Plants and Animal Responses

• What is abiotic stress?


  Non-living environmental factor that could harm a plant e.g mineral deficiency, drought, depleted oxygen supply, pollution
• How do plants respond to abiotic stress and herbivory?

  • May produce antifreeze enzymes.
  • May contain bitter-tasting tannins
  • May contain bitter-tasting nitrogen compounds called alkaloids
  • Release cell-signalling pheromones to trigger defensive responses in other organisms
• How does Mimosa pudica respond to being touched?


  Seismonasty (touch sensitivity) causes leaves to fold
• What is plant tropism?


  Directional growth response of plants

  • Phototropism: response to light
  • Geotropism: response to gravity
  • Hydrotropism: response to water
  • Thermotropism: response to temperature
  • Thigmotropism: response to touching a surface or object
• How is leaf loss (leaf abscission) in deciduous plants controlled?


  1. As leaf ages, cytokinin and auxin levels lower, ethene levels increases
  2. Triggers production of cellulase enzymes which weaken leaves by breaking down cell walls in abscission layer
  3. Leaves break from branch. Below abscission layer, suberin layer forms to prevent entry of pathogens
• List the functions of gibberellins.


  Stimulate:

  • germination
  • elongation at cell internodes
  • fruit growth
  • rapid growth/flowering
• How is germination stimulated?


  1. Seed absorbs water, activating embryo to secrete gibberellins
  2. Gibberellins diffuse to aleurone layer which produces amylase
  3. Amylase diffuses to endosperm layer to hydrolyse starch
  4. Hexose sugars act as respiratory substrate to produce ATP as ‘energy currency’
• List the functions of auxins.


  • Involved in trophic responses e.g IAA
  • Control cell elongation
  • Suppress lateral buds to maintain apical dominance
  • Promote root growth e.g in rooting powders
• Explain why shoots show positive phototropism


  1. Indoleactetic acid (IAA) diffuses to shaded side of shoot tip
  2. As IAA diffuses down shaded side, it causes active transport of H+ ions into cell walls
  3. Disruption of H-bonds between cellulose molecules and action of expansins make cell more permeable to water. (Acid growth hypothesis)
  4. Cells on shaded side elongate faster due to higher turgor pressure
  5. Shoot bends towards light
• Explain why roots show positive gravitropism


  1. Gravity causes IAA to accumulate on lower side of the root
  2. IAA inhibits elongation of root cells
  3. Cells on upper side of root elongate faster so the root tip bends downwards
• How do hormones stimulate stomata to close?


  1. Abscisic acid binds to complementary receptors on guard cell membrane causing calcium ion channels on tonoplast to open. Calcium ions diffuse from vacuole into cytosol
  2. Positive feedback triggers other ion channels to open. Other ions e.g potassium diffuse out of guard cell
  3. Water potential of guard cell becomes more positive. Water diffuses out via osmosis.
  4. Guard cells become flaccid so stomata close
• What is apical dominance?


  Phenomenon where during growth of shoot, growth of side shoots does not take place. Maintained by the action of auxin, abscisic acid and cytokinins.
• Explain the experimental evidence that auxins maintain apical dominance.


  Auxin production in apex maintains high levels of abscisic acid. Inhibits growth of side shoots.

  When apex is removed:

  1. auxin levels drop thus causing abscisic acid levels to drop
  2. cytokinins (initially concentrated near auxin reserve in bud) diffuse evenly to promote bud growth in other parts of plant = lateral buds
• Explain the experimental evidence that gibberellins control stem elongation and germination.


  Stem elongation: tall plants have higher gibberellin concentration than dwarf plants

  Germination: mutant seeds with non-functional gibberellin gene do not germinate unless gibberellin is applied externally. Inhibitors of gibberellin production to prevent germination
• How are auxins and cytokinins used commercially?


  Auxins: rooting powder, growing seedless fruit, herbicides, low concentrations prevent leaf and fruit growth, high concentrations promote fruit drop

  Cytokinins: prevent yellowing of lettuce leaves, promotes shoot growth
• How are gibberellins and ethene used commercially?


  Gibberellins: delay senescence in citrus, elongation of apples and grape stalks, brewing beer for malt production, increase sugar cane yield, speed up seed formation in conifers, prevent lodging

  Ethene: speeds ripening, promotes lateral growth, promotes fruit drop
• Outline the gross structure of the mammalian nervous system.


  See below

  
• Name the 2 main divisions of the mammalian nervous system


  Structural organisation:

  • Central nervous system: comprised of brain and spinal cord. Specialised system of nerve cells processes stimuli and propagates impulses.
  • Peripheral nervous system: all neurones that are not part of the CNS
• Name 2 main divisions of the peripheral nervous system


  Functional organisation:

  • somatic (under conscious control)
  • autonomic (not under conscious control)
• Name the 2 main divisions of the autonomic nervous system.


  • Sympathetic: often stimulates effectors (flight-or-fight response), neurotransmitter noradrenaline, ganglia near CNS
  • Parasympathetic: often inhibits effectors (rest/digest response), neurotransmitter acetylcholine, ganglia far from CNS

  Act antagonistically to regulate response of effectors
• Describe the gross structure of the human brain.


  2 hemispheres joined by band of nerve fibres (corpus callosum). Divided into lobes.

  • Parietal lobe at the top of the brain: movement, orientation, memory, recognition
  • Occipital lobe at the back of the brain: visual covert processes signals from the eye
  • Temporal lobe beneath the temples: processes auditory signals
• Identify the location and function of the cerebellum.


  • Controls execution (not initiation) of movement e.g timing, balance, coordination, posture.
  • Possible role in cognition e.g attention and language
• Identify the location and function of the cerebellum


  • Controls execution (not initiation) of movement e.g timing, balance, coordination, posture
  • Possible role in cognition e.g attention and language

  
• Identify the location and function of the medulla oblongata


  • Controls a range of autonomous functions such as breathing and heart rate (location of cardioacceleratory/deceleratory centres)

  
• Identify the location and function of the cerebrum


  • Uppermost part of the brain is organised into lobes which control voluntary functions e.g initiating movement, speech, thought.

  
• Identify the location and function of the hypothalamus


  • Includes anterior pituitary gland (secretes metabolic and reproductive hormones)
  • Involved in thermo and osmoregulation

  
• Outline what happens in a simple reflex arc


  • Receptors detect stimulus -> sensory neurone -> relay neuron in CNS coordinates response -> motor neuron -> response by effector
  • Survival benefit: rapid response to potentially dangerous stimuli since only 3 neurons involved, instinctive
• Describe the knee jerk reflex.


  Important for maintaining posture and balance

  1. Tapping patellar tendon stimulates stretch-mediated receptors
  2. Impulse travels sensory to motor with no interneuron. Quadriceps contract. Inhibits antagonistic hamstring contraction.

  Diagnostically useful: multiple kicks = symptom of cerebellar disease, lack of reflex = nervous problems
• What is the ‘fight or flight’ response?

  If brain perceived threat, it stimulates stress responses involving adrenaline.

  Triggers physiological changes to prepare body: pupil dilation, inhibition of digestive enzyme, higher heart rate and stroke volume, greater blood flow to brain for metal awareness, faster metabolic rate.
• Use secondary messenger model to explain how adrenaline works.


  1. Adrenaline 1st messenger. Hormone-receptor complex forms
  2. Conformational changes to receptor activates G-protein
  3. Activates adenylate cyclase which converts ATP to cyclic AMP (cAMP)
  4. cAMP 2nd messenger. Activates protein kinase A pathway
  5. Results in glycogenolysis
• Describe the 3 types of muscle tissue


  A: striated skeletal muscle consists of multinucleated cells. Antagonistic muscle pairs enable movement

  B: smooth involuntary muscle enable walls of blood vessels and intestines to contact

  C: cardiac muscle consists of branches multinucleated cells. Myogenic contraction = heartbeat

  
• Describe the gross structure of skeletal muscle


  Muscle cells are fused together to form bundles of parallel muscle fibres (myofibrils)

  Arrangement ensures there is no point of weakness between cells.

  Each bundle is surrounded by enodmycium: loose connective tissue with many capillaries
• Describe the microscopic structure of skeletal muscle

  Myofibrils: site of contraction

  Sarcoplasm: shared nuclei and cytoplasm with lots of mitochondria and endoplasmic reticulum

  Sarcolemma: folds inwards towards sarcoplasm to form tranverse (T) tubules
• Describe a diagram to show the ultrastructure of a myofibril.


  Z-line: boundary between sarcomeres

  I-band: only actin (appears light under optical microscope)

  A-band: overlap of actin and myosin (appears dark under optical microscope)

  H-Zone: only myosin

  
• How is muscle contraction stimulated?


  1. Neuromuscular junction: action potential = voltage-gated calcium channels open
  2. Vesicles move towards and fuse with presynaptic membrane
  3. Exocytosis of acetylcholine which diffuses across synaptic cleft
  4. Acetylcholine binds to receipts on sodium channel proteins on skeletal muscle cell membrane
  5. Influx of sodium = depolarisation
• Explain the role of calcium ions in muscle contraction.


  1. Action potential moves through T-tubules in sarcoplasm = calcium channels in sarcoplasmic reticulum open
  2. Calcium binds to troponin triggering conformational change in tropomyosin
  3. Exposes binding sites on actin filaments so actinomyosin bridges can form
• Outline the ’sliding filament theory’


  1. Myosin head with ADP attached forms across bridge with actin
  2. Power stroke: myosin head changes shape and loses ADP, pulling actin over myosin
  3. ATP attaches to myosin head, causing it to detach from actin
  4. ATPase hydrolyses ATP -> ADP and inorganic phosphate so myosin head can return to original position
  5. Myosin head re-attaches to actin further along filament
• How does sliding filament action cause a myofibril to shorten?


  • Myosin heads flex in opposite directions = actin filaments are pulled towards each other.
  • Distance between adjacent sarcomere Z lines shortens
  • Sliding filament action occurs up to 100 times per second in multiple sarcomeres
• Explain the role of creatine phosphate in muscle contraction.


  Phosphorylates ADP directly to ATP when oxygen for aerobic respiration is limited e.g during vigorous exercise.
• State the name and location of the 2 nodes involved in heart contraction.

  Sinoatrial node (SAN): within the wall of the right atrium

  Atrioventricular node (AVN): near lower end of right atrium in wall that separates the 2 atria