F215 Module 4 Responding to the Environment

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Tropisms are directional growth responses. There are many types, including:

Phototropism - growing towards/away from light

Geotropism - roots growing towards gravity

Chemotropism - growing towards/away from chemicals

Thigmotropism - climbing plants wind around solid structures for support

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Plant hormones vs animal hormones

Plant hormones:

  • Produced by many different cell types
  • Transported by diffusion, mass flow in the xylem/phloem, active transport..
  • Receptors on target tissue(s) bind to the hormones, eliciting a response

Animal hormones:

  • Produced by specialised endocrine glands
  • Travel in the blood
  • Receptors on target tissue(s) bind to the hormones, eliciting a response
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Plant hormones, effects and commercial uses


  • Effect - promote cell elongation, inhibit sideshoot growth and leaf abscission
  • Uses - promote flowering and seedless fruit production, used in rooting powder to promote root growth


  • Effect - promote cell division
  • Uses - stops lettuce yellowing, cuttings are treated to promote growth of buds and shoots


  • Effect - promote seed germination and stem growth
  • Uses - delay citrus senescence, promote elongation of apples, speeds up malt brewing


  • Effect - promotes fruit ripening
  • Uses - a liquid spray for fruit, promotes lateral bud growth

Abscissic acid

  • Effect - inhibits germination and growth, closes stomata when low water availability
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Meristems and Auxin

Apical meristems - the tips of roots and shoots

Lateral bud meristems - the buds of side shoots

Lateral meristems - a cylinder around the outside of roots and shoots, widens them

Intercalary meristems - between nodes (where buds branch off the stem)

How do Auxins stimulate growth?

  • Promote active transport of hydrogen ions into the cell wall
  • The low pH is optimum for expansins (wall loosening enzymes)
  • Expansins (and hydrogen ions) break the cellulose bonds, so the walls can expand.
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When light is coming from the top, auxin is evenly distributed down both sides of the plant and each side elongates at the same rate - the shoot grows straight

When light is coming from the side, auxin moves down the shaded side only, so the shaded side elongates more than the light up side - the shoot grows towards the light

Leaf Abscission

  • The low nutrient availability causes a drop in cytokinin concentration - nutrients move out of the leaf, into the stem.
  • A drop in cytokinin concentration causes a drop in auxin concentration, which causes an increase in ethene concentration
  • The cellulase concentration increases - cells in the abscission zone are digested
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Gibberellins and Apical Dominance


Gibberellins affect both elongation and division, which was proved using experiments. 2 seperate plants were grafted together. A lacks the precursor molecule that is converted to gibberellins using an enzyme, and B lacks the enzyme to do this - so both are short. When they're grafted together, they grow to a normal size.

Apical Dominance

The growing apical bud inhibits growth of the sideshoots. There is evidence to suggest this is due to auxin  - when the shoot is cut off, there is no auxin and the sideshoots grow. When the cut tip is covered with auxin paste, the side shoots don't grow. However, there is also contradictory evidence, so further research is necessary.

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The Autonomic Nervous System

This is the name for all neurones from the CNS to non-voluntary (smooth/cardiac) muscle.

It is divided into two types - sympathetic and parasympathetic

Sympathetic - active during stress, noradrenaline as the neurotransmitter, increase HR, dilate pupils, ******

Parasympathetic - active during relaxation, acetylcholine as the neurotransmitter, decrease HR, contract pupils, arousal.

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The brain


  • Largest part of the brain
  • Responsible for conscious thought, overriding refleces, association and intelligence

Cerebellum - responsible for fine motor control, movement, posture

Medulla oblongata - controls HR, breathing, the fight-or-flight response

Hypothalamus - homeostasis, regulates endocrine glands

Corpus callosum - connects the two hemispheres

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Innate behaviours

Innate --> Stereotypical in all members of a species, known from birth, passed on through reproduction.

Reflex --> e.g when there is a pressure channge, fish flick their tails to dart away.

Taxis --> a directional movement response. e.g ants move towards sweet substances

Kinesis --> a non-directional movement response. e.g woodlice will increase movement until it finds itelf in dark, damp conditions

Fixed action pattern --> a response to a non-immediate stimulus. e.g the waggle dance in bees indicates the direction of a food source.

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Learned Behaviours

Developed through environmental interaction, taught to offspring.

Habituation --> e.g birds learn to ignore scarecrows as they don't bring reward/punishment

Classical conditioning --> e.g cats run to the kitchen when a tin is opened

Operant conditioning --> e.g humans learn to avoid food that makes them ill

Imprinting --> e.g young ducks follow the first moving thing they see (their parents)

Insight learning --> problem solving using reasoning. e.g chimpanzees stack boxes to reach high hanging bananas

Latent (exploratory) learning --> e.g young rabbits learn the layout of their burrow to avoid future predation

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The elbow joint

The elbow is a synovial joint - requires a lot of movement.

Cartilage close to each bone reduces friction. The synovial membrane produces synovial fluid. The synovial fluid lubricates. The ligament prevents dislocation.

Transmission of an action potential at a neuromuscular junction

1. An action potential arrives at the synaptic knob.

2. Vesicles fuse to the membrane and acetyl choline is released.

3. Acetylcholine binds to receptors on the sarcolemma.

4. The depolarisation travels down T tubules.

5. The depolarisation causes the release of calcium ions from the sarcoplasmic reticulum

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Types of Muscle


  • Many mitochondria, many nuclei, sarcoplasmic reticulum
  • Striated
  • Responsible for movement of the skeleton at the joints - moving the limbs
  • Contracts quickly and powerfully, but fatigues easily.


  • Forms intercalated disks (dark rings)
  • Striated
  • Myogenic
  • Keeps blood flowing
  • Contracts continually and powerfully throughout life.


  • Spindle shaped cells
  • Non striated
  • Responsible for peristalsis and vasoconstriction
  • Contracts slowly, but fatigues slowly
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The fight or flight response

The hypothalamus:

  • Stimulates sympathetic nervous activity, activating glands and smooth muscle, and stimulating the adrenal medulla to release adrenaline and noradrenaline
  • Stimulates the pituitary to release CRF, causing the release of ACTH, which stimulates the adrenal cortex to release around 30 hormones.

Effects of the fight or fight response:

  • Pupils dilate
  • HR and BP increase
  • Glucose concentration increases
  • Metabolic rate increases
  • Hairs stand erect
  • Endorphins are released in the brain
  • Sweat production increases
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Social Behaviour in Gorillas

  • Gorillas live in troops of around 10, consisting of one silverback male, several females and their offspring.
  • The alpha male protects the group and leads them in search of food and shelter
  • When males are old enough, they leave and start a new troop.
  • Allogrooming - the gorillas pick parasites off each other, which reinforces relationships.
  • The mother is in constant contact with the young for 5 months, then the young does not venture more than 5m from the mother for the first 12 months.
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