Sensitivity in plants

Question 1

Explain why a plant may have not flowered

Answer 1

• not enough time in dark
• not enough Pfr concerted to Pr
• Pfr levels remain too high
• once Pfr below a certain level, flowering occurs
• flowering stimulated by fall in Pfr

Question 2

What do the phytochrome convert into

Answer 2

• Pr is converted into Pfr in red light
• Pfr is converted into Pr in far red light
• Pfr slowly turns to Pr when in darkness

Question 3

What light does daylight contain

Answer 3

• red light

- Pr turns to Pfr

Question 4

What light stimulates germination

Answer 4

• red light stimulates germination
• far red light inhibits germination
• Pr turns to Pfr in red light, indicating higher levels of Pfr stimulate germination

Question 5

What phytochrome stimulates flowering in long day plants

Answer 5

• require Pfr

- plants flower in summer when nights are short so there is less time for Pfr to convert to Pr

Question 6

What phytochrome stimulates flowering in short day plants

Answer 6

• requires Pr

- plants flower in autumn when nights are long so there is more time for Pfr to convert to Pr

Question 7

Suggest benefits to plants being able to respond to changes in day length

Answer 7

• flowering occurs at the right time
• flowers when insects available
• seeds germinate at the right time
• day length changes to a sets pattern, short days in winter, long day in summer
• reliable compared to less regular stimuli, e.g. temperature

Question 8

What is the control of flowering

Answer 8

• a certain period of darkness is required to cause flowering
• it is enough stimulus if only part of the plant, e.g. leaf, is in the dark for the critical period
• leaf is a photoreceptor
• phytochrome in leaves
• signal must be passed to site of flower production from leaves

Question 9

Describe the mechanism that causes bending of shoots

Answer 9

• phototropism
• light causes auxin moves laterally away from light
• high concentration of auxin away from light
• auxin diffuses down shoot towards zone of elongation (auxin synthesised in meristem at tip of shoot)
• auxin stimulates cell elongation
• H+ pumps in membrane activated, H+ ions enter lowering pH, providing optimum pH for enzyme to break cellulose microfibrils, allowing cells to absorb water and expand

Question 10

How do auxins cause cell elongation

Answer 10

• auxin diffuses to zone of elongation
• H+ pumps in membrane activated
• H+ ions enter
• pH lowers
• optimum pH for enzymes to break down cellulose microfibrils
• as fewer cross links, cells can absorb water and expand
• cell wall flexible

Question 11

Compare response of shoot to light with hormonal coordination in animals

Answer 11

• both chemical
• both transported away from production site
• diffusion in plants, blood system in animals
• slower in plants, some animal hormones faster
• some animal hormones has a shorter term effect
• plant response only involves growth/cell elongation, animal hormones do not just effect growth
• comparison of stimuli, e.g….