6. Organisms Response

increases chance of survival

stimulus, receptor, coordinator, effector, response

simple, directional responses determined by the direction of the stimulus. Used by simple mobile organisms who move towards or away from a stimulus.

move towards a favourable stimulus

move away from an unfavourable stimulus

light

non-directional simple response used by simple mobile organisms who change the speed of movement or the rate of direction change, in response to a non-directional stimulus.

If it is surrounded by negative stimuli, the rate of turning decreases to keep it moving in a relatively straight line to increase the chances of it finding a new location with favourable conditions.

humidity and temperature

growth of part of a plant in response to a directional stimulus.

growth towards stimulus

growth away from stimulus

grow towards light, away from gravity

grow away from light, towards gravity

• Controls cell elongation in shoots • Inhibits the growth of cells in the roots.

• Cells in the tip of the shoot produce IAA, which is then transported by diffusion down the shoot. • The IAA is initially transported evenly throughout all regions as it begins to move down the shoot. • Light causes the movement of lAA from the light side to the shaded side of the shoot where it accumulates. • A greater concentration of IAA builds up on the shaded side of the shoot than on the light side. • As IAA causes elongation of shoot cells, the cells on the shaded side elongate more. • The shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light.

• Cells in the tip of the root produce IAA, which is then transported by diffusion along the root. • The IAA is initially transported to all sides of the root. • Gravity influences the movement of IAA from the upper side to the lower side of the root. • A greater concentration of IAA builds up on the lower side of the root than on the upper side. In root cells, IAA inhibits the elongation so the cells on the lower side elongate less than those on the upper side. • The relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity.

• one direction • increases plasticity of cell walls • young cell walls • acid growth hypothesis (a.t of hydrogen ions from cytoplasm into spaces in the cell wall causing the cell wall to become more plastic)

the potential difference across the membrane due to a difference in charge between the inside and outside of a cell when it is at rest (inside is negatively charged relative to the outside).

change in the potential difference when a stimulus is detected as the cell membrane is excited and becomes more permeable, allowing more ions to move in and out of the cell

the level the generator potential reaches to trigger an action potential.

electrical impulse along a neurone triggered if the generator potential is big enough and reaches the threshold level. Action potentials are all one size, so the strength of the stimulus is measured by the frequency of the action potentials.