Biology (Combined Science)
Key biology topics for combined science: cells, organisation, infection, bioenergetics, and ecology.
In this topic
Cell Biology
All living organisms are made of cells. Animal cells have: cell membrane, cytoplasm, nucleus, mitochondria, and ribosomes. Plant cells also have: cell wall, permanent vacuole, and chloroplasts.
Substances move in and out of cells by diffusion (high to low concentration, passive), osmosis (water through a partially permeable membrane), and active transport (low to high, requires energy).
Cells divide by mitosis (growth/repair — 2 identical cells) and meiosis (gametes — 4 different cells, half chromosomes).
Key Points
- Know the differences between animal, plant, and bacterial cells
- Diffusion, osmosis, and active transport — know examples of each
- Mitosis = growth; Meiosis = gametes
- Stem cells can differentiate into specialised cells
Example Questions
3Explain why root hair cells use active transport to absorb mineral ions from the soil.
The concentration of mineral ions is higher inside the root hair cell than in the soil. Mineral ions must move against the concentration gradient, which requires energy from respiration.
[3 marks]
2State two differences between mitosis and meiosis.
Mitosis produces 2 genetically identical cells; meiosis produces 4 genetically different cells. Mitosis maintains chromosome number; meiosis halves it.
[2 marks]
Bioenergetics
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (endothermic, in chloroplasts) Limiting factors: light intensity, CO₂ concentration, temperature.
Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (exothermic, in mitochondria) Anaerobic respiration (animals): glucose → lactic acid (less energy released) Anaerobic respiration (yeast): glucose → ethanol + CO₂ (fermentation)
Key Points
- Photosynthesis takes in energy; respiration releases energy
- Respiration occurs in ALL living cells continuously
- Anaerobic respiration produces less energy than aerobic
- Exercise causes breathing rate and heart rate to increase
Example Questions
2A farmer grows crops in a greenhouse. Suggest two ways to increase crop yield.
1) Add artificial lighting to increase light intensity for photosynthesis. 2) Increase CO₂ concentration using a heater or gas supply to increase the rate of photosynthesis.
[2 marks]