1. Explain why haemoglobin is said to have a "quaternary structure".
Answer
It is made up of more than one polypeptide chain.
2. The figure below shows the dissociation curve for human haemoglobin. Describe how the dissociation curve would change when a greater concentration of carbon dioxide is present and what this effect is called.
Answer
The curve would shift to the right, and it is called the Bohr effect.
3. The figure below shows a section of phloem tissue. Identify the structures labelled A–C.
Answer
A = Sieve plate,
4. A fetus obtains its supply of oxygen from its mother, via the placenta. Fetal haemoglobin is different from adult haemoglobin. The figure below shows the oxygen dissociation curves of adult and fetal haemoglobin. Identify which line represents fetal haemoglobin.
Answer
Line X because this has a higher affinity for oxygen than the maternal haemoglobin.
5. Explain the advantage of fetal haemoglobin having a different dissociation curve from maternal haemoglobin.
Answer
Fetal haemoglobin has a higher affinity for oxygen at low partial pressures, which means it can become fully saturated with oxygen from the mother's blood in the placenta. Because the fetus has a very high respiration rate, it requires plenty of oxygen to support its development.
6. At birth, the amount of fetal haemoglobin present in a baby's blood falls quite rapidly, and is replaced with adult haemoglobin. Suggest the advantage of this.
Answer
After birth, babies begin using their lungs for gas exchange, where the partial pressure of oxygen is higher than in the maternal blood of the placenta.
7. Describe the role of hydrogen ions in the process of translocation.
Answer
Hydrogen ions (H+) are actively transported out of the companion cell into the source, creating a concentration gradient between the source and the companion cell. This gradient drives the co-transport of H+ and sucrose into the companion cell, which then leads to the diffusion of sucrose into the phloem (sieve tube element).
8. Explain how sucrose is transported from the sieve tubes at the source to the sink.
Answer
At the source, the sieve tubes have a low water potential. Water moves in from the xylem by osmosis, down the water potential gradient. This creates a high hydrostatic pressure, generating a pressure potential gradient that drives the movement of the solution away from the source. At the sink, sucrose is actively transported out of the phloem, which increases the water potential there.
9. The figure below shows an investigation into the process of translocation. The investigators used a technique called ringing. This is when the outer tissues of the stem are removed including the phloem leaving only the xylem intact. The ring was removed below the lowest leaf on the plant. The second drawing shows the results after one day. Explain how this investigation gives evidence that sucrose is transported in the phloem.
Answer
There is no bulge below because water can move through the undamaged xylem. However, phloem damage halts the movement of the sucrose solution above the cut, causing a buildup of solution and resulting in bulging.