Biology 5: Section 1

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Stimulus & Response

  • Stimulus is a change in an organisms environment that can be detected by receptor cells
  • Receptor cells detect a stimulus and initiates a nerve impulse
  • Kinesis is a change in the speed of random movement in response to an envrinmental stimulus
  • Taxis is a directed movement towards or away from a stimulus
  • There are positive and negative taxis
  • Tropisms are usually seen in plants, they can respond by growing in a particular direction

Examples of Tropisms:

  • Chemotropism
  • Geotropism
  • Hydrotropism
  • Phototropism
  • Thigmotropism (touch)
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Reflex Arc

Fixed movement in response to a stimulus

  • Sensory Neurone - Nervecel that carries impulses from a receptor to the CNS
  • Central Nervous System (CNS) - Brain & Spinal Cord - Processes incoming information & prodces a response
  • Motor Neurone - Nerve cell carries impulses from the CNS to the effectors
  • Effectors - Organs that bring about a response
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The Control of Heartbeat

  • The heart is myogenic
  • The cardiovascular centre in the medulla of the brain is responsible for maintaining the heart rate to the needs of the body, but it modifies rather than initiates the heartbeat
  • Carotid & Aortic bodies contain chemoreceptor cells that are sensitive to CO2 levels in the plasma - more CO2 increases the speed of impulses
  • Pressure receptors (baroreceptors) in carotid sinus transmit impulses to the cardiovascular centre when blood pressure rises - part of negative feedback system, maintains blood pressure
  • Two antagonistic nerves leading from the cardiovascular centre to the sino-atrial node
  • A sympathetic nerve, carrying impulses that speed up the heart
  • A parasympathetic nerve, carrying impulses that slow down the heart
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The Pacinian Corpuscle

  • Function is to detect pressure and vibration - it is a mechanoreceptor
  • Each corpuscle consists of a single sensory neurone surrounded by 20-60 lamellae of connective tissue
  • Change in pressure on the corpuscle is transmitted through to the sensory neurone, which deforms, causing stretch-mediated sodium ion channels in the axon membrane to open
  • This allows sodium ions to diffuse in, creating a generator potential, once threshold reached, impulse passes through the sensory neurone
  • Respond to changes in environment, not constant stimuli
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The Retina & Visual Acuity

  • Single layer of light sensitive receptor cells
  • Rods are more sensitive than cones and can function in dim light
  • Fovea consists only of cone cells

Reasons for increased sensitivity:

  • Pigment in rod cells more easily bleached
  • Many rods converge onto one neurone, all contribute to generator potential

Visual Acuity

  • Ability to distinguish objects that are close together
  • Cones give us high visual acuity
  • Many rods converge onto one neurone
  • One cone converges onto one neurone
  • Greatest concentration of cones are at fovea, when light falls on this point we see in great detail
  • Cones send more information to the brain per unit area of the retina than rod cells
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Mammalian Hormones

  • Most hormones are water soluble & travel in the blood
  • When they encounter a cell with the correct receptor protein they fit into the receptor, but they do not enter the cell
  • The binding activates an enzyme inside the cell, Adenyl Cyclase, which in turn converts ATP in the cytoplasm into cyclic AMP which acts as a second messenger, the hormone being the first messenger
  • Cyclic AMP brings about an effect by activatory specific enzymes or enzyme pathways, both insulin & glucagon act in this way
  • Lipid hormones are known as steroids e.g. Oestrogen, Progesterone & Testosterone
  • Being lipids they can pass straight through cell membrane
  • Once inside bind to receptor, hormone-receptor complex passes into the nucleus where it directly affects gene expression
  • Nervous communication is brought about by electrical signals, called impulses, which are transmitted down elongated specialised cells called neurones
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Local Chemical Mediators

  • Histamine - released by mast cells in response to damage causes familiar symptoms of allergy
  • Prostaglandins - Lipid for contraction & relaxation of smooth muscle, the dilation and constriction of blood vessels, control of blood pressure & degree of inflammation
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Plant Hormones

  • Auxins control plant growth including tropisms, differntiation of tissues and abassion (leaf & fruit fall)
  • Darwin found that a message is sent from the tip, which passes down the stem and causing bending
  • Plants do not have neurones so communication must be chemical
  • Auxin diffuses down the stem, accumulates on dark side, so they bend towards the light
  • Root must grow down & shoot must grow upwards, auxin responsible for controlling this geotropism
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Nerve Impulses

A neurone is a nerve cell capable of carrying impulses. All neurones have the same features:

  • A cell body that contains the nucleus & other organelles
  • Dentrites that take impulses towards the cell body
  • An axon that takes impulses away from the cell body
  • Synapses that junction with other neurones or effectors

The Resting Potential

1. Active Transport:

  • used to pump 3 sodium ions out and 2 potassium ions in
  • more positive in than pass out - unequal exchange

2. Facilitated Diffusion

  • sodium ions leak by diffusion into the cell
  • potassium ions leak out, down respective gradients
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Nerve Impulses 2

The Action Potential

  • Brought about by a quick reversal in the permeability of the axon membrane
  • This allows sodium ions to flow into the axon, making the inside positive with respect to the outside
  • Voltage gated, change shape in respect to voltage

Depolarisation

  • when neurone stimulated, voltage changes
  • few sodium channels notice change, sodium diffuses in
  • if reaches threshold (-50mV) rest of channel open for 0.5ms
  • sodium ions diffuse in, making inside more positive
  • the more sodium ions there are, the more the voltage changes, more channels open and more sodium ions diffuse in

Repolarisation

  • When membrane potential reaches 0V potassium channels open for 0.5ms
  • This causes potassiu ions diffuse out, makes inside more negative again
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Nerve Impulses 3

Re-Establishing the Resting Potential

  • Potassium channels remain open once -70mV reached
  • Causes hypepolarisation when the pd reaches approx -80mV
  • Potassium channels then close

Refractory Period

  • Absolute Refractory Period - imposible to create another impulse
  • Relative Refractory Period - possible to create another impulse, the stimulus must be greater than normal

Faster Transmission:

  • Greater Diameter
  • Less Synapses
  • Higher Temperature
  • Myelinated
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Nerve Impulses 4

Saltatory Conduction

  • Occurs in myelinated neurones, action potential jumps from node to node
  • Greatly increases speed of transmission, exchange only occurs at nodes
  • When axon potential is present at one node, influx of sodium ions causes the displacement of potassium ions down the axon
  • This diffusion of potassium down the axon makes the next node more positive and depolarises it until the threshold is reached
  • In this way the impulse quickly jumps from node to node at speeds upto 100ms-1
  • Energy efficient in terms of ATP usage
  • Only small part of axon used, fewer ions need to be pumped back
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The Synapse

  • Action potential arrives at synaptic knob
  • Calcium channels open, so calcium ions flow into synaptic knob
  • The calcium ions cause vesicles containing a transmitter substance to move to the presynaptic membrane
  • The vesicles fuse with the presynaptic membrane and discharge the transmitter into the synaptic cleft
  • Molecules of transmitter diffuse across the gap and fit into specific receptor sites on the postsynaptic membrane
  • The permeablilty of the postsynaptic membrane changes, causing a movement of ions.
  • Sodium ions flow inwards, building up a charge known as EPSP (Excitory Postsynaptic Potential)
  • If EPSP reaches threshold, action potential generated in neurone
  • The transmitter substance is broken down by enzyme in the cleft
  • Products of breakdown are reabsorbed into synaptic knob, where they are re-synthesised using energy from ATP synthesised by the mitochondria
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The Synapse 2

Inhibition at Synapse

  • Synapses allow us to select pathways, therefore more synapses need inhibiting than need stimulating
  • Inhibitory neurones make it more difficult for an action for an action potential to be generated
  • The neurone transmitters from these synapses open potassium and chloride channels rather than sodium channels, causes IPSP (Inhibitory Postsynaptic Potential) in which the postsynaptic membranes are hyperpolarised (to approx -90mV) rather than polarised
  • The balance of inhibition and stimulation received at a particular synapse will determine whether an action potential is generated or not

Temporal Summation 'in time' - where two or more impulses arrive in quick succession down the same neurone

Spatial Summation 'in space' - where two or more impulses arrive at the same time down different neurones

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Muscle

Smooth Muscle

  • Generally found in tubular organs such as the intestines, blood vessels and reproductive system where its function is peristalsis

Cardiac Muscle

  • Only found in the heart - never tires

Skeletal Muscle

  • Attatched to bone via tendons where its function is to produce movement and maintain posture
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Skeletal Muscle

Sliding filament theory of muscle contraction

  • An impulse arrives down a motor neurone + terminates at the neuromuscular junction
  • The synapse secretes acetylcholine
  • Acetylcholine fits into receptor sites on the motor end plate
  • The binding causes a change in the permeability of sacroplasmic recticulum, resulting in an influx of calcium ions into the myofilament
  • The calcium ions bind to the Troponin, changing its shape
  • Troponin displaces the tropmyosin, so that the myosin heads can bind to the actin
  • The myosin head pulls backwards, so that actin is pulled over the myosin, this is the 'power stroke'
  • An ATP molecule becomes fixed to the myosin head causing it to detach from the actin
  • The splitting of ATP provides the energy to move the myosin head back to its original position
  • The myosin head again becomes attatched to the actin, but further again.
  • In this way, the actin is quickly pulled over the myosin in a rachet motion, shortening the sacromere, the whole filament and the muscle
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The Role of ATP and Phosphocreatine

  • ATP is resynthesised using phosphate derived from the splitting of phosphocreatine
  • The ATP/PC system can provide enough energy for maximum effort for 10 seconds
  • After this (for upto 1 minute) ATP is supplied from glycolysis
  • This is anaerobic, lactate build up is a painful problem
  • After approx 1 minute ATP is provided from aerobic respiration, this only allows 60-70% effort but is endless along as fuel is available, as long as there is no lactate build up
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Slow and Fast-Twitch Fibres

Fast-Twitch Fibres

  • Contract quickly and powerfully, but fatigue quickly
  • Rely on glycolysis for their ATP, so lactate builds up rapidly
  • Atheletes who specialise in power events tend to have more fast-twitch fibres

Slow-Twitch Fibres

  • Contract more slowly, producing less power
  • Do not tire as quickly as fast-twitch, keep going for long periods
  • Slow-twitch fibres rely on anaerobic respiration for their ATP
  • Contain more mitochondria than fast-twitch fibres
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Homeostasis

Homeostasis is the ability of an organism to maintain its internal conditions within certain limits. Examples of homeostasis include:

  • Maintaining the pH of blood and body fluids between 7.3 and 7.45
  • Maintaining the core body temperature around 37 degrees celsius
  • Maintaining blood glucose levels between 4 & 11 millimoles per litre

Internal conditions are not absolutely constant, but must be kept within certain limits

Negative Feedback

Most examples of homestasis involve negative feedback. When a factor changes, the homeostatic mechanism acts to reverse that change and bring things back to normal

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Regulation of Body Temperature

Endotherm - Mammals & Birds

  • usually control their body temperature regardless of external temperature
  • control their body temperature by physiological and behavioural means

Ectotherm - Amphibians & Reptiles etc

  • can only regulate their body temperature by behavioural means
  • generally their body temperature is usually similar to their environment

Behavioural thermoregulation in mammals

  • the temperature of blood is detected by the hypothalmus, inside is the thermoregulatory centre, which has two parts: heat loss and heat gain centres
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Too Cold

We get too cold when the heat loss from our body exceeds what is generated and the temperature of the blood falls. This drop is detected by the hypothalmus, and signals pass from the heat gain centre to bring about the following responses:

1. Shivering - rapid contraction and relaxation of muscles creates heat, which is distributed to the rest of the body in the blood

2. Vasoconstriction - constriction of the muscular arteriole walls re-directs blood away from the skin, keeping the warmer blood to the centre of the body

3. Hairs stand on end - hairs are raised by erector pili muscles. In hairy mammals this traps a layer of insulating air, in humans gives us 'goose pimples'

4. Increasing basal metabolic rate (BMR) - Acheived by adrenaline in short term, acheived by hormone thyroxine in long term

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Too Hot

We get too hot when we generate more heat than we lose. During exercise, for example, the temperature of the blood rises. The heat loss centre brings about these responses:

1. Sweating - secretion of salty sodium solution from millions of sweat glands that cover our bodies. Sweat coats the body as it evaporated, as sweat takes energy when it evaporates

2. Vasodilation - the arterioles dilate, allowing more blood to flow through the capillaries of the skin. The heat in the blood is transferred to the sweat, and is lost from the body as the sweat evaporates

3. Lowering of hairs - this happens as the erector pili muscles relax

4. Basal metabolic rate lowers

We tend to overheat when we excersise because movement of muscles creates heat. It is basic thermodynamics: no energy conversion is 100% efficient - some energy is lost as heat. Exercise involves two energy conversions: 1st, when chemical energy in glucose is transferred to ATP via respiration and, 2nd, when the energy in the ATP is used to produce movement in muscles

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Regulation of Blood Glucose Levels

Too much blood glucose - hyperglycaemia - will lower the water potential of the blood and produce symptoms of thirst (and as a result of fluid intake) frequent urination

Too little blood glucose - hypoglycaemia - will produce symptoms of dizziness, tiredness, lack of concentration, irritablilty & in extreme cases, coma and death. This is because the brain must have glucose, it cannot use alternative fuel sources

Islets of Langerhan in pancreas make & secrete enzymes, Alpha-cells secrete glucagon, Beta-cells secrete insulin. These hormones are antagonistic, they have opposing effects

Definitions:

  • Glycogenesis - the production of glycogen by the polymerisation of glucose
  • Glycogenolysis - the breakdown of glycogen to release glucose
  • Glyconegenesis - the production of glucose from non-carbohydrate sources (e.g. lipid or protein) this happens during fasting/dieting/starvation when glucose and glycogen levels are low 
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Regulation of Blood Glucose Levels 2

If blood sugar levels are too high..

Blood sugar levels usually rise for a few hours after a meal, as the sugars and starches are digested into glucose and absorbed in to the blood. The high levels are detcted by the Beta-cells themselves which respond by secreting insulin. This hormone travels in the blood and fits into specific receptor proteins on the membranes of cells througout the body, but notably those of the liver

If blood sugar levels are too low..

The low levels are detected by Alpha-cells, which respond by secreting glucagon. This peptide hormone travels in the blood & fits into receptor proteins in the same cells that respond to insulin - those that contain stored glycogen Glucaon acts via a second messenger activating enzyme pathways that breakdown glycogen. Glucose then passes into the blood, thereby raising blood glucose levels

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Diabetes

Type 1 - Insulin dependent diabetes, cannot make insulin and so glucose cannot pass from blood into cells. Blood glucose rises causing hyperglycaemia. Symptoms include:

  • Thirst - the high glucose levels decrease the water potential of the blood, stimulating the sensation of thirst
  • Glucose in urine - the kidneys usually absorb all glucose, but can't due to high levels
  • Weight loss - when starved of their main fuel, cells respire other fuels such as lipids
  • Breath smells of ketone (fruity) - ketones are by-product of lipid metabolism
  • Excessive urination - a concequence of increased fluid intake

Only about 10-15% of all diabetics have Type 1 diabetes, it tends to develop at a young age, before 40, often as a child. It is an auto-immune disease in which the bodies own immune system destroys the insulin producing cells. Treatment usually involves injections.

Type 2 - This is known as late onset diabetes and is becoming increasingly common due to a high rate of obesity. It is also becoming more common in younger people. Of all diabetics, 85-90% have Type 2 diabetes. The problem is that the body does not make enough insulin, or that the cells do not respond to it properly. Treatment usually involves a combination of diet, exercise & weight loss 

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The Oestrus Cycle

The Pituitary Gland

  • Key endocrine gland that controls the activities of many other endocrine glands
  • Secretes follicle stimulating hormone (FSH) and leuteinising hormone (LH)
  • Both hormones are classed as gonadotropins

In males a straight forward negative feedback system keeps testosterone levels relatively steady. The hypothalmus is sensitive to testosterone levels in the blood. If too much the hypothalmus will inhibit production, if not enough, stimulates production

Control of Mammalian Oestrus - Essential Features:

  • An oocyte (egg cell or ovum) is matured and released during each cycle from an ovary
  • The lining of the uterus is prepared to receive a fertilised embryo
  • If the egg is not fertilised, the lining of the uterus is lost during menstruation and the cycle repeats itself
  • Menstruation is only in primates, not all mammals
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The Events of the Menstrual Cycle

Day 1 - Beginning of menstruation (a period). The endometrium (uterus lining) comes away. t the same time, the pituitary gland secretes FSH. This stimulates the development of an ovum in the ovary. The ovum becomes surrounded by layers of cells that form a follicle. The follicle develops a blood supply and begins to secrete oestrogen

Day 5 - Menstruation ends when most of the endometrium has been lost but the inner layer remains intact. Oestrogen stimulated mitosis in the endometrium which begins to thicken

Days 5 to 10 - Oestrogen levels rise. This inhibits the secretion of two pituitary hormones, FSH & LH, a negative feedback. The more oestrogen the more FSH & LH inhibited

Days 10 to 14 - In the ovary, oestrogen production by the follicle cells reaches a peak, and the rise of oestrogen levels in the bloos exerts a positive feedback effect. The hypothalmus releases more GnRH, and the responsiveness of the pituitary gland to GnRH increases. The result is a large surge of LH secretion, and a smaller surge of FSH secretion from the pituitary gland. A few hours later the ovum is released from a mature ovarian follicle - the moment of ovulation - and the remains of the follicle turn into a corpus luteum ('yellow body')

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The Events of the Menstrual Cycle 2

Days 15 to 21 - The corpus luteum begins to secrete progesterone as well as continuing to secrete oestrogen. Progesterone stimulates the endometrium to mature and become more glandular by secreting specific glycoproteins (protein molecules with sugar groups attached). By about day 21 the endometrium is ready for implantation of an embryo

N.B. Progesterone inhibits FSH. This means that no new follicles develop while there is a corpus luteum present in the ovary

If the egg is not fertilised:

The corpus luteum degenerates and stops secreting progesterone. The inhibition of FSH is lifted, as it the protection that progesterone gave to the endometrium. Menstruation begins and FSH is secreted again, stimulating the development of an ovum

If the egg is fertilised:

It is essential that the menstrual cycle stops as it would kill the embryo. To achieve this, the implanted blastocyst gives out a hormone, HCG, that acts as a signal to the corpus luteum to keep secreting progesterone. The detection of HCG is the basis of pregnancy tests

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Enzymes Involved in Oestrus Cycle

FSH - Gonadotrophin

  • Secreted by Anterior Pituitary
  • Effect - Stimulates follicle to develop in ovary, helps stimulate ovulation

Oestrogen - Steroid

  • Secreted by Follicle in Ovary
  • Effect - Stimulates repair of endometrium

LH - Gonadotrophin

  • Secreted by Anterior Pituitary
  • Effect - Surge of LH around day 14 triggers ovulation

Progesterone - Steroid

  • Secreted by Corpus Luteum in Ovary
  • Effect - Stimulates maturation of and maintains endometrium; inhibits FSH
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