| 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 ) |
