- Created by: MillieJohnston
- Created on: 20-11-15 03:52
- includes all factors including biotic and abiotic that affect the lives of an organism
- physical factors that can act as a stimuli for an organism's response.
Light - photo
Gravity - geo
Temperature - thermo
Water - hydro
Current - rheo
Chemicals - chemo
Touch - thigmo
Most organism's try to keep within a narrow range to keep within the optimum rangeof TOLERANCE. Factor too extreme - physcological stress or death may result.
- way organism's respond to each other and other species.
Intraspecific relationships (within species)
Interspecific relationships (between different species)
Organism's Response to Environment
- organism's response to signals from it's environment = behaviour.
- can be innate (genetic base) or learnt (from experience).
- For an organism to be successful, they must SURVIVE and REPRODUCE to contribute their alleles to the gene pool.
- To do this, an organism must be able to:
1. find favourable & avoid unfavourable conditions.
2. ensure supplies of nutrients, energy, water and oxygen.
3. reduce competition.
4. avoid predators or reduce herbivory.
5. find a mate of the same species.
- the way an organism has adapted to the habitat it lives in.
- combination of where and how it there i.e. adaptations (structural, behavioural, physiological and life cycle).
- includes role that an organism performs in its natural community.
- the niche an organism would occupy if all necessary environmental conditions were present.
- the limits to the fundamental niche are set by the limits of the organisms physiological tolerances to the abiotic factors.
- the actual niche that an organism occupies. Not as extensive as the fundamental niche.
- boundaries are set typically by the biotic factors.
Biological Timing Responses of Animals
- there are many regular rhythms in our world and living things must be adapted to these to survive.
These rhythms include:
- Day and night - caused by the rotation of the Earth on its axis.
- Seasons - caused by the tilt of the Earth as it travels around the sun.
- Months - caused by the orbit of the Moon around the Earth.
- Tides - caused by the action of the Sun and the Moon on the oceans of the Earth.
- to be able to predict and prepare for environmental changes that result from the Earth's cycles, animals must have some form of internal biological clock.
- a biological clock is an internal timing system that continues to operate even without external time cues.
- helps to control the timing of plant and animal activities.
Biological Clock Uses
- Controlling daily rhythms of the body - sleep, pulse rate, blood pressure, body temperature, blood cell count, urine composition, metabolic rate and sex drive.
- Reproduction timing - animals come to heat, carry out courtship rituals or release egg and sperm at the same time.
- Migration preparation - laying down fat reserves, etc.
- Winter preparation - storing food, increasing thickness of fur, changing colour of fur, hibernating.
- Navigation - by the sun or the stars.
The biological clock in verterbraes in located in the brain.
- consists of an area called the SCN (SupreChiasmic Nuclei) located in the hypothalamus.
- the SCN controls all of the body's daily or circadian rhythms, such as walking and sleeping.
For sleep: in the morning, light entering the eye is detected by the SCN which sends a message to the pineal gland. This causes the pineal gland to stop producing the hormone melatonin, which makes you wake up. At night, the absence of light causes the SCN to signal the pineal gland to PRODUCE melatonin, which induces sleep.
- the SCN is the body's master clock.
- it responds to the cycle of day and night and synchronises cellular clocks found in all of the different tissues throughout the body.
- these cellular clocks are genetically controlled by genes that switch on and off over a 24 hour period.
- an endogenous rhythm is a cyclical body process that is controlled by an internal biological clock.
- endogenous rhythms run for a time period that is APPROXIMATELY equal to the Earth's natural cycles.
A rhythm is usually endogenous if one of the following apply:
1. the rhythm is not exactly the same length as a natural Earth rhythm i.e. does not exactly match the daylength.
2. When an organism is kept in constant conditions and the the rhythm persists although the period may change.
3. When an organism is moved from one part of the world to another, the rhythm will still persist but will gradually change to match the conditions of the new loaction.
- an exogenous rhythm is a rhythm that is controlled only by an external stimulus.
- it DOES NOT involve a biological clock and it stops when the stimulus stops.
- circadian rhythms are daily rhythms that have a period of approx. 24 hours.
They determine when animals are active:
Diurnal - active during the day
Nocturnal - active at night
Crepuscular - active at dawn and dusk
Arrhythmic - no regular pattern
- they allow animals to predict daybreak so they can begin to search for food and therefore maximise feeding time and hence their success.
- a rhythm that is synchronised with the ebb and flow of the tides.
- has a period of 12.4 hours, tidal activity is a zeitgeber.
- circatidal rhythms allow an animal to predict the changing tide so that they can prepare to feed or avoid predation.
- crabs and many other intertidal animals are active when covered by the high tide.
- some species are active at low time, e.g. sandhoppers.
- these rhythms are strongly endogenous and will continue to persist for several weeks after an animal has been removed from the shore.
- behaviour synchronised with phases of the moon.
- has an approximate period of 29.5 days.
- zeitgeber is usually the light of the moon.
- allows animals to predict spring high tides of the full moon to maximise their chances of successful reproduction e.g. the grunion fish.
- allows animals to predict the onset of seasons and prdict favourable conditions.
- Slowing down of metabolism in winter, called diapause in insects.
- Must be able to predict winter to store food.
- Triggered by a decrease in day length.
- Summer hibernation e.g. snails withdraw into shells and secrete to prevent desiccation.
- Triggered by shortening day length or drop in temperature.
- Ensures offspring are born when environmental factors are most favourable.
- Animals detect increase in daylength, temperature, rainfall, food supply, etc.
- This causes secretion of hormones which prep sex organs for courtship and reproduction.
Period of the rhythm - the time it takes to complete one cycle of activity.
Phase shift - when the onset of the rhythm is changed so that it starts earlier or later.
Free running period - when the biological clock is running without any cues from th environment and so is running free.
Entrainment - the resetting of the biological clock on a regular basis, so that it becomes synchronised with the environment. This is done with a zeitgeber.
Zeitgeber - a german word meaning 'time giver'. This is the environmental factor that resets the biological clock - usually a change in light or temperature.
Photoperiod- means daylength. The responses of animals and plants to day or night length are called photoperiodic responses.
- orientation is a behavioural response, involving movement, that an animal makes to an abiotic factor (stimulus) in its environment.
There are 4 types of responses:
- taxes (singular = taxis)
- kineses (singular = kinesis)
- a taxis is the movement of an animal towards or away from a stimulus coming from one direction.
- movement towards = positive response.
- movement away = negative response.
- each stimulus is given a prefix such as photo- for light.
Taxic responses allow animals to move out of unfavourable conditions and into favourable ones. This increases their chances of survival.
- a kinesis is a non-directional response to a change in intensity of a stimulus.
* positive or negative responses are not used *
- the animal may change its rate of movement. It may move faster or slower or it may make random turning movements.
- an animal will tend to move more quickly or make more turns when it is uncomfortable and will slow down when it is comfortable, therefore will usually end up in more favourable conditions and stay there.
TWO MAIN TYPES:
Orthokinesis - used to describe a change in the speed of an animal's movement in relation to a change in intensity of a stimulus. The more unfavourable the stimulus, the faster the speed of movement.
Kinokinesis - used to describe a change in the amount of random turning of an animal in relation to a change in intensity of a stimulus. The more unfavourable the stimulus, the more random turns the animal makes.
- navigation is the process in which an animal finds its way from one location to another using environmental cues.
- animals must be able to navigate to find their way home or to migrate from one geographic location to another.
-navigation, like orientation behaviour, is innate, not learnt. However, experience can improve an animal's ability to navigate.
Methods of navigation:
- solar navigation
- stellar navigation
- magnetic fields
- chemical navigation
- sound navigation
Most animals use a combination of methods depending on the conditions and how close they are to home.
- the ability of an individual to return to its home site after it has bee away to look for food or mates.
- homing usually occurs on a daily basis and often involves the animal navigating over unfamiliar territory.
- many different species display the ability to home e.g. 'homing' pigeons, cats, bees, ants, etc.
- the animal recognises familiar landmarks and uses these to guide itself to its destination.
- normally used over short distances.
- long distance migrants may use landmarks such as coasts, islands or mountain ranges to assist them in finding their way home.
- piloting is where an animal moves from one familiar landmark to another until it reaches its destination.
- the sun moves across the sky from east to west and during the day many animals are able to use the sun as a compass for navigation.
- this requires an animal to have an inbuilt sense of timing (biological clock) so it can compensate for the sun's changing position during the day.
- honeybees and many birds use this method. They are able to measure the angle between their direction of movement and the sun as they fly.
- some animals that travel over long distances use star constellation patterns for navigating at night.
- in the northern hemisphere they use the north celestial pole star.
- many animals are able to use the Earth's magnetic field lines to navigate.
- these animals have an inbuilt magnetic comapass (iron compounds in the brain) that enables them to sense the Earth's magnetic field.
- many animals are able to use chemical (scent) trails to find their way home.
- animals are able to do this with their natural phermones.
- sonar navigatio uses sound.
- some animals are able to find their way with echolocation.
- this involves emitting soun waves which bounce back between objects. The time difference between the sound and it's echo allows the animal to position itself in relation to objects around it.
- the regular, annual or seasonal mass movements made by animals from their breeding area to another area where they do not breed.
- occurs in a north-south direction so that the animal can take advantage of optimum food supplies and temperatures.
- in true migration, the animal makes a 'round trip' and returns to its breeding grounds.
- in some species, the return trip takes place at another stage of the life cycle.
- in monarch butterflies, migration takes place over several generations.
- the ability to navigate over long distances. Most animals use a combination of navigation methods. Navigation is innate, not learnt, however success improves with experience.
- Animals need to prepare for migration. Their biological clock sets in action activities such as: 1. laying down fat layers to ensure energy supplies for journey and 2. moulting of flight feathers and replacement with new ones to ensure maximum flight efficiency.
Migration Advantages and Disadvantages
- remain in favourable temperatures year round.
- grow larger.
- prodduce more offspring.
- constant food supply.
- colonisation of new areas.
- reduced predation, disease and parasitism.
- greater genetic mixing.
- better breeding conditions.
- may get lost or blown off course.
- may get eaten by unfamiliar predators.
- huge energy investment.
- may use up too much energy and die of exhaustion.
- may starve on the way.
- occurs between individuals of different species.
- mutualism - when both animals benefit from the relationship.
- commensalism - when one animal benefits while the other is unaffected.
- parasitism - parasites live and feed on/inside a host that they do not want to kill.
- predation - when an animal hunts and kills another animal for food.
- competition - occurs when resources become limited.
- a predator is an animal that hunts and kills another animal for food.
- predators display specific adaptations for hunting and killing their prey and their prey displays counter adaptations to reduce the risk of successful predation.
- these adaptations evolve over time through co-evolution.
Predator strategies include:
- the letting the prey come to you by sifting the environment, dangle bait, webs and traps and lying in ambush.
- moving after prey so predators must have the right appendages to catch prey e.g. claws, teeth, etc. Hunting in swarms and teams and using tools are ways of moving after prey.
Defence Against Predation
- predators recognise their prey by their silhouette (shape), eyes and bulk. Prey animals often have adaptations to disguise these things.
- camouflage - using cryptic shape and colourisation.
- visual deception - having large eyespots to confuse predator or a fake head so that the predator attacks the non-vital part of the body, allowing escape.
- startle display - flashing bright colours or rising up.
- pretending to be inedible - looking like a stick or bird dropping.
- mimicry - where one organism closely resembles or imitates another.
- warning (aposematic) colouration - highly poisonous animals advertise that fact with bright colour and stripes.
- warning sounds - roars and squeaks, etc.
- offensive weapons - horns and spikes, etc.
- firing chemicals - exploding hot chemicals into the predator.
- curling up.
- retreating into a shell.
- hiding with camouflage.
- group defence.
- keeping watch.
Two kinds of mimicry:
1. BATESIAN MIMICRY
- where harmless animals imitate poisonous ones.
e.g. monarch butterfly (poisonous) and the viceroy butterfly (not poisonous).
2. MULLERIAN MIMICRY
- where several unpalatable or poisonous animals take on the same colouration.
e.g. bees, wasps, hornets, monarch butterfly caterpillars are all black and yellow.
Predators kmow to avoid certain colours because they are not nice to eat.
Predator - Prey Relationships
- predators are important in controlling prey populations.
- predators typically kill the weaker or older individuals so are important 'selecting agents', removing 'less fit' individuals and their genes from the prey population gene pool.
- in nature, predators and their prey usually come into balance with each other and their populations follow a predator - prey curve.
- there are always more prey than predators and a peak in prey numbers is usually followed by a similar peak in predator numbers.
- competition occues when resources such as food, water, nests, etc. become limited.
- competition reduces the 'fitness' of the competing species and may restrict the species' potential niches.
- when competition is extreme, one species may out-compete and eliminate the other.
Gause's Competitive Exclusion Principle
- Gause's Principle - no two species with identical ecological niches can co-exist for long in the same place or "complete competitors cannot co-exist."
- Interspecific competition can be reduced by two species having differences in niche e.g. feeding on different parts of the same plant or at different times of a day.
- Niche differences are selected for by natural selection.
Co-operative Groups - Intraspecific Relationship
- many groups live in social groups for all or part of their lives.
- groups usually have some degree of organisation and individuals co-operate or assist each other in various ways.
- group formation and maintenance result from behavourial adaptations of individuals within a species.
- individuals with behavourial adaptations that enhance the survival and reproductive success of the group are 'selected for'.
- the advantages of group living must outweigh disadvantages for this way of life to continue.
Advantages of Group Formation
- Hunting - greater ability to find food and capture prey.
- Defence - greater ability to detect predators. Some individuals can be on lookout while others eat.
- Protection - younger and weaker members of the group are kept on the inside while moving or feeding.
- Breeding - mates more readily available, shared care of the young, greater learning oppurtunities for the young.
- Confusion - shoals of fish, flocking birds, quick moving meerkats confuse predators and make them less likely to target one individual.
- Clumping - to conserve moisture or body heat.
- Role Specialisation - different individuals take on different roles within the group.
Disadvantages of Group Formation
- greater intraspecific competition as the group becomes larger and resources become limited.
- increased risk of contracting and spreading disease and parasites.
- overcrowding increase conflict in the group. In some situations this can put the young at greater risk of being harmed/eaten by adults.
- access to mates may be restricted to only high ranking individuals.
Altruism and Kin Selection
Altruism is self-selecting behaviour towards other individuals for the benefit of the group
- many animals will put themselves at risk to help others in the group but usually benefit in some way themselves.
Kin Selection is altruism towards realatives (kin). Because related individuals share genes in common, altruism towards relatives indirectly benefits the individual by ensuring these genes survive and are passed on.
Courtship and Pair-Bonding
- to mate, animals must invade each other's personal space (a threat).
To mate requires co-operation, temporary suppression of aggressive behaviour and a system of communication and species recognised.
Potential partners must make sure they are of the same species, both fertile and are both fully prepared to mate.
- it is usually the female who chooses the male and the male competes for her.
- a male can gain an advantage over another male by fighting or ritualised combat OR special displays and adornment.
- sexulal competition of males has led to the evolution of brilliant colours, antlers and ornaments. This shows the 'fitness' of the male.
- courtship ensures two animals are of the same species and ready to start ovulation. Aggression is reduced by dances, signals, chemical pheromones or touching. This allows a strong pair-bond to form so mating is possible. Pair bond = stable relationship between animals of the opposite sex that ensures co-operative behaviour of mating and rearing of the young.
- successful reproduction is essential for the survival of the species.
Two different strategies exist:
- R-strategy - production of large numbers of young with little or no investment of time and energy in raising the. This provides little risk to the parents but few young survive.
- no parental care = produce large numbers.
- K-strategy - production of few offspring with a large investment of time and energy in raising them.This strategy has a higher risk to the parents but has a higher success rate. The young often have colours or behaviours which reduce aggressive behaviour in adults and trigger parental care responses instead.
- lots of parental care = only a few offspring produced.
Different Reproductive Strategies
- Monogamy - one female and one male
- Polygyny - one male and many females. No care of females or young by males.
- Polygamy - one male and many females. Male guards protects females and young.
- Polyandry - one female and many males. Male often cares for young.
- Polygynandry - promiscuous mating where many males mate with many females. No pair-bonding and very little care of young.
- Co-operative - one female and one male. Young fed and cared for by parents and other members of the group.
- Social Insects - only queen reproduces. All other members of group develop from unfertilised eggs and are infertile.
- aggressive responses are responses that animals make towards other animals as a result of competition for limited resources.
- competition is always more intense between individuals of the same species.
- aggressive responses cause physiological stress (poor health, greater risk of disease and mortality, etc) within a group and if individuals die, their genes are lost from the gene pool, so natural selection has favoured behaviours that act to reduce aggression in groups.
- aggression within a group is reduced by agonistic behaviour.
- this is where animals use postures, gestures or vocalisations that threaten another animal, rather than actually fighting.
- the threatened animal will usually back off or show submissive behaviours so that serious attacks do not occur.
- agonistic behaviour tends to be ritualised and the dominant and submissive postures recognised and acted on all by all members of the group.
- a territory is an area established by an individual or group for feeding, mating and rearing young.
- it is marked and defended against other individuals or groups of the same species.
Home range - a larger area covered regularly in search for food and mates. Home ranges may overlap and are not defended. Territory - part of the home range that is marked and actively defended. Territories contain the nest site and do not overlap. Other individuals or groups are excluded.
- provide safe place for animals to court, mate and rear young with no interference.
- spreads individuals out which reduces spread of disease and parasites.
- animal has a familiar area to find food, water and protection from predators.
- most successful male holds the best territory and is the one to breed the most, ensuring best genes are handed on to offspring and enhances reproductive success of group.
- males unable to secure and defend a territory are usually unable to attract a mate.
- hierarchies occur in groups when individuals have a specific rank or status in the group.
- in a linear hierarchy, individuals are ranked from the highest (alpha) to lowest (omega).
- in complex hierarchies, the alpha individual may be supported by other individuals in its control of the group.
- ranking determines an individuals access to resources such as water, food and mates.
- ranking is determined by a combination of sex, age, experience, size and strength.
- animals compete for position in the hierarchy often using trials of strength. Younger individuals may challenge older ones for rank.
Advantages and Disadvantages:
- each individual in the hierearchy "knows its place" so conflict and aggression is reduced.
- rankings may change over time and younger lower ranked individuals may challenge higher ranked individuals to improve their ranking.
- more subordinate individuals may be denied access to some resources and mating.
- plants differ from animals in that they are autotrophs - carry out photosynthesis to make food in the chloroplasts of their cells.
- plant structures and responses are adaptations for ensuring successful photosynthesis.
- plants need to be able to obtain light, minerals, water, CO2 and O2 from their environment and need a large surface area:volume ratio to do this efficently. This achieved by having a finely divided body shape (branches and leaves).
- all plants have the same needs for their environment (sunlight, water, minerals, CO2, O2, space).
- inter-specific and intra-specific competition occur when any of these factors are in short supply.
- plants have a range of adaptations and strategies for reducing competition.
- allelopathy is when a plant species produces and releases a chemical that prevents or inhibits the growth of other plant species in the area around it.
Adaptations to Different Light Intensities
- plants growing in reduced light grow larger leaves containing more chlorophyll to make the most of the avaliable light.
Stratification - the vertical layering of plants that occurs in a forest. Dominant trees form a canopy and change the conditions for plants growing in the lower layers.
Commensalism - epiphytes perch on tall trees to gain more sunligh and lianas climb up tall trees for sunlight and support. These tall trees are unaffected by these plants.
Parasitism - plants are parasitic on other plants if they feed on the sap of the plant and carry out little of their own photosynthesis.
Mutualism - both organisms benefit such as root nodules on legumes contain nitrogen-fixing bacteria which supplies the plant with nitrates .
Plant - Animal Relationships
- animals that eat plants
- grazers, browsers, sap suckers, nectar feeders, etc.
- seed dispersal - plants have a wide variety of adaptations to ensure that seeds are dispersed away from the parent plant to avoid competition.
- thorns, spines, stings, low growing points, divarication, waxy cuticles, chemicals
Plants that Eat Animals
- an adaptation of plant sto obtain nitrates and phosphates needed for growth
Orientation Responses of Plants
- plants respond to a number of different factors in their environment.
- two main types of responses are tropisms and nastic responses.
Types of stimuli:
Photo - light
Geo - gravity
Hydro - water
Chemo - chemical
Thigmo - touch
- a tropism is a growth response towards or away from a stimulus coming from one direction.
Taxes - the movement of a whole organism towards or away from a stimulus, normally applies to animals but in plants it is restricted to single-celled or simple algae such Euglena
- the response of plants to stimuli such as light, temperature or touch that do not come from a particular direction are called nastic responses.
- the plant responds rapidly to a change in intensity of the stimulus.
- these are not growth responses and are reversible.
- result from changes in tugor pressure in cells.
For example, the opening and closing of flowers in response to light intensity.
Control of Plant Growth
- plant growth is controlled mainly by hormones.
"a hormone is a chemical produced in one part of an organism and transported to another part of the organism where it produces a response."
Five important groups of plant hormones are:
- ethylene gas
- abscisic acid
- auxins are one of the most important groups of plant hormones.
- they are produced in the tips of roots and shoots and transported throughout the plant.
- they cause cells to elongate and differentiate (develop into different types of cells).
- they are responsible apical dominance and plant tropisms.
- the main auxin in plants is called IAA (indole acetic acid).
Auxins and Phototropism
- auxin is produced in the growing tip of a shoot and diffuses down through the shoot cells causing them to elongate.
- if light is shone on a shoot from one direction, auxin moves to the dark side of the stem and causes the cells on this side to elongate. This has the effect of causing the shoot to bend towards the light.
Auxin in Shoots and Roots
- the effect of auxin depends on its concentration and where it is located in the plant.
- ROOT growth is stimulated by LOW concentrations of auxin. Medium and high concentrations inhibit root growth.
- growth of side branches from LATERAL BUDS is stimulated by MEDIUM concentrations of auxin. High concentrations inhibit lateral bud growth.
- STEM (apical bud) growth is stimulated by HIGH concentrations of auxin. Very high concentrations inhibit growth of stems.
Auxins and Geotropism
- when a seed germinates, the shoot always grows upwards and the root always grows downwards no matter which way the seed is planted.
- shoots show negative geotropism while roots show positive geotropism.
- seedlings detect gravity using small starch grains in their cells called statoliths which sink with gravity to the lower side of the cell.
- they respond to gravity using auxins which also settle to the lower sides of the shoot and root cells.
Auxin and Geotropism:
- the shoot and root tips as they emerge from the seed contain high concentrations of auxin which diffuses from the tips along the root and shoot cells and settles with gravity to the lower sides of the shoot and root. High levels of auxin in the lower side of root cells inhibit root elongation causing root to bend downwards. High levels of auxin in the lower side of shoot cells stimulate shoot cell elongation causing the shoot to bend upwards.
- another effect of auxin is that it prevents the growth of side branches in plants. This is called apical dominance.
- the top growing tip of the plant (apical meristem) produces high concentrations of auxin which inhibits the growth of lateral buds into side branches.
- as auxin diffuses down the stem, it becomes less concentrated so side branches begin to grow lower down.
- this produces the typical triangular shape of many trees.
- if the apical bud at the top of the plant is removed, the high concentration of auxin is removed so the lateral buds start to grow and produce side branches.
- this produces a 'bush'.
- another plant hormone called cytokinin, which is produced in the roots, has the opposite effect to auxin and stimulates growth of lateral buds, adding to the bushy effect.
- gibberellins are produced in young leaves and buds as well as the tips or roots and shoots.
- they promote rapid growth of stems, especially between nodes (where the side branches and leaves come off the stem), by causing cell elongation.
- dwarf varieties of some plants result from a lack of gibberellins.
- they also promote growth of seeds and buds.
Cytokinis are produced mainly in the roots of plants and transported to other parts of the plant through the xylem.
- they are dound abundantly in fruits, seeds and roots where they promote cell division.
- cytokinins can slow the process of aging in plants. If applied to leaves they can prevent yellowing in autumn and abscission (leaf fall).
- cytokinins can be used to stimulate growth in cuttings and tissue culture.
- varying the concentration of cytokinins and uxins can cause plant tissue to grow shoots or roots.
high auxin + low cytokinin = root growth
low auxin + high cytokinin = stem growth
equal concentrations = callus growth (undifferentiated with no roots or stems).
Abscisic Acid (ABA)
- abscisic acid generally acts as an inhibiting hormone: acting against auxins, gibberellins and cytokinins which tend to promote growth.
- abscisic acid promotes formation of abscission zones which is the layer of cells where fruit and leaves fall off a tree.
- abscisic acid promotes dormancy of seeds and buds over winter.
- it is also involved in closing the stomata to reduce water loss in drought condition.
- ethylene gas is produced in ripe fruit and causes surrounding fruit ti ripen.
- putting a rip apple in a bag with an unripe kiwifruit will cause the kiwifruit to quickly ripen in a few days.
- ethylene gas also causes the bananas in a fruit bowl to go brown.
- the action of ethylene gas can be inhibited by placing fruit in a high concentration of CO2. This is used to prevent the rippening of fruit druring shipping or storage.
Biological Rhythms in Plants
- like animals, plants respond to the Earth's cycles and show:
Circadian Rhythms in Plants
- circadian rhythms in plants are endogenous, controlled by a biological clock.
- opening and closing of flowers - many plants open their flowers during he day when their pollinators are active and close them at night. Some moth spollinated species open at night and close during the day. This is called photonasty.
- nectar or scent production - many flowers produce nectar or scent for only a few hours a day and pollinators learn when this will occur.
- following the sun - some flowers turn their faces to the sun and follow its position during the day.
- drooping of leaves - many plants droop their leaves at night. /this may help to reduce water loss or reduce absorption of moonlight which could upset the photoperiodic responses of the plant to day or night length.
- photosynthesis - occurs only if sufficient light is present. This is an exogenous rhythm that continues only when light is present.
Circalunar and Circatidal Rhythms
Plants do not show obvious monthly rhythms but in all cultures there are calenders for planting certain plants according to the full moon.
Marine algae show tidal rhythms, particularly in the timing of the release of the eggs and sperm for reproduction.
Circannual Rhythms in Plants
- circannual rhythms in plants occur in response to the seasonal changes in daylength or night length so are called photoperiodic responses.
- growth patterns
- dormancy (a period of metabolic inactivity in seeds)
- abscission of leaves (leaf fall in autumn)
- vernilisation of seeds (the need for a period of chilling in the soil over the winter months before germination will occur)
Flowering in Plants
Photoperiodism is the response of plants to changes in day length.
- it has been found that flowering plants actually depends on night length rather than day length.
There are 3 types of plants:
Short day plants - require a short day and a long night length before flowering will occur. Flower in autumn, winter and early spring.
Long day plants - requires a long day and short night length for flowering. Glower in late spring and summer.
Day neutral plants - flowering unaffected by day or night length. Flowering can occur in any season.
Phytochrome and Flowering
- the ability of plants to respond to day and night length is controlled by a light-absorbing pigment present in shoot tips and leaves called phytochrome.
- phytochrome exists in 2 different forms which are named after the wavelength of light the absorb: Pr which absorbs red light and Pfr which absords far-red light.
- the two forms of phytochrome can change from one to the other (interconvertible).
- when Pr absorbs red light it changes to Pfr
- when Pfr absorbs far-red light it changes to Pr.
- sunlight contains more red than far-red light, so during the day Pr absorbs red light and changes into Pfr form which builds up in the leaves of the plant.
- at night there is very little light but there is more far-red light than red light present, so Pfr absorbs the far-red light and slowly changes back to the Pr form.
- a plant "measures" daylength by the amount of phytochrome that exists in each form.
- when nights are short, less Pf bulids up in the leaves while more builds up in long nights.
Short Day vs. Long Day Plants
Short day plants will flower when enough Pr accumulates. This happens when the nights are long because the Pfr that has built up during the day slowly converts back to Pr at night. Exposing these plants to even a flash of light (or red light) at night will prevent them from flowering because this quickly changes their Pr back to Pfr.
Long day plants will flower when Pfr levels are high and Pr levels are low. This happens when the nights are short because there is not as much dark time for Pfr to convert back to Pr. These plants can be made to flower in winter instead of summer by interruoting a long night with a flash of light.
Phytochrome and Seed Germination
- most seeds will only germinate if exposed to light. Burying them too deeply in the soil nay prevent this.
- exposure to red light stimulates germination.
- exposure to far-red light inhibits germination.
- seeds contain the two forms of phytochrome Pr and Pfr. When seeds are exposed to red light, Pr is converted to Pfr, the active form of phytochrome that stimulates germination.
- Pfr is thought bind the proteins associated with DNA and activate or inhibit the transcription of specific genes.
- most seeds will not germinate when heavily shaded by leaves of other plants. This is because these leaves absorb red light and prevent it from reaching the seeds.