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Supervisor comment: 'sub-headings are a useful crutch while you are learning to structure an essay'

Supervisors may have different views on how your essay should be presented. If you are in doubt, then ask what they are expecting. You may be asked for handwritten essays which will affect the editing process and the diagrams you are able to produce, but this is useful practice for exam writing. If your essay is word-processed, then utilise the footnotes to acknowledge sources if appropriate. For ease of reading, current convention is to leave a line space between paragraphs rather than to indent each first line.

Supervision essays are normally 3-6 A4 pages in length, or 1500 words. As long as you are concise, this length is sufficient to develop your argument effectively.


Practice using headings

Writing in the Sciences often benefits from the inclusion of headings as long as these enhance rather than detract from your argument. Many supervisors are in favour of sub-headings, especially if at this stage in your studies they help you to retain your focus in each section. It is, however, advisable to consider this as a temporary strategy whilst you are developing a more sophisticated style of writing.

Where would you insert headings in the following extract taken from How are proton gradients established and used by cells? The extract is paragraphed as the writer intended.

These membrane potentials as they are termed can exist across any membranes with the ability to pump ions against their gradients. One proton related use of these gradients is in secondary active transport. Unlike primary active transport, secondary active transport does not directly require the energy released from the hydrolysis or ATP to power transportation across the membrane. Instead the energy for this process is the free energy, as mentioned above, released when the ion moves down its electrochemical gradient. This is then coupled with the movement of another solute against its gradient and across the membrane. The movement of this solute maybe in the same direction as the passive movement of the ion (symporter) and in the opposite direction (antiporter), but either way the mechanism is still very similar.

The mechanism of secondary active cotransport of protons is seen in Escherichia Coli cells taking up lactose and other sugars. The bacteria from a H+ gradient across the membrane using ATP from the oxidation of various fuels to power the pumping of protons. The cotransporter protein involved is called lactose permease, it has a structure as such that between the two main regions of the protein there is an opening into which lactose can fit. This binding pocket faces the outside of the cell. Proteins diffuse from the extracellular fluid down their electrochemical gradients into the permease binding site where they bind to a carboxyl group. The proton is followed by lactose which also binds causing a conformational change releasing the lactose to the other side of the cell along with the proton. As both the ion and the cotransported substance are moving in the same direction the lactose permease enzyme can be described as a symporter.

A simpler use of proton gradients is in the formation of microenvironments with the cell that have differing internal and external pHs. As pH is directly related to H+ concentration controlling the number of protons either side of a membrane allows pH to be altered. An example of this use of proton gradients is in the intracellular lysosomes. Here a special type of proton pump known as a V-ATPase uses ATP to translocate protons into the endosome thus increasing the H+ concentration. As a result the pH is lowered to about pH5 in contrast to the cytosolic pH of about 7. This low internal pH is required by the lysosomal enzymes in order to function, and without it, as seen when a lysosome ruptures, the enzymes cannot function.

In bacteria proton gradients are at times formed by running ATP synthetic enzymes in the reverse direction. ATP synthase is a transmembrane protein that couples the energy released by the movement of protons down their gradients with the formation of ATP from ADP and P. This flow of protons is known as the proton motive force and is used to drive thermodynamically unfavourable actions such as the formation of ATP.

When driven in reverse, the hydrolysis of ATP can provide energy to pump H+ ions. Though this process seems a contradictory use of ATP, it is used in important processes such as flagellar rotations, providing movement and delivering nutrients to the cell. The rotation of the flagellum is brought about when proteins flow down there electrochemical gradients and through a rotary ATPase protein at the base. The energy from the chemiosmosi, as the movement is termed, causes the base of the protein to rotate and thus turns the flagellum.

Fermenting bacteria often use this reversal of ATP synthase method to create their proton gradients because they have no access to oxygen and therefore have none of the adaptations for H+ gradient formation seen in aerobic cells such as complex electron transport chains.

In some prokaryotes such as cyano bacteria, their H+ gradients are created by photophosphorylation whereby light is used to power proton pumping in an electron transport chain similar to that seen in photosynthesis.


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