CELLS

1. A cell is the basic, living, structural and functional unit of the body. A generalized cell is a composite that represents various cells of the body. Cytology is the science concerned with the study of cells. The principal parts of a cell are the plasma (cell) membrane, cytoplasm, organelles, and inclusions. Extracellular materials are manufactured by the cell and deposited outside the plasma membrane.
2. The plasma (cell) membrane, surrounds the cell and separates it from other cells and the external environment. It is composed primarily of proteins and phospholipids. According to the fluid mosaic model, the membrane consists of a phospholipid bilayer with integral and peripheral proteins. Functionally, the plasma membrane facilitates contact with other cells, provides receptors, and mediates the passage of materials.
3. The membrane's selectively permeable nature restricts the passage of certain substances. Substances can pass through the membrane depending on their molecular size, lipid solubility, electrical charges, and the presence of carriers.
4. Passive movement of materials through the cell membrane involve the kinetic energy of individual molecules. Diffusion is the net movement of molecules or ions from an area of higher concentration to an area of lower concentration until an equilibrium is reached. In facilitated diffusion, certain molecules, such as glucose, combine with a carrier to become soluble in the phospholipid portion of the membrane. Osmosis is the movement of water through a selectively permeable membrane from an area of higher water concentration to an area of lower water concentration. During osmosis in an isotonic solution, red blood cells maintain their normal shape; in a hypotonic solution, they undergo hemolysis; in a hypertonic solution, they undergo crenation. Filtration is the movement of water and dissolved substances across a selectively permeable membrane by pressure.
5. Active processes involve the use of ATP by the cell. Active transport is the movement of ions across a cell membrane from lower to higher concentration. Endocytosis in the movement of substances through plasma membranes in which the membrane surrounds the substance, encloses it, and brings it into the cell. Phagocytosis is the ingestion of solid particles by pseudopodia. It is an important process used by white blood cells to destroy bacteria that enter the body. Pinocytosis is the ingestion of a liquid by the plasma membrane. In this process, the liquid becomes surrounded by a vacuole. Receptor-mediated endocytosis in the selective uptake of large molecules and particles by cells.
6. Cytoplasm is the substance inside the cell that contains organelles and inclusions. It is composed mostly of water plus proteins, carbohydrates, lipids, and inorganic substances. The chemicals in cytoplasm are either in solution or in a colloid (suspended) form. Functionally, cytoplasm is the medium in which chemical reactions occur.
7. Organelles are specialized portions of the cell that carry on specific activities. They assume specific roles in cellular growth, maintenance, repair, and control.
8. The nucleus is usually the largest organelle, the nucleus controls cellular activities and contains the genetic information. Cells without nuclei, such as mature red blood cells, do not grow or reproduce. The parts of the nucleus include the nuclear membrane, karyolymph, nucleoli, and genetic material (DNA), comprising the chromosomes. Chromosomes consist of DNA and histones and consist of subunits called nucleosomes.
9. Ribosomes are granular structures consisting of ribosomal RNA and ribosomal proteins. They occur free (singly or in clusters) or in conjunction with endoplasmic reticulum. Functionally, ribosomes are the sites of protein synthesis.
10. The ER is a network of parallel membranes continuous with the plasma membrane and nuclear membrane. Granular or rough ER has ribosomes attached to it. Agranular or smooth ER does not contain ribosomes. The ER provides mechanical support, conducts intracellular nerve impulses in muscle cells, exchanges materials with cytoplasm, transports substances intracellularly, stores synthesized molecules. and helps export chemicals from the cell.
11. The Golgi complex consists of four to eight stacked, membranous sacs called cisternae. In conjunction with the ER, the Golgi complex secretes proteins and lipids and synthesizes and secretes glycoproteins. It is particularly prominent in secretory cells such as those in the pancreas or salivary glands.
12. Mitochondria consist of a smooth outer membrane and a folded inner membrane surrounding the interior matrix. The inner folds are called cristae. The mitochondria are called ''powerhouses of the cell'' because ATP is produced in them. They are endosymbionts and are actually cells living within cells.
13. Lysosomes are spherical structures that contain digestive enzymes. They are formed from Golgi complexes. They are found in large numbers in white blood cells, which carry on phagocytosis. If the cell is injured, lysosomes release enzymes and digest the cell. Thus they are called ''suicide packets.''
14. Peroxisomes are similar to Iysosomes, but smaller. They contain enzymes (e.g., catalase) involved in the metabolism of hydrogen peroxide.
15. Together microfilaments and microtubules form the cytoskeleton. Microfilaments are rod -like structures consisting of the protein actin or myosin. They are involved in muscular contraction, support, and movement. Microtubules are cylindrical structures consisting of the protein tubulin. They support, provide movement, and form the structure of flagella, cilia, centrioles, and the mitotic spindle.
16. The dense area of cytoplasm containing the centrioles is called a centrosome. It is located near the nucleus. Centrioles are paired cylinders arranged at right angles to one another. They assume an important role in cell reproduction.
17. Cilia and flagella are cellular projections have the same basic structure and are used in movement. If the projections are few and long, they are called flagella. If they are numerous and hairlike, they are called cilia. The flagellum on a sperm cell moves the entire cell. The cilia on cells of the respiratory tract move foreign matter trapped in mucus along the cell surfaces toward the throat for elimination.
18. Cell inclusions are chemical substances produced by cells. They are usually organic and may have recognizable shapes Examples are melanin, glycogen, lipids, and mucus.
19. Extracellular materials are all the substances that lie outside the cell membrane. They provide support and a medium for the diffusion of nutrients and wastes. Some, like hyaluronic acid and chondroitin sulfate, are amorphous. Others, like collagenous, reticular, and elastic fibers, are fibrous.
20. Plant cells differ mainly from animal cells by the presence of chloroplasts and cell walls. Cell walls are mainly made from cellular secretions called cellulose, lignin, and pectin. During cell division plant cells form a cell plate rather than undergoing cytokinesis.