Membranes are an essential component of cells. The plasma membrane forms the border between a cell and its environment. Membranes inside eukaryotic cells divide the cytoplasm into compartments. The basic structure of all biological membranes is the same. A bilayer of phospholipids and other amphipathic molecules forms a continuous sheet that controls the passage of substances despite being 10 nanometres or less Phospholipid molecules have a phosphate "head" and two hydrocarbon "tails". The tails of the phospholipids are hydrophobic and interact with each other to form the core of biological membranes. Due to this, the membrane core has low permeability to all hydrophilic particles, including ions with positive or negative charges and polar molecules such as glucose. There are usually aqueous solutions on either side of cell membranes. These solutions are in a liquid state, so both water molecules and hydrophilic solutes are in continuous random motion. The solutes nearest to the membrane surface might penetrate between the hydrophilic phosphate heads of the phospholipids, but if they reach the hydrophobic core of the membrane they are drawn back to the aqueous solution outside the membrane. The hydrophobic hydrocarbon chains that form the core of the membrane do not repel hydrophilic solutes but they are more attracted to each other, and the solutes are much more attracted to water outside the membrane. Molecular size also influences membrane permeability. The trend is that the larger the molecule, the lower the permeability. For example, water molecules which are only slightly larger than single oxygen atoms, pass through membranes more easily than large molecules such as glycogen or protein.

Membranes

Essential components of cells that form boundaries between cells and their environment.

Plasma membrane

The boundary between a cell and its surroundings.

Eukaryotic cells

Cells with internal membranes that divide the cytoplasm into compartments.

Biological membranes

Share a basic structure of a bilayer of phospholipids and amphipathic molecules.

Phospholipids

Molecules forming a continuous sheet in membranes, controlling substance passage and are very thin.