Starter: - Mrs Jones A-Level Biology

Starter: - Mrs Jones A-Level Biology

Starter: How do plants co-ordinate their response to the environment? What are plants sensitive to? WHY? 1. Plants use chemicals/hormones to co-ordinate growth, development and responses to the environment 2. Light: direction, intensity, wavelength, length of

exposure; Gravity, Water, Temperature, Touch; Internal chemicals Why? To maximise survival/photosynthesis/reproduction. Homework 2 Prepare a table stating the functions of each of the plant growth regulators and how they are each used commercially Must be no more than an A3 page These HWs are for after the mocks..

What do we do after the Mocks?? Learning Outcomes Explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress. Plant Responses Even though most plants are firmly rooted in the ground they are capable of making adjustments to changes in their external environment:

Plants have evolved a wide range of responses to a large variety of stimuli, this helps them to To cope with changing conditions To maximise photosynthesis: more light, water, minerals Survive long enough to reproduce: to ensure germination in suitable conditions; pollination; seed dispersal etc. Avoid abiotic (non-living chemical and physical parts of the environment) stress Predation pressure. Avoid being eaten: herbivory (he act of eating plants and a herbivore is an animal that eats plants) /grazing

Plant responses are mainly manifested as changing patterns of growth: Can you give some changes in plants that manifest themselves as growth patterns? Plant responses can be: 1. Tropisms (we look at in detail): slower growth responses 2. Life cycle responses: seasonal changes in environment as cues for starting/ending a life cycle stage. Mediated by plant growth factors (phytochromes) Include: Flowering, photoperiodic responses, dormancy, germination and leaf fall

3. Rapid response to environmental stimuli: such as closing of stomata in water loss, opening and closing flowers in response to temperature, and nastic responses. Reduce plants exposure to abiotic stress or grazing pressure 4. Plant competition and Allelopathy: Can compete with other plants to gain access to resources. Some plants can produce chemicals that inhibit the growth of neighbouring plants (chemical inhibition is allelopathy). Plants compete for light, so can grow aggressively to shade out slow growing competitors 5. Plant responses to herbivory: Evolutionary adaptations to grazing/being eaten to enable them to survive constant cropping. Rapid growth in

grasses; sharp spines/thorns to deter browsers or toxins in leaf tissue (eucalyptos) Relatively sudden physiological changes (flowering) Steady growth responses in a directional fashion (tropisms) Seasonal changes (dormancy, germination, leaf fall) Responses to herbivory (evolutionary adaptations such as spines on cacti, rapid growth such as constantly grazed or mown grasses, toxins in leaves e.g. eucalyptus) Plant competition (aggressive growth to shade other plants, chemical inhibitors to prevent neighbouring plant growth)

Relatively rapid response (stomatal closure during low water availability, flower closure due to light e.g. crocus and tulip) Movements in a non- directional fashion (nastic movements; Mimosa leaf closure if touched but independent of stimulus direction) Sensitivity in plants A plants responses to the external environment are mainly growth responses Plants must respond to:

Light Gravity Water Chemicals Touch

Plants communicate by plant growth regulators. (hormones) Learning Outcomes Define the term tropism. Tropisms: plant directional growth responses to environmental stimulus. The direction f the response is determined by the direction of the stimulus. Explain how plant responses to environmental

changes are coordinated by hormones, with reference to responding to changes in light direction. Plant growth regulators Like human hormones, plant growth regulators are: Chemical messengers Transported away from their site of manufacture To act on other parts of the plant; target cells/tissues They are produced in a variety of tissues in the plant (hormones: endocrine glands)

At target cells they bind to specific receptors on plasma membrane, to ensure they only act on the correct tissues They can travel around the plant by: diffusion, active transport, transpiration, translocation in xylem/phloem Many different PGR: action can amplify each others effects (synergy); cancel out each others effects(antagonism). They can influence: cell division, cell elongation or cell differentiation Interaction of plant growth regulators

Synergism 2 or more act together to reinforce an effect Antagonism Have opposing actions and inhibit (diminish) each others effects. So Why plant growth regulators, not hormones?

Exert influence by affecting growth Produced in a region of plant structure by unspecialised cells Some are active at the site of production Not specific can have different effects on different tissues Quick exam question.. Plants coordinate their responses to environmental stimuli using hormones. Mammals also co-ordinate their responses to some stimuli using hormones.

State 3 differences in the ways in which plant and mammalian hormones operate 3 marks all points must show a clear comparison between mammals (M) and plants (P) 1 (M) made in endocrine glands versus (P) made in many plant tissues ; 2 (M) move in blood versus (P) move, in xylem / in phloem / from cell to cell ; ( accept diffusion/through plasmodesmata from cell to cell; accept translocation/

transpiration stream) 3 (M) act on, a few / specific / target, tissues versus (P) act on most tissues / can act in cells where produced ; 4 (M) act more rapidly ; ORA must be comparative such as faster in mammals Tropisms Slower responses resulting in directional growth is a directional growth response in which the direction of the response is determined by the direction of the external stimulus

Phototropism: movement of plants in response to light (photosynthesis). Shoots grow towards light=+ve phototrophic Geotropism: roots grow towards pull of gravity: anchors, minerals, water(photosynthesis, support, cool) =+ve geotrophic Chemotropism: on a flower pollen tubes grow down the style attracted by chemicals, towards ovary where fertilisation takes place. = +ve chemotropism Thigmotropism: (touch) shoots of climbing plants (ivy) wind around other plants/structures for support

Pollen tube growth http://www.bbc.co.uk/wales/eclips/pag es/eng_11to14_bio_greenplants_fertili zation.shtml Phototropism Phototropism is the response of plant organs to the direction of light. A shoot shows positive

phototropism A root negative phototropism Other plant movements: Nasties! Nastic Movements: Non-directional movements The movement of part of a plant in response to a stimulus, but the direction of the response is not

related to the direction of the stimulus Rapid reversible movement Response to touch/temperature changes Often brought about by changes in turgidity in cells examples Venus fly trap shutting Leaves closing Petals closing

Nastic Movements Can you think of a nastic movement made by marram grass? Describe the response and its adaptive value to the plant. Phototropism Explain how plant responses to environmental changes are coordinated by hormones, with

reference to responding to changes in light direction. Plant hormones and their effects HORMONE EFFECTS and site of production Auxins (IAA) Promote cell elongation; inhibit growth of side shoots i.e. responsible for apical dominance; inhibit leaf abscission; promotes development of cambium. Found in young leaves and buds.

Cytokinins Promote cell division. Developing tissue. Prevent senescence of leaves and fruit. Gibberellins Cell elongation and growth. Promote seed germination; growth of stems by mobilising food stores during germination. Abscisic acid Inhibits seed germination and growth; causes stomatal closure

during low water availability. Found in leaf chloroplasts. Ethene Promotes fruit ripening; promotes leaf fall. Found in old leaves. Auxin The most important auxin produced by plants is IAA indole-3-acetic acid It is a phytohormone (plant growth substance) It was the first discovered and most studied

How do plants grow? The cell wall in plants limits the cells ability to divide and expand, so Plants have special zones of growth: meristems New cells are produced and cells increase in size Meristems: closely packed, small, undifferentiated cells with thin cell walls and no large vacuoles

Apical meristem: zone of cell division Plant growth Plant growth occurs at meristems Apical meristem: tips of roots and shoots (longer) Lateral bud meristems: in buds: side shoots Lateral meristems: cylinder near outside of roots and shoots: wider Intercalary meristems: between nodes, where leaves, buds

branch off stems: shoot gets longer Cell division happens just behind the apex, then cell elongation behind that Just behind meristem is the zone of elongation Cells have thin primary cell walls Develop small vacuoles Absorb water by osmosis Large permanent cell vacuoles

form As absorb water increase in size: as walls are flexible Cell wall impregnated cellulose so no longer flexible How do plants grow? Darwins experiment.. In a phototrophic response, a shoot bends towards the light source

Experiments were carried out on cereal seedlings Darwins conclusions A growth stimulus is produced in the tip of the coleoptile Growth stimulus is transmitted to the zone of elongation Cells on the shaded side of the coleoptile elongate more than the cells on the other side.

Boysen-Jensens experiment Boysen-Jensens experiment Boysen-Jensens conclusions Materials which are not permeable to water can stop the curvature response in some circumstances

Materials which are permeable to water do not interfere with the curvature response Wents experiment Wents conclusions Wents conclusions Angle of curvature is related to the number of tips used Number of tips used relates to the concentration of

auxin in the agar block Curvature response is due to a chemical which moves from the tip and affects cell elongation What are you learning from these? Co-ordination in plants: 1. Something in tip is influencing the growth and curving towards light

2. Tip is producing a chemical that can diffuse in and out of agar block 3. In uni-directional light more of the chemical passes down the shaded side of the shoot Phototropin, auxin and phototropism What causes

phototropisms? A shoot bends towards light Because the cells elongate more on the shaded side Evidence from all the experiments suggested that a chemical: auxin, was transported to the shaded side Rate of elongation of the cells promoted Shoot bends towards light How light causes redistribution of auxin is unknown 2 enzymes identified: phototropin 1 and 2. Their activity is promoted by blue light: wavelength 400-450nm Lots of phototropin on the light side, less on dark

side=gradient which causes redistribution of auxin Phototropin, auxin and phototropism Phototropins Proteins that act as receptors for blue light In plasma membrane of certain cells in plant shoots Become phosphorylated when hit by blue light If light is directional, then the phototropin on the side receiving the light becomes phosphorylated.

Phototropin, auxin and phototropism Phosphorylation of phototropin brings about a sideways movement of auxin More auxin ends up on the shady side of the shoot than on the light side Involves transporter proteins in the plasma membranes of some cells in the shoot, these actively move auxin out of the cell

The presence of auxin stimulates cells to grow longer Where there is more auxin there is more growth Auxin action Auxin binds to receptors in plasma membranes of cells in the shoot. This affects the transport of ions through the cell membrane Build up of hydrogen ions in the cell walls The low pH activates enzymes that break cross-linkages

between molecules in cellulose walls Cell wall loosening Cell takes up water by osmosis, cell swell and become longer Permanent effect Investigating phototropism Read then: Complete the worksheet

1. A healthy coleoptile was decapitated and the tip was placed on a block of agar jelly. The two were then placed back on the cut end of the coleoptile as shown in Diagram 1. coleoptile tip agar block Diagram 1 asymmetric agar block

Diagram 2 After 5 hours the agar jelly was removed and placed asymmetrically, without the original coleoptile tip, on a different decapitated coleoptile as shown in Diagram 2. The whole experiment was conducted in total darkness. (a) Describe and explain the appearance of the coleoptile in Diagram 2 after 24 hours. (3) (b) (b) Explain how the plant growth substance illustrated in this experiment brings about the effects described in your answer to question 1.(a). (3 marks) TOTAL 6 marks

The coleoptile would bend over towards the right (1) because the plant growth substance, auxin, produced in the tip diffuses into the agar jelly.(1) When this is placed on the decapitated coleoptile the auxin diffuses down the left hand side of the coleoptile causing more growth on that side and hence a bending over towards the right hand side. (1)

EXAMINER: 1. (a) An excellent response covering all the main points expected. The student has addressed both the describe and explain parts of the question and has hence scored all 3 marks available. Auxin softens the cellulose microfibrils in the cell wall. The cell walls become less turgid and so take up water resulting in elongation of the cells. Since this occurs on the left hand side of the coleoptile this side elongates more than the right

hand side causing the coleoptile to bend towards the right. (3) Learning Outcomes: Shedding leaves Outline the role of hormones in leaf loss in deciduous plants. Leaf Abscission Abscission (from the Latin ab, meaning away, and scindere, meaning to cut) is the shedding of various parts of an

organism, such as a plant dropping a leaf, fruit, flower, or seed Trees in temperate countries shed their leaves in autumn. Survival advantage

Reduces water loss through leaf surfaces Avoids frost damage Avoid fungal infections through damp, cold leaf surfaces Plants have limited photosynthesis in winter Abscission and hormones Many different plant hormones control abscission Auxin Inhibits abscission Ethene (gas)

Increase in ethene production inhibits auxin production Abscisic Acid Inhibits growth (antagonistic to Giberillins and Auxin) stress hormone Control stomatal closure Plays a role in leaf abscission Cytokinins Stop leaves of deciduous trees from aging (senescing) by making sure the leaf is a

sink for phloem transport. If cytokinin production slows then the supply of nutrients slows and senescence begins This is usually followed by leaves being shed Stages in leaf abscission As leaves age, rate of auxin production at tip of leaf declines Cells in abscission zone are more sensitive to ethene production Drop in auxin causes an increase in ethene production

More ethene produced, inhibits auxin production This increases production of enzyme cellulose, which digests the cell walls in abscission zone Separates the petiole from the stem Leaf Abscission Abscission Layer

The abscission layer is made of thin-walled cells Weakened by enzymes that hydrolyse polysaccharides in their walls Layer is so weak that the petiole breaks Leaf falls off Tree grows a protective layer where the leaf will break off Cell walls contain suberin Leaves a scar which prevents the entry of pathogens

Auxins Synthesised in shoot or root tips. Most common form is IAA (indole-3-acetic acid a.k.a. indoleacetic acid) Main effects of auxins include:

Promote stem elongation Stimulate cell division Prevent leaf fall Maintain apical dominance. Learning outcomes Evaluate the experimental evidence for the role of

auxins in the control of apical dominance and gibberellin in the control of stem elongation. Auxins and Apical Dominance Auxins produced by the apical meristem Auxin travels down the stem by diffusion or active transport Inhibits the sideways growth from the lateral buds This is called apical dominance

Apical Dominance Apical Dominance Evidence for mechanism (1) If the tip is cut off of two shoots Indole-3-acetic-acid (IAA) is applied to one of them, it continues to show apical dominance The untreated shoot will branch out sideways

Evidence for mechanism (2) If a growing shoot is tipped upside down Apical dominance is prevented Lateral buds start to grow out sideways This supports the theory Auxins are transported downwards, and can not be transported upwards against gravity Direct causative link no more:

new Cutting shoot tip off a kidney bean plant increased auxin concentration in lateral buds So now scientists believe 2 hormones may be involved: Abscisic acid inhibits bud growth High conc of auxin in shoot keeps abscisic acid levels high in bud Remove tip, abscisic acid levels drop and buds grow Cytokinins also promote bud growth: apply cytokinin to buds can override the apical dominance effect. High concs of auxin make the shoot apex a sink for

cytokinins produced in the roots. When tip is removed the cytokinin spreads more easily around the plant promoting bud growth Gibberellins and stem elongation Gibberellin (GA) increases stem length Increases the lengths of the internodes Stimulating cell

division Stimulating cell elongation Evidence for GA and stem elongation Dwarf beans are dwarf because they lack the gene of producing GA Mendels short pea plants lacked the dominant allele that encodes for GA Plants with higher GA concentrations are taller

Gibberellins and stem elongation Gibberellins were identified (GA) Tested on a selection of plant varieties Applied to dwarf maize the plants grew taller Suggested that GA is responsible for stem growth BUT: because GA3 can cause stem elongation does not mean that it does so

in nature So an experiment needed to be set up to met the natural criteria: natural levels of GA, and in parts of plant it is normally found in! Gibberellins and stem elongation So an experiment needed to be set up to met the natural criteria: natural levels of GA, and in parts of plant it is normally

found in! HOW? Compared Ga1 concentrations of tall pea plants (homozygous Le Le) to dwarf plants (homozygous recessive le le) which were otherwise IDENTICAL Plants with higher concentrations of GA1 were taller To show GA1 directly caused stem growth: needed to know how GA1 was formed

Action of GA Worked out that: Le allele was responsible for producing the enzyme that converted GA20 to GA1 They then chose a pea plant with a mutation that blocks GA production between ent-kaurene and GA12aldehyde The plants produced no GA and grew to 1cm in height BUT if you graft a shoot onto a homozygous le plant which cannot convert GA20 to GA1, it grows tall The shoot has no GA20 of its own, but it does have the enzyme to convert GA20 to GA1, and it can use the

unused GA20 from the normal plant Because the shoot grew tall= GA1 causes stem elongation Action of GA GA causes growth at internodes by stimulating cell elongation (by loosening cell walls) Cell division (by stimulating the production of a protein that controls the cell cycle) Affects gene expression Moves through plasma membrane into cell

Binds to a receptor protein, which binds to other receptor proteins eventually breaking down DELLA protein. DELLA proteins bind to transcription factors If DELLA protein is broken down, transcription factor is released and transcription of the gene can begin In both plants and animals, chemical messengers help to transfer information from one part of the organism to another to achieve coordination. The table below lists some of these chemicals together

with their functions. Complete the table. 5 marks Name of chemical messenger ................................................................ insulin glucagon ................................................................ ................................................................

function controls water permeability of collecting ducts in kidney ................................................................ ................................................................ ................................................................ ................................................................ stimulates stomatal closure during water stress controls apical dominance

name of chemical messenger ADH/ anti diuretic hormone insulin glucagon ABA / abscisic acid auxin / IAA function controls water permeability of collecting

ducts in kidney reduces blood sugar levels / correct mechanism to achieve this increases blood sugar levels / correct mechanism to achieve this stimulates stomatal closure during water stress controls apical dominance

Exam questions All must do: Animals and plants respond to changes 6 marks Other questions Learning Outcomes Describe how plant hormones are used commercially. Commercial use of Auxins

Sprayed onto developing fruits to prevent abscission (fruit and leaf drop) Sprayed onto flowers to initiate fruit growth without fertilisation Parthenocarpy promotes the growth of seedless fruits Applied to the cut end of a shoot to stimulate root production Synthetic auxins are used as selective herbicides Commercial use of Ethene

Fruits harvested before they are ripe allows them to be transported without deteriorating, these are sprayed with ethene to promote ripening at the sale point. E.g. bananas from the Caribbean Commercial use of Gibberellin Sprayed onto fruit crops to promote growth Sprayed onto citrus trees to allow fruit to stay on the trees longer

Sprayed onto sugar cane to increase the yield of sucrose Used in brewing, where GA is sprayed onto barley seeds to make them germinate, amylase is produced, starch is broken down into maltose, the action of yeast on the maltose produces alcohol. Commercial use of cytokinins Delay leaf senescence can be sprayed on lettuce leaves to prevent them from yellowing Can be used in tissue culture to mass produce

plants

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