LESSON ASSIGNMENT
LESSON 2 Physiology of Cells and Miscellaneous Tissues.
LESSON ASSIGNMENT Paragraphs 2‑1 through 2‑35.
LESSON OBJECTIVES
After completing this lesson, you should be able to:
2‑1. Match the major components of a "typical" animal cell with their
functions.
2‑2. Identify important functions of ATP and ADP.
2‑3. Match the names of the fluid compartments with their descriptions.
2‑4. Identify a general requirement for electrolytes, and match terms
related to tonicity with their descriptions.
2‑5. Identify functions and characteristics of water.
2‑6. Identify examples of homeostasis and feedback mechanisms.
2‑7. Match terms related to the movement of materials into and out of
cells with their descriptions or examples.
2‑8. Match terms related to membrane potentials, cell growth, and cell
multiplication with their descriptions.
2‑9. Match types of tissues with their characteristics.
SUGGESTION
After completing the assignment, complete the exercises at the end of this
lesson. These exercises will help you to achieve the lesson objectives.
LESSON 2
PHYSIOLOGY OF CELLS AND
MISCELLANEOUS TISSUES
Section I. CELLS
2‑1. THE CELLULAR LEVEL
a. The individual cell is the unit of structure of all living things. An entire organism may consist of a single cell (unicellular) or many cells (multicellular).
b. In human beings and other multicellular organisms, the cells tend to be organized in specific ways. A group of like cells performing a particular function is referred to as a tissue. An organ is a discrete structure composed of several different tissues together. An organ system is a group of organs together performing an overall function. (An example of an organ system is the digestive system.) The individual organism is the combination of all of these things as a discrete and separate entity.
c. Although all living matter is composed of cells, animal cells and plant cells are significantly different from each other. Not only do plant cells contain chlorophyll, a green coloring matter; plant cells also have a cell wall around them which is made up of a very complex carbohydrate known as cellulose. Neither chlorophyll nor a cell wall is present in connection with animal cells.
2‑2. THE MAJOR COMPONENTS OF A "TYPICAL" ANIMAL CELL
A "typical" animal cell is illustrated in Figure 2‑1.
a. Cell Membrane. As its outer boundary, the animal cell has a special structure called the cell or plasma membrane. All of the substances that enter or leave the cell must in some way pass through this membrane.
b. Protoplasm. The major substance of the cell is known as protoplasm. It is a combination of water and a variety of materials dissolved in the water. Outside the cell nucleus (see below), protoplasm is called cytoplasm. Inside the cell nucleus, protoplasm is called nucleoplasm.
c. Organelles. Within the cytoplasm, certain structures are called organelles. These organelles include structures such as the endoplasmic reticulum, ribosomes, various kinds of vacuoles, the Golgi apparatus, mitochondria, and centrioles.
(1) The endoplasmic reticulum resembles a circulatory system for the individual cell. It is a network composed of unit (single‑thickness) membranes.
Figure 2-1. A “typical” animal cell.
(2) The ribosomes are granular particles concerned with protein synthesis. They may be found free, clustered, or attached to the endoplasmic reticulum.
(3) The vacuoles are small spaces or cavities within the cytoplasm. These serve functions at the cellular level such as digestion, respiration, excretion, and storage.
(4) The Golgi complex is a portion of the endoplasmic reticulum that aids in the final preparation of certain proteins and mucus‑like substances and in the movement of these substances. It is best-developed in secretory cells.
(5) The mitochondria are the "powerhouses" of the cell. They "recharge" ADP molecules to form ATP molecules.
(6) There are ordinarily two centrioles. These organelles play a major role in cell division.
d. Nucleus. Within the cell is the nucleus. This structure has a nuclear membrane separating it from the cytoplasm. Within the nucleus is the chromatin material, made up of the protein deoxyribonucleic acid (DNA). At the time of cell division, this chromatin material is aggregated into individual structures known as chromosomes. Each chromosome has a set of specific genes, which determine all of the physical and chemical characteristics of the body, which represent its structure and function.
2‑3. ENERGY
a. We mentioned in lesson 1 that the human body depended upon external sources for energy. Plants use solar radiation to make glucose and other nutrients. The human body takes glucose and other nutrients directly or indirectly from plants. The body receives oxygen from the air. The energy that was once derived by plants from solar radiation is released within human cells by the process of metabolic oxidation. This involves the combination of glucose and other nutrients with oxygen, releasing the stored energy.
b. The mitochondria of the cells use this released energy to form ATP molecules from ADP molecules. Adenosine diphosphate is converted to ATP by the addition of a "part of a molecule" called a phosphate radical. The binding of this phosphate radical requires a large quantity of energy, which can be released later when the phosphate radical is separated off. Adenosine triphosphate provides energy for cellular processes such as active transport of substances across membranes, synthesis of chemical compounds for the body, and mechanical work (such as muscle contraction). When an Adenosine triphosphate molecule provides energy for such a process, it loses a phosphate radical and becomes ADP. Then, the cycle begins again as ADP is converted into ATP within the mitochondria.
c. Certain cells, such as muscle cells and nerve cells, require great amounts of energy. Such cells have well‑developed mitochondria.
Section II. BODY FLUIDS
2‑4. INTRODUCTION
Approximately 56 percent of the human body consist of fluids. Soft tissues consist almost completely of fluids. These body fluids are composed largely of water. Thus, water is the major component of living substances.
2‑5. FLUID COMPARTMENTS
Regarding the human body, we speak of fluid compartments or spaces. These are intracellular fluid, the interstitial fluid, and the circulating (plasma) fluid. See Figure 2‑2 for a scheme of the body fluids and fluid compartments.
Figure 2-2. Scheme of the body fluids and fluid compartments.
a. Within the cell, we have seen that the major constituent is water. This fluid is called intracellular fluid ("within the cell").
b. Therefore, all other fluids are extracellular. The extracellular fluids are found in two different compartments.
(1) The tissue fluid is located among but not within the cells of the body. It is therefore called interstitial or intercellular fluid.
(2) In some systems, fluids serve as a vehicle to carry items around the body. These systems are called circulatory systems. The circulating fluid is called the plasma‑‑the non-cellular component of blood.
2‑6. ELECTROLYTES
Within the fluids of the body, there are certain chemicals known as electrolytes. Electrolytes are chemicals that dissociate ("break up") into ions ("charged particles") when they are dissolved. To maintain life and good health, electrolytes must be in balance. That is, they must be present in certain proportions and concentrations in each fluid compartment.
2‑7. WATER
As we have mentioned, water is the main constituent of the human body.
a. Some Physical Characteristics. Water has several important physical characteristics that make it extremely useful to the body.
(1) First, it is a fluid. Therefore, it has the capacity to flow.
(2) Secondly, it is often called the "universal solvent." This refers to its ability to dissolve so many substances within itself. Thus, water is an excellent vehicle for the circulatory systems.
(3) Water is very useful in the temperature control mechanisms of the body. This is because of its heat‑carrying capacity and its tendency to remove large numbers of calories during evaporation.
b. Sources. Water thirst and water satiation is controlled by special centers in the hypothalamus of the brain. The human body obtains water in two primary ways:
(1) Most items that humans drink or eat consist largely of water.
(2) A second source of water is metabolic oxidation. This water is referred to as metabolic water. As various food substances are oxidized within the individual cell, water is one of the main by‑products.
c. Losses.
(1) Perspiration. Water is continuously lost from the body in the form of perspiration or sweat. With high surrounding temperatures and/or vigorous exercise, the sweat is obvious. This is called sensible perspiration. Otherwise, the sweat is usually not obvious, and there is a low level of water loss. This is called insensible perspiration.
(2) Respiration. The surfaces of the lungs must be moist to ensure the passage of gases to and from the blood. Air is moistened within the respiratory passages and the alveoli of the lungs. Thus, moisture passes out of the body along with the exhaled breath.
(3) Urination. Water is also lost from the body in the form of urine. Urine carries nitrogenous wastes of protein metabolism, dissolved in the water.
(4) Vomiting and diarrhea. During vomiting and diarrhea, the body loses large quantities of water and dissolved electrolytes. In infants and the elderly, this loss of water and electrolytes can be very dangerous. Sometimes, even death may result.
2‑8. DISSOLVED SUBSTANCES
As mentioned before, one of the characteristics of water that makes it so desirable is its capacity to dissolve almost anything ("universal solvent").
a. Gases. Oxygen and carbon dioxide are exchanged between air in the lungs and the blood. They are also exchanged between the blood and the individual cells of the body. At least in part, these gases are carried as dissolved substances in the water of the blood.
b. Nutrients. By nutrients, we mean the end products of digestion, and vitamins and minerals from the digestive system. By being dissolved in the water of the blood, these nutrients are distributed to the individual cells of the body.
c. Wastes. Wastes result from the metabolic processes of the body. Wastes are picked up from the individual cells and delivered dissolved in the water to the excretory organs of the body, such as the kidneys.
d. Hormones. Hormones are carried from the endocrine glands to specific target organs while dissolved in the water of the blood.
2‑9. TISSUE FLUID CYCLE
That portion of the extracellular fluid found among the cells is called the tissue fluid, or interstitial fluid. Tissue fluid originates primarily with a fluid portion of the blood that escapes into the tissues from the capillaries. Part of this escaped fluid enters the beginning of the venous vessels. However, a large percentage of the tissue fluid is picked up by another circulatory system, the lymphatic system. Thus, there is a continuous flow of the fluids throughout the body. In addition, the intracellular fluid and the immediate extracellular fluid are continually being exchanged.
Section III. HOMEOSTASIS
2‑10. INTRODUCTION
a. The body fluids play an important role in homeostasis. Homeostasis is the body's tendency to maintain a steady state. The tissue fluid forms the immediate environment of the living cell. In order to maintain the life processes of the individual cells, there must be appropriate concentrations of oxygen, carbon dioxide, nutrients, electrolytes, and other substances within the tissue fluid.
b. One of the chief functions of any organ system is to help to maintain this steady state. For example, the digestive system helps to maintain a steady concentration of nutrients. The respiratory system helps to maintain steady concentrations of oxygen and the removal of carbon dioxide.
c. All organ systems are at least partially controlled by a feedback mechanism. A feedback mechanism resembles the household thermostat. When the concentration of a substance is too low, the feedback mechanism stimulates an increased production and/or distribution. Once the level returns to normal, the feedback mechanism signals a decrease in production. There is a similar feedback mechanism for body temperature.
2‑11. WATER BALANCE
The body has a natural requirement for a certain amount of water to continue its processes properly. Lack of fluid in the circulatory system can result in heart failure. Excessive amounts of fluid in the tissue spaces cause swelling of the body, known as edema. There are feedback mechanisms to maintain water balance.
2‑12. ELECTROLYTE BALANCE
The electrolytes must also be in balance. Electrolyte balance is an important consideration when fluids are administered to a patient. See Figure 2‑3 for an explanation of tonicity.
+ + electrolytes ----> direction of water flow (osmosis)
Figure 2-3. Tonicity (cell with semipermeable membrane, nonpermeable to
electrolytes present).
a. Hypertonicity. If the overall concentration of electrolytes is greater in the tissue fluid surrounding a cell than it is in the intracellular fluid within the cell, the tissue fluid is hypertonic (noun: hypertonicity). The cell tends to be destroyed by loss of its fluid to the hypertonic environment.
b. Hypotonicity. If the overall concentration of electrolytes as less in the tissue fluid than it is in the intracellular fluid within the cell, the tissue fluid is hypotonic (noun: hypotonicity). In a hypotonic environment, fluid will enter a cell and cause it to swell and burst.
c. Isotonicity. If the concentrations of electrolytes are the same in the tissue fluid and the intracellular fluid, the situation is balanced (homeostatic). That is, the fluids are isotonic.
2‑13. MOVEMENT OF MATERIALS INTO AND OUT OF THE CELL
We noted earlier that all substances that enter or leave the cell must pass through the cell membrane in some way.
a. Semipermeability. The permeability of a membrane is its capacity to allow materials to move through it. Since the cell membrane of animal cells is selective and does not allow all materials to pass through it, we say that it is semipermeable (noun: semipermeability).
b. Diffusion. Some materials readily pass through the membrane from an area of higher concentration to an area of lower concentration. This process is called diffusion. When materials require help to pass through the cell membrane, the process is referred to as facilitated diffusion.
c. Active Transport. In certain situations, materials pass through the cell membrane against the concentration gradient. In this case, an expenditure of energy is required. The process is called active transport. An example is the sodium/potassium pump, in which the sodium ions are forced out of the cytoplasm of the cell and into the surrounding tissue fluid and potassium ions are pumped back into the cell cytoplasm.
d. Osmosis. Sometimes a substance is not able to pass through the cell membrane. When the concentration of this substance is greater on one side of the cell membrane than the other, water will tend to pass through the membrane to the area of greater concentration. This process is called osmosis. This process involves the concept of tonicity, discussed in paragraph 2‑12.
e. Pinocytosis and Phagocytosis. Sometimes, the cell membrane will engulf a minute amount of tissue fluid and its contents. This process is called pinocytosis. During pinocytosis, the cell membrane produces a vacuole to contain the engulfed material. When the cell membrane engulfs larger particles, such as bacteria or other cells, the process is called phagocytosis. After either pinocytosis or phagocytosis, digestive fluids may pass from the cytoplasm into the vacuole. The end products of digestion are absorbed from the vacuole into the cell cytoplasm.
2‑14. MEMBRANE POTENTIALS
In living cells, there is generally a higher concentration of positively charged ions on the outside of the cell and a higher concentration of negatively charged ions on the inside of the cell. Thus, there is a concentration gradient (an electrical potential or polarity) across the membrane that we call the membrane potential that creates an electrical gradient.
a. Resting Potential. When the cell is in a resting state, the membrane potential is maintained by the sodium/potassium pump. The sodium/potassium pump actively transports 3 positive sodium ions (Na+) to the outside of the cell membrane and 2 potassium ions to the inside of the cell membrane. This results in a negative charge inside the cell and a positive charge outside the cell, producing a potential or polarity across the membrane.
b. Action Potential. The electrical activity that occurs in a stimulated neuron or muscle fiber is called the action potential. This involves depolarization and subsequent repolarization. First, sodium ions move into the cell by diffusion. This reverses the polarity (depolarization). Second, potassium moves out of the cell by diffusion that causes repolarization. The sodium/potassium pump then restores the ionic balance by actively (energy required) pumping sodium back out and potassium back into the cell. These various electrical potentials can be measured with appropriate instruments.
Section IV. CELL GROWTH AND MULTIPLICATION
2‑15. CELL GROWTH
a. The individual cells have the capacity to grow. They do this by acquiring various substances from the blood and converting them into appropriate cellular elements.
b. Sometimes, a tissue such as muscle tissue will increase in mass without an increase in the number of units. This condition is called hypertrophy.
2‑16. CELL MULTIPLICATION
a. On the other hand, if an increase in tissue mass results from a greater number of cells, we refer to this as hyperplasia.
b. Cell multiplication is accomplished through a process called mitosis. In mitosis, the genetic material of the cell is doubled. Then, the cell divides into halves. One‑half of the genetic material goes into each of the two daughter cells. In this manner, the two new cells each have the same genetic composition as the original cell.
Section V. EPITHELIAL CELLS AND TISSUES
2‑17. INTRODUCTION
Tissues are groups of like cells together performing a common function or functions. The epithelial tissues are specialized to cover surfaces and line cavities. They are also secretory.
2‑18. EPITHELIAL CELL TYPES
By observing microscopic preparations of epithelial tissues, one can classify the cells of epithelial tissues into three general types: columnar, cuboidal, and flat (squamous).
2‑19. EPITHELIAL TISSUE TYPES
If an epithelial tissue consists of a single layer of cells, it is called a simple epithelial tissue. When there are several layers of cells, it is called a stratified epithelial tissue. In both cases, the epithelial tissue is further identified by the type of epithelial cell that forms the outermost layer of the tissue. For example, the outer layer (epidermis) of the skin is a stratified squamous epithelium; squamous cells form the outermost of many layers.
2‑20. LINING OF SEROUS CAVITIES
The many serous cavities of the body are lined with a simple squamous epithelium. This epithelial tissue also secretes a serous fluid to act as a lubricant, reducing frictional forces of organs moving against each other. An example is the outer surfaces of the lungs, which move on the inside of the chest wall (within the pleural cavity) during breathing.
2‑21. OUTER SURFACE OF THE BODY
The outer layer of the skin is a stratified squamous epithelium. In it, there are many layers of cells. The outermost layers consist of squamous, or flat, cells.
2‑22. SECRETORY PROCESSES
Secretory epithelial cells, such as those in various glands, have a well‑developed Golgi complex. In one type of secretory cells, the secretions are passed through the cell membrane. In another type of secretory cells, those of the sebaceous glands, a portion of the cell containing the secretion is sloughed off from the cell.
Section VI. FIBROUS CONNECTIVE TISSUE
2‑23. INTRODUCTION
Tissues that generally support the body parts in various ways are known as the connective tissues (CT).
a. All of these connective tissues are characterized by having the major substance outside of the cell but formed by the cell. This extra‑cellular material is called the matrix.
b. One type of CT is called the fibrous connective tissue (FCT). In FCT, the cell known as the fibroblast forms a long narrow thread‑like structure known as the fiber. During the life of the individual, the fibroblast actually moves up and down the fiber. During this movement, it keeps the fiber in repair and restructures it in response to the stresses applied to the body.
2‑24. TYPES OF FIBROUS CONNECTIVE TISSUE FIBERS
Two types of fibers are formed‑‑the collagen or white fibers and the elastic or yellow fibers. The collagen fibers are limited in stretchability, particularly when compared to the elastic fibers.
2‑25. FIBROUS CONNECTIVE TISSUES
The fibers of the FCT are variously organized to perform particular functions.
a. Loose Areolar Fibrous Connective Tissue. In some locations, the fibers are loosely arranged with spaces between them. This tissue serves as filler material in the spaces between the organs. This loose areolar FCT is also found between the skin and the underlying structures of the body. Thus, the skin is able to move more or less freely over the surface of these structures.
b. Dense Fibrous Connective Tissue. The fibers of dense FCT are closely packed and more or less parallel. As membranes, dense FCT envelops areas or structures of the body (as in capsules around organs). Other examples of dense FCT are ligaments and tendons. A ligament is a band of dense FCT that holds the bones together at a joint. A tendon attaches a muscle to a bone.
2‑26. LENGTH AND TENSION
As a collagen fiber is increased in length, the tension (resistance to stretch) increases considerably. This can be shown by a length‑tension (L‑T) curve diagram, similar to the one in Figure 2‑4.
Figure 2-4. Length tension curve of an FCT fiber.
2‑27. TEMPERATURE AND TENSION
a. The degree of elasticity (stretchability) of an FCT is more or less proportional to its temperature. The cooler it is, the less stretchable and the more subject to damage it is. On the other hand, as the fiber becomes warmer, its stretchability and resistance to damage increase.
b. This characteristic is the basis of warm‑up exercises before participating in strenuous activities such as sports. By exercising to the point of sensible perspiration (para 2‑7c), the body, temperature is raised to the desired level. At this level, the FCT are able to stretch and withstand the various forces applied to them.
Section VII. FATTY TISSUES
2‑28. INTRODUCTION
Another supportive tissue of the body is fatty tissue (fat connective tissue). Here, the matrix is a lipid material, but found within the cell rather than outside of the cell.
2‑29. LIPIDS
Lipids are fats, oils, and similar compounds such as fatty acids. Lipids are stored mostly in the form of neutral fat. Neutral fat consists of triglycerides, a type of molecule formed from glycerol (a type of alcohol) and three fatty acids. According to the length of each fatty acid, the triglyceride may be a liquid (oil) or a solid (fat). The triglycerides are kept in a liquid form, and even in cold weather, their lengths are adjusted in order to maintain a liquid state.
2‑30. BROWN FAT AND YELLOW FAT
There are two types of fat within the body‑‑‑brown fat and yellow fat. Both are excellent means of energy storage. When metabolized, they both yield large amounts of energy, especially when compared to carbohydrates. Brown fat is more common in infants and children, whereas adults tend to have mostly yellow fat.
2‑31. TURNOVER OF FATS
Fats are essentially a temporary storage phenomenon. There is a continuous turnover of the triglycerides. There is a complete turnover within a 3‑week period.
2‑32. SOURCES
The diet is the major source of fat in the human body. Fats may be taken in as fats or converted from other substances, such as carbohydrates.
2‑33. OBESITY
Obesity occurs when excessive amounts of fats and/or carbohydrates are taken into the body. When the energy contained in these compounds is not used in bodily activities, the surplus is generally stored as the triglycerides in fatty tissues of the body.
2‑34. STORAGE OF FAT‑SOLUBLE SUBSTANCES
a. A number of fat‑soluble substances may be stored in the fat of the body. Vitamins A and D are fat-soluble.
b. In addition, organophosphoric compounds of modern pesticides are often stored in human fat. Although these compounds may have been required in food production, they may also be ingested along with the food.
c. The storage of such substances becomes particularly important when an individual goes on a crash diet. As fat is lost during such a diet, these fat‑soluble substances are released into the general system. They may reach dangerous levels. In addition, the organs supported by the fat may become loose within the body and subject to injury.
2‑35. CHOLESTEROL
Cholesterol is a special lipid‑type substance. It is very important for the proper functioning of several structures and processes of the body, particularly in the liver. However, there are some indications that, in some individuals, excessive cholesterol may be damaging to the cardiovascular system of the body.