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This means that it is easier to determine the source of the light. As a result, cone cells are responsible for higher visual acuity since they allow you to better distinguish between two points Light is concentrated by a lens to the centre of the eye called the fovea. This region receives a high light intensity and therefore has more cone cells. The peripheries of the eye receive a low light intensity and therefore consist mainly of rod cells.

Principles of coordination In mammals, there are two main forms of coordination: The nervous system — Uses nerve cells that can pass electrical impulses along their length. The result is the secretion of chemicals by the target cells called neurotransmitters.

The response is quick, yet short lived and only acts on a localised region of the body. The hormonal system — Chemicals are transported in the blood plasma which then reach target certain cells, thus stimulating them to carry out a function.

The responses due to secretion of hormones often act over a longer period of time, yet are slower to act. Chemical mediators Nervous and hormonal forms of communication are only useful at coordinating the activities of the whole organism. At the cellular level they are complimented by chemical mediators.

Chemical mediators are secreted by individual cells and affect other cells in the immediate vicinity. A common example of this type of coordination is the inflammation of certain tissues when they are damaged or exposed to foreign agents.

Two examples of chemical mediators are: Histamine — Stored in white blood cells and is secreted due to the presence of antigens. Histamine causes dilation of blood vessels, increased permeability of capillaries and therefore swelling the infected area. Prostaglandins — Found in cell membranes and cause dilation of small arteries and arterioles.

They release due to injuries and increase the permeability of capillaries. They also affect blood pressure and neurotransmitters. In doing so they relieve pain. Hormonal system Nervous system Communication by chemicals Communication by nervous impulses.

IAA is used to ensure that plant shoots grow towards a light source. Cells in the tip of the shoot produce IAA, which is then transported down the shoot. The IAA is initial transported to all sides as it begins to move down the shoot 3. Light causes the movement of IAA from the light side to the shaded side of the shoot. A greater concentration of IAA builds up on the shaded side of the shoot 5.

The cells on the shaded side elongate more due to the higher concentration of IAA 6. The shaded side of the root therefore grows faster, causing the shoot to bend towards the source of light IAA can also effect the bending of roots towards gravity.

However in this case it slows down growth rather than speeds it up. IAA decreases root growth and increases shoot growth Section Can remove cell debris and are associated with nerve regeneration.

A nerve impulse is not an electrical current! It is a self-propagating wave of electrical disturbance that travels along the surface of an axon membrane. Nerve impulse — temporary reversal of the electrical p. This is called the sodium potassium pump. Sodium being positively charged causes the axon to become more positive in charge.

The myelin sheath — Prevents the action potential forming in myelinated areas of the axon. The action potential jumps from one node of Ranvier to another salutatory conduction — this increases the speed of the impulse as less action potentials need to occur 2.

The greater the diameter of the axon the greater the speed of conductance — due to less leakage of ions from the axon 3. Temperature — Higher temperature, faster nerve impulse. Energy for active transport comes from respiration. Respiration like the sodium potassium pump is controlled by enzymes.

Refractory period After an action potential, sodium voltage-gated channels are closed and sodium cannot move into the axon. It is therefore impossible during this time for a further action potential to be generated.

This time period, called the refractory period serves two purposes: It ensures that an action potential can only be propagated in one direction — An action potential can only move from an active region to a resting region.

It produces discrete impulses — A new action potential cannot be generated directly after the first. It ensures action potentials are separated from one another. It limits the number of action potentials — action potentials are separated from one another, therefore there is a limited amount that can pass along a neuron in a given time.

All or nothing principle Nervous impulses are all or nothing responses. A stimulus must exceed a certain threshold value to trigger an action potential A stimulus that exceeds the threshold value by a significant amount, will produce the same strength of action potential as if it has only just overcome the threshold value A stimulus can therefore only produce one action potential An organism can perceive different types of stimulus in two ways: The number of impulses in a given time larger stimulus, more impulses per second Having neurons with different threshold values — depending on which neurons are sending impulses, and how frequently impulses are sent, the brain can interpret the strength of the stimulus Section This means that several responses can be combined to give on single response Neurotransmitters are made in the presynaptic cleft only When an action potential reaches the presynaptic knob, it causes vesicles containing the neurotransmitter to fuse with the presynaptic membrane The neurotransmitter will the diffuse across the synaptic cleft The neurotransmitter then bind with receptors on the postsynaptic membrane, in doing so generating a new action potential in the postsynaptic neuron Features of synapses Unidirectionality.

Eventually the neurotransmitter will accumulate so as to overcome the threshold value of the postsynaptic membrane. Therefore generating a new action potential Inhibition Some postsynaptic membranes have protein channels that can allow chloride ions to diffuse into the axon making it more negative than usual at resting potential.

This type of hyperpolarisation inhibits the postsynaptic neuron from generating a new action potential. The importance of these inhibitory synapses is that it allows for nervous impulses to be controlled and stopped if necessary Transmission across a synapse When the neurotransmitter across a synapse is the chemical acetylcholine it is called a cholinergic synapse Acetylcholine is made up of acetyl ethanoic acid and choline Cholinergic synapses are more common in vertebrates Cholinergic synapses occur in the central nervous system and at neuromuscular junctions 1.

When an action potential reaches the presynaptic knob, calcium channels open allow calcium to diffuse into the presynaptic knob 2.

The influx of calcium ions causes presynaptic vesciles containing acetylcholine to fuse with the presynaptic membrane, releasing the neurotransmitter into the synaptic cleft 3. Acetylcholine diffuses across the cleft and fuses with receptor sites on sodium channels found on the presynaptic membrane.

When they do so, the sodium channels open, allowing sodium ions to diffuse along their concentration gradient into the postsynaptic knob. The influx of sodium ions, generates a new action potential in the postsynaptic neuron 5.

Acetylcholinesterase hydrolyses acetylcholine back into the acetyl and choline which will the diffuse back across the synaptic cleft into the presynaptic neuron. In this way acetylcholine can be recycles and reused and also is prevented from continuously generating new action potentials on the postsynaptic neuron. ATP is released by mitochondria, providing energy to recombine acetyl and choline. Follow 1 Our teachers have told us absolutely nothing about how to write a decent, let alone good, essay in biology paper 3.

Has anyone got any tips on how to format the essay and write it to a fairly good standard? I think I'm failing really badly at the moment. Zozaaaa Follow 4 followers 14 badges Send a private message to Zozaaaa. Follow 2 It is pretty simple, if you're like me and struggle with application, but know content like the back of your hand then you'll be fine.

First off, you're gonna need to write 5 or 6 fairly detailed paragraphs on the topic of the essay. If you do other essay subjects at sixth form then this will come in handy, write up a quick plan and think up where your topic links to. You get 3 marks for QWC, you do lose marks however for writing things which are incorrect, so do try and remember your content. For example, here is the question I had in my mock and here is one of the paragraphs I wrote which got high marks so you can see how I kind of structured it and linked it back to the question: The way in which water and the regulation of water content are important to organisms [25 marks] Additionally, water is important for active loading of sucrose in the phloem.

Sucrose is actively loaded into the phloem as the phloem has a high water potential and therefore a low solute potential next to the source end of the phloem. This enables sucrose molecules to travel down the concentration gradient into the phloem.

The sink end of the phloem has a low water potential as water diffuses from the phloem back into the adjacent xylem by osmosis to increase the solute potential. This enables sucrose to travel down the concentration gradient and diffuse into the sink cell where it can later be hydrolysed into glucose which can be used in photosynthesis in the plant.

Therefore, water regulation in plants enables transport of molecules used in metabolic reactions around the plant. I hope this was of some help, most of your marks are for scientific content, a few for qwc, a few for off spec stuff.

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