Medical brain notes 27

  A ll living organisms are made of cells and cell   products. This simple statement, called the cell the-ory, was first proposed more than 150 years ago. You may think of a theory as a guess or hypothesis, and sometimes this is so. A scientific  theory , however, is actually the best explanation of all available evidence. All of the evidence science has gathered so far sup-ports the validity of the cell theory. Cells  are the smallest living subunits of a multicel-lular organism such as a human being. A cell is a com-plex arrangement of the chemicals, is living, and carries out specific activities. Microorganisms, such as amoebas and bac-teria, are single cells that function independently. Human cells, however, must work together and func-tion 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 microsco...

  CELL STRUCTURE Despite their many differences, human cells have sev-eral similar structural features: a cell membrane, a nucleus, and cytoplasm and cell organelles. 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 mem-brane  is made of phospholipids, cholesterol, and pro-teins. The arrangement of these organic molecules is shown in Fig. 3–1. The phospholipids are diglyc-erides, and form a bilayer, or double layer, which makes up most of the membrane. 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 pro-teins have several functions: Some form  channels  or  pores  to permit passage of mater...

  CELL MEMBRANE Also called the  plasma membrane , the  cell mem-brane  is made of phospholipids, cholesterol, and proteins. The arrangement of these organic molecules is shown in Fig. 3–1. The phospholipids are diglyc-erides, and form a bilayer, or double layer, which makes up most of the membrane. 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 pro-teins have several functions: Some form  channels  or  pores  to permit passage of materials such as water or   ions; others are carrier enzymes or transporters  that also help substances enter the cell. Still other proteins, with oligosaccharides on their outer surface, are  anti-gens , markers that identify the cells of an individual as   “self.” Yet another group of proteins serves as  recep-tor sites...

  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 (Fig. 3–2). A  nucleolus  is a small sphere made of DNA, RNA, and protein. The nucleoli form a type of RNA called ribosomal RNA, which becomes part of ribosomes (a cell organelle) and is involved in protein synthesis. The nucleus is the control center of the cell because it contains the chromosomes. Even with a microscope, the 46  chromosomes  of a human cell are usually not visible; they are long threads called  chro-matin . Before a cell divides, however, the chromatin   coils extensively into visible chromosomes. Chromo-somes are made of DNA and protein. Some chromo-somal proteins provide the structural framework for the coiling of the chromatin into chromosomes so...

  CYTOPLASM AND CELL ORGANELLES Cytoplasm  is a watery solution of minerals, gases,   organic molecules, and cell organelles that is found between the cell membrane and the nucleus.  Cytosol  is the water portion of cytoplasm, and many chemical reactions take place within it. Cell organelles are intracellular structures, often bounded by their own membranes, that have specific functions in cellular metabolism. They are also shown in Fig. 3–2. 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 on its surface, whereas smooth ER has no ribosomes. As a network of inter-connected tunnels, the ER is 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 ...

  CELLULAR TRANSPORT MECHANISMS Living cells constantly interact with the blood or tis-sue fluid around them, taking in some substances and secreting or excreting others. There are several mech-anisms 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 sec-tions 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 concen-tration (that is, with or along a concentration gradi-ent ). Diffusion occurs because molecules have free   energy; that is, they are always in motion. The mole-cules in a solid move very slowly; those in a l...

  THE GENETIC CODE AND PROTEIN SYNTHESIS The structure of DNA, RNA, and protein was described. We will review some of the essentials here, and go a step further with a simple description of how all of these organic molecules are involved in the process of protein synthesis. DNA AND THE GENETIC CODE DNA  is a double strand of nucleotides in the form of   a  double helix , very much like a spiral ladder. The uprights of the ladder are made of alternating phos-phate groups and deoxyribose sugar molecules. The rungs of the ladder are made of the four nitrogenous bases, always found in complementary pairs: adenine with thymine (A–T) and guanine with cytosine (G–C). Although DNA contains just these four bases, the bases may be arranged in many different sequences (reading up or down the ladder). It is the sequence of bases, the sequence of A, T, C, and G, that is the  genetic code . The DNA of our 46 chromosomes may   also be called our  genome , which is th...