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AQA Categories Archives: 3.6 Organisms respond to changes in their internal and external environments

Control of blood water potential

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 9 of 9 TOPICS

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

Control of blood water potential:

The nephron starts in the cortex (the outer layer of the kidney) at the Bowman’s capsule and then starts to weave where this area is called the proximal convoluted tubule. The nephron from the proximal convoluted tubule enters the medulla (the inner layer of the kidney) where it straightens and descends, curves and ascends back up into the cortex. The ‘U-looking’ bit of the nephron is called The loop of Henlé. The nephron then weaves again which is called distal convoluted tubule and then becomes the collecting duct and descends to become ureter which goes into the bladder. NB: Proximal convoluted tubule and distal convoluted tubule can be abbreviated into PCT and DCT but if you are going to use these abbreviations then you should know what the letters stand for as the whole name can be given in an exam question. The whole of the nephron from the Bowman’s capsule to the collecting duct wrapped around with capillaries to filter the blood to make a filtrate and also to reabsorb any useful molecules for the body called selective reabsorption where at the end it is called renal vein.

The Bowman’s capsule has capillaries in the Bowman’s space called glomerulus which supplied with blood from the renal arteriole called the afferent arteriole. The blood then leaves the glomerulus via an arteriole called the efferent arteriole. A glomerular filtrate is made by a process called ultrafiltration. The afferent arteriole has a larger diameter than the efferent arteriole and so the blood in the afferent arteriole has a higher hydrostatic pressure than the blood in the efferent arteriole. This means that urea, water, amino acids and glucose are forced out of the capillaries into the nephron (Bowman’s capsule). Large proteins and blood cells cannot be forced through as they are too large to go through ultrafiltration. Between the capillary and the Bowman’s capsule the small molecules have to go through 3 layers: the endothelium of the capillary wall, the basement membrane made of collagen in the middle of the capillary and the Bowman’s capsule and the epithelial cells that line the Bowman’s capsule. The filtrate that is in the nephron is called the glomerular filtrate.

Selective reabsorption takes place from the proximal convoluted tubule, loop of Henlé, distal convoluted tubule and collecting duct into the blood . The endothelium cells in the proximal convoluted tubule have microvilli which provide a large surface area to absorb the useful molecules. These include:

  • Glucose by facilitated diffusion and active transport. Active transport is also needed for the reabsorption of glucose because facilitated diffusion increases the amount of glucose in the medulla compared to the nephron therefore another process is needed to absorb glucose up the concentration gradient.
  • Water by osmosis from the proximal convoluted tubule, loop of Henlé, distal convoluted tubule and collecting duct. The remaining filtrate in the nephron is then sent down the ureter into the bladder.

The loop of Henlé is very important as it plays a role in absorbing water into the capillaries. The loop of Henlé has a descending limb and an ascending limb which makes up the loop. In the ascending limb, Na+ ions are pumped out by active transport out of the ascending limb into the medulla up its concentration gradient. The water potential in the medulla lowers and is lower than the water potential in the ascending limb but the membrane of the ascending limb is impermeable to water therefore water cannot exit the nephron. Only very few Na+ ions come into the descending limb but does not change anything as it has minimal effect. The high concentration of Na+ ions in the medulla causes water to leave the descending limb by osmosis. The filtrate in the descending limb becomes more concentrated but cannot really leave, down its concentration gradient into the medulla as it is relatively impermeable permeable to the Na+ ions. Water is then reabsorbed into the capillary called the vasa recta (the capillary that surrounds the whole of the nephron) by osmosis from the medulla. The filtrate then flows down the descending limb to the bottom of the loop of Henlé where at this point the concentration of Na+ ions is the highest. Na+ ions diffuse out of the nephron into the medulla down its concentration gradient. The medulla now has two regions of Na+ ions where the region that is closest to the bottom of the loop of Henlé which has the highest amount of Na+ ions in the nephron, has the larger concentration. Therefore a concentration gradient is made in the medulla of Na+ ions which allows the maximum amount of water to leave the distal convoluted tubule and along the whole of the collecting duct.

Osmoregulation is the process where organisms regulate the water content in their bodies. This is needed to keep the water content in the body relatively constant. Osmoreceptors in the hypothalamus of the brain detect a change in the water content and so send an electrical impulse to the posterior pituitary gland which releases antidiuretic hormone and in an amount depending on the water potential in the blood. The kidneys respond to this change and so change how concentrated the urine will be. NB: Antidiuretic hormone can be abbreviated into ADH but make sure that when you do use the abbreviated version, you know what it stands for as it can get confusing if the full name was to be given in the exam. If the water potential in the blood is too low the osmoreceptors shrink as water leaves down its concentration gradient from the cell to the blood by osmosis. An electrical impulse is sent to the posterior pituitary gland where ADH is released into the blood in high quantities. This makes the membrane of the collecting duct more permeable to water and so the water leaves by osmosis into the blood making the urine more concentrated. If the water potential of the blood is high the osmoreceptors in the hypothalamus become turgid and sends an electrical impulse to posterior pituitary gland where ADH is made in minimal quantities. This makes the membrane of the collecting duct relatively impermeable to water where not much water is reabsorbed making the urine less concentrated.

 

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Control of blood glucose concentration

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 8 of 9 TOPICS

 

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

Control of blood glucose concentration:

Factors that affect glucose concentration are food, medication, activity, biological, environmental and many more.

Insulin is released from the beta cells of the islets of Langerhans in the pancreas when high levels of glucose are detected. It binds insulin receptors of the liver which stimulates the conversion of glucose to glycogen (glycogenesis) and glucose to fat. This causes the level of glucose to drop. If the blood glucose concentration becomes too low, it is detected by the alpha cells of the islets of Langerhans in the pancreas. These produce glucagon which binds to receptors of the liver cells. This stimulates the conversion of glycogen to glucose (glycogenolysis) and using other biological molecules such as amino acids and glycerol to convert them into glucose (gluconeogenesis). This will cause an increase in the level of glucose. This whole process is known to be multiple feedback as the levels of glucose fluctuate around the normal level.

Insulin that is secreted by the beta cells also binds to insulin receptors of body cells. This increases the rate of absorption of glucose into the body cells particularly muscle cells and is used for respiration.

The adrenal glands are found on top of each kidney and it consists of the adrenal cortex and adrenal medulla:

  • Adrenal cortex: This releases cortisol which is secreted when glycogen stores run out. It stimulates gluconeogenesis which increases glucose levels quickly.
  • Adrenal medulla: This releases adrenaline and causes glycogenolysis. NB: Adrenal medulla may be abbreviated into just medulla. However it may get confusing with the medulla oblongata in the brain which is also abbreviated into medulla. It is better to learn adrenal medulla than just medulla.

How does adrenaline cause glycogen to turn into glucose: It is not a one step process which is as follows. Adrenaline, the first messenger, binds to receptors on the cell surface membrane to create hormone-receptor complex. The formation of the complex causes the enzyme adenylate cyclase/adenylyl cyclase to be activated. NB: Some textbooks and websites may say that a protein is activated by the complex which in turn stimulates the enzyme adenylate cyclase/adenylyl cyclase. The protein does not need to be known for AQA. It catalyses the conversion of ATP into cyclic AMP which is the second messenger. NB: Cyclic AMP can be abbreviated into cAMP. If you want to use this make sure you know that the little c stands for cyclic. Cyclic AMP activates an enzyme, Protein Kinase A/Protein Kinase, in the cell which starts a cascade of reactions. The last reaction to occur is the conversion of glycogen into glucose (glycogenolysis).

Diabetes/Diabetes mellitus is a disorder where the blood glucose levels are not regulated properly. There are two types:

  • Type 1/insulin-dependent: This occurs suddenly in childhood. The body is unable to produce any insulin and is thought to be because the beta cells of the islets of Langerhans in the pancreas have been targeted by the immune system. The levels of glucose can be controlled by regular injections with insulin and the careful management of diet and exercise.
  • Type 2/insulin-independent: This occurs mainly in people over the age of 40 however it is becoming increasingly more common in adolescents too. This condition arises when the insulin receptors are no longer responsive to insulin or an inadequate amount of insulin is being made by the beta cells. This too can be controlled by the careful management of diet and exercise.

A TIP TO LEARN THE TRICK WORDS (GLUCAGON, GLYCOGENESIS, GLUCONEOGENESIS, GLYCOGENOLYSIS):

  • Glucagon is the hormone which increases the level of glucose (gluc- for glucose and -on at the end shows the level of glucose has been turned on)
  • Glycogenesis is where glycogen is being made (glyco- for glycogen and -genesis means made)
  • Gluconeogenesis is where glucose is made from non-carbohydrates such as amino acids and glycerol (gluco- is for glucose, -neo- means new where a new source is used to make glucose and -genesis means made)
  • Glycogenolysis is where glycogen is broken down into glucose (glyco- is for glycogen and -lysis means breaking down)

 

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Principals of homeostasis and negative feedback

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 7 of 9 TOPICS

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

 

 

Principals of homeostasis and negative feedback:

Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits.

Not maintaining a stable core body temperature and pH may cause an enzyme to slow its activity down or to denature.

Stable glucose concentration allows cells to respire and keeps the water potential constant. This prevents shrivelling and bursting of cells.

Positive feedback is when levels rise of a certain factor but does not fall in the end and keeps rising to the normal. An example is during the first part menstrual cycle. Before ovulation, small amounts of oestrogen are secreted from the ovary. This stimulates GnRH from the hypothalamus in the brain and LH from the pituitary gland, also in the brain. GnRH also stimulates the release of LH. LH causes oestrogen to be secreted where the level of oestrogen rise. This has a knock-on effect by raising the levels of GnRH and LH. The level keep rising which is called positive feedback. NB: This example, as well as the one for negative feedback and multiple negative feedback, does not need to be known off by heart but is an example for you to understand what is meant by positive feedback. Other examples may be given in the exam.

Negative feedback is when levels fall and keep falling to the normal. An example is the second part to the menstrual cycle. After ovulation, the corpus luteum is formed in the ovary and begins to secrete progesterone in response to high levels of LH. This inhibits the release of GnRH from the hypothalamus in the brain and LH from the pituitary gland in the brain. The levels of both GnRH and LH decrease further known as negative feedback.

Multiple negative feedback is where negative feedback used to bring levels back to the normal. NB: Remember that positive feedback is not part of multiple negative feedback even though levels rise in some parts. The levels rise and stop at the normal and do not keep on rising like positive feedback. A combination of the two allows more control than just using one because the levels can be more stable around the normal and is less likely to get the levels too high or too low of a certain factor. An example of this is the menstrual cycle put together. A combination of the two allows levels of LH to fluctuate from high to low depending on the stage of the menstrual cycle.

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Synaptic transmission

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 5 of 9 TOPICS

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

 

 

 

Synaptic transmission:

The structure of a cholinergic synapse and neuromuscular junction should be known. The acetylcholine receptor in the first image on the left is more better known as nicotinic cholinergic receptor.

In a cholinergic synapse (this is the only synapse you need to know) an action potential increases permeability of the presynaptic membrane by stimulating the Ca2+ ion gated channels to open. This causes an influx of Ca2+ ions into the presynaptic knob down its concentration gradient by facilitated diffusion. The high concentration of Ca2+ ions causes the vesicles of acetylcholine (neurotransmitters) to fuse with the presynaptic membrane. NB: It is best to say acetylcholine than Ach because it gives you more of an understanding and helps with questions if it says ‘acetylcholine’ instead of Ach. If you are going to use Ach it is important that you know what it is. Acetylcholine leaves the presynaptic knob by exocytosis into the synaptic cleft. Acetylcholine diffuses across the synaptic cleft and binds to the cholinergic receptors causing the Na ligand gated channels to open. This causes an influx of Na+ ions into the postsynaptic neurone making the postsynaptic neurone depolarised and if the threshold is met, an action potential is generated. The acetylcholine is removed from the synaptic cleft by the enzyme acetylcholine esterase into products by complementary shapes to prevent a continuous impulse. NB: Acetylcholine esterase can be abbreviated into Ache however it is best also to refer to this enzyme as acetylcholine esterase as it will help you in questions that have this name. The products are actively transported into the presynaptic knob by the use of Pi from ATP into vesicles to make acetylcholine. The Ca2+ ions are actively transported out of the presynaptic knob by the use of Pi from ATP.

Above is an example of excitatory neurotransmitters. This is where the postsynaptic neurone is depolarised leading to an action potential being fired when the threshold is met. Neurotransmitters can also be inhibitory where they hyperpolarise the postsynaptic neurone by opening the K= ion gated channels open.

Neuromuscular junctions work in exactly the same way however:

  • Postsynaptic membrane: The postsynaptic membrane of the muscle is deeply folded to form clefts. This is where acetylcholine esterase is stored. NB: It is important that you say postsynaptic membrane of the muscle and not postsynaptic membrane of a neurone as a postsynaptic neurone is not involved in a neuromuscular junction.
  • Receptors: There are many more receptors on the postsynaptic membrane of the muscle than on the postsynaptic membrane of a neurone.
  • Neurotransmitters: The acetylcholine are excitatory in every neuromuscular junction whereas in the synapse it can be excitatory or inhibitory.

Spatial summation is where many presynaptic neurones connect to one postsynaptic neurone. A small amount of excitatory neurotransmitters can be enough for the threshold to be met in the postsynaptic neurone and causing an action potential to be created. If some neurotransmitters are inhibitory then the overall effect may not be an action potential as it will be difficult to meet the threshold in the postsynaptic neurone. Temporal summation is where there is a quick-fire of two or more action potentials arriving at the same time from one presynaptic neurone. This means more neurotransmitters are released into the cleft making an action potential more likely to occur as the threshold may be met.

Some drugs mimic or inhibit the action of neurotransmitters:

  • If a drug causes an action potential to be triggered, then this is because the drug and the receptor have complementary shapes where it is mimicking the neurotransmitter. These type of drugs are said to be agonists.
  • If a drug does not cause action potential but it is binded to the receptors, then this means that the drug is complementary to the receptor but blocks the receptor so not many receptors are activated. These type of drugs are said to be antagonists.
  • If a drug binds to an acetylcholine esterase, then this means fewer enzyme-substrate complexes will be formed with acetylcholine creating a continuous impulse.
  • If more receptors are stimulated, then this is because the drug releases more neurotransmitters than usual.
  • If less receptors are stimulated, then this is because the drug inhibits the release of neurotransmitters.

NB: Recall of names of drugs and the mechanism of drugs do not need to be recalled in the exam. A piece of information will be given in the exam about a drug and its mechanism and only you have to explain why that has happened which are the bullet points above. These are the only explanations you need to know and are highlighted in green.

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Skeletal muscles are stimulated to contract by nerves and act as effectors

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 6 of 9 TOPICS

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

 

 

 

Skeletal muscles are stimulated to contract by nerves and act as effectors:

Muscles act as antagonistic pairs against an incompressible skeleton where if one muscle contracts (agonist) it pulls the bone and the other muscle relaxes (antagonist). An example is biceps and triceps in the arm. NB: Muscles do not push bones and only pull. Skeletal muscles are attached to bones by tendons. Ligaments are attached from one bone to the other.

The gross structure is as follows: The muscle is made up of bundles which are packaged together. These bundles contain fibres/fascicles which are packaged in a connective tissue and have a distinct stripy look called striated tissue. These fibres/fascicles are known as muscle cells and have a membrane called sarcolemma, a cytoplasm called the sarcoplasm and organelles called myofibrils. These myofibrils are made up of 2 types of proteins: actin and myosin.

The microscopic structure should be known which is of the sarcomere. This is from one Z-line to another Z-line and is one contractile unit of many in a myofibril which are long and cylindrical in shape which shorten to create contraction. Between the two Z-lines in the middle is a line called the M-line. Adjacent to the Z-lines inside are I-bands (light bands), one on each side, which are lighter in colour as they only contain actin. The A-band (dark band) is from one I-band to the other and is darker in colour as it contains actin and myosin. H-zone is in the middle of the A-band and contains only myosin.

The sliding filament theory is as follows: At a neuromuscular junction on a fibre, an action potential is made and travels along the postsynaptic membrane and into the t-tubules which are indentations of the muscle cell. Sarcoplasmic reticulum, located at the t-tubules, release Ca2+ ions into the sarcoplasm when an action potential passes and is used to bind to tropomyosin which is wrapped around actin covering myosin-head binding sites. When it does bind to tropomyosin, it changes shape to expose the myosin-head binding sites so that the myosin heads can attach to form actinomyosin bridges. NB: The myosin heads actually bind to a protein called troponin on tropomyosin but this does not need to be known for AQA. The myosin heads swing backwards pulling the actin forwards and then detaches by the hydrolysis of ATP by ATP hydrolase. NB: This enzyme is only active when there is an influx of Ca2+ ions out of the sarcoplasmic reticulum. This process of attaching and detaching continues in a prose called a power stroke and stops until the sarcomere has shortened so much that the I-bands have gone smaller and the H-zone has gone smaller with the A-band and Z-lines staying the same. The Ca2+ ions are actively transported into the sarcoplasmic reticulum using ATP. The sarcomere returns to its original length when the antagonist muscle contracts.

The role of ATP in muscle contraction is to provide the energy as well as the many roles stated in the above paragraph. Phosphocreatine/Creatine phosphate is used when ATP is short in supply even after anaerobic respiration has taken place. It donates a phosphate group to ADP to make more ATP. NB: Phosphocreatine may be abbreviated into PCr however this may get confusing as it may look like a compound of phosphorous and chromium. Plus the full name may be given in the exam so learning the full name will avoid confusion.

 StructureLocationGeneral properties
Fast twitch muscle fibresThey have high stores of phosphocreatine/Creatine phosphate, low levels of myoglobin so they are white in colour, have many enzymes used for anaerobic respiration, relatively large, have few mitochondria, have fewer capillariesMainly in the legs of sprintersMakes a large amount of ATP because of the huge amount of anaerobic respiration but in low quantities. Phosphocreatine/ Creatine phosphate is for a supply of ATP when it runs out. Muscles can get fatigued quickly because of the high amounts of lactate. Creates fast contraction
Slow twitch muscle fibresThey have low store of phosphocreatine/ Creatine phosphate, high levels of myoglobin so they are red in colour, have few enzymes used for anaerobic respiration, relatively small, have a lot of mitochondria, have a lot of capillariesMainly in muscles that give posture and in leg muscles of long distance runnersMakes a large amount of ATP in high quantities compared to one cycle of anaerobic respiration where many ATP is made during oxidative phosphorylation. Muscles do not get fatigued unlike fast twitch muscles because of low levels of lactic acid. Creates slow contraction

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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Nerve impulses

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 4 of 9 TOPICS

 

 

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

Nerve impulses:

For AQA, you only need to know the structure of a motor neurone which is below.

The resting potential is where the membrane permeability differentiates Na+ and K+ ions so that at any given time there are more Na+ ions in the tissue fluid than in the axon with more K+ ions in the axon than in the tissues fluid. This is done when Na-K pump (or sodium-potassium pump) actively transports the ions up their electrochemical gradient (concentration gradient) where for every 3Na+ ions are pumped out, 2K+ ions are pumped in. The build up of K+ ions inside the axon allows the ions to go down their electrochemical gradient (concentration gradient) by facilitated diffusion through the K+ ion channels. There are not many Na+ ion channels meaning not many Na+ ions can diffuse into the axon by facilitated diffusion. This makes the tissue fluid more positive relative to the axon. The membrane is said to be polarised and the potential difference across the membrane is about -70mV (millivolts).

Depolarisation is where a stimulus causes the Na+ gated channels to open and K+ ion gated channels to remain close. NB: K+ ion channels and K+ ion gated channels are two different channels that do similar roles but are used in different situations. They should not be confused over. K+ ion gated channels can be referred to as K+ ion channels but if a question did say K+ ion gated channels then it should not cause confusion. My advice would be to use K+ ion gated channels for depolarisation and K+ ion channels for resting potential. This same tip goes for Na+ ion channels and Na+ ion gated channels. This causes Na+ ions to diffuse into the axon by facilitated diffusion down their electrochemical gradient (concentration gradient) making the axon more positive relative to the tissue fluid. The membrane is said to be depolarised and the potential difference across this axon now is about 35mV. At first only a few Na+ ion gated channels open but when the threshold is met (about -55mV) many more Na+ gated ion channels open and there is a bigger chance of an action potential (impulse) to occur depending on how big the stimulus is. This is passed along the axon in one way causing more Na+ ion gated channels to open.

Repolarisation occurs after depolarisation where the K+ ion gated channels open and Na+ ion gated channels are closed causing K+ ions to diffuse in by facilitated diffusion down their electrochemical gradient (concentration gradient) into the axon. This causes the potential difference to drop as these potassium ions diffuse out the K+ ion channels when there is a high concentration inside the axon. However the K+ ion gated channels remain open for too long making the potential difference to drop below -70mV.

The last stage is the refractory period where it restores the resting potential by closing both the Na+ ion gated channels and K+ ion gated channels as they are ‘recovering’. Only the Na-K pump (sodium-potassium pump) is used as well as the K+ ion channels to get the potential difference back up to -70mV.

The wave of impulses are discrete (It is not a continuous flow) because of the refractory period making the Na+ ion gated channels and K+ ion gated channels closed. This is also the reason why the impulse travels in one direction and limits the frequency of impulse transmission.

Myelinated neurones (neurones that have Schwann cells wrapped around its axon) transmit impulses quicker than non-myelinated neurones. This is because the signal jumps from node to node known as saltatory conduction where depolarization occurs only at the nodes of Ranvier. In non-myelinated axons the whole of the axon must be depolarized. As well the axon being myelinated or not myelinated affecting the speed of conductance, there are two other factors that affect the speed of conductance too:

  • Diameter of the axon: The bigger the diameter of the axon the faster the speed of conductance. This is because there is less friction with the walls of the axon.
  • Temperature: The higher the temperature the faster the rate of conductance. This is because there is a faster rate of diffusion into the axon of sodium ions.

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Survival and response

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 1 of 9 TOPICS

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

 

 

 

Survival and response:

Organisms increase their chance of survival by responding to changes in their environment.

In flowering plants, specific growth factors move from growing regions to other tissues, where they regulate growth in response to directional stimuli e.g. light and gravity.

Phototropism is where shoots and roots grow in response to light. This is because of a plant hormone (auxin) called indoleacetic acid which is produced in the meristem of shoots (NB: Indoleacetic acid can be abbreviated into IAA and the meristem is the tip of shoots. This is the only plant auxin that you need to know for AQA). IAA has different effects in shoots and roots:

  • Shoots: IAA diffuses to the side of the shoot behind the meristem (in cell elongation) that is shaded. This promotes growth and the cells elongate causing the shoot to bend towards light. This is called positive phototropism.
  • Roots: IAA diffuses to the side of the root that is shaded. This inhibits growth making the root bend away from light. This is called negative phototropism.

Geotropism/Gravitropism is where shoots and roots grow in response to gravity. This is also because of an auxin called IAA. IAA also has different effects in shoots and roots:

  • Shoots: IAA diffuses to the side of the shoot that is towards gravity behind the meristem (in cell elongation). This promotes growth making the shoot grow away from gravity. This is called negative geotropism/gravitropism.
  • Roots: IAA diffuses to the side of the root that is towards gravity. This inhibits growth making the root bend towards gravity. This is called positive geotropism/gravitropism.

Taxis is a directional response in movement because of a stimuli. Positive taxis is a directional movement towards a stimuli with negative taxis being a directional movement away from stimuli. NB: The different types of taxis do no need to be known but must be familiarised with. This includes chemotaxis – chemically generated taxis, phototaxis – light-sensitive taxis, geotaxis – taxis in response to gravity and rheotaxis – taxis in response to movement.

Kinesis is a non-directional response in movement because of stimuli. There is no positive or negative kinesis but is random in order to increase the survival chances of an organism. If an organism is in an environment where it is increasing its survival chances then it would turn slowly with many turns to keep in the same spot. If it is in danger then it would turn faster with less turns to exit.

Reflex arc is a process to create a protective effect. It starts with a stimulus which is picked by the receptors in the skin. A nerve impulse is carried along the sensory neurone to the spinal cord. It then travels through the relay/intermediate neurone which connects the sensory neurone to the motor neurone. The nerve impulse then travels along the motor neurone which connects the spinal cord to the biceps which is an effector. The muscle contracts creating a response which pulls the finger away. NB: Details of the spinal cord and, dorsal and ventral roots are not required.

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Control of heart rate

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 3 of 9 TOPICS

 

 

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

Control of heart rate:

Action potentials originate in the sinoatrial node and travel across the wall of the atrium to the atrioventricular node on the right side of the heart. This passes slowly giving time for the atria to contract and empty all the blood into the ventricle. This is then passed along the atrioventricular bundle into the interventricular septum along the bundle of His. This bundle separates into two at the apex (bottom) of the ventricles and goes upwards along each of the ventricle walls. The action potentials travel along this pathway and is carried deeper into the ventricular walls by the Purkinje fibres. This provides unison of a strong contraction by both the ventricles and allowing the ventricles to empty properly.

Chemoreceptors, located in the aorta and the carotid artery, monitor the CO2 levels, O2 levels and pH levels in the blood. When the receptor picks up the stimulant it transfers it to the sensory neurone to the medulla oblongata (also known as medulla) which also has chemoreceptors. The impulse then travels along the sympathetic neurone or parasympathetic neurone depending on the stimuli to the sinoatrial node. The effector, the cardiac muscle, can have two responses depending on the stimulus and route it takes:

  • If there are low levels of O2 and pH with high levels of CO2 then the impulse will go through the sensory neurone to the medulla and through the sympathetic neurone releasing noradrenalin from the sinoatrial node (an excitatory neurotransmitter) which makes the heart rate faster.
  • If there are high levels of O2 and pH with low levels of CO2 then the impulse will go through the sensory neurone to the medulla and through the parasympathetic neurone releasing acetylcholine from the sinoatrial node (an inhibitory neurotransmitter) which makes the heart rate slower. This is so the levels of O2 and pH can lower and CO2 levels can be raised.

Baroreceptors, located in the carotid artery and aorta only, detect changes in the pressure in the blood. The routes are exactly the same for which the nerve impulses take:

  • If there is high blood pressure the route it follows is through the parasympathetic neurone which releases acetylcholine from the sinoatrial node slowing the heart rate down to lower the blood pressure.
  • If there is low blood pressure the route it follows is through the sympathetic neurone which releases noradrenalin from the sinoatrial node making the heart rate faster to raise the blood pressure.

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Receptors

Organisms respond to changes in their internal and external environments (AQA A2 Biology) PART 2 of 9 TOPICS

 

 

 

 

TOPICS: Survival and response  Receptors  Control of heart rate  Nerve impulses  Synaptic transmission  Skeletal muscles are stimulated to contract by nerves and act as effectors  Principals of homeostasis and negative feedback  Control of blood glucose concentration  Control of blood water potential

Receptors:

The simple of a Pacinian corpuscle should be known which is illustrated below.

Receptors are specific as they only respond to specific stimuli. An example is the Pacinian corpuscle which is a mechanoreceptor that only responds to mechanical stimuli. The connective tissue round the neurone ending deforms when pressure is applied opening the stretch-mediated sodium ion channels. This causes an influx of Na+ ions into the axon and depolarising the axon membrane. The change in potential difference up to this point is called the generator potential. If the threshold is met an action potential is created.

Rods and cones are found in the photosensitive layer in the peripheral area of the eye. The pigments, sensitivity to light, acuity and sensitivity to colour must be known of rods and cones to identify any differences:

RODS: (Pigment – rhodopsin, high sensitivity to light, low acuity, low sensitivity to colour)

  • Pigments: Rods contain a pigment called rhodopsin.
  • Sensitivity to light: Many rods are connected to one nerve fibre by spatial summation. Light can be detected from a wide area and the combination of these stimulatory effects of each rod makes them sensitive.
  • Acuity: Because of spatial summation, each rod will have a different stimulatory effect. These are then combined into one image by the brain which comes out in a blur.
  • Sensitivity to colour: Only one type of pigment is found in all of rods and lets us see things in black and white. An example is in the dark where the rods are only in use and not cones. Not much colour can be seen.

CONES: (Pigment – iodopsin, low sensitivity to light, high acuity, high sensitivity to colour)

  • Pigments: Cones contain one type of the three types of pigment called iodopsin.
  • Sensitivity to light: Only one cone is attached to its own nerve and so there is only stimulatory effect created.
  • Acuity: Cones are not connected to each other which means only one impulse travels from one cone to make one image therefore the image is more clear.
  • Sensitivity to colour: Each type of the three types of pigments is found in one cone and is responsible for one of the three primary colours (blue, green and red). The pigment is bleached when light at a certain wavelength corresponds to the colour.

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