Transport — IB Diploma Biology HL

Revision notes

9 short notes, in the order of the specification. Each one in short:

1. Adaptations of capillaries

   Capillaries are the narrowest blood vessels and are adapted for exchange of materials between blood and the internal or external environment. Branching and narrow diameters give a large surface area, walls one cell thick give a short diffusion distance, and fenestrations (pores) in some capillaries allow particularly rapid exchange.

2. Structure and adaptations of arteries and veins

   Practical

   Arteries carry blood away from the heart at high pressure and have thick walls of smooth muscle and elastic tissue relative to a narrow lumen. Veins return blood to the heart at low pressure and have thin, flexible walls, a wide lumen and valves that prevent backflow, while surrounding muscles squeeze blood towards the heart.

3. Pulse rates and occlusion of the coronary arteries

   Practical

   Pulse rate is measured by feeling the carotid or radial pulse with the fingertips and counting beats per minute. Occlusion of the coronary arteries is blockage by fatty plaque or a blood clot, which cuts the supply of oxygen to cardiac muscle and can cause a heart attack, where part of the heart muscle dies.

4. Water transport in xylem

   Water moves from roots to leaves because transpiration from leaf cell walls draws water out of the xylem by capillary action, creating tension (negative pressure potential). This tension pulls the water column up, and cohesion between water molecules keeps it continuous. Xylem vessels are hollow tubes with lignified walls and pits.

5. Tissues in dicotyledonous stems and roots

   Practical

   In a transverse section of a dicotyledonous stem, vascular bundles are arranged in a ring, each with phloem towards the outside and xylem towards the inside, surrounded by cortex and an outer epidermis. In a dicotyledonous root, xylem and phloem are in a central vascular region, with xylem in a star shape, surrounded by a wide cortex and epidermis.

6. Tissue fluid and lymph

   HL only

   Tissue fluid is formed by pressure filtration of plasma through capillary walls, promoted by high blood pressure from arterioles. It bathes cells, supplying oxygen and glucose and receiving carbon dioxide and wastes. Lower pressure near venules lets most of it drain back into capillaries, and the excess drains into lymph ducts, returning to the blood.

7. Single and double circulation and the mammalian heart

   HL only

   Bony fish have a single circulation: blood passes through the heart once, from heart to gills to body. Mammals have a double circulation: blood passes through the heart twice, through separate pulmonary and systemic circuits. The mammalian heart delivers pressurized blood using myogenic cardiac muscle, a pacemaker, thick-walled ventricles, valves, a septum and coronary vessels.

8. Stages in the cardiac cycle

   HL only

   The cardiac cycle is the sequence of events in one heartbeat. The sinoatrial node initiates it: the atria contract (atrial systole), then the ventricles contract (ventricular systole), closing the atrioventricular valves and opening the semilunar valves, then the heart relaxes (diastole). Blood pressure is given as systolic over diastolic, for example 120/80 mm Hg.

9. Root pressure and phloem transport

   HL only

   Root pressure is a positive pressure potential in the xylem, generated when roots actively transport mineral ions into the xylem so that water follows by osmosis. It moves water when transpiration is insufficient. Phloem sieve tubes have sieve plates, little cytoplasm and no nucleus, and companion cells with many mitochondria load sugars through plasmodesmata.