Control and Coordination in Plants

2026 Syllabus Objectives

By the end of this topic, you should be able to:

  1. 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
  2. Explain the role of auxin in elongation growth by stimulating proton pumping to acidify cell walls
  3. Describe the role of gibberellin in the germination of barley

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1. The Venus Fly Trap – A Rapid Plant Response

What is the Venus fly trap?

The Venus fly trap is a carnivorous plant – this means it can catch and digest small animals (mainly insects) to get nitrogen compounds that are often scarce in the soil where it grows. This gives it an advantage in nutrient-poor environments.

Structure of the trap

The Venus fly trap has specialized leaves that work like traps. Here's how they're built:

  • Two lobes – The leaf is divided into two halves (lobes) on either side of a central ridge called the midrib
  • Red coloring inside – The inside surface of each lobe is red, which helps attract insects
  • Nectar-secreting glands – Around the edges of the lobes are glands that produce sweet nectar. This nectar attracts insects to land on the leaf
  • Sensory hairs – Each lobe has three stiff hairs that can detect touch. These are the trigger for closing the trap
  • Interlocking hairs – There are also non-sensory hairs around the edges that interlock when the trap closes, creating a cage to prevent the insect from escaping

How the trap closes – Step by step

The closure of the Venus fly trap is a fascinating process that involves electrical signals, similar to how nerves work in animals:

Step 1: The insect lands An insect is attracted by the nectar and red color, and lands on the open leaf.

Step 2: Stimulation of sensory hairs When the insect touches one of the three sensory hairs on a lobe, something important happens in the cells at the base of that hair.

Step 3: Calcium channels open The touch causes calcium ion channels (tiny doorways in the cell membrane) to open. Calcium ions rush into the cells at the base of the hair.

Step 4: Receptor potential forms The influx of calcium ions creates a receptor potential – this is a small electrical change in the cell.

Step 5: Action potential is triggered Here's the clever part: the trap won't close from just one touch. This prevents the plant from wasting energy on false alarms (like a raindrop or piece of debris). An action potential (a strong electrical signal) only occurs if:

  • Two different sensory hairs are touched at the same time, OR
  • One sensory hair is touched twice within about 30 seconds

If this condition isn't met, the trap resets and the process starts over.

Step 6: Signal spreads and trap closes Once an action potential is generated, it spreads rapidly across all the cells of the trap. The cells at the base of the lobes suddenly change shape, causing the two lobes to snap shut along the midrib. The leaf changes from a convex shape (curving outward) to a concave shape (curving inward), trapping the insect inside.

Step 7: Sealing and digestion begin When the insect is trapped and continues to move inside (continuing to stimulate the sensory hairs), more calcium ions enter gland cells. This triggers exocytosis (the release of substances from cells). Vesicles containing digestive enzymes are released into the trap.

Step 8: Digestion and absorption The trap stays tightly closed for up to a week. During this time:

  • The digestive enzymes break down the insect's body
  • The plant absorbs the nutrients (especially nitrogen compounds) from the digested insect
  • After absorption is complete, the trap reopens and is ready to catch another insect

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