Coordination in Plants (Chapter 15)

Question 1

How does the Venus fly trap respond to the stimulation of its hairs before closing?

Answer 1

1) deflection of a sensory hair activates Ca2+ channels in cells at the base of the hair which open so that Ca2+ ions flow in to generate a receptor potential
2) if 2 of the hairs are stimulated within a period of 20-35s, or one hair is touched twice, action potentials travel across the trap

Question 2

How does the Venus fly trap close?

Answer 2

1) first, there is a release of elastic tension in the cell walls - however the trap is not completely closed
2) the prey inside the trap further stimulates the inner surface of the lobes, ∴ triggering further action potentials
3) this forces the edges of the lobes together, sealing the trap to form an external ‘stomach’ in which prey digestion occurs

Question 3

How does the Venus fly trap digest its prey?

Answer 3

1) further deflections of the sensory hairs stimulate the entry of Ca2+ ions into gland cells - these stimulate the exocytosis of vesicles containing digestive enzymes
2) the traps stay shut for up to a week for digestion to take place

Question 4

How does auxin promote elongation growth?

Answer 4

1) molecules of auxin bind to a receptor protein on the CSM OR auxin enters a cell by diffusion
2) the binding of auxin stimulates ATPase H+ pumps to move H+ ions across the CSM from the cytoplasm into the cell wall ∴ acidifying the cell wall
3) expansins (proteins in cell walls) are activated by the decrease in pH and loosen the linkages (H-bonds) between cellulose microfibrils and the matrix that surrounds them
4) the cells absorb water by osmosis and the pressure potential cause the wall to stretch, ∴ these cells become longer (elongate)

Question 5

How do expansins work?

Answer 5

1) expansins disrupt the non-covalent interactions between cellulose microfibrils and surrounding substances
2) this disruption occurs briefly so that microfibrils can move past each other, allowing the cell to expand without losing much of the overall strength of the wall

Question 6

How does seed germination occur?

Answer 6

1) the absorption of water at the beginning of germination stimulates the embryo to produce GAs
2) these diffuse to the aleurone layer and stimulate the cells to synthesise amylase
3) the amylase mobilises energy reserves by hydrolysing starch molecules in the endosperm, converting them into soluble maltose molecules
4) these are then converted into glucose and transported to the embryo, where they can be respired to produce energy as the embryo begins to grow e.g. for mitosis, allowing germination to occur

Question 7

How does GA cause the synthesis of amylase?

Answer 7

By regulating genes involved in the synthesis of amylase

1) application of GA causes an increase in transcription of mRNA coding for amylase
2) GA has this effect by causing the breakdown of DELLA proteins which inhibit the binding of PIF - a transcription factor
3) by causing the breakdown of DELLA, GA allows PIF to bind to its target promoter
4) transcription of the gene can then take place, resulting in an increase in amylase production

Question 8

How does GA promote stem elongation?

Answer 8

1) the dominant allele of a gene which partially controls tallness in pea plants (Le) regulates the synthesis of the last enzyme in a pathway that produces an active form of GA (GA1)
2) active GA stimulates cell division and cell elongation in the stem, causing the plant to grow tall

Question 9

What happens when there is a mutation of the gene with the Le/le alleles?

Answer 9

1) a substitution mutation in this gene gives rise to the recessive form of this gene (le) which causes a change in amino acid from alanine to threonine in the primary structure of the enzyme near its active site, producing a non-functional enzyme
2) homozygous plants (lele) are genetically dwarf as they do not have the active form of GA

Question 10

What does coordination in plants involve?

Answer 10

The use of electrical impulses for fast responses and hormones (plant growth regulators) for coordinating slower responses to stimuli

Question 11

What factors to plants respond to?

Answer 11

Gravity, light, water availability, changes in [CO2], grazing by animals and infection by fungi/bacteria

Question 12

How are some changes brought about in plants?

Answer 12

By quick changes in turgidity e.g. stomata responding to changes in humidity, [CO2] and water availability

Question 13

Describe the resting/action potentials in plants

Answer 13

• Plant cells have electrochemical gradients across their CSM and resting potentials (like animal cells)
• Plant action potentials are triggered when the membrane is depolarised
• The depolarisation results from an outflow of negatively charged Cl- ions (not influx of Na+)
• Repolarisation is achieved by the outflow of K+ ions

Question 14

How are action potentials transmitted in plants?

Answer 14

• Plants do not have specific nerve cells, but many of their cells transmit waves of electrical activity that are very similar to those transmitted along neurones of animals
• The action potentials travel along the cell membranes of plant cells and from cell to cell through plasmodesmata that are lined by cell membrane
• Action potentials normally last much longer and travel more slowly than in animal neurones

Question 15

What are triggers of action potentials in plants?

Answer 15

Chemicals coming into contact with a plant’s surface e.g. low pH of acid rain

Question 16

What is the Venus fly trap?

Answer 16

A carnivorous plant that obtains a supply of nitrogen compounds by trapping and digesting small animals, mostly insects

Question 17

Describe the structure of the Venus fly trap leaf

Answer 17

• The specialised leaf is divided into two lobes either side of a midrib
• The inside of each lobe is often red and has nectar-secreting glands around the edge to attract insects
• Each lob has 3 stiff sensory hairs that respond to being deflected
• The outer edges of the lobes have stiff hairs that interlock to trap the insect inside
• The surface of the lobes has many glands that secrete enzymes for the digestion of trapped insects

Question 18

Summarise the mechanism of action of the Venus fly trap

Answer 18

The touch of an insect on the sensory hairs on the inside of the folded leaves stimulates action potentials that travel very fast across the lead causing it to fold over and trap the insect

Question 19

What happens when the second trigger takes too long to occur after the first for the Venus fly trap?

Answer 19

The trap will not close and a new time interval starts

Question 20

What happens in the hair is deflected a third time?

Answer 20

The trap will still close

Question 21

What is the time between stimulus and response for the Venus fly trap?

Answer 21

0.5s

Question 22

How long does it take for the trap to close and trap the insect?

Answer 22

Less than 0.3s

Question 23

Describe the changes in the shape of the leaf of the Venus fly trap when it is open vs when it closes

Answer 23

Open - lobes of leaves bulge upwards and are convex in shape

Closed - lobes rapidly change into a concave shape, bending downwards so the trap snaps shut

Question 24

What is needed to seal the trap?

Answer 24

Ongoing activation of the trigger hairs by the trapped prey

Question 25

What happens once the insect is digested?

Answer 25

The cells on the upper surface of the midrib grow slowly so that leaf reopens and tension builds in the cell walls of the midrib so the trap is set again

Question 26

How are Venus fly traps adapted to avoid closing unnecessarily and wasting energy?

Answer 26

1) the stimulation of a single hair does not trigger closure - this prevents the traps closing when it trains or when a piece of debris falls into the trap
2) the gaps between the stiff hairs that form the ‘bars’ of the trap allow very small insects to crawl out as the plant would waste energy digesting a very small meal

Question 27

What is responsible for most communication in plants

Answer 27

Plant hormones/plant growth regulators i.e. chemicals

Question 28

Describe the features of plant growth regulators

Answer 28

• Unlike animal hormones, plant growth regulators are not produced in specialised cells within glands, but in a variety of tissues
• They move in the plant either directly from cell to cell (by diffusion or active transport) or are carried in the xylem or phloem sap
• Some don’t move far from the site of synthesis and have their effects on nearby cells

Question 29

What does ABA do?

Answer 29

Control the response of plants to environmental stresses e.g. shortage of water

Question 30

How do plant hormones work?

Answer 30

1) they interact with receptors on the surface of cells, in the cytoplasm or in the nucleus
2) these receptors usually initiates a series of chemical or ionic signals that amplify and transmit the signal within the cell

Question 31

What are auxins?

Answer 31

Plant hormones that influence many aspects of growth, including elongation growth, which determines the overall length of roots and shoots

Question 32

Where is auxin synthesised?

Answer 32

In the growing tips (meristems) of shoots and roots, where the cells are dividing

Question 33

How is auxin transported?

Answer 33

It is transported back down the shoot or up the root (from the tip) by active transport from cell to cell and to a lesser extent in phloem sap

Question 34

Where does growth in plants occur?

Answer 34

At meristems (those in shoot and root tips)

Question 35

What three stages does growth occur in?

Answer 35

1) cell division by mitosis
2) cell elongation by absorption of water (involves auxins)
3) cell differentiation

Question 36

What are Gibberellins (GAs)?

Answer 36

Plant growth regulators involved in seed germination and controlling stem elongation

Question 37

Where are GAs synthesised and found?

Answer 37

• Synthesised in most parts of plants
• Found in stems, where they have an important role in determining their growth
• Present in especially high concentrations in young leaves and seeds

Question 38

What happens when active GA is applied to plants which would normally remain short e.g. cabbages?

Answer 38

It can stimulate them to grow tall

Question 39

What happens when the seed is shed from the parent plant and why is this useful?

Answer 39

• The seed is in a state of dormancy - it contains very little water and is metabolically inactive
• This is useful because it allows the seed to survive in adverse conditions e.g. through a cold winter, only germinating when the temp rises in spring

Question 40

Describe the structure of a seed

Answer 40

• The seed contains an embryo, which will grow to form the plant when the seed germinates
• The embryo is surrounded by endosperm, which is an energy store containing starch
• On the outer edge of the endosperm is a protein-rich aleurone layer
• The whole seed is covered by a tough, waterproof, protective layer