The Cell PDF

Physiology Dr. Mohamed Balfas The Cell Cells are the smallest living subunits of the body. Human body is a multicellular organism. Microorganisms, such as bacteria, are single...

Physiology Dr. Mohamed Balfas The Cell Cells are the smallest living subunits of the body. Human body is a multicellular organism. Microorganisms, such as bacteria, are single cells that function independently. Human cells, however, must work together and function interdependently. 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 micrometers (formerly called microns). One micrometer (μm) = 1/1,000,000 of a meter. One exception is the human ovum or egg cell, which is just visible to the unaided eye. 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. The function of human cells differ from one type to another, for example; muscle cells perform contraction, red blood cells carries oxygen, intestinal cells digest and absorb nutrients…..etc. Cell structure: Despite their many differences, human cells have several similar structural features: a cell membrane, a nucleus, a cytoplasm and cell organelles. Red blood cells are an exception, because they have no nuclei or cell organelles when mature. Cell membrane: The cell membrane (also called plasma membrane) forms the outer boundary of the cell and surrounds the cytoplasm, cell organelles, and nucleus. It is made of phospholipids, cholesterol, and proteins. The phospholipids form a 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 (i.e., selectively permeable). The presence of cholesterol decreases the fluidity of the membrane, thus making it more stable. Figure 1: cell membrane structure. 1 Physiology Dr. Mohamed Balfas Proteins of the cell membrane are classified into two types: - Integral (transmembrane) proteins; most of them are glycoproteins with oligosaccharides. - Peripheral membrane proteins; present mainly on the inside surface of the membrane. The functions of membrane proteins include the following: 1- Channels (pores); for passage of small water soluble substances such as sodium ions (Na+) and potassium ions (K+), across the cell membrane. 2- Carrier or transporters; help in the transfer of larger water-soluble substances across the cell membrane. 3- Antigens; integral proteins with oligosaccharides acts as antigens—markers that identify the cells of the body as “self.” 4- Receptors; chemical substances, like hormones, can bind to these receptor proteins in order to initiate their physiologic changes inside the cell. 5- Enzymes; enzyme proteins in the cell membranes may act in certain chemical reactions. For example, an enzyme on the membrane of a cell can terminate the effect of a hormone—whose job is done—to prevent further excessive stimulation of the cell. 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 (Figure 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 (46 chromosomes). Chromosomes are made of DNA. DNA in the nucleus of each cell contains all of the genetic information for all human traits. 2 Physiology Dr. Mohamed Balfas Figure 2: The cell and its organelles. 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 (or intracellular fluid) is the water portion of cytoplasm without the cell organelles. The cell organelles are intracellular structures that have specific functions in cellular activities. The endoplasmic reticulum (ER) is an extensive network of membranous tubules. Rough ER has numerous ribosomes on its surface, whereas smooth ER has no ribosomes. Proteins were synthesized by the ribosomes on the rough ER while lipids were 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. A proteasome is a barrel-shaped organelle made of enzymes (protease enzymes) that cut apart protein molecules that are no longer needed or those that are damaged. 3 Physiology Dr. Mohamed Balfas Golgi complex, consists of a series of flat, membranous sacs, somewhat like a stack of saucers. It receives protein and/or lipid-filled vesicles that bud from the ER; then it modifies, sorts, and packaging them for transport. Mitochondria are oval or spherical organelles 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. Mitochondria contain their own genes in a single DNA molecule and duplicate themselves when a cell divides. An individual’s mitochondrial DNA (mDNA) is of maternal origin, that is, from the mitochondria that were present in the ovum, or egg cell, which was then fertilized by a sperm cell. The mitochondria of the sperm cell usually do not enter the ovum during fertilization, because they are not found in the head of the sperm with the chromosomes. Lysosomes are single-membrane structures that contain digestive enzymes. When certain white blood cells engulf bacteria, the bacteria are digested and destroyed by these lysosomal enzymes. Many of our cells are capable of dividing, or reproducing, themselves. Centrioles are a pair of rod-shaped structures perpendicular to one another. Their function is to organize the spindle fibers during cell division. Figure 3: Centrioles and spindle fibers. Cilia and flagella are mobile thread-like projections through the cell membrane. Cilia are usually shorter than flagella, and an individual cell has many of them on its free surface. The cilia of a cell beat in unison and sweep materials across the cell surface. 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 movement for the sperm cell. 4 Physiology Dr. Mohamed Balfas Figure 4: (a) The cilia, and (b) the flagella of sperms. Microvilli are folds of the cell membrane on the surface of a cell. These folds greatly increase the surface area of the membrane, and are part of the cells lining organs that absorb materials. The small intestine, for example, requires a large surface area for the absorption of nutrients, and many of its lining cells have microvilli. 5