All Living Things Obtain Resources (PDF)

All living things must be able to obtain the resources they need to continuously support their metabolism and growth (including reproduction). This is no trivial matter, as even when the resources are present in ample supply in the environment, the organism must still be able to bring them in and di...

All living things must be able to obtain the resources they need to continuously support their metabolism and growth (including reproduction). This is no trivial matter, as even when the resources are present in ample supply in the environment, the organism must still be able to bring them in and distribute them to areas of need fast enough to keep up with demand. The ability to obtain and deliver adequate nourishment to all parts of the body is a major factor in determining the size to which both individual cells and multicellular organisms can grow. For sufficiently small organisms living in a sufficiently wet environment, adequate nourishment may simply be a matter of taking up certain dissolved molecules or nutrients (such as oxygen, nitrate, or sugar) from the surrounding water across the organism’s outer surface, and then allowing them to disperse throughout the body on their own. Prokaryotes, unicellular eukaryotes, and some very thin multicellular organisms, such as algae, sponges, and flatworms, use this approach successfully for at least some of the resources they need. Even nonvascular land plants—the bryophytes—can get away with simply absorbing water and nutrients across all the surfaces of their body, provided their environment is moist enough and they remain small enough. Simple uptake and passive spread of goods generally becomes less viable as organisms grow larger because the interior volume they need to nourish increases relatively more rapidly than does the surface area across which they can take up goods (Fig. 7.1), and absorption alone simply isn’t an option for organisms that live in dry conditions regardless. Thus, organisms that have a large number of deeply interior cells and those that live in truly dry conditions must be able to actively pull resources in from the environment and distribute them throughout their bodies. Kinetic energy causes random dispersal of solutes and gases Recall that kinetic energy is specifically the energy associated with motion. Heat is the term we use to refer specifically to the kinetic energy associated with chaotic motion. All free atoms and molecules are subject to constant random motion, and the warmer the system is, the more vigorously they move. This constant molecular motion is essential not only to the chemistry of life but also the uptake and transport of goods by living things. Random motion is a regular feature of the molecular world Atoms and molecules are in constant motion, due to the constant impacts of energy they receive in various forms from the environment. This background motion is mainly random: atoms and molecules vibrate and careen off each other chaotically, without any uniformity of direction. We can see the effects of these myriad molecular collisions in the phenomenon of Brownian motion, which is a slight jiggling of microscopic objects that occurs in water and is visible under a microscope. Rigid chemical binding prevents atoms and molecules in solid substances from moving much, but once the bonds begin to break (as occurs, for example, when a substance melts or catches fire), they are more free to randomly disperse throughout the liquid or especially gas over time. The hotter the system, the more vigorously the atoms and molecules will randomly move about, and the more rapidly they will disperse. Evaporation is the dispersal of water molecules into air The energy imparted on a molecule of water when another object collides with it may be strong enough to break it free of the hydrogen bonds that otherwise hold it to surrounding water molecules. When this happens, one of two fates will occur for that water molecule: it will either re-form hydrogen bonds with surrounding water molecules, thus staying in solution, or it will be knocked free into the air as water vapor (gas). This second fate, in which water molecules transition from liquid to gas, is called evaporation. Anything that causes water to move more vigorously will increase the rate at which it evaporates: heat, wind, or even physical splashing will all act to increase the rate of evaporation from a pool of water. The amount of water vapor already present in the air will also play a role, since the drier the air is, the more likely it is that an evaporated molecule will remain as vapor, rather than forming hydrogen bonds with surrounding water molecules and condensing into a droplet large enough to fall back to Earth. Diffusion is the net movement of atoms or molecules down their concentration gradient due to random motion Random motion causes free atoms and molecules to become randomly dispersed throughout a solution or gas over time. The random nature of this dispersal necessarily means that, at any given moment in time, more molecular bits will be moving out of a cluster of high concentration than are moving into it. This nonrandom transfer of atoms or molecules from a region of high concentration to low is called diffusion. Diffusion is a slow process over large distances, but is much faster in a gas than in a liquid, because the molecules are spaced much farther apart in a gas allowing the atoms or molecules more freedom of movement. Note that although diffusion represents a directional transport of molecular bits down their concentration gradient, this does not mean that the bits move together in only direction; indeed, their direction of movement remains fundamentally random. Rather, diffusion simply reflects an overall shift from a less random to a more random distribution over time as atoms and molecules disperse, which results in a net nonrandom transfer of bits into regions of lower concentration on balance. Once the bits are evenly dispersed, the nonrandom net transfer effect of diffusion will cease, even though the random motion of the bits will continue. Osmosis is the diffusion of liquid water Water molecules that are present in liquids or gases are just as subject to random motion as any other component of the system, and thus, water too will disperse and diffuse. The diffusion of water in solution from areas of higher to lower water concentration is called osmosis. When considering osmosis, it is important to be mindful that water concentration is inversely related to the total overall solute concentration of a solution—that is, that a higher solute concentration equates to a lower water concentration (lower water purity). From the perspective of water concentration, only the total abundance of solutes dissolved in the solution matters, regardless of what they are. The total overall solute concentration of a solution is measured in units of osmolarity (Osm, in units of moles per liter), which is like the common concentration unit of molarity (M) but measured with regard to all of the various solutes taken together rather than just one. Water will thus diffuse from regions of lower osmolarity to regions of higher osmolarity. To quickly reference relative osmotic conditions, biologists refer to the relative concentration of solutes in one solution compared to another as its tonicity: Hypertonic = hyperosmotic: The solution has a higher solute concentration (higher osmolarity) than the reference solution Hypotonic = hypoosmotic: The solution has a lower solute concentration (lower osmolarity) than the reference solution Isotonic = isoosmotic: The solution has the same solute concentration (same osmolarity) as the reference solution. Water molecules pass somewhat easily through most cell membranes, since they are small enough and sufficiently weak in polarity to slip fairly easily between the hydrophobic fatty acid tails of the membrane interior. However, osmosis into or out of cells can be greatly enhanced by aquaporin channels, which create passageways for water to flow through cell membranes even more readily. Diffusion alone can support the uptake and distribution of water and nutrients only in organisms with very thin bodies Diffusion through solutions is a relatively slow and inefficient process, but even slow shifts can be effectively rapid over a short enough distance. Like a snail that may take hours to cross a yard but passes a single blade of grass in mere seconds, diffusion is very slow over long distances but occurs in practically an instant over the few nanometers it takes to cross a cell membrane. This rapid short-distance transfer of goods from areas of higher to lower concentration by diffusion is an essential process in all cells. For example, during aerobic cellular respiration cells consume oxygen and produce carbon dioxide. As oxygen levels fall and carbon dioxide levels rise in a cell, diffusion acts to rapidly supply fresh oxygen to the cell from surrounding higher-oxygen fluids, while eliminating excess carbon dioxide. In some cases, diffusion may be all that is necessary to keep a unicellular or even multicellular organism supplied with the water and at least some of the goods they need from the surrounding environment, provided they live in water and have bodies that remain thin enough for their surface area to volume ratios to remain favorable. Remarkable unicellular examples of this principle are found among the protists of the genus Acetabularia, which are green algae with palm tree or flower-like morphologies that can grow as tall as 10 cm in height, despite their bodies consisting of just a single cell with a single nucleus. This growth is possible because of their thin morphologies and use of photosynthesis to produce sugar over their entire body. Single-celled algae of the genus Caulerpa can grow even larger by this same strategy but with a coenocytic body, meaning it is a single cell with multiple nuclei, effectively like a multicellular organism made up of one continuous cell. In 2022, scientists reported the discovery of a bacterium that is able to grow to as long as 2 cm in size—far exceeding typical prokaryotic limits—by filling most of its interior volume with a water-filled sac that keeps all of its biomolecular guts pressed close to the cell’s membrane (a finding that even challenges our basic notions of prokaryotic life, given the presence of such a complex internal membranous structure)! Some multicellular aquatic organisms, including algae, flatworms, sponges, and jellies, have evolved the same kinds of strategies to enable all of their active cells to remain in close proximity to the surrounding water even as they grow larger in size overall. Most algae and flatworms have very flat bodies, while sponges have bodies that are perforated with channels through which water perfuses such that no active cell is located more than a few cells away from the external environment. Giant kelps and jellies are able to achieve both great size and volume by producing an inert material that fills most of their interiors but requires little maintenance, around which the active cells are layered and remain in close contact with the surrounding water. Living things use energy to actively transport goods and control water movement Although diffusion is an essential process in all cells, it cannot supply deep interior spaces or distant cells at the rates they need, and living things must frequently transport goods against their concentration gradients. In these cases, organisms must expend energy to actively bring in and distribute goods. The expense of energy to move goods against their concentration gradients, or more rapidly than would be achieved by diffusion alone, is called active transport. All cells bind molecules from surrounding fluids and expend energy to move them across the cell membrane. This form of active transport is the only means by which most bacteria and fungi acquire nutrients; as decomposers, they feed by secreting digestive enzymes into the environment and then absorbing the nutrients that are released. Many eukaryotic cells also use a form of active transport called phagocytosis, in which the cell engulfs whole chunks of food that are then digested internally. Phagocytosis is widespread among the protists and is even utilized by some cells of multicellular organisms, as in the various food particle ingesting cells of sponges or pathogen consuming white blood cells of vertebrates. Eukaryotic cells further use energy to rapidly distribute these goods throughout their cells via cytoskeletal filaments and transport vesicles, enabling eukaryotic cells to generally grow much larger than their prokaryotic counterparts. Living things use active transport to control the movement of water as well, in two main ways. First, living things actively transfer solutes to adjust the osmolarities of cells or parts of cells, thereby creating conditions for water to follow by osmosis. Second, many multicellular organisms utilize tube-shaped cells and tissues to rapidly pipe large volumes of water (along with any dissolved goods it carries) from one part of the organism to another. This bulk flow through vascular tissues is driven by differences in water pressure along the tubular vessels, from regions of higher osmotic pressure to lower osmotic pressure. This pressure gradient may be driven by solute concentrations within tissues and through physical squeezing. 7.2. Vascular plants and fungi achieve uptake and bulk flow through active transport of solutes and osmotic pressure gradients Vascular tissues are the tissues responsible for conducting (piping) fluids through multicellular organisms. In the case of plants, this means xylem and phloem, which run parallel to each other in vascular bundles that form the veins of plants (Fig. 7.2). Xylem conducts water and minerals from the roots to the shoots of plants, while phloem moves sugars from leaves or storage tissues to other areas. Both xylem and phloem are composed largely of long, tubular, essentially hollow cells that act like a series of straws through which the fluids—called sap—flow. Sap movement through vascular plants is achieved through a combination of active and passive processes: energy is used to actively transport solutes into different parts of the plant, and water follows passively down its osmotic gradient. However, the mechanisms by which this flow is achieved differ between the xylem and phloem. Vascular tissues allow land plants to grow in more places and to larger sizes by enabling them to obtain water and nutrients from deep in the soil and pipe it to potentially great heights, while efficiently moving sugar from leaves to whichever tissues need it. Multicellular fungi, being filamentous organisms with bodies composed of chains of cells, achieve similar osmotic pressure flow of fluid through their hyphae, such that they too can acquire resources in one part of their body and rapidly distribute them throughout, even without any dedicated vascular tissues. The movement of water through plants is driven by water potential Recall that water tends to exhibit net movement from regions of higher water concentration (lower osmolarity) to lower water concentration (higher osmolarity) due to diffusion, which is a process known as osmosis. Water can also be forced to move by physical pressure, either by squeezing it out of an area (positive pressure) or sucking it into an area (negative pressure or tension). The overall tendency for water to move out of a given area is referred to as water potential, ψ (psi), which is reported in units of pressure (the standard international unit of which is the Pascal, Pa). More positive values of ψ indicate a greater tendency for water to push out of an area, whereas more negative values of ψ represent a greater tendency for water to pull into an area. The actual direction that water moves will depend on differences in ψ between areas: water always moves from areas of higher (more positive, or less negative) water potential to lower (less positive, or more negative) water potential. Although this principle effectively applies everywhere, it is mainly in vascular plants (and, to a lesser extent, fungi) that water movement is described in terms of water potential, probably because of the multiple pressure factors that conspire to cause water movement in plants (including osmolarity, water pressing against cell walls, tension due to evaporation, and gravity). Vascular plants adjust the internal solute concentrations of their roots to control water uptake from the soil For the roots of a plant to take up water from the surrounding soil, the water potential inside the roots must be less than that of the surrounding soil. Soil water is full of dissolved ions, including such nutrients as nitrate, phosphate, and iron, making the water potential of the soil water negative. Plants make the water potential of their roots even more negative by actively transporting these minerals from the surrounding soil water into their own cells, thus creating conditions for the soil water to passively follow by osmosis. This uptake of nutrients and absorption of water is accomplished mainly by very thin, high surface area projections on root cells called root hairs that penetrate into the interstitial spaces between the soil particles. Plants also exploit a symbiotic relationship with certain fungi, called mycorrhizae, to further enhance their ability to take up water and nutrients; the hyphae of the fungi are even finer than root hairs, making them even more able to infiltrate all the little nooks and crannies of the soil interstices, and also produce digestive enzymes that further liberate nutrients from organic matter and minerals that otherwise wouldn’t be available. Transpiration is the upward pull of water from the roots to the shoots of plants through xylem due to the evaporation of water from leaves When water enters the roots of a plant by osmosis, it flows into xylem cells where it is drawn up the plant under tension, carrying minerals along with it. This tension is caused by the evaporation of water from stomata, which are pores in the leaves of plants that open to allow for ventilation of gases between the leaves and surrounding air. Plants must periodically open their stomata to bring in fresh CO 2 and flush out excess O2 that has built up as a byproduct of photosynthesis, during which time they lose water to evaporation as well. Because of hydrogen bonding between water molecules, the evaporation of water causes an upward tug on the water in the xylem cells below as the vapor escapes, pulling the xylem sap upward like a chain to replace the water lost. This water transport mechanism is called transpiration, and it does not require any regular inputs of energy from the plant to occur—once water has entered the roots, evaporation will take care of the rest. The upward tug of transpiration is supported and assisted by capillary action within the xylem cells, in which the adhesion of water molecules to the sides of the cell walls (due to hydrogen bonding with the cellulose) helps to support the weight of the water chain and may even wick some of it upward. It should be noted that, although transpiration is a clever and effective way to carry water and minerals from the soil to the shoots of plants, it is a fundamentally wasteful process: most of the water that is transported up a plant by transpiration simply goes to replacing the water that was lost to transpiration. Put another way, most of the water taken in by a vascular plant simply passes right through it. If plants did not lose so much water leaving their stomata open for gas exchange in the first place, they wouldn’t need to take in so much water from the soil to replace it. Thus, adaptations that enable plants to slow their rates of transpiration and conserve water are beneficial, particularly in dry environments. To reduce transpiration rates and conserve water, the stomata of plants are usually located on the shaded lower surfaces of their leaves, rather than in the direct heat of the Sun, and plants generally keep their stomata closed as much as possible, especially during the heat of the daytime. Each stoma is formed by two guard cells that swell into a turgid kidney shape when filled with water, thus creating the opening between them (Fig. 7.3). When the guard cells lose water, they shrink and become flaccid such that the opening collapses. Plants regulate the opening and closing of stomata by actively transporting solutes into or out of the guard cells, thereby controlling the flow of water into or out of them by osmosis. Because transpiration is a passive process that is driven by evaporation, the xylem cells that conduct the sap need only be thin hollow straws through which water can be drawn. They do not even need to be alive! Xylem contains two types of pipe cells bundled into its tissue, both of which are effectively dead and have small holes in their ends that allow water to flow freely from one to the next. Tracheids are very thin, tapered cells that are abundant in both gymnosperms and angiosperms. Vessel elements have far larger volumes to support a much greater flow of water through each cell and are mainly found in angiosperms (with most gymnosperms not having them at all). The cell walls of both tracheids and vessel elements are thickly reinforced with cellulose and sometimes even stronger lignin so that the tension of the water pulling through them will not cause them to collapse (like a straw that is sucked on too hard). Transpiration is only possible because water is cohesive, resulting in a chain of water being pulled upward. Of course, there are limits to the mass of water that can be pulled upward before the hydrogen bonds between them will snap and the chain will collapse under its own weight. How, then, are trees able to grow so tall? The answer is that in addition to being cohesive, water is adhesive: water molecules stick not only to each other but to other polar compounds as well, including the cellulose and lignin that make up the walls of the xylem cells. This enables the walls of the xylem cells to bear the brunt of the weight of the xylem sap, acting as “handholds” for the water molecules within. Because xylem cells are extremely thin, a high percentage of the water molecules in the sap within them is able to directly cling to the walls of the xylem, providing much more support than larger vessel elements do. Together, this makes tracheids much less prone than vessel elements to the problem of cavitation (breaks or voids occurring in the water column within). Vessel elements, for their part, carry more sap per cell and are able to replace water lost to evaporation more rapidly (potentially supporting a greater number of stomata for gas exchange) but are much more subject to the problem of cavitation, with more dire consequences to the plant should it occur, given the larger volume of water transport that is disrupted when an embolism forms. Translocation is the transport of sugars from sources to sinks in plants through phloem due to osmotic pressure gradients Phloem tissue is responsible for distributing sugar produced in the leaves to other parts of the plant in a process called translocation. Unlike xylem sap, which always flows up the plant, phloem sap may move in whichever direction is necessary to redistribute sugar from areas of abundance to need, such as into or out of storage sites. Also unlike xylem flow, the movement of sugar through the phloem requires constant inputs of energy to achieve, via a mechanism known as the pressure flow hypothesis. During translocation the phloem-conducting cells, called sieve elements, expend energy to actively transport sugar from the source cells into the phloem sap. This is called phloem loading, and it has the effect of greatly increasing the osmolarity of the phloem sap in that region of the phloem tissue. The resulting reduction in water potential causes water from the surrounding area, including nearby xylem, to flow into the phloem sap by osmosis, causing pressure to build against the walls of the sieve elements. This increase in pressure pushes the phloem sap down the sieve elements, away from the source of sugar loading, carrying the sugar along with it. When it reaches the sink, the sugar is actively transported out of the phloem into the cells that need it, prompting water to also depart by osmosis, thereby maintaining the sugar and pressure gradient within the phloem. The flow therefore continues as long as active transport of sugar continues at each end of the phloem. Because translocation relies on constant active transport of sugars, the sieve tube elements—unlike the tracheids or vessel elements of xylem—must be alive to keep the process going. However, they must also still be mainly hollow so that the sap can flow through them, which limits their ability to carry much of the machinery for regular cellular functions. Sieve elements therefore have companion cells positioned alongside them that manage most cellular functions for them, providing them with the ATP, nutrients, and various other goods and regulatory functions they need to live. Multicellular fungi transport goods via a pressure flow mechanism similar to that of plant phloem Bacteria and fungi are decomposers, meaning they secrete digestive enzymes into the environment that break down matter to liberate nutrients, which they then absorb. Multicellular fungi have a great advantage over bacteria and even their yeast cousins, in that they can acquire nutrients and water with one part of their body and transport it to another. This means multicellular fungi are not limited to moist nutrient-rich environments the way bacteria and yeast generally are, and that they can grow to as large as many acres or even miles underground, infiltrating any food and water sources they encounter as they grow while using the nutrients to feed further expansion. Fungi likely achieve bulk flow transport by osmotic pressure gradients in a manner similar to that of the pressure flow hypothesis of plant translocation, with active uptake or internal liberation of solutes driving the influx of water that in turn builds up pressure against the fungal cell walls to force fluid movement. Because the pressure gradients seem to mainly push fluid out to the tips of the hyphae and even help them to extend and penetrate into new areas during growth, this transport has been dubbed tip-directed mass flow. Animals achieve uptake and bulk flow through physical force The main superpower of the animal kingdom is our incredible ability to move. Our muscles enable us to physically shift all manner of body parts in ways far beyond what other multicellular organisms can achieve. This is evident, for example, in the myriad ways animals take in, process, and internally distribute resources. Like plants and fungi, animals can use cellular transport to control osmotic conditions and achieve some transport, but most animals can also use the physical movement of muscles to consume food and water and then physically pump them to different parts of the body. The manner in which sponges acquire nutrients reflects their status as barely complex multicellular animals Sponges were the first animals to appear in the fossil record, and they remain prevalent across the Earth’s seafloors today. It isn’t obvious that sponges are animals; after all, they are sessile (don’t move); don’t have anything like a head, mouth, or circulatory system; and even lack nerves, muscles, and reproductive organs. The great Greek philosopher and father of zoology, Aristotle, thought they were plants! However, later discoveries, such as their use of collagen and a gastrulation-like stage during their embryonic development, confirmed that they are indeed animals. Although it would be misleading to call modern sponges more primitive than other animals, we can certainly say they are simpler. Sponges lack true tissues, and consist of two layers of cells separated by a jelly-like matrix coated over a skeleton of microscopic spicules, which are made of calcium carbonate, glass (SiO2), or protein. Numerous open pores and channels perforate their bodies, resulting in the spongy quality we think of in sponges. These channels are important in sponge feeding. Sponges are suspension or filter feeders, meaning they obtain their food by filtering it out of the water. To do this, sponges have flagellated collar cells lining their channels, called choanocytes, that beat flagella in unison to pull water past, trapping tiny particles of organic matter in the collar structure that fringes each choanocyte as it flows by. Sponges exhibit intracellular digestion, meaning that once a cell captures food particles, it engulfs them by phagocytosis and then digests them internally, as many protists do. Choanocytes engulf and digest some of the food particles for their own needs and then pass the remainder on to other cells, which will do the same. In this way, although they work together to acquire food, once captured each cell is ultimately responsible for obtaining its own nourishment from it. Animals actively ingest food and usually digest it internally The intracellular digestion scheme used by sponges is unusual among the animals. While many of the cells of other animals can also digest matter within intracellular compartments (vacuoles), they do this to break down and recycle or dispose of waste material rather than digest food. Instead, other animals use a more centralized strategy of food and water consumption with extracellular digestion, in which the cells of specialized tissues secrete digestive chemicals and enzymes that break food down, after which the nutrients released are absorbed and distributed to the rest of the animal’s body. In most animals, muscles are used to consume the food through a mouth and pass it in bulk into a digestive tube or chamber for internal digestion, though a few (such as sea stars and spiders) digest the food outside of their body and then consume the resulting nutrient slurry. The need to more efficiently acquire nutrients has been a strong driver of animal evolution, and today animals exhibit a wide variety of feeding strategies using different kinds of mouth parts to exploit diverse food sources. Suspension or filter feeders obtain food particles by sifting them from water, while deposit feeders sift food from sediment or soil. Fluid feeders consume liquids, such as nectar from flowers, sap from trees, or blood from mammals. Bulk feeding occurs when animals ingest great chunks of food at a time, and includes such strategies as suction feeding, in which an animal snorts in and swallows a whole organism; rasping, in which an animal uses its mouthparts to scrape and consume bits of its prey at a time; and biting, in which jaws or jaw-like structures are used to either forcefully cut or tear pieces of an organism free or gape open to consume the organism whole, representing the most effective and versatile form of bulk feeding that has evolved. Most animals pass food into a digestive organ to be broken down once it has been consumed. In some animals, such as the cnidarians, there is only one opening into this organ that acts as both the mouth and anus, such that any undigested solid waste remaining afterward is expelled out the same way that it came in. However, most animals (including all of the bilaterians) have a proper gut, in which food is brought in through a mouth, nutrients are extracted from the food and absorbed as it passes through the digestive organs, and the leftover waste matter is expelled out an anus. A proper gut consists of three general processing zones which generally correspond with each of these steps: food is first taken into the foregut, which includes the mouth and any food storage and breakdown chambers; passed through the midgut, where nutrients are absorbed while the food may be broken down further; and finally exits through the hindgut, which recovers water and sometimes a few additional nutrients from the digestive mix and holds the resulting solid waste until it is excreted out the anus. In vertebrates, the mouth, esophagus, and stomach together represent the foregut, the small intestine is the midgut, and the large intestine, rectum, and anus make up the hindgut. Passage of the digestive slurry or chyme (pronounced KAIm, which rhymes with time) through the digestive system is controlled by progressive muscular wave or squeezing contractions, called peristalsis and catastalsis, respectively, while circular muscles called sphincters regulate the passage of the chyme from one part of the gut to another, by constricting or dilating the openings between them. Prokaryotes are important partners in animal digestion Animal bodies are home to a teeming microbiome of prokaryotic cells, most of which live in the gut. While some of these microbes are pathogenic, the vast majority are helpful. Although we have long known that bacteria may cause disease, we are just now beginning to understand the myriad diseases that instead may actually be caused by a lack of the right kinds of microbes. The prokaryotes that populate animal guts aid digestion by helping to break down biomolecules, especially those that the animal lacks the enzymes to digest on its own, though unfortunately, some of them produce upsetting gases and odors in the process. Along the way, the microbiome also produces a wide variety of metabolites that appear to have wide ranging impacts on the biochemistry and thus well being of their animal hosts. A rich carpet of beneficial bacteria within animal guts even helps to protect them from harmful germs, by denying the pathogens a foothold in the gut ecosystem, should they happen to find their way in. All terrestrial and most aquatic animals use a circulatory system to rapidly distribute goods throughout their bodies Diffusion alone can sufficiently hydrate and nourish some effectively thin aquatic animals, such as sponges, jellies, flatworms, and tardigrades, but all other animals require a vascular system to distribute goods from the point of intake to the rest of the body via bulk flow. Animals achieve this circulation by squeezing muscular tubes to generate pressure, thereby forcing fluids to move about the body. Animal circulatory systems may be open or closed. In an open circulatory system, as found in arthropods and most mollusks, muscular tubes called aortic arches rhythmically squeeze to flush a bath of circulatory fluid around the animal’s organs, like jets swirling a hot tub. In a closed circulatory system, as found in invertebrates like annelid worms and cephalopod mollusks (squids, octopuses, and the chambered nautilus) along with all of the vertebrates, the circulatory fluid is instead contained within long tubes called vessels throughout, kept moving by one or more aortic arches or more robust and internally complex hearts. The circulatory fluid of a closed circulatory system is called blood, and the vessels that carry blood away from the main heart under high pressure are called arteries. Arteries branch into a network of very fine hair-like vessels, called capillaries, which spread throughout the tissues to exchange water, oxygen, carbon dioxide, and other goods and wastes with surrounding fluids and cells by diffusion. These capillaries flow back into larger veins that return the blood to the heart under low pressure. Both arteries and veins are frequently surrounded by smooth muscle that enables them to dilate or constrict as needed to control the flow of blood to different parts of the body. Arteries are tough and elastic to resist the pulses of high pressure generated in them by the beating of the heart, whereas veins—which are not subject to pressure bursts—do not need to be robust or resilient against rupture, but instead need to have one-way valves to help prevent backflow of blood. In a closed circulatory system, water and goods are constantly exchanged between blood and the interstitial fluid, or lymph, that fills the spaces between surrounding body cells, but because they never directly mix the blood and lymph always remain physically and chemically different from each other. This is not the case in open circulatory systems, where the circulatory and interstitial fluids are effectively one and the same. The single pervasive body fluid of an open circulatory system is therefore often referred to as hemolymph (HEE- muh-limf) rather than blood. The internal cavity or chamber that holds the bath of hemolymph around the organs is, accordingly, called a hemocoel (HEE-muh-seel). Circulatory fluids often utilize oxygen-binding proteins to increase the amount of oxygen they can carry Oxygen gas (O2) enters into circulatory fluids by diffusion. The amount of dissolved oxygen that the fluid can hold on its own is not enough to meet the demand in most animals, however. Even when fully saturated, the dissolved oxygen in the fluid will usually be depleted well before it is able to reach all of the cells over the course of its circulation. To solve this problem, circulatory fluids usually contain an oxygen-binding protein that acts as an oxygen sponge or bank to take up and sequester oxygen, later releasing it back to the surrounding fluid as needed to replace oxygen that is taken up by cells along the way. In this way, the circulatory fluid can carry and deliver many times more molecules of oxygen than it can without such proteins. Vertebrate and invertebrate blood differ fundamentally in that vertebrate blood is rich in infection-fighting white blood cells, and red blood cells that carry stored oxygen. The oxygen-binding protein of vertebrate blood is called hemoglobin and is held inside the red blood cells, which are given their red color by iron in the heme group of hemoglobin, which is the part responsible for binding the oxygen. Vertebrate muscles use a closely related iron- containing protein called myoglobin to also bind and store oxygen, but with an even higher affinity for oxygen, ensuring that it will transfer from blood to muscle rather than the other way around. In addition to its various blood cells, vertebrate blood also carries numerous proteins, some of which play roles in immune defense, blood clotting, and damage repair. In contrast, while the circulatory fluids of invertebrates also contain various proteins, they have no cells. Among these free-circulating proteins, hemolymph either carries no oxygen- binding proteins, or uses ones with a copper-based oxygen binding group, resulting in fluids that are clear to yellowish green or blue, rather than red, in color. Animals move water or air across a respiratory surface to take up oxygen and eliminate carbon dioxide The diffusion of gases between an organism and surrounding water or air is called respiration. In the case of animals, this means taking up fresh oxygen gas (O 2) and eliminating carbon dioxide waste (CO2) in support of aerobic cellular respiration. For respiration to occur, fresh air or water must move past the tissues responsible for exchanging the gases. The movement of the external gas-containing medium across the organism’s respiratory surface is called ventilation. As we have seen, plants ventilate by opening their stomata to allow fresh air to flush into their leaves while their waste gases flush out. Animals, for their part, often use body movements to achieve ventilation across a variety of respiratory surfaces. Water-breathing animals take up oxygen through their skin or gills Many invertebrates that live in aquatic or very moist environments, including some with closed circulatory systems, like the annelid worms, achieve respiration by diffusion across the skin. In these cases, just allowing the surrounding environment to naturally mix and flow by the body of the organism may provide sufficient ventilation on its own, though the animal may enhance this by simply moving about. Among the vertebrates, amphibians also frequently achieve a measure of respiration in this manner. Most aquatic animals, invertebrate and vertebrate alike, exchange gases with water through structures called gills. Gills are made up of very fine layers of tissue perfused with hemolymph or capillaries. Their thinness ensures rapid diffusion of gases across the tissues, and they also have large surface areas to maximize exchange with the surrounding water. Gills may be internal or external. External gills simply billow out in the surrounding water for ventilation, with no protection for their delicate tissues. Internal gills instead have a covering that protects them from damage, but also inhibits water flow past them. Most animals with internal gills therefore have a mechanism for generating water flow past their gills to improve ventilation. Fish, for example, have internal gills covered by a structure called the operculum, which can open slightly to allow water to pass out. To ventilate, fish gulp or swim forward to bring water in through their mouths, which then passes past their gills and out of the body from under the operculum. This results in continuous ventilation, in which fish enjoy a constant unidirectional flow of fresh, oxygen-rich water across their gills. The capillaries of the gills also carry the blood through them in the opposite direction of the water flowing past, thereby increasing oxygen uptake efficiency through the principle of countercurrent exchange, in which exchange across a surface is enhanced by a more favorable diffusion gradient along its entire length (in this case, because the fresh, oxygen-rich water entering the gill bed encounters the mostly-oxygenated blood that is on its way out, after having already taken up much oxygen from the water, while in turn the stale, largely oxygen-depleted water exiting the gill bed at the other end has a little oxygen yet to give to the highly deoxygenated blood that is just returning from the body for oxygenation). Air-breathing animals take up oxygen through lungs In principle, gills can diffuse oxygen from air just as well as from water, but in practice, without proper support their thin tissues collapse and stick together out of water, like a wet tissue, thus blocking gas exchange. They also dry out rapidly in air. Some aquatic animals, such as crabs, get around these problems by having rigid support structures that hold the gills open and a protective carapace that traps moisture, enabling them to regularly venture onto land for a time. Breathing air full-time, however, requires yet additional innovation, with more robust tissues that are even better protected from desiccation, as realized in lungs. Lungs are respiratory organs in which the respiratory surfaces are fully internalized, and therefore work only in air, as this full enclosure is only made possible by the ease with which air can be moved to achieve ventilation. The arachnid arthropods (spiders, scorpions, ticks, and mites) use book lungs that are structured very much like internal gills, being likewise composed of stacks of thin respiratory membranes, but with their respiratory surfaces being separated by channels of air rather than flowing water. Most air-breathing vertebrates greatly boost their oxygenation capacity through lungs that instead work like a bellows, physically expanding to draw fresh air into tubes that lead to the respiratory surfaces. In the case of amphibians, reptiles, and mammals, the tubes that bring air into the lungs end in sacs called alveoli, the walls of which are packed with capillaries for gas exchange. In this scheme, fresh air is carried from the windpipe or trachea into the lungs, where oxygen and carbon dioxide are exchanged between the air and blood by diffusion across the capillary walls of the alveoli. In amphibians, the air is forced into the lungs by gulping and swallowing air, which is very inefficient. In reptiles and mammals, air is instead drawn in by expanding the lungs during inhalation, and then pushed back out the way it came in by contracting the lungs during exhalation. Although this scheme is highly effective overall, the reversal of air flow in the lungs, or tidal breathing, introduces some inefficiency, since it results in discontinuous ventilation in which fresh oxygen is supplied to the lungs only during the inhalation part of the cycle—meaning the body effectively receives oxygen only half the time. This inefficiency is compounded by the mixing of fresh air brought in by each inhalation with a small volume of residual stale air that cannot be fully expelled during exhalation, and yet further by the fact that the alveoli must be robust enough to handle the physical strain of expansion and contraction, resulting in slower uptake of oxygen than would be achieved if they could be thinner. Birds also utilize lungs, and even though they too breathe in and out through a trachea, they have evolved an ingenious scheme that results in circular breathing for much more efficient continuous ventilation across the lungs. To achieve this, the lungs themselves do not expand and contract, but rather air sacs positioned around them do, arranged in such a way that they pull and push air continuously in the same direction across the capillaries of the lungs, which have no alveoli. Air thus enters the trachea and flows through a loop that traverses the lungs, before being expelled again through the trachea, so that birds continue to receive a fresh supply of oxygen regardless of whether they are inhaling or exhaling. This not only allows for constant oxygenation of the blood, but also eliminates the problem of residual air sitting in the lungs, and—because they are immobile—allows the lungs to harbor much finer respiratory surfaces, allowing for more efficient gas exchange by diffusion. Together, these adaptations provide birds with the most efficient respiratory system of all the vertebrates, which supports the great energy demands of flight, even in the lower oxygen of high altitudes. Interestingly, fossil evidence reveals that their dinosaur ancestors almost certainly had this same respiratory arrangement, which may explain how dinosaurs were able to grow so much larger than is typical of animals today, even despite the lower oxygen levels of their time. Some terrestrial invertebrates, most notably insects, have evolved another way to breathe air, by ventilating it directly to their muscles through spiracles that line their bodies, rather than distributing oxygen from a centralized set of lungs. Most generally, spiracles are any openings that carry air or water through tubes to a respiratory surface, which includes the holes that bring water to the gills of some fish, the pores that bring air into the channels of arachnid lungs, and even the blowholes of whales. The spiracle respiratory system of insects is unique, however, in that their muscles serve as the respiratory surface directly, rather than relying on gills or lungs as a middleman. Insects can increase the rate that air flushes through their spiracles by expanding and contracting their thorax, akin to the bellows action of vertebrate lungs. Decentralizing oxygen uptake in this way allows insects to deliver oxygen to their muscles at a rate that supports a high level of activity, despite the circulatory inefficiency of their open circulatory system. Animal activity levels generally track with the kinds of circulatory systems they have One of the circulation challenges animals confront is the need to supply their muscles with oxygen at a high enough rate to sustain their use. The open circulatory systems used by many invertebrates are fairly inefficient for delivering fresh oxygen to tissues, since their constant free mixing of hemolymph under low pressure does not allow any way of keeping oxygenated and deoxygenated fluid separate and also provides no way of especially directing it to particular tissues in need. Some invertebrates, such as insects, get around this by piping oxygen directly to their muscles, rather than relying on hemolymph circulation to deliver it. Most other invertebrates with open circulatory systems are simply limited to less active lifestyles. Closed circulatory systems, in contrast, can push blood through the body more rapidly under higher pressure, allow for different blood vessels to be dilated or constricted to control blood supply to different tissues and divert more blood to regions of need, and keep oxygenated blood from mixing with and being diluted by deoxygenated body fluids. Taken together, these abilities enable closed circulatory systems to generally support higher levels of activity in animals. However, the degree to which closed circulatory systems can achieve these things is dependent on the number and types of hearts present., with certain kinds of heart arrangements supporting greater levels of sustained activity than others. The number of chambers in a heart determines the number of pressurized circuits it can support The several aortic arches present along the circulatory systems of most invertebrates are essentially thickened muscular vessels, which constrict regularly to squeeze the circulatory fluid through them. Together, the aortic arches produce many jets that effectively slosh the bath of hemolymph around in an open circulatory system or push blood through the different parts of a simple closed circulatory system. However, blood can be more powerfully and efficiently pushed through the vessels of a closed circulatory system by the larger, more muscular and capable structures of hearts. Hearts usually have multiple chambers that forcefully squeeze to efficiently push blood in a particular direction, under greater pressure and over greater distances than aortic arches can achieve. Chambers that receive blood in the heart are called atria, and those that push blood out are called ventricles. Blood is received into atria from veins under low pressure, which constrict to squeeze the blood into ventricles that then forcefully push the blood out to the tissues through arteries under high pressure. Flaps of tissue between the chambers are pushed open by the pressure to allow the blood to pass through in one direction but then close when the pressure shifts to the other side, preventing backflow. The rate of blood circulation through the whole system can be increased by accelerating the heartbeat, and to specific tissues by dilating just the vessels that supply and drain them. The flow of deoxygenated blood into the respiratory organs is called pulmonary circulation, whereas the flow of oxygenated blood out to the rest of the body is called systemic circulation. Pressure is necessarily lost from an artery when it divides into the fine network of capillaries that are used to exchange goods with the surrounding tissues, just as a matter of fluid dynamics. Thus, the only way for an animal to deliver blood under high pressure to both the pulmonary and systemic capillaries is to either have multiple hearts to pressurize each circuit separately, which is what some invertebrates with closed circulatory systems do, or have a single heart with enough chambers to support multiple circulatory loops (Fig. 7.4), which is mainly something that vertebrates do, although there are exceptions—squids and octopuses, for example, actually use both strategies, having three hearts, only one of which is multi-chambered (with three). The most important circulation loop to pressurize is typically the pulmonary circuit, to ensure that blood is being oxygenated quickly and thoroughly before being sent out to the rest of the body, and this is therefore the loop that is typically prioritized in a species’ plumbing when they lack the anatomy to pressurize more than one. A heart that has only one or two chambers can only pressurize one circulation loop, with two chambers (both an atrium and ventricle) enabling this to happen more effectively. This is the case with fish, whose single two-chambered heart drives rapid oxygenation of blood through the gills but then can do nothing to help deliver the oxygen to the rest of the body (Fig. 7.4a). The weak systemic circulation that results can be aided by the general squeezing effects of muscle movements as the fish swims. Amphibians and most reptiles have a single three-chambered heart (Fig. 7.4b), in which the atrium is divided into two separate atria. In this heart, one atrium receives oxygenated blood returning from the lungs and the other receives deoxygenated blood returning from the systemic circulation. Both atria drain their blood into the same shared ventricle, causing the oxygenated and deoxygenated blood to mix before it is pressurized to flow out two openings, one leading to the lungs and the other to the rest of the body. This heart enables blood to be delivered under high pressure to both the pulmonary and systemic circuits but at the cost of delivering partially oxygenated blood to both. Some reptiles today, such as turtles, have an incomplete septum (partition) in the ventricle that partially divides it, reducing the mixing considerably. When both the atrium and ventricle of a heart are fully divided by a complete septum, the result is a four-chambered heart with two atria and two ventricles that, together, support two fully independent circulatory loops (Fig. 7.4c). This is the kind of heart that mammals, crocodiles, and birds have, and is the most efficient and capable heart that has evolved. In a four-chambered heart, blood from the pulmonary circuit drains into one atrium, while blood returning from the systemic circuit drains into the other, with each passing from there into a separate ventricle to be pressurized to either the lungs or body. A heart with four chambers is therefore able to pump both fully deoxygenated blood under high pressure to the respiratory capillaries and fully oxygenated blood to the systemic capillaries, without any loss of efficiency due to mixing in between. The lymphatic system of vertebrates returns interstitial fluid to the blood Water and dissolved molecules that ooze out of capillaries collect in the spaces between the cells of surrounding tissues, where they bathe, nourish, and pick up waste products from the cells. This interstital fluid is called lymph, and vertebrates have a separate network of vessels called the lymphatic system that helps to drain and return this fluid to the blood. The lymphatic system does not have any pumping mechanisms, and thus lymph flows through its vessels only under low pressure, but as with the systemic circulation of fish its movement is greatly aided by the squeezing effects of regular body movements. It is for this reason that we may experience swelling of our lower extremities when we remain upright but inactive for long periods of time, causing lymph to pool under the draw of gravity. Blockages of the lymphatic system can result in massive accumulations of lymph, as occurs in the pronounced and disfiguring edemas of elephantiasis. The lymphatic system is also important in immune defense, since it has a multitude of lymph nodes that contain white blood cells to destroy pathogens that have collected in it. The water vascular system of echinoderms is a unique circulatory scheme that is also used for locomotion The echinoderms—sea stars, sea urchins, sea cucumbers, and related organisms—have a unique bulk transport system that also serves as a hydraulic movement system. This water vascular system, as it is known, consists of an open body cavity that fills with fluid, akin to an open circulatory system, paired with a network of tubes through which fluid is actively pumped, like a closed circulatory system. In sea cucumbers, this fluid is produced internally, like blood, but in the other echinoderms it is mainly just seawater pulled in from the surrounding ocean. The water vascular system takes up and distributes oxygen and other goods like a circulatory system, but its most remarkable purpose is actually locomotion: by squeezing sacs in the system, echinoderms force fluid into numerous flexible tube feet, causing them to become rigid and extend. Through this squeezing, a sea star can generate enough force to pry open the shells of prey, after which they extend their stomach out of their body and drop it onto their victim to digest it externally. The tube feet, along with numerous small projections that cover the organism’s body, also do double-duty as major sites of gas exchange for respiration, in keeping with the other major function of the system.