OCR A, A-level Biology Module 3 Summary

OCR A, A-level Biology Module 3 Summary



1. Need for Specialised Transport Systems

 High metabolic demand

 Low surface area to volume ratio (SA:V)

 Molecules (e.g., enzymes) need transport between sites

 Waste products must be removed

 Diffusion alone is insufficient



2. Types of Circulatory Systems

 Open: Transport medium pumped directly into body cavity (e.g.,
insects use haemolymph)

 Closed: Transport medium enclosed in vessels

 Single: Blood passes through the heart once per circuit

 Double: Blood passes through the heart twice per circuit



3. Arteries

 Small lumen

 Endothelium

 Thick muscular wall

 Elastic tissue (withstands high pressure, allows recoil)

 Collagen outer layer

4. Arterioles

 Link arteries to capillaries

 Can vasodilate and vasoconstrict

5. Capillaries

 Very small, thin-walled

,  Enable material exchange (gases, nutrients, waste)

 Large SA:V ratio

 Short diffusion distance

 Fenestrations present

6. Veins

 Endothelium

 Thin elastin and smooth muscle layer

 Valves prevent backflow of blood



7. Functions of Blood

 Transport of O₂ and CO₂

 Transport of nutrients and waste (e.g., nitrogenous)

 Transport of hormones and other messengers



8–12. Tissue Fluid Formation & Pressures

 Hydrostatic pressure – pushes fluid out

 Oncotic pressure – draws water in (due to plasma proteins)

 Arteriole end: Hydrostatic > Oncotic → Fluid leaves capillaries

 Venous end: Oncotic > Hydrostatic → Fluid re-enters capillaries



13. Erythrocyte Adaptations

 Biconcave shape

 No nucleus → Increases SA:V ratio

14. Haemoglobin Reaction

 Hb + 4O₂ → Hb(O₂)₄ (Oxyhaemoglobin)

15. Positive Cooperativity

 Binding of one O₂ increases Hb's affinity for more O₂