different alleles for a gene (Bb).
  • Genotype: The genetic makeup of an organism (e.g., BB, Bb, bb).
  • Phenotype: The observable characteristics resulting from the genotype (e.g., brown eyes, blue eyes).
  • Punnett Squares: Used to predict the probability of offspring genotypes and phenotypes.

Natural Selection

A mechanism for evolution proposed by Charles Darwin.

  1. Variation: Individuals within a species show variation due to mutations and sexual reproduction.
  2. Competition: Organisms compete for limited resources (food, water, mates, shelter).
  3. Survival of the Fittest: Individuals with advantageous traits are better adapted to their environment, more likely to survive, and reproduce.
  4. Inheritance: These advantageous traits are passed on to offspring.
  5. Evolution: Over generations, the frequency of these advantageous traits increases in the population, leading to evolution.

Ecology

Ecosystems

A community of living organisms (biotic factors) interacting with their non-living environment (abiotic factors, e.g., temperature, light, water).

Food Chains and Webs

  • Food Chain: Shows the flow of energy from one organism to another (e.g., Grass → Rabbit → Fox).
  • Producers: Organisms that make their own food (e.g., plants).
  • Primary Consumers (Herbivores): Eat producers.
  • Secondary Consumers (Carnivores/Omnivores): Eat primary consumers.
  • Tertiary Consumers: Eat secondary consumers.
  • Food Web: Interconnected food chains in an ecosystem.

Cycles

  • Carbon Cycle: The movement of carbon through the atmosphere, oceans, land, and living organisms (photosynthesis, respiration, combustion, decomposition).
  • Water Cycle: The continuous movement of water on, above, and below the surface of the Earth (evaporation, condensation, precipitation, transpiration).

Human Impact on the Environment

  • Deforestation: Clearing of forests for other land uses, leading to habitat loss, reduced biodiversity, increased CO2, and soil erosion.
  • Pollution: Contamination of the environment (air, water, land) by harmful substances (e.g., sulfur dioxide leading to acid rain, plastics).
  • Global Warming: Increase in Earth's average temperature due to enhanced greenhouse effect from increased greenhouse gas emissions (e.g., CO2, methane) primarily from burning fossil fuels.
  • Biodiversity Loss: Reduction in the variety of life on Earth, caused by habitat destruction, pollution, climate change, and overexploitation.

Practice Questions

  1. State three differences between an animal cell and a plant cell.
  2. What is the function of the nucleus in a cell?
  3. Define 'magnification' in microscopy.
  4. Name the three main types of enzymes and the nutrients they break down.
  5. Write the word equation for photosynthesis.
  6. List three limiting factors for the rate of photosynthesis.
  7. Where in the cell does aerobic respiration primarily occur?
  8. What is the product of anaerobic respiration in human muscle cells?
  9. Define homeostasis.
  10. Name the two main parts of the Central Nervous System (CNS).
  11. Describe the role of insulin in blood glucose regulation.
  12. What is the difference between a dominant and a recessive allele?
  13. If a homozygous dominant individual (BB) is crossed with a homozygous recessive individual (bb), what will be the genotype of all offspring?
  14. State two sources of variation within a species.
  15. Outline the main steps of natural selection.
  16. What is an ecosystem?
  17. Give an example of a producer in a food chain.
  18. Name two human activities that contribute to global warming.
  19. Describe one way deforestation impacts the environment.
  20. What is the purpose of decomposers in an ecosystem?

Answer Key

  1. Plant cells have a cell wall, a large permanent vacuole, and chloroplasts, which animal cells do not.
  2. The nucleus controls cell activities and contains the genetic material (DNA).
  3. Magnification is the number of times larger an image appears compared to the actual size of the object.
  4. Amylase (carbohydrates), Protease (proteins), Lipase (fats).
  5. Carbon dioxide + Water --(light energy)--> Glucose + Oxygen.
  6. Light intensity, carbon dioxide concentration, temperature.
  7. Mitochondria.
  8. Lactic acid.
  9. Homeostasis is the maintenance of a constant internal environment in an organism.
  10. Brain and spinal cord.
  11. Insulin is a hormone that lowers blood glucose levels by signalling cells to take up glucose from the blood and promoting its conversion into glycogen for storage in the liver and muscles.
  12. A dominant allele expresses its characteristic even if only one copy is present, while a recessive allele only expresses its characteristic if two copies are present (i.e., homozygous recessive).
  13. All offspring will be heterozygous (Bb).
  14. Mutation and sexual reproduction (which includes meiosis and random fertilisation).
  15. Individuals in a population show variation; individuals with advantageous traits are better adapted and more likely to survive and reproduce; these advantageous traits are passed on to offspring; over time, the frequency of advantageous traits increases in the population, leading to evolution.
  16. An ecosystem is a community of living organisms (biotic factors) interacting with their non-living environment (abiotic factors).
  17. Any plant or organism that produces its own food, e.g., grass, algae, oak tree.
  18. Burning fossil fuels (e.g., for energy, transport), deforestation, industrial processes, agriculture (e.g., livestock methane).
  19. Deforestation leads to habitat loss, reduced biodiversity, increased atmospheric carbon dioxide (as fewer trees absorb CO2), and soil erosion.
  20. Decomposers (bacteria and fungi) break down dead organic matter, recycling nutrients back into the soil, making them available for producers.
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