5.1.5 Plant Responses Flashcards
Define cell signalling
Communication between cells that allow cells to work together to trigger a response or reaction inside the cell
Why do plants respond to their environment?
- To avoid abiotic stress
- To maximise photosynthesis by obtaining more light + water
- To avoid herbivores
- To ensure germination in suitable conditions, pollination, seed dispersal
What are plant hormones and how do they work
Chemical messengers
Produced in one part of a plant but have effects in the other
Affect activity of target tissue
Have long-lasting and widespread effects
How are target cells able to be affected by hormones?
They contain specific receptors on their membranes with a complementary shape to the hormones
Similarities between plant + animal hormones
- Both bind to specific + complementary shaped receptors, causing enzyme reactions insde cells
- Act on several target tisseus
- Only need to be present in small concentrations to have an effect
- May involve switching genes on/off
Differences between plant + animal hormones
- Produced in endocrine glands in animals; produced in many tissues in plants
- Move in blood in animals; move in xylem + phloem or from cell to cell in plants
- Act on few + specific target tissues in animals, act on most tissues + cells where they are produced in plants
What do antagonism + synergism mean in relation to hormones?
Antagonism: Cancel each other out/ have opposite effects
Synergism: Amplify each other/ work together
Where are food stores in dicots + monocots?
Dicots: 2xCottyledons
Monocots: Endosprem
Explain the steps of seed germination (And the use of germination)
- Seed starts germinating by absorbing water
- Water activates embryo to produce gibberelin
- Gibberelin activates genes to make enzymes that hydrolyse good stores for use
- Enzymes include protease/amylase/maltase
- Food used to respire make ATP for building shoots + roots
What is antagonistic to gibberelins?
ABA (abscisic acid)
What part of the plant do gibberelins affect?
The length of internodes (region between leaves) + the stem
What are meristems?
Group of unspecialised cells that can divide + differentiate
What are apical meristems?
Tips of roots + shoots making them grow longer
What are lateral bud meristems?
Buds that could form a side shoot (If apical MS removed)
What are lateral meristems?
Circular band makes roots + shoots grow wider
What are intercalary meristems?
Between nodes, helps shoots get longer
Where are auxins made?
Tips of shoots + tips of roots
Where do auxins move from shoots + roots tips?
Down from shoot tip
Up from root tip
How do auxins move around the plant?
From cell to cell using diffusion or active transport
Can move in transport tissue (xylem + phloem)
How do auxins cause cell elongation
- Made in shoot tip + travel down by diffusion or active transport to zones of elongation
- Binds to specific receptors on cell membrane
- Cause active transport of H+ by ATPase enzyme into cell wall
- Low pH optimal for expansion enzymes that loosen cell wall
- Expansion enzymes break hydrogen bonds between micro fibrils present in cellulose
- Allows for cell elongation
What is cell elongation proportional to?
Auxin concentration
How do auxins cause positive phototropism?
- Auxin produced in tip - moves down via diffusion or active transport
- Light causes movement of auxin from light side to shaded side
- Higher auxin concentration in shaded size causes cell wall loosening, so greater elongation on shaded side
- Shoot bends towards the light
How do auxins cause positive geotropism?
- Auxin produced by root tip
- Gravity causes higher concentration of auxin at lowest point
- Less auxin concentration at atop of root
- Causes cell elongation/ growth
- Shoot points downwards
What is apical dominance?
When high levels of auxins supress growth of lateral shoots
How does apical dominance occur?
- Auxin produced in apical bud
- Growth in main shoot stimulated by auxin made in the tip
- Hormone inhibits growth of lateral shoots
- Down the stem auxin concentration decreases
- Lower down stem = more lateral buds
What are the effects of removing the apical shoot/auxin producing cells?
- Auxin production stops
- Apical dominance stopped
- Lateral bud develops
- Side shoots grow
- Plants become bushy
Define tropism
Directional growth of plants determined by the direction of an external stimulus (abiotic or biotic)
What is the point of phototropism + geotropisn?
Phototropism: Shoots point towards light to enable photosynthesis
Geotropism: Roots grow downwards to find minerals water + to anchor into soil
Define hydrotropism
Roots grow towards water
Define chemotropism
Growth of pollen tubes towards ovary
Define thigmotropism
Growth towards solid structures to gain support
Why do plants grow more rapidly in the dark?
- To reach light to photosynthesise
- Auxin not destroyed by light (More present in the dark)
- Moves down from shoot tip causing cell elongation
Describe two practical investigations to investigate geotropism in roots and shoots?
In shoots
- Rotating drum used (clinostat)
- Plants grow so gravitational stimulus is applied evenly to all sides of plant
- Shoot grows straight
In Roots
- Seeds placed in petri dishes
- Petri dishes stuck to wall + rotated at 90degree intervals
- Geotropism seen
List abiotic factors for plants
- Changes in day length
- Cold + heat
- Lack of water
- Excess water
- High winds
- Change in salt concentration
Why do deciduous trees loose their leaves in winter?
- Winter causes sunlight decrease + less heat energy available
- Less photosynthesis occurs
- Amount of glucose required to maintain leaves + produce chemicals from chlorophyll to prevent freezing > than amount of glucose created by photosynthesis
- also less likely to blow over in the wind
What is plant senescence and what does it cause?
Ageing in plants
- Chlorophyll degradation reveals carotenoids underneath
- Cause of autumn leaf colour
- Followed by leaves sheding (abscission)
Explain the process of abscission
- Cytokinins inhibit senescing by making the leaf a sink for phloem nutrients
- Cytokinin production drops in Autumn so senescing begins
- Auxins support cells in the abscission cells normally; senescing reduces auxin production
- Low auxins increases production of Ethene
- Ethene causes increase in cellulase enzymes to break down cell walls in abscission zone
- Petiole + stem seperate
What protects the plant when leaves fall?
Fatty material deposited on cells either side of separation layer, forming a protective scar, preventing entry of pathogens
How is freezing prevented in plants?
- Cytoplasm contains vascuolar sap containing solutes
- Solutes lower freezing point of water (like salt on roads)
- Polysaccharides, amino acids, sugars, proteins act a ‘anti-freeze’
- Decrease in temp + reduction in day length cause genes responsible for chemicals that make plants frost resistant to activate
- ABA known stimulates cold protective responses
How does stomata close + open?
- Opens in light, closes in dark
- Large vacuoles take up water, turning them turgid
- Thick inner wall + thin outer walls; forcing them to bend outwards + open
Why are stomata opened + closed?
Opened: Cools plant as water evaporates from cells in leaves in transpiration
Close: Conserves water
What hormone controls stomata size
ABA hormone produced by leaf cells due to abiotic stress
How does ABA close stomata?
- Soil water levels fall, causing the release of ABA
- ABA transported to leaves
- ABA binds to specific shaped receptors on plasma membranes of guard cells
- Causes calcium ion channels to open and calcium ions to enter guard cells
- K+, NO3-, Cl- ions leave guard cell, increasing water potential of guard cells
- Water leaves guard cell to surrounding cell by osmosis so they become less flaccid, closing the stomata
List physical defences of plants
- Thorns
- Barbs
- Spikes
- Spiny leaves
- Fibrous + inedible tissues
- Hairy leaves
- Stings
What are tannins?
Part of a group of chemicals called phenols
Bitter taste puts animals off eating leaves
Toxic to insects - bind to enzymes in saliva
Tea + red wine rich in enzymes
What are alkaloids?
Bitter tasting, nitrogenous compounds
Many act as drugs (caffeine, nictoine, morphine, cocaine)
Caffeine is toxic to fungi + insects
Nicotine is a toxin made by tobacco plant roots
What are terpenoids?
Form essential oils
Toxins to insects + fungi
Pyrethrin produce by chrysanthemums interferes with insect nervous system
Citronella produce by lemon grass, repel insects
What are pheremones
A chemical made by one organism which affects the social behaviour of other members of the same species
(Plants don’t rely on these much)
What are volatile organic compounds (VOCs)
Act like pheromones between themselves and other organisms, particularly inspects
Usually happens when the plant detects an attack by an insect - happens through chemicals in the insects saliva
How do some plants respond when touched?
Fold down and collapse, frightening herbivores + dislodges small insects
What are climacteric fruits?
Fruits that continue to ripen once picked
How is Ethene used in fruit transport?
Fruit picked when not ripe as it is hard so less easily damaged in transport - Ethene then applied to ripen fruit
How can auxin be used for micropropagation?
Causes production of roots
Makes i easier to make new plants from plant cuttiangs
Easier to develop cuttins to sell + for plant growers to take their own cuttings
Essential for micropropagation
How can dicot auxins be used as weedkiller?
Affects metabolism, so grwoth rate is unsustainable so they die
What are the advantages of using auxins as weedkillers?
Simple, cheap to make, low toxicity to mammals + are selective
Commerical uses of Auxin
- Prevents leaf + fruit abscision
- High conc promotes fruit drop
- Promotes flowering
- Makes unpollinated flowers make seedless fruit
- Increased fruit size
- Artifical auxin used as herbicides
Commerical uses of Cytokinins
- Delays leaf senescence
- Promotes bud + shoot growth in cuttings
- Large no gorwing buds = more plant curttings = more plants
Commerical uses of Gibberellins
- Delay senescence in citrus fruit
- Stalk grwoth of grapes so less comapct + bigger grapes
- Speed up starch hydrolysis by amylase to maltose in barley for beer
- Sugar cane stem frwoth at node
- MS for greater sugar producction
- Increase seed production
Commercial use of Ethene
- Speeds up fruit ripening
- Promotes fruit + leaf abscision
- Promotes female sex in cucumbers
- Prootes lateral bud growth in some plants
- Slows fruit ripening
- Silver salts inhibit ethene production for flowers shelf life
