Survival and response

Survival tactics

Living organisms need to respond to their environment, and more specifically changes in local environmental factors which act as stimuli. If their responses are helpful, they are more successful and they continue to exist. If they do not respond at all or they make a less helpful response then they may suffer problems.

This is essentially an aspect of survival of the fittest which results in the evolution of living organisms to suit their environment, which is a fundamental concept in Biology. The responses are often simply seen as adaptations.

Plants need light for photosynthesis, but too much exposure can lead to dehydration
They also need water and mineral salts, which generally come from a different direction.
They need to be anchored in the ground, but being rooted to the spot means their options are limited.
And for climbing plants the efficiency of their grip is a matter of life and death.

Small animals and plants that can move have more opportunities and they must monitor their immediate environment, whether it is light and open, dark and enclosed, dry or wet.

Larger animals have specialised nervous systems to monitor their environment (both external and internal) but these can operate at an automatic - almost instinctive - level as well as being fine-tuned as a result of experience during development.

Flowering plants and tropisms

Flowering plants move parts of themselves - leaves, flowers, roots and shoots - by growing in different directions, in response to stimuli which are also directional.

These directional growth movements are called tropisms. Phototropism is growth movement in response to light; gravitropism (geotropism) is in response to gravity. Other examples of tropisms include thigmotropism (growth around obstacles, as shown by climbing plants), hydrotropism (growth towards water) and chemotropism (growth towards specific chemicals).

Tropisms can be directed towards the source of a stimulus (positive tropisms) or away from it (negative tropisms). Diatropisms are responses resulting in parts of plants growing at right angles to the direction of the stimulus, as shown by leaves. Negative halotropism is movement (of roots) away from salt, as may occur in a salty or brackish environment, such as a sea lagoon or estuary.

So plant shoots show both positive phototropism and negative gravitropism - they grow towards the light and upwards, whereas roots show positive gravitropism - they grow downwards - and possibly negative phototropism - so that they grow away from light.

These growth responses are regulated by specific chemical growth factors, also known as plant hormones or phytohormones.

The growth factors are produced in growing regions called meristems and they move to other developing tissues, causing different patterns of cell elongation, leading to curvature of structures.

Nice or nasty?

Plants can also respond to stimuli by making nastic movements which differ from tropic movements in that the direction of nastic movements is independent of the position of the stimulus.
These are generally caused by tension resulting from osmotic gradients within hinge sections, and may be triggered by release of potassium & chloride ions or calcium ions, rather like action potentials within nerve cells. Other chemical growth factors such as ethylene also have an effect in some cases.

Examples include
Photonasty: response to light
Gravinasty/geonasty: response to gravity
Thigmonasty/seismonasty/haptonasty: response to contact or vibration.
Some plants living in water or permanently wet conditions such as bogs do not receive enough nitrates: they compensate by trapping and digesting insects.
The Venus flytrap plant Dionaea muscipula and bladderwort Utricularia (aquatic plant) have quick-acting traps for this purpose.
Mimosa pudica, the sensitive plant, folds its leaves inwards when touched - presumably an adaptation to reduce damage by herbivores.
Nyctinasty: movements at night or in the dark.
Some plant species close their leaves or flowers in the dark: circadian 'sleeping movements'. This may reduce cooling and exposure to herbivores at night.
Hyponasty: upward bending of the leaves and the elongation of the petioles.
This may raise plants' leaves above water when submerged.

Charles Darwin, together with his son Francis, performed various observations on several species of plants: tracing the movement of plant shoots - 'circumnutation' - as they turned during the day in response to illumination by the sun. This was published in 1880, just 2 years before his death.

As part of this work, he investigated the growth of coleoptiles: the tubular sheaths which grow out of germinating seeds of grasses and cereals such as oats. From these, the main shoot of the plant grows later.
He concluded that "sensitiveness to light is confined to a small part of the plant; and that this part when stimulated by light, transmits an influence to distant parts, exciting them to bend."

Darwin's experiments using oat coleoptiles

Darwin subjected these coleoptiles to different treatments:
A - Coleoptile with no modifications bends towards the light.
B - Coleoptile with tip cut off shows no response to light.
C - Coleoptile with tip covered with opaque cover shows no response to light.
D - Coleoptile with transparent cover over tip bends towards the light.
E - Coleoptile with base covered with opaque cover bends towards the light.
Conclusions:
> Coleoptiles bend towards a directional light stimulus

> The tip of the plant is active in initiating this response (it 'perceives' the light)
> The reaction (growth response) happens further down the stem.

This led Darwin to suggest that there was an 'influence' that moves from the site of perception to the site of reaction.

Peter Boysen-Jensen cut the tips off coleoptiles and placed a thin piece of gelatin or mica between the coleoptile and the lower shoot.

Mica is a light mineral which can be split into thin sheets. It is a barrier to the movement of liquids. Gelatin sets to form a layer of jelly. Being water-based, it allows soluble substances to pass through by diffusion.
Conclusion
> A water-soluble substance passes from the tip to the elongation zone.

In a further experiment, he inserted mica halfway into the shoots, either on the illuminated or non-illuminated sides.
Conclusion
> The substance only passes from the tip to the elongation zone down the non-illuminated side of the shoot. It does not move across as it passes down.

Indoleacetic acid (IAA)

Indoleacetic acid (IAA) - also known as auxin - is the main plant growth hormone or chemical growth factor.

It has different effects on plant shoots and roots.

Other phytohormones

Gibberellins e.g. Gibberellic acid may have a number of effects on plant development. They can stimulate rapid stem and root growth, induce mitotic division in the leaves of some plants, and increase seed germination rate. They have been widely used in horticulture and malting.

Abscisic acid (ABA) - named originally after its role in the abscission (dropping, shedding) of plant leaves - is involved in many developmental processes in plants, including seed and bud dormancy, the control of organ size and stomatal closure.

Cytokinins (CK) are a class of plant growth substances that promote cell division in plant roots and shoots. They appear to interact with auxin, generally having opposite effects.

Ethylene (ethene) - C₂H₄ - governs the development of leaves, flowers, and fruits. It may also promote, inhibit or induce senescence.

Indole 3-acetic acid is acetic acid (ethanoic acid) - CH₃COOH - with an indole group (a six-membered benzene ring fused to a five-membered pyrrole ring) substituting for one of the hydrogens on the methyl group.

Its molecular structure is quite similar to the amino acid tryptophan.

It is the main component of rooting compounds used by gardeners to propagate plants from cuttings. Other indole compounds e.g. indole-butyric acid are sometimes used for this purpose.

Several synthetic compounds with a similar molecular structure ( e.g. '2,4-D' and '2,4,5-T') have been used as herbicides.

Shoots

Auxin produced in the stem tip promotes cell elongation in a region just a few mm back from the tip. Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant.

This difference in cell elongation on the two sides of the stem causes curvature of the shoot tip towards the light: positive phototropism. This is advantageous to the plant as it maximises exposure to light for photosynthesis.

A higher concentration of IAA causes more cell elongation in shoots (on the shaded side).

Auxin movement - sideways and down in shoots

It is said that IAA moves from the growing regions of a plant shoot to other tissues by diffusion but simple diffusion alone does not explain the uneven distribution of auxin when a shoot receives more light from one side than the other.

It was once thought that auxin is degraded in the light and this causes the differential growth of the two sides of the shoot.

More recently it has been shown that lateral, directional auxin transport in the tip of the shoot causes the asymmetric auxin distribution which then moves back down towards the region of elongation, and this causes tropism. It is probable that phototropin molecules - sensitive to blue light - initiate a response involving protein molecules which carry auxin molecules from one side of a cell to another. A number of auxin influx and efflux carrier molecules have been identified so the movement is essentially a form of facilitated diffusion.

A modern explanation is that auxin moves to the darker side because auxin efflux carriers - PIN and others - become attached to cell membranes on the shaded side. Its location is said to be polarised, as light represses the transcription of PINOID (PID) kinase enzyme which catalyses the production of PIN.

In the elongation zone, auxin causes a H⁺ pump in the cell membrane to secrete H⁺ into the cell wall, activating 'expansin' enzymes which break bonds between cellulose molecules. Cellulose in the cell wall becomes softened, allowing the cell to expand under turgor pressure due to osmosis.

These images show PIN3.GFP, a green fluorescent protein (GFP)-tagged auxin-responsive marker, in endodermis cells of the upper hypocotyls in Arabidopsis.
a shows start (in dark)
b & c have been subjected to unidirectional light from the left for 4 and 12 hours,
d & e have light from all sides for 4 and 12 hours
[arrows show direction of illumination, open arrowheads depict outer side of endodermis cells, scale bars, 25 µm]
(g-k) Quantification of PIN3.GFP signal intensities of a-e.
Fluorescence measurements, in arbitrary fluorescence units (a.u.), from (1)outer, (2)inner cell sides of the left hypocotyl side and (3)inner, (4) outer cell sides of the right hypocotyl side
There is a gradual PIN3 polarization (weak outer and strong inner signal) only at the illuminated hypocotyl side after unilateral illumination with white light.
This indicates that light regulates the subcellular distribution of PIN3, and that light-dependent changes in PIN3 localization may redirect auxin fluxes during the phototropic response.

Roots

Roots tend to grow downwards, but some grow a certain distance sideways before curving downwards. This tends to take roots towards water and anchor the plant in the ground. Uptake of water and anchorage are obvious advantages to the plant.

If a seedling plant is placed so that its root is horizontal, the new section of root that grows curves downwards. This is explained by differential elongation of cells on the two sides of the root, due to different distribution of auxin.

Flow of auxin in the root tip


It is thought that auxin moves to the lower side so more IAA/auxin accumulates there. This causes the lower side to grow less/slower as the cells there do not expand so much.

A higher concentration of IAA causes less cell elongation in roots (on the under side). This may be seen as inhibition of growth, or less or slower growth on the lower side

The fall and rise of auxin in roots

There is a flow of materials from the shoot - above ground - to the root - below ground.

Sugars resulting from photosynthesis are translocated via the phloem (within the stele in the centre of the root) to the root growing tip. In some cells (columella cells), starch grains form, and amyloplasts containing them form into dense statoliths which accumulate on the lower sides of gravity-sensing cells called statocytes.

Amyloplasts may become attached to actin fibres leading from the cell membrane, so displacement of a root from the vertical may cause strain in the cell membrane (rather like stretch-mediated channels in Pacinian corpuscles in skin). This may change membrane potential and/or activate auxin efflux channels which move auxin in different directions in different cell types.

In fact auxin also flows from the shoot to the root down the stele in the centre of the root. At the tip of the root it is moved sideways then upwards through the epidermis to the zone of elongation where cells change in length, and differences in auxin concentration on either side can cause the root to curve so that it is pointing downwards. Auxin also appears to be recycled downwards from this zone, presumably increasing the concentration in the root tip.

The root cell types responding to gravity stimulus in Arabidopsis


(A) Longitudinal view of an Arabidopsis primary root showing meristem, Distal and central elongation zones, maturation zone.
(B) Gravity sensing in roots occurs in the central columella cells of the root cap (inset). The root cap consists of four layers of cells important for root gravitropic response namely S1, S2, S3, S4, respectively. However, the central columella cells from S1 and S2 (the encircled cells) play major role for gravity sensing.
(C) Columella cell (cartoon) contains starch-filled organelles called amyloplasts, nucleus, vacuole, actin filaments, endoplasmic reticulum (ER), plasma membrane (PM), and cell wall.

Taxes and kineses

These forms of behaviour are responses shown by simple motile organisms, i.e. those that can move from place to place. This sort of movement is called locomotion. These may be small animals, bacteria or simple unicellular organisms - including photosynthetic protoctistans and algae.

Taxes

A taxis is a directional movement of a (whole) organism in response to a stimulus. It often serves to cause movement towards favourable stimuli or away from unfavourable ones. Aerotaxis, phototaxis, and chemotaxis are common examples.

It is known that phytoplankton in sea and freshwater consist largely of motile species (dinoflagellates). These move using their flagella, rising to higher levels in the water when exposed to sunlight, and sink or swim to lower levels when light levels fall, or when ultraviolet levels rise. This behaviour optimises exposure to light for photosynthesis. And zooplankton, which feed on them, show similar behaviour. This phototactic behaviour underlies productivity in aquatic ecosystems.

Fine-tuning the response

These responses are generally innate (unlearned) but they may be modified in certain circumstances. This is called behavioural plasticity.

It has been found that the small nematode worm Caenorhabditis elegans can learn to associate a particular temperature with the presence or absence of food. If the worms have recently been well fed in an environment at a constant temperature they form an association between this temperature and food. When deprived of food for a short time and placed on a surface with a temperature gradient, they will migrate to a region with the same temperature to which they had become accustomed.


This diagram shows normal thermotactic behaviour ('isothermal tracking') - movement towards the same temperature in which the worms have been cultured and fed - in this case 17 °C.
A mutant variety shows 'athermotactic behaviour' - with no preference for any temperature.

The normal response has the advantage that the worms will tend to stay in the optimal position combining the availability of food with this temperature. In the absence of food they will respond to temperature, which may coincidentally also lead them to more food.

Some of the behaviour patterns mentioned in this section may be modified under certain circumstances. For instance, some animals may show a different response when fed to their normal hungry response! And insects may show different behaviour at different stages in their life cycle.

Kineses

A kinesis is also a movement of a (whole) organism in response to a stimulus, but it is non-directional.

In this case the stimulus causes movement to vary in a different way (i.e. not towards/away from the stimulus):
Orthokinesis is speeding up or slowing down of movement. This is often accompanied by
Klinokinesis, which is (randomly) turning in different directions, changing in response to the strength of the stimulus. The frequency or rate of turning is proportional to the stimulus intensity.

As a result the organism tends to remain in a favourable environment, although they will probably keep moving slowly.

Although a kinesis is not actually oriented with the direction of the stimulus, it can still result in migration which has a general directional component, so it is possible to describe the effect as positive or negative.

A positive photokinesis is defined as an increase in the speed of movement in the presence of light, and negative photokinesis covers the slower movement of an organism upon entering an illuminated area relative to its movement in the dark or in dim light.

Hygrokinesis is movement in response to (air) humidity.
This is sometimes linked with thigmokinesis - movement in response to surfaces - ruts in the ground surface (and sometimes edges of containers in the lab).

Choice chambers

These types of locomotory behaviour can be tested using choice chambers or mazes.
Experimental animals might be

• woodlice (although it is often noted that different species - see opposite - show slightly different responses to environmental factors so it is best to check their identity first!)
• flour beetles (Tribolium spp.)
• maggots (larvae of the blowfly Calliphora vomitoria which can be obtained from anglers' supply shops)

Choice chambers can be made up from cardboard or wooden boxes divided into compartments, or specially made plastic structures.

The environmental conditions within each section can be varied: water (soaked into cotton wool) for high humidity, silica gel for low humidity. Other chemicals can be chosen to give intermediate values of relative humidity. Alternatively sections may be shaded or covered with different coloured filters on the outside.

Inside the container there should be a layer of plastic mesh to prevent animals from falling (or burrowing) into the contents of the sections below.

A number of experimental animals are placed into the apparatus via the central hole. They are then given some time to migrate to their chosen area within the apparatus, then the numbers in each section are counted. Counts can be taken again after suitable intervals.

Statistical testing

It is usual to perform a χ² (chi-squared) test on the results.

A null hypothesis might be that the animals will not distinguish between the environments in the sections, so they will be evenly distributed (equal numbers in each section).

See Practical suggestions below

Animal antics

The simple reflex

A reflex, or reflex action, is an involuntary and nearly instantaneous response to a stimulus - usually resulting in a movement which is beneficial or helpful in some way.

The stimulus is detected by a sensory neuron(e) which sends a nervous impulse, generated by a receptor, to another (relay) neurone, which sends an impulse to a motor neurone which stimulates an effector organ (muscle or gland). The combination of three neurones is called a reflex arc, and it causes a more rapid response than movement initiated in the brain by conscious thought.

The initial nervous impulse from a sensory organ (responding to heat, pain or some other threat) is sent up a nerve to the spinal cord, and this causes another impulse to be sent back from here down a nerve to an appropriate part of the body. Impulses are also sent up the spinal cord to the brain but this is a secondary action and the information is processed more slowly.

These 'hard-wired' responses generally have a protective function.
Examples include:
The withdrawal reflex, such as moving the hand away from a hot object
The grasp reflex - a response shown by baby humans and monkeys where it is important to cling on to mothers' fur.
There are several other primitive reflexes shown by babies which gradually become modified in normal development. There are also a large number of reflex actions known which cover the basic functioning of the body's processes throughout life.

A reflex action is a more or less automatic response, as opposed to actions controlled by the brain, which are effectively learned responses. These are slower, because more neurones are involved and impulses travel over greater distances.

Too much information?

Here is a graphic I made to support another specification
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Other reflex actions

Knee jerk reflex (patellar reflex) when stretch receptor on leg muscle is stimulated, muscle is stimulated to restore position: this may be a result of a reflex arc with only 2 cells

Coughing or sneezing, caused by irritants in the nasal passages.

Iris pupil reflex in response to variation in light entering eye

Blinking when an object e.g insect comes very near to the eye.

These can all be seen to have a protective function to the body.