CIE AS/A Level Biology -15.2 Control and coordination in plants- Study Notes- New Syllabus
CIE AS/A Level Biology -15.2 Control and coordination in plants- Study Notes- New Syllabus
Ace A level Biology Exam with CIE AS/A Level Biology -15.2 Control and coordination in plants- Study Notes- New Syllabus
Key Concepts:
- describe the rapid response of the Venus fly trap to stimulation of hairs on the lobes of modified leaves and explain how the closure of the trap is achieved
- explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls
- describe the role of gibberellin in the germination of barley (see 16.3.4)
Rapid Response of the Venus Flytrap
🧩 Overview
- Venus flytrap (Dionaea muscipula) is a carnivorous plant.
- Uses modified leaves (lobes) to capture insects.
- Responds rapidly to mechanical stimulation of sensory hairs on the inner surface of lobes.
⚡ Mechanism of Trap Closure
- Stimulation of Hairs: Each lobe has 3–4 trigger hairs. Two touches within 20–30 s or one strong touch required. Touch generates receptor potential → threshold reached → action potential initiated.
- Action Potential Transmission: Action potential spreads through lobes. Rapid electrical signal triggers cell turgor changes.
- Rapid Closure: Inner surface cells lose turgor (water exits), outer surface maintains pressure. Differential turgor → lobes snap shut in <1 second.
- Trap Sealing: Edges of lobes interlock to form a chamber. Glands secrete enzymes to digest prey.
📊 Summary Table
| Step | Event | Outcome |
|---|---|---|
| Stimulation | Hairs touched twice within 20–30 s | Receptor potential generated |
| Action potential | Spreads across lobe | Rapid signal triggers turgor changes |
| Turgor change | Inner cells lose water | Lobes snap shut |
| Sealing | Edges interlock | Prey trapped for digestion |
✨ Key Idea: The Venus flytrap’s rapid response is a turgor-driven movement triggered by action potentials, allowing it to capture prey efficiently.
Role of Auxin in Elongation Growth
🧩 Overview
- Auxins are plant hormones that regulate cell elongation.
- Growth occurs primarily in shoots and involves the acid growth hypothesis.
⚡ Mechanism of Auxin-Induced Elongation
- Auxin Binding: Auxin binds to receptors in the plant cell membrane of the elongation zone.
- Activation of Proton Pumps: Auxin stimulates H⁺-ATPase (proton pumps) in the plasma membrane. Pumps transport H⁺ ions from cytoplasm into the cell wall (apoplast).
- Acidification of Cell Wall: Lower pH in the wall (≈4.5–5) activates expansins, enzymes that loosen cellulose and hemicellulose fibres.
- Cell Wall Loosening: Loosened wall becomes more flexible and less rigid.
- Water Uptake & Cell Elongation: Osmotic uptake of water increases turgor pressure, stretching the flexible wall → cell elongation.
📊 Summary Table
| Step | Event | Outcome |
|---|---|---|
| Auxin binding | Auxin attaches to receptors | Activates proton pumps |
| Proton pumping | H⁺ ions pumped into cell wall | Cell wall acidifies |
| Wall loosening | Expansins activated | Cell wall becomes flexible |
| Water uptake | Osmosis increases turgor | Cell elongates |
✨ Key Idea: Auxin promotes elongation growth by acidifying the cell wall, allowing turgor pressure to stretch the wall and increase cell length.
Role of Gibberellin in Barley Germination
🧩 Overview
- Gibberellins (GA) are plant hormones that stimulate seed germination.
- In barley, they activate enzymes that mobilise food reserves in the endosperm.
⚡ Mechanism in Barley Germination
- Water Uptake (Imbibition): Dry barley seed absorbs water → seed swells. Embryo becomes metabolically active.
- Gibberellin Production: Embryo synthesises GA. GA diffuses from embryo to aleurone layer of endosperm.
- Enzyme Activation: GA triggers gene expression in aleurone cells → synthesis of hydrolytic enzymes, mainly α-amylase.
- Starch Breakdown: α-Amylase hydrolyses starch into maltose and glucose. Sugars provide energy for embryo growth and radicle emergence.
- Seedling Growth: Sugars fuel cell division and elongation → radicle and shoot emerge.
📊 Summary Table
| Step | Event | Outcome |
|---|---|---|
| Imbibition | Seed absorbs water | Embryo activates metabolically |
| GA production | Embryo synthesises GA | Diffuses to aleurone layer |
| Enzyme activation | GA stimulates α-amylase production | Starch converted to sugars |
| Energy supply | Sugars fuel growth | Radicle and shoot emerge |
✨ Key Idea: Gibberellins are essential in barley germination because they link water absorption to enzyme production, mobilising stored food for growth.