A semi-permeable membrane, also called a selectively permeable membrane,
is a membrane that allows certain molecules or ions to pass through it while blocking others.
One example of a semi-permeable membrane is a phospholipid bilayer -
a group of phospholipids consisting of a phosphate head and two fatty acid tails, arranged into a double layer,
with the hydrophilic phosphate heads exposed to the water content outside and within the cell,
and the hydrophobic tails hidden on the inside.
In general, small, non-charged molecules such as oxygen
or carbon dioxide, can freely cross the membrane without an input of energy.
They’re able to slip between the heads of the phospholipids
and pass through the hydrophobic tails through a process known as diffusion.
These molecules are able to move down their concentration gradient
as they move from an area of high concentration
to an area of lower concentration.
In contrast, ions and polar molecules have difficulty crossing a membrane. To move as quickly
as is necessary,
they are often assisted across the plasma membrane by carrier proteins.
The proteins that conduct this form of transport are often called pumps
because they use energy to force molecules or ions to move from an area of lower concentration
to an area of higher concentration.
This is commonly referred to as up, or against, the concentration gradient.