SL Paper 3
Two models of plasma membrane structure are shown.
[Source: diagram from article published in The American Journal of Pathology, 65, J Singer and G Nicolson,
The structure and chemistry of mammalian cell membranes, 427–437, Copyright Elsevier (1971)]
Phospholipase C is an enzyme that digests the polar heads of phospholipids. Scientists used phospholipase C to test these models of membrane structure. They found that the enzyme could digest the heads of phospholipids in the plasma membranes of red blood cells.
State the scientists who proposed model A.
Label the model A diagram to show a region of protein.
Label the model B diagram to show a phospholipid.
Deduce one conclusion about the structure of the plasma membrane reached by the scientists from their results.
Suggest one reason for maintaining a pH of 7.5 throughout the experiment.
State one technological improvement, other than enzymatic digestion, that led to the falsification of previous models to determine the current model of membrane structure.
An experiment was carried out on osmosis in carrot (Daucus carota) root tissue and potato (Solanum tuberosum) tuber tissue. Similar sized pieces of tissue were cut and soaked in different sucrose solutions for 24 hours. The results are shown in the graph below.
Using the graph, estimate isotonic sucrose solutions for potato tissue and carrot tissue.
Potato:
Carrot:
Suggest a reason for the difference in the isotonic points for the potato and the carrot tissues.
From the evidence provided by the graph, evaluate the reliability of these data.
Explain one reason for calculating the percentage change in mass.
The micrograph shows mitosis in a cell of an onion (Allium cepa) root tip.
Deduce, with a reason, which stage of mitosis is shown.
The cells visible in the onion root tip were classified and counted.
Calculate the mitotic index.
Membrane structure can be investigated using a technique known as freeze fracture.
Cells are frozen and then split. Fracturing often occurs between the two phospholipid layers of membranes in the cell. An electron micrograph of such a fractured membrane is shown.
[Source: © Science Photo Library]
Using the scale bar, calculate the magnification of the image.
Explain how electron micrographs such as this helped to falsify the Davson–Danielli model of membrane structure.
Explain how the amphipathic nature of phospholipids allows them to form bilayers.
The micrograph shows a section of an organ in the human body.
State from which organ the section was taken.
Identify the layer of tissue found at X.
The actual length of the structure labelled Y is 0.8 mm between the two black lines. Calculate the magnification of the micrograph. Working should be shown.
The image is a transmission electron micrograph of part of two adjacent pancreatic cells.
Using the scale bar, determine the diameter of the nucleus of the cell on the left, giving the units.
Deduce the magnification of the image.
Identify the structure labelled A.
The micrograph shows a transverse section through blood vessels of a mammal.
[Source: This book was originally published by OpenStax College, released under the CC-By license: https://creativecommons.org The eBook was adapted by Frank Lee.]
Identify the vein by labelling it with the letter V.
Distinguish between the vein and the artery with reference to structures visible in the micrograph.