Download How do poikilothermic organisms tolerate

Plagiarism and referencing exercise
A class has been set an essay to write, entitled ‘How do poikilothermic organisms tolerate freezing
temperatures?’. It’s a short essay, and should be around 500 words in length. Two students, Archibald and
Beauregard have each independently gone to the library and, knowing that part of the science of ecology
involves studying how organisms interact with their environments, have looked in two ecology textbooks
for information to help them with their essays. Both of the textbooks have sections dealing with
temperature, and both of these sections have some information on life at low temperatures. The sections
from both books relevant to the essay are reproduced on pages 2-3, and the essays that the two students
wrote, using exactly the same source material are on pages 4-6.
You should read through the source material, and then carefully read through both essays. When you’re
reading through the essays, think about the following points.
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•
•
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How has the student used the source material?
How similar is the essay to the source material, and is the degree of similarity acceptable?
How well does the essay actually answer the question that is its title?
Is the use of references in the essay correct?
What mark would you give the essay?
In your workbook, write down your opinion of each essay, answering each of the above points.
Source 1: pages 59-61 from Begon, M., Harper, J.L. and
Townsend, C.R. (1996) Ecology. Third Edition. Blackwell
Science
Source 2: page 99 from Krebs, C.J. (1985) Ecology: The Experimental Analysis of Distribution and
Abundance. Third Edition. Harper and Row.
How do poikilothermic organisms tolerate freezing temperatures?
By Archibald
The greater part of our planet is below 5oC and cold is the fiercest and most widespread enemy of life. We
inhabit a cold planet and should regard those organisms that are best able to cope with cold temperatures as
being its most successful colonisers (Russell, 1990). There are two quite distinct types of damage (chilling
and freezing) at low temperatures that can be lethal, either to tissues or two whole organisms.
Cold itself can have lethal physical and chemical consequences even though ice may not be formed. Water
may supercool to temperatures at least as low as –40oC, remaining in an unstable liquid form in which its
physical properties change in biologically significant ways. Many organisms are damaged by exposure to
temperatures that are low, but above freezing point – ‘chilling injury’. The nature of the injury seems to be
associated with the breakdown of membrane permeability and the leakage of specific ions such as calcium
(Minorsky, 1985).
Ice seldom forms in an organism until the temperature has fallen several degrees below zero. Pure water
then separates out of the solid phase and the concentration of solutes in the liquid phase rises. When ice
forms in plant or animal tissues it is almost always extracellular water that freezes. It is very rare for ice to
form within cells and it is then inevitably lethal, but the freezing of extracellular water is one of the factors
that prevents ice forming within the cells themselves (Franks et al., 1990).
The damage from freezing is greatest if supercooling has occurred and ice forms suddenly. For this reason,
mechanisms that encourage the formation of ice at temperatures only just below 0oC protect an organism
from freezing damage. Certain bacteria (Pseudomonas syringae and Erwinia herbicola) are able to
synthesize materials that catalyse the formation of ice at temperatures as high as –4oC. Some species of
bivalves and gastropods also produce ice-nucleating proteins that induce the formation of extracellular ice
during the winter (Johnston, 1990).
Insects have been shown to have two strategies that allow survival through the low temperatures of winter.
A ‘freeze-avoiding’ strategy uses low-molecular-weight polyhydric alcohols (e.g. glycerol and sorbitol)
that prevent the formation of intracellular ice by depressing both the freezing point and the supercooling
point and uses specialized proteins to prevent ice nuclei from forming. A contrasting, ‘freeze tolerant’
strategy encourages the formation of extracellular ice but protects the cell membranes from damage when
water is withdrawn from the cells (Storey, 1990).
Some Antarctic fishes are able to avoid freezing, even though they live in water full of small ice crystals (1.9oC). In these fishes occur glycoprotein molecules that appear to have as their sole function the lowering
of the freezing point of the tissue fluids (DeVries 1971). The glycoprotein molecules contain a large
number of –OH groups that are essential for antifreeze activity. The concentrations of these biological
antifreezes seem to be proportional to the freezing dangers encountered by the fish species (Hochachka and
Somero 1973, p.265).
Bibliography
Begon, M., Harper, J.L. and Townsend, C.R. (1996) Ecology. Third Edition. Blackwell Science.
De Vries (1971) Glycoproteins as biological antifreeze agents in Antarctic fishes. Science 172: 1152-1155
Franks, F., Mathias, S.F. & Hatley, R.H.M. (1990) Water, temperature and life. Philosophical Transactions
of the Royal Society, Series B, 326, 517-533
Hochachka, P.W. and G.N. Somero (1973) Strategies of Biocehmical Adaptation. Saunders, London.
Johnston, I.A. (1990) Cold Adaptation in marine organisms. Philosophical Transactions of the Royal
Society, Series B, 326, 655-667.
Krebs, C.J. (1985) Ecology: The Experimental Analysis of Distribution and Abundance. Third Edition.
Harper and Row.
Minorsky, P.V. (1985) An heuristic hypothesis of chilling injury in plants: a role for calcium as the primary
physiological transducer of injury. Plant Cell and Environment 8, 75-94.
How do poikilothermic organisms tolerate freezing temperatures?
By Beauregard
Much of our planet experiences cold temperatures for at least part of the year, and so many organisms are
exposed to temperatures capable of causing damage to tissues or killing whole animals or plants. Cold
temperatures can damage organisms in two ways: chilling and freezing. Chilling damage is caused when
the organism experiences temperatures that do not cause water in the organism to freeze but which are cold
enough to change the physical properties of the water in the animal enough to interrupt the normal
physiological functioning of the organism. This may occur at temperatures above freezing, or it may
happen when the temperature is below freezing but the water in the organism has not frozen: a state known
as ‘supercooling’. Freezing damage occurs when the water in the organism actually turns into ice. Ice
crystals can form extracellularly, in which case they will change the concentration of the solutes in the
remaining fluid as water turns to ice, or they can form intracellularly. Intracellular ice crystal formation is
usually lethal as it disrupts cell membranes. (Begon et al. 1996).
Organisms which are poikilotherms, i.e. which tend to have internal temperatures similar to those in the
environment, can follow two different strategies in order to avoid being damaged by freezing when
temperatures fall below 0oC (chilling damage seems to be largely confined to tropical organisms such as
bananas). These two strategies are freeze avoidance and freeze tolerance. Freeze avoidance is just that- the
organism avoids freezing, usually by producing some form of ‘antifreeze’ molecules that stop ice forming
within the organism. This can be achieved by lowering the freezing and supercooling points of water by
producing polyhydric alcohols such as glycerol and by producing specialised proteins that prevent ice
nuclei forming (Begon et al. 1996). Many Antarctic fish are known to be able to survive in water that is
well below freezing point (-1.9oC) which may itself contain many small ice crystals. These fish produce
glycoprotein molecules that have many –OH groups that give them an antifreeze effect, thus lowering the
freezing point of the fluids in the animals tissues (DeVries 1971, cited in Krebs, 1985).
Freeze tolerant organisms, on the other hand, avoid damaging intracellular ice formation by encouraging
extracellular ice formation. This leads to dehydration of the intracellular fluids as the concentration of
extracellular solutes increases and water is drawn out of the cells by osmosis. This removal of intracellular
water means that damaging ice crystals are not able to form within the organisms’s cells. Such freeze
tolerant organisms also produce substances such as glycerol which help protect the complex molecules and
structures within the cells from damage during this dehydration process. Many plants and also some species
of insects and molluscs show this freeze tolerance, and many require a period of ‘cold hardening’ before
they are fully freeze tolerant, during which they accumulate these defensive compounds (Begon et al.
1996). Interestingly, some bacterial species such as Pseudomonas syringae are also able to protect
themselves from damaging intracellular ice formation by promoting the formation of ice crystals around
their cells (Schnell and Vali 1976, cited in Begon et al. 1996).
Bibliography
Begon, M., Harper, J.L. and Townsend, C.R. (1996) Ecology. Third Edition. Blackwell Science.
Krebs, C.J. (1985) Ecology: The Experimental Analysis of Distribution and Abundance. Third Edition.
Harper and Row.