Survival and response
All living organisms can detect stimuli. By responding to these stimuli, they increase their chances of survival.
A stimulus is: a detectable change in the internal or external environment of an organism that produces a response.
Examples of stimuli in:
Plants - light, temperature, water, gravity
Animals - chemicals (smell), temperature, pressure, touch, light
Taxes and Kineses
These are simple innate forms of behaviour that rely on simple reflex actions. They allow mobile organisms to respond to environmental changes and maintain them in favourable conditions for survival.
Innate: inborn/instinctual. Does not have to be learnt.
2 such types of behaviour are TAXES and KINESES. They have certain characteristics:
- They are genetically determined, and cannot be adapted or modified to suit changing circumstances.
- They show a stereotyped pattern, similar in all members of that species, although some differences may exist between the males and females.
- The behaviour often consists of a chain of reflexes in which each link in the chain provides the stimulus for the commencement of the next.
- They are typical of simple organisms with a short life cycle, where learning has little opportunity to occur. (The behaviour may also be important in the early life of verterates before learning takes over)
A taxis (or tactic response) is a simple, directional response to a stimulus, and is carried out by the whole organism. The direction of the stimulus determines the direction of the response.
Taxes may be positive (towards a favourable stimulus) or negative (away from an unfavourable stimulus).
Identify the type of behaviour and advantages in each of the following:
- Single-celled algae/protoctista move towards light: POSITIVE PHOTOTAXIS. Advantage: Allows photosynthesis to make organic compounds so increases survival chances.
- Earthworms move away from light: NEGATIVE PHOTOTAXIS. Advantage: Better able to conserve water (avoid dehydration), find food and avoid predators.
- Male insects of some species fly towards chemicals secreted by mature females of the same species: POSITIVE CHEMOTAXIS. Advantage: Allow them to breed/mate and find a mate of the same species.
- Some bacteria move towards a high concentration of glucose: POSITIVE CHEMOTAXIS. Advantage: Glucose is a soluble food source and used in respiration.
Hydrotaxis = water
Thermotaxis = temperature
Aerotaxis = oxygen
Geotaxis = gravity
A kinesis is a non directional response to a stimulus. The rate of movement or rate of change of direction is related to the intensity of the stimulus. A kinesis results in an increase in random movement.
E.g. woodlice move faster and change direction less often in drier conditions.
Explain how this increases their chance of survival:
- Non-directional response to humidity = hygrokinesis
- In dry air - woodlice move faster and turn less often to increase their chance of moving AWAY from drier air. This will increase survival chances by reducing water loss.
- Also humidity higher is concealed places so safe from predators.
- As air becomes more humid they more slower and turn more often so they're more likely to stay in that area.
Way to remember:
kinetic, energy, movement
Plants are usually held firmly in position by their roots and are, therefore, unable to move from one place to another. They rely on growth to respond to different directional stimuli to maintain their roots and shoots in a favourable environment.
A tropism is a plant's growth response to an external, directional stimulus. The response takes the form of bending/turning in a certain direction.
If the growth is towards the stimulus = positive response/tropism
If the growth is away from the stimulus = negative response/tropism
Plan shoots grow towards light: positive phototropism. survival value: leaves absorb light for photosynthesis
Plant roots grow away from the light: negative phototropism. survival value: increases probability that roots grow into soil - for anchorage and increased chances of absorbing water and ions
Young roots of seedlings grow towards gravity: positive geotropism.
Young stems of seedlings grow away from gravity: negative geotropism. survival value: grow out of the soil - leaves exposed to the Sun
Plant roots grow towards water: positive hydrotropism. survival value: root systems grow and spread in soil where there's more water
geotropism = response to gravity. phototropism = response to light
hydrotropism = response to water
- Charles Darwin was 1 of the first scientists to investigate the response of plant shoots to light
- he carried out experiments with oat seedlings
- when an oat seed germinates it produces a cylindrical 'coleoptile' enclosing the primary shoot of the young oat plant
- this is useful to investigate growth as, in normal conditions, it grows upright
Conclusions made from Darwins results:
- Stimulus is detected by the TIP of the shoot
- The response (bending towards the light) is in the region just BELOW THE TIP - this is known as the GROWING REGION
Plant growth regulators
Further investigations have shown that plants produce 'chemical messengers' to allow them to respond to external stimuli. These are called plant growth regulators rather than hormones because:
- They affect GROWTH of the whole plant or part of a plant
- They are not secreted by glands and do not travel in a transport system
- They can sometimes affect the tissues that release them
They are produced in the GROWING REGIONS of the plant, and they can either STIMULATE or INHIBIT growth.
Auxins were the 1st class of plant growth regulators to be discovered.
One of the main auxins is IAA (Indole acetic acid), which controls directional growth seen in tropisms in flowering plants in the following way:
- IAA is produced in the tip of the shoot (or root)
- It is moved to other parts of the plant by diffusion (or sometimes active transport) over short distances, and via the phloem over long distances
- Tropisms in the shoot and root are due to an UNEVEN DISTRIBUTION of IAA, causing uneven growth of different parts of the plant
- Growing region: just below the tip. Auxin diffuses from tip to here
- Phloem - transports sugars
- More IAA/auxin is concentrate/redistributed to the shaded side of plant and it bends towards light
- Auxin causes cells to get longer and shaded side so it grows towards light. Auxin stimulates cell elongation
Role of IAA in controlling phototropisms
a) in shoots. (Positive phototropic response) (cell elongation takes place-due to auxin:shoot grows towards light)
Positive phototropism is observed when a young shoot bends towards light directed at it from one side (unilateral exposure). This growth response is due to the following sequence of events:
- IAA is produced by cells in the tip of the shoot and is then transported down to the growing region (just below tip), where growth is stimulated by causing more cell elongation to occur
- The IAA is transported evenly when it begins to move down the shoot (as it would in uniform light or darkness)
- Light from one side causes the movement of IAA from the light side to the shaded side of the shoot
- The shaded side grows at a fasrer rate than the illuminated side, causing the shoot to bend and grow towards the light
b) in roots. (negative phototropic response)
- When unilateral light is shone on the root, the auxin moves away from the light source, as in the shoot, and accumulates on the shaded side
- However, in the root, this high auxin conc. on the shaded side inhibits growth (the root cells respond differently)
- Therefore, the shaded side grows at a slower rate, causing the root to bend away from the light: a negative phototropic response
Shoot: IAA causes shaded side to grow faster
Root: IAA causes shaded side to grow slower
Role of IAA in controlling geotropisms
A geotropism is the growth of plant roots and shoots in response to gravity. If a seedling is placed horizontally and left to grow, the root grows DROWNWARDS and the shoot grows UPWARDS. This is thought to be due to the pull of gravity causing a redistribution of auxin in both the root and the shoot - IAA moves DOWN to the lower side. However, the effects are different in the shoot compared to the root:
Auxin inhibits growht in root (so grows down)
Auxin stimulates growth in root (so grows up)
Auxin distributed to pull of gravity
In response to gravity:
Roots are said to be POSITIVELY geotropic (towards pull of gravity)
Shoots are said to be NEGATIVELY geotropic (against pull of gravity)
Suggesting explanation for the different gravitational response in the root and the shoot:
- Shoot - auxin STIMULATES growth - cells lower side grow faster (more cell elongation) so shoot curves upwards
- Root - auxin INHIBITS growth - cells on lower side grow more slowly (less cell elongation) so root curves downwards
Nervous control in mammals
Organisation of the nervous system
Structurally, the nervous system may be diviced into 2 sections: -
CENTRAL NERVOUS SYSTEM - BRAIN AND SPINAL CORD
PERIPHERAL NERVOUS SYSTEM - SENSORY AND MOTOR NERVES
Functionally, the nervous system can be divded into 2 different systems:-
SOMATIC NERVOUS SYSTEM - Carrying out mainly VOLUNTARY functions (conscious control)
AUTONOMIC NERVOUS SYSTEM - Carrying out INVOLUNTARY functions (Subconscious control) - e.g. glands, breathing, heart rate etc.
Autonomic also has the :
sympathetic (releases adrenaline) (fight or flight) increases heart rate, breathing etc)
parasympathetic (releases noradrenaline) (relaxed state) - decreases heart rate, digestion back to normal etc)
Each system works in the same way, with input from stimuli being detected by receptors and responses coordinated and then carried out by effectors:
STIMULUS (light) -> RECEPTOR (photoreceptor in retina/rods&cones) -> CO-ORDINATOR (CNS-brain) -> EFFECTOR (Iris muscle) -> RESPONSE (Iris muscle contracts-pupil constricts)
Nerve cells (neurones) pass electrical impulses along their length and stimulate their target cells by secreting chemical NEUROTRANSMITTERS directly onto them. This results in RAPID, SHORT-LIVED and LOCALISED REPONSES.
Spinal reflexes (Don't go to the brain)
The simplest form of nervous response to a stimulus is a reflex action
A reflex action is a rapid, short-lived response to a specific stimulus
A SIMPLE REFLEX is innate and always results in the SAME reponse to a particular stimulus
Examples: knee jerk, swallowing, blinking, pain withdrawal, flinching, pupil dilation etc.
Simple reflexes are important because:
- They are INVOLUNTARY actions that do not involve decision making areas of the brain, leaving them free to carry out more complex responses
- They are very RAPID, as they involve fast electrical impulses and very short neurone pathways
- They are usually concerned internally with the CONTROL of continuous and repetitve actions, such as breathing, control of heart rate etc, ESSENTIAL FOR SURVIVAL
- They are effective from birth and DO NOT HAVE TO BE LEARNED. They protect the body from harmful stimuli, thus avoiding damage and increasing survival chances.
The Reflex Arc
The shortest PATHWAY taken by the impuluses in a simple reflex is known as the REFLEX ARC (the simplest type is a monosynaptic reflex as it involves only a sensory and a motor neurone and hence one synapse) (eg knee jerk)
sensory neurone: carries impulse from receptors to CNS (brain/spinal cord)
motor neurone: carries impulse from CNS to an effector (muscle/gland)
Reflexes involve either the spinal cord or the brainstem. Most spinal reflexes involve 3 neurones, with 2 synapses. The sensory and motor neurones are linked by a relay neurone (or interneurone) within a region of the spinal cord called the grey matter. (The spinal cord is a column of nervous tissue that lies inside the vertebral column for protection). (look at pg 10 for diagram)
The withdrawal reflex is an example of this type of reflex, where the hand or foot is drawn away from a stimulus that could cause damage.
E.g. hot plate being picked up, temp and pain receptors in your fingers would detect this stimulus and generate impulses in the sensory neurone.
A SENSORY neurone enters the spinal cord via the DORSAL root which has a swelling, called the DORSAL ROOT GANGLION, contained the cell bodies of these neurones. The neurone synapses with a RELAY NEURONE which, in turn, synpases with a MOTOR NEURONE. All synapses occur in the grey matter.
The MOTOR NEURONE carried impulses to the EFFECTOR (e.g. muscle cells in the arm and hand) which then contract, causing you to drop the plate. In practice, many neurones (forming separate reflex arcs) would be involved in this process, but each has an involuntary function to protect your body from harm.
Sensory information concerning this reflex action will be carried up to the brain, making you aware of what has happened, and you may then modify your response.
THE AUTONOMIC NERVOUS SYSTEM - part of motor neurone
- This regulates internal glands and muscles, which are normally beyond our conscious control, although some control may be learned over time (e.g. bladder and anal sphincter muscles)
- Many reflex actions are thus part of the ANS
- ANS is made up of 2 antagonistic systems, the SYMPATHETIC and PARASYMPATHETIC divisions
- Neurotransmitter: Noradrenaline
- General effect: Excitatory
- Dominant during: Stress, danger, excitement, fight or flight. Heart rate INCREASES.
- Neurotransmitter: Acetylcholine
- General effect: Inhibitory
- Dominant during: Rest. Heart rate DECREASES.
Adrenaline = hormone. Noradrenaline = neurotransmitter.
Explain why they are described as antagonistic:
Actions normally oppose each othe - sympathetic prepares for activity; parasympathetic slows down and conserves energy. They work together.
Control of heart rate
Sinoatrial node (SAN) on right atrium in the co-ordination of the heart beat. The SAN is the natural pacemaker of the heart. It does NOT require nervous stimulation to cause it to conduct impulses across the atria and then to the ventricles, causing them to contract. This results in the rhythmic contractions of the CARDIAC CYCLE.
Average heart rate of healthy adult during rest: 60-80 bpm.
Atrial systole -> Ventricular systole -> Diastole
Although the heart muscle can contract without nervous stimulation, impulses from the ANS are required to change the RATE of the heart beat.
Advantage to an athlete of an increase in heart rate during exercise: Increased cardiac output, more blood flow to muscles, more oxygen and glucose to muscles, leading to increased rates of respiration and more ATP for muscle contraction.
Changes to the heart rate are controlled by the CARDIAC CENTRE in a region of the brain called the MEDULLA OBLONGATA. The cardiac centre sends impulses to the SAN via 2 different nerves of the ANS (which originate from different parts of the cardiac centre)
- A SYMPATHETIC NERVE is responsible for INCREASING the heart rate (acts as an accelerator)
- A PARASYMPATHETIC NERVE is reponsible for DECREASING the heart rate
MORE FREQUENT IMPULSES in the SYMPATHETIC nerve and/or fewer impulses in the parasympathetic nerve will result in a FASTER heart rate.
Conversely, MORE FREQUENT IMPULSES in the PARASYMPATHETIC nerve and/or fewer impulses in the sympathetic nerve will result in a SLOWER heart rate.
The cardiac centre receives information from two main sources:
- CHEMORECEPTORS (detect changes in H+ ion conc/pH) in the walls of the CAROTID ARTERIES (that supply blood to the brain) and the AORTA (leaving the left ventricle of the heart). The receptors are clustered together to form structures called the CAROTID and AORTIC BODIES. These chemoreceptors are sensitive to changes in BLOOD pH resulting from changes in CARBON DIOXIDE CONC.
- PRESSURE RECEPTORS (baroreceptors), also found in the carotid and aortic bodies, which are sensitive to changes in BLOOD PRESSURE.
As CO2 increases:
- Chemoreceptors detect change -> Increased action potential frequency in sensory neurone -> Medulla oblongata -> Sympathetic nerve -> Increase heart rate
As CO2 decreases:
- Chemoreceptors detect change -> Decreased action potential frequency in sensory neurone -> Medulla oblongata -> Vagus nerve -> Decrease heart rate
Control by chemoreceptors:
- chemoreceptors are sensitive to changes in pH caused by changes in the CO2 conc of the blood. CO2 dissolves in water to form a weak acid (carbonic acid). This can then dissociate to form hydrogen ions.
- More CO2 = higher pH
- Blood CO2 level increases, causing the H+ ion conc to increase and the pH to DECREASE.
- This is detected by chemoreceptors which send more impulses in sensory neurones to the cardiac centre in the medulla.
- This responses by increasing the frequency of impulses via the sympathetic nerve to the SAN which increases the heart rate.
- The increased blood flow leads to more CO2 being removed by the lungs. The blood CO2 level decreases, and pH increases back to normal levels.
- Chemoreceptors reduce their input to medulla which, in turn, reduces frequency of impulses via sympathetic nerve to SAN and heart rate returns to normal.
control by baroreceptors (bp pressure receptors)
Effect of blood pressure on heart rate:
- Fall in blood pressure in aorta detected by baroreceptors
- Impulses carried in sensory neurones to CARDIAC CENTRE (Cardioacceleratory centre) in medulla
- MORE FREQUENT IMPULSES along SYMPATHETIC/accelerator nerve to SAN
- NORADRENALINE released from sympathetic nerve endings onto SAN
- SAN increases its RATE of activity causing an INCREASE IN HEART RATE and BLOOD PRESSURE
- INCREASE in blood pressure in aorta detected by baroreceptors
- Impulses carried in sensory neurones to CARDIAC CENTRE (cardioinhibitory centre) in medulla
- MORE FREQUENT IMPULSES along PARASYMPATHETIC/vagus nerve to SAN
- ACETYLCHOLINE released from parasympathetic nerve endings onto SAN
- SAN decreases its RATE of activity causing a DECREASE IN HEART RATE and BLOOD PRESSURE
Components of control system involved when there's an INCREASE in BP:
- Stimulus: HIGH BP
- Receptor: BARORECEPTOR in aorta and carotid arteries
- Co-ordinator: Cardiovascular centre in medulla
- Effector: SAN
- Response: DECREASE in heart rate and stroke volume, leading to a decrease in blood pressure