the cell ultrastructure

• Lysosomes are membrane-bound organelles that contain digestive enzymes to break down ingested materials or damaged organelles within cells.
• The scientific study of cells - their structures, functions, and activities is called cytology.
• The cell is the smallest unit of life.
• In 1590, Francis and Zacharias Janssen constructed the first compound microscope.
• The cell as a structural unit was first observed by Robert Hooke in thin slices of cork and other plant tissues in 1665.
• Other inclusions in plant cells are starch granules, inorganic deposits such as calcium oxalate crystals, crystals of calcium carbonate, calcium sulfate and silica, gluten and aleuron grains, and oil globules.
• Waste vacuoles serve as organs of excretion, metabolic trash cans.
• At about the same time (1674), the Dutch lensmaker Leeuwenhoek reported seeing "animalcules" or single-celled organisms through his microscope.
• Marcello Malphigiani used a microscope to examine and describe thin slices of animal tissues from brain, liver, kidney, spleen, lungs, and tongue.
• In time, the quality of microscopes improved, leading to the establishment of the concept of the cell theory in the early 1800s.
• Theodor Schwann, a zoologist, wove the various loose ends into a coherent theory which proposed that all organisms are composed of cells and the product of cells.
• In 1831, Robert Brown described and named the nucleus of the cell.
• At the end of the decade, Purkinje proposed using the term protoplasm to describe the living material within the cell.
• Thomas Huxley later called protoplasm the "physical basis of life".
• Rudolf Virchow stated an important principle: All cells arise from pre-existing cells.
• Louis Pasteur's experiment marked the end of a belief in spontaneous generation theory and the establishment of the principle of biogenesis.
• Plant cells are encased in an outer protective and supporting structure called the cell wall.
• The cell wall is composed of the polysaccharide cellulose in the form of threadlike structure called fibril.
• The first portion of the cell wall laid down by a young cell is the primary wall.
• Between the walls of adjacent cells is an intercellular layer called middle lamella which contains pectin.
• The middle lamella binds the cells together.
• The cells of the more woody portion of plants form an additional layer called secondary wall.
• Some unicellular organisms such as bacteria and blue-green algae do not show a distinct separation of nuclear and cytoplasmic constituents.
• This is located internal to the primary wall and contains lignin which adds stiffness and rigidity to cell walls.
• The cell membrane, also known as the plasma membrane or plasmalemma, is the living boundary between the external and internal environment of the cell.
• In the matrix of the mitochondria are a number of enzymes of the Krebs cycle, salts, water.
• Cells without nuclei are called procaryotic cells in contrast to those with nuclei called eucaryotic cells.
• The nucleolus (little nucleus) is a highly compact and darkly staining spherical body which consists largely of RNA and proteins and is believed to be the site of ribosomal manufacture.
• Many cells bear specialized structures of locomotion called cilia or flagella which are vibratile extensions of the cell surface.
• The nuclear membrane has a double unit membrane structure and communicates with an internal membrane network, the endoplasmic reticulum by minute pores.
• The Golgi apparatus (Golgi bodies or dictyosomes) were discovered by Camilo Golgi and consist of flattened sacs which appear as parallel membranes, cluster of small, tightly packed vesicles and large clear vacuoles.
• Research shows that the mitochondria are the principal sites for cellular respiration.
• Mitochondria are round or filamentous structures with predominantly fat-protein composition and are found in all cells.
• The most conspicuous protoplasmic structure is a spherical or ellipsoid body called the nucleus which is the reproductive, metabolic, and dynamic center of the cell.
• The cytoplasm, the largest volume of the cell, is a translucent, colorless, viscous mass where most of the organelles of the cell are situated.
• The outer layer of the mitochondrial surface is covered with a membrane, while the inner surface is much folded and forms projections called cristae that extend into the interior fluid or matrix.
• Chromosomes are principal nuclear structures which consist of DNA and proteins and are conspicuous only during cell division.
• The cristae is the site of protein synthesis for respiration.
• The mitochondria, as the respiration centers, have been aptly called the "powerhouse of the cell".
• The soluble portion of the cytoplasm, the cytosol, has a very high protein concentration exceeding 20% and is the site of metabolic enzymes.