GENERAL HISTOLOGY

Detailed explanation-1: -The main function of a semi-permeable membrane in cells is to separate the cell from the environment while allowing for the controlled transport of molecules in and out of the cell.

Detailed explanation-2: -Protein pumps move ions and other solutes across membranes at concentration gradients of up to a million times. This job requires energy from a variety of sources, including light, oxidation-reduction processes, and, most often, ATP hydrolysis .

Detailed explanation-3: -Cell membranes serve as barriers and gatekeepers. They are semi-permeable, which means that some molecules can diffuse across the lipid bilayer but others cannot.

Detailed explanation-4: -Small nonpolar molecules can easily diffuse across the cell membrane. However, due to the hydrophobic nature of the lipids that make up cell membranes, polar molecules (such as water) and ions cannot do so. Instead, they diffuse across the membrane through transport proteins.

Detailed explanation-5: -Diffusion, osmosis and facilitated diffusion are passive means to get things across the membrane. There are energy consuming means also. These would fall under the heading of active transport. The really big stuff (e.g. proteins and polysaccharides) does not get in and out of a cell by passing through the membrane.