5.5 - Plant and Animal Responses

Question 1

Why do plants respond to stimuli?

Answer 1

To increase their chance of survival by responding to changes in their environment.

Question 2

Give an example of plants responding to a biotic stress.

Answer 2

Herbivory – being eaten by plants.

Question 3

Give an example of plants responding to an abiotic stress.

Answer 3

Anything that is non-living – drought, extreme cold.

Question 4

Describe 3 chemical defences of plants to herbivory.

Answer 4

Tannins – both taste bitter & make the plant hard to digest.
Alkaloids - taste bitter, noxious smell & poisonous characteristics.
Pheromones - signalling chemicals that produce a response in otherorganisms.

Question 5

Describe a physical defence of plants to herbivory & name a plant that does this.

Answer 5

Some plants fold up in response to being touched e.g. Mimosa pudica.

Question 6

Why do plants fold up in response to herbivory?

Answer 6

May knock off the insect eating them or scare the organism eating them.

Question 7

What is the name given to a plant responding to touch or contact?

Answer 7

Thigmotropism.

Question 8

What are tropisms?

Answer 8

A directional growth response to a stimulus.

Question 9

What is geotropism, phototropism & hydrotropism?

Answer 9

A directional growth response to gravity, light or water.

Question 10

Describe a positive and negative tropism.

Answer 10

Positive – grows towards the stimulus.

Negative – grows away from the stimulus.

Question 11

Where are plant hormones produced? Where do they act?

Answer 11

Produced in growing regions & move to regions where they are needed. Specific hormones have specific shapes that only bind to complementary receptors on the plasma membrane of target cells.

Question 12

Where does cell division take place in a plant?

Answer 12

Only in meristematic regions of the plant – tip of shoots & roots, cambium & pericycle.

Question 13

How does growth occur elsewhere in the plant? What is this called?

Answer 13

Growth occurs by increasing the size of the cell by making cell walls loose & stretchy & absorbing water into the vacuole. This is called cell elongation.

Question 14

What effect does Indoleacetic acid (IAA) have on the growth of shoots & roots?

Answer 14

Stimulates elongation in shoots. Inhibits elongation in roots.

Question 15

Where is IAA produced?

Answer 15

Meristematic tissue – tips of shoots and roots.

Question 16

How is IAA moved around the plant to control growth over short & long distances?

Answer 16

Over short distances - moves by diffusion & active transport.
Over long distances - moves via the phloem.

Question 17

Where does IAA move to in shoots in response to light?

Answer 17

To the shaded areas to stimulate elongation in these cells.

Question 18

Where does IAA move to in response to gravity?

Answer 18

It accumulates on the lower side & stimulates cell elongation in shoots or inhibits cell elongation in roots.

Question 19

What are the roles of plant hormones?

Answer 19

Role in: Leaf loss. Stimulated by: Ethene. Inhibited by: Auxins.
Role in: Seed germination. Stimulated by: Gibberellins. Inhibited by: Abscisic acid.
Role in: Stomatal closure. Stimulated by: Abscisic acid. Inhibited by: Not inhibited, only stimulated.
Role in: Apical dominance. Stimulated by: Auxins stimulate the growth of the apical bud. Inhibited by: Auxins inhibit the growth of side shoots from the lateral buds.
Role in: Stem elongation. Stimulated by: Gibberellins. Inhibited by: Not inhibited, only stimulated.

Question 20

What are the commercial uses of plant hormones?

Answer 20

Hormone: Ethene. Commercial use: Fruit ripening. Effect: Stimulates enzymes that - break down cell walls; break down chlorophyll; convert starch into sugars.
Hormone: IAA (auxin). Commercial use: Rooting powder. Effect: Stimulates cuttings to grow roots; lots of the same plant (cloning) can be grown quickly & cheaply.
Hormone: High levels of IAA. Commercial use: Selective weed killer. Effect: Stimulates the plant shoots of broad-leaved plants (dandelion) to grow so rapidly that it exhausts the plant & it dies. IAA has little effect on narrow leaved plants.

Question 21

Explain the control of leaf loss by plant hormones.

Answer 21

Ethene stimulates abscission cells (layer of cells at the bottom of the leaf stalk) to expand, breaking cell walls & cause the leaf to fall.
Auxins are produced by young leaves & inhibit leaf loss.

Question 22

Explain the control of seed germination by plant hormones.

Answer 22

Gibberellins are produced by the embryo. Gibberellic acid (GA) stimulates production of amylase (and other enzymes) by the aleurone layer. Amylase breaks down starch to maltose in the endosperm. Maltose is then broken down into glucose (by maltase) & used to provide energy for growing embryo.

Question 23

Explain the control stomatal closure by plant hormones.

Answer 23

Abscisic acid binds to receptor on guard cell membrane. The receptors activate a cascade of events that result in specific ion channels opening. Ions move out of the guard cell, raising the water potential. Water leaves guard cells by osmosis. Guard cells become flaccid & stomata close.

Question 24

What would trigger the stomata to close?

Answer 24

When there is too much water loss through transpiration or in response to detecting an invading pathogen.
Darkness / night time when no photosynthesis takes place so no need for gas exchange.

Question 25

What is apical dominance?

Answer 25

The growth of the main, central stem of the plant is dominant over (i.e., grows more strongly than) other side stems. Apical dominance prevents side shoots from growing.

Question 26

In what way does apical dominance help a plant to survive?

Answer 26

Allows a plant to grow very fast, past the smaller plants where there is competition for light. Saves energy as prevents the side shoots from the same plant competing with the shoot for light.

Question 27

Why do side shoots grow when you remove the apical bud?

Answer 27

The apical bud produces auxins. If removed side shoot growth will no longer be inhibited and start growing by cell division & elongation.

Question 28

Why do tall trees have large side shoots towards the base?

Answer 28

Auxins become less concentrated as they move away from the apical bud to the rest of the plant. At the base of the tree there is a very low auxin concentration and so side shoot growth is not inhibited.

Question 29

Why are dwarf varieties of plants short?

Answer 29

A mutation will result in a non-functional gene for gibberellin. Without this hormone, stem elongation cannot be stimulated.

Question 30

What happens when dwarf varieties of plant are treated with gibberellin?

Answer 30

Their stems elongate and they get taller.

Question 31

What is the autonomic nervous system?

Answer 31

Part of the nervous system responsible for controlling involuntary motor activities.

Question 32

What is the central nervous system?

Answer 32

Brain and spinal cord.

Question 33

What is the peripheral nervous system?

Answer 33

Sensory and motor neurons connecting receptors and effectors.

Question 34

What is the somatic nervous system?

Answer 34

Motor neurons under unconscious control.

Question 35

Describe the division of the nervous system.

Answer 35

CNS and PNS. PNS divided into sensory and motor systems. Motor system divided into somatic - neurones connecting muscles under conscious control, one neuron for each effector. Autonomic - neurones connecting muscles and glands under unconscious control - two or more neurones for each effector, neurones connected via ganglia.

Question 36

Describe the subdivision of the autonomic nervous system.

Answer 36

Sympathetic. Stress - fight, flight, flirt, prepares body for activity. Effects: Increase in heart rate; pupils dilate; ventilation increases; reduced digestion; orgasm. Noradrenalin as neurotransmitter.
Parasympathetic. Relax - energy conservation. Effects: decrease in heart rate; pupils constrict; ventilation rate decreases; increased digestion; arousal. Acetylcholine as neurotransmitter.

Question 37

Name the four main structures of the brains and outline their roles.

Answer 37

Cerebrum: Organises higher thought processes, conscious thought and memory, emotional responses.
Cerebellum: Coordinates movement and balance.
Hypothalamus and pituitary complex: Organises homeostatic responses.
Medulla oblongata: Coordinates autonomic responses.

Question 38

What is the cerebral cortex?

Answer 38

Outermost layer of cerebrum. Thin layer of nerve cell bodies. Subdivided into areas with specific responsibilities:
Sensory - size and complexity linked to sensitivity of receptors.
Association - compares inputs with experiences to make judgements.
Motor - sends action potentials to effectors, size and complexity linked to the complexity of movements/parts of body; left hand side of cortex coordinates right hand side of body and vice versa.

Question 39

What is the cerebellum?

Answer 39

Contains over half of all neurones in the brain. Coordinates balance and movement. Receives and interprets from many receptors including retina, inner ear and muscle spindle fibres.

Question 40

How do the cerebral cortex and cerebellum coordinate movement?

Answer 40

Cerebral cortex processes conscious decision to contract muscle. Cerebellum coordinates complex responses. Connected by the pons. Coordination and balance require practice; skills are learnt. Programming of coordinated response strengthened through practice and is automatic.

Question 41

What is the role of the hypothalamus?

Answer 41

Hypothalamus controls homeostatic responses. Temperature regulation. Osmoregulation. Contains sensory receptors. Acts by negative feedback.

Question 42

What is the role of the pituitary gland?

Answer 42

Acts in conjunction with the hypothalamus. Posterior lobe linked to hypothalamus by neurosecretory cells. Hormones made in hypothalamus pass through neurosecretory cells to pituitary gland. Anterior lobe produces hormones which are released into blood by releasing factors.

Question 43

What is the role of the medulla oblongata?

Answer 43

Controls non-skeletal muscle. Cardiac, smooth muscle. Regulates vital processes. Cardiac centre - heart rate. Vasomotor centre - circulation and blood pressure. Respiratory centre - rate and depth of breathing.

Question 44

What is a reflex action?

Answer 44

A response to a change in environment that does not involve brain processing. The brain is informed of the reflex but does not coordinate it. Important for survival.

Question 45

Outline the blink reflex.

Answer 45

Cranial reflex - passes through the brain but does not involve thought processes. Receptor and effector in same place - reflex arc. Stimulated by: Object in eye, bright light, loud sounds, sudden movement close to eye.

Question 46

Outline the corneal reflex.

Answer 46

Contact with cornea distorts receptors. Sensory neurone enters via pons. Synapse connects sensory neurone to relay neurone, action potential passed to motor neurone. Eyelid blinks. Rapid (0.1s).

Question 47

How is the corneal reflex overridden?

Answer 47

Myelinate neurone in pons pass action potential from sensory neurone to cerebral cortex. Thought processes can override reflex via inhibitory action potentials.

Question 48

Outline the knee jerk reflex.

Answer 48

Spinal reflex, coordination of balance. Muscle in quadriceps (front of thigh) attaches to patella tendon. Patella tendon stretched over knee cap and connects lower leg bones to knee. When patella tendon is stretched spindle fibres detect increase in muscle length. Unexpected stretch causes reflex contraction of quadriceps. Leg straightens, balance regained.
Note reflex has no relay neurone, no synapse so quicker response, reflex cannot be inhibited.
Inhibitory action potentials are sent to synapse in the reflex arc to prevent contraction of opposing muscles during walking and running.