本文是留学生入学coursework范例，题目是“Example Biology Coursework（生物学作业示例）”，“稳态”一词最早是由沃尔特·坎农在1929年创造的，字面意思是“稳定的状态”。它描述了一种动态平衡，通过调节和控制在设定的限度内保持内部的恒定。在整个人体和其他生物体中有许多自我平衡控制的例子，如pH值、压力和温度。
The Basic Mechanisms of Homeostasis体内平衡的基本机制
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.
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).
对内稳态很重要的一个概念是反馈电路的过程;包括受体，效应器和控制中心。受体负责检测身体的变化，而效应器则负责纠正这种变化。控制中心将这两者组织在一起来引出反应。内稳态中最常见的控制形式是负反馈，即内稳态系统的过剩或不足触发其自身的调节。下面的图表说明了关于温度控制的这个概念(图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’.
图1是一个相当复杂的流程的简单表示。在这里，几种负反馈电路涉及温度控制归纳为一。下丘脑是一个综合的受体和控制中心，既能识别极端的温度变化，又能触发身体效应来纠正这种变化。图1显示了人体对体温下降的反应，这导致器官加快新陈代谢，从而引起颤抖。另一个影响可能是皮肤上的毛细胞迫使它们的毛发向上，在身体表面形成一层困住的空气。这样的效果应该会使体温再次上升到最佳的37°C，导致反馈开关电路“关闭”。如果这没有发生，电路将继续引导效应器加热身体，因为反馈将不会被“关闭”。
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.
然而，最近的研究为已接受的内稳态定义增加了另一个维度。研究昼夜节律(24小时人体周期)的科学家指出，人体内部环境并没有完全恒定的“正常”设定值。例如，他们发现人体温度的设定点以24小时为周期变化，在36°C和37°C之间波动。这项研究的结果是，目前的观点认为，虽然体内平衡控制着“环境中每分钟的波动”，但昼夜节律随着时间的推移控制着身体的总体编程。
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例1:控制血糖水平
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.
人体有许多机制来调节能量分子的储存和释放。有时，一个人摄入的热量会超过立即消耗的量，所以糖会以糖原(葡萄糖的一种聚合物)的形式储存在肝脏和肌肉细胞中。然而，其他活动增加的时期可能需要能量的突然释放，此时储存在肝脏中的糖原首先被氧化。显然，这是内稳态的另一个例子，如图2所示。
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.
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.
如果血糖水平过高，胰岛素分泌增多，胰高血糖素分泌减少。这导致细胞从血液中吸收葡萄糖，而肝脏将葡萄糖转化为糖原。然而，在低血糖时，胰高血糖素的释放增加，激活肝糖原分解为葡萄糖，葡萄糖被释放到血液中。这是一个很好的负反馈控制的例子，比如降低血糖，会进一步抑制胰岛素的分泌。
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例3:控制植物的水分流失
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.
植物需要在保存水分的需要和光合作用的需要之间找到平衡。蒸腾作用使水分被动地从植物中被拉上来，因为水分通过叶片扩散出去。这些气孔通过周围保护细胞的作用打开和关闭，如图3所示。
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.
气孔的打开和关闭已被证明受光照浓度的影响。当光照时，由于淀粉转化为苹果酸，保卫细胞液泡中的溶质浓度增加，质膜中的质子泵受到刺激。质子泵从保卫细胞中除去氢离子(H+)，钾离子(K+)随之流入细胞。氯离子(Cl-)也通过另一个泵响应H+浓度差进入细胞。这些离子和苹果酸盐在保卫细胞液泡中的积累足以导致保卫细胞内的水势下降。然后水通过渗透作用流入，导致刚才描述的膨胀和打开孔隙。由于这个开放过程发生在光线下，正好相反的情况发生在晚上。当光丢失时，将Cl-和K+传导出保卫细胞的通道打开，水分丢失，细胞变得松弛和关闭。
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.
Concluding Remarks结束语
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.
自20世纪中期首次被发现以来，体内平衡的概念已经得到了很好的发展。我们在这篇文章中已经看到，反馈回路在稳态过程中起着重要的作用，这个过程是由探测器和效应激素以及其他由控制中心激活的分子的作用控制的。正在进行的研究也表明，天生的昼夜节律影响体内平衡的过程，导致内部条件的最佳设置点每天都在变化。
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