The importance of cell membranes inside and outside cells:
Cell membranes surround cells and some organelles in eukaryotes, and act as a barrier
between the cell and its environment. The cell surface membranes partially permeable
letting some molecules through but not others, such as lipids, amino acids and ions so
therefore is critical in controlling which substances enter and leave the cell. In this essay I
will discuss the importance of cell membranes inside and outside of cells and the role that
they have in the body.
The structure of the cell membrane is crucial for the transport and exchange of substances
into and out of cells. The cell membrane is composed of phospholipid molecules which have
fatty acid ‘tails’ which are hydrophobic so repel water and phosphate ‘heads’ which are
hydrophilic so attract water. These phospholipoids are arranged into a bilayer with the
heads facing outwards and the tails facing inwards. The phospholipids arrangement in the
cell membrane and the alternating hydrophobic and hydrophilic regions of the bilayer are
important as they allow small lipid soluble, non-polar molecules such as oxygen and carbon
dioxide to diffuse across cell membranes freely from a region of higher concentration to a
region of lower concentration. This process is used in the lungs, whose function is the gas
exchange of carbon dioxide and oxygen across the epithelium of the alveoli. Oxygen can
diffuse through the red blood cells cell membrane, and the alveolar cell membrane into the
alveoli, where it can then be used in processes such as aerobic respiration, which occurs in
the inner mitochondrial membrane. Carbon dioxide leaves the alveoli and diffuses through
the cell membrane into the red blood cells, to prevent the build-up of carbonic acid, which
could denature proteins in cells.
The phospholipid bilayer is also embedded with channel and carrier proteins which are used
for the efficient transport of molecules into and out of cells. Carrier proteins are involved in
facilitated diffusion – (the movement of particles down a concentration gradient involving
carrier proteins) and active transport, (the movement of molecules from a low to a high
area of concentration using energy from the synthesis of ATP). They bind to complementary
molecules and change shape to transport specific water -soluble molecules such as sodium
ions through the membrane and release them into/out of the cell. Channel proteins also
transport water soluble, polar molecules through the membrane but instead have pores
which the molecule travels through. Channel proteins only transport molecules by
facilitated diffusion, not active transport. These processes are important for organisms so
molecules and substances can be transported effectively to different cells in the body for
specific uses. An example is the cotransport of glucose (and also amino acid) molecules from
the lumen of the ileum into the bloodstream in the small intestine. Sodium ions are actively
transported out of the epithelial cells through the cell membrane into the blood by sodium
potassium pump carrier proteins, this lowers the concentration of sodium ions in the cell
creating a concentration gradient. Sodium ions then diffuse from the lumen of the ileum
Cell membranes surround cells and some organelles in eukaryotes, and act as a barrier
between the cell and its environment. The cell surface membranes partially permeable
letting some molecules through but not others, such as lipids, amino acids and ions so
therefore is critical in controlling which substances enter and leave the cell. In this essay I
will discuss the importance of cell membranes inside and outside of cells and the role that
they have in the body.
The structure of the cell membrane is crucial for the transport and exchange of substances
into and out of cells. The cell membrane is composed of phospholipid molecules which have
fatty acid ‘tails’ which are hydrophobic so repel water and phosphate ‘heads’ which are
hydrophilic so attract water. These phospholipoids are arranged into a bilayer with the
heads facing outwards and the tails facing inwards. The phospholipids arrangement in the
cell membrane and the alternating hydrophobic and hydrophilic regions of the bilayer are
important as they allow small lipid soluble, non-polar molecules such as oxygen and carbon
dioxide to diffuse across cell membranes freely from a region of higher concentration to a
region of lower concentration. This process is used in the lungs, whose function is the gas
exchange of carbon dioxide and oxygen across the epithelium of the alveoli. Oxygen can
diffuse through the red blood cells cell membrane, and the alveolar cell membrane into the
alveoli, where it can then be used in processes such as aerobic respiration, which occurs in
the inner mitochondrial membrane. Carbon dioxide leaves the alveoli and diffuses through
the cell membrane into the red blood cells, to prevent the build-up of carbonic acid, which
could denature proteins in cells.
The phospholipid bilayer is also embedded with channel and carrier proteins which are used
for the efficient transport of molecules into and out of cells. Carrier proteins are involved in
facilitated diffusion – (the movement of particles down a concentration gradient involving
carrier proteins) and active transport, (the movement of molecules from a low to a high
area of concentration using energy from the synthesis of ATP). They bind to complementary
molecules and change shape to transport specific water -soluble molecules such as sodium
ions through the membrane and release them into/out of the cell. Channel proteins also
transport water soluble, polar molecules through the membrane but instead have pores
which the molecule travels through. Channel proteins only transport molecules by
facilitated diffusion, not active transport. These processes are important for organisms so
molecules and substances can be transported effectively to different cells in the body for
specific uses. An example is the cotransport of glucose (and also amino acid) molecules from
the lumen of the ileum into the bloodstream in the small intestine. Sodium ions are actively
transported out of the epithelial cells through the cell membrane into the blood by sodium
potassium pump carrier proteins, this lowers the concentration of sodium ions in the cell
creating a concentration gradient. Sodium ions then diffuse from the lumen of the ileum
