IB Biology B3.2: Transport worked solutions

Veins carry blood at low pressure back to the heart, and their valves stop it flowing backwards. Arteries have thick walls and no valves along their length (B). Capillary walls are one cell thick and have no elastic fibres (C). Veins carry blood at lower pressure than arteries (D).

A thick wall round a small lumen is the section of an artery, and a thin wall round a wide lumen is a vein. Valves cannot be seen in a transverse section (C), the shape of the lumen alone does not make a vein (B), and a capillary is only one cell thick (D).

Xylem vessels are dead, hollow and lignified. Phloem and cortex are living tissues, and the pith is made of thin-walled living cells.

Award 1 for a feature and its function, twice. Lignified, thick walls resist the low pressure caused by the transpiration pull, so the vessel does not collapse (1). The lumen is empty because the cells are dead, so there is little resistance to flow (1). End walls are lost, so the water forms a continuous column (1). Pits let water move sideways (1). A feature without its function earns nothing.

Phloem carries sucrose from sources to sinks. Xylem carries water and minerals from the roots to the leaves, not sucrose (A) or amino acids (D), and phloem does not carry water upwards from the roots (B).

The atrioventricular valve shuts when the ventricular pressure exceeds the atrial pressure. The semilunar valve stays shut while the ventricular pressure is below the aortic pressure. Both are therefore shut.

Oxygenated blood leaves the lungs in the pulmonary vein, enters the left atrium, passes to the left ventricle and leaves by the aorta. The pulmonary artery carries blood to the lungs (B), and the right side of the heart carries deoxygenated blood (C, D).

Any two. In a mammal blood passes through the heart twice in each complete circuit, in a fish once (1). A mammal has separate pulmonary and systemic circuits (1). The blood is pumped again after the lungs, so it reaches the body at high pressure, whereas fish blood loses pressure in the gills (1). Oxygenated and deoxygenated blood are kept apart in the mammal heart (1).

(a) The coronary arteries supply the heart muscle (1). (b) A block cuts off the oxygen and glucose, so the heart muscle cells beyond the block stop contracting and die, which is a heart attack (1).

The sinoatrial node is the pacemaker: its cells contract by themselves and set the rate. The AV node only delays the impulse (A), and the bundle of His and Purkinje fibres only carry it to the ventricles (C, D).

Any five, each a feature linked to a function. Artery: a thick wall containing collagen resists the high pressure (1). Elastic fibres stretch and recoil, which maintains the pressure and smooths the flow (1). A narrow lumen for the wall thickness keeps the pressure high (1). Smooth muscle can change the lumen to control flow (1). Capillary: a wall one cell thick gives a short diffusion distance (1). Branching gives a large surface area (1). A narrow lumen slows the blood so there is time for exchange (1). Pores (fenestrations) make exchange faster (1). Maximum 5.

Any five. Water evaporates from the walls of mesophyll cells and leaves through the stomata (1). This lowers the pressure and creates a tension in the xylem (1). Water molecules are cohesive because of hydrogen bonds, so the column is pulled up as one chain (1). Water adheres to the lignified wall, which helps to hold the column up (1). Lignified walls stop the vessel collapsing under the tension (1). Water enters the root by osmosis, aided by active uptake of mineral ions, and replaces what is lost (1).

(a) Ratio = wall thickness / lumen diameter, in the same units. For P, \dfrac{12}{8}=1.5 (1). For Q, \dfrac{3}{18}=0.17 (1). (b) P has the larger ratio, so its wall is thick for its lumen and it is the artery (1). (c) 12\times\dfrac{1.0}{20}=0.60\,\mathrm{mm} (1).

(a) Any two. Companion cells have many mitochondria to supply ATP for active loading (1). They have a large membrane surface area with protein pumps (1). Plasmodesmata link them to the sieve tube element (1). (b) Sucrose is loaded actively at the source, which lowers the water potential in the sieve tube (1). Water enters by osmosis, so the hydrostatic pressure rises (1). Sap flows by mass flow to the sink, where sucrose is removed and the pressure is lower (1).

(a) The highest aortic pressure in the data is 118\,\mathrm{mmHg} (1). (b) At 0.2\,\mathrm{s} the ventricular pressure (110) exceeds the aortic pressure (108), so the semilunar valve is open (1). At 0.4\,\mathrm{s} the ventricular pressure (98) is below the aortic pressure (103), so it is shut (1). (c) Diastolic pressure is the lowest aortic value, 79\,\mathrm{mmHg}, so pulse pressure =118-79=39\,\mathrm{mmHg} (1).

(a) Any two: plasma proteins, clotting factors, red blood cells (1 each). (b) At the arteriole end the blood is at high hydrostatic pressure (1), which forces fluid out of the capillary through the thin wall, leaving large proteins and blood cells behind (1).

(a) Award 1 for a circular outline with a thin epidermis and a wide cortex in proportion, 1 for a ring of bundles drawn as outlined regions (no individual cells) and 1 for correct labels with lines that touch the tissue (3). (b) Xylem lies towards the pith and phloem towards the outside (1).

Any five. Independent variable: the length of time spent stepping, for example 0, 1, 2 and 3 minutes (1). Dependent variable: pulse rate in bpm (1), measured at the radial artery with fingertips over 30 s and doubled (1). Controls: same person, bench height and stepping rate, and the same rest period of 5 minutes between trials (1). Repeat each condition at least three times and calculate a mean (1). Compare means with the range or a statistical test (1).

Award 1 for two circuits that each pass through the heart (right side to lungs, left side to body), 1 for the four named vessels in the correct places (pulmonary artery to the lungs, pulmonary vein from the lungs, aorta from the left ventricle, vena cava to the right atrium) and 1 for arrows that show the direction of flow.

Any four. The sinoatrial node, the pacemaker, starts each beat by contracting on its own (1). The impulse spreads across the atria and they contract (1). The atrioventricular node delays the impulse, so the atria finish contracting first (1). The impulse then passes down the bundle of His and along the Purkinje fibres (1), so the ventricles contract from the apex upwards (1).

(a) The incidence per 1000 men is \dfrac{60}{4}=15, \dfrac{140}{4}=35 and \dfrac{260}{4}=65 (2, 1 for two correct). (b) \dfrac{65-15}{15}\times100=333\% (1). (c) The incidence rises with the cholesterol level, a strong positive correlation (1). A mechanism supports a link, because cholesterol builds the atheroma that blocks coronary arteries (1). The study is observational, so other differences, such as smoking, age or diet, may also differ between the groups, and a correlation alone does not prove a cause (1).

Any six, in order. Atrial contraction fills the ventricle (1). The ventricle contracts and its pressure rises above the atrial pressure, so the AV valve shuts (1). Pressure keeps rising until it exceeds the aortic pressure, so the semilunar valve opens (1). Blood is ejected into the aorta (1). The ventricle relaxes and its pressure falls below the aortic pressure, so the semilunar valve shuts (1). The pressure falls below the atrial pressure, so the AV valve opens and the ventricle refills (1).

(a) With no leaves there is no transpiration pull (1). The roots pump mineral ions into the xylem by active transport, which lowers its water potential (1). Water enters by osmosis and the resulting positive pressure, root pressure, pushes the sap up and out (1). (b) Cyanide stops ATP production, so the active transport of ions stops (1). The ion concentration in the xylem falls, less water enters by osmosis and the pressure falls (1).

(a) Removing the phloem breaks the pathway for sucrose, so it cannot pass the ring and accumulates above it (1). The radioactive sucrose is found only above the ring, which shows that sucrose moves down the trunk in the phloem (1). (b) The xylem lies deeper in the trunk and was not removed, so water and minerals still reach the leaves (1) and the leaves stay healthy for a time (1). (c) The roots below the ring, as sinks, receive no sucrose and will eventually starve (1).

(a) \dfrac{0.90}{3.0}=0.30 (1) and \dfrac{0.20}{5.0}=0.04 (1). (b) X has the larger ratio and a thick wall round a small lumen, so it is the artery; Y has a thin wall round a wide lumen, so it is the vein (1, 1 for the evidence). (c) Skeletal muscles squeeze the thin wall and push the blood on (1), and valves stop it flowing backwards, so the blood moves towards the heart at low pressure (1).

(a) A proton pump uses ATP to move H^{+} out of the companion cell (1). A co-transporter then carries sucrose into the cell with H^{+} flowing back down its gradient (1). (b) Root cells pump mineral ions into the xylem by active transport (1), so the water potential of the sap falls and water enters by osmosis, which raises the pressure (1). (c) Both processes use ATP made by aerobic respiration in mitochondria (1), so cells that are active in loading or pumping have many mitochondria (1).

(a) The capillary wall is one cell thick and lies against the alveolus wall, so oxygen and carbon dioxide diffuse a short distance (1). Branching gives a large surface area for exchange (1). (b) Blood loses pressure in the lung capillaries and returns to the heart (1), where it is pumped again, so it reaches the muscles at high pressure (1). (c) Oxygen diffuses from the alveolus into the blood in the lungs and is carried to the muscles (1), and carbon dioxide returns in the opposite direction to be breathed out (1).

(a) Water moves from the soil into the root hair cell, from the higher to the lower water potential, by osmosis (1). (b) -0.2-(-1.2)=1.0\,\mathrm{MPa} (1). (c) Water moves down the gradient, from soil to root, stem and leaf, by osmosis (1). Evaporation from the leaf keeps its water potential low, so the gradient is maintained (1). (d) If salt lowered the soil to -1.5\,\mathrm{MPa} the soil would have a lower water potential than the root cell (1), so water would leave the root and the plant would wilt (1).