Briefly describe the structure of a eukaryotic plasma membrane, identifying at l

Question

Briefly describe the structure of a eukaryotic plasma membrane, identifying at least two component macromolecules and describing what their basic function is within the membrane. Summarise the manner in which small molecules are permitted to pass through the membrane ensuring you explain the difference between passive, facilitated, active, and coupled transport by comparing and contrasting them. Finally, explain the difference between endocytosis and exocytosis and provide an example of a molecule (remembering to state its function) that is transferred using this method.

Explanation / Answer

Plasma membrane forms the external boundary of cells. Its structure is explained by Fluid-msaic model, proposed by Singer and Nicolson, according to this model -

Plasma membrane is a selectively permeable membrane that allows certain molecules to pass through and blocks the passage for large uncharged molecules like glucose and sucrose, ions like Na+, K+, Cl- but allows hydrophobic molecules like O2, CO2 and small uncharged polar molecules like urea, glycerol.

The transport across the membrane occurs through either the lipid bilayer or through the transport protein. Transport across the membrane -

Endocytosis - It is taking in of the material from the surrounding by the cell. Inernalization is done by the membrane bound vesicles derived by the invagination of plasma membrane, followed by pinching off and release of free vesicles into the cytoplasm. It can be phagocytosis and pinocytosis. Example - Uptake of LDL byclathrin mediated edocytosis.

Exocytosis - This process releases the contents out of the cell by the fusion of the membrane of transport vesicles and the palsma membrane. It is the secretory pathway for secreting products such as hormones, neurotransmitters or digestive enzymes

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