1. The figure below shows the oxygen dissociation of three different animals. Explain why it is necessary for different animals to have haemoglobin that has different affinities for oxygen.
Answer
Different organisms live in different environments that vary in their partial pressure of oxygen (i.e. higher altitudes), different types of haemoglobin with different affinities for oxygen are advantageous in different environments.
2. Describe the properties of water that enable transpiration to occur.
Answer
Water is polar and therefore has strong cohesion between molecules. This allows water to be drawn up in a continuous column of water from the roots to the leaves in the xylem.
3. Arteries and veins carry out different roles within our bodies. Describe and explain how their structures relate to their function.
Answer
Arteries: Thick elastic walls, so they can stretch and recoil with changing pressure. A smooth endothelium, so friction is reduced. A thick muscle layer, so pressure is maintained.
Veins: Valves, so backflow of blood is prevented. Walls are thinner, with less muscle and elastic tissue, because they do not have to withstand high pressure.
4. A capillary lumen is estimated to be around 8 μm. A red blood cell's diameter is approximately 6.2–8.2 μm. The figure below shows the cross-section of a capillary. Use the information provided and the diagram to describe two features that adapt the capillary for rapid and efficient diffusion.
Answer
The capillary wall is one cell thick, which means the diffusion distance is very short. The lumen is very narrow, so red blood cells are pressed against the wall and pass through one cell at a time, ensuring that diffusion has time to occur.
5. The figure below shows the relationship between transpiration and water uptake for one plant over a day. Describe the relationship between water uptake and transpiration.
Answer
The rate of water uptake and rate of transpiration show the same pattern, when one increases so does the other. There is a time delay between the two, water uptake lagging behind transpiration.
6. Explain the relationship between water uptake and transpiration.
Answer
Water diffuses out of the leaf by evaporation. This loss is replaced by water from the xylem, moving down a water potential gradient. The movement pulls water up the xylem in a continuous column, due to the cohesive properties of water and hydrogen bonding. Water enters the root by osmosis. There is a time delay between these processes because time is required for one to trigger the other.
7. The figure below shows the gills of a lugworm. Lugworms live in burrows on sandy beaches. They obtain oxygen from seawater when the sand is covered by the tide. The burrow has two openings. Seawater enters by one opening and leaves by the other, circulated by movements of the lugworm's body. The blood in the lugworm's gills and the seawater flow in different directions. Explain the advantage of this to the lugworm.
Answer
Countercurrent flow mechanism. Maintains a concentration gradient all the way along the gill. Enables more oxygen to be absorbed and more carbon dioxide to be removed.
8. When the tide goes out, the lugworm's burrow is filled with air rather than seawater. Air contains a much higher concentration of oxygen than seawater. However, the lugworm is unable to absorb significant amounts of oxygen from air. Explain why.
Answer
The gills stick together and are not separated in the absence of water, reducing the surface area available for gas exchange. In addition, fresh air does not circulate within the burrow, so the oxygen in the air becomes depleted.
9. The figure below shows the oxyhaemoglobin dissociation curve for a lugworm and a human. Describe how lugworm haemoglobin is different from human haemoglobin.
Answer
Lugworm haemoglobin has a higher affinity for oxygen and becomes fully saturated at a much lower partial pressure of oxygen.
10. Explain the advantage to the lugworm of having this kind of haemoglobin.
Answer
Haemoglobin becomes fully saturated when the tide is in, acting as an oxygen store. It does not dissociate or unload oxygen until the partial pressure of oxygen is very low, which occurs when the tide is out. This allows the lugworm to survive during low tide.