Draw a labelled diagram to show the human ventilation system.

Outline anaerobic cell respiration in plant cells.

Explain the concept of homeostasis, using the control of blood sugar as an example.

Award [1] for each structure clearly drawn and correctly labelled.

trachea;

bronchi;

bronchioles;

lungs;

alveoli – shown enlarged as inset;

diaphragm;

intercostal muscles;

abdominal (wall) muscles;

Award [3 max] for diagrams that do not show correct connections or proportions.

anaerobic (cell) respiration in the absence of oxygen;

glycolysis / breakdown of glucose molecules;

leads to the production of pyruvate;

also known as fermentation;

production of small yield/two ATP (molecules per molecule of glucose respired);

produces ethanol;

produces carbon dioxide;

occurs in cytoplasm;

example of anaerobic respiration in plants (e.g. waterlogged roots);

maintaining the internal environment constant/between narrow limits;

example (other than blood sugar) of blood pH / oxygen and carbon dioxide concentrations / body temperature / water balance;

involves negative feedback;

where a variation from the normal (blood sugar level) triggers the correction mechanisms;

controlled by both nervous and endocrine systems;

blood sugar above normal stimulates insulin release;

insulin secreted by β cells in the (islets of the) pancreas;

insulin lowers blood sugar;

by converting to glycogen/fat / increased respiration;

blood sugar below normal stimulates glucagon release;

glucagon secreted by α cells in the (islets of the) pancreas;

glycogen converted to glucose; causes increased level of glucose in the blood;

In the diagram of the human ventilation system, alveoli needed to be shown as an inset to gain their mark. This was consistent with the Teacher’s notes for A.S. 6.4.4 in the Subject Guide. Many candidates included intercostal muscles but it was difficult to show them clearly. Abdominal (wall) muscles were not shown. The quality of the ventilation diagrams was generally lower than for the membrane diagrams. Correct labels must correspond to recognizable structures.

Uncertainty as to the type of anaerobic respiration found in plants may have put off some candidates since, for A.S. 3.7.3., the Teacher’s notes only mention yeast and humans. Nonetheless, some candidates gained full marks knowing those aspects common to both pathways. Also, candidates may have associated yeast with plants, thereby describing the alcohol fermentation pathway. At least one candidate was able to cite waterlogged roots as a place where anaerobic respiration would occur in plant cells.

Many candidates did quite well on this question, showing good knowledge and understanding of homeostasis. The mark scheme provided ample opportunities for many high scoring answers. Negative feedback was frequently included but "controlled by both nervous and endocrine systems" was rarely seen.