The cell membrane lets cells be specialized units, protects cytosolic contents, and maintains the
cell's structure. The membrane acts as interface between the cell and the environment. It is a
phospholipid layer that maintains homeostasis of a cell by regulating the molecules that get in or out of a
cell. Due to the semipermeability of cell membranes, they have ability to control what molecules go
through. Essentially, some molecules an easily go in and out while others need special structures to go in
and out of cells. Other molecules require energy to pass through a cell membrane. Each membrane has
an appropriate combination of these structures to ensure the internal environment of the cell is correct.
The phospholipid bilayer of cell membranes is impermeable to ions, amino acids, glucose and other
water-soluble molecules. Transport of such ions and molecules across the membrane is facilitated by
transport proteins from the bilayer (Passive Transport and Active Transport). Thereby, since different
types of cells need different mixtures of compounds with low molecular weight, the cell membrane of each
type of cell has certain transport proteins that let only specific molecules or ions to cross.
There are two major methods that molecules pass across a cell membrane, and the difference is
about if or not cell energy is needed. Simple diffusion and other passive mechanisms do not require
energy, whereas active transport consumes energy (Passive Transport and Active Transport). Simple
diffusion occurs when molecules are driven by concentration gradient to pass through the lipid bilayer.
The process depends on simple factors such as lipid solubility, temperature, molecule size, surface area,
concentration gradient, and membrane thickness. In real life, it is possible for water molecules and ions to
pass through the membrane using protein channels. This is also a passive process that not require
involve assisting the solutes move across the cell membrane. The mechanism depends on interaction of
a carrier protein. Essentially, the carrier protein facilitates movement of ions or molecules across the cell
membrane by binding with them chemically and transporting them via the cell membrane in that form. The
capacity of facilitated diffusion depends on the rate at which molecules can percolate via the channels
(Yartsev). Therefore, passive diffusion rate increases with concentration gradient, but facilitated diffusion
has a limitation beyond which additional concentration change produces no membrane permeability
The proteins that allow facilitates diffusion may exert control over what goes into the cell. This
regulation is achieved by a combination of charge selectivity and pore size. Protein channels that facilitate
diffusion are usually selective for specific molecules.ion diffusion though the channels must be controlled
(Yartsev). Hence, ions channels are gated by voltage gating or ligand gating.
Primary active transport involves utilization of energy stored as Adenosine triphosphate to allow transport
of ions across a cell membrane against the ion species' concentration gradient. In contrast, secondary
active transport utilizes a protein carrier to facilitate transport but rather than using energy from direct
breakdown of ATP, it relies on concentration gradient already established by primary active transport
Use of an ATP-powered protein transporter or exchange protein is impractical if molecules are
large. In that situation, the endocytosis method is used. In this process, the whole membrane experiences
conformal change to allow it engulf a molecule and then move the membrane-bounded molecule into the
cell (Yartsev). The molecule bounded by the membrane is named an endosome.
“Passive Transport and Active Transport Across a Cell Membrane Article.” Khan Academy, n.d.,
transport-and-active-transport-across-a-cell-membrane-article. Accessed 16 September 2020.
Yartsev, Alex. “Transport of Substances Across Cell Membranes.” Deranged Physiology, n.d.,