The Basic Mechanisms of Homeostasis
Overview of homeostasis
The term homeostasis was first coined by Walter Cannon in 1929 to literally mean ‘steady state’. It describes the dynamic equilibrium by which internal constancy is maintained within set limits by regulation and control. There are many examples of homeostatic control throughout the human body and in other living organisms, such as pH, pressure, and temperature.
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A concept important to homeostasis is the process of feedback circuits; involving a receptor, an effector, and a control centre. A receptor is responsible for detecting a change in the body, while the effector corrects this. The control centre organises these two together to elicit the response. The most common form of control in homeostasis is known as negative feedback, in which an excess or deficit in a homeostatic system triggers its own regulation. The diagram below illustrates this concept in reference to the control of temperature (Figure 1).
Figure 1 is a simple representation of a rather complicated process. Here, the several types of negative feedback circuits involved in temperature control have been summarised into one. The hypothalamus is a combined receptor and control centre, both recognising extremes of temperature change, and triggering bodily effectors to correct the changes. Figure 1 shows the responses to a decrease in body temperature, which directs organs to increase metabolism, thus causing shivering. Another effect would be causing hair cells on the skin to force up their hairs, creating a trapped layer of air across the body surface. Such effects should then cause the body temperature to rise to the optimal 37°C again, causing feedback to switch the circuit ‘off’. If this does not occur, the circuit will continue to direct effectors to warm the body because the feedback will not be switched ‘off’.
Recent research, however has added another dimension to the accepted definition of homeostasis. Scientists studying circadian rhythms (24-hour bodily cycles) have pointed out that the internal environment does not have completely constant ‘normal’ set point. They have found, for example, that the set point for human body temperature varies over a 24 hour cycle, fluctuating between 36°C and 37°C. As a result of this research, current thinking suggests that while homeostasis controls the ‘minute-by-minute fluctuation in the environment’ , circadian rhythms control the body’s general programming over time.
In this essay, we will concentrate on two examples of homeostasis, one that occurs in humans and one which occurs in plants. Firstly, we will discuss the control of blood glucose levels in mammals, and then will look at the role of plant stomata in regulating water loss.
Example 1: Control of blood glucose levels
The human body has a number of mechanisms in place to regulate the storage and release of molecules for energy. Sometimes, an individual will consume more calories than can be immediately used, so sugars will be stored in the form of glycogen (a polymer of glucose) in liver and muscle cells. Other periods of increased activity may however, require the sudden release of energy, whereby glycogen is initially oxidised from the stores in the liver. Clearly, this is another example of homeostasis and it is outlined in Figure 2.
Two enzymatic hormones are utilised by the body to control the interchange of glucose as an energy molecule and glycogen as a storage molecule. The first, insulin, lowers blood glucose levels by promoting its conversion to glycogen. The second, glucagon, increases glucose levels by allowing glycogen to be phosphorylated. Both of these hormones are produced and released by specialised cells in the pancreas known as Islets of Langerhans. Insulin is released from β-cells, and glucagon is released from α-cells.
Figure 2: Blood glucose control by insulin and glucagon
If the blood glucose level is too high, more insulin and less glucagon is released. This causes cells to take in glucose from the blood, while the liver converts glucose to glycogen. During low levels of blood glucose however, glucagon release increases, activating the breakdown of glycogen to glucose in the liver, and glucose is released into the blood. This is a good example of negative feedback control, as the lowering of blood glucose, for example, inhibits further insulin secretion.
Importantly, insulin is dependent upon calcium. This is because glucose activates calcium channels. When glucose levels are high, the subsequent release of calcium results in calcium binding to calmodulin. Together, the two molecules promote insulin vesicles to be released from the pancreas. This demonstrates the negative feedback system discussed in the overview.
Example 3: Control of water loss by plants
Plants need to balance their need to conserve water with their need to photosynthesise energy. Transpiration causes water to be pulled up through the plant passively as water diffuses out through the leaves. These pores are opened and closed by the action of surrounding guard cells, located as illustrated in Figure 3.
Figure 3: Drawing of stoma & guard cells
These guard cells can take on two extremes of conformation; either flaccid, to close the stoma, or turgid, to open the stoma. When guard cells take in water via osmosis, they swell, become turgid, and are forced to bulge outwards into a kidney shape, opening the stoma. They adhere to this shape both because the two cells are attached to each other at either end, and because cellulose microfibrils constrain them. However, if the guard cells lose their water content, they shrink and become flaccid, closing the stoma so that water cannot leave.
The opening and closing of the stomata have been shown to be affected by light concentrations. When illuminated, the concentration of solutes in the guard cell vacuoles increases because starch is converted to malic acid, and a proton pump in the plasma membrane is stimulated. The proton pump removes hydrogen ions (H+) from the guard cells, and in response, potassium ions (K+) flow into the cell. Chloride ions (Cl-) also flow into the cell via another pump in response to the H+ concentration difference. The accumulation of these ions and malate in the vacuole of the guards cells is enough to cause the water potential to drop within the guard cells. Water then flows in by osmosis, leading to the turgidity just described and opening the pore. As this opening process occurs in light, exactly the opposite happens at night. As light is lost, channels open to conduct Cl- and K+ out of the guard cells, water is lost, and the cells become flaccid and close.
Another stimulus for the closing of stomata is an emergency response to the plant wilting from lack of water. In this case, CO2 concentration increases inside the leaf cells, and alongside the wilting, causes the plant to release the hormone abscisic acid (ABA). This diffuses into guard cells and activates the loss of Cl- and K+, effectively mimicking the night time action of the stomata.
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The idea of homeostasis has been well-developed since it was first identified in the mid-1900s. We have seen in this essay that feedback loops play an important part in homeostatic processes, and that the process is controlled by the action of detector and effector hormones and other molecules activated by control centres. Ongoing research also indicates that innate circadian rhythms affect the processes of homeostasis, causing the optimal set point for internal conditions to vary on a daily basis.
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