Question

Compare/Contrast the following: protein/enzyme, ligand/substrate, channel protein/carrier protein, exocytosis/endocytosis.

Answer 1

Both proteins and enzymes are biological macromolecules which participate in metabolic process of the cell. The similarity between the two is that both are made up of Amino acids. Enzymes are also proteinaceous in nature. But the difference between them is that enzymes play only catalytic role. They speed up the rate of a biological reaction by lowering the activation energy which is required to start the reaction. But on the other hand proteins not only play enzymatic role, but also participate in transportation of molecules, structural support to the cell, Motor proteins etc.

A ligand is a molecule which binds to to its receptor which is located either on the plasma membrane or on the nuclear membrane or even mitochondrial membrane or chloroplast membrane or any other membrane of the cell. Where as substrate is a molecule which binds to an enzyme which is present in the cytoplasm of the cell or matrix of the organelles. The similarity between ligand and substrate is that both bind to their respective molecules as if a key fits into a lock.

Channel proteins and carrier proteins both are transport proteins and help in transportation of molecules across the cell membrane. But channel proteins are those which are always open whereas carrier proteins are those which are not always open. Molecules can move across channel proteins at anytime and there is no conformational change in the channel proteins during transportation. Where as carrier proteins open only when they are required and change their confirmation for transportation of molecule.

Cells take up molecules across its surface with the help of two processes, exocytosis and endocytosis. Exocytosis is the process by which cell expels molecules outside it and endocytosis is the process by which a cell takes of molecules inside it.

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