5.1.5

• Physical defences of plants

  • Barbs
  • Spikes
  • Spiny leaves
  • Inedible tissue
• Tannins


  A group of phenols produced by plants that are toxic to microorganisms and larger herbivores
• Alkaloids


  A group of bitter-tasting, nitrogenous compounds found in growing tips and flowers, and peripheral cell layers of stems and roots, acting as a feeding deterrent
• Terpenoids


  A group of compounds that are toxic to insects and fungi
• Pheromones


  Chemicals released by one plant that can affect the physiology of another plant
• Response to water stress or herbivory


  Causes the release of pheromones which warn other plants
• Cabbage attacked by caterpillars


  Releases a chemical signal which attracts the parasitic wasp Cotesia glomerata
• Tropisms


  Directional growth responses to external stimuli in plants
• Phototropism


  • Leaves grow towards the light
  • Roots grow away from the light
• Geotropism


  • Roots grow towards gravity
• Chemotropism


  Causes pollen tubes to grow down the style, attracted by chemicals, towards the ovary where fertilisation occurs
• Thigmotropism


  Buds of shooting plants grow toward a particular direction in response to touch, allowing them to wind around a solid structure
• Nastic responses


  Non-directional responses to external stimuli
• Testing phototropism


  Germinate two seeds, one under unilateral light and one with directional light, and compare the horizontal distance between two marked points on the stem to measure the distance the plant has grown toward the light
• Testing geotropism


  Use a clinostat, which slowly rotates, to change the directional response of the plant to geotropism, causing it to spiral
• Abscission


  Leaves respond to falling auxin concentrations by producing the gaseous plant hormone ethene, which cells in the abscission zone at the base of the leaf are sensitive to, initiating enzymes that digest the cell walls
• Stomatal closure


  Occurs as a result of low light concentrations, to cool the plant or conserve water, largely under the control of the hormone ABA
• Gibberellins


  Promote seed germination by enabling the production of amylases and proteases, which break down starch to glucose and storage proteins to amino acids, providing a respiratory substrate for the embryo
• ABA


  Antagonistic to gibberellins, preventing seed germination
• Auxins


  Plant hormones responsible for regulating plant growth, inhibiting lateral bud growth and promoting main apical shoot growth
• Removing apical bud


  Inhibits root growth, but replacing auxin artificially at low concentrations restores it
• Gibberellins


  Cause growth in the internodes of stems by stimulating cell elongation through the loosening of cell walls and cell division
• Genetically modified plant embryos to not produce gibberellin


  Seed will not germinate
• Applying gibberellin biosynthesis inhibitors to seeds


  They do not germinate
• Applying gibberellin to seeds


  They germinate
• Ripening


  Can be suppressed for transport and promoted nearer the point of sale using gibberellins and ethene gas
• Hormone rooting powders


  Increase the success of micropropagation to clone commercially favourable plants using cuttings from shoots
• Hormonal weedkillers


  Synthetic dicot auxins are applied as weedkiller, absorbed by broad-leaved plants but not narrow-leaved monocot staple plants, increasing their growth rate unsustainably causing them to die
• Central Nervous System (CNS)

  Consists of the brain and spinal cord, largely made up of unmyelinated relay neurones
• Peripheral Nervous System


  Ensures rapid communication between sensory receptors, the CNS, and effectors, composed of both sensory and motor neurones
• Somatic Nervous System


  • Conducts action potentials under voluntary control, such as skeletal muscles
• Autonomic Nervous System


  • Conducts action potentials not under voluntary control from the CNS to effectors such as glands, cardiac, and smooth muscle
  • Subdivided into sympathetic and parasympathetic systems
• Cerebrum


  Largest part of the brain, organises most of the higher thought processes
• Cerebellum


  Coordinates movement and balance
• Hypothalamus and Pituitary Complex

  Organises homeostatic responses and control various physiological processes
• Medulla Oblongata


  Coordinates many of the autonomic responses
• Cerebral Cortex


  • Outermost layer of the nerve cell bodies in the cerebrum, involved in conscious thought, actions, emotional responses, intelligence, reasoning, judgement, decision making, and factual memory
• Cerebellum


  Involved with balance and fine coordination of movement, providing complex movements initiated by the cerebral cortex
• Hypothalamus and Pituitary Complex

  Controls homeostatic mechanisms, detecting changes in body temperature and water potential, and initiating responses mediated by the nervous system or hormonal system
• Medulla oblongata


  Controls non-skeletal muscles through the autonomic nervous system and contains centres for coordinating vital processing