Plant Responses Flashcards
Explain why plants need to be able to respond to their environment
- To cope with changing conditions
- Avoid abiotic stress
- To maximise photosynthesis or to obtain more light / water / minerals
- Avoid herbivory
- To ensure germination in suitable conditions
Give examples of abiotic stresses
- Change of day length
- Temperature
- Water levels
- Wind
- pH
Summaries the physiological adaptations plants show to cope with abiotic stress
- Thick waxy cuticles
- Fine hairs on leaves
- Sunken stomata
- Wilting in hot, dry conditions
Name the type of plant that keeps their leaves all year round
Coniferous
Name the type of plant that lose their leaves in winter
Deciduous
Explain why deciduous trees lose their leaves in winter
- Rate of photosynthesis decreases as day length reduces and temperatures fall
- Amount of glucose produced by photosynthesis decreases
- Amount of glucose needed increases
- Needed for respiration to maintain leaves through winter and produce chemicals to
prevent damage from freezing - More efficient to lose leaves and become dormant until days lengthen and temperatures
increase in spring
What is photoperiodism?
Response of plant to lack of light (length of period of darkness)
What are phytochromes?
- Pigments in leaves
- Detect light levels
Describe the different types of phytochromes
Pr (inactive form)
- Pfr (active form)
Give examples of plant responses affected by length of darkness
- Dormancy of leaf bud
- Timing of flowering
- Tuber formation in preparation for winter
Define abscission
- Leaf fall
- Occurs in deciduous trees in autumn
Which plant hormones are involved in abscission?
- Auxin
- Ethene
Describe the process of abscission in deciduous trees
- Triggered by falling light levels
- Decreased concentration of auxin
- Leaves produce ethene
- Initiates gene switching in abscission zone at base of leaf stalk
- Gene switching causes production of new enzymes
- Enzymes weaken cell walls in outer layer of abscission zone (‘separation layer’)
- Vascular bundles sealed off on stem side of separation layer
- Layer forms protective scar when leaf falls preventing pathogen entry
- Cells in separation zone retain water and swell
- Puts more strain on outer layer
- Further abiotic factors finish process (e.g. strong wind)
- Strain too much and leaf separates from plant leaving waterproof scar
Explain how plant hormones protect plant cells in freezing conditions
- Hormones (e.g. abscisic acid) trigger gene switching
- Plants produce more sugars and proteins
- Lower the freezing point of the cytoplasm
- Protect cells against damage by ice crystals if they do freeze
How else can plants protect themselves from freezing?
- Water in intercellular spaces freezes
- Energy released raises temperature of cells
- Solute concentration in cytoplasm and vacuoles maintains lower freezing point
How do stomata respond to abiotic stress?
They close
Name the hormone responsible for the closure of stomata
Abscisic acid (ABA)
Explain how ABA causes stomatal closure
- ABA produced by roots
- In response to low water levels
- ABA transported to leaves
- Binds to receptors on plasma membrane of guard cells
- Causes ions to diffuse out guard cell
- Water follows by osmosis
- Guard cells become less turgid
- Change in shape leads to closure of stomata
Define herbivory
Process by which herbivores eat plants
Outline the physical defences some plants have against herbivory
- Thorns
- Spiny leaves
- Fibrous, inedible tissue
- Stings
- Hairy leaves
Summarise the various chemical defences plants have against herbivory
- Tannins
- Have bitter taste
- Toxic to insects
- Alkaloids
- Affect metabolism of animals, acting as poison
- Prevent germination in plants
- Terpenoids
- Act as toxin to insects and fungi
How can plant chemical defences be used by humans?
- Tannins
- Flavour tea and red wine
- Alkaloids
- Used in coffee (caffeine)
- Contained in cigarettes (nicotine)
- Terpenoids
- Insect repellent
Define pheromone
- Chemical made by an organism
- Affects social behaviour of other members of same species
Why is it necessary for pheromones to be volatile?
Must travel through the air to carry message
Give an example of how plants use pheromones to defend themselves
- Maple trees produce pheromone when attacked by insects
- Absorbed by leaves on other branches and nearby trees
- Leaves produce protective chemicals, e.g. callose
Define volatile organic compound (VOC)
- Chemical made by an organism
- Affects social behaviour of other species
Outline how some plants use VOCs to defend themselves
- Apple tree attacked by spider mites produce VOCs that attract predatory mites
- Predatory mites destroy spider mites attacking tree
- Wheat seedlings produce VOCs when attacked by aphids
- VOCs repel other aphids from the plant
Define meristems
- Regions of small, undifferentiated (totipotent) cells
- Continue to divide and grow throughout lifetime of plant
Where are apical meristems found in plants?
- Tip of root
- Tip of stem
- Cause lengthening of plant
- Produces new leaves and flower
Where are lateral meristems found in plants?
Cambium (within the stem)
- Cause widening of plant
- Produces bark on trees
List organs found in plants
- Roots
- Stems
- Flowers
- Bulb
What does a shoot consist of?
- Stems
- Leaves
Define tropism
A plant’s directional response to a stimulus
Give three different tropisms
- Geotropism - response to gravity
- Phototropism - response to light
- Thigmotropism - response to touch (e.g. climbing plants)
Define apical dominance
- Plant growth primarily occurs at tip of shoot
- Horizontal growth (e.g. branches, leaf stems) inhibited
How does auxin stimulate cell elongation in shoots?
- Auxin activates proton pumps in plasma membrane
- Causes H+ ions to be pumped from cytoplasm to cell wall
- Low pH breaks bonds in cell wall fibres, loosening them
- Provides optimum pH for enzymes that keep cell wall elastic
- Alters gene expression patterns to increase growth rate
- Inhibits lateral growth
- Growth of horizontal branches/shoots prevented
Outline how the stem of a plant may bend towards the light
- Auxins (plant hormones) concentrate on the side furthest from light
- Causes cell lengthening
- Stem bends towards the light
Explain auxin’s role in phototropism
- Positive phototropism is growth towards light
- Auxin is a plant hormone
- Produced by shoot tip (apex)
- Diffuses down shoot on shaded side of stem
- Causes active transport of H+ from cytoplasm to cell wall
- Decrease in pH breaks bonds between cell wall fibres
- Softens cell walls
- Provides optimum pH for enzymes that keep cell wall elastic
- Gene expression altered by auxin to promote cell growth
- Causes cells on darker side to elongate
- Shoot bends towards light
What effect does auxin have in roots?
- Accumulates on lower side of plant in response to gravity (geotropism)
- Inhibits cell elongation
- High concentrations limit growth
- Lower side of root grows slower and roots turn downwards
Describe the ways in which a plant responds to overcrowding by other plants
- Positive phototropism
- Auxin produced at shoot tip
- Shoots bend towards light
- Plants grow taller
- Positive thigmotropism
- Climbing plants climb other plants
- Negative gravitropism
- Grow roots towards water and minerals
Suggest how hormones alter a plant’s growth if the top of the plant shoot is removed - Less auxin produced
- Apical dominance stopped
- Lateral buds develop
- Plant becomes bushy
Suggest how hormones alter a plant’s growth if the top of the plant shoot is removed
- Less auxin produced
- Apical dominance stopped
- Lateral buds develop
- Plant becomes bushy
Explain why most plants have more lateral shoots lower down the stem
- Auxin primarily produced at shoot tip
- High concentration of auxin inhibits lateral growth
- Causes apical dominance
- Auxin diffuses down stem
- Concentration of auxin lower towards bottom of stem
- Lateral growth not inhibited
- Lateral shoots grow more
Why is it beneficial for a plant to grow towards the light?
Maximises amount of photosynthesis
Define internode
Region between leaves on a stem
Explain how plants are able to grow in the dark
- Growth caused by gibberellin plant hormone
- Causes rapid cell elongation of internodes
- Results in fast upwards growth in dark conditions
Why is it important for plants to grow in the dark?
- Seedlings need to break through soil
- Compete with other seedlings and plants for light
Describe the structure of a seed
- Testa - seed coat that protects embryonic plant
- Cotyledon - food store for seed, forms embryonic leaves
- Plumule - embryonic shoot
- Radicle - embryonic root
Outline how and where energy is stored in plants
- Glucose from photosynthesis stored as starch
- Starch stored in chloroplasts and seeds/tubers
- Lipids stored in seeds
Describe the process of germination
- Food source used up
- Radicle grows down
- Shoot grows up
- Plant starts to photosynthesise
What conditions are needed for germination to occur? Use WOW to help you remember!
- Water - rehydrates the seed and activates metabolic processes
- Oxygen - required for aerobic respiration as seed germinates
- Warmth - for enzyme activity
- Each seed type has specific temperature requirements to ensure seeds germinate
at the correct time of year
What are gibberellins?
- Plant hormones
- Required for germination
- Stimulate production of amylase and protease enzymes
Outline the metabolic processes that occur in starchy seeds during germination
- Water absorbed by the seed activates metabolism
- Gibberellin synthesised
- Stimulates production of amylase
- Amylase digests starch (stored in cotyledon) to maltose
- Maltose converted to glucose by maltase
- Glucose used in aerobic respiration
- Glucose used in synthesis of cellulose
How can the rate of germination be measured?
Measure rate of seed growth over a set period of time
Outline the experimental evidence for the role of gibberellins in seed germination
- Mutant seeds that cannot produce gibberellins created
- Seeds do not germinate
- If gibberellins applied artificially, seed germinates
- Gibberellin synthesis inhibitors applied to seed
- Seed cannot produce gibberellins
- Seed does not germinate
- If inhibitor removed, seeds germinate
Describe the use of auxins in industry
- Rooting powder
- Selective weedkillers
- Development of seedless fruit
Describe the use of giberellins in industry
- Delays fruit ripening
- Increases fruit size
- Speeds up the brewing process
Describe the use of ethene in industry
- Controlled ripening
- Fruit dropping
- Leaf fall
Describe the use of cytokinins in industry
- Prevent ageing of ripened fruit
- Control tissue development in micropropagation
How are auxins used in propagation?
- Added to rooting powder
- Presence of auxin stimulates cut shoots to form roots
- Increases likelihood of successful propagation
How are auxins used as weedkillers?
- Artificial auxins created that are specific for weed plants
- Not taken up by crop plants
- Cause unsustainable growth in weeds
- Weeds die
Outline why both plants and animals need to be able to respond to changes in their environment
- To avoid abiotic stress
- To avoid being eaten
- To access resources
