Question

In which type of passive transport, simple diffusion or facilitated diffusion a carrier protein is required for transport across the membrane.

Answer 1

Transportation is the process of movement or distribution of different materials or goods from one place to another. In common, humans make use of different types/kinds of transportation. Comparatively, the living system, including plants, animals, and humans has a unique network of the transportation system, which is involved in circulating food, minerals, hormones, oxygen, carbon dioxide, waste products, etc.

Complete solution:
A. Passive transport is the process of movement of ions and other molecular substances within the cells across the concentration gradient, without any external energy. It is also called passive diffusion. There are four types of passive transport: Simple Diffusion, Facilitated Diffusion, Filtration and Osmosis.
i. Simple diffusion - The difference in the concentration of the two areas is termed as concentration gradient and the process of diffusion continues until this gradient neutralizes. Diffusion occurs in liquids and gases because their particles move randomly from one place to another. It is an important process in living things as it is required for different life processes. The substances pass in and out of the cells through simple diffusion.
ii. Facilitated diffusion - It is the passive movement of molecules along the concentration gradient. It is a selective process, i.e., the membrane allows only selective molecules and ions to pass through it. It prevents other molecules from passing by the membrane. The electric charge and pH maintain the diffusion across the membrane. Hydrophilic, polar or charged molecules cannot cross the membrane. Therefore, a carrier protein moves into a cell through facilitated diffusion.

Note: The important examples of facilitated diffusion are listed below:
i. Glucose Transporter - Facilitate the transport of glucose across the plasma membrane. These are present in the plasma membrane. It binds to glucose molecules and transports them across the lipid bilayer.
ii. Aquaporins - These proteins facilitate/ease the transport of water beyond the lipid bilayer. If the proteins forming aquaporins are mutated, it might result in diseases like diabetes insipidus.
iii. Ion Channels - These are transmembrane proteins that allow the selective transport of ions and solutes across the plasma membrane. These ionic pumps keep the concentration of the extracellular fluid different from that of the cytosol.