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Homeostasis: Exam and practical skills

Learn more about the key skills you will need in order to carry out effective research on homeostasis.

Describing control of blood glucose level

Work is usually marked for scientific content, understanding and some discretion is used with poor spelling, but this is one topic where the names of the key molecules and hormones need to be spelt correctly. Understanding the derivation of the words can be useful.

‘Gluco’ or ‘glyco’ means glucose, ‘neo’ means new, ‘lysis’ means splitting, ‘genesis’ means birth or origin. Therefore ‘glyco-gen’ is a molecule that can be hydrolysed to release glucose, as can be seen in the process of ‘glycogenolysis’, which is the splitting of glycogen to make glucose.

If the word ‘glycagon’ is used where the answer should be ‘glucagon’ then no marks will be awarded. Remembering the words glucogenesis and glycogenolysis may be helpful but it is more important to understand the actual processes and why they are needed in homeostasis.

It is important to fully describe the modes of action of insulin and glucagon. Insulin does not convert glucose to glycogen itself. It promotes the uptake of glucose into the liver cells by attaching to receptors on the plasma membrane, which then triggers an increase in membrane permeability to glucose. After the glucose enters the cell, the activity of enzymes converting glucose to glucose phosphate increases. (This maintains a high diffusion gradient between the blood and liver cells). Other enzymes then convert the glucose phosphate to glycogen, which is stored in the cell. This has the effect of removing glucose from the blood and returning the concentration of blood glucose to its normal level. This lower blood glucose concentration stops the release of insulin from the beta cells of the pancreas (negative feedback).

Analysing samples and quantitative methods for glucose concentration

Analysing samples for their glucose concentration is a practical skill that is often tested in practical examinations, and knowledge of the procedure can be tested in written examinations. There are a number of different ways that the concentration can be estimated. Glucose is a reducing sugar that can donate electrons to other chemicals. In the case of Benedict’s reagent the electron is donated to the copper II ions in an alkaline solution of copper II sulfate. When the solution is heated to a high temperature, it forms an insoluble precipitate of copper I oxide, which changes the original colour of the solution from blue through green, yellow, brown to red depending on the amount of reducing sugar present. This gives a semi- quantitative estimation of the amount of reducing sugar present, if compared to set of test tubes containing known amounts of glucose. Reagent test strips e.g. Clinistix can also give a semi-quantitative test of glucose concentrations and are often used by doctors to give a fast estimation of urine glucose concentrations.

There are a number of different methods that can be used to give a more accurate estimation of glucose concentration. Glucose reacts with the potassium manganate VII solution and reduces the purple manganate (MnO4-) ions to colourless manganese ions (Mn2+). The time taken for the solution to decolorize is directly proportional to the amount of glucose in solution. A standard curve can be prepared by making up a range of solutions of known glucose concentrations and timing how long they take to decolorize. The unknown concentration can then be estimated by reading the glucose concentration from the standard curve equivalent to the time taken for the unknown solution to decolorize.

A compound called DNS (3,5 dinitrosalicylate) is reduced to ANS by glucose and this compound has a red colour. A calibration curve can be made using a range of known sugar solutions and reacting them with the DNS solution in a boiling hot water bath. The tubes are cooled and the absorbance read in a colorimeter with a blue filter (540nm) and a suitable blank. The unknown solution is treated in the same way and the concentration of glucose is read from the calibration curve obtained.

There is an alternative version of Benedict's reagent that can be used for quantitative testing. It contains potassium thiocyanate and does not form red copper oxide. The presence of reducing sugar is measured by the loss of the blue colour of copper sulfate in the reagent. A white precipitate is formed which will easily settle out or can be removed by filtration. The blue colour remaining in the supernatant can be read in a colorimeter. (A standard curve should be prepared as in the previous examples.)

Microscopic structure of the pancreas

It is expected that students have examined and can draw accurately from the microscope, the structures of the tissue of the main homeostatic organs: e.g. kidney, liver and pancreas. The pancreas is made up of the exocrine tissue, which is made of cells that manufacture and secrete enzymes and the endocrine tissue, which are called the Islets of Langerhans. The exocrine tissue cells are arranged in groups called an acinus (plural acini), which lie next to small ducts through which the secreted enzymes flow to the pancreatic duct. The islet cells generally take up less stain so can be seen as paler areas amongst the more densely stained exocrine tissue.

A drawing of any tissue needs to include an informative title with some indication of the scale or magnification of the drawing. The drawing should be of a reasonable size – at least a third of an A4 sheet of paper. It is helpful to make faint marks to indicate the height and width of the drawing and a faint outline can always be removed later. All lines need to be clearly drawn with a sharp pencil and there should be no shading included unless it is very helpful: e.g. in a soil profile. It should be noted that low power tissue plans should not include any individual cells, just the extent of each individual tissue type. Any label lines should be drawn in pencil and be horizontal.