from nourishment is used for organism function, growth, re- production, while the fundamental physics underlying the relation between form. Animal - Form and function: To stay alive, grow, and reproduce, an animal must find Actively moving animals can feed on organisms that do not move, a rich . it is known that there is a correlation between learning and memory capacity. approach to form and function, I distinguish two classes of ses focus on the relationship between the organ- features of the organism can then be examined.
Here, our principal goal is to explore how it is that both geometries of life coexist on Earth, whether intermediate geometries are possible, and what all this implies for evolution of life on Earth. Living organisms span an impressive range of body mass, shapes, and scales. They are inherently complex, they have been shaped by history through evolution and natural section, and they continually extract, transform, and use energy from their environment.
The most prevalent large multicellular organisms on Earth, namely plants and animals, exhibit distinct shapes, as determined by the distribution of mass over the volume. Animals are able to move and are approximately homogeneous in their mass distribution—yet they have beautiful fractal transportation networks. Plants are rooted organisms with a heterogeneous self-similar fractal geometry—the mass of the tree is more concentrated in the stem and branches than in the leaves.
The approximate power law dependence of the metabolic rate, the rate at which an organism burns energy, on organism mass has been carefully studied for nearly two centuries and is known as allometric scaling 4 — In an influential series of papers, West and coworkers 111214 — 16 suggested that fractality was at the heart of allometric scaling.
Inspired by these papers, a contrasting view was presented 13which argued that, although fractal circulatory networks may have advantages, quarter-power scaling came built in with the directed transport of nutrients. However, this latter paper was necessarily incomplete because it did not address the distinct geometries of animals and trees. We then turn to a consideration of the sharp differences in the geometries of animals and trees and argue that the evolution of organismal forms follows from a rich interplay of geometry, evolutionary history, developmental symmetry, and efficient nutrient acquisition.
Despite their independent evolution and different metabolisms, vascular plants and bilaterian animals share major design features, namely, an internal mass comprising organized cells capable of metabolic and bioenergetic activities, a transport mechanism for distributing molecules and energy within itself, and a surface capable of exchanging matter and energy with the environment.
Regardless of the shape differences observed between these two groups, the physics associated with the transformation, transport, and exchange of matter and energy must unavoidably impose physical constraints on their designs. An organism is akin to an engine—part of the energy obtained from nourishment is used for organism function, growth, reproduction, while the rest is dissipated through its surface.
We consider the hypothesis of the survival of the fittest in terms of energy metabolism and postulate that an organism with a higher energy intake would have a competitive advantage over another organism of similar mass performing energetically suboptimally, and explore its consequences. Consider an isotropic 3D organism of spatial extent h whose volume V scales as. Generalization to organisms with distinct scaling along the three different directions is straightforward. We make the simplifying assumption that the consumption and metabolic activity is distributed uniformly in space and in time or suitable averaging is used.
We denote the basal metabolic rate of the organism by B and its mass by M. B is a measure of the energy being delivered to the organism per unit time and ought to be proportional to the energy dissipated through its surface. Our goal is to understand the ideal dependence of B on M in the scaling regime. The characteristic time scale associated with the organism is known to scale as —it is a measure of how long it would take for energy proportional to M to be dissipated at a rate of B.
Henceforth, proportionality constants, which serve to fix the correct units of various quantities related through scaling relations, will be omitted for the sake of simplicity. The number of metabolites, N, consumed in the organism per unit time is proportional to B. Let us defineso that a single metabolite is consumed per unit time in the local region surrounding each site of an grid. Each of these sites can be thought of as being within a service volume, in which one metabolite is consumed per unit time, of linear spatial extent.
Partitioning a hydroskeleton into many small, separate, but coordinated units facilitates locomotion. In an earthwormfor example, a front group of segments narrows together, thereby elongating that part of the worm.
If there were no partitions between the segments, the fluid would flow farther back, providing little elongation. Widened segments behind these initial segments anchor the worm, and its head moves forward.
The process then reverses in a wave, and the posterior end moves forward. Metamerismor the partitioning of the coelom, is thought to have evolved in ancestral annelids to improve their ability as burrowers in the bottom mud of the ocean. It undoubtedly explains the unrivaled success of this phylum among worms and helps to explain the extraordinary success of one of its relatives, the arthropods, which remained segmented even after the skeletal function of the coelom was lost.
Elastic skeletons do not change shape but simply bend when a muscle contracts. Muscle relaxation results either from a muscle contracting in the opposite direction to its antagonist or from the skeleton resuming its original position. The tentacles of many hydrozoan coelenterates, the mesoglea of jellyfishthe hinge of clamshells, and the notochord of chordates are examples.
The high-pressured coelom contained in the rigid but flexible cuticle of nematodes also functions like an elastic skeleton.
Rigid, jointed skeletons achieve movement through a lever system. The elements of the skeleton are rigid segments attached together by flexible joints. Muscles span the joints and attach at each end to different elements.
The more stable attachment site of a muscle is called the origin, the other the insertion. One muscle contracts and moves the skeletal element on which it is inserted, and an antagonistic muscle contracts and moves the skeletal element in the opposite direction. The biceps and triceps of the upper arm in humans are such a set of antagonistic muscles that bend and straighten, respectively, the lower arm. The control of movement can be quite precise with jointed skeletons.
Muscles can bend or rotate skeletal elements whose length, shape, and number contribute to the resulting action. The dexterity of the hands is an example of the complexity of controlled movements made possible by a jointed skeleton. Important to the speed and force of a movement are the length of the skeletal element and the size of the contracting muscle. Short limbs with thick muscles have more power than long limbs with slender muscles, but the latter have more speed.
Limbs thus reveal a great deal about how an animal moves. Likewise, the relative massiveness of jaws reflects the toughness of the food eaten. Two animal phyla, Chordata vertebrates only and Arthropodaexploit jointed skeletons. Although the skeleton is internal in vertebrates and external in arthropods, the principles of movement are the same.
A jointed skeleton is ideal for moving on land because adaptations for protection against dehydration such as the cuticle do not interfere with the action of the skeletal system. Indeed, the arthropod cuticle serves jointly a protective and a skeletal role.
Moreover, the diverse range of precise movements made possible by this skeleton facilitates all sorts of locomotory patterns: Jointed skeletons are also used directly for feeding jaws. Arthropod jaws are derived from legs, while vertebrate jaws are derived from gill arches. Translating movement into locomotion and feeding Although all animals can move, not all locomote or displace the body over a distance.
Locomotion serves the animal in finding food and mates and in escaping predators or unsuitable habitats. These functions of locomotion are typically correlated among different animals, so that those using the same mechanism of locomotion usually also feed, seek mates, and avoid danger in similar ways.
Some of the correlations between mode of locomotion and mode of feeding are described here, but space precludes discussion of the rich diversity found among animals past and present.
Locomotory strategies for finding or gathering food include the following techniques. Sitting still and waiting for food to arrive is particularly prevalent in aquatic habitats but is not rare on land.
Sessile animals tend to develop strong defenses that are sometimes incompatible with effective locomotion. They rely on water or air currents or on the locomotion of their potential prey to bring food within reach. Because food may come from any direction, many sessile animals evolve radial symmetry. Settlement may be permanent or temporary, but in all cases one stage of the life cycle is capable of moving actively or passively from its place of origin. The choice of attachment site can also be active or passive; passive choice is often associated with an ability to grow in such a way as to maximize feeding efficiency.
As with plants, passive settlers do well only with luck. The retention of locomotory capabilities requires energy and nutrients that can otherwise be diverted into growth or the production of offspring. Sessile feeders need to move if feeding and resting sites differ. Sessile animals include filter feeders, predators, and even photosynthesizers; the latter include corals that house symbiotic algae.
Internal parasites are usually sessile because they live within their lifetime food supply. Mobile animals that pursue sedentary strategies for seeking prey include web-spinning spiders a terrestrial mode of filter feeding or deep-sea fishes with morphological adaptations that lure prey. Burrowing animals typically eat the rich organic substrates they move through. Others burrow for protection and either temporarily emerge and gather organic sediments at the top of their burrows or pump water with potential food through the burrow.
Instead of digging or finding burrows, some animals move into the centre of sponges, where they find protection and a renewing source of food. Active movement in search of food requires energy, but this expenditure is more than made up for by an ability to seek out areas of concentrated food. This method of feeding applies to burrowing animals that eat the substrate through which they move, as well as to animals that move over solid surfaces, swim, or fly.
Actively moving animals can feed on organisms that do not move, a rich variety coating virtually the entire solid surface of Earthfrom the depths of the oceans to the peaks of many mountains.
Form, function, and evolution of living organisms
The main problem with this most productive avenue of food gathering is protection. Shells and poisons are the major types of defenses, although innovative detoxification metabolism and jaws of various kinds breach the defenses in part. This is an escalating battle in which the defenses, as well as the weapons to penetrate them are continually improving.
Nudibranchs, shell-less marine snails, incorporate the defensive stinging cells of prey cnidarians into their own skin. Poisonous plants are eaten by specialized insects that avoid or detoxify the poison. In fresh water, for reasons not known, the arms race has not proceeded as far as in the sea. Cooperation of individuals enables social animals to obtain food in novel ways.
Uncannily like humans, some ants farm and herd other organisms for food. For example, some cultivate a fungus on leaves they cannot directly digest, while others herd aphids from which they milk nectar actually the phloem sap of plants.
Some ants even raid the nests of other species and make slaves of them. Another form of cooperation is the mutualism between species that trade advantage for advantage.
Some fishes feed on parasites on the surfaces of other fishes, which benefits all but the parasites. In many animals, including termites and ruminants, microorganisms thrive in the gut and digest cellulose for them.
The nervous system Coherent movement results only when the muscles receive a sensible pattern of activating signals for example, antagonists must not be activated to contract simultaneously. Animals use specialized cells called neurons to coordinate their muscular activity; nerves are bundles of neurons or parts thereof.
Neurons communicate between cells by chemical messengers, but within a single cell often extremely long they can send high-speed signals through a wave of ionic polarization analogous to an electric current along their membranes, a property inherent in all cells but developed for speed in nerve cells by special modifications. A system of communication requires three parts: In animals, sensory nerves and organs such as eyes collect the information; associative nerves usually concentrated into a brain integrateevaluate, and decide its relevance; and effector or motor nerves convey decisions to the muscles or elsewhere.
Although all three parts of the nervous system have kept pace with increases in the size and complexity of animals, the simplest systems found among animals those of parazoans and coelenterates are nevertheless capable of intricate feats of coordination.
All ends of a coelenterate bipolar neuron can both receive and transmit an impulse, whereas the unipolar neurons of more derived animals receive only at one end dendrite and transmit at the other axon. A neuron can have multiple dendrites and axons. The earliest animals were probably radial in design, so that bipolar neurons arranged in a netlike pattern made sense.
In such a design, a stimulus impinging at any point on the body can travel everywhere to alert a simple array of myofilaments to contract simultaneously. In the case of directed locomotion and relevant sensory input received at the head end of a bilateral animal, unidirectional transmission of nerve impulses to muscles becomes the only way to communicate effectively.
Structure & function — Science Leadership Academy @ Center City
The location of the brain in the head also reflects efficiency and the speed of receipt of information, because this position minimizes the distance between sensory and associative neurons as well as concentrates these two functions in a small, protected part of the body. In most animals nerve cells cannot be replaced if lost, although axons can be. Nerve cells tend to be concentrated centrally in ganglia or nerve cords, with long axons extending peripherally.
Although certain animals may lose tails or limbs to predators or in accidents and then regenerate them, loss or damage to the central nervous system means death or paralysis. The nervous system uses the transmission properties of neurons to communicate. To pass to the next cell at a synapse, where an axon meets a dendrite, a chemical transmitter is required.
Although chemical transmission is considerably slower than the ionic wave, it is more flexible. For example, learning involves in part increasing the sensitivity of a particular nerve pathway to a stimulus. The sensitivity of a synapse can be altered by increasing the amount of transmitter released from the axon per impulse received, increasing the number of receptors in the dendrite, or changing the sensitivity of the receptors. Bridging the synapse directly by the formation of membrane-bound gap junctionswhich connect adjacent cells, enables an impulse to pass unimpeded to a connecting cell.
The increase in speed of transmission provided by a gap junction, however, is offset by a loss in flexibility; gap junctions essentially create a single neuron from several.
The same result can be achieved more effectively by lengthening the axons or dendrites, making some nerve cells metres in length.
Situations arise where gap junctions become desirable, however. Gap junctions are found in vertebrate cardiac and smooth muscles, both of which transmit impulses along their cells to others. This ability makes these muscles somewhat independent of nervous-system control. A body can thus be kept partly functioning for some time without the activity of a brain.
Nerve impulses travel faster along axons of greater diameter or along those with good insulation against ion leakage except at spaced nodes required for recharging.
Form, function, and evolution of living organisms
Vertebrates use their unique myelinated axons to increase the transmission rate of nerve impulses, whereas invertebrates are limited to using axons of greater diameter. As a result, vertebrates can concentrate more small neurons into a body of a particular size, with the potential for greater complexity of behaviour. Memory is still a poorly understood aspect of the nervous system.
As in learning, both short- and long-term memories seem to involve alterations in the ease with which subsequent impulses travel a particular pathway after it has been used. Transfer of memory through direct ingestion of the brain has not been confirmed experimentally.
Although the underlying mechanisms are only dimly understood, it is known that there is a correlation between learning and memory capacity. The capacities for both increase with the number of associative neurons and the number of branches or interconnections formed. Since learning is a process of associating incoming cues with appropriate motor or internal response, greater memory capacity of a brain gives a more rapid learning process. Memory of inappropriate responses to an incoming set of cues can be used without motor repeat.
The degree to which the neurons of a brain develop interconnections is correlated with the complexity of its environs while growing. Basic, repeated behaviours are inherited or learned by the development of fixed pathways by which an environmental signal reaches the motor nerves rapidly with little or no variation reflex arcs.
Nonreflex behaviour requires a decision to be made in the brain, with the resulting pathway to the motor nerves becoming more fixed habitual as one particular decision seems always to be correct. Reflexes are faster than decisions, but their relative adaptiveness depends on context. Animals vary in the degree to which they use reflexes or make decisions, patterns that are strongly correlated to brain size.
Habitual actions are perhaps the most prevalent response, a compromise between the speed of a response and its appropriateness to context. The senses Appropriate behaviour relies on receiving adequate information from the environment to alert an animal to the presence of food, mates, or danger. Although sensory nerves carry this information to the brain, they do not always directly perceive the external world. Other modified cells intervene to convert light waves into vision, pressure waves in air or water into sound, chemicals into smell or taste, and simple contact into touch.
Some animals have other senses, as for electric or magnetic fields. In visionfor example, a photosensitive molecule changes shape and thereby sets off a chain of reactions that ultimately depolarize the dendrite of a sensory nerve. The associative neurons in the brain interpret the pattern of incoming impulses into a composite picture. The accuracy of what is seen increases with brain size and the complexity of the visual gathering system, or eyes. Animal eyes range from being able to discern only the presence or absence of light to being able to see objects in vivid colour and great detail.
Some animals see in ranges beyond unaided human vision. Pollinating insects in particular discern the colour of flowers differently than do humans; the ultraviolet reflection patterns of flowers do not always coincide with their coloured ones.
Bees and birds perceive polarized light and can orient themselves by it. Some animals perceive long wavelengths, which are associated with heat infraredand can locate the presence of warm-blooded prey by such a mechanism. Chemoreceptors are usually little-modified sensory neurons, except for the taste receptors of vertebrates, which are frequently replaced cells in synaptic contact with permanent sensory neurons.
Chemoreception is based on the recognition of molecules at receptor sites, lipid-protein complexes that are liberally scattered on the dendrites of a sensory neuron. When the receptor recognizes one particular molecule by shape and sometimes chemical compositionit fires an impulse. The pattern of firings set off in the receptors of a certain molecule provides the information that the brain interprets as an odour or a taste. The details of how animals smell and taste are not as well understood as are the other senses.
In many animals, chemoreceptors are not concentrated into obvious organs as they are in vertebrates, making even their location difficult to discern. Sounds are waves of molecular disturbance that move through air, water, or solids, and their perception by animals simply uses sensitive mechanoreceptors. Loud sounds can also be felt by the general touch receptors of the body and thereby influence its sense of well-being.
Sound receptors are sensitive hair cells or membranes that depolarize a sensory neuron when bent by the passage of a sound wave. Direct deformation of the dendritic membrane or release of transmitters by the hair cells fire the sensory neurons. Aside from a few insects, only vertebrates have organs with which to hear.
Fishes and aquatic amphibians use a lateral-line systemand other vertebrates use ears; both organs use hair cells as phonoreceptors. Sound waves directly stimulate the hair cells of lateral-line systems, while sound waves only indirectly stimulate the hair cells of ears through an amplifying system of membranes and bones, which reaches a peak of complexity in mammals.
Sound is the preferred medium of communication between animals that hear. It can be used over longer distances than vision, and it can be used when vision is not possible. The signals decay more rapidly than do those of odours, and therefore the information can be more precise.
Mechanoreceptors also respond to touch, pressure, stretching, and gravity.Biology: Cell Structure I Nucleus Medical Media
They are located all over the body and enable an animal to monitor its state at any moment. Much of this monitoring is subconscious but necessary for normal functioning. Mechanoreceptors are often just sensory nerves, but other cells may be involved. Unlike other senses, that of touch is found in all animals, even sponges, where it reflects a general cellular trait of eukaryotes. Hormones Hormones are the chemical integrators of a multicellular existence, coordinating activities from daily maintenance to reproduction and development.
The neurotransmitters released by axons are one class of chemical communicators that act on an adjacent cell, usually a muscle cell or another neuron. Hormones are a mostly distinct class of chemical communicators secreted by nerves, ordinary tissue, or special glands; they act on cells far removed from the site of their release.
They can be proteins, single polypeptides, amines, or steroids or other lipids.