9.1 Chemical control

Question 1

How does apical dominance occur?

Answer 1

• High level of auxins → Increased levels of abscisic acid → stops lateral buds growing
• High levels of auxins → Decrease the level of cytokinins → reduce bud growth

Question 2

How does IAA cause cell elongation?

Answer 2

• Cells in the shoot tip produce IAA
  IAA diffuses back down to the zone of elongation
• IAA molecules bind to specific receptor sites on the cell membrane activating the pumping of H+ ions into the cell wall spaces
• This changes the pH to around 5 which is optimum for enzymes which break bonds between the cellulose microfibrils
• This means the cell can absorb more water by osmosis causing the cell wall to stretch and elongate

Question 3

Why do shoots grow towards the light?

Answer 3

• When a shoot is exposed to light, the auxins tend to diffuse to the dark side of the shoot
• Greater concentration of auxins in the zone of elongation on the darker side
• Stimulates the cells to grow so the shoot bends and grow towards the light
• The transport then becomes asymmetric (Same on both sides of the shoots) and the shoot continues to grow towards the light

Answer 4

• Seed absorbs water and swells and activates the embryo
• Embryo secretes that diffuses to aleurone layer
• Gibberellin stimulates amylase production that diffuses into the endosperm and breaks down the food store/starch
• Enzymes produced in response to gibberellin digests the endosperm
• Products released from endosperm are used by embryo to make new cells germinate

Question 5

How do plants grow?

Answer 5

• Cell division takes place in meristems
• Meristem zones → Division, elongation, differentiation
• Meristems are found in the tips of roots/shoots
• Examples of plant hormones → Auxins, gibberellins, cytokinins

Question 6

Role of cytokinins

Answer 6

• Help to retain nutrients on the leaves
• In Autumn cytokinins drop, leaves die and fall of in autumn to lower water loss through transpiration

Question 7

Homeostasis

Answer 7

Maintaining a constant internal environment

Question 8

Changes in temperature and PH

Answer 8

• Channel proteins and Enzymes are effected (breaking bonds in the tertiary structure)
• Changes can cause a reduction in the rate of reaction (reduced kinetic energy) or denaturation

Question 9

Changes in the water potential of tissue fluid

Answer 9

• Can cause a change in volumes of cells due to osmosis - they can expand and shrink
• This us particularly influenced by glucose concentration

Question 10

Changes in glucose concentration

Answer 10

• Affects the supply of glucose for respiration

Question 11

How can these changes be managed?
(Temp, PH, tissue fluid and glucose conc)

Answer 11

• The Receptor detects change from the optimum eg. hypothalamus detects a change in temperature
• The coordinator links the receptor to the effector
• An effector is a Muscle or gland which brings changes to return system to optimum

Question 12

Negative feedback

Answer 12

• System deviates from the optimum
• Change is detected by the receptor
• A change is then produced which returns the system back closer to the optimum

Question 13

Positive feedback

Answer 13

• System deviates from the optimum
• Change is detected by the receptor
• A change is then produced which causes an even greater deviation from the optimum E.g. more sodium channels open in a membrane

Question 14

Explain the Importance of maintaining blood pH in homeostasis (4 marks)

Answer 14

• PH is optimum for enzymes
• (Too high/ too low) PH denatures enzymes/alters tertiary structure/alters the shape of active site/enzyme
• Substrate cannot bind
• Slower rate of reaction
• Fewer successful collisions/ Fewer enzyme substrate complexes formed