Proteins that act as channels or pumps work in one direction. Carbohydrates, attached to lipids or proteins, are also found on the exterior surface of the plasma membrane.
These carbohydrate complexes help the cell bind substances in the extracellular fluid that the cell needs. This adds considerably to the selective nature of plasma membranes. Recall that plasma membranes have hydrophilic and hydrophobic regions.
This characteristic helps the movement of certain materials through the membrane and hinders the movement of others. Lipid-soluble material can easily slip through the hydrophobic lipid core of the membrane. Substances such as the fat-soluble vitamins A, D, E, and K readily pass through the plasma membranes in the digestive tract and other tissues. Molecules of oxygen and carbon dioxide have no charge and pass through by simple diffusion. Polar substances, with the exception of water, present problems for the membrane.
The selective permeability of the plasma membrane helps the latter to carry out an important function of controlling or regulating the passing through of substances between the external environment and the cytoplasm. See also:. A typical eukaryotic cell is comprised of cytoplasm with different organelles, such as nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, and so on.
The cellular contents are surrounded by a double layer, cell membrane. These cellular structures and cell junctions are elaborated in this tutorial Read More. Generation of resting membrane potential. Why is the cell membrane selectively permeable? Don't Memorise. Jan 4, Explanation: Cell membranes are also called selectively permeable membranes, because they are selective in allowing entry of particles into the cell.
Consider an example: What were to happen to a human being who would consume anything he comes across? That is the reason we humans only consume a well balanced diet of food items. Related questions How is the cell membrane affected by temperature?
How does the cell membrane change shape?
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