Flashcards in Coordination in Plants (Chapter 15) Deck (40)
How does the Venus fly trap respond to the stimulation of its hairs before closing?
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
How does the Venus fly trap close?
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
How does the Venus fly trap digest its prey?
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
How does auxin promote elongation growth?
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)
How do expansins work?
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
How does seed germination occur?
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
How does GA cause the synthesis of amylase?
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
How does GA promote stem elongation?
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
What happens when there is a mutation of the gene with the Le/le alleles?
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
What does coordination in plants involve?
The use of electrical impulses for fast responses and hormones (plant growth regulators) for coordinating slower responses to stimuli
What factors to plants respond to?
Gravity, light, water availability, changes in [CO2], grazing by animals and infection by fungi/bacteria
How are some changes brought about in plants?
By quick changes in turgidity e.g. stomata responding to changes in humidity, [CO2] and water availability
Describe the resting/action potentials in plants
- 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
How are action potentials transmitted in plants?
- 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
What are triggers of action potentials in plants?
Chemicals coming into contact with a plant's surface e.g. low pH of acid rain
What is the Venus fly trap?
A carnivorous plant that obtains a supply of nitrogen compounds by trapping and digesting small animals, mostly insects
Describe the structure of the Venus fly trap leaf
- 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
Summarise the mechanism of action of the Venus fly trap
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
What happens when the second trigger takes too long to occur after the first for the Venus fly trap?
The trap will not close and a new time interval starts
What happens in the hair is deflected a third time?
The trap will still close
What is the time between stimulus and response for the Venus fly trap?
How long does it take for the trap to close and trap the insect?
Less than 0.3s
Describe the changes in the shape of the leaf of the Venus fly trap when it is open vs when it closes
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
What is needed to seal the trap?
Ongoing activation of the trigger hairs by the trapped prey
What happens once the insect is digested?
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
How are Venus fly traps adapted to avoid closing unnecessarily and wasting energy?
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
What is responsible for most communication in plants
Plant hormones/plant growth regulators i.e. chemicals
Describe the features of plant growth regulators
- 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
What does ABA do?
Control the response of plants to environmental stresses e.g. shortage of water