Description of the plasma membrane of a eukaryotic cell.

1. Structure of a eukaryotic plasma membrane and two component
macromolecules
The plasma membrane is “a phospholipid bilayer with embedded proteins that separate the
internal contents of the cell from its surrounding environment” (OpenStax College, Biology.
Pg. 113). The phospholipid bilayer is a macromolecule made out of a hydrophilic head
composed of a phosphate and a glycerol group. The hydrophobic tails, one is made of a
saturated fatty acid and the other of an unsaturated fatty acid (which presents a kink in the
chain due to the cis double bond). They arrange themselves in a double layer structure with
the hydrophilic heads pointing toward the exterior and the hydrophobic tails pointing inward
away from water.
Two kinds of proteins are present in the plasma membrane:
1. Integral membrane proteins, which are permanently attached to the
membrane. Its function “include transporters,
linkers, channels, receptors, enzymes, structural membrane-anchoring
domains, proteins involved in accumulation and transduction of energy, and
proteins responsible for cell adhesion.” (Wikipedia, 2018. Integral membrane
protein).
2. Peripheral membrane proteins: are proteins which adhere only temporarily
to the plasma membrane. Function: “the reversible attachment of proteins to
biological membranes has shown to regulate cell signaling and many other
important cellular events, through a variety of mechanisms. For example, the
close association between many enzymes and biological membranes may
bring them into close proximity with their lipid substrate(s).” (Wikipedia,
2018. Peripheral membrane protein).

2. Membrane transport mechanisms:
Transportation across the cell membrane is a very careful regulated mechanism.
We could say in brief there are 4 ways of membrane transport:
1. Simple diffusion: it is the simplest way and consists in the pass of molecules
that are small and non-polar. It does not require energy.
2. Facilitated diffusion: it is the pass of polar molecules and larger ions through
the membrane by a membrane transport channel. An example is the transport
of GLUT4 so important in diabetes.
3. In the primary active transport molecules move against their gradient
coupled to the hydrolysis of ATP. This means (contrary to the two previous
ways) that requires energy. An example of this is “the sodium-potassium pump
(Na+/K+ ATPase) that helps maintain resting potential in the cell. This
protein uses the energy released from hydrolysis of ATP (adenosine
triphosphate) to pump three sodium ions out of and two potassium ions into
the cell.” (Khan Academy. Passive transport and active transport across a cell
membrane article).
4. Secondary active transport “moves multiple molecules across the
membrane, powering the uphill movement of one molecule(s) (A) with the
downhill movement of the other(s) (B). For example, SGLT2 is a glucose
transporter that allows glucose (Molecule A) into our cells (against its
gradient) by bringing in a sodium molecule (Molecule B) as well.” (Khan
Academy).

3. Endocytosis and Exocytosis:
This occur for bulk transport. There are two forms of endocytosis, one is phagocytosis,
where the cell engulfs a solid (a bacteria for instance) and wraps it around with cell
membrane forming a food vacuole. When the cell captures a liquid doing the same
mechanism as the phagocytosis but in this case englobing a liquid substance, we call it
pinocytosis.
Exocytosis is a form of active transport in which the cell transports large molecules out of the
cell. An example of this is the release of proteins outside of the cell to be used for certain
functions. Another example is found in neurons that form synaptic vesicles to release
neurotransmitters.
RESOURCES:
1. Khan Academy. Passive Transport and Active transport Across the Cell Membrane.
Retrieved from: https://www.khanacademy.org/test-prep/mcat/cells/transport-across-
a-cell-membrane/a/passive-transport-and-active-transport-across-a-cell-membrane-
article
2. OpenStax College, (May 30, 2013). Biology. OpenStax College.
http://cnx.org/content/col11448/latest
3. University of Washington. Membrane Transport Mechanisms. Retrieved from:
https://courses.washington.edu/conj/bess/transport/summary/membrane-
transport.html
4. Wikipedia. (2018). Integral Membrane Protein. Retrieved from:
https://en.wikipedia.org/wiki/Integral_membrane_protein