response to stimuli

• a stimulus is a detectable change in the internal or external environment of an organism that leads to a response in the organism
• a response is a behaviour that is manifested by an organism in response to an external or internal stimulus
• Being able to respond to stimuli is advantageous to organisms. They can detect and move away from harmful stimuli, like extremes of temperature. But they can also detect and move towards stimuli like food sources which aids survival. These organisms that survive have a greater chance of reproducing and passing on their alleles to the next generation. Selective pressures favour a group of organisms with more appropriate responses
• A taxis is a simple response whose direction is determined by the direction of a stimulus. A motile organism responds directly to environmental changes by either moving towards a favourable stimulus, or away from an unfavourable one. Positive Taxis = towards, negative taxis = away from
• taxis example: single celled algae move towards light in a positive phototaxis.
• Kinesis is a response where an organism changes its speed of movement, or frequency of direction change. If an organism crosses a dividing line between a favourable and unfavourable environment, its rate of turning increases so that its chance of returning to the favourable environment faster increases. If further from favourable environment rate of turning may fall, but speed increases. Important when stimulus is less directional
• Tropism: growth of a plant in response to a directional stimulus. With the plant growing towards or away from a stimulus. i.e plant shoots grow towards light and plant roots away from it. Leaves are therefore most accessible to sunlight, for photosynthesis. While roots are more likely to grow into soil where they can absorb mineral ions and water. This increases chance of plant survival.
• plants respond to: light - shoots grow towards light for photosynthesis. gravity - plants must be firmly anchored in soil, so roots must sense gravity and grow in the direction of its pull. water - almost all plant roots grow towards water, in order to absorb and use it for metabolic processes and support
• plants response to external stimuli involve plant growth factors. They exert their influence by affecting growth, and are produced in small quantities. One type of plant growth factors is auxins, which include IAA which controls plant cell elongation
• phototropism in flowering plants: unilateral light. Cells in shoot tip produce IAA which is transported down shoot. Initially IAA is evenly distributed throughout all regions, but unilateral light stimulus causes movement of IAA from light to shaded side of the shoot. So IAA concentration greater on shaded than light side. IAA cases cell elongation in shoots, so cells on shaded side elongate after and the shoot tip bends towards the light. In roots IAA inhibits cell elongation, so build up of IAA on the shaded side causes light side to elongate faster and the root tip to bend away from light.
• gravitropism in flowering plants: Cells in the root tip produce IAA which is transported along root to all sides and regions. Gravity influences IAA movement from upper to lower side of root. IAA concentration builds up and is higher on lower than upper root side. IAA inhibits root cell elongation, so cells on upper side where concentration is lesser elongate faster. so upper side longer and root tip bends down towards gravity. In shoots the greater IAA concentration on the lower side causes faster cell elongation and shoot tip to bend up, away from gravity. So roots grow in to soil.
• IAA in plant cell elongation: IAA increases the plasticity of plant cell walls, only occurs on young cell walls as with maturity comes rigidity. Does so via the acid growth hypothesis, where hydrogen ions are actively transported from the cytoplasm to spaces in the cell wall. So, the cell wall becomes more plastic and can hence elongate by expansion.
• the nervous system can be divided into the central nervous system (the brain and spinal cord) and the periphery nervous system (nervous that connect the receptors and effectors to the CNS). The periphery nervous system can be divided into sensory and motor neurones. And motor neurones can be further subdivided to the somatic (under voluntary control) and autonomic (carries impulses to glands, smooth muscle, and cardiac muscle, under involuntary control)
• reflexes are rapid, short lived, localised and involuntary. relax arc: stimulus activates receptor which generates nerve impulses in the sensory neurone. The intermediate neurone acts as a coordinator, linking the sensory and motor neurones in the spinal cord. The motor neurone then carries nerve impulses from the spinal cord to a muscle, which acts as an effector which is stimulated to contract. Resulting in a response
• reflex arcs are important as: They are involuntary, so don't require decision making and the brain can concentrate on more complex responses. Hence, the brain is not overloaded with situations where the response is always the same. They protect the body from harm, and are active from birth so don't have to be learnt. They are fast, as the neurone pathway is short with only one or two synapses, very important in withdrawal reflexes. The absence of the decision making process also makes them rapid.
• features of sensory reception: - specific to a single type of stimulus, which won't respond to others. i.e the pacinian corpuscle only responds to mechanical pressure, not light, heat or sound. - produces a generator potential by acting as a transducer. Stimuli involve a change in some form of energy, receptors must act as transducers to convert this to another form which can be interpreted by the body. This energy is in the form of a nervous impulse called a generator potential.
• A pacinian corpuscle responds to mechanical stimuli, they are made up of a sensory neurone surrounded by rings of concentric connective tissue, each separated by a layer of gel.
• The pacinian corpuscle has tretch-mediated sodium channel in plasma membrane. In its normal state stretch mediated sodium channels of membrane around the neurone of pacinian corpuscle are too narrow for sodium ions to pass along them. Pacinian corpuscle has resting potential. Pressure applied to the pacinian corpuscle, deformed and the membrane around the neurone becomes stretched. Stretching widens sodium channels in membrane so sodium ions diffuse into neurone. influx of sodium ions changes potential of membrane, depolarising it, producing a generator potential. creates action potential
• The light receptors in the eye are found on the retina and split into two main types, the rods and the cones. both act as transducers converting light energy into the electrical energy of a nerve impulse
• rod cells cannot distinguish between light wavelengths, only provide black + white vision. multiple rod cells connected to single bipolar synapse, all contributing to meeting threshold light value to create generator potential. retinal convergence so there's greater chance threshold exceeded due to summation, so rod cells allow us to see in low light intensity. generator potential is created when rhodopsin pigment broken down. Rod cells = low visual acuity, same neurone generates single impulse, regardless of how many/which rod cells stimulated. brain can't distinguish between light sources.
• cone cells are divided into three types, each responding to different range of light wavelengths. Allows us to see in colour, depending on proportion stimulated. Individually connected to the bipolar synapse, so only respond to high light intensity required to breakdown iodopsin pigment. Each type of cone cell has differing iodopsin. Give high visual as if two adjacent cone cells stimulated, the brain receives to impulses. Cone cells concentrated on the fovea which receives the highest light intensity. While at the peripheries only rod cells are found as light intensity received is low
• The autonomic nervous system is self governing, controlling involuntary activities of internal glands and muscles. it has two divisions: the sympathetic nervous system - stimulates effectors at times of stress, speeding up activity like heart rate and diverting blood flow away from the digestive tract and reproductive organs towards the brain and skeletal muscles. It heightens our awareness. The parasympathetic nervous system - inhibits effectors at times of rest, slowing down activity. Conserves energy and replenishes the body's reserves. The actions of each system are antagonistic.
• Both the sympathetic and parasympathetic nervous system are active at all times. The activities of internal muscles and glands are therefore regulated by a balance of the two systems
• autonomic control of the heart rate: cardiac muscle is myogenic, with contraction initiated from within the muscle itself, rather than neurogenic. the right atrium contains the sinoatrial node, which is where the initial stimulus for contraction originates. it has its own basic rhythm of stimulation that determines the beat of the heart. hence it is often referred to as the pacemaker
• control of the heart rate by the sinoatrial node; Wave of electrical excitation spreads out from the SAN across both atria, leading them to contract. a layer of non-conductive atrioventricular septum tissue prevents the wave crossing to the ventricles. The excitation wave enters the atrioventricular node, between the two atria. after a short delay this conveys wave of electrical excitation down the ventricles via the purkyne tissue muscle fibres. the bundle of his conducts the wave from the atrioventricular septum to the ventricle base. where the bundle branches into smaller purkyne fibres.
• control of the heart rate continued: the wave of excitation is released from the purkyne tissue, causing both ventricles to contract quickly at the same time from the bottom of the heart upwards.
• modifying resting heart rate: essential heart rate can be altered to meet varying demand for oxygen. changes to heart rate are controlled by the medulla oblongata region of the brain. it has two centers concerned with heart rate, a centre to increase heart rate linked to the sinoatrial node via the sympathetic nervous system. And a centre to decrease heart rate linked to the sinoatrial node by the parasympathetic nervous system. Which of the centres is stimulated depends on the nerve impulses they receive from the chemical and pressure receptors in the blood
• control of heart rate by chemoreceptors; Found in walls of the carotid arteries and aorta, are sensitive to changes in blood pH. When blood co2 concentration is higher than normal pH falls. detected by chemoreceptors, which increase frequency of impulses to the medulla oblongata. this centre increases frequency of impulses via the sympathetic nervous system to the SAN. electrical waves produced faster by the SAN, and heart rate increases. increased blood flow so more co2 removed in the lungs, so concentration falls and pH returns to normal. chemoreceptors then reduce nerve impulse frequency.
• control by pressure receptors: found within the walls of the caratoid arteries and the aorta. when blood pressure higher than normal, pressure receptors transmit more nervous impulses to the centre in the medulla oblongata that decreases heart rate. Sends impulses to the SAN via the parasympathetic nervous system, decreases the rate of heart beats. When blood pressure is lower than normal, pressure receptors transmit more impulses to the heart rate increase centre of the medulla oblongata, which sends impulses to the SAN via the sympathetic nervous system to increase the rate of heart beats.