HSC Biology
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Module 1: Cells as the Basis of LifeCell Structure
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Cell Function4 Topics
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Module 2: Organisation of Living ThingsOrganisation of Cells2 Topics
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Nutrient and Gas Requirements6 Topics
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Transport2 Topics
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Module 3: Biological DiversityEffects of the Environment on Organisms2 Topics
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Adaptations
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Theory of Evolution by Natural Selection
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Evolution - the Evidence
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Module 4: Ecosystem DynamicsPopulation Dynamics
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Past Ecosystems
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Future Ecosystems
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Module 5: HeredityReproduction
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Cell Replication
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DNA and Polypeptide Synthesis
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Genetic Variation
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Inheritance Patterns in a Population
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Module 6: Genetic ChangeMutation1 Topic
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Biotechnology
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Genetic Technologies
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Module 7: Infectious DiseaseCauses of Infectious Disease
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Responses to Pathogens
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Immunity
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Prevention, Treatment and Control
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Module 8: Non-infectious Disease and DisordersHomeostasis
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Causes and Effects
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Epidemiology
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Prevention
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Technologies and Disorders
Gas Exchange in Animals and Plants
Plants
Leaf epidermis
A single-cell layer that covers the leaves. It forms a divide between the plant and its external environment.
Stomata
The leaf epidermis is covered with tiny spores called stomata. The stomata allow gases, including water vapour, to move in and out of the leaf. Each stomata has a guard cell on each side.
- Carbon dioxide enters through open stomata
- Water loss is prevented with closed stomata
- These objectives must be balanced

Stomata open and close with the help of guard cells, which control the diameter of the pore by changing shape. When guard cells take up water, they swell and become tight, opening the pore. Inversely, when the guard cells lose water, they shrink and reduce the pore diameter.
- Gains water = wider pores
- Loses water = smaller pores
- Stomata are open in the light and close in the dark
Mammals
Gaseous exchange occurs in all animals and involves the movement of gases between the internal and external environments by diffusion across cell membranes.
Oxygen is essential for all cells to carry out cellular respiration to release energy from the nutrients they have consumed. The respiratory system enables the exchange of gases between an organism and its environment.
Fish
Fish must obtain oxygen in order to remove carbon dioxide. Water flowing over them ensures maximum oxygen uptake. As the water is only flowing in one direction, the water can enter and flow over the gills and then leave via the gill slit.
Insects
Insects do not have lungs or capillaries for gas exchange. This occurs in the spiracles, which are breathing pores.