Transport and Circulation
What this domain tests
This domain covers water and food transport in plants and the human circulatory system: blood composition, heart chambers, double circulation, cardiac cycle, blood pressure and lymph.
Core rules
- Xylem carries water and minerals upward, driven by transpiration pull, cohesion-tension and root pressure; it is dead tissue at maturity.
- Phloem translocates sucrose and amino acids from source to sink by pressure flow; companion cells provide energy.
- Blood: plasma (about 55%), RBCs (haemoglobin, no nucleus in mammals), WBCs for defence, platelets for clotting.
- Human heart: right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.
- Double circulation: pulmonary and systemic circuits; this keeps oxygenated and deoxygenated blood separate.
- Cardiac cycle about 0.8 s; SA node is the natural pacemaker; normal blood pressure about 120/80 mm Hg.
- Lymph carries fat from the intestine, returns tissue fluid to blood, and contains lymphocytes.
Worked example
Why does the left ventricle have the thickest wall? Reasoning: it pumps blood into the aorta and around the entire systemic circuit, so it must generate far higher pressure than the right ventricle, which sends blood only the short distance to the lungs. The extra cardiac muscle mass therefore matches the greater workload.
Traps that cost marks
Defining an artery as a vessel carrying oxygenated blood - the pulmonary artery carries deoxygenated blood; a vein is defined by flow towards the heart. Other slips: saying the left ventricle pumps to the lungs; forgetting the SA node's pacemaker role; claiming xylem transports food.