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level: 14.2 Plant growth factors

Questions and Answers List

level questions: 14.2 Plant growth factors

QuestionAnswer
What stimuli do plants respond to?- Light - Gravity - Water
Light- Shoots grow towards light (positively phototropic) cause light is needed for photosynthesis
Gravity- Plants need to be firmly anchored in soil - Roots are sensitive to gravity + grow in the direction of its pull (positively gravitropic)
Water- Almost all plant roots grow towards water (positively hydrotropic) - In order to absorb it for use in photosynthesis, other metabolic processes + support
Plant growth factors- Hormone-like substance - Used to respond to external stimuli - May be made by cells located throughout plant rather than particular organs - Unlike animal hormones, some affect tissues that release them rather than acting in a distant target organ - Produced in small quantities
Indolacetic acid (IAA)- Auxins - Controls plant cell elongation
Positive phototropism- Plant grows towards light that is directed at it from one side (unilateral light)
Phototropism in flowering plants- Cells in tip of shoot produce IAA, transported down s - IAA is initially transported evenly to all regions - Light causes movement of IAA from light side to darker side of s - A greater conc. of IAA builds up on the darker side than on the lighter side - As IAA causes elongation of shoot cells, greater conc. of IAA on darker side, cells on this side elongate more - Darker side of s elongates faster, causing s tip to bend towards light
Extra info- IAA also controls bending of roots - Whereas a high conc. of IAA increases cell elongation in shoots, it inhibits cell elongation in roots - As a result, in roots the elongation of cells is greater on the light side than on the shaded side - So roots bend away from light (negatively phototropic)
Gravitropism in flowering plants- Cells in root tip produce IAA, transported along root - IAA initially transported to all sides of root - Gravity influences movement of IAA from upper side to lower side of root - Greater conc. of IAA builds up on lower side of root than upper side - As IAA inhibits elongation of root cells, greater conc. of IAA on lower side, cells on this side elongate less - The relatively greater elongated cells on upper side causes root to bend downwards towards force of gra.
Extra info- In shoots, greater conc. of IAA on lower side increases cell elongation - Causes this side to elongate more than upper side - As a result, shoot grows upwards away from gravity
Role of IAA in elongation growth- Has no. of effects on plant cells including increasing the plasticity of cell walls - Only occurs on young cell walls where cells are able to elongate - As cells mature they develop greater rigidity, so older parts of the shoot/root wont be able to respond - Hypothesis of how IAA increases plasticity of cells, acid growth hypothesis
Acid growth hypothesis- Involves AT of H ions from cytoplasm into spaces in cell wall - Causing cell wall to become more plastic - Allowing cell to elongate by expansion - Elongation of cells on one side of stem/root can lead to bending, so caused by uneven distribution of IAA
Choice chamber meaning- Container with different compartments to create different environmental conditions - Can be used to investigate how animals respond to conditions like light intensity
Using a choice chamber to investigate animal responses to light intensity- Constrct choice chamber - Cover 1 half of lid with black paper, making 1 side dark - Put damp filter paper in both sides of base - Place 10 woodlice on mesh in centre of chamber - Cover chamber with lid - After 10 mins, take off lid, record no. of woodlice on each side - Repeat after moving woodlice back to centre - Should find woodlice end up on dark side
Using a choice chamber to investigate animal responses to humidity- Place some damp filter paper in 1 side of base - Place a desiccating (drying) agent in other side - Don't cover lid with paper - Put lid on, leave chamber for 10 mins to stablise before carrying out steps above ( CAN DO SIMILAR EXP. WITH MAZE )