“ The property of selective permeability exists only in living plasma membrane’’. Design an experiment with a suitable illustration to verify the statement.
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Selective Permeability Definition
Selective permeability is a property of cellular membranes that only allows certain molecules to enter or exit the cell. This is important for the cell to maintain its internal order irrespective of the changes to the environment. For example, water, ions, glucose and carbon dioxide may need to be imported or exported from the cell depending on its metabolic activity. Similarly, signaling molecules may need to enter the cell and proteins may need to be released into the extracellular matrix. The presence of a selectively permeable membrane allows the cell to exercise control over the quantum, timing and rate of movement of these molecules.
Structure of Selectively Permeable Membranes
Cell membranes are not easily visualized using light microscopes. Therefore, hypotheses about their existence only arose in the late 19th century, nearly two hundred years after the first cells has been observed. At various points, different models have attempted to explain how the structure of the membrane supports its function. Initially, the membrane was supposed to be a simple lipid layer demarcating the cytosol from the extracellular region. Afterwards, models included semipermeable gel-like regions in a lipid sea to explain the movement of water but not charged particles. Thereafter, the presence of pores was proposed, allowing small molecules to move freely.
Function of Selective Permeability
Selective permeability is crucial for creating a distinctly different environment inside the cell as compared to the extracellular matrix. It is equally relevant in maintaining the integrity of various organelles inside the cell. Each organelle is a small compartment with a specialized function, requiring optimal concentrations of proteins, small molecules and ions. For instance, cellular respiration inside a mitochondrion requires that the proteins that aid this process be selectively imported into the organelle, and its internal chemistry should remain unaffected by the other metabolic processes of the cytoplasm. Similarly, after a neuron transmits an electrochemical signal, it needs to recover and return to its resting potential to enable the next round of excitatory activity. The same happens in every cardiac muscle cell each time the heart beats. These rapid and large scale changes in electrochemical properties of these cells are necessary for their function and need the presence of a membrane that is selectively permeable.
Active And Passive Transport Across Selectively Permeable Membranes
Passive transport is of two types – free diffusion or facilitated diffusion – and movement is always along a concentration gradient. Free diffusion is seen most often in the movement of uncharged molecules such as carbon dioxide or ethanol across the cell membrane, without the involvement of any other molecules.
Facilitated diffusion requires the presence of another molecule, usually a protein, that acts as a carrier and helps the substrate cross the cell membrane. Carrier proteins bind to the substrate on one side of the membrane and change conformation to release the substrate on the other side. Classic examples of facilitated diffusion are the movement of oxygen through binding to haemoglobin or the transport of water through minute pores formed by aquaporins.
The diffusion of water can be observed at the macroscopic level as well. For example, when seeds swell after being soaked in water, we are seeing the overall effect of water entering the cell. Similarly, fruits left in a dry environment, such as a refrigerator, shrivel and shrink as they lose water. Many organisms, including humans, have a waxy coating over their skin to minimize water loss from their cells in a dry environment.
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