The Cells

The Cells

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The Cells...

Cells:

All living organisms are made of cells and cell products. This simple statement, called the Cell Theory, was first proposed over 150 years ago. You may think of a theory as a guess or hypothesis, and sometimes this is so. A theory, however, is actually the best explanation of all the available evidence. All of the evidence science has gathered so far supports the validity of the Cell Theory.

Cells are the smallest living subunits of a multicellular organism such as a human being. A cell is a complex arrangement of the chemicals discussed in the previous chapter, is living, and carries out specific activities. Microorganisms, such as amoebas and bacteria, are single cells that function independently. Human cells, however, must work together and function interdependently. Homeostasis depends upon the contributions of all of the different kinds of cells.

Human cells vary in size, shape, and function. Most human cells are so small they can only be seen with the aid of a microscope and are measured in units called microns (1 micron = 1/25,000 of an inch - see Appendix 1: Units of Measure). One exception is the human ovum or eggshell, which is about 1 millimeter in diameter, just visible to the unaided eye. Some nerve cells, although microscopic in diameter, may be quite long. Those in our arms and legs, for example, are at least 2 feet (60 cm) long.

With respect to shape, human cells vary greatly. Some are round or spherical, others rectangular, still others irregular. White blood cells even change shape as they move.

CELL STRUCTURE

Despite their many differences, human cells have several similar structural features: a cell membrane, cytoplasm and cell organelles, and a nucleus. Red blood cells are an exception because they have no nuclei when mature. The cell membrane forms the outer boundary of the cell and surrounds the cytoplasm, organelles, and nucleus.

CELL MEMBRANE

Also called the plasma membrane, the cell membrane is made of phospholipids, cholesterol, and proteins. The phospholipids permit lipid soluble materials to easily enter or leave the cell by diffusion through the cell membrane. The presence of cholesterol decreases the fluidity of the membrane, thus making it more stable. The proteins have several functions: Some form pores or openings to permit passage of materials; others are enzymes that help substances enter the cell. Still other proteins, with oligosaccharides on their outer surface, are antigens that identify the cells of an individual as “self”. Yet another group of proteins serves as receptor sites for hormones. For most hormones to bring about their specific effects they must first bond to a particular receptor on the cell membrane. This bonding then triggers chemical reactions within the cell membrane or the interior of the cell.

Although the cell membrane is the outer boundary of the cell, it should already be apparent to you that it is not a static or wall-like boundary, but rather an active, dynamic one. The cell membrane is selectively permeable, that is, certain substances are permitted to pass through and others are not. These mechanisms of cellular transport will be covered later.

NUCLEUS

With the exception of mature red blood cells, all human cells have a nucleus. The nucleus is within the cytoplasm and is bounded by a double-layered nuclear membrane with many pores. It contains one or more nucleoli and the chromosomes of the cell.

A nucleolus is a small sphere made of DNA, RNA, and protein. The nucleoli form type of RNA called ribosomal RNA, which becomes part of ribosome’s ( a cell organelle) and is involved in protein synthesis.

The nucleus is the control center of the cell because it contains the chromosomes. The 46 chromosomes of a human cell are usually not visible; they are long threads called chromatin. When a cell divides, however, the chromatin coils extensively into visible chromosomes. Chromosomes are made of DNA and protein. Remember from our earlier discussion that the DNA is the genetic code for the characteristics and activities of the cell. Although the DNA in the nucleus of each cell contains all of the genetic information for all human traits, only a small number of genes (a gene is the genetic code for one protein) are actually active in a particular cell. These active genes are the codes for the proteins necessary for the specific cell type.

CYTOPLASM AND CELL ORGANELLES

Cytoplasm is watery solution of minerals, gases, and organic molecules that is found between the cell membrane and the nucleus. Chemical reactions take place within the cytoplasm, and many of the cell organelles are found here. Cell organelles are intracellular structures, often bounded by their own membranes, that have specific roles in cellular functioning.

The endoplasmic reticulum (ER) is an extensive network of membranous tubules that extend from the nuclear membrane to the cell membrane. Rough ER has numerous ribosomes in its surface, whereas smooth ER has no ribosomes at all. As a network of interconnected tunnels, the ER serves as a passageway for the transport of the materials necessary for cell function within the cell. These include proteins synthesized by the ribosomes on the rough ER, and lipids synthesized by the smooth ER.

Ribosomes are very small structures made of protein and ribosomal RNA. Some are found on the surface of rough ER, while others float freely within the cytoplasm. Ribosomes are the site of protein synthesis.

The Golgi apparatus is a series of flat, membranous sacs, somewhat like a stack of saucers. Carbohydrates are synthesized within the Golgi apparatus, and are packaged, along with other materials, for secretion from the cell. To secrete a substance, small sacs of the Golgi membrane break off and fuse with the cell membrane, releasing the substance of the exterior of the cell.

Mitochondria are oval or spherical organelles within the cytoplasm, bounded by a double membrane. The inner membrane has folds called cristae. Within the mitochondria, the aerobic (oxygen-requiring ) reactions of cell respiration take place. Therefore, mitochondria are the site of ATP (and hence energy) production. Cells that require large amounts of ATP, such as muscle cells, have many mitochondria to meet their need for energy.

Lysosomes are single membrane structures within the cytoplasm that contain digestive enzymes. When certain white blood cells engulf bacteria, the bacteria are digested and destroyed by these lysosomal enzymes. Worn-out cell parts and dead cells are also digested by these enzymes, which is necessary before tissue repair can begin, but which contributes to the process of inflammation in damaged tissues.

Centrioles are a pair of rod-shaped structures perpendicular to one another, located just outside the nucleus. Their function is to organize the spindle fibers during cell division.

Cilia and flagella are mobile thread-like projections through the cell membrane. Cilia serve the function of sweeping materials across the cell surface. They are usually shorter than flagella, and an individual cell has many of them. Cells lining the fallopian tubes, for example, have cilia to sweep the egg cell toward the uterus. The only human cell with a flagellum is the sperm cell. The flagellum provides motility, or movement, for the sperm cell.

CELLULAR TRANSPORT MECHANISMS

Living cells constantly interact with the blood to tissue fluid around them, taking in some substances and secreting or excreting others. There are several mechanisms of transport that enable cells to move materials into or out of the cell: diffusion, osmosis, facilitated diffusion, active transport, filtration, phagocytosis, and pinocytosis. Some of these take place without the expenditure of energy by the cells. But others do require energy, often in the form of ATP. Each of these mechanisms is described in the following sections and an example is included to show how each is important to the body.

DIFFUSION

Diffusion is the movement of molecules from an area of greater concentration to an area of lesser concentration (that is, with or along a concentration gradient). Diffusion occurs because molecules have free energy, that is, they are always in motion, The molecules in a solid move very slowly; those in a liquid move faster, and those in a gas move faster still, as when ice absorbs heat energy, melts, and then evaporates. In fig. 3-3, a sugar cube in a glass of water is shown. As the sugar dissolves, the sugar molecules collide with one another. These collisions spread out the sugar molecules until they are evenly dispersed among the water molecules. The molecules are still moving, but as some go to the top others go to the bottom, and so on. Thus, an equilibrium (or steady-state balance) is reached.

Diffusion is a very slow process, but may be an effective transport mechanism across microscopic distances. Within the body, the gases oxygen and carbon dioxide move by diffusion. In the lungs, for example, there is a high concentration of oxygen in the alveoli (air sacs) and a low concentration of oxygen in the blood in the surrounding pulmonary capillaries. The opposite is true for carbon dioxide: A low concentration in the air in the alveoli and a high concentration in the blood in the pulmonary capillaries. These gases diffuse in opposite directions, each moving from where there is more to where there is less. Oxygen diffuses from the air to the blood to be circulated throughout the body. Carbon dioxide diffuses from the blood to the air to be exhaled.

OSMOSIS

Osmosis may be simply defined as the diffusion of water through a selectively permeable membrane or barrier. That is, water will move from an area with more water present to an area with less water. Another way to say this is that water will naturally tend to move to an area where there is more dissolved material, such as salt or sugar. If a 2% salt solution and a 6% salt solution are separated by a membrane allowing water but not salt to pass through it, water will diffuse from the 2% salt solution to the 6% salt solution. The result is that the 2% solution will become more concentrated, and the 6% solution will become more dilute.

In the body, the cells lining the small intestine absorb water from digested food by osmosis. These cells have first absorbed salts, have become more “salty,” and water follows salt into the cells. The process of osmosis also takes place in the kidneys, which reabsorb large amounts of water (many gallons each day) to prevent its loss in urine.

ACTIVE TRANSPORT

Active transport requires the energy of ATP to move molecules from an area of lesser concentration to an area of greater concentration. Notice that this is the opposite of diffusion, in which the free energy of molecules causes them to move to where there are fewer of them. Active transport is therefore said to be movement against a concentration gradient.

In the body, nerve cells and muscle cells have “sodium pumps” to move sodium ions (Na+) out of the cells. Sodium ions are more abundant outside the cells and they constantly diffuse into the cell, their area of lesser concentration. Without the sodium pumps to return them outside, the incoming sodium ions would bring about and unwanted nerve impulse or muscle contraction. Nerve and muscle cells constantly produce ATP to keep their sodium pumps working and prevent spontaneous impulses.

Another example of active transport is the absorption of glucose and amino acids by the cells lining the small intestine. The cells use ATP to absorb these nutrients from digested food, even when their intracellular concentration becomes greater than their extra-cellular concentration.

FILTRATION

The process of filtration also requires energy, but the energy needed does not come directly from ATP. It is the energy of mechanical pressure. Filtration means that water and dissolved materials are forced through a membrane from an area of higher pressure to an area of lower pressure.

In the body, blood pressure is created by the pumping of the heart. Filtration occurs when blood flows through capillaries, whose walls are only one cell thick and very permeable. The blood pressure in capillaries is higher than the pressure of the surrounding tissue fluid. In capillaries throughout the body, blood pressure forced plasma and dissolved materials through the capillary membranes into the surrounding tissue spaces. This creates more tissue fluid and is how cells receive glucose, amino acids and other nutrients. Blood pressure in the capillaries of the kidneys also brings about filtration, which is the first step in the formation of urine.

PHAGOCYTOSIS AND PINOCYTOSIS

These two processes are similar in that both involve a cell engulfing something. An example of phagocytosis is a white blood cell engulfing bacteria. The white blood cell flows around the bacterium, taking it in and eventually digesting it.

Other cells that are stationary may take in small molecules that become absorbed or attached to their membranes. The cells of the kidney tubules reabsorb small proteins by pinocytosis, so that the protein is not lost in urine.