Stimuli and Response Flashcards
What is kinesis?
Kinesis is the random non-directional movement of a whole organism in response to a stimulus. If the organism is in unfavourable conditions, it moves fast and turns rarely (to move away from the area). If the organism is in favourable conditions, it moves slowly and turns a lot (to stay in the same place).
What is taxis?
Taxis is a directional response where a whole organism moves either towards (positive taxis) or away from (negative taxis) a stimulus.
What is a reflex?
An involuntary response which follows a specific pattern in response to a stimulus.
What is a tropism?
A directional response that involves an organism either growing towards (positive) or away from (negative) a stimulus, such as light or gravity (in plants)
What are auxins?
Auxins are a group of hormones in plants that control many processes, such as growth. They are sometimes refereed to as growth factors because they directly affect growth and are made my cells all over the plant, rather than only being made in particular organs. Unlike animal hormones, some growth factors can directly affect the tissues that released them, as opposed to a target organ.
What is a phototropism?
A response in which parts of a plant grow towards or away the direction in which light is coming.
What is geotropism/gravitropism?
A response in which parts of a plant grow towards/away from gravity.
What is indoleacetic acid (IAA)?
IAA is an auxin produced in shoot tips which causes cell elongation is the shoot tips. IAA inhibits cell elongation in the roots. Gravity causes IAA to diffuse down from the shoot tips to other parts of the plant.
How does phototropism occur in shoots?
IAA accumulates on the shady side of the plant, causing cell elongation. This causes the shoot tip to bend towards the sunlight (as the shady side is growing faster than the brighter side). This is positive phototropism.
How does phototropism occur in roots?
IAA accumulates on the shady side of the roots, causing the inhibition of cell elongation. This means that the shaded side of the plant grows slower than the brighter side, so the root grows away from the light. This is negative phototropism.
How does gravitropism occur in shoots?
IAA accumulates on the lower side of the shoot, which causes cell elongation. This means that the shoot bends upwards (against gravity), as the lower side is growing faster than the upper side. This is negative geotropism.
How does gravitropism occur in roots?
Gravity pulls IAA to the lower side of the root, which inhibits cell elongation. This causes the root to bend downwards (with gravity). This is positive geotropism.
What is the acid growth hypothesis?
IAA increases plasticity of the plant cell wall by actively transporting H+ from the cytoplasm into spaces in the cell wall. This allows the cell to elongate by expansion. This is easier in younger cell walls, rigidity develops with maturity so roots and shoots become less responsive.
What are the benefits of positive phototropism in plants?
Leaves exposed to more sunlight and so carry out more photosynthesis. Flowers can be seen by insects for pollination. Plants get higher for better seed dispersal.
What are the benefits of positive gravitropism in plants?
By growing deeply into the soil, the root fixes the plant into the ground firmly. Roots are able to reach more water and roots have a larger surface area for more diffusion and osmosis.
Describe the method for the choice chamber practical (CP10).
Place a wet paper towel over half of a choice chamber and put a thin mesh on top. Place the lid on the choice chamber and put a set number (e.g. 20) of the organism you are studying (e.g. woodlice, maggots) into the chamber. Place a dark paper covering on half of the choice chamber, such that the four sections each have different conditions (dark and damp, dark and dry, light and damp and light and dry). Observe the movements of the organisms and after 5 minutes, count how many individuals are in each section. Repeat the experiment to obtain a total of 3 trials.
What are the expected results for the choice chamber practical?
You would expect the organisms to show a tactic or kinetic response to the stimulus and move towards the dark and damp conditions, as these are the most favourable (to avoid being spotted by predators and to avoid drying out). You would therefore expect to find more individuals in the darker and darker sections of the choice chamber.
Why might you not obtain the results you expect in the choice chamber practical?
The organisms are likely to be distressed by the situation, so may not act as they naturally would. If you don’t wash the mesh between trials, they may do the same thing every time, as they leave scent trails which they are more likely to follow than to go to other parts of the mesh. There may also be not enough organisms (to small a sample size) or not enough trials.
What is the central nervous system?
The CNS consists of the brain and spinal cord and contains relay neurones which connect sensory neurones to relevant effectors.
What is the peripheral nervous system?
The peripheral nervous system contains all the other nerves in the body including sensory neurones and motor neurones. The PNS contains the sensory nervous system and motor nervous system (which contains voluntary and autonomic parts (the autonomic part contains the sympathetic and parasympathetic nervous system)).
What are dendrites?
Dendrites are fine extensions of the dendrons, which are larger extensions of the cytoplasm of the neurone. They carry electrical impulses towards the cell body from neighbouring neurones.
What is the axon?
Long thread-like extension of the cell membrane and cytoplasm. Transmits the nerve impulse from the cell body down to an effector.
What are the myelin sheath?
A sheath of lipid wrapped around the axon, with gaps (Nodes of Ranvier). Insulates the neurone and allows saltatory condition (this speeds up electrical transmission).
What are branched axon terminals?
Branched extensions of the axon at the end of the neurone. This carries an impulse to an effector such as a gland or muscle, or to other neurones at synapses.
What are Schwann cells?
Schwann cells surround the axon and protect it, provide electrical insulation, carry out phagocytosis and play a role in nerve cell regeneration. The membranes of Schwann cells from the myelin sheath, which is rich in lipid.
What is a neurone?
Specialised nerve cells that are adapted to carrying rapid electrochemical changes called nerve impulses from one part of the body to another.
What are sensory (afferent) neurones?
Transmit electrical impulses from receptors to the CNS. The cell body is located the the middle of the neurone on a side branch while there is a long dendron and a shorter axon than usual.
What are relay (association) neurones?
Found within the CNS, transmits electrical impulses between sensory neurones and motor neurones. Relay neurones are very short compared to other neurones and they don’t have long dendrons or axons.
What is a motor (efferent) neurone?
Transmits electrical impulses from the CNS to effectors (usually muscles or glands). Has dendrites and a long axon, but very short dendrons leading to the cell body.
What are some important features of reflexes that help survival?
Reflexes are involuntary actions which are automatic and faster than voluntary actions as they require no processing. They are sent to the spinal cord or unconscious areas of the brain, involving short neuronal pathways. This reduces damage to tissues (e.g. from burning), allow organisms to escape from predators and form a part of homeostasis.
What is the reflex arc?
The path information takes from receptor to effector in a reflex response.
Describe the reflex arc.
There is a detectable change in the surroundings which acts as a stimulus. Recepotirs detect this and an electrical impulse is sent from the receptor along a sensory neurone to a relay neurone in the CNS via diffusion of neurotransmitter across a synapse. The electrical impulse is transmitted to the relevant motor neurone, which transmits the impulse to an effect or. This effector brings about a response (e.g. contracting a muscle to move your hand away from a hot object).
What are the differences between the endocrine and nervous systems?
E- communication by hormones that travel in the blood plasma
N - communication by electrical impulses down neurones
E - the response is slow
N- the response happens instantly
E - response is long-lasting
N - response n is short-lasting
E - hormones can have a widespread effect on many cells or organs
N - the impulse acts on one or a few calls only
E- effect may be permanent
N - effect is not permanent
What is a nerve impulse?
The change in electrical charge that moves along a neurone in response to a stimulus.
What is the resting potential?
The electrical potential across the plasma membrane of a cell that is not conducting an impulse. (-70 mV)
What is the sodium-potassium pump?
An intrinsic protein found in neurone cell membranes which uses ATP to actively transport 2 K+ into the axon and 3 Na+ out of the axon. This helps to contribute to the resting potential and is used to maintain resting potential faster depolarisation.
How is the resting potential generated in a neurone?
Ions are charged, so cannot diffuse directly across the phospholipid bilayer. There are sodium-gated channels and potassium-gated channels which allow sodium ions to leak into the axon and K+ to leak out of the axon down their concentration gradients (even when the channels are closed). The bilayer is 100x more permeable to K+ than Na+, giving an overall negative charge inside the axon (resting potential). The Na+/K+ pump also contributes to the resting potential.
Describe the formation of an action potential.
At resting potential (-70mV) Na+ voltage-gated channels are closed. A stimulus causes the membrane at one part of the neurone to increase permeability to Na+ (sodium channels open and Na+ enters the axon down their electrochemical gradient by diffusion. The membrane is depolarised and when the potential reaches -30mV (threshold), more Na+ channels open and the inside of the cell becomes more positively charged (positive feedback). At around +40mV, the Na+ channels close and the K+ channels open. K+ rush out down their electrochemical gradient, making the cell more negative (repolarisation). There is an undershoot meaning the axon becomes more negative than usual (hyperpolarisation). The K+ channels close and this triggers a wave of depolarisation in the next part of the neurone. The Na+/K+ pump restores resting potential.
How is the action potential propagated along the axon?
Na+ ions enter the axon by diffusion when the Na+ channels open after a stimulus. The Na+ are attracted sideways to the negative parts of the axon (at resting potential). This sets up a localised current, which causes the opening of adjacent voltage-gated Na+ channels further along the axon. As these regions depolarise, the regions before then repolarise as K+ leaves the axon through potassium-gated channels.
Why do myelinated neurones conduct impulses faster than unmyelinated neurones?
There are gaps in the myelin sheath called Nodes of Ranvier. Action potentials only occur at the nodes, so the nerve impulse jumps from node to node (saltatory conduction). Therefore, the action potential does not need to depolarise the whole neurone (only the Nodes of Ranvier), this speeds up conduction.