Communication and Homeostasis


Through chemical signalling, cells are able to communicate with adjacent cells and distance cells. Cells can communicate through different hormones and nervous impulses. This is essential to coordinate the activities of different organs.

Neuronal Communication

The nervous system is made up of the peripheral and central nervous system. The PNS includes the receptors, sensory and motor neurones, whilst the CNS is the coordination centres such as the brain and spine.

The electrical impulses that pass along neurones is due to movement of ions across membranes. Therefore, transport across a membrane (active transport, co-transport, diffusion and facilitated diffusion) and plasma membrane structure are key to understanding this topic.

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Myelinated Neurone

The structure of a myelinated motor neurone can be seen below.

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The cell body of the neurone contains the organelles found in a typical animal cell, including the nucleus. It is in the cell body where proteins and neurotransmitter chemical are made.

Dendrites are the branched extensions protruding from the cell body. These carry action potentials to surrounding cells.

The axon is the conductive, long fibre that carries the nervous impulse along the motor neurone.

Schwann cells wrap around the axon to form the myelin sheath, which is a lipid and therefore does not allow charged ions, or the impulse, to pass through it. There are gaps between these myelin sheath, called nodes of Ranvier.

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Resting Potential

When a neurone is not conducting an impulse, there is a difference between the electrical charge inside and outside of the neurone, this is known as the resting potential.

There are more positive ions,__ Na+__ and K+, outside compared to inside, therefore the inside of the neurone is comparatively more negative at -70mV.

The resting potential is maintained by a sodium-potassium pump, involving active transport and therefore ATP. The pump moves 2K+__ions in and __3 Na+ ions out. This creates a concentration gradient and results in K+ diffusing out and Na+ __diffusing in, however because the membrane is more permeable to __K+ more are moved out resulting in the -70mV.

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Action Potential

When the potential of the neurone changes, this is an action potential, and results in a nervous impulse. Changes in membrane permeability lead to this depolarisation and generation of an action potential. Throughout depolarisation, the Na+ continues to rush inside until the action potential reaches its peak and the sodium gates close. If the depolarisation is not high enough to exceed a threshold, then an action potential and the impulse are not produced. This is called the All-or-None Principle. This is important, as it makes sure that animals only respond to large enough stimuli, rather than responding to every slight change in the environment which would overwhelm them.

A stimulus provides energy that can cause the sodium voltage-gated channels (proteins in the membrane that only open when a certain voltage is reached) in the axon membrane to open. This causes Na+ to diffuse in, which increase the positivity inside of the axon. This causes more voltage-gated channels to open, so even more Na+ diffuse in. When a threshold of +40mV is reached inside the axon, the voltage-gated sodium channels close and instead voltage-gated potassium ion channels open. This results in potassium ions diffusing out, and the axon becoming negative again and repolarises the axon. Temporarily the axon becomes more negative than the -70mV and is hyperpolarised. The potassium ion gates will now close and the sodium-potassium pump restores normal activity to reform the resting potential.

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Once an action potential is generated it moves along the axon like a Mexican wave.

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This will occur along the entire axon in a non-myelinated neurone, whereas it is much quicker in myelinated axons. Action potentials can only occur at the nodes of Ranvier, where there is no myelin insulating. Therefore, the localised action potentials will jump from node to node and travels much quicker. This is known as saltatory conduction.

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In addition to myelination and saltatory conduction increasing the speed of conductance, temperature and axon diameter also have an impact. The higher the temperature, the faster the ions will diffuse and therefore increase the conductance. The wider the diameter of the axon, the faster the rate of conductance. This is because the larger the diameter, the less likely it is for ions to leak across the membrane and effect the potential.

Refractory Period

Straight after an action potential has been generated, the membrane enters a refractory period when it can’t be stimulated, because sodium channels are recovering and can’t be opened. This means that the voltage-gated sodium channels are closed. This is important for many reasons.

Firstly, it ensures that discrete impulses are produced, meaning that an action potential cannot be generate immediately after another one and this makes sure that each is separate from another.

Secondly, it ensures that action potentials travel in one direction. This stops the action potential from spreading out in two directions and prevent a response.

Finally, it limits the number of impulse transmission. This is important to prevent over reaction to a stimulus and therefore overwhelming the senses.

What is the term given to the potential when the neurone is at rest?
What is the term given to the potential when a stimulus is detected?
What is the term given to the period after a stimulus generates a response?
Refractory period
What determines whether the sodium channels within axon membranes open or close?
What is the name of the insulating layer that wraps around an axon?
Your answer should include: Myelin / Sheath
Where in a motor neurone is the nucleus found?
Your answer should include: Cell / Body


Homeostasis is the maintenance of a constant internal environment via physiological control systems. These control systems keep temperature, blood pH, blood glucose and water potential within set limits.

Importance of Homeostasis

If body temperature is too low there will be insufficient kinetic energy for enzyme-controlled reactions, and if body temperature is too high then enzymes will denature. Either way, metabolic reactions could slow to the point that cells die. Alterations in blood pH will also result in enzymes denaturing.

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Glucose is needed for respiration, so a lack of glucose in the blood could result in cell death. If blood glucose levels are too high, then this will lower the water potential of the blood and water will leave surrounding cells by osmosis and prevent normal cell function. If the water potential of the blood is too low, water will move into cells by osmosis and can cause them to burst.

Negative Feedback

Negative feedback is how a deviation from the normal values are restored systems to their original level. This involves the nervous system and often hormones too, as shown in the flow diagram.

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Positive Feedback

Positive feedback is when a deviation from the normal values triggers a response to increase the deviation further, for example during childbirth.

Excretion as an Example of Homeostatic Control

Mammalian Liver

The liver is essential for glycogen storage, detoxification and in the formation of urea. The liver contains a range of different enzymes that make these processes possible.

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The ornithine cycle (urea cycle) is how urea is produced from ammonia, ready to be transported to the kidneys and excreted. Excess proteins are deaminated in the liver (broken down to amino acids), which are then converted into ammonia. Ammonia is highly toxic, which is why it is converted to urea before being transported in the blood.

Detoxification is the neutralisation/breakdown of unwanted chemicals such as alcohol, drugs, hormones and toxins produced in chemical reactions in the body.

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Mammalian Kidney and Osmoregulation

This is the process of controlling the water potential of the blood.

The nephron is structure in the kidney where the blood is filtered, and useful substances are reabsorbed into the blood. This is shown in the diagram below.

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Endotherms are able to regulate their own body temperature through a nervous response. Peripheral receptors in the skin detect a change in the external temperature. This sends an impulse along a sensory neurone to the brain, where the hypothalamus coordinates the impulse. This will trigger a response, which if you are too hot could be for glands to produce more sweat, vasodilation or behavioural (e.g. move to the shade, remove clothes, fan yourself). If you are too cold the responses are for glands to produce less sweat, muscles to contract and relax (shivering) to generate heat and behavioural (wear more clothes).

Ectotherms cannot regulate their internal temperature and can only control it through their behaviour. This is why cold blooded animals, ectotherms, often bask on hot rocks to absorb the radiating heat.

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The graph shows how endotherms, the mouse, are able to regulate their temperature so there is little fluctuation even when the external temperature increases:Communication and Homeostasis, figure 2

Nephron Structure

  1. The Bowman’s (renal) capsule- Ultrafiltration occurs here because the afferent arteriole (entering the glomerulus) is wider than the efferent arteriole (leaving the glomerulus) creating high hydrostatic pressure. Small molecules and water are forced out of the capillaries into the renal capsule, and large proteins and blood cells remain in the blood.
  2. Proximal convoluted tubule__ - The walls are made of microvilli epithelial cells to provide a large surface area for diffusion of glucose into the cells from the __PCT. Glucose is then actively transported out of the cells into the intercellular space to create a concentration gradient. Glucose can then diffuse into the blood again.
  3. Loop of Henle__ __- Sodium ions are actively transported out of the ascending limb into the medulla to create a low water potential. Water moves out of the descending limb and out of the distal convoluted tubule and collecting duct by osmosis due to this water potential gradient.

The liquid remaining in the collecting duct forms the urine. It contains water, dissolved salts, urea and other substances such as hormones.

More or less water can be reabsorbed at the collecting duct depending on the water potential of the blood. The hypothalamus in the brain monitors water potential of blood. If the water potential of the blood decreases, water will move out of the osmoreceptor cells by osmosis resulting in the cells shrivelling. This triggers other cells in the hypothalamus, which send a signal to the posterior pituitary gland. The posterior pituitary will then release antidiuretic hormone (ADH) into the blood.

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ADH will bind to complementary receptors that are only located on target cells in the DCT and collecting duct. When it binds is activates adenyl cyclase to make cAMP. This activates an enzyme which causes vesicles containing aquaporins to fuse with the membrane. Aquaporins and channel proteins that allow water to transport across the membrane. As a result, the membrane becomes more permeable to water and more will leave to be reabsorbed back into the blood.

If kidney failure occurs then the blood is not filtered properly and this can lead to a buildup of urea in the blood and an electrolyte imbalance (mineral ions). Two potential treatments for this are dialysis; a machine that the patient’s blood is passed through 3 to 4 times per week which performs the function of a kidney to filter the blood. A second option is a kidney transplant, but this depends on the availability of a kidney from a donor that is a match.

Urine in Diagnosis

Due to urine being composed of substances filtered out of the blood, it can be used to test for diabetes, pregnancy, anabolic steroids and drugs.

A pregnancy test uses monoclonal antibodies to detect the presence of the human growth human, which is produced by pregnant women. A monoclonal antibody is a single type of antibody that can be isolated and cloned. This is used for targeting medication to specific cell types by attaching a therapeutic drug to an antibody and medical diagnosis. Creating monoclonal antibodies requires mice to produce the antibodies and tumour cells, which leads to ethical debates as to whether this use of animals is justified to enable the better treatment of cancers in humans and to detect disease.

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Synapse and Neuromuscular Junction

Synapses are the gaps between the end of the axon of one neuron and the dendrite of another one. Here the action potential is transmitted as neurotransmitters that diffuse across the synapse.

This process occurs as follows:

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Neuromuscular junction

This is a synapse that occurs between a motor neurone and a muscle and is very similar to a synaptic junction.

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Neuromuscular JunctionCholinergic Synapse
Unidirectional due to the neurotransmitter receptors only being on the post synaptic membrane
Only excitatoryCould be excitatory or inhibitory. Inhibitory synapses cause chloride ions to move into the postsynaptic neurone and potassium ions to move out. The combined effect of negative ions moving in and positive ions moving out makes the membrane potential increase to -80mV, hyperpolarisation, and therefore an action potential is highly unlikely.
Connects motor neurone to musclesConnect two neurones, which could be sensory, relay or motor.
This is the end point for the action potentialA new action potential is generated in the next neurone.
Acetylcholine binds to receptors on muscle fibre membranesAcetylcholine binds to receptors on post-synaptic membrane of a neurone,


Summation is the rapid build-up of neurotransmitters in the synapse to help generate an action potential by two methods; spatial or temporal summation. This is needed because some action potentials do not result in sufficient concentrations of neurotransmitter being released to generate a new action potential.

Spatial summation: many different neurones collective trigger a new action potential by combining the neurotransmitter they release to exceed the threshold value.

Temporal summation: One neurone releases neurotransmitter repeatedly over a short period of time to add up to enough to exceed the threshold value.

Hormonal Communication

Blood Glucose Control

Blood glucose will increase following ingestion of food or drink containing carbohydrates and will fall following exercise or if you have not eaten.

The pancreas detects changes in the blood glucose levels and contains endocrine cells in the Islets of Langerhans which release the hormones insulin and glucagon to bring blood glucose levels back to normal. Adrenaline is released by adrenal glands if you body anticipates danger and results in more glucose being released from stores of glycogen in the liver.

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Key Terms:

  1. Glycogensis (genesis means to make)

This is the process of when excess glucose is converted to glycogen in blood glucose is higher than normal. This occurs in the liver and the liver can store 75-100g of glycogen. This would last you 12 hours at rest if you do not eat.

  1. Glycogenolysis (lysis means to breakdown)

This is the breakdown of glycogen back into glucose in the liver. This occurs when blood glucose is lower than normal.

  1. Gluconeogensis (Amino acids to glucose)

This is the process of creating glucose from non-carbohydrate stores in the liver. This will occur if all glycogen has already been hydrolysed back into glucose and your body still needs more glucose.

The above processes are controlled by the three hormones insulin, adrenaline and glucagon.

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The Action of Insulin

Beta cells in the Islets of Langerhans detect if blood glucose is too high and will secrete insulin. Insulin will decrease blood glucose in the following ways:

Attaching to receptors on the surfaces of target cells. This changes the tertiary structure of the channel proteins resulting in more glucose being absorbed by facilitated diffusion.

More protein carriers are incorporated into cell membranes so that more glucose is absorbed from the blood into cells.

Activating enzymes involved in the conversion of glucose to glycogen. This results in glycogenesis in the liver.

The Action of Glucagon:

Attaching to receptors on the surfaces of target cells. When glucagon binds it causes a protein to be activated into adenylate cyclase and to convert ATP in a molecule called Cylic AMP (cAMP). cAMP activates an enzyme, protein kinase, that can hydrolyse glycogen into glucose.

Activating enzymes involved in the conversion of glycerol and amino acids into glucose.

The Role of Adrenaline by:

Adrenaline attaches to receptors on the surfaces of target cells. This causes a protein (G protein) to be activated and to convert ATP into cAMP. cAMP activates an enzyme that can hydrolyse glycogen into glucose. This is known as the second messenger model of adrenaline and glucagon action, because the process results in the formation of cAMP, which acts as a second messenger.


This is when blood glucose cannot be controlled.

Type I diabetes is due to the body being unable to produce insulin, it starts in childhood and could be the result of an autoimmune disease where the beta cells were attacked. Treatment involves injection in insulin.

Type II diabetes is due to receptors on the target cells losing their responsiveness to insulin, it usually develops in adults because of obesity and poor diet. It is controlled by regulating intake of carbohydrates, increasing exercise and something insulin injections.

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The Human Brain

The human brain is made up of billions of neurones and coordinates responses.

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The key structures are the cerebrum, cerebellum, medulla oblongata, hypothalamus and the pituitary gland.

Cerebrum - This is the largest part of the brain, shown in two shades of grey above. The lighter grey is the outer layer, known as the cerebral cortex. It is made up of many folds and is split into two hemispheres. The functions range from controlling conscious thoughts, language, intelligence, personality, high-level functions and memory.

Cerebellum - Shown in orange above and looks like a mini cauliflower. This coordinates movement, muscles and controls balance.

Medulla Oblongata - The orange structure above the label for spinal cord. This is the centre of control for unconscious activities, such as breathing and heart rate.

Hypothalamus - This small part of brain is responsible for homeostasis, such as temperature and water balance.

__Pituitary Gland __- The small lobed grey structure in the diagram above, is known as the master gland because it secretes many hormones to coordinate several responses such as the oestrous cycle and osmoregulation.

Control of the Heart

Cardiac muscle is myogenic, but the rate of contraction is controlled by a wave of electrical activity. The sinoatrial node (SAN) is in the right atrium and is known as the pacemaker. The SAN will release a wave of depolarisation across the atria, causing it to contract. The atrioventricular node (AVN) is located near the border of the right and left ventricle within the atria still.

The medulla oblongata in the brain controls the heart rate, via the autonomic nervous system. There are two parts: a centre linked to the sinoatrial node that increases heart rate via the sympathetic nervous system and another that decreases heart rate via the parasympathetic nervous system.

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The heart rate changes in response to pH and blood pressure, and these stimuli are detected by chemoreceptors and pressure receptors in the aorta and carotid artery.

Response to Pressure

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Response to pH

The pH of the blood will decrease during times of high respiratory rate, due to the production of carbon dioxide or lactic acid.

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Why do hormones only affect their target cells and not other cells?
Your answer should include: Complementary / Receptors / Cell / Surface / Membrane
Why is it harmful if there is too much glucose in the blood?
Your answer should include: Reduces / Water / Potential / Osmosis
Why is it harmful if there is not enough glucose in the blood?
Where is ADH released from?
Your answer should include: Pituitary / Gland
What does ADH stand for?
Your answer should include: Anti-Diuretic / Anti / Diuretic / Hormone
What effect does ADH have on the collecting duct?
Your answer should include: Permeable / Water

Plant and Animal Responses

Response in Flowering Plants

Tropism is the term given to when plants respond, via growth, to stimuli. Tropisms can be positive or negative, growing towards or away from a stimulus. Plants respond to light, gravity and water.

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Tropisms are controlled by specific growth factors and one key example is indoleacetic acid (IAA). IAA is a type of auxin and can control cell elongation.



Plants need light for photosynthesis which is why plants grow and then bend towards light. This is controlled by IAA. This is positive phototropism.

Shoot tip cells produce IAA, which causes cell elongation in shoots, and this diffuses to other cells. If there is unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there. The IAA causes the cells on the shaded side to elongate more and this causes the plant to bend towards the light source.

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Roots do not photosynthesise and do not require light and are more able to anchor the plant if they are deep in the soil away from light. In roots a high concentration of IAA inhibits cell elongation, causing roots cells to elongate more on the lighter side and so the root bends away from light. This is negative phototropism.



IAA will move from the upper side to the lower side of a shoot. If a plant is vertical, this causes the plant cells to elongate and the plant grows upwards. If a plant is on its side, it will cause the shoot to bend upwards. This is negative gravitropism.

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IAA moves to the lower side of roots so that the upper side elongates and the root bends down towards gravity and anchors the plant in. This is positive gravitropism.


This is a plant hormone that helps initiate germination in seeds and initiates stem growth in already germinated plants.

Do to their impacts, both auxins and gibberellins are used commercially to promote seed germination and plant root and shoot growth.

Responses and Reflexes

A stimulus is a detectable change in the environment. These changes can be detected by cells, which are called receptors. This triggers a response through the flow chart shown below. The effector is the name given to the structures that cause the response, and these are muscles or glands.

Stimulus __–>__ Receptor __–>__ Coordinator–> ____Effector–> Response

Organisms increase their chance of survival by responding to stimuli via different response mechanisms.

Simple Reflex

In mammal the simplest response to a stimulus is a reflex. This is a rapid, involuntary response to danger which by-passes the brain. It is rapid because it only involves three neurones and therefore has few synapses. Synapses slow down responses as electrical energy is converted to chemicals that diffuse across the synapse. Because no decision is involved, the response is also rapid and prevents the brain from being overloaded with situations to decide responses too. Once the stimulus is detected by the receptor, an impulse is passed along the sensory neurone to a relay neurone, located in the spine. The relay neurone passes the impulse onto a motor neurone which is connected to an effector. For example, if the stimulus is a hot object the effector would be the muscles in your hand and arm and the response would be that they contract to move your hand away from the dangerous hot object.

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Muscles act in antagonistic pairs against an incompressible skeleton to create movement. This can be automatic as part of a reflex response or controlled by conscious thought.

Below is the ultrastructure of a myofibril.

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Muscle fibres are made up of millions of myofibrils which collectively bring about the force to cause movement. Myofibrils are made up of fused cells that share nuclei and cytoplasm, known as sarcoplasm, and there is a high number of mitochondria. Myofibril are made up of two key types of protein, myosin and actin, that form a sarcomere.

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Sliding Filament Theory

When an action potential reaches a muscle, it stimulates a response. Calcium ions enter and cause the protein tropomyosin, that block binding sites for the myosin head of the actin, to move and uncover the binding sites. Whilst ADP is attached to the myosin head, it can bind to the binding site on the actin to form a cross-bridge. The angle created in this cross-bridge creates tension and as a result the actin filament is pulled and slides along the myosin. In doing so the ADP molecules is released. An ATP molecule can then bind to the myosin head and causes it to change shape slightly and as a result it detaches from the actin. Within the sarcoplasm there is the enzyme ATPase, which is activated by the calcium ions, to hydrolyse the ATP on the myosin head into ADP and releases enough energy for the myosin head to return to its original position. This entire process repeats continually whilst the calcium ions remain high, and therefore whilst the muscle remains stimulated by the nervous system.

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Active muscles therefore require a high concentration of ATP. In times when aerobic respiration cannot create enough ATP to meet this demand, anaerobic respiration. The chemical phosphocreatine, which is stored in muscles, assists this by providing phosphate to regenerate ATP from ADP.

Slow and Fast Skeletal Muscle Fibres

Slow-twitch fibresFast-twitch fibres
StructureContains a large store of myoglobin, a rich blood supply and many mitochondria.Thicker and more myosin filaments, a large store of glycogen, a store of phosphocreatine to help make ATP from ADP and a high concentration of enzymes involved in anaerobic respiration.
LocationCalf musclesBiceps
General propertiesContract slower and can respire aerobically for longer periods of time due to the rich blood supply and myoglobin oxygen store. These muscles are adapted for endurance work, like marathons.Contract faster to provide short burst of powerful contraction. These are adapted for intense exercise, such as sprinting or weight-lifting.