1. 1
   The marine bacterium Vibrio fischeri produces light (bioluminescence) only in some conditions. Each cell continuously releases a small signalling molecule called an autoinducer, which diffuses freely across membranes. In open seawater, where there are about 100 V. fischeri cells per cm³, the cells do not glow. In the light organ of the Hawaiian bobtail squid, the population reaches about 10¹¹ cells per cm³ and the bacteria glow brightly. In a laboratory, medium from a dense glowing culture was passed through a filter that removes all cells and was then added to a sparse, non-glowing culture. The sparse culture began to glow within an hour.

   (a)

   What term is used for the autoinducer when it binds to its receptor?

   [1 mark]

   • ALigand
   • BSubstrate
   • CAntigen
   • DSecond messenger

   (b)

   Why do V. fischeri cells glow in the squid light organ but not in open seawater?

   [1 mark]

   • AThe squid supplies the enzymes needed to make light
   • BOnly at high cell density does the autoinducer concentration become high enough to activate the receptors
   • CCells in seawater lack the genes for bioluminescence
   • DThe autoinducer is only released when cells are inside the squid

   (c)

   Explain the result of adding the filtered medium to the sparse culture.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A student compares four signalling chemicals in the human body. Insulin is a protein secreted by pancreatic cells into the blood and acts on liver, muscle and fat cells. Acetylcholine is released from a motor neuron into a synaptic gap about 20 nm wide and acts on a muscle fibre. Interleukin-2 is a small protein released by an activated helper T-lymphocyte that stimulates nearby lymphocytes to divide. Calcium ions are released from stores inside a muscle fibre after it is stimulated and trigger its contraction.

   (a)

   Which of the four signalling chemicals is a cytokine?

   [1 mark]

   • AInsulin
   • BAcetylcholine
   • CInterleukin-2
   • DCalcium ions

   (b)

   What happens when acetylcholine binds to its receptor on the muscle fibre?

   [1 mark]

   • ATyrosine residues inside the fibre are phosphorylated, moving glucose transporters
   • BAcetylcholine enters the nucleus and promotes transcription
   • CA G protein activates the production of cAMP, which opens calcium channels
   • DA channel in the receptor opens and positive ions diffuse into the fibre, changing the membrane potential

   (c)

   Contrast the way insulin and acetylcholine reach their target cells and the extent of their effects.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Liver cells were incubated with adrenaline (epinephrine) labelled with a radioactive isotope. All the radioactivity remained outside the cells or on the outer surface of the plasma membrane. The concentration of cyclic AMP (cAMP) in the cells was 0.2 pmol per mg of protein before adrenaline was added, 5.0 pmol per mg 30 seconds after, and 3.1 pmol per mg after 2 minutes; the rate of glycogen breakdown rose over the same period. Cells treated with a drug that inactivates G proteins showed no rise in cAMP and no glycogen breakdown when adrenaline was added. Cells given a cAMP analogue that can cross membranes broke down glycogen even without adrenaline.

   (a)

   Outline how binding of adrenaline to its receptor leads to an increase in cAMP inside the cell.

   [3 marks]

   (b)

   Evaluate the evidence that cAMP acts as a second messenger carrying the adrenaline signal into the cell.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   Insulin is a protein hormone of 51 amino acids; oestradiol is a steroid hormone with a relative molecular mass of 272. When muscle cells were given labelled insulin, the label was found only on the outer surface of the plasma membrane, and within 10 minutes the cells took up glucose three times faster. When hypothalamus cells were given labelled oestradiol, the label was found in the cytoplasm and then the nucleus. Responses to oestradiol took several hours to appear and were prevented by a drug that blocks transcription, whereas the response to insulin was not affected by this drug.

   (a)

   Compare and contrast the mechanisms by which insulin and oestradiol act on their target cells, using the evidence.

   [6 marks]

   (b)

   Outline the effects of oestradiol and progesterone on their target cells, and distinguish between positive and negative feedback in the regulation of cell signalling, using one example of each.

   [6 marks]

   Total for question 4: 12 marks

End of questions