AQA A-level Biology Responding to Changes A* Notes

Responding To

Chges in the
Environment

, Survival d Response
STIMULUS: a detectable change in the internal or external environment of an organism

• causes the organism to respond
• The internal environment consists of the surroundings of the cells in a multicellular organism

RESPONSE: what occurs as a result of the stimulus
NATURAL SELECTION:
• organisms with alleles, enabling them to respond to stimuli, have an increased chance of survival
• More likely to reproduce
• Passing on those alleles to their offspring
• Increasing the frequency of those alleles in the population

• Animals have hormonal communication + NS (more rapid)
• Usually have many different receptors + control effectors
• Each receptor and effector linked to a central coordinator
• Coordinates connect the info from the receptor w/ Appropriate effector
• sequence of events can involve either chemical control or nerve cells
Summarised as:
Stimulus -> Receptor -> Coordinator -> Effector -> Response

STIMULUS: change in the organisms, external internal environment that brings about a response
Change in light , temp , pressure, pain, sound etc


RECEPTOR: a sense cell, which is stimulated by a specific stimulus, ultimately resulting in the
production of a nervous impulse
Rod cells in retina , chemoreceptors in tongue + nose, pressure receptors in the skin

COORDINATOR: formulate a suitable response to a stimulus by connecting information from
receptor with the appropriate effector
Brain , spinal cord
EFFECTOR: a structure that produces a response
Usually muscle or gland

RESPONSE: muscle contracts or gland, secretes a substance
Pancreas secrete insulin if blood glucose levels rise

, 🌍 💧
Tropisms

Movement of a part of a plant, almost always by growth in response to a directional external
stimulus

• ensure that the shoots and roots of a plant are maintained in a suitable environment

• coordinated by means of plant growth factors
• Comparable in effect to animal hormones

* AUXIN

• either negative (away from stimulus)
• Positive (towards stimulus)


1. PHOTOTROPISMS -> responses to light

2. GEOTROPISMS -> responses to direction of gravity

3. HYDROTROPISMS -> responses to water

, Growth factors in Flowering Plts
• plants do not have nervous system
• Use plant growth factors to stimulate growth:
• Chemicals which have effect on tissues

• produced in cells thought the plant
• (not like animals- localised glands)
• have an effect on tissues that produced them + other nearby tissues

Auxin
• auxins chemo which can induce cell elongation in stems
• IAA = type of auxin (plant growth hormone)

• usually affect other processes asw as cell elongation of stem cells

Rts

• high conc of IAA decreases growth in roots
• Explains why roots show neg phototropism


Light
• more IAA in roots
• As darker under soil
• Causes less elongation of
cells
SHOOT




ROOTS
& •
•
more IAA on shaded side
Less elongation
• cells longer • Shorter cells
• More elongation • Growth inhibited