Part 4 Unit 1 Coordination in Plants 2024-25 PDF

Academic Year 2024-25 Unit: Control, Coordination and Response Grade: MYP 5 Key concept: System Related concept: Environment Global context: Identities and relationships SOI: Organisms are more likely to survive in a system when the...

Academic Year 2024-25 Unit: Control, Coordination and Response Grade: MYP 5 Key concept: System Related concept: Environment Global context: Identities and relationships SOI: Organisms are more likely to survive in a system when they are adapted to interact with their surrounding and respond to changes in their environment Background information: Connection with SOI: Like all organisms, plants detect and respond to stimuli in their environment. A plant’s primary means of response is to change how it is growing. Their responses are generally controlled by hormones, which are chemical messenger molecules. Plants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive. Purpose: The following notes should be used by students for reference and revision. The notes explore the concepts of coordination in plants which help them in adapting and survival by responding to different stimuli COORDINATION IN PLANTS Coordination is the ability to use different parts of the plant together, smoothly and efficiently. In plants, coordination is due to the result of a chemical system, wherein plant hormones or phytohormones have a major role. TROPISMS OR TROPIC MOVEMENTS Video: Tropisms: https://www.youtube.com/watch?v=PxSkuyjZ3MM The Growth movement of a plant in response to its stimulus, based on the direction of the stimulus, determines the direction of growth of the plant, referred to as TROPISM. This Growth pattern of a plant can be based on the nature of the tropism. If the Plant bends or grows towards the stimulus, it has a positive effect on growth termed as Positive Tropism. If the plant bends and grows away from the direction of stimulus, it has a negative effect on growth termed as Negative Tropism. MOVEMENT IN PLANTS Plants exhibit two types of movements. Growth-dependent movements called the Tropic Movements (towards or away from a stimulus) Non-growth dependent movements called the Nastic Movements (independent of stimulus) Tropism are responses by part of a plant toward of away from a stimulus coming from one direction. The movement is always a growth movement. Like animals, plants are able to response to their environment, although usually with much slower responses than those of animals. In general, plants respond to stimuli by changing their rate or direction of growth. They may grow either towards or away from a stimulus. These responses are called tropisms. PHOTOTROPISM This is the movement of the plant in response to light. This is a positive tropism, as the shoot of the plant grows towards sunlight. GEOTROPISM This is the movement of the plant in response to Gravity. This is a positive tropism, as the root of the plant grows downwards into the soil. CHEMOTROPISM This is when a plant responds to a Chemical. It too is a form of positive tropism. Eg: the Pollen tube in a Flower grows toward the Ovules for Fertilization. HYDROTROPISM Roots always grow in the direction of the source of water for the development of the plant. In doing so they against the law of Gravity. They can grow upwards or sideways depending on the water source. They are said to be Hydrotropic. THIGMOTROPISM We may have come across situations or plants, that which upon touching close their leaves. Some plants have weak stems called Tendrils and use the support of stronger plants by winding around them and helping them grow. This is an example of positive tropism, as the weaker plant (tendril) responds to the stimulus by clinging and winding on it, enabling it to grow. NASTIES OR NASTIC MOVEMENTS When the response by a plant to a stimulus is ignored or not directed toward the stimulus, it is called a Nastic Movement. Generally, plants, their entire body, always grow in the direction of the stimulus, but in nastic movements, parts of the plant tend to behave differently, like the closing of Petals of a flower or the folding of the leaves. This behavior is called Thigmonasty. In certain cases, the petals of a flower open according to the intensity of light. The brighter the light, the petals remain open. If light fades, then the petals close. This behavior is called Photonasty. "Coordination and Response pdf. 11 Mar. 2014. Weebly.com. 13 May 2019 accessed May 10, 2020 TROPIC RESPONSES Two important stimuli for plants are light and gravity. "Coordination and Response pdf. 11 Mar. 2014. Weebly.com. 13 May 2019 accessed May 10, 2020 Shoots normally grow towards light. Roots do not usually respond to light, but a few grow away from it. Shoots tend to grow away from the pull of gravity, while roots normally grow towards it. "Coordination and Response pdf. 11 Mar. 2014. Weebly.com. 13 May 2019 accessed May 10, 2020 Coordination and Response pdf. 11 Mar. 2014. Weebly.com. 13 May 2019 accessed May 10, 2020 Plant hormones or phytohormones Video: Plant Hormones: https://www.youtube.com/watch?v=8Ji3g4yp4VE They are responsible for the control and coordination of plants. There are different types of hormones, which affect the growth of a plant. Phytohormones are chemical compounds which are released by stimulated cells. These hormones are diffused around the plant cells. They have a role to play in the cell division, cell enlargement, cell differentiation, fruit growth, falling of leaves, ripening of fruits, ageing of plants etc. The different types of phytohormones are: 1. Auxins 2. Gibberellins 3. Cytokinins 4. Abscisic acid Auxins: They help in the cell growth at the shoot tips. By elongating the cells, they help in the growth process. Video: Plant Growth: Auxins and Gibberellins: https://www.youtube.com/watch?v=EZ5tU45Ti_g Darwin did the first experiments to study the effects of Auxin Shoots and roots that have their tips removed will not respond to light or gravity because the part that produces auxins has been cut off. Shoots that have their tips covered with opaque material grow straight upwards when exposed to one-sided light, because the auxin distribution is not influenced by the light. Auxins are plant growth substances, produced by the shoot and root tips of growing plants. - Auxins in the shoot → stimulate cell growth, by the absorption of water. - Auxins in the root → slow down the cell growth. Auxin in phototropism 1. If a shoot is exposed to light from one side More auxins are moving in the shaded side (from the tip of the shoot) On this side, cells are stimulated to absorb more water, plant grows more Shoot bends toward the light. This is called positive phototropism. 2. If a root is exposed to light in the absence of gravity More auxins are moving in the shaded side (from the tip of the root) On this side, cells are stimulated to absorb less water, plant grows less Root bends away from the light. This is called negative phototropism. Auxin in geotropism 1. If a shoot is placed horizontally in the absence of light: Auxins accumulate on the lower side of the shoot, due to gravity. Cells on the lower side grow more quickly The shoot bends upwards. This is called negative geotropism. 2. If a root is placed horizontally in the absence of light: Auxins accumulate on the lower side of the shoot, due to gravity. Cells on the lower side grow more slowly The shoot bends downwards. This is called positive geotropism.  µ Coordination and Response pdf. 11 Mar. 2014. Weebly.com. 13 May 2019 accessed May 10, 2020 Gibberellins: These hormones are responsible for the cell growth in the stem, seed germination, and flowering. Cytokinins: They promote cell division in plants. They also promote the opening of the stomata and delay ageing in leaves. Abscisic acid: This hormone inhibits the growth of the plant. And therefore, it promotes dormancy in seeds and buds. The detachment of fruits, flowers, and falling of leaves etc. are promoted by this hormone. Ethylene: Ethylene (C2H4) is a simple gaseous hydrocarbon that has profound effects upon plant growth and development. Although most popularly associated with ripening, ethylene plays a role throughout the entire life of the plant. Effects of weed killers Weed killers (herbicide) are synthetic plant hormones, similar to auxins. If these chemicals are sprayed on to plants they can cause rapid, uncontrolled growth and respiration, resulting in the death of the plant. Some plant species are more sensitive than others to synthetic plant hormones, so weed killers can be selective. Many weed killers kill only broad-leaved plants (dicotyledons), leaving grasses (monocotyledons) unharmed. Guiding questions: 1. How does having more auxin on the dark side of a shoot tip cause it to grow toward the light? 2. Charles Darwin conducted an experiment on phototropism in the 19th century. He placed canary grass (Phalaris canariensis) seedlings in box open on one side in front of a south-west window. Curvature towards the light was traced and accurately measured. The diagrams shown below illustrate the curvature of three seedlings after eight hours. Allott, Andrew, and David Mindorff. MYP Biology. Oxford University Press, 2017. The graph shows average shoot curvature over time.  Allott, Andrew, and David Mindorff. MYP Biology. Oxford University Press, 2017. a. What was the time delay before the shoots start to curve towards the light? b. Analyze and evaluate the effect of light on shoot curvature based on the graph given. At what time was the shoot curving most rapidly? c. Describe the pattern of shoot curvature shown in the graph. d. If a small aluminium foil cap is placed over the top of the shoots, they do not respond to unilateral light. What do you conclude from this? Bibliography Jones, Mary, and Geoff Jones. Cambridge IGCSE Biology. Cambridge University Press, 2014. "Coordination and Response pdf. 11 Mar. 2014. Weebly.com. 13 May 2019 accessed May 10, 2020