Section 6 ( Organisms Response to Changes within Environments ), Topics 14 and 15
A stimulus is a change detected in either the internal or external environment or organisms,
and requires an appropriate response. Being able to respond to stimuli is a vital
characteristic of life, increasing chances of survival and reproduction and the ability to pass
their genes onto their offspring. Hence, the selection pressure constantly favours organisms
with genes allowing more effective responses.
Stimuli are detected by highly specific receptor cells, leading to coordinators formulating
responses. The coordination itself may be involving in small molecules, or larger scale
responses involving organ systems. The response itself is carried out by effectors, which can
be muscles or glands. Two means of communication within large multicellular organisms
are:
Hormones ( Endocrine System )
o Slow process found in both plants and animals.
Neurones ( Nervous System )
o Fast process found in animals.
The more simple forms of response are Taxes, Kinesis and Tropisms.
Taxis
A taxis is a simple response, in which the response is determined by the direction of the
stimulus. Hence, organisms use taxis responses by changing their whole body direction
either towards a favourable change or away from an unfavourable change.
Examples of this are:
Algae moving towards light ( positive phototaxis )
Earthworms moving away from light ( negative phototaxis )
Bacteria moving towards glucose ( positive chemotaxis )
Kinesis
A Kinesis is a response, in which the response is the constant frequent change in the speed
and direction of the organism. Hence, organisms use kinesis when the location of the
favourable change is unknown, and increasing the speed at which it moves and changing
direction increases the chances of returning back to the favourable environment.
An example is woodlice. When in dry conditions, woodlice use kinesis responses by
increasing their speed at which they move and change direction. This increases their chance
of returning back to the damp environment. Once returned, they will decrease their kinesis
response in order to remain in the damp conditions.
, Tropisms
A Tropism is the growth of a part of a plant in response to a directional stimulus. Different
organs of a plant either grow towards or away from the stimulus.
Plant shoots grow towards light ( positive phototropism ) and away from gravity (
negative gravitropism )
o Leaves are in the most favourable position to capture light for
photosynthesis.
Plant roots grow away from light ( negative phototropism ) and towards gravity (
positive gravitropism )
o Roots have a higher chance of entering soil to absorb more water and
mineral ions.
Plant Growth Factors
Plants use growth factors in order to respond to their environments. Growth factors can be
classed as the plant equivalent of hormones in animals. Due to the lack of a nervous system
and muscle ( but with glands ), plants respond to their environment via:
Light
o Photosynthesis
Gravity
o Stability in the soil
o Water and mineral ions
Water
o Physical support
o Photosynthesis and other metabolic processes
Growth factors have the ability to:
Bring about changes by affecting growth.
Be produced by cells throughout the plant, not just one region.
Bring about changes to the tissues that produced them ( unlike hormones ).
Plant growth factors are usually produced in small amounts, in which Indoleacetic Acid
( IAA ) is an example of. It belongs to a group of substances called Auxins. IAA controls plant
cell elongation, and is broken down by sunlight. IAA in the shoot causes cell elongation, and
IAA in the root inhibits cell elongation.
Tropisms by Indoleacetic Acid
, Shoots' response to IAA ( assuming one sided light ):
Cells located in the tip of the shoot produce IAA, which it flows down the shoot.
IAA is evenly distributed around shoot, but is broken down by sunlight.
Due to uneven light distribution, light breaks down IAA unevenly. A greater
concentration of IAA is on the shaded side.
Cells on the shaded side elongate at a faster rate.
Shoot tip bends towards light.
Roots' response to IAA ( assuming horizontal ):
Cells located in the tip of the shoot produce IAA, which it flows down the shoot.
IAA flows towards the roots, and is under the influence of gravity.
Due to the root growing horizontally, IAA builds up on the lower side of the root.
Greater concentration of IAA on the lower side, inhibiting cell elongation.
Upper side grows at a faster rate, so root tip bends towards gravity.
Acid Growth Hypothesis
Alongside cell elongation, IAA also has effects on plant cells by increasing their plasticity
( ability to stretch ) of cell walls. This is mainly seen among young plant cells. As cells
mature, plasticity decreases and a greater rigidity property is developed, so they cannot
respond. This is called the acid growth hypothesis. In simple terms, hydrogen ions are
transported from the cytoplasm into the cell wall, increasing the cell wall's flexibility.
Reflex Arc
A reflex arc is a type of nervous response to a stimulus. The following is the composition of
the Nervous System in a mammalian body.
Nervous Organisation
The Nervous System has two major divisions:
Central Nervous System ( CNS )
o Brain and Spinal Chord
Peripheral Nervous System ( PNS )
o Pairs of nerves that originate from the CNS.
The Peripheral Nervous System is in itself divided into two:
Sensory Neurones
o Carry nerve impulses from receptor cells to the CNS.
