AQA GCSE Biology Plant Hormones PDF Revision Notes

Head to www.savemyexams.com for more awesome resources AQA GCSE Biology Your notes Plant Hormones Contents Plant Hormones Required Practical: Plant Growth Uses of Plant Hormones...

Head to www.savemyexams.com for more awesome resources AQA GCSE Biology Your notes Plant Hormones Contents Plant Hormones Required Practical: Plant Growth Uses of Plant Hormones Page 1 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Plant Hormones Your notes Tropisms Controlled by Hormones Plants produce plant hormones called auxins to coordinate and control growth Plants need to be able to grow in response to light (phototropism) and gravity (gravitropism or geotropism) The shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight – shoots show a positive phototropic response and a negative gravitropic response Roots need to grow downwards into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles so roots show a negative phototropic response and a positive gravitropic response Gravitropism and phototropism table Auxins are produced in the tips of the shoots and the roots; they diffuse to the cells behind the tips and have the following effects: In the shoots auxin promotes cell elongation; more auxin = more cell elongation = more growth Page 2 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources In the roots auxin inhibits cell elongation; more auxin = less cell elongation = less growth The distribution of auxin in the shoots is affected by light and gravity, whereas the distribution in the roots is primarily affected by gravity alone Your notes If a shoot or root is placed on its side, auxins will accumulate along the lower side as a result of gravity; so the uppermost side has a lower auxin concentration In the shoots, the lower side grows faster the upper side, so the shoot grows upwards In the roots, the lower side grows slower than the upper side (as auxin inhibits cell elongation and growth in the roots), so the root grows downwards Unequal distributions of auxin cause unequal growth rates in plant roots and shoots How auxins control growth in the shoots Auxin is mostly made in the tips of the growing shoots diffuses to the region behind the tip Auxin stimulates the cells behind the tip to elongate (get larger); the more auxin there is, the faster they will elongate and grow This is an important point - only the region behind the tip of a shoot is able to contribute to growth by cell division and cell elongation If light shines all around the tip, auxin is distributed evenly throughout and the cells in shoot grow at the same rate - this is what normally happens with plants growing outside When light shines on the shoot predominantly from one side though, the auxin produced in the tip concentrates on the shaded side, making the cells on that side elongate and grow faster than the cells on the sunny side This unequal growth on either side of the shoot causes the shoot to bend and grow in the direction of the light Page 3 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Positive phototropism in plant shoots is a result of auxin accumulating on the shaded side of a shoot Auxin Plant Hormones Higher tier only Auxins are just one example of hormones found in plants Gibberellins are important in initiating seed germination – the process that occurs when a seed starts to grow Gibberellins also have a role in inducing flowering and the growth of fruit Ethene is a gas released by plants which controls cell division and ripening of fruits Both gibberellins and ethene are used commercially – see Uses of Plant Hormones Page 4 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Required Practical: Plant Growth Your notes Effect of Light or Gravity Aim: To investigate the effect of light or gravity on the growth of newly germinated seedlings In order to investigate the effect of gravity on germinated seedlings you will: Set up two petri dishes with 3 mustard seeds and allow them to germinate Place one dish on a clinostat and the other to a support on its side Record the direction of growth of both the shoots and the roots for each seed Method Page 5 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Page 6 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Investigating the gravitropic response (set-up) Add some damp cotton wool to two petri dishes Your notes Place 3 bean seedlings in the cotton wool in each petri dish A - radicle facing downwards B - horizontally C - radicle (root grows from here) facing upwards Cover each dish with a lid Attach one petri dish to a support so that it’s on its side Attach the second petri dish to a clinostat (as shown in the diagrams above). Place both in a light-proof box (so that the seedlings are in complete darkness), leave for two days and then observe growth of the seedlings Analysis of results Page 7 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Investigating the gravitropic response (results) In the first petri dish all radicles (roots) have grown downwards (positive gravitropic response) regardless of which way they were initially facing (horizontal, up or down) and all plumules (shoots) have grown upwards (negative gravitropic response) In the second petri dish, all radicles and all plumules have all grown neither up nor down but straight outwards in whichever direction they were placed as the effect of gravity has been cancelled out by the revolving of the clinostat - they have shown no gravitropic response at all The experiment needs to be done in a lightproof box in order to control the effect of light on the growth of the seedlings Light is a control variable when investigating gravity Page 8 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Examiner Tips and Tricks Make sure you know what a clinostat is and what it does (it cancels out the effect of gravity). Your notes Page 9 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Uses of Plant Hormones Your notes Uses of Plant Hormones Higher tier only Plant hormones can be extracted or artificially made and used by gardeners and farmers in horticulture and agriculture to usefully control plant growth to obtain larger yields for example The use of auxins, ethene and gibberellins commercially has been very beneficial is helping producing food and plants for decoration However the everyday use of hormones as weed killers can have a negative effect on biodiversity; as the growth of unwanted but natural plants such as weeds is inhibited Many different species of plants are classed as weeds commercially, but to other organisms they are a food source and potential habitat, so destroying them can have negative effects on other organisms in the ecosystem Auxins Higher tier only Auxins can be used as selective weed killers; negatively affecting the growth of broad-leaved plants which are weeds in comparison to the narrow-leaved grasses and cereals grown as crops for food production (which are desired) The growth of weeds is controlled by farmers who don’t want their yields to be smaller as a result of competition between crops and weeds for space and nutrients from the soil Selective weedkillers disrupt the growth of weeds only, causing them to die However once applied to a crop their spread cannot be controlled, and they could affect other plant species negatively If a gardener or farmer wants to easily and cheaply produce lots of clones of a desirable plant, then they can take cuttings of the plant and dip the tips in auxins which are sold as ‘rooting powders’ as they encourage the rapid development of roots The same principle as above can be used to clone plants in the lab; auxins in this way are used to promote growth in tissue culture where scrapings of cells can be taken from a desirable plant and used to produce clones in a petri dish that are then planted and allowed to grow in soil Ethene Higher tier only Page 10 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Ethene is used in the food industry to control ripening of fruit during storage and transport It is far more effective to transport unripe fruit, as ripe fruit is softer and therefore more easily damaged Your notes and spoiled The production of ethene can be inhibited to delay ripening of fruits in storage; this can either be achieved directly by adding chemicals that prevent ethene from being produced, or by reacting ethene in the air around fruit with substances that can remove it When ripening needs to be encouraged (eg. when fruit is in the supermarket), artificially produced ethene gas can be released to speed up the process Gibberellins Higher tier only Gibberellins can be used to: End seed dormancy, as a high concentration of gibberellin promotes seed germination. Gibberellin levels naturally rise after a period of dormancy (exposure to cold and dry conditions) – usually, dormancy ends with an intake of water into the seed and warmer weather Promote flowering regardless of the weather conditions the plant is in Increase fruit size – higher levels of gibberellin promote the development and growth of fruit Page 11 of 11 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers