Chapter 2
IN THIS CHAPTER
Seeing what your body does automatically every day
Finding out what goes on inside of every cell
Discovering the importance of homeostasis
Building and maintaining your parts
This chapter is about your life as an organism. As Chapter 1 explains, organism is the fifth of five levels of organization in living things. Although the word organism has many possible definitions, for the purposes of this chapter, an organism is a living unit that metabolizes and maintains its own existence.
In this chapter, you see why your to-do list, crowded as it is, doesn’t include items such as Take ten breaths every minute or At 11:30 a.m., open sweat glands. The processes that your body must carry out minute by minute to sustain life, not to mention the biochemical reactions that happen millions of times a second, can’t be left to the distractible frontal lobes (the conscious, planning part of your brain). Instead, your organs and organ systems function together smoothly to carry out these processes and reactions automatically, without the activity ever coming to your conscious attention. All day and all night, year in and year out, your body builds, maintains, and sustains every part of you; keeps your temperature and your fluid content within some fairly precisely defined ranges; and transfers substances from outside itself to inside, and then back out again. These are the processes of metabolism and homeostasis.
The laws of thermodynamics are the foundation of how the physics and chemistry of the universe are understood. They’re at the “we hold these truths to be self-evident” level for chemists and physicists of all specialties, including all biologists. The first law of thermodynamics states that energy can be neither created nor destroyed — it can only change form. (Turn to Chapter 16 for a brief look at the first law and other basic laws of chemistry and physics.) Energy changes form continuously — within stars, within engines of all kinds, and, in some very special ways, within organisms.
The most basic function of the organism that is you on this planet is to take part in this continuous flow of energy. As a heterotroph (an organism that doesn’t photosynthesize), you ingest (take in) energy in the form of matter — that is, you eat the bodies of other organisms. You use the energy stored in the chemical bonds of that matter to fuel the processes of your metabolism and homeostasis. That energy is thereby transformed into matter called “you” (the material in your cells), matter that’s “not you” (the material in your exhaled breath and in your urine), and some heat radiated from your body to the environment.
Plants convert light energy from the sun into the chemical energy in carbohydrates, which comprise most of the matter of the plant bodies, recycling the waste matter (carbon dioxide) of your metabolic processes. Energy goes around and around, and some of it is always flowing through your body, being transformed constantly as it does so. You, my friend, are part of a cycle of cosmic dimensions!
The word metabolism describes all the chemical reactions that happen in the body. These reactions are of two kinds — anabolic reactions make things (molecules), and catabolic reactions break things down.
Your body performs both anabolic and catabolic reactions at the same time, around the clock, to keep you alive and functioning. Even when you’re sleeping, your cells are busy. You just never get to rest (until you’re dead).
Chapter 11 gives you the details on how the digestive system breaks down food into nutrients and gets them into your bloodstream. Chapter 9 explains how the bloodstream carries nutrients around the body to every cell and carries waste products to the urinary system. Chapter 12 shows you how the urinary system filters the blood and removes waste from the body. This chapter describes the reactions that your cells undergo to convert fuel to usable energy. Ready?
Even when your outside is staying still, your insides are moving. Day and night, your muscles twitch and contract and maintain “tone.” Your heart beats. Your blood circulates. Your diaphragm moves up and down with every breath. Nervous impulses travel. Your brain keeps tabs on everything. You think. Even when you’re asleep, you dream (a form of thinking). Your intestines push the food you ate hours ago along your alimentary canal. Your kidneys filter your blood and make urine. Your sweat glands open and close. Your eyes blink, and even during sleep, they move. Men produce sperm. Women move through the menstrual cycle. The processes that keep you alive are always active.
Every cell in your body is like a tiny factory, converting raw materials to useful molecules such as proteins and thousands of other products, many of which we discuss throughout this book. The raw materials (nutrients) come from the food you eat, and the cells use the nutrients in metabolic reactions. During these reactions, some of the energy from catabolized nutrients is used to generate a compound called adenosine triphosphate (ATP). This molecule is the one your cells can actually use to power all those chemical reactions.
ATP works like a rechargeable battery. It contains three phosphates aligned in a row (see Figure 2-1). Breaking one of them off accesses the energy, leaving behind adenosine diphosphate (ADP) and a phosphate (P) by itself. However, just like you can plug in your phone to recharge its battery, the energy in the bonds of glucose is used to reattach the P — re-creating ATP (albeit in an incredibly complicated way).
© John Wiley & Sons, Inc.
FIGURE 2-1: The chemical structure of ADP and ATP.
The reactions that convert fuel (specifically glucose) to usable energy (ATP molecules) include glycolysis, the Krebs cycle (aerobic respiration) and anaerobic respiration, and oxidative phosphorylation. Together, these reactions are referred to as cellular respiration. These are complex pathways, so expect to take some time to understand them. See Figure 2-2 and refer to it as many times as necessary to understand what happens in cellular respiration. (Note: Alcohol fermentation is included for reference but does not occur in the human body.)
© John Wiley & Sons, Inc.
FIGURE 2-2: Cellular respiration: glycolysis, aerobic (Krebs cycle) and anaerobic respiration, and oxidative phosphorylation, all of which convert energy from fuel into ATP.
Here, we focus on the ins and outs of the three main components of cellular respiration.
Starting at the top of Figure 2-2, you can see that glucose — the smallest molecule that a carbohydrate can be broken into during digestion — goes through the process of glycolysis, which starts cellular respiration and uses some energy (ATP) itself. Glycolysis occurs in the cytoplasm and doesn’t require oxygen. Two molecules of ATP are required to start each molecule of glucose rolling down the glycolytic pathway; although four molecules of ATP are generated during glycolysis, the net production of ATP is two molecules. In addition to the two ATPs, two molecules of pyruvic acid (also called pyruvate) are generated. They move into a mitochondrion and enter the Krebs cycle.
The Krebs cycle is a major biological pathway in the metabolism of every multicellular organism. It’s an aerobic pathway, requiring oxygen.
As the pyruvate enters the mitochondrion, a molecule called nicotinamide adenine dinucleotide (NAD+) joins it. NAD+ is an electron carrier (that is, it carries energy), and it gets the process moving by bringing some energy into the pathway. The NAD+ provides enough energy that when it joins with pyruvate, carbon dioxide is released, and the high-energy molecule NADH is formed. Flavin adenine dinucleotide (FAD) works in much the same way, becoming FADH2. The product of the overall reaction is acetyl coenzyme A (acetyl CoA), which is a carbohydrate molecule that puts the Krebs cycle in motion.
Oxidative phosphorylation, also called the electron transport chain (ETC), takes place in the inner membrane of the mitochondria. The electron carriers produced during the Krebs cycle — NADH and FADH2 — are created when NAD+ and FAD, respectively, are “reduced.” When a substance is reduced, it gains electrons; when it’s oxidized, it loses electrons. (Turn to Chapter 16 for more information about such “redox reactions.”) So NADH and FADH2 are compounds that have gained electrons, and therefore, energy. In the ETC, oxidation and reduction reactions occur repeatedly as a way of transporting energy. At the end of the chain, oxygen atoms accept the electrons, producing water. (Water from metabolic reactions isn’t a significant contributor to the water needs of the body.)
As NADH and FADH2 pass down the respiratory (or electron transport) chain, they lose energy as they become oxidized and reduced, oxidized and reduced, oxidized and … . It sounds exhausting, doesn’t it? Well, their energy supplies become exhausted for a good cause. The energy that these electron carriers lose is used to add a molecule of phosphate to adenosine diphosphate (ADP) to make it adenosine triphosphate — the coveted ATP. For each NADH molecule that’s produced in the Krebs cycle, three molecules of ATP can be generated. For each molecule of FADH2 that’s produced in the Krebs cycle, two molecules of ATP are made.
Sometimes oxygen isn’t present, but your body still needs energy. During these times, a backup system, an anaerobic pathway (called anaerobic because it proceeds in the absence of oxygen) exists. Lactic acid fermentation generates NAD+ so that glycolysis, which results in the net production of two molecules of ATP, can continue. However, if the supply of NAD+ runs out, glycolysis can’t occur, and ATP can’t be generated.
This occurs most often in muscle cells during periods of intense exercise. The byproduct of this reaction, lactic acid, builds up in the muscle, contributing to muscle fatigue (the inability of a muscle cell to contract). Thus, this process cannot be maintained for extended periods of time.
Chemical reactions are not random events. Any reaction takes place only when all the conditions are right for it: All the required reagents and catalysts are close together in the right quantities; the fuel for the reaction is present, in sufficient amount and in the right form; and the environmental variables are all within the right range, including the temperature, salinity, and pH. The complicated chemistry of life is extremely sensitive to the environmental conditions; the environment being the body itself. Homeostasis is the term physiologists use to reference the balance of all the variables. Numerous homeostatic mechanisms are employed by our bodies to keep everything in check; otherwise, all the reactions that comprise our metabolism cannot occur.
The following sections look at a few important physiological variables and how the mechanisms of homeostasis keep them in the optimum range in common, everyday situations.
All metabolic reactions in all organisms require that the temperature of the body be within a certain range. Because we humans are homeotherms or “warm-blooded,” we maintain a relatively constant body temperature regardless of the ambient temperature. We do this by regulating our metabolic rate. The large number of mitochondria per cell enables a high rate of metabolism, which generates a lot of heat.
Another way we control our body temperature is by employing adaptations that conserve the heat generated by metabolism within the body in cold conditions or dissipate that heat out of the body in overly warm conditions. A few of the specific adaptations include the following:
A watery environment is a requirement for a great proportion of metabolic reactions (the rest need a lipid, or fatty, environment). The body contains a lot of water: in your blood, in your cells, in the spaces between your cells, in your digestive organs, here, there, and everywhere. Not pure water, though. The water in your body is a solvent for thousands of different ions and molecules (solutes). The quantity and quality of the solutes change the character of the solution. Because solutes are constantly entering and leaving the solution as they participate in or are generated by metabolic reactions, the characteristics of the watery solution must remain within certain bounds for the reactions to continue happening.
Glucose, the fuel of all cellular processes, is distributed to all cells dissolved in the blood. The concentration of glucose in the blood must be high enough to ensure that the cells have enough fuel. However, extra glucose beyond the immediate needs of the cells can harm many important organs and tissues, especially where the vessels are tiny, as in the retina of the eye, the extremities (hands and, especially, feet), and the kidneys. Diabetes is a disease in which there is a chronic overconcentration of glucose in the blood.
The amount of glucose in the blood is controlled mainly by the pancreas. Absorption by the small intestine puts the glucose from ingested food into the blood. Insulin is a hormone released into the blood from the pancreas in response to increased blood glucose levels. Most cells have receptors that bind the insulin, which allows glucose into the cells for cellular respiration. The cells of the liver, muscles, and adipose tissue (fat) take up the glucose and store it as glycogen (see Chapter 3). At times when your intestines aren’t absorbing much glucose, like hours after a meal, the production of insulin is suppressed and the stored glucose is released into the blood again. Refer to Chapter 8 for more information about the pancreatic control of blood glucose levels.
How does the pancreas know when to release insulin and how much is enough? How does the kidney know when the salt content of the blood is too high or the volume of the blood is too low? What tells the sweat glands to open and close to cool the body or retain heat? Well, read on!
Every homeostatic mechanism employs three parts: a receptor, an integrator, and an effector. Numerous receptors, or sensors, are strategically placed throughout your body. Some respond to chemical changes (like pH), others to mechanical ones (like blood pressure), and there are many more. These receptors are specialized nervous cells and communicate to the brain — the integrator — any changes from our balance. The brain processes all the incoming information and “decides” if a response is warranted. If it is, a message will be sent out via neurons or hormones to the effectors, which carry out the body’s response (the effect).
My, how you’ve changed, and are still changing! Growing up, growing old, and just living every day, you’re building new parts and replacing old ones. From conception to early adulthood, your body was busy making itself: everything from scratch.
But the job wasn’t finished when you were fully grown. Complex living tissues and organs almost all require replacement parts at some time, and many require them all the time. This necessity is one of the defining characteristics of organisms — the ability to organize matter into the structures that compose themselves and to replace and renew those structures as required, as we describe in the following sections.
You began life as a single cell and built yourself from there, with some help from your mom to get started. Your body developed along a plan, building a backbone with a head at the top and a tail at the bottom (somehow, you lost the tail). Now look at you: 100 trillion cells, almost every one with its own special structure and job to do. Good work! Find more about the processes of development in Chapter 15.
Just like the organism they’re a part of, many kinds of cells have a life cycle: They’re born, they develop, they work, they get worn out, and they die. For an organism to continue its life cycle, these cells must be replaced continuously, either by the division of the same cell type or by the differentiation of stem cells. These relatively undifferentiated cells wait patiently until they’re called upon to divide. Some of the daughter cells differentiate into their specific programmed type while others remain stem cells and wait to be called upon the next time. Stem cells are an active area of research in physiology and in the field of regenerative medicine.
Some of the cell and tissue types that must be continuously replaced are
Some other types of tissue replace their cells at a very slow rate, such as the following:
Your body repairs some tissues as necessary, such as after an injury:
When you have a tiny, superficial surface wound (a little scratch), the epidermis simply replaces the damaged cells. In a few days, the scratch is gone. But when the wound is deep enough that blood vessels are damaged, the healing process is a little bit more involved. Turn to Chapter 9 for information on blood and blood vessels.
The immediate rush of blood washes debris and microbes out of the wound. Then, the vessels around the wound constrict to slow down the blood flow. A type of “formed element” in the blood called platelets sticks to the collagen fibers that make up the vessel wall, forming a natural band-aid called a platelet plug.
After the platelet plug forms, a complex chain of events results in the formation of a clot that stops blood loss altogether. This chain of events is called the clotting cascade or coagulation cascade. Enzymes called clotting factors initiate the cascade. Here’s a rundown of what happens, focusing on the most important steps:
Underneath the scab, the blood vessels regenerate and repair themselves, and in the dermis, cells called fibroblasts spur the creation of proteins to fill the space in the damaged layers. Scars are created to provide extra strength to skin areas that are deeply wounded. Scar tissue has many interwoven collagen fibers, but no hair follicles, nails, or glands. Feeling maybe lost in the area covered with scar tissue if the nerves are damaged.
As we mention earlier in the chapter, almost all tissues and organs require replacement parts at some time. However, here are some exceptions: