Cell Theory
Cell Theory
Cell Theory
Cell theory refers to the idea that cells are the basic unit of structure in every living thing. Development of this theory during the mid 17th century was made possible by advances in microscopy. This theory is one of the foundations ofbiology. The theory says that new cells are formed from other existing cells, and that the cell is a fundamental unit of structure, function and organization in all living organisms.
History
The cell was discovered by Robert Hooke in 1665. He examined (under a coarse, compound microscope) very thin slices of cork and saw a multitude of tiny pores that he remarked looked like the walled compartments a monk would live in. Because of this association, Hooke called them cells, the name they still bear. However, Hooke did not know their real structure or function.
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cells (which were actually non-living cell walls) was published inMicrographia.[2] His cell observations gave no indication of the nucleus and other organelles found in most living cells. The first person to make a compound microscope was Zacharias Jansen, while the first to witness a live cell under a microscope was Antonie van Leeuwenhoek, who in 1674 described the algae Spirogyra and named the moving organisms animalcules, meaning "little animals". saw bacteria.
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Cell theory was in contrast to the vitalism theories proposed before the discovery of cells.
The idea that cells were separable into individual units was proposed by Ludolph Christian Treviranus
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and Johann Jacob Paul Moldenhawer.[6] All of this finally led to Henri Dutrochet formulating
one of the fundamental tenets of modern cell theory by declaring that "The cell is the fundamental element of organization".[7] The observations of Hooke, Leeuwenhoek, Schleiden, Schwann, Virchow, and others led to the development of the cell theory. The cell theory is a widely accepted explanation of the relationship between cells and living things. The cell theory states: All living things or organisms are made of cells and their products. New cells are created by old cells dividing into two. Cells are the basic building units of life.
The cell theory holds true for all living things, no matter how big or small, or how simple or complex. Since according to research, cells are common to all living things, they can provide information about all life. And because all cells come from other cells, scientists can study cells to learn about growth, reproduction, and all other functions that living things perform. By learning about cells and how they function, you can learn about all types of living things. Credit for developing cell theory is usually given to three scientists: Theodor Schwann, Matthias Jakob Schleiden, and Rudolf Virchow. In 1839, Schwann and Schleiden suggested that cells were the basic unit
of life. Their theory accepted the first two tenets of modern cell theory (see next section, below). However the cell theory of Schleiden differed from modern cell theory in that it proposed a method of spontaneous crystallization that he called "Free Cell Formation".
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come from pre-existing cells, thus completing the classical cell theory.
Classical interpretation
1. All living organisms are made up of one or more cells. 2. Cells are the basic unit of life. 3. All cells arise from pre-existing cells. 4. The cell is the unit of structure, physiology, and organization in living things. 5. The cell retains a dual existence as a distinct entity and a building block in the construction of organisms.
Modern interpretation
The generally accepted parts of modern cell theory include: 1. The cell is the fundamental unit of structure and function in living organisms. 2. All cells arise from pre-existing cells by division. 3. Energy flow (metabolism and biochemistry) occurs within cells. 4. Cells contain hereditary information (DNA) which is passed from cell to cell during cell division. 5. All cells are basically the same in chemical composition in organisms of similar species. 6. All known living things are made up of one or more cells. 7. Some organisms are made up of only one cell and are known as unicellular organisms. 8. Others are multicellular, composed of a number of cells. 9. The activity of an organism depends on the total activity of independent cells.
Types of cells
Cells can be subdivided into the following subcategories: 1. Prokaryotes: Prokaryotes lack a nucleus (though they do have circular DNA) and other membrane-bound organelles (though they do contain ribosomes). Bacteria and Archaea are two domains of prokaryotes. 2. Eukaryotes: Eukaryotes, on the other hand, have distinct nuclei bound by a nuclear membrane and membrane-bound organelles (mitochondria, chloroplasts, lysosomes, rough and smooth endoplasmic reticulum, vacuoles). In addition, they possess organized chromosomes which store genetic material.
CELL THEORY
Definition: The Cell Theory is one of the basic principles of biology. Credit for the formulation of this theory is given to german scientists Theodor Schwann, Matthias Schleiden, and Rudolph Virchow. The Cell Theory states: All living organisms are composed of cells. They may be unicellular or multicellular. The cell is the basic unit of life. Cells arise from pre-existing cells. The modern version of the Cell Theory includes the ideas that: y Energy flow occurs within cells. y Heredity information (DNA) is passed on from cell to cell. y All cells have the same basic chemical composition.
y y y
y y y
One or more per cell Spherical shape Denser than surrounding cytoplasm
Chromosomes
- Usually in the form of chromatin - Contains genetic information - Composed of DNA - Thicken for cellular division - Set number per species (i.e. 23 pairs for human)
Nuclear membrane
- Surrounds nucleus - Composed of two layers - Numerous openings for nuclear traffic
Nucleolus
- Spherical shape - Visible when cell is not dividing - Contains RNA for protein manufacture
y y y y
Collective term for cytosol and organelles contained within Colloidal suspension Cytosol mainly composed of water with free-floating molecules Viscosity constantly changes
Centrioles
- Paired cylindrical organelles near nucleus - Composed of nine tubes, each with three tubules - Involved in cellular division - Lie at right angles to each other
Chloroplasts
- A plastid usually found in plant cells - Contain green chlorophyll where photosynthesis takes place
Cytoskeleton
- Composed of microtubules - Supports cell and provides shape - Aids movement of materials in and out of cells
Endoplasmic reticulum
- Tubular network fused to nuclear membrane - Goes through cytoplasm onto cell membrane - Stores, separates, and serves as cell's transport system - Smooth type: lacks ribosomes - Rough type (pictured): ribosomes embedded in surface
Golgi apparatus
- Protein 'packaging plant' - A membrane structure found near nucleus - Composed of numerous layers forming a sac
Lysosome
- Digestive 'plant' for proteins, lipids, and carbohydrates - Transports undigested material to cell membrane for removal - Vary in shape depending on process being carried out - Cell breaks down if lysosome explodes
Mitochondria
- Second largest organelle with unique genetic structure - Double-layered outer membrane with inner folds called cristae - Energy-producing chemical reactions take place on cristae - Controls level of water and other materials in cell - Recycles and decomposes proteins, fats, and carbohydrates, and forms urea
Ribosomes
- Each cell contains thousands - Miniature 'protein factories' - Composes 25% of cell's mass - Stationary type: embedded in rough endoplasmic reticulum - Mobile type: injects proteins directly into cytoplasm
Vacuoles
- Membrane-bound sacs for storage, digestion, and waste removal - Contains water solution - Contractile vacuoles for water removal (in unicellular organisms)
Cell wall
- Most commonly found in plant cells - Controls turgity - Extracellular structure surrounding plasma membrane - Primary cell wall: extremely elastic - Secondary cell wall: forms around primary cell wall after growth is complete
Plasma membrane
- Outer membrane of cell that controls cellular traffic - Contains proteins (left, gray) that span through the membrane and allow passage of materials - Proteins are surrounded by a phospholipid bi-layer.
CELL STRUCTURE
Cell Wall
y y
y y y
Support (grow tall) Protection allows H2O, O2, CO2 to diffuse in & out of cell
Cell Membrane
All cells
Plant - inside cell wall Animal outer layer; cholesterol Double layer of phospholipids with proteins Selectively permeable
y y y
Support Protection Controls movement of materials in/out of cell Barrier between cell and its environment Maintains homeostasis Controls cell activities Contains the hereditary material of the cell
Nucleus
y y
y y
y y
Nuclear membrane
Clear, thick, jellylike material (cytosol) Organelles found inside cell membrane Contains the cytoskeleton fibers Network of tubes or membranes Smooth w/o ribosomes Rough with embedded ribosomes Connects to nuclear envelope & cell membrane Small bodies free or attached to ER Made of rRNA & protein
y y
Ribosome
All cells
y
Synthesizes proteins
Mitochondrion
y y y
Peanut shaped Double membrane Outer membrane smooth Inner membrane folded into cristae
Breaks down sugar (glucose) molecules to release energy Site of aerobic cellular respiration
Vacuole Plant cells have a single, large vacuole Animal cells have small vacuoles Lysosome Plant uncommon Animal common
y y y
Store food, water, metabolic & toxic wastes Store large amounts of food or sugars in plants Breaks down larger food molecules into smaller molecules Digests old cell parts
Green, oval containing chlorophyll (green pigment) Double membrane with inner membrane modified into sacs called thylakoids Stacks of thylakoids called grana & interconnecte d Gel like innermost substance called stroma Found inside the cell's nucleus May have more than one Disappear during cell division
Uses energy from sun to make food (glucose) for the plant Process called photosynthe sis Release oxygen
nucleolus
Make ribosomes
Golgi Apparatus
y
Have a cis & tran s face Modify proteins made by the cells Package &
export proteins
Movement
Have a 9-2 arrangement of microtubules Long, but few in number Paired structures near the nucleus Made of a cylinder of microtubule pairs
Movement
Centrioles
Animal cells
Cytoskeleton
y y
All cells
Strengthen cell & maintains the shape Moves organelles within the cell
Rudolph Virchow (1855), who is my personal hero, was a German physician and pathologist who stated that all living cells come only from other living cells. He included this idea in his concepts of disease, even entitling the textbook of pathology he published in 1858, Cellular Pathology. His lectures set the ground work for studies of the causes of diseases and the science of cell biology. The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells. All cells come from preexisting cells. Vital functions of an organism occur within cells, and all cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells. The word cell comes from the Latin cellula, a small room. The name was chosen by Robert Hooke when he compared the cork cells he saw to the small rooms monks lived in.