Cell theory, cell specialization, and cell replacement

Cell theory1. All living things are composed of cells.2. Cells are the smallest unit of life.3. Cells only arise from pre-existing cells.Functions of life Metabolism: the web of all enzyme-catalysed reactions in a cell or organism. Growth: change in shape/size. Reproduction: living things reproduce offsprings, either sexually or asexually. Response: living things respond and interact with the environment. Homeostasis: the maintenance of a single internal. Nutrition: providing a source of compounds with many chemical bonds that can be broken down to provide the organism the energy necessary to maintain life. Excretion: the removal of metabolic waste. Cell sizeLimitations to the size of a cell:  As a cell gets larger, the surface area to volume ratio decreases. The rate at which materials enter and leave the cell depends on its surface area. The rate at which materials are used or produced depends on its volume. As a cell gets larger, it may not be able to take in materials or excrete waste substances quickly enough. Cells that regenerate heat may not be able to lose temperature quickly enough.  The surface area to volume ratio is important as it explains many phenomena apart from the maximum cell sizes. Units of cell:1 meter = 1000 millimetres1 millimeters = 1000 micrometers1 micrometers = 1000 nanometersCalculating magnification:Magnification = Size of image / Size of specimenScale bar: a line added to the drawing or micrograph to represent the actual size of the structures.Emergent propertiesEmergent properties arise from the interaction of component parts: the whole is greater than the sum of its parts.Multicellular organisms are capable of completing functions that individual cells could not undertake.In multicellular organisms:Cells group together to form tissues.Organs are then formed of the functional grouping of multiple tissues.Organs that interact may form organ systems (carry out specific body functions)Organ systems carry out functions of life required by an organism.Specialized tissue can develop by cell differentiation in multicellular organismsIn human 220 distinct highly specialized cell types have been recognized.All specialized cells and the organs constructed from them have developed as a result of cell differentiation.Differentiation involves the expression of some genes and not others in a cell’s genome All individual cells of an organism share an identical genome - each gene contains the entire set of genetic instructions for that organism. But not all genes are expressed to all cells. In embryonic stem cells, the entire genome is active. Newly formed cells receive signals that deactivate genes. Active genes are usually packaged in an expanded and accessible form (euchromatin), while inactive genes are mainly packaged in a condensed form (heterochromatin). The fewer active genes a cell possesses, the more specialized it will become. As genes are expressed cell differentiation begins: cell's metabolism and shape change to carry out specialized functions. Stem cellsAre unspecialized cells that:Continuously divide and replicate.Can differentiate to any specialized cells.Totipotent: can differentiate to any type of cell.Pluripotent: can differentiate to many types of cell.Multipotent: can differentiate to a few closely-related types of cell.Unipotent: can regenerate but can only differentiate into their associated cell types.Striated muscles Challenge the idea that a cell has one nucleus. Cell muscles have more than one nucleus per cell. Cell muscles called fibres can be very long (300mm). They are surrounded by a plasma membrane but they are multi-nucleated. This doesn't conform the the standard view of a small single nucleus within a cell. A septate fungal hyphae Challenges the idea that a cell is a single unit. Fungal hyphae are very large with many nuclei and a continous cytoplasm.  The turbular systems of hyphae form dense networks called mycelium. They are multi-nucleated. They have cell walls composed of chitin. The continous cytoplasm has no end of cell wall or membrane. Giant algae A single-celled organism that challenges both the ideas that cells must be simple in structure and small in size. Gigantic size (5 - 100mm). Complex in structure, composed of 3 parts:  1. bottom rhizoid 2. long stalk 3.top umbrella of branches that may fuse into a cap. The single nucleus is in the rhizoid. Paramecium

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