22964 questions

What is an action potential, and what steps are involved?

Action potentials are changes of electrical potential in a cell. The normal resting membrane potential inside cells is around -70mV. This membrane potential is maintained through the action of the Na+K+ATPase pump (which moves 3Na+ out and 2 K+ in), leaky Na channels (that allow influx of Na+) and leaky K+ channels (that allow efflux of K+). The net movement of Na+ and K+ is balanced at this point.An action potential starts when Na+ channels open and Na+ starts moving into the cell. This starts depolarisation of the cell, where the membrane potential starts increasing. At around -30mV a threshold is met. This is an all or nothing response, if enough stimulus is present all the voltage-gated Na+ channels open causing a rapid depolarisation of the cell. When the membrane potential reaches around +30/40mV the Na+ channels close, and the voltage-gated K+ channels open at the same time. As K+ starts moving into the cell, it starts repolarisation of the cell - the membrane potential starts moving down. The K+ channel remain open past -70mV and eventually close when the potential reaches -90mV - this is known as hyperpolarisation. The action of the Na+K+ATPase pump then brings the cell back to its resting membrane potential (-70mV). At this point a second action potential can occur.
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Claudia L.

Answered by Claudia, Human Biology GCSE tutor with MyTutor

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How do you ask questions in French?

There are three ways of asking questions in French, and you must choose the right form according to the context and the person you are speaking to.The easiest and most used way, especially in spoken French, is to write or say a statement, and put a question mark at then end. When speaking you would simply say that statement as a question: 'tu travailles ce soir ?' Are you working this evening? NOTE: This is very informal French and you must avoid this form when writing in exams.To get around this in an exam you can use the second way to ask a question: by taking the sentence in the first example and putting 'est-ce que'. 'est-ce que tu travailles ce soir ?' Are you working this evening? NOTE: This is the form you should learn for your exams as it is standard French.The final way of asking a question is with the use of inversion.You take the verb and subject pronoun and switch them around:'Travailles-tu ce soir ?' This way of asking a question will be found in written French and any questions in French will be written with this form so it should be learnt and recognised.
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Christian H.

Answered by Christian, French A Level tutor with MyTutor

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Explain the sliding-filament theory

Annotated diagram of a sarcomere. Explaining the key steps
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Joonatan P.

Answered by Joonatan, Biology IB tutor with MyTutor

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Explain the stages of aerobic respiration that occur in the mitochondria of eukaryotes. [8]

Aerobic respiration is the controlled released of energy in cells by converting organic molecules into ATP. It invovles the reduction and oxidation of electron carriers. Firstly the link reaction occurs where pyruvate from glycolysis is converted into acetyl coenzyme A, this is done by removing CO2 from a pyruvate and reducing a NAD molecule. Next in the Krebs cycle a four carbon molecule combines with the acetyl coenzyme A and a 6 carbon molecule is formed. Next the molecule is decarboxylated twice meaning two CO2 molecules are released and a 4 carbon compound remains. 3 reduced NAD molecules and a reduced FAD molecule are formed. There is some ATP also generated during the Krebs cycle. The four carbon molecule that remains will now start the next cycle with a new acetyl coenzyme A. Next the electron transport chain occurs, the now reduced NAD and FAD molecules carry the electrons to the inner membrane of the mitochondria. The electrons are deposited to carriers which enable proton pumping to occur, protons accumulate in the inter-membrane space and generate potential energy. Through diffusion the protons now pass through the ATP synthase enzyme embedded into the membrane and using their kinetic energy create ATP by combining an ADP molecule with an extra inorganic phosphate, this process is known as chemiosmosis. Finally as the electrons travel from carrier to carrier they reach the final terminal acceptor oxygen.
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Joonatan P.

Answered by Joonatan, Biology IB tutor with MyTutor

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How is different an NFA from a DFA?

The difference between an NFA (non-deterministic finite state automaton) and a DFA (deterministic finite state automaton) is that while in the former we can have more than one transition with the same label from the same state, in the latter each label has only one transition associated with it.
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Michelangelo P.

Answered by Michelangelo, Computing A Level tutor with MyTutor

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What is the geometrical meaning of the derivative of a function?

Since the definition of derivative is the limit of the incremental rapport as h approaches 0 of ( f(x + h) - f(x) ) / h ), the geometrical meaning of a derivative is the slope of the tangent line in a general point (x, y) of the function (we assume the the function is sharp through all the points in its domain).
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Michelangelo P.

Answered by Michelangelo, Further Mathematics A Level tutor with MyTutor

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How is the human body used in Wilfred Owen's poem 'Mental Cases'?

The human body is broken down in 'Mental Cases' and is detached from the soldiers. Owen dehumanises the men, to better establish how the horror of the First World War is impacting them. He compares them to some sort of demon, unable to view them as human any more, 'Sure we have perished / Sleeping, and walk hell; but who these hellish?'. He struggles to identify with them, as established by the opening rhetorical question of 'Who are these?' Additionally, even the men's faces are no longer human, 'Thus their heads wear this hilarious, hideous, / Awful falseness of set-smiling corpses'. Owen describes the human body using abstract terms to dehumanise the soldiers, highlighting how their trauma is impacting them and making them forever different from the rest of humanity.
Beyond the soldiers, Owen uses the body as a way of describing the world around the patients. Here, he is highlighting how the war has bled into all of the world the soldiers are experiencing and they are unable to get the images of death out of their minds, 'Sunlight seems a bloodsmear: night comes blood-black; / Dawn breaks like a wound that bleeds afresh'. The trauma the soldiers have experienced is causing them to visualise their memories everywhere and they are unable to view reality without remembering images of death and bodies. This makes clear Owen's anti-war message, and shows a great awareness for the effects PTSD has on these patients.
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Callum W.

Answered by Callum, English A Level tutor with MyTutor

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How do I start a plan for an exam essay?

First, read the question and identify what it is asking you to do. Then decide what your argument is going to be e.g. if the question is asking you to assess the importance of the Russo-Japanese war to the Russian Revolution, decide whether you think it s the most important factor or if there were others which would better explain the conflict. Organise your points into the order you want to make them e.g. least to most important factor and assign roughly one paragraph to each point. Decide what facts or historical opinions you want to use to back up your point, analyse them and decide how you are going to show that they illustrate you argument. The introduction should briefly give the historical context of the essay and outline the points you wish to make as well as your overall argument. The conclusion should summarise how the points you have made prove your own argument. Given the time constraints in the exam, try to be brief and avoid overly descriptive sections, analysis is key.
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Rebecca H.

Answered by Rebecca, who has applied to tutor History Scottish Highers with MyTutor

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