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| Biological Processes |
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| The movement of certain substances will have to occur against a concentration gradient. As such the movement requires some biological process involving expenditure of energy. |
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| It is a process by which certain substances like glucose molecules are transported against the concentration gradient. The carrier proteins in the cell membrane carry out the process. |
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| The substance present in the medium binds with the active site on the carrier protein forming a complex. The carrier protein now undergoes conformational changes carrying the substance through a channel and then releasing it on the cytoplasmic side of the membrane. The energy required for this process is provided by ATP molecules. |
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| It is a process, which involves intake of a substance due to a pushing in of the cell membrane. |
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| fig. 15.6 - Endocytosis |
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| Endocytosis is of the following two types. |
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| Pinocytosis |
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| It is the nonspecific intake of a tiny droplet of the extracellular fluid which otherwise cannot pass through the membrane. The process involves the formation of a tiny pinocytic vesicle by an in pushing of the cell membrane. The vesicle with the fluid gets pinched off from the membrane. The process is commonly known as 'cell drinking'. |
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| Phagocytosis |
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| It is the intake of a solid material by a cell. Just like pinocytosis, it involves the invagination of cell membrane in the region where the macromolecule is present. The invagination traps the solid and gets pinched off to form a phagocytic vesicle (or phogosome). The process is commonly called as 'cell eating'. This is the process by which an Amoeba ingests its solid food. This is the process by which WBC engulf pathogenic bacteria. |
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| It is the process in which materials either waste products or secretions are eliminated from a cell. The process involves the substance being surrounded by a membranous vacuole or vesicle. It moves to the surface of the cell and fuses with the cell membrane. The substance gets released to outside. This process is therefore commonly known as 'cell vomiting'. |
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| fig. 15.7 - Exocytosis |
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| Endocytosis results in the loss of small segments of cell membrane. The internalised membrane gets replaced by the fusion of exocytotic vesicles with the cell membrane. The process enables the cells to regain their surface area volume ratio. |
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