Transport in mammals and plants

Mammals

• The cardiac cycle: atrial systole, ventricular systole, diastole. The sinoatrial node (pacemaker) starts the heartbeat; the atrioventricular node delays the impulse; the Purkyne fibres (bundle of His) carry it to the ventricle walls, which contract from the apex upwards.
• Cardiac output = stroke volume × heart rate.
• Haemoglobin carries oxygen; the oxygen dissociation curve is S-shaped (cooperative binding). The Bohr effect: higher CO₂ shifts the curve to the right, releasing more oxygen in active tissues. Fetal haemoglobin has a higher affinity for oxygen.
• Carbon dioxide is carried mainly as hydrogencarbonate ions (carbonic anhydrase in red cells), with the chloride shift.
• Tissue fluid forms at the arterial end of capillaries by hydrostatic pressure and returns by osmosis at the venous end; excess drains into lymph.

Plants

• Water moves through roots by the apoplast pathway (cell walls) and symplast pathway (cytoplasm); the Casparian strip in the endodermis forces water into the symplast.
• Water rises in xylem by cohesion–tension (transpiration pull), aided by root pressure and adhesion.
• Translocation in phloem: sucrose is actively loaded into sieve tubes at sources, water follows by osmosis, and the mass flow of sap carries it to sinks.
• Xerophytes (e.g. plants of the dry lowveld) reduce water loss: thick cuticles, sunken stomata, rolled leaves, hairs, and reduced leaf area.

Check yourself

1. Name the pacemaker of the heart.
2. Explain the Bohr effect.
3. What is the role of the Casparian strip?