Chapter 1. Homeostasis Module

Introduction

Integrating material from chapters 1, 5, 26, 29, 35, 36, 37, 38, 39, 40, and 41.

Core Concepts:

• H.1 Homeostasis is a fundamental feature of cells and organisms.
• H.2 The plasma membrane maintains homeostasis at the level of the cell.
• H.3 Water balance in animals is a form of homeostasis.
• H.4 Water balance in plants is a form of homeostasis.
• H.5 Many organ systems maintain homeostasis in animals.

Environmental conditions are always changing. It can be hot or cold; dry or humid; sunny or rainy, or even snowy. We sometimes have beautiful summer days and occasionally violent storms. Taking a longer view, we have ice ages punctuated by warmer interglacial periods. For aquatic organisms, temperature, pH, and salinity of the water can vary widely from place to place and from time to time.

By contrast, the environment within cells and organisms is remarkably constant. Whether we consider temperature, water balance, pH, or a number of other key parameters, these stay for the most part within a very narrow range of conditions. This constancy is critical for life itself because chemical reactions and physiological processes can often only take place under these conditions.

We might think of something constant as static or fixed, but this is not the case for cells and organisms. Because the environment outside is often in a state of flux, there is an active and dynamic interplay between the outside and internal environment to maintain these constant conditions within the cell or organism. Like standing on one foot, maintaining an internal steady state takes constant small adjustments. In addition, it takes work in the face of changing environmental conditions. That is, a cell or organism expends a considerable amount of energy to achieve this kind of balance. The active maintenance of a stable internal environment is called homeostasis.

How does a cell or organism maintain homeostasis? What parameters are stably maintained? What organ systems are involved in maintaining homeostasis in animals? We explore answers to these questions in this module.

H.1 An Overview of Homeostasis

The concept of homeostasis was first described as regulation of the body’s “interior milieu” in the late 1800s by the French physiologist Claude Bernard, who is often credited with bringing the scientific method to the field of medicine. The term “homeostasis” was coined by the American physiologist Walter Cannon, whose book The Wisdom of the Body (first published in 1932) popularized the concept. We can think of homeostasis in terms of the conditions within an individual cell or within an organism.

There are many conditions inside of a cell that are actively maintained in a narrow window. Ion concentrations are often kept relatively constant, as they are important for many cellular functions. For instance, the firing of action potentials by neurons requires particular ion concentrations on either side of the membrane (Chapter 35). Temperature and pH inside cells are other parameters that do not vary much because enzymes often work effectively only within narrow temperature and pH ranges. Recent evidence suggests that nerve cell firing rates are maintained at a steady level in the brain of rats, regardless of sensory stimulus or deprivation, or whether the animals are awake or asleep.

Homeostasis is also maintained for the whole body. Many physiological parameters are maintained within a narrow range of conditions throughout the body, including pH, temperature, and ion concentrations. For example, some animals are faced with long periods of drought that challenge their ability to remain hydrated and maintain a stable water and ion balance (Chapter 41). These animals must rapidly respond by changing the permeability of their skin and respiratory organs so that they can retain as much water as possible.

H.2 Cellular Homeostasis

In chapter 1, we discussed three key features of cells: a stable archive of information, a plasma membrane that separates the inside of a cell from the outside, and the ability to harness energy from the environment. It is the second feature, the plasma membrane, that maintains homeostasis within cells. Cells are not closed systems independent of the environment. On the contrary, there is an active and dynamic interplay between cells and their surroundings that is controlled by the plasma membrane.

The plasma membrane of cells maintains homeostasis because it is selectively permeable. This means that it lets some molecules in and out freely; it lets others in and out only under certain conditions; and it prevents other molecules from passing through at all.

The membrane’s ability to act as a selective barrier is the result of the combination of lipids and embedded proteins of which it is composed. The hydrophobic interior of the lipid bilayer prevents ions as well as charged or polar molecules from diffusing freely across the plasma membrane. Furthermore, many macromolecules such as proteins and polysaccharides are too large to cross the plasma membrane on their own. By contrast, gases, lipids, and small polar molecules can move freely across the lipid bilayer. Protein transporters in the membrane allow the export and import of molecules, including certain ions and nutrients, that cannot cross the cell membrane on their own.

The identity and abundance of these membrane-associated proteins vary among cell types, reflecting the specific functions of different cells. For example, cells in your gut contain membrane transporters that specialize in the uptake of glucose, whereas nerve cells have different types of ion channel that are involved in electrical signaling.

The simplest form of movement into and out of cells is passive transport. Passive transport works by diffusion, which is the random movement of molecules. Molecules are always moving in their environments. For example, molecules in water at room temperature move around at about 500 m/sec, which means that they can move only about 3 molecular diameters before they run into another molecule, leading to about 5 trillion collisions per second. The frequency with which molecules collide has important consequences for chemical reactions, which depend on the interaction of molecules (Chapter 6).

Diffusion leads to a net movement of the substance from one region to another when there is a concentration gradient in the distribution of a molecule, meaning that there are areas of higher and lower concentrations. In this case, diffusion results in net movement of the molecule from an area of higher concentration to an area of lower concentration (Fig. 5.9).

Some molecules diffuse freely across the plasma membrane as a result of differences in concentrations between the inside and the outside of a cell. Oxygen and carbon dioxide, for example, move into and out of the cell in this way. Certain hydrophobic molecules, such as triacylglycerols (Chapter 2), are also able to diffuse through the cell membrane, which is not surprising since the lipid bilayer is likewise hydrophobic.

Some molecules that cannot move across the lipid bilayer directly can move passively toward a region of lower concentration through protein transporters. When a molecule moves by diffusion through a membrane protein and bypasses the lipid bilayer, the process is called facilitated diffusion. Diffusion and facilitated diffusion both result from the random motion of molecules, and net movement of the substance occurs when there are concentration differences (Fig. 5.10). In the case of facilitated diffusion, the molecule moves through a membrane transporter, whereas in the case of simple diffusion, the molecule moves directly through the lipid bilayer.

Membrane transporters are of two types. The first type is a channel, which provides an opening between the inside and outside of the cell within which certain molecules can pass, depending on their shape and charge. Some membrane channels are gated, which means that they open in response to some sort of signal, which may be chemical or electrical (Chapter 9). The second type of transporter is a carrier, which binds to and then transports specific molecules. Membrane carriers exist in two conformations, one that is open to one side of the cell, and another that is open to the other side of the cell. Binding of the transported molecule induces a conformational change in the membrane protein, allowing the molecule to be transported across the lipid bilayer, as shown on the right in Fig. 5.10.

Up to this point, we have focused our attention on the movement of molecules (the solutes) in water (the solvent). We can take a different perspective and focus instead on water movement. Water itself also moves into and out of cells by passive transport. Although the plasma membrane is hydrophobic, water molecules are small enough to move passively through the membrane to a limited extent by simple diffusion. In addition, many cells have specific protein channels, known as aquaporins, for transporting water molecules. These channels allow water to move more readily across the plasma membrane by facilitated diffusion than is possible by simple diffusion.

The net movement of a solvent such as water across a selectively permeable membrane such as the plasma membrane is known as osmosis. As in any form of diffusion, water moves from regions of higher water concentration to regions of lower water concentration (Fig. 5.11). Because water is a solvent within which nutrients such as glucose or ions such as sodium or potassium are dissolved, water concentration drops as solute concentration rises. Therefore, it is sometimes easier to think about water moving from regions of lower solute concentration toward regions of higher solute concentration. Either way, the direction of water movement is the same. During osmosis, the net movement of water toward the side of the membrane with higher solute concentration continues until it is opposed by another force. This force could be pressure due to gravity (in the case of Fig. 5.11) or the cell wall (in the case of plants, fungi, and bacteria, as described below).

Passive transport works to the cell’s advantage only if the concentration gradient is in the right direction, from higher on the outside to lower on the inside for nutrients that the cell needs to take in, and from higher on the inside and lower on the outside for wastes that the cell needs to export. However, many of the molecules that cells require are not highly concentrated in the environment. Although some of these molecules can be synthesized by the cell, others must be taken up from the environment. In other words, cells have to move these substances from areas of lower concentration to areas of higher concentration. The “uphill” movement of substances against a concentration gradient, called active transport, requires energy. The transport of many kinds of molecules across membranes requires energy, either directly or indirectly. In fact, most of the energy used by a cell goes into keeping the inside of the cell different from the outside, a function carried out by proteins in the plasma membrane.

During active transport, cells move substances through transport proteins embedded in the cell membrane. Some of these proteins act as pumps, using energy directly to move a substance into or out of a cell. A good example is the sodium-potassium pump (Fig. 5.12). Within cells, sodium is kept at concentrations much lower than in the exterior environment; the opposite is true of potassium. Therefore, both sodium and potassium have to be moved against a concentration gradient. The sodium-potassium pump actively moves sodium out of the cell and potassium into the cell. This movement of ions takes energy, which comes from the chemical energy stored in ATP. Active transport that uses energy directly in this manner is called primary active transport. Note that the sodium ions and potassium ions move in opposite directions. Protein transporters that work in this way are referred to as antiporters. Other transporters move two molecules in the same direction, and are referred to as symporters or cotransporters.

Active transport can also work in another way. Because small ions cannot cross the lipid bilayer, many cells use a transport protein to build up the concentration of a small ion on one side of the membrane. The resulting concentration gradient stores potential energy that can be harnessed to drive the movement of other substances across the membrane against their concentration gradient.

For example, some cells actively pump protons (H⁺) across the cell membrane using ATP (Fig. 5.13a). As a result, in these cells the concentration of protons is higher on one side of the membrane and lower on the other side. In other words, the pump generates a concentration gradient, also called a chemical gradient because the entity forming the gradient is a chemical (Fig. 5.13b). We have already seen that concentration differences favor the movement of protons back to the other side of the membrane. However, the lipid bilayer blocks the movement of protons to the other side and therefore stores potential energy, just like a dam or battery.

In addition to the chemical gradient, another force favors the movement of protons back across the membrane: a difference in charge. Because protons carry a positive charge, the side of the membrane with more protons is more positive than the other side. This difference in charge is called an electrical gradient. Protons (and other ions) move from areas of like charge to areas of unlike charge, driven by an electrical gradient. A gradient that has both charge and chemical components is known as an electrochemical gradient (Fig. 5.13b).

If protons are then allowed to pass through the cell membrane by a transport protein, they will move down their electrochemical gradient toward the region of lower proton concentration. These transport proteins can use the movement of protons to drive the movement of other molecules against their concentration gradient (Fig. 5.13c). The movement of protons is always from regions of higher to lower concentration, whereas the movement of the coupled molecule is from regions of lower to higher concentration. Because the movement of the coupled molecule is driven by the movement of protons and not by ATP directly, this form of transport is called secondary active transport. Secondary active transport uses the potential energy of an electrochemical gradient to drive the movement of molecules; by contrast, primary active transport uses the chemical energy of ATP directly.

The use of an electrochemical gradient as a temporary energy source is a common cellular strategy. For example, cells use a sodium electrochemical gradient generated by the sodium-potassium pump to transport glucose and amino acids into cells. In addition, cells use a proton electrochemical gradient to move other molecules and, as we discuss in Chapter 7, to synthesize ATP.

Many cells use active transport to maintain a constant size. Consider human red blood cells placed in a variety of different solutions (Fig. 5.14). If a red blood cell is placed in a hypertonic solution (one with a higher solute concentration than that inside the cell), water leaves the cell by osmosis and the cell shrinks. By contrast, if a red blood cell is placed in a hypotonic solution (one with a lower solute concentration than that inside the cell), water moves into the cell by osmosis and the cell lyses, or bursts. Animal cells solve the problem of water movement in part by keeping the intracellular fluid isotonic (that is, at the same solute concentration) as the extracellular fluid. Cells use the active transport of ions to maintain equal concentrations inside and out, and the sodium-potassium pump plays an important role in keeping the inside of the cell isotonic with the extracellular fluid.

Human red blood cells avoid shrinking or bursting by maintaining an intracellular environment isotonic with the extracellular environment, the blood. But what about a single-celled organism, like Paramecium, swimming in a freshwater lake? In this case, the extracellular environment is hypotonic compared to the concentration in the cell’s interior. As a result, Paramecium faces the risk of bursting from water moving in by osmosis. Paramecium and some other single-celled organisms contain contractile vacuoles that solve this problem. Contractile vacuoles are compartments that take up excess water from inside the cell and then, by contraction, expel it into the external environment. The mechanism by which water moves into the contractile vacuoles differs depending on the organism. The contractile vacuoles of some organisms take in water through aquaporins, while the contractile vacuoles of other organisms first take in protons through proton pumps, with water following by osmosis.

A third example comes from the domain Archaea. One group of archaeons illustrates the limits of tolerance to salinity, or saltiness. The Haloarchaea are halophilic (salt-loving) photoheterotrophs that use the protein bacteriorhodopsin to absorb energy from sunlight. They live in waters that are salty enough to precipitate table salt (NaCl), maintaining osmotic balance by accumulating ions (especially K⁺) and organic solutes in their cytoplasm. Just as extreme acidophiles require acid conditions, these extreme halophiles require high salt conditions and cannot live in dilute water.

H.3 Water Balance in Animals

Water and electrolytes are two key substances that allow cells and organisms to maintain their size and shape, as well as to carry out essential functions. Life originated and diversified in a watery environment. Consequently, the chemical functions of cells and organisms crucially depend on the properties of water (Chapter 2) and the relative amounts of water and electrolytes inside a cell.

Animal life on Earth can exist in a wide range of environments by adopting different means of osmoregulation, which is the regulation of osmotic pressure. Osmoregulation can be thought of as the regulation of water content, keeping internal fluids from becoming too concentrated (high osmotic pressure) or too dilute (low osmotic pressure). Osmoregulation, like energy balance (Chapter 40), is a form of homeostasis. High osmotic pressures inside a cell can damage the cell, sometimes even causing it to burst and thus disrupt some of the animal’s functions. Low osmotic pressure can lead to dehydration. Although cells and tissues can tolerate dehydration, often as high as 50% or more, excessive dehydration impairs a cell’s metabolic function because many chemical reactions, such as hydrolysis reactions that break down proteins and nucleic acids into individual subunits, depend on the presence of water.

How do cells control their internal osmotic pressure? We have seen that water follows solutes across selectively permeable cell membranes. To regulate water and solute levels, and hence the osmotic pressure inside cells, the cell controls the solute concentration of the inside of the cell relative to the outside. At the level of the cell, then, osmoregulation is achieved by the movement of solutes, particularly electrolytes.

At the level of the organism, osmoregulation is achieved by balancing input and output of water and electrolytes. Let’s first consider how animals gain and lose water. Animals gain water by drinking water that is less concentrated in solutes (hypotonic) than their body fluids. Humans and many other vertebrates, both freshwater and terrestrial, gain most of their water this way. Animals also gain water through the food they eat. Finally, animals produce water as a product of cellular respiration (Chapter 7). This is the primary source of water for some animals adapted to deserts and oceans, where drinkable water is not readily available. Animals lose water in their urine and feces. Terrestrial animals lose water through evaporation from their lungs, and, in the case of humans, by sweating. Freshwater fish gain water through their gills, whereas most marine animals lose water through their gills.

Now let’s consider how animals gain and lose electrolytes. Animals gain electrolytes in the food they eat. Marine animals can gain electrolytes by drinking salt water, which has a higher solute concentration than their body fluids. Marine aquatic animals also gain electrolytes as hypertonic water moves across their gills. In contrast, freshwater aquatic animals lose electrolytes through diffusion across their gills into the hypotonic watery environment. Humans lose electrolytes as well as water when they sweat. Drinking beverages high in electrolytes is helpful before demanding physical activities—including sports—because sweating during these activities can result in substantial water and electrolyte loss. Electrolytes are also lost by specialized glands (discussed below) and in urine and feces.

There are two basic ways in which animals maintain homeostasis with respect to water and electrolytes. Some animals match their internal osmotic pressure to that of their external environment: These animals are osmoconformers. They keep their internal fluids at the same osmotic pressure as the surrounding environment, which reduces the movement of water and solutes into or out of their bodies. Because the solute concentrations inside and outside osmoconformers are similar, osmoconformers don’t have to spend a lot of energy regulating osmotic pressure. However, they have to adapt to the solute concentration of their external environment. Osmoconformers tend to live in environments like seawater that have stable solute concentrations, because maintaining stable internal solute concentrations is easier in stable environments.

Although osmoconformers generally match the overall concentration of their tissues with their external environment, they often expend energy to regulate the concentrations of particular ions such as sodium, potassium, and chloride and of other solutes such as amino acids and glucose. For example, whereas the intracellular space of nearly all multicellular animals has a relatively high concentration of potassium ions and low concentration of sodium ions, the extracellular space has concentrations that are just the opposite—low in potassium and high in sodium. This means that these cells are actively pumping sodium out and potassium in (Chapter 5).

Most marine invertebrates, such as sea stars, mussels, lobsters, and scallops, are osmoconformers, matching their total intracellular solute concentration to the solute concentration of seawater. These organisms maintain high concentrations of sodium and chloride in their cells to achieve an overall solute concentration close to that of seawater. Consequently, they have few specializations for osmoregulation beyond the need to regulate specific internal ion concentrations relative to their environment. Some marine vertebrates are also osmoconformers. Hagfish and lampreys, for example, match their total internal solute concentration to that of seawater by maintaining high internal concentrations of particular electrolytes, just as marine invertebrates do.

Other osmoconformers, like sharks, rays and coelacanths, match seawater’s solute concentration by maintaining a high internal concentration of a compound called urea. Urea is a waste product of protein metabolism that many other animals excrete, so it does not build up to high concentrations (section 41.2). By retaining this solute, these marine vertebrates are able to achieve osmotic equilibrium with the surrounding seawater.

The second way in which animals achieve water and electrolyte homeostasis is by maintaining internal solute concentrations different from that of the environment. These animals are osmoregulators. Osmoregulators expend considerable energy pumping ions across cell membranes in order to regulate the movement of water into or out of their bodies. Some freshwater and marine fish that are osmoregulators are estimated to expend 50% of their resting metabolic energy on osmoregulation. However, the ability to regulate osmotic pressure allows osmoregulators to live in diverse environments—salt water, fresh water, or land.

Teleosts (or bony fish, the largest group of marine vertebrates) are osmoregulators. So are all freshwater and terrestrial animals. Marine bony fish maintain concentrations of electrolytes much lower than the surrounding seawater, whereas freshwater fish and amphibians maintain concentrations that are higher than the surrounding freshwater.

Let’s consider how osmoregulators achieve osmotic balance in salt water, in fresh water, and on land. In salt water, marine organisms live in an environment that is hypertonic relative to their internal fluids, like the cytoplasm of the cell, blood, and other internal fluids. These organisms maintain internal electrolyte concentrations lower than the surrounding salt water. They are thus faced with the problem of water loss (across their gills, as well as in their urine and feces) and electrolyte gain (across their gills and in seawater that they drink). As a result, they need to take in as much water as possible and eliminate excess electrolytes. They obtain water primarily through the food they eat and cellular respiration. They avoid water loss by producing concentrated urine (discussed in section 41.2).

Bony fish have evolved specialized gills that allow them to excrete excess electrolytes as well as to breathe. The thin gill filaments, with their extensive surface area, are useful sites for electrolyte transport out of the body in addition to being well adapted for gas exchange (Fig. 41.2). Specialized cells in the gills called chloride cells pump chloride ions into the surrounding seawater. By pumping negatively charged chloride ions out of the body, chloride cells create an electrical gradient that is balanced by positively charged sodium ions moving out of the body as well (Fig. 41.2a).

In fresh water, the environment is hypotonic relative to an organism’s internal fluids. In this case, the problem is just the opposite as that faced by marine organisms: water gain and salt loss. As a result, freshwater organisms tend to minimize their water intake and maximize water elimination. For example, in contrast to marine fish, they do not drink water. In addition, freshwater fish produce very dilute urine (discussed in section 41.2).

To maintain electrolyte concentration higher than the surrounding water, freshwater fish have gill chloride cells with reversed polarity relative to marine bony fish. Their chloride cells pump chloride ions (with sodium ions following) into the body to counter their ongoing loss from the gills into the surrounding fresh water (Fig. 41.2b).

Nearly all amphibians also live in freshwater environments and therefore face challenges similar to those facing freshwater fish. They take up electrolytes across their skin and produce dilute urine.

On land, the challenge is water loss. Terrestrial animals lose water in their urine and feces, through evaporation from their lungs, and, in humans, by sweating. To counter these losses, they drink water that is hypotonic relative to their body fluids. They also acquire water by eating food and by producing water during cellular respiration. They produce concentrated urine to minimize water loss.

Many marine birds have evolved specialized nasal salt glands to rid themselves of the high salt content of their diet (Fig. 41.3). These salt glands actively pump ions from the blood into the cells that make up the gland, and then excrete salt from the body through the gland and the nasal cavity. Their salt glands allow these birds to gain net water intake by drinking seawater. In contrast, humans and many other animals that live in non-marine environments are incapable of deriving useful net water gain by drinking seawater because they cannot eliminate salt in high enough concentration. The high concentration of magnesium sulfate in seawater results in diarrhea if too much seawater is consumed, exacerbating the net loss of water in the feces.

An extreme example of the ability of an animal to regulate water and electrolyte levels is provided by the life cycle of salmon. Mature salmon swim in the salt water of the open ocean but return upriver to spawn in fresh water (Fig. 41.4). During the course of their return, they swim from high-salt ocean water through water of intermediate saltiness into freshwater rivers and streams that have little salt. After spawning, the young feed and grow in fresh water for up to 3 years, eventually returning to the open ocean, where they live until they become reproductively mature and return to their home river system to spawn again.

These changes in aquatic habitat demand that salmon make dramatic adjustments in the way they handle water and solutes. When in the ocean, salmon must avoid loss of water and uptake of electrolytes. However, when mature salmon migrate upriver to spawn and as young salmon continue to grow in freshwater streams and rivers, the challenge is reversed: They must avoid gaining too much water and losing critical electrolytes across their gills. In salmon, radical changes in osmoregulation are required over the course of an animal’s lifetime to enable it to reproduce in freshwater streams and return to the sea for further growth and maturation.

H.4 Water Balance in Plants

Water balance is essential for plants as well as animals, but photosynthesis makes water balance especially challenging because large surface areas must be exposed to the air in order to obtain sunlight and carbon dioxide (CO₂). The evolution of vascular plants transformed the physical and biological environment on land. Vascular plants keep their photosynthetic cells hydrated with water drawn from the soil and thus can carry out photosynthesis even when the soil surface is dry and can sustain the water content of photosynthetic leaves elevated as much as 100 meters into the air. In many ways, the capacity to control the uptake and loss of water made possible the extraordinary evolutionary success of vascular plants.

Fig. 29.3 shows the general features of a vascular plant. Aboveground, we see three major types of organ—leaves, stems, and reproductive organs—which collectively form the shoot. Roots, which are generally hidden from view in the ground, make up the fourth major organ system. The leaf is the principal site of photosynthesis in vascular plants. A cross section shows the leaf’s three major tissues: sheets of cells called the epidermis line the leaf’s upper and lower surfaces; loosely packed photosynthetic cells make up the mesophyll (literally, “middle leaf”); and the system of vascular conduits called veins connects the leaf to the rest of the plant (Fig. 29.4). In this section, we examine how these organ systems work together to allow for photosynthesis without drying out.

Water is the resource that most often limits a plant’s ability to grow and function. We see this easily when we forget to water our houseplants or garden. And when you drive across a continent, you see that wet places look very different from dry ones. Water has such a significant effect on the growth and functioning of plants because plants use large amounts of water. Why do plants require so much water?

The answer is not because of water's role as an electron donor in photosynthesis (Chapter 8). That process accounts for less than 1% of the water required by vascular plants. Instead, most of a plant’s need for water arises as a consequence of CO₂ uptake from the air.

As we saw in Chapter 25, CO₂ is a minor constituent of air, about 400 parts per million at present. This low concentration limits the rate at which CO₂ can diffuse into the leaf. Therefore, how fast a plant can take up CO₂ depends in large part on the degree to which leaves can expose their photosynthetic cells to the surrounding air. If we look inside a leaf, we see that each mesophyll cell is largely surrounded by air. The mesophyll cells obtain the CO₂ that they need for photosynthesis directly from these air spaces. If the air spaces within the leaf were completely sealed off, photosynthesis would quickly run out of CO₂ and come to a halt. But they’re not sealed off—the leaf’s air spaces are connected to the air surrounding the leaf by pores in the epidermis. As photosynthesis draws down the concentration of CO₂ molecules within the leaf’s air spaces relative to the concentration of CO₂ in the outside air, the difference in concentration between the inside and the outside of the leaf causes CO₂ to diffuse into the leaf, replenishing the supply of CO₂ for photosynthesis.

The ability of leaves to draw in CO₂ comes at a price: If CO₂ can diffuse into the leaf, water vapor can diffuse out (Fig. 29.4). Furthermore, water vapor diffuses out of a leaf at a much faster rate than CO₂ diffuses inward. Molecules diffuse from regions of higher concentration to regions of lower concentration, and the rate of diffusion is proportional to the difference in concentration. On a sunny summer day, the difference in water vapor concentration between the air spaces within a leaf and the air outside can be more than 100 times larger than the difference in concentration of CO₂. Add the fact that water is lighter than CO₂, and so diffuses 1.6 times faster for the same concentration gradient, and it becomes clear why, on a sunny summer day, several hundred water molecules are lost for every molecule of CO₂ acquired for photosynthesis.

The loss of water vapor from leaves is referred to as transpiration and the rates at which plants transpire are often quite large. A sunflower leaf, for example, can transpire an amount equal to its total water content in as little as 20 minutes. To put this rate of water loss in perspective, you would have to drink about 2 liters per minute to survive a similar rate of loss. Vascular plants can sustain such high rates of water loss because they can access the only consistently available source of water on land: the soil. Water moves from the soil, through the bodies of vascular plants, and then, as water vapor, into the atmosphere. Therefore, the challenge of keeping photosynthetic cells hydrated is met partly by the continual supply of water from the soil. As we discuss next, it is also met by limiting the rate at which water already in leaf tissues is lost to the atmosphere.

As noted above, small pores in the epidermis allow CO₂ to diffuse into the leaf (Fig. 29.5a). These pores are called stomata (singular, stoma). Stomata can be numerous, as there can be hundreds per square millimeter. Yet because each one is small, less than 1 to 2% of the leaf surface is actually covered by these pores. Thus, even with stomata, the epidermis is a significant barrier to the diffusion of CO₂ and water vapor.

The epidermis with its stomatal pores represents a compromise between the challenges of providing food (CO₂ uptake) while preventing thirst (water loss). However, because both the dryness of the air and the wetness of the soil are variable, leaves must be able to alter the porosity of the leaf epidermis to maintain a balance between the rates of water loss to the atmosphere and water delivery from the soil. They can do this by opening and closing their stomata.

Stomata are more than just holes in the leaf epidermis; they are hydromechanical valves that can open and close. Each stoma consists of two guard cells surrounding a central pore. The guard cells can shrink or swell, changing the size of the pore between the guard cells (Fig. 29.5b). How does this valve system work?

Cellulose consists of long molecules that provide strength to plant cell walls. In guard cells, the cellulose molecules are oriented radially – that is, wrapped around the cell. This makes it easier for swelling guard cells to expand in length rather than in girth. Because the guard cells are firmly connected at their ends, this increase in length causes them to bow apart, opening the stomatal pore. Conversely, a decrease in guard cell volume causes the pore to close.

Guard cells control their volume by altering the concentration of solutes, such as potassium ions (K⁺) and chloride ions (Cl^–), in their cytoplasm. ATP is needed used to drive the uptake of solutes across the plasma membrane against their concentration gradient. An increase in solute concentration causes water to flow into the cell by osmosis, whereas a decrease in solute concentration causes water to flow out of the cell. Recall from section H.2 and Chapter 5 that osmosis is the diffusion of water across a selectively permeable membrane, such as the cell’s plasma membrane, from a region of higher water concentration to a region of lower water concentration. An increase in the concentration of solutes in guard cells is accompanied by a corresponding decrease in the concentration of water molecules. Thus, as guard cells add solutes, water diffuses into the cells, causing them to swell.

Stomata close by the same process, only in reverse. Solutes leave the guard cells, and as the solute concentration decreases, water diffuses out of the cells, causing them to shrink.

To function effectively, stomata must open and close in response to both CO₂ and water loss. Stomata are thus key sites for the processing of physiological information. Light stimulates stomata to open, while high levels of CO₂ inside the leaf (a signal that CO₂ is being supplied faster than photosynthesis can take it up) cause stomata to close. Guard cell volume also changes in response to signaling molecules. For example, abscisic acid, a hormone produced during drought, causes stomata to close. Thus, stomata open when the conditions for photosynthesis are favorable, and close when a water shortage endangers the hydration of the leaf.

The water balance of a plant is strongly dependent on the relative investment in producing roots versus leaves. A plant with a higher root:shoot ratio is better able to supply water to its leaves when the soil is dry. When a plant experiences drought, it produces more roots, and these roots penetrate farther into the soil, in part because they produce fewer lateral branches. By producing deeper roots, a plant increases its chances of reaching moister soil.

The root cap appears to play an important role in allowing roots to sense and respond to the amount of water in the soil. Because the root cap is not well connected to the plant’s vascular system, the water status of its cells provides a good indication of the moisture content of the soil. As soils dry, root cap cells produce a hormone called abscisic acid, abbreviated as ABA (Table 31.1). Abscisic acid stimulates root elongation, leading to roots that penetrate deeper into the soil. It also triggers stomata to close, reducing the demand for water.

Plants can also respond to soil drying by producing fewer leaves. Branching allows plants to support more leaves, but producing branches too close together results in leaves that overlap and shade one another. Because of the formation of axillary buds, seed plants have the potential to produce a new branch at each point where a leaf was attached. To avoid overlap, only a few axillary buds actually develop into branches. Which become branches is determined by the interaction of several hormones, including both cytokinins and auxin, which are produced by shoot meristems.

Recent studies indicate that this top-down control is complemented by the action of another hormone, strigolactone. This hormone is made in roots and transported upward in the xylem. Plants that are unable to synthesize strigolactone produce many more branches than do wild-type plants. This indicates that strigolactone inhibits the outgrowth of axillary buds. One hypothesis is that strigolactones are transported to the shoot when either water or nutrients are limiting. Thus the plant would stop growing new branches whenever the root system is unable to supply the water and nutrients they require.

H.5 Homeostasis in Animals

Water balance is essential for life, but it is not the only form of homeostasis. Organisms keep many conditions relatively constant in their bodies. Here, we discuss how organs and organ systems actively work to maintain homeostasis in animals.

How is homeostasis maintained? Homeostatic regulation often depends on negative feedback (Fig. 35.18). In negative feedback, a stimulus acts on a sensor that communicates with an effector, which produces a response that opposes the initial stimulus. For example, negative feedback is used to maintain a constant temperature in a house. Cool temperature (the stimulus) is detected by a thermostat (the sensor). The thermostat sends a signal to the heater (the effector), producing heat (the response). In this way, a stable temperature is maintained (Fig. 35.18a).

In a similar way, humans and other mammals maintain a steady body temperature even as the temperature outside fluctuates. Nerve cells in the hypothalamus (located in the base of the brain) act as the body’s thermostat (Fig. 35.18b). When a decrease in the temperature in the environment causes a drop in body temperature, the lowered body temperature signals the hypothalamus to activate the somatic nervous system to induce shivering and the production of metabolic heat, as discussed in Chapter 40. At the same time, the hypothalamus activates the autonomic nervous system, causing peripheral blood vessels to constrict. The reduction in blood flow near the body’s surface reduces heat loss to the surrounding air. By contrast, an increase in temperature signals sweat glands to secrete moisture and peripheral blood vessels to dilate to aid heat loss from the skin.

Homeostatic regulation, therefore, relies on negative feedback to maintain a set point, which in this case represents an animal’s preferred body temperature. The ability to maintain a constant body temperature is known as thermoregulation, and it is just one of many physiological set points that the body actively maintains.

The autonomic nervous system plays a key role in homeostasis. The autonomic nervous system (Fig. 35.17) controls internal functions of the body such as heart rate, blood flow, digestion, excretion, and temperature. It includes both sensory and motor components, which usually act without our conscious awareness. The autonomic nervous system, in turn, is divided into two major subdivisions, a sympathetic division and a parasympathetic division. Both divisions continuously monitor and regulate internal functions of the body. Generally, sympathetic and parasympathetic nerves have opposite effects, but not always. For example, sympathetic neurons stimulate the heart to beat faster, whereas parasympathetic neurons cause the heart to beat slower.

The sympathetic nervous system generally results in arousal and increased activity. It is the pathway activated when animals are exposed to threatening conditions, resulting in what is referred to as the “fight or flight” response. As part of this response, the sympathetic nervous system increases heart rate and breathing rate, stimulates glucose release by the liver, and inhibits digestion. The parasympathetic nervous system, in contrast, slows the heart and stimulates digestion as well as metabolic processes that store energy—in other words, it enables the body to “rest and digest.”

In addition to having different functions, sympathetic and parasympathetic nerves also have different anatomical distributions, as shown in Fig. 35.17. Sympathetic nerves leave the CNS from the middle (thoracic and lumbar) region of the spinal cord, forming ganglia along much of the length of the spinal cord. Parasympathetic nerves leave the CNS from the brain by cranial nerves and from lower (sacral) levels of the spinal cord.

Because an animal’s need for O₂ varies with activity level, animals adjust their respiratory rate to meet their cells’ changing demand for O₂. Respiration is a unique physiological process in that it is controlled by both the voluntary and involuntary components of the nervous system (Chapter 35). In sleep, breathing is maintained at a resting rate by the involuntary part of the nervous system. Indeed, in most circumstances, breathing is controlled unconsciously.

The regulation of blood O₂ levels is a key example of homeostasis, like core body temperature (Chapter 35). Recall that homeostasis often depends on sensors that monitor the levels of the chemical being regulated. In the case of breathing, the sensors are chemoreceptors located within the brainstem and in sensory structures called the carotid and aortic bodies that are located in the neck and near the heart (Fig. 39.10). The carotid bodies sense O₂ and proton (H⁺) concentrations of the blood going to the brain, and the aortic bodies monitor their levels in blood moving to the body. In contrast, chemoreceptors in the brainstem sense CO₂ and H⁺ concentrations. The most important factor in the control of breathing is the amount of CO₂ in the blood. If the concentration of CO₂ in the blood is too high, chemoreceptors in the brainstem stimulate motor neurons that activate the respiratory muscles to contract more strongly or more frequently. Stronger or faster breathing rids the blood of excess CO₂ and increases the supply of O₂ to the body.

Breathing can also be controlled voluntarily. It is a simple matter to voluntarily hold your breath. Holding the breath makes it possible for humans and marine mammals to dive under water; it is also critical to the production of speech, song, and sound for communication. Sound is produced by voluntarily adjusting the magnitude and rate of airflow over the vocal cords of mammals, the syrinx of songbirds, or the glottal folds of calling amphibians, such as some toads and frogs.

Endocrine control of internal body functions is also central to homeostasis. Without some way to maintain a stable internal environment, changing environmental conditions would lead to dangerous shifts in an animal’s physiological function. For example, an animal’s body weight depends on the regulation of its energy intake relative to energy expenditure. Disruption of hormones that regulate appetite and food intake can lead to obesity on the one hand or weakness and lethargy on the other. Similarly, hormones that regulate the concentration of key ions in the body, such as Na⁺ and K⁺, are fundamental to healthy nerve and muscle function (Chapters 35 and 36) and fluid balance within the body (Chapter 41).

How does the endocrine system maintain homeostasis? Maintaining homeostasis depends on feedback from the target organ to the endocrine gland that secretes the hormone. Because hormones are transmitted through the bloodstream, this feedback can occur over varying distances within the body, coordinating the function of several organs at any one time. In response to this feedback, the endocrine gland modifies its own subsequent production of hormone, either increasing or decreasing it.

Homeostasis typically depends on negative feedback, as we saw earlier for the control of both the temperature in a house and the core body temperature of an animal.

Many physiological parameters – such as blood glucose and calcium levels – are maintained at relatively steady levels by negative feedback. For example, a change in glucose or calcium levels causes a response that brings the levels back to the starting point, called the set point. In the case of the endocrine system, a change in level (the stimulus) is detected by a sensor in the endocrine organ (Fig. 38.4). The endocrine organ (the effector) releases a hormone, and the hormone causes a response in the body that opposes the initial stimulus such that the glucose or calcium levels move back toward the set point. The reversal of these levels causes secretion of the hormone to be decreased and there is a limit to further change. Constant feedback between the response and sensor maintains a set point. That is, the maintenance of homeostasis is an active process.

Let’s examine an example in more detail. Like core body temperature, the amount of glucose in the blood of animals is maintained at a steady level (Fig. 38.5). If glucose levels are too low, cells of the body do not have a ready source of energy. If glucose levels are too high for too long, they can damage organs.

Maintaining steady blood glucose levels is challenging because glucose levels rise immediately after a meal when glucose is absorbed into the bloodstream from the intestine, then fall as glucose is taken up by cells to meet their energy needs. How then are constant levels maintained in the body? After a meal, when blood glucose rises, β (beta) cells of the pancreas secrete the hormone insulin, which circulates in the blood (Fig. 38.5a). In response to insulin, muscle and liver cells take up glucose from the blood and either use it or convert it to a storage form called glycogen (Chapter 7). In this way, insulin guards against high levels of glucose in the blood.

What happens if blood glucose levels fall too low several hours after a meal? In this case, a different population of cells in the pancreas, called α (alpha) cells, secretes the hormone glucagon, which has effects roughly opposite to those of insulin (Fig. 38.5b). Glucagon stimulates the breakdown of glycogen into glucose and its release from muscle and liver cells. The result is that blood glucose levels rise. In both cases, the stimulus (either high or low blood glucose levels) is sensed by cells of the pancreas (β or α cells) and triggers a response (secretion of insulin or of glucagon, bringing blood glucose levels back to the set point). Note that in each case the response feeds back to the secreting cells to reduce further hormone secretion.

When the control of blood glucose levels by insulin fails, a disease called diabetes mellitus results. When untreated, diabetic individuals excrete excess glucose in their urine because blood glucose levels are too high. Diabetes causes cardiovascular and neurological damage, including loss of sensation in the extremities, particularly the feet.

Like core body temperature (Fig. 40.5), blood glucose levels (Chapter 38), and blood pressure (Chapter 39), the energy balance of an organism is often maintained at a constant level. An animal in energy balance takes in over time the same amount of calories of energy from food that it uses over time to meet its metabolic needs. Energy balance can be thought of as a form of homeostasis. We consider sources of energy, or energy intake, and ways in which energy is expended, or energy use.

For animals, the source of energy is the diet. In turn, energy is used to do work—maintain tissues, grow, move about, and the like. Energy required for basic life processes accounts for the majority (about 70%) of energy use. Interestingly, digestion and absorption of food themselves require energy, and this use is included in the 70% figure. The balance (about 30%) can be used for physical activity, with higher levels of activity using more energy, as we saw.

When energy intake does not equal energy used, there is an energy imbalance. If an animal eats more food than it requires, energy stores such as fat deposits grow over time. The result is that the body shifts its metabolism mostly to anabolic processes that build energy stores. Many animals achieve a net positive energy balance during the late summer and fall when food is plentiful, before it becomes scarce in winter. Other animals maintain a constant energy intake throughout the year by migrating to areas with more abundant food and avoiding colder temperatures that require increased energy expenditure to remain active and warm. Still other animals hibernate, or become less active, to conserve their energy use over the winter.

Animals that cannot acquire enough food are in negative energy balance and become undernourished. During prolonged periods of an inadequate food supply or starvation, an animal consumes its own internal fuel reserves. Starvation forces animals to deplete their glycogen and fat reserves first, and then, if no food is found, to resort to protein stores, primarily in muscle tissue. This ultimately leads to muscle wasting. Undernourishment and starvation are particularly serious human health problems in many developing countries, especially those ravaged by war and political instability.

Humans, like most other animals, store excess food calories as fat. We do so because over much of our evolutionary history food was less abundant and more unpredictable in its availability than it is today. With the development of agriculture and domestication of livestock, food supplies rapidly increased, and many human populations were able to grow and consume increasing amounts of food. With the rise of mechanized agricultural food production and the availability of highly processed foods developed by the modern food industry, excessive intake of food calories has led to an increasing and now critical public health problem: obesity.

Obesity is now an epidemic in many industrialized nations. In the United States, about 36% of the adult population is considered obese. Obesity is a major public health concern because it increases the risk of diabetes, heart disease, and stroke, and contributes to a shorter life-span. For most animals, however, acquiring food and storing its products efficiently in the body allow them to have a fuel reserve to meet seasonal energy requirements. This rationing of stored energy remains an essential part of their metabolic and digestive physiology.

Because the kidneys receive a large fraction of the blood leaving the heart, they are well suited for monitoring changes in blood pressure and maintaining it at relatively constant levels, another example of homeostasis. Specialized cells of the efferent arteriole leaving the glomerulus of each nephron form the juxtaglomerular apparatus (Fig. 41.22). In response to a drop in blood pressure, these cells secrete the hormone renin. Renin converts angiotensinogen, which is produced in the liver and circulates in the blood, into angiotensin I. Angiotensin I in turn is converted by an enzyme in the lungs and elsewhere to its active form, angiotensin II. Angiotensin II acts on the smooth muscles of arterioles throughout the body, causing them to constrict, thus increasing blood pressure and directing more blood back to the heart.

Angiotensin II also stimulates the release of the hormone aldosterone from the adrenal glands (Fig. 41.22). Aldosterone stimulates the distal convoluted tubule and collecting ducts to increase the reabsorption of electrolytes and water back into the blood. Blood volume increases and therefore blood pressure increases, too. The kidneys, therefore, play many roles. In addition to eliminating wastes and other toxic compounds, they maintain homeostasis in several ways, including through the maintenance of water and electrolyte balance and the regulation of blood volume and blood pressure.

As we saw earlier, the maintenance of a constant core body temperature is a form of homeostasis and requires balancing heat production and heat loss. Animals can be categorized by the sources of most of their heat. Animals that produce most of their own heat as by-products of metabolic reactions, including the breakdown of food, are endotherms. These animals usually, but not always, maintain a constant body temperature that is higher than that of their environment. Mammals, including humans, and birds are endotherms.

By contrast, animals that obtain most of their heat from the environment are ectotherms. Ectotherms often regulate their body temperatures by changing their behavior—moving into or out of the sun, for example. Think of a turtle or some other reptile sunning itself on a rock: It is raising its body temperature. The temperatures of these animals usually fluctuate with the outside environment. Most fish, amphibians, reptiles, and invertebrates are ectotherms.

Endotherms are sometimes referred to as “warm-blooded” and ectotherms as “cold-blooded,” but these terms are misleading because, depending on environmental conditions, cold-blooded animals can have core body temperatures higher than warm-blooded animals. It should also be kept in mind that these thermoregulatory mechanisms are not completely distinct, but instead represent extremes of a continuum.

Nevertheless, thermoregulatory mechanisms have profound implications for an animal’s metabolic rate. Endotherms have a higher metabolic rate than ectotherms. As a result, they are able to be active over a broader range of external temperatures than ectotherms. Both the activity level and metabolic rate of ectotherms increase with increasing body temperature. However, ectotherms have metabolic rates that are about 25% of endotherms of similar body mass and at similar body temperatures. Although they can achieve activity levels similar to those of endotherms when their body temperatures are similar, ectotherms cannot sustain prolonged activity. Therefore, ectotherms such as lizards or insects rely on brief bouts of activity, followed by longer periods of inactivity compared to endotherms.

Endotherms benefit from having an active lifestyle. However, their high metabolic rate requires long periods of foraging to acquire enough food to keep warm and be active. By contrast, although ectotherms cannot sustain activity for long periods, they can survive much longer periods without food.

As we discussed, the control of core body temperature, or thermoregulation, is a form of homeostasis and requires the coordinated activities of the nervous, muscular, endocrine, circulatory, and digestive systems. We summarize how these systems work together in endotherms and ectotherms in Fig. 40.5.

Core Concepts Summary

Homeostasis is the active maintenance of a stable internal environment.

Homeostasis is an essential feature of living organisms.

Homeostasis is observed for cells and organisms.

The plasma membrane is a selective barrier that controls the movement of molecules between the inside and the outside of the cell.

Selective permeability results from the combination of lipids and proteins that makes up cell membranes.

Passive transport is the movement of molecules by diffusion, the random movement of molecules. There is a net movement of molecules from regions of higher concentration to regions of lower concentration.

Passive transport can occur by the diffusion of molecules directly through the plasma membrane (simple diffusion) or be aided by protein transporters (facilitated diffusion).

Active transport moves molecules from regions of lower concentration to regions of higher concentration and requires energy.

Primary active transport uses energy stored in ATP; secondary active transport uses the energy stored in an electrochemical gradient.

Animal cells often maintain size and shape by protein pumps that actively move ions in and out of the cell.

Osmoregulation is the regulation of osmotic pressure inside cells or organisms.

Osmoconformers maintain an internal solute concentration similar to that of the environment, whereas osmoregulators have an internal solute concentration different from that of the environment.

Both osmoconformers and osmoregulators regulate the levels of particular solutes, especially sodium, potassium, and chloride.

Osmoregulators that live in high-salt environments excrete excess electrolytes and minimize water loss.

Osmoregulators that live in low-salt or freshwater environments excrete excess water and minimize electrolyte loss.

Osmoregulators that live on land minimize water loss.

The low concentration of CO₂ in the atmosphere forces plants to expose their photosynthetic cells directly to the air. The outward diffusion of water vapor leads to a massive loss of water.

The waxy cuticle on the outside of the epidermis slows rates of water loss from leaves but also slows the diffusion of CO₂ into leaves.

Stomata are pores in the epidermis that open and close, allowing CO₂ to enter into the leaf but also allowing water vapor to diffuse out of the leaf.

A plant with a high root:shoot ratio is better able to supply water to its leaves when the soil is dry.

Plants can also respond to drought by producing fewer leaves.

The nervous system helps to regulate physiological functions to actively maintain stable conditions inside a cell or an organism.

The autonomic system regulates body functions through opposing actions of the sympathetic and parasympathetic divisions.

The regulation of blood oxygen levels is an example of homeostasis and is regulated by chemoreceptors located within the brainstem and carotid and aortic bodies that are located in the neck and near the heart.

Blood glucose levels are maintained in a narrow range by negative feedback.

Energy balance is a form of homeostasis, and depends on the energy intake and use.

The kidneys help to regulate blood volume and pressure by secreting renin, leading to the production of angiotensin II, which constricts blood vessels, and aldosterone, which increases absorption of salt and water by the kidneys.

Thermoregulation is a form of homeostasis; endotherms maintain elevated body temperature by metabolic heat production and ectotherms depend on external heat sources to warm their bodies.