A-LevelBiologyBiological molecules9700/22 · May/June 2025 · Q27 Marks
The transport of respiratory gases involves blood plasma and red blood cells. Red blood cells contain the globular protein, haemoglobin.
- (a)Describe features of a haemoglobin molecule that are typical of a globular protein, other than having an approximately spherical shape.[2]
- (b)A number of substances are involved in the transport of respiratory gases. Complete Table 2.1 by stating the name of the substance that matches the description of its role in the transport of respiratory gases. The first row has been completed for you. [Table 2.1][5]
Mark scheme
Marking points from the official mark scheme (May/June 2025). The mark or mark type is shown in brackets.
- [1] amino acids with hydrophilic R groups to, exterior / outside, (of molecule) A facing, watery / aqueous, environment / cytosol / cytoplasm OR amino acids with hydrophobic R groups to, interior / centre / AW, (of molecule)
- [1] soluble (in water / cytoplasm / cytosol) ; A forms colloidal suspension physiological / metabolic / dynamic, (role)
- [1] ref. to two or more bond types holding (3D) shape / tertiary structure / quaternary structure in place
- [1] chloride ion / Cl¯
- [1] carbaminohaemoglobin
- [1] hydrogencarbonate (ion) ; A HCO3
- [1] oxyhaemoglobin
- [1] hydrogen ion(s); A H+/proton
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