C2.1 Chemical signalling (HL). Practice questions with markscheme.

• C: glucose per cAMP molecule = 3.0 × 10⁸ ÷ 6.0 × 10⁵ = 500, and glucose per receptor with epinephrine bound = 3.0 × 10⁸ ÷ 40 = 7.5 × 10⁶, so each bound receptor leads to millions of glucose molecules because the signal is amplified at each stage of the cascade; A divides by the number of G proteins activated (8.0 × 10²) instead of the number of receptors; B divides the wrong way round; D gives the number of cAMP molecules made per receptor (6.0 × 10⁵ ÷ 40 = 1.5 × 10⁴), not glucose per cAMP molecule;

• B: a run of about 20 hydrophobic amino acids can span the hydrophobic core of the membrane, so P is a transmembrane receptor with its binding site outside the cell, suited to a hydrophilic peptide hormone that cannot cross the membrane; Q has no such stretch, so it is not held in a membrane but lies in the cytoplasm or nucleus, where it can bind a hydrophobic steroid hormone that has diffused into the cell;

• D: nicotinic acetylcholine receptors are ligand-gated (cation) channels; binding opens the channel, Na⁺ entry depolarizes the postsynaptic membrane;

• (a) [2]
  • the response decreases as the distance from the source increases;
  • the decrease is steepest close to the source / the response falls by more than half (to 37 %) within the first 100 µm;
  • the response is (almost) zero, 5 % or less, at 300 µm and beyond;
• (b) [2]
  • untreated slice: 70 µm (accept 65 to 75 µm);
  • slice treated with the drug: 175 µm (accept 165 to 185 µm);
• (c) [2]
  • in the untreated slice Z is broken down by the enzyme as it diffuses, so its concentration falls quickly with distance;
  • with the enzyme inhibited, Z lasts longer, so it diffuses further from the source;
  • (so) more Z reaches and binds to the receptors of distant cells, so the response is larger at every distance beyond the source (e.g. 20 % rather than 2 % at 400 µm);

• (a) [2 max]
  • cytokines are (small) proteins secreted by many types of cell, especially cells of the immune system, whereas hormones are secreted by endocrine glands, OWTTE;
  • cytokines usually act locally on nearby cells (or on the cell that secreted them), whereas hormones are transported in the blood to distant target cells;
  • cytokines coordinate immune responses / inflammation, whereas hormones regulate processes such as growth, metabolism and reproduction;
  • cytokines are secreted in response to a stimulus such as infection, whereas many hormones are secreted continuously at a regulated rate, OWTTE;

  Credit only explicitly comparative statements. Award converse statements.

• (b) [2 max]
  • calcium ions act as signals inside cells / are intracellular signalling chemicals, rather than being secreted to act on other cells;
  • a rise in the calcium ion concentration in the cytoplasm (by entry from outside the cell or release from the endoplasmic reticulum) triggers a response;
  • example: entry of calcium ions into a presynaptic neuron triggers release of neurotransmitter by exocytosis OR calcium ions released inside a muscle fibre trigger contraction OR entry of calcium ions into an egg cell triggers the cortical reaction;

  Accept any valid named example.

• (a) [2 max]
  • in negative feedback the product/response inhibits the pathway, whereas in positive feedback the product/response stimulates the pathway;
  • negative feedback counteracts change / returns the system towards a set point, whereas positive feedback amplifies change / moves the system further from the starting point;
  • negative feedback maintains stability, whereas positive feedback drives a process rapidly to completion, OWTTE;

  Credit only explicitly comparative points.

• (b) [2]
  • negative feedback example: thyroxine inhibits TRH/TSH secretion OR insulin secretion falls as blood glucose is lowered OR oestradiol reducing GnRH secretion, OWTTE;
  • positive feedback example: oxytocin in childbirth (contractions stimulate further oxytocin release) OR high oestradiol stimulating GnRH/the LH surge OR ethylene stimulating ripening and further ethylene production;

  One mark for a correctly outlined example of each. Accept other valid examples.

• both are carried in the blood and so reach cells throughout the body;
• both are effective at very low concentrations;
• both bind to a specific receptor whose binding site is complementary in shape to the hormone;
• both act only on cells that possess the appropriate receptor, so cells without it give no response;
• both change the activity of the target cell, e.g. by altering the quantity of a protein that the cell makes;
• both are broken down or removed, so the response does not continue indefinitely;

Compare requires similarities only; each point must refer to both types of hormone. A statement that the receptors are in different locations is a contrast and gains no credit here.