A2 Biology OCR, Communication and homeostasis

All living things need to maintain conditions inside their cells because of enzyme activities. Enzymes need specific conditions - suitable conditions, suitable pH, freedom from toxins and aqueous solution must be present.

External environments: As external environments change it puts organism under stress. The environmental change is the stimulus and the way in which the organism changes its behavious is its responce. A change in stimulus must be monitored and organism must respond to change.

Internal environments: In animals, the intenal cells are bathed in tissue fluid. This is its environment. The activities of the cells alter their own environment. The composition of tissue fluid is maintained by blood as it flows through body transporting substance. It gives nutrients and removes waste so cells maintain good health.

Coordination: A multicellular organism needs cells ro be specialised to perfom different functions. A good communication system is required to ensure all parts work together effectively. A good system will cover whole body, enable cells to communicate with eachother, enable specific and rapid communication and enable both long and short term responces.

Cells comunicate with eachother by the process of cell signalling. This is a process in which one cell will release a chemical that is detected by another cell. The second cell will respond to the signal released by the first cell.

There are two major systems of communication that work by cell signalling:

The neural system is an interconnected network of neurones that signal to eachother across synapse junctions. The neurones can conduct a signal very quickly and enable rapid responces to stimuli that may be changing quickly.

The hormonal system uses blood to transport signals. Cells in an endocrine organ release the signal (a hormone) directly into the blood. It is carried all over the body but is only recognised by speciific target cells. The hormonal system enables longer term responces to be coordinated.

Homeostasis is the maintenance of the internal environment in a constant state, despite external changes. These conditions may include body temperature, blood glucose concentration, , water potential of blood, blood pressure and carbon dioxide concentration.

Negative feedback us a process that brings about a reversal of any change in conditions. It ensures that an optimum steady state can be maintained, as the internal environment is returned to its original set of conditions after any change. It is essential for homeostasis.   stimulus > receptor > communication pathway (cell signallling) > effector > responce 

Positive feedback is a process that increases any change detected by the receptors. It tends to be harmful and does not lead to homeostasis. Benefitial when oxytocin when presence increases uterine contractions so baby can be born.

An ectotherm is an organism that relies on external sources of heat to regulate its body temperature.

Advantages to ectotherm: use less food in respiration, survive for longer without eating, greater proportion of energy from food can be used for growth.

Disadvantages to ectotherm: less active in morning so to move to warm up which puts them at greater risk of being eaten, not capable of activity during winter as need to preseve energy

Temperature regulation in ectotherms:

Adaptation                                        How it helps regualte temperature               Example

Expose body to sun                         Enables heat to be absourbed                     Snakes

Orientate body to sun                      Expose larger SA for more heat                   Locusts

Hide in burrow                                 Reduces absorbtion                                      Lizards

Alter body shape                             Alter SA to sun                                              Horned lizards

An endotherm is an organism that can use internal sources of heat, such as heat generated from metabolism in the liver, to maintain its body temperature.

Advantages of endotherm: faiarly constant body temperature, activity possible when external temperature is cold, can inhabit colder parts of country.

Disadvantages of endotherm: lots of energy used to maintain body temperature in cold, more food required, less energy used for growth

Temperature regulation in endotherms: (Physiological then behavioural)

Sweat glads - secrete more sweat so evaporates and looses heat and cool down

Lungs, nose, mouth - panting increases evaporation from tounge and moiset surfaces

Skeletal contractions - shiver to generate more heat

Move into shadow or sun, orientate body to sun, remain inactive and increase SA

Control of temperature regulation: (happens in hyperthalamus)

Rise in temperature - thermoregulatory centre in hypothalumus detects change - nervous and hormonal system carry signals to skin, liver and muscles - less heat generated and more lost

Fallin temperature - thermoregulatory centre in hypothalumus detects change - nervous and hormonal system carry signals to skin, liver and muscles - more heat generated and less lost

Role of peripheral temperature receptors:

The thermoregulatory centre in the hypothalumus monitors blood temp and detects change internally. An early warming ot hypothalumus will respond more quickly to change.

If the extremeties start to cool down or warm up, this leads to eventually affecting core body temperature. Peripheral receptors in skin monitor temperature and info is fed to thermoregulatory receptor. The brain can also initiate behavioural mechanisms for maintaining body temperature, such as moving to shade.

Sensory receptors:

Sensory receptors are specialised cells that can detect changes in our surroundings. They are transducers that convert one form of energy to another. Ech transducer adapts to a change, eg pressure or light levels, and each change is called a stimulus.

Changing membrane permeability:

Neurones (nerve cells) have specialised channel proteins that are specific either to Na or K ions. They have a gate that open or close. When open, the permeability to that ion increases. They're usually closed. Nerve cells also contain carrier proteins that actively transport 2 Na+ out of cell and 3 K+ into cell. The inside of the cell is negative compared to outside = polarised

An impulse is created by altering the permeability of the nerve cell membrane to Na ions. As Na channels open the permeability to Na increases and sodium ions move across membrane down conc gradient to cell. The movement creares a potential difference (charge) and inside becomes less negative = depolarisation

A polarised membrane is one that had a potential difference across it. This is the resting potential.

Depolarisationis the loss of polarisation across a membrane. It refers to the period when sodium ions are entering the cell making the inside less negative with respect to the outside.

Generator potentials

Receptor cells respond to changes in the environment, The gated sodiium ion channels open, allowing sodium ions to diffuse across membrane into the cell.

A small change in potenial caused by one or two sodium ion channels opening is called a generator potential. The larger the stimulus, the more gated channels will open.

If enough sodium ions entre the cell, the potential difference changes significanty and initiates an action potential.

3 types of neurone:

Sensory neurone carries action potential from a sensory receptor to CNS. The motor neurones carry an actional potential from the CNS to effect (eg muscle or gland). The relay neurone connect sensory and motor neurone.

Most neurones have similar basic structure to carry impulses around body. Their features:

• very long so can transmit action potenital over a long distance
• the cell surface membrane has gated ion channels that control Ca, Na and K
• they have Na/K ion pumps that use ATP to actively transport Na ions out and K into cells
• maintain potential differnce across their surface membrane
• surrounded by a fatty sheath called myelin sheath, that insulate the neurone from electrical activity in nearby cells. Gaps are called nodes of ranvier
• have a cell body that conatins the nucleus, mitochondria and ribosomes
• numerous dendrites connect to other neurones

The resting potentialis the potential difference or voltage across the neurone cell membrane while the neurone is at rest. It is about -60mV inside the cell compared with the outside. Other cells may also maintain a resting potential that might change under certain circumstances.

• When a neurone is not transmitting an action potential it is said to be at rest. In fact, it is actively transporting ions across its cell surface membrane.
• Na/K ion pumps use ATP to pump 3 Na ions out and 2 K ions in. The plasma membrane is more permeable to K ions than to Na ions and many diffuse out again.
• The cell cytoplasm also contains large organic anions (negatively chargered ions), hence the inside of the cell is maintained at a negative potential compared with the outside. The cell membrane is said to be polarised.
• The potential difference acrss the cell membrane is about -60mV. This is called resting potential.

The action potentialis the depolarisation of the cells membrane so that the inside of the cell membrane so that the inside is more positive than th eoutside, with a potential differnce across the membrane of +40 mV. This can be transmitted across the axon or dendron plasma membrane.

At rest the gated Na ion channels are kept closed. The Na/K ion pumps uses ATP to actively transport three Na+ out for every two K+ ions brought into axon. A few K ions diffuse back out as some K channels open. If some of the Na channels are opened then Na ions will quickly diffuse down their conc gradient into the cell from the surrounding tissue fluid. This causes depolarisation of the membrane. In the generator region of the receptor cells the gated channels are opened by energy changes in the environment. E.g. the gates in a Pacinian corpusclem which detects pressure, are opened by deformaton. The gates further along the neurone are opened by changes in the potential difference acros the membrane. They are called voltage-gated channels and these respond to depolarisation of the membrane.

There is an all or nothing principle - if the depolarisation is large enough to reach threshold potential it will open some nearby voltage gated channels. This will cause a large influx of Na ions, reaching +40mV. Once this value has been reached an action potential will be transmitted

Ionic movements: action potential consists of a set of ionic movements

• Membrane starts in resting state - POLARISED with the inside being -60mV compared to outside
• Na ion channels open and some Na ions diffuse into the cell
• The membrane DEPOLARISES - it becomes less negative with respect to the outside and reaches threshold vaule of -50mV
• Volatge gated Na ions channels open and many Na ions flood in. As more Na ions entre the cell becomes positively charged inside compared to outside.
• The Na ions channels close and K ions channels open
• K ions diffuse out of the cell brining the potential difference back to negatice inside compared with outside = REPOLARISATION
• The potential differejce overshoots slightly making the cell HYPERPOLARISED
• The origional potential differnce is estored so that the cell returns to its resting state

After an action potential, Na and K are in wrong places so must be restored by Na/K pumps. For a short time, it is impossible to stimulate another action potential = REFRACTORY PERIOD. This ensures that action potentials are transmitted in only one direction

Transmission of acton potentials: Local currents

The opening of Na ions channels at a point of the neurone upsets the Na/K balance created by the pumps. This creates local currents in the cytoplasm. These local currents cause Na+ channels further along the membrane to open.

• When an action potential occurs the Na+ channels open at a particular point
• This allows sodium ions to diffuse across the membrane from the region of higher concentration outside the neurone, into the neurone
• The movement of sodium ions into the neirone upsets the balace of ionic concentrations created by sodium and potassium pumps
• The concentration of Na ions inside the neurone rises at the point where the sodium ion channels open
• This causes the sodium ions to diffuse sideways, aways from this region of increased concentration
• The movement of charged particles is a current called a local current

Transmission of action potentials: Voltage gated Na+ channels

Further along the membrane are more gated Na+ channels. These gates are operated by changed in the voltage across a membrane. At rest the voltage will be a resting potential. The movement of Na ions along the neurone (away from high conc) alters the potential difference across the membrane. When potential difference is reduced the gates open, this allows Na ions to enter neurone further along membrane. The action potential has moved along the neurone.

Transmission of action potentials: The myelin sheath

The myelin sheath is an insulating layer of fat. Na/K can't diffuse through fatty layer, therefore the ionic movements that create the action potential cant occur over much of the length of the neurone. The gaps between the Schwann cells are called Nodes of Ranvier. In myelinated neurones the local currents are elongated and Na ions diffuse along the neurone from one node to the next. This means the potential appears to jump. This is saltatory conduction.

Advantages to saltatory conduction - speeds up the transmission of the action potential as action potential can only occur at the gaps. Myelinated conduct action potentials quicker than non-myelinated.

A synapse is a junction between two or more neurones. It is where one neurone can communicate with or signal to another neurone. Between 2 neurones is a gap called a synaptic cleft. An action potential is produced by movements of ions across the neurone membrane. The action potential cannot bridge the gap between neurpones. Instead the presynaptic action potential causes the release of a neurotransmitter that generated an action potential in the postsynaptic neurone.

The synaptic knob

The knob contains many specialised features - many mitochondria (indicating than an active process, needing ATP, is involved), a large amount of smotth endoplasmic reticulum, vesicles of a chemical called acetylcholine, voltage gated calcium ion channels in the membrane

The post-synaptic membrane

The postsynaptic membrane contains specialised sodium ion channels that respond to the neurontransmitter substance. Two polypeptided have a special receptor site, specific to acetycholine. The receptors have a complementry shape to acetylcoline molecule and bind to site. When acetylcoline binds to the two receptors the sodium ion channels open.

Transmission across the synapse

• An action potential arrives at the synaptic knob
• The voltage gated calcium ion channels open
• Calcium ions diffuse into synaptic knob
• The calcium ions cause the synaptic vesicles to move to and fuse with the presynaptic membrane
• Acetylcholine is released by exocytosis
• Acetylcholine molecules diffuse across the cleft
• Acetylecholine molecules bind to the receptor sites on the sodium ion channels in the postsynaptic membrane
• The sodium ion channels open
• Sodium ions diffuse across the postsynaptic membrane into the postsynaptic neurne
• A generator potential or excitatory postsynaptic potental is created
• If sufficient generator potentials combine then the potential across the postsynaptic membrane reaches the threshold potential
• A new action potential is created in the postsynaptic neurone
• Once and action potential is achieved it will pass down the postsynaptic neurone

The role of acetylcholinesterase

Acetylcholineis an enzyme in the synaptic cleft. It breaks down the transmitter substance acetylcholine.

Acetylcholineasterase is an enzyme found in the synaptic cleft. It hydrolyses the acetylcholine to ethanoic acid and choline. This stops the transmission of signals so that the synapse does not continue to produce action potentials in the postsynaptic neurone.

The ethanoic acid and choline are recycled. They re-enter the synaptic knob by diffusion and are recombined to acetylcholine using ATP from respiration in the mitochondria.

The recycled acetylcholine is stored in synaptix vesicles for future use.

The main role is to connect twoi neurones together so that a signal can be passed from one to the next. However, they are also able to perform other functions:

• Several presynaptic may converge to one post synaptic neurone. This allows several parts of the nervous system to create same responce (eg - several stimuli warning of danger)
• One presynaptic neurone might diverge to several postsynaptic neurones, allowing one signal to be transmitted to several parts of nervous system (eg - reflex arc, goes to brain and muscle)
• Sysnapses ensure that signals are transmitted in one direction
• Synapses can filter out unwanted signals
• Low-level signals can be amplified by a porcess called summation. If a low-level stimulus of persistant it will generate several siccessive action potentials in ithe presynaptic neurone. Summation can also occur when several presynaptic neurones each release small vesicles
• Acclimatisation - after repeated stimulation a synapse may run out of vesicles containing the transmitter substance, the synapse is then fatigued. This means it no longer responds to the stimulus.

• The complex interconnections of the nervous system are not the only way that different messages can be conveyed using the same signals. A signal coming from the light receptors in the eye to the relevent optical centre in the brain will inform the brain that light is falling on the eye. However, it says nothing about the intensity of the light.

• When a stimulus is at a higher intensity the sensory receptors will produce more generator potentials. This will cause more frequent action potentials in the sensory neurone. When these arrive at a synapse they will cause more besicles to be released. In turn, this creates a higher frequency of action potentials in the postsynaptic neurone, Out crain can determine the intensity of the stimulus from the frequency of signals arriving. A higher frequency of signals means a more intense stimulus.

• Around one third of the peripheral neurones in vertebrates are myelinated. That is, they are insulated by an individual myelin sheath. The sheath is created by a series of seperate cells called Schwann cells. These are wrapped around the neurone so the sheath actually consists of several layers of membrane and thin cytoplasm from the Schwann cell. At intervals along the neurone there are gaps in the myelin sheath. These are called the nodes of Ranvier. Each node is very short.
• The remainder of the peripheral neurones and the neurones found in the CNS are not meylinated. Non-myelinated neurones are still associated with Schwann cells byr several merones may be enshrouded in one loosely wrapped schwann cell. This means that the action potential moved along the neurone in a wave rather than jumping from node to node as seen in myelinated neurones.

Advantages to myelination:

They can transmit an action potential much quicker. Also myelinated neurones carry signals from sensory receptors to the CNS and from CNS to receptors. They carry signals over long distances so increased speed means it reaches the end quicker, enabling a more rapid responce. Non-myelinated neurones are shorted and are used in coordinating body functions, eg breathing.

The endocrine system is another communication system in the body.It uses the blood circulation to transport its signals.The blood transports material all over the bodty therefore any signal released into blood will be transported everywhere - the signals released are called hormones.

Hormonesare molecules that are released by endocrine glands directly into the blood. They act as messangers, carrying a signal from the endocrine gland to a specific target organ or tissue.

Endocrine:Endocrine glads release or secrete their hormoness directly into the blood. They are ductless

Exocrine:Secretes molecules into a duct that carries the molecules to where they are used

Targeting the signal:The cells recieving a hormone signal must posses a specific complementary receptor on their plasma membrane. The hormone binds to this receptor. Ech hormones is different from all other. This emans that a hormone can tracel around in th eblood without affecting cells that do not posses that receptor. The cells that hold the receptor are called target cells.

The first messengeris the hormone that ransmits a signal around the body.

The secondary messenger is cAMP, which transmits a signal inside the cell.

Adrenaline is an amino acid. It is unable to enter the target cell. Therefore it must cause an effect inside the cell without entering the cell itself. The adreneline receptor on the outside of the vell suface membrane has a shape complementry to the adrenaline molecule. This receptor is asociated with an enzyme on the inner surface of the cells membane. The enzyme is called adenyl cyclase.

Adrebaline in the blood binds to its specific receptor in the cell surface membrane. The adrenaline molecule is called the first messanger.When it binds to the receptor it activated the enzyme anenyl cyclase. The adenyl cytclase convert ATP to cyclic AMP. The cAMP is the secondary messengerinside the cell. The cAMP can then cuase an effect indide the cell by activating enzyme action.

The adrenal glands are found lying just above the kidneys - one on each side of the body. Each gland can be divided into a medulla region and a cortex region.

The adrenal medulla:the medulla is found in the centre of the glad. The cells release the hormone adrenaline in response to stress such as pain or shock. The effecrs of adrenaline are widespread and most cells have adrenaline receptors. They prepare the body for activity by:

• relax smooth muscle in the bronchioles
• increase stroke volume of heart
• incease heart rate
• dilate pupils
• cause body hair to erect
• stimulate conversion of glyogen to glucose

The adrenal cortex: The cortex uses cholesterol to produce certain steroid hormines. The have a variety of roles - the moneralocorticoids help to control the concentrations of sodium and potassium in blood and the glucocorticoids help to control the metabolism of carbohydrates and proteins in the liver.

The pancreas is a small rogan lying below the stomach. It is an unsual organ in that it had both exocrine and endocrine functions.

The majority of cells in the pancreas manufacture and release digestive enzymes. This is the exocrine function of the pancreas. The cells are found in small groups surrounding tiny tubules into which they secrete digestive enzymes. The tubules join up to create the pancreatic ductwhich carries the fluid containing enzymes to first part of the small intestine. The fluid contains amylase, trypsinogen and lipase. The fluid also contaubs Na hydrocarbonate which makes it alkaline. This helps neurtralise the contents of digestive system that have left the acid environment of stomache.

The islets of Langerhansare small patches of tissue in the pancreas that have an endocrine function. Alphaand betacells are found in the Islets of Langerhans.

• Alpha cells secrete the hormone glucagon
• Beta cellssecrete the hormone glucogon
• Insulinis the hormone, released from the pancreas, that causes blood glucose levels to go down
• Glucagonis the hormone that causes blood glucose levels to rise

If concentration of blood glucose rise or falls away from the acceptable concentration than the alpha and beta cells in islets of langerhans detect the change and respond by releasing a hormone.

A high blood glucose concentrationis detected by beta cells. In responce the beta cells secrete insulin into body. The target cells are the liver cells (hepatocytes), muscle cells and other cells in brain. These posses specific receptors for insulin. When blood pass these cells, the insulin binds to receptors. This activates adenyl cyclase so generates cAMP which creates a series of reaction.

Insulin has many effecrs on the cell:more glucose channels are placed into the cell surface membrane, more glucose enters the  ell, glucose in the cell is converted to glycogen for storage (glycogenesis), more glucose is converted to fats, moore glucose used in respiration.

A low blood glucose concentration  is detected by alpha cells. In responce the alpha cells secrete the hormone glucagon. Its target cells are hepatocytes which posses yje specific receptor for glucogon.

Glucagon effects cells:convert glycogen to glucose, use more fatty acids in respiration and production of glucose by conversion from amino acid and fats (glucogenesis)

Insulin brings about the effects that reduce the blood glucose concentration. If the blood glucose concentrations is too high then it is important that insulin is released from beta cells and vice versa.

The control of insulin secretion:

• The cell membranes of the beta cells contain both K+ and Ca+ ion channels.
• The potassium ions channels are normally open and the calcium closed. Potassium ions diffuse out of the cell making the inside of the cell more negative, at rest the potential difference is about -70mV.
• When glucose concentrations outside of cell are high, glucose molecules diffuse into cell.
• The glucose is quickly used in metabolism to produce ATP.
• The extra ATP causes the potassium channels to close
• The potassium can no longer diffuse out and this alters the potential differnce across the cell membrane - it becomes less negative
• This change in potential differene opens the calcium ion channels.
• Calcium ions enter the cell anf cause the seretion of insulin by making the vesicles containing insulin move to the cell surface membrane and fuse with it, releasing insulin by exocytosis.

Diabetes mellitusis a disease in which the body is no longer able to control its blood glucose concentration. Itcan lead to very high concentrations (hyperglycaemia) of glucose after a meal rich in sugars. It can also lead to the concentrations dropping low (hypoglycamia) after exercise.

Type I:It is often called juvenile-onset diabetes because it usually starts in childhood. It is thought to be a result of an autoimmune response in which the bodys own immune stsem attacks the beta cekks abd destroys them. It may also result from a viral attack. The body is no longer able to manufacture sufficient insulin and cant store excess glucose and glycogen.

Type II:A person with type II diabetes can still produce insulin however as people age the responsiveness to insulin declines. This is probably because the specific receptors decline and the cells loose their ability to respond to insulin in blood. The levels secreted by beta cells may also decline. Certain factors are known to cause earlier onset of type II diabetes:

• obesity
• diet high in sugars
• being Asian or Afro-Caribbean
• family history

• Type II diabetes is usually treated by carefull monitoring and control of diet. Care is taken to match carbohydrate intake and use. This may be supplemented by insulin injections or use of other drugs which slow down absorption of glucose from digestive system
• Type I is treated by insulin injections. The bloof glucose conc must be monitored and correct doses of insulin administrated so it remains fairly stable.

Source of insulin:More recently, insulin can be produced by bacteria that have been genetically engineered to manufacture human insulin. The advantages to using genetically engineering is......  -  it is an exact copy so therefore is faster acting and more effective, less chance of developing tollerance to insulin, less chance of rejection,, lower risk of infection, cheaper to manufacture the insulin than extract from animals, manufacture process is more adaptable, less likley to have moral rejections by using bacteria.

Stem cells:Research has shown that use of stem cells can be used to treat type I diabetes. Stem cells are undifferntiated and can be introduced to develop into a variety of cells. Scientists have found precursor cells in pancreas of mice and many be true stem cells. If similar they could be developed to produce new beta cells in patients with type I diabetes.

The heart pumps blood around the circulatory system, supplying muscles with oxygen and nutrients and removing waste products, eg carbon dioxide and urea. The requirements of cells vary according to activity level, eg when exercising your body needs more oxygen and nutrients, therefore it is important that the heart can adapt to meet requirements of the body.

How the heart adapts to supply more oxygen and glucose:

• Increase the number of beats per minute (increase heart rate)
• The heart can increase the strength of its contractions
• Increase the volume of blood pumped per beat (increase stroke volume)

Control of heart rate:

• The heart muscle reqponds to the presence of the hormone adrealine
• It contains its own pacemaker - SA node. The SAN can initiate and action potential, causing heart to contract
• The heart is supplied by nerves from medulla oblongata of the brain. They dont initiate a contraction but can affect the frequency of them. Impulses sent down accelerator nerve increase and vagus nerve decrease frequency.

Interaction between control mechanisms:

Various factors that affect heart rate must interact in a way to ensure heart beats at appropriate rate. At rest, it is controlled by SAN (60-80bpm) howver the frequency can be controlled by the cardiovascular system in medualla oblongatta.

There are many factors that affect heart rate:

• Movements is detected by stretch receptors, these tell cardiovascular system that more oxygen will be needed so increases heart rate.
• When we exercise muscles produce more CO2. This reduces the pH and rge change is detected by chemoreceptors which increases heart rate.
• When we stop exercise, CO2 falls. This reduces activity of accelorator pathway so HR falls.
• Adrenaline is secreted to respond to stress or excitement. The presence of adrenaline increases heart rate, so helps prepare body for activity.
• Blood pressure is monitored by stretch receptors in waklls of carotid sinus. If blood pressure rises too much by exercise, the stretch receptors send signals to CV centre. This responds by reducing heart rate.