Stimulus and response

• RESPONSE TO STIMULI


  14.1: Survival and response

  14.2: Plant growth factors

  14.3: A reflex arc

  14.4: Receptors

  14:5: Control of heart rate
• 14.1 SURVIVAL AND RESPONSE

  Learning objectives:

  > Define a stimulus and response.

  > Examine the advantage to organisms of being able to respond to stimuli.

  > Describe taxes, kinesis and tropisms.

  > Explain how each type pf response increases an organism's chances of survival.
• What is a stimulus?

  A change in the environment that is detected by an organism. It can either be internal in multicellular organisms and internal or external in any organism.
• What is a receptor?


  An organ or specialsed cell that is able to respond to light, heat, or other external stimulus and transmit a signal to a sensory nerve.
• What is a response?

  A reaction to a stimulus that is detected by the receptor. This may be movement of the organism or a change in behavior.
• What is an effector?


  Muscles or glands that bring about a response. A muscle causes a movement and a gland secretes a hormone into the bloodstream, which elicits a chemical response.
• What is the stimulus-response order?

  Stimulus > Receptor > Coordinator > Effector > Response
• What is meant by the term taxis?

  A response that involves movement in a specific direction.
• Name the types of taxes

  Photo - Light

  Geo - Gravity

  Hydro - Water

  Chemo - Chemical

  Thermo - Heat
• Give an example of positive taxis.

  Positive chemotaxis: mobile bacteria moving to an area where there is a higher concentration of glucose.
• Give an example of negative taxis.

  Negative phototaxis: Earthworms will move away from light. This increases their chances of survival because it takes them into the soil, where they are better able to conserve water, find water and avoid some predators.
• What is meant by the term kinesis?


  A form of response where the organism has more rapidly random movements. Both the speed and frequency of direction change increase.
• Give an example of kinesis.


  Woodlouse will move to a dry area and it will speed up and change direction more frequently in order to increase the chance it enters a damp region which are its favoured conditions.
• What is meant by the term tropism?

  The growth of part of a plant in response to a directional stimulus (i.e light or water). In most cases the plant grows towards (positive tropism) or away from (negative tropism) the stimulus.
• What tropisms do plant shoots show?

  Positive phototropism: plant shoot grows towards light.

  Negative gravitropism: plant shoot grows away from gravity.

  This is so that all their leaves are in the most favourable position to capture light for photosynthesis.
• What tropisms do plant roots show?

  Negative phototropism: plant roots grow away from light

  Positive gravitropism: plant roots grow towards gravity.

  This is to increase the probability that the roots will grow into the soil, where they are better able to absorb water and mineral ions.
• Summary questions for 14.1: Survival and response


  For the following statements, name the type of response described and the survival value of the response.
• Some species of bacteria move away from the waste products that they produce.

  Negative chemotaxis - wastes are often removed from an organism because they are harmful. Moving away prevents the waste harming the organism and so increases the chance of survival.
• The sperm cells of a moss plant are attracted towards a chemical produced by the female reproductive organ of another moss plant.


  Positive chemotaxis - increases the chances of sperm cells fertilising the egg cells of other mosses and so helps to produce more moss plants/future generations. Cross-fertilisation increases genetic variability, making species better able to adapt to future environmental changes.
• The young stems of seedlings grow away from gravity.

  Negative gravitropism - takes the seedlings above the ground and into the light, where they can photosynthesise. More photosynthesis means more carbohydrate and so a better chance of survival.
• 14.2 PLANT GROWTH FACTORS

  > Describe the stimuli that plants respond to.

  > Describe plant growth factors such as IAA.

  > Explain phototropism in flowering plants.

  > Explain gravitropism in flowering plants.
• What are the three factors that plants respond to in the environment?

  1. Light - shoots grow towards light because light is needed for photosynthesis.

  2. Gravity - plants need to be firmly anchored in the soil. Roots are sensitive to gravity and grow in the direction of its pull.

  3. Water - almost all plant roots grow towards water in order to absorb it for use in photosynthesis and other metabolic processes, as well as for support.
• What are plant growth factors?

  They exert their influence by affecting growth and they may be made by cells located throughout the plant rather than in a particular organs. Plant growth factors affect tissues that release them rather than acting on a distant target organ.
• Give an example of a plant growth factor.

  Indoleacetic acid (IAA).

  IAA controls cell elongation and belongs to a group of substances called auxins.

  Uneven distribution of IAA causes uneven growth of the plant.
• Recap: What is a tropism?

  A directional growth of a plant in response to a directional stimulus.
• Explain phototropism in flowering plants.

  The response of shoots of flowering plants to unilateral light is due to the following events:

  1. Cells in the tip of the shoot produce IAA, which is transported down the shoot.

  2. The IAA is initially transported evenly throughout all regions as it begins to move down the shoot.

  3. Light causes the movement of lAA [rom the light side ro the shaded side of the shoot.

  4. A greater concentration of IAA builds up on the shaded side of the shoot than on the light side.

  5. As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot. the cells on this side elongate more.

  6. The shaded side of the shoot elongates faster than the light side. causing the shoot tip to bend towards the light.

  
• Explain another factor that IAA controls.

  IAA also controls the bending of ROOTS in response to light. A high concentration of IAA increases cell elongation in shoots, it inhibits cell elongation in roots.
• Explain gravitropism in flowering plants

  Horizontally-growing root:

  1. Cells in the tip of the root produce IAA, which is then transported along the root.

  2. The IAA is initially transported to all sides of the root.

  3. Gravity influences the movement of IAA from the upper side to the lower side of the root.

  4. A greater concentration of IAA builds up on the lower side of the root than on the upper side.

  5. As IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side.

  6. 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.

  
• How do the shoots grow upwards from the force of gravity?

  In shoots, the greater concentration of IAA on the lower side increases cell elongation and causes chis side to elongate more than the upper side. As a result, the shoot grows upwards away from the force of gravity.
• What is the role of IAA in elongation growth?

  The transport of IAA is in one direction. namely away from the tip of shoots and roots where it is produced.

  As the cells mature they develop greater rigidity therefore older pans of the shoot/root will not be able to respond. The proposed explanation of how IAA increases the plasticity of cells is called the acid growth hypothesis. It involves the active transport of hydrogen ions from the cytoplasm into spaces in the cell wall causing the cell wall to become more plastic allowing the cell to elongate by expansion.
• Summary questions for 14.2: Plant growth factors
• Explain how the movement of IAA in shoots helps a plant to survive.

  More lAA moves towards the shaded side of shoots than the light side when the light is unidirectional. In response to this uneven distribution of lAA, the cells on the shaded side elongate faster than those on the light side and the shoot bends towards the light. Th is ensures that the shoot and the leaves attached to it have a greater chance of being well illuminated. As light is essential for photosynthesis, the process by which organic material for respiration is manufactured. the plant has a greater chance of survival.
• Suggest two advantages to a plant of having roots that respond to gravity by growing in the direction of its force.

  Response ensures that roots grow down into the soil, anchoring the plant firmly and bringing them closer w water (needed for photosynthesis).See an expert-written answer!We have an expert-written solution to this problem!
• Consider the following facts about IAA:

  i. They are easily made synthetically

  ii. They are readily absorbed by plants

  iii. They are not easily broken down

  iv. They are lethal to some plants in low concentrations

  v. Narrow-leaved plants are less easily killed than broad-leaved plants. Suggest ways in which these facts might be relevant to agricultural practice.


  i. The fact that IAA is readily absorbed, easily synthesized and is lethal to plants in low concentrations makes it useful as a herbicide.

  ii. The fact that it more readily kills broad-leaved plants than narrow-leaved ones is an advantage because many agricultural crops are narrow leaved while the weeds that compete with them are broad-leaved.

  iii. As a result. application of IAA at appropriate concentrations will kill only the weeds with little, or no harm to the crop.

  iv. As IAA is nor easily broken down means it will persist in the soil and continue to act as a selective weedkiller for some time.

  v. This may prevent a broad-leaved crop being grown on the land for some time after application of IAA. There is also a danger that IAA might accumulate along food chains with possible harm to animals in those chains.See an expert-written answer!We have an expert-written solution to this problem!
• Discovering the role of IAA in tropisms questions:
• Which of Darwin's three experiments acted as a control?

  Experiment 1
• Suggest an explanation for the results in experiment 5.

  As mica conducts electricity it will not prevent electrical messages passing from the shoot tip but it will prevent chemical messages passing. As there is no response, the message must be chemical and must pass down the shaded side.
• Suggest an explanation for the results in experiment 7.

  Displacement of the tip means that the chemical initially only moves down the side of the shoot that is in contact with the tip. This side grows more rapidly, causing bending away from that side.
• Suggest reasons for using a glass plate in experiments 9 and 10.

  It prevents chemicals I IAA, but not light, passing from one side to the other.
• State which of the three theories the results tend to support. Give reasons for your answer.

  Results support the hypothesis that lAA is transported from the lighter side to the shaded side of the shoot.See an expert-written answer!We have an expert-written solution to this problem!