Module 5.5 - Plant And Animal Responses - Plants

Question 1

Cytokinins

Answer 1

Promote cell division
Delay leaf senescence (loss of chlorophyll)
Overcome apical dominance
Promote cell expansion

Question 2

Abscisic acid

Answer 2

Inhibits seed germination and growth
Cause stomata to close in low water availability
Inhibit bud growth

Question 3

Auxins

Answer 3

Promote cell elongation
Inhibit growth of side shoots
Inhibit leaf abscission

Question 4

Gibberellins

Answer 4

Promote seed germination

Growth of stems

Question 5

Ethene

Answer 5

Causes leaf abscission

Causes fruit ripening

Question 6

Why do plants need to respond to their environment?

Answer 6

Avoid abiotic stress
Maximise photosynthesis by obtaining more light/water/minerals
Avoid herbivory/grazing
Ensure germination occurs in suitable conditions

Question 7

How do plants avoid abiotic stress?

Answer 7

Higher temperatures - thicker waxy layer
Very windy - more lignification
Drought - root growth slows, stomata close (abscisic acid)

Question 8

How can plants maximise photosynthesis?

Answer 8

Tropisms
Phototropism
Geotropism
Hydrotropism
Chemotropism
Thigmotropism

Question 9

How can plants avoid herbivory/grazing?

Answer 9

Thigmonasty - folding in response to touch
Chemical defences (e.g. tannins - make leaves taste bad and defend roots against pathogens; alkaloids - make tips of roots and shoots and flowers taste bitter; pheromones - can be produced when one leaf is eaten to communicate with other leaves to produce chemical defences)

Question 10

Process of plant hormones action

Answer 10

Made in many plant tissues and act on very wide range of target tissues
Move in xylem or phloem tissue by mass flow up and down the plant
Move by diffusion or active transport from cell to cell
Bind to complementary receptors on plasma membranes
Binding causes series of enzyme controlled reactions (sometimes causing genes to be switched on/off) to bring about the response

Question 11

Commercial uses of auxins

Answer 11

Plant cuttings dipped in rooting powder containing low concentrations of auxin to promote root growth
Weed killer - promotes rapid shoot growth - plant can’t support itself so falls and dies
Help make seedless fruits

Question 12

Commercial uses of cytokinins

Answer 12

Prevent yellowing of lettuce leaves (senescence)

Added in tissue culture to promote many side shoots - can be grown into new plants - higher production of new plants

Question 13

Commercial uses of gibberellins

Answer 13

Elongate internodal cells in grape stalks - grapes spread out and get bigger
Elongation of internodal cells in sugar cane - more sugar
Beer production needs malt - gibberellins added to encourage barley seed to make amylase so starch is broken down into maltose - germination then stopped by drying

Question 14

Commercial uses of ethene

Answer 14

Speed up ripening
Promote fruit abscission
Cold conditions, little oxygen and high carbon dioxide prevents ethene synthesis so can prevent fruit ripening during shipping

Question 15

Experimental evidence for the role of auxins in the control of apical dominance

Answer 15

Tip cut off plant caused lateral growth, implies auxins are found in tip and cause apical dominance
To prove that another hormone wasn’t made which promotes lateral growth, auxin transport inhibitors applied below the tip to stop auxins moving down the stem
Side shoots still grew even though tip wasn’t removed
Suggests that auxins cause apical dominance

Question 16

Experimental evidence for the role of gibberellins in the control of stem elongation

Answer 16

If GA is applied to a normally very short plant, the internodal regions of the stem grew, suggesting that gibberellins cause stem elongation
In naturally occurring plants, there are higher levels of GA found in tall plants than in dwarf plants of the same species, suggesting that gibberellins cause stem elongation

Question 17

Experimental evidence for the role of gibberellins in the control of seed germination

Answer 17

Application of gibberellins to a seed can cause early germination
Mutants lacking gibberellin can’t germinate on their own but can be induced to germinate by by applying gibberellins
Abscisic acid inhibits gibberellins - if abscisic acid is added during fruit ripening, seeds inside fruit will begin germinating during ripening

Question 18

Auxin mechanism for elongation

Answer 18

Auxins are produced at apex of shoot and diffuse down shoot to zone of elongation and bind to receptors on cells
Causes H+ ions to be actively transported into cell wall
Low pH causes wall loosening enzymes to work by breaking bonds with cellulose, making walls more flexible
Water enters cell and flexible wall allows cell to elongate

Question 19

Auxin mechanism for phototropism

Answer 19

Auxins produced at apex of shoot
If more light is coming from one side phototropin enzymes are activated more on this side
Phototropins cause PIN proteins to transport more auxins to shaded side
Auxins bind to receptors on cells on shaded side, causing H+ ions to be actively transported into cell walls
Low pH causes wall loosening enzymes to work by breaking bonds with cellulose, making walls more flexible
Water enters cells and flexible walls allow cells to elongate
Cells on shaded side of shoot elongate more quickly, causing shoot to bend towards light

Question 20

What did Darwin’s work on phototropisms show?

Answer 20

Tip was responsible for phototropism

Question 21

What did Boysen-Jensen’s work show about phototropisms?

Answer 21

Shows a substance responsible doe phototropism (auxins) must pass from tip down to cause response
Used gelatin (permeable) and mica (impermeable) blocks to separate tip from rest of shoot

Question 22

Auxin mechanism for geotropism

Answer 22

Auxins produced at apex of shoot
If roots are lying flat (e.g. seed has been deposited on side), auxin collects on lower side
Auxin inhibits cell elongation in roots so lower side cells do not elongate but upper side cells do so root bends downwards
Auxin promotes elongation in shoots so lower side elongates faster and shoots bend up