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The membrane structure refers to the cell membrane.

The cell membrane is semi-permeable, this means that it allows certain substances in and out:

Three interesting points about a cell membrane can be found below:

1. Passive diffusion and osmosis: Some substances (small molecules, ions) such as carbon dioxide (CO2), oxygen (O2), and water, can move across the plasma membrane by diffusion, which is a passive transport process. Because the membrane acts as a barrier for certain molecules and ions, they can occur in different concentrations on the two sides of the membrane. Such a concentration gradient across a semipermeable membrane sets up an osmotic flow for the water.

2. Transmembrane protein channels and transporters: Nutrients, such as sugars or amino acids, must enter the cell, and certain products of metabolism must leave the cell. Such molecules are pumped across the membrane by transmembrane transporters or diffuse through protein channels. These proteins, also called permeases, are usually quite specific, recognizing and transporting only a limited food group of chemical substances, often even only a single substance.

3. Endocytosis: Endocytosis is the process in which cells absorb molecules by engulfing them. The plasma membrane creates a small deformation inward, called an invagination, in which the substance to be transported is captured. The deformation then pinches off from the membrane on the inside of the cell, creating a vesicle containing the captured substance. Endocytosis is a pathway for internalizing solid particles (cell eating or phagocytosis), small molecules and ions (cell drinking or pinocytosis), and macromolecules. Endocytosis requires energy and is thus a form of active transport.

The cellular membrane is made of phospholipids, proteins, cholesterol, and more. So there are several ways stuff can get in and out. Polar molecules either diffuse through the membrane slowly or go through protein channels or other proteins such as aquaporins which allow water to travel through the membrane quickly. Nonpolar items such as hormones (not all hormones) can enter a cell through the membrane quickly by diffusion through the layer of phospholipids. Big items such as starch need transport proteins and such. Imagine there are 1 million people in a room and someone yells fire. All the fat people will have trouble squishing through the doors together but skinny people can get by easier. On top of that, if there are more doors, more people can exit. That is why there are folds in the membrane to allow for more surface area.

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It is made out of a phospholipid bilayer which is able to repel water. It has protiens that dot it that allow substance that it needs in and keeps bad things out. This is called selective permeability.