Objectives
- 1.
Recognize the need to maintain the constancy of the internal environment of the body and the concept of homeostasis.
- 2.
Describe the hierarchical view of the body as an ensemble of distinct compartments.
- 3.
Describe the composition and structure of the lipid bilayer membranes that encompass cells and organelles.
- 4.
Explain why the protein-mediated transport processes that regulate the flow of water and solutes across biomembranes are essential to all physiological functions.
Homeostasis enables the body to survive in diverse environments
Humans are independent, free-living animals who can move about and survive in vastly diverse physical environments. Thus we find human habitats ranging from the frozen tundra of Siberia and the mountains of Nepal a
a The adaptability of humans is remarkable: humans can survive on Mount Everest, which, at 29,028 feet above sea level, is at the cruising altitude of jet airplanes. At the summit the temperature is approximately −40° Celsius (same as −40° Fahrenheit), the thin atmosphere supplies only about one third of the oxygen at sea level, and the relative humidity is zero.
to the jungles of the Amazon and the deserts of the Middle East. Nevertheless, the elemental constituents of the body are cells, whose survival and function are possible only within a narrow range of physical and chemical conditions, such as temperature, oxygen concentration, osmolarity, and pH. Therefore the whole body can survive under diverse external conditions only by maintaining the conditions around its constituent cells within narrow limits. In this sense the body has an internal environment, which is kept constant to ensure survival and proper functioning of the body’s cellular constituents. The process whereby the body maintains constancy of this internal environment is referred to as homeostasis. bb The concept of the internal environment was first advanced by the 19th-century pioneer of physiology Claude Bernard, who discussed it in his book, Introduction à l’étude de la médecine expérimentale in 1865. Bernard’s often-quoted dictum is: “The constancy of the internal environment is the prerequisite for a free life.” ( “La fixeté du milieu intérieur est la condition de la vie libre.” from Leçons sur les phénomènes de la vie communs aux animaux et aux végétaux, 1878.) The term “homeostasis” was introduced by Walter B. Cannon in his physiology text, The Wisdom of the Body (1932).
When homeostatic mechanisms are severely impaired, as in a patient in an intensive care unit, artificial life support systems become necessary for maintaining the internal environment.Achieving homeostasis requires various component physiological systems in the body to function coordinately. The musculoskeletal system enables the body to be motile and to acquire food and water. The gastrointestinal system extracts nutrients (sources of both chemical energy, such as sugars, and essential minerals, such as sodium, potassium, and calcium) from food. The respiratory (pulmonary) system absorbs oxygen, which is required in oxidative metabolic processes that “burn” food to release energy. The circulatory system transports nutrients and oxygen to cells while carrying metabolic waste away from cells. Metabolic waste products are eliminated from the body by the renal and respiratory systems. The complex operations of all the component systems of the body are coordinated and regulated through biochemical signals released by the endocrine system and disseminated by the circulation, as well as through electrical signals generated by the nervous system.
The body is an ensemble of functionally and spatially distinct compartments
The organization of the body may be viewed hierarchically ( Fig. 1.1 ). The various systems of the body not only constitute functionally distinct entities, but also comprise spatially and structurally distinct compartments. Thus the lungs, the kidneys, the various endocrine glands, the blood, and so on are distinct compartments within the body. Each compartment has its own local environment that is maintained homeostatically to permit optimal performance of different physiological functions.
Compartmentation is an organizing principle that applies not just to macroscopic structures in the body, but to the constituent cells as well. Each cell is a compartment distinct from the extracellular environment and separated from that environment by a membrane (the plasma membrane ). The intracellular space of each cell is further divided into subcellular compartments (cytoplasm, mitochondria, endoplasmic reticulum, etc.). Each of these subcellular compartments is encompassed within its own membrane, and each has a different microscopic internal environment to allow different cellular functions to be carried out optimally (e.g., protein synthesis in the cytoplasm and oxidative metabolism in the mitochondria).
The biological membranes that surround cells and subcellular organelles are lipid bilayers
Diverse “integral” membrane proteins are inserted into the lipid bilayer membranes that separate cells and subcellular compartments from the surrounding environment. Many of these proteins are transmembrane proteins that mediate the transport of various solutes or water across the bilayers. Ion channels and ion pumps are examples of such transport proteins. Other transmembrane proteins have signaling functions and transmit information from one side of the membrane to the other. Receptors for neurotransmitters, peptide hormones, and growth factors are examples of signaling proteins.
Biomembranes are formed primarily from phospholipids but can also contain cholesterol and sphingolipids
Most of the lipids that make up biomembranes are phospholipids. These amphiphilic (or amphipathic ) phospholipids consist of a hydrophilic (water-loving), or polar, phosphate-containing head group attached to two hydrophobic (water-fearing), or nonpolar, fatty acid chains. The phospholipids assemble into a sheet or leaflet. The polar head groups pack together to form the hydrophilic surface of the leaflet, and the nonpolar hydrocarbon fatty acid chains pack together to form the hydrophobic surface of the leaflet. Two leaflets combine at their hydrophobic surfaces to form a bilayer membrane.
The bilayer presents its two hydrophilic surfaces to the aqueous environment, whereas the hydrophobic fatty acid chains remain sequestered within the interior of the membrane ( Fig. 1.2 ). The individual lipid molecules within the bilayer are free to move and are not rigidly packed. Therefore the lipid bilayer membrane behaves in part like a two-dimensional fluid and is frequently referred to as a fluid mosaic.
