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The Paper provided to you by samedayessay. 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.

Sodium channels on the postsynaptic membrane are now closed due to the absence of acetylcholine attached to receptor sites. Cardiac muscle which is found only in the heart Smooth muscle which is found in the walls of blood vessels Skeletal muscle which is attached to bone and is the only type of muscle under conscious control Muscles are made up of many muscle fibres called myofibrils.

If the cells of muscles were joined together from the end of one cell to another, the point between cells would be a point of weakness Because of this, the muscle cells are fused together into muscle fibres Cells of the same myofibrils share the same nuclei as well as cytoplasm sarcosplasm. Within the sacroplasm are many mitochondria as well as endoplasmic reticulum Microscopic structure of skeletal muscle Myofibrils are made up of two types of protein filament Actin — thinner, consists of two strands twisted around each other Myosin — thicker and is made up of long rod shaped fibres with bulbous heads projecting outwards Myofibrils have coloured bands.

The isotropic I bands appears lighter since it consists only of actin no overlap The anisotropic A bands are darker since this is where acting and myosin overlap The H zone is the region in the centre of the sarcomere that is lighter in colour since there is only myosin The z line lies at the centre of the I bands Types of muscle fibre Slow-twitch fibres — Contract more slowly, less powerful.

Large store of myoglobin, Supply of glycogen, Rich supply of blood vessels, Numerous mitochondria Fast-twitch — Contracts more rapidly with more power but only for a short period of time.

Adapted for intense exercise by:. Having hicker and more numerous myosin filaments, having a high concentration of enzymes used for anaerobic respiration, a large store of phosphocreatine to provide phosphate to make ATP Neuromuscular junctions Many neuromuscular junctions are spread through the muscle for simultaneous contraction Each muscle fibre has one motor neuron associated with it.

The muscle fibre and the neuron make up one motor unit When only a small force is needed only a few motor units are stimulated When a nerve impulse reaches the neuromuscular junction, the synaptic vesicles join with the presynaptic membrane and release acetylcholine which diffuses across to the postsynaptic membrane and stimulates it to allow sodium ions to enter.

The acetylcholine is then broken down by Acetylcholinesterase and then diffuses back into the presynaptic neuron. The I band becomes narrower The z lines move close to one another The h band becomes narrower The a band does not change as this band is determined by the width of the myosin Myosin is made up of two different types of protein 1. A fibrous protein arranged into the filament called the tail 2. A globular protein that forms a head at each end.

Stimulation, contraction and relaxation Muscle stimulation When an action potential reaches the neuromuscular junctions, Calcium ion channels open and calcium ions move into the synaptic knob The Calcium ions cause the synaptic vesicles to move to the presynaptic membrane and fuse with it releasing acetylcholine Acetylcholine diffuses across the synaptic cleft and binds with receptors on the sodium voltage gated channels on the postsynaptic membrane causing it to depolarise.

Muscle contraction The action potential movies through the fibres by travelling through T — tubules that branch through the sarcoplasm The action potential moves through the tubules until it reach the sarcoplasmic reticulum The action potential opens calcium ions in the sarcoplasmic reticulum Calcium ions diffuse out into the muscle Calcium ions cause tropomyosin to change shape and so that the binding sites on the actin filament are exposed An ADP molecule that is attached to the myosin heads allows it to form a cross bridge with actin by binding with the receptor site Once the cross bridge is formed, the myosin head changes shape and slides the actin across.

The myosin head now has a new ADP molecule that will allow it to bind with a new receptor site somewhere along the actin filament Muscle relaxation When the muscle is not being stimulated, the sarcoplasmic reticulum actively transport calcium ions back into it The lack of calcium ions means that tropomyosin can establish its original position, covering the myosin head binding sites Energy supply Energy is needed for the movement of myosin heads and the active transport of calcium ions ATP often needs to be generated anaerobically.

Maintaining the volume, chemical make up and other factors of blood and tissue fluid within restricted limits There are continuous fluctuations; however, they occur around a set point Homeostasis is the ability to return to that set point thus maintaining equilibrium The importance of homeostasis. Enzymes and other proteins are sensitive to changes in pH and temperature Water potential of blood and tissue fluid should be kept constant to ensure cells do not burst or shrink due to a net movement of water osmosis Maintaining a constant blood glucose concentration ensures that the water potential of the blood remains the same Independence of the external environment — a wider geographical range and therefore a greater chance of finding food shelter, etc Mammals — homeostasis allows them to tolerate a wide range of conditions Control mechanisms The set point is monitored by: Controller — brain analyses and records information from a number of different sources and decides on the best course of action 3.

Effector — brings about the change to return to set point 4. Feedback loop — informing the receptor of the changes in the system brought about by the effector Section Can sometimes change colour to alter heat that is radiated Regulation of body temperature in Endotherms Most heat gained through internal metabolic activities.

Temperature range — 35 — 44 oC — Compromise between higher temperature where enzymes work more rapidly and the amount of energy needed hence food to maintain that temperature Conserving and gaining heat in response to a cold environment Long term adaptations: V ration Therefore mammals and birds in cold environments are relatively large Smaller extremities e. Behavioural mechanisms — bathing in the sun Decreased sweating. Loss of heat in response to a warm environment Long term adaptations: V ratio so smaller animals are found in warmer climates Larger extremities Light coloured fur to reflect heat Vasodilation — Arterioles increase in diameter, more blood reaches capillaries, more heat is therefore radiated away Increased sweating — Heat energy is required to evaporate sweat water.

Energy for this comes from the body. Therefore, removes heat energy to evaporate water Lower body hair — Hair erector muscles relax. Hairs flatten, reduces the insulating layer of air, so more heat can be lost to the environment Behavioural mechanisms — seeking shade, burrows, etc Control of body temperature.

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Assessment guides: essays We’ve created these essay resources to support your teaching of the new AS and A-level Biology specifications and help you prepare students for the essay in A-level Paper 3.

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