This document discusses stimulus-response mechanisms in organisms. It defines key terms like stimulus, response, and coordination. It describes common external and internal stimuli and how organisms coordinate responses through receptors and effectors. Locomotory responses to stimuli are called taxes, including phototaxis, chemotaxis, aerotaxis, etc. Growth responses in plants are called tropisms, like phototropism, geotropism, and hydrotropism. Tropisms are influenced by plant hormones like auxins. The document compares taxes and tropisms, and also discusses nastic responses in plants mediated by changes in turgor pressure.

1. RECEPTION, RESPONSE
AND CO-ORDINATION
Mr. Peter Kanyoro

2. Introduction
• Stimulus – A condition arising from the environment (internal or external) which produces a
change in activity of a part of or the whole organism.
• Response – It is the actual change in activity by the organism due to stimulus/stimuli.
• Irritability/sensitivity/responsiveness – The ability of an organism to detect and respond to
changes occurring in the environment.
• For reception and response to occur, there are; receptors (parts that perceive changes) and
effectors (parts that bring about response to changes).

3. Common stimuli
• External; temperature changes, light, pH, humidity, mineral salt concentration, population
density and presence/absence of predators.
• Internal; waste accumulation, gas concentration, presence/absence of internal parasites,
temperature changes and glucose concentration in the blood.

4. Co-ordination
• It is the mechanism by which systems of organisms are integrated to bring
about the appropriate response to stimuli.
Or;
• Working together of parts of body of organisms in an orderly and organized
manner to bring about the correct response to stimuli.
• E.g. On touching a hot object the skin - receptor and hotness (high temp) –
stimulus, a person withdraws the hand – response by the help of muscles –
effector.

5. Responses
• Grouped according to causative stimulus.
• Involve movement of whole or part of organism.
• Are of two types;
i. Positive – movement is towards the stimulus.
ii. Negative – movement is away from the stimulus.
• Major response are;
a. Taxes
b. Tropisms

6. Taxes (Sing; taxis)
• Locomotory responses of whole organism or the motile cell e.g. a gamete to
a unidirectional stimulus.
• Unidirectional stimulus – a stimulus that gets to the organism from one
direction.
• Types of taxes include;

7. Phototaxis
• Response to variation in light intensity.
• Movement towards light is positive phototaxis e.g. in euglena and fruit flies
• Movement away from light is negative phototaxis e.g. in woodlice, maggots
and termites.

8. Chemotaxis
• Response to variation in chemical substances.
• Positive chemotaxis is seen in male gametes of bryophytes and pteridophytes
towards ova.
• Negative chemotaxis is seen in mosquitoes moving away from insect
chemical repallants.

9. Aerotaxis
• Response to variation in oxygen concentration.
• Positive aerotaxis – an amoeba (aerobic) moves from an area of low oxygen
concentration to an area of high oxygen concentration.

10. Osmotaxis
• Response to variation in osmotic pressure.
• Negative osmotaxis is shown in marine crabs burrowing into sand to avoid
dilution of their body fluids.

11. Rheotaxis
• Response to variation in direction of water or air currents.
• Fish and planarian worms show negative rheotaxis by moving against water
currents. Also butterflies and moths fly against direction of wind.
• Some birds e.g. crows may show positive rheotaxis by moving towards the
direction of wind for easy flight

12. Thermotaxis
• Locomotory response to temperature changes.
• E.g. paramecium moves from a place with low temperature to that of
moderate temperature.

13. Importance of Tactic Responses
• Enable organisms to escape from harmful stimuli e.g. excessive heat.
• Enable the organisms to seek favorable/appropriate habitats and acquire
resources e.g. nutrients.
• Enable important life processes such as fertilization to occur e.g. chemotaxis.

14. In Plants
• Unlike animals, plants lack organized nervous and sensory systems.
• Sensitivity is by responses in certain parts.
• Responses are in form of growth movements aka tropisms.

15. Tropisms
• Growth movements in response to unidirectional external stimuli.
• Are expressed by growth curvatures of plant parts.
• If the growth curvature is towards the stimulus, the response is positive.
• The response is negative when curvature is away from the stimulus.
• Are classified according to the causative stimulus.
• Are slow as growth rate is controlled by plant hormones (auxins)

16. Types of Tropisms
Phototropism
• Growth curvature in response to the direction and intensity of light.
• Shoots show positive phototropism while roots show negative
phototropism.

17. Chemotropism
• Growth curvature in response to gradient of chemical concentration.
• A developing pollen grain growing towards embryo is a positive
chemotropism.
• It is a response to chemicals secreted by the embryo sac.

18. Geotropism
• Growth curvature in response to gravity.
Give examples of positive and negative geotropism!
• Roots are positively geotropic whiles shoots are negatively geotropic.

19. Hydrotropism
• Growth curvature in response to water or moisture.
• Roots are positively hydrotropic.

20. Thigmotropism aka haptotropism
• Growth curvature in response to contact with a solid objects.
• It is shown by tendrils and climbing stems which twine around objects such
as branches or stems of other trees.
• Root tips grow avoiding solid objects such as rocks showing negative
thigmotropism.

21. Survival Value of Tropic Responses
• Phototropism exposes leaves in position for maximum light absorption and enhance
photosynthesis.
• Hydrotropism enables the plant roots to seek water.
• Thigmotropism enables herbaceous (non-woody) plants to obtain mechanical
support.
• Geotropism enables plant roots grow deep in soil thus offering firm anchorage.
• Chemotropism enables growth of pollen tube towards embryo sac thus enhance
fertilization.

22. Comparison of Tropic and Tactic responses
Similarities
• Both are adaptive responses that enable the organisms to survive better in
their environments.
• Both are due to similar external stimuli such as water and light.
• Both are directional i.e. they follow a certain direction when responding to
stimuli.
• Both are caused by unidirectional stimuli.

23. Differences between Tropic and Tactic Responses.
Tropisms
• Are growth responses thus more
permanent.
• Responses are slow.
• Are brought about by growth
hormones.
Taxes
• Are locomotory responses thus
temporary.
• Responses are fast.
• Are not influenced by growth
hormones.

24. Nastic Responses.
• Non-directional movements of plant parts in response to different stimuli.
• Caused by turgor pressure changes at petal and leaf bases.
• The bases have pressure sensitive swellings called pulvini.
• Pulvini cause the movements as they lose or gain turgidity.
• Examples; leaf-folding during hot weather, opening and closing of flowers in
response to light intensity and leaf-closing in response to touch e.g. in Mimosa pudica.
• Types of nastic movements are;

25. Nyctinasty (sleep movement)
• Movements in response to variation in light intensity and temperature changes of
day and night e.g. in sunflowers.
• Photonasty – response to variation in light intensity. Flower is open in presence of
light and closed in absence.
• Thermonasty – response to temperature changes. Some plants have their flowers
open when temperature increases and close when it decreases.

26. Haptonasty
• Response to touch e.g. in Mimosa pudica.
• In Mimosa pudica leaves close rapidly when they or stem is touched.
• An insectivorous plant called venus (Dionaea) fly trap also show haptonasty.
• It grows in nitrogen deficient soils.
• It traps and digest insects to replenish itself with nitrogen.
• When insect land on its sensitive hairs on leaves, the mid-rib cells lose water rapidly
thus turgor pressure.
• Trap springs inward and close leaf and spines interlock.

27. Chemonasty
• Movement in response to presence of certain chemical substances of
nitrogenous compounds such as urea and ammonium compounds.
• Sundew plants (Drosera) show chemonasty.
• When insect trapped by plant’s tentacles, it provides the substances that
stimulate release of digestive enzymes by plant.

28. Hydronasty
• Response due to changes in humidity.
• It is seen in flowers such as of Dandelion.
• They close when the air is moist.

29. Survival value for Nastism
• Helps in protection of inner parts of flowers.
• Helps to reduce the transpiration thus excessive loss of water.
• Helps in temperature regulation.
• Helps in obtaining some mineral nutrients that are limited in the soil.

30. Co-ordination in Plants.
• Growth responses in plants are coordinated by growth hormones;
• GHs; auxins, gibberellins, cytokinins and florigens.
• Tropisms are mostly influenced by auxins.

31. Role of Auxins in Tropisms.
• Indole-acetic acid (IAA) is the common auxin.
• Auxins are produced at apical meristems of shoots and roots.
• They move backward to the region of cell elongation where they exert their
effects.
• They move by diffusion from cell to cell in one direction.
• Shoots produce more auxins than roots thus more amounts are translocated
from them to roots.

32. Role of Auxins…
• IAA stimulate growth in both shoots and tips at cell elongation region.
• Required in very small amounts to cause the response.
• Roots are however more sensitive to IAA than shoots; amounts that cause
maximum shoot growth inhibits root growth.

33. Auxins and Phototropism
• When light is uniformly distributed, auxins produced at shoot apex are
evenly translocated down the shoot.
• There is equal growth rate in the zone of cell elongation.
• Leads to normal growth/increase in height of shoot.

34. Auxins and Phototropism…
• When light is unidirectional, auxins migrate laterally to the darker side.
• Auxins are more concentrated on darker side stimulating rapid cell
elongation on that side.
• Growth rate is faster on this side hence the shoot tip bends/curves towards
light source/direction of the light.
• This explains the positive phototropism in shoot.