Download The plasma membrane is made up of a phospholipid

The plasma membrane is made up of a phospholipid bilayer that
regulates the concentration of substances that can permeate a cell.
LEARNING OBJECTIVE [ edit ]
Detail the structure and purpose of the plasma membrane of a cell
KEY POINTS [ edit ]
All eukaryotic cells have a surrounding plasma membrane, which is also known as the cell
membrane.
The plasma membrane is made up by a phospholipid bilayer with embedded proteins that
separates the internal contents of the cell from its surrounding environment.
Only relatively small, non-polar materials can easily move through the lipid bilayer of the plasma
membrane.
Passive transport is the movement of substances across the membrane that does not require the
use of energy while active transport is the movement of substances across the membrane using
energy.
Osmosis is the diffusion of water through a semi-permeablemembrane down its concentration
gradient; this occurs when there is an imbalance of solutes outside of a cell compared to the
inside the cell.
TERMS [ edit ]
hypotonic
Having a lower osmotic pressure than another; a cell in this environment causes water to enter
the cell, causing it to swell.
phospholipid
Any lipid consisting of a diglyceride combined with a phosphategroup and a
simple organic molecule such as choline or ethanolamine; they are important constituents of
biological membranes
hypertonic
having a greater osmotic pressure than another
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The Plasma Membrane
Despite differences in structure and
function, all living cells in multicellular
organisms have a surrounding plasma
membrane (also known as the cell
membrane). As the outer layer of your skin
separates your body from its environment,
the plasma membrane separates the inner
contents of a cell from its exterior
environment. The plasma membrane can
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be described as a phospholipid bilayer with embedded proteins that controls the passage of
organic molecules, ions, water, and oxygen into and out of the cell. Wastes (such as carbon
dioxide and ammonia) also leave the cell by passing through the membrane.
Eukaryotic Plasma Membrane
The eukaryotic plasma membrane is a phospholipid bilayer with proteins and cholesterol embedded in it.
The cell membrane is an extremely pliable structure composed primarily of two adjacent
sheets of phospholipids. Cholesterol, also present, contributes to the fluidity of the
membrane. A single phospholipid molecule consists of a polar phosphate "head," which
is hydrophilic, and a non-polar lipid "tail," which is hydrophobic. Unsaturated fatty
acids result in kinks in the hydrophobic tails. The phospholipid bilayer consists of two
phospholipids arranged tail to tail. The hydrophobic tails associate with one another, forming
the interior of the membrane. The polar heads contact the fluid inside and outside of the cell .
Phospholipid Bilayer
The phospholipid bilayer consists of two adjacent sheets of phospholipids, arranged tail to tail. The
hydrophobic tails associate with one another, forming the interior of the membrane. The polar heads
contact the fluid inside and outside of the cell.
The plasma membrane's main function is to regulate the concentration of substances inside
the cell. These substances include ions such as Ca++, Na+, K+, and Cl–; nutrients including
sugars, fatty acids, and amino acids; and waste products, particularly carbon dioxide (CO2),
which must leave the cell.
The membrane's lipid bilayer structure provides the cell with access control through
permeability. The phospholipids are tightly packed together, while the membrane has a
hydrophobic interior. This structure causes the membrane to be selectively permeable. A
membrane that has selective permeability allows only substances meeting certain criteria to
pass through it unaided. In the case of the plasma membrane, only relatively small, non-polar
materials can move through the lipid bilayer (remember, the lipid tails of the membrane are
nonpolar). Some examples of these materials are other lipids, oxygen and carbon dioxide
gases, and alcohol. However, water-soluble materials—such as glucose, amino acids, and
electrolytes—need some assistance to cross the membrane because they are repelled by the
hydrophobic tails of the phospholipid bilayer.
Transport Across the Membrane
All substances that move through the membrane do so by one of two general methods, which
are categorized based on whether or not energy is required. Passive (non-energy requiring)
transport is the movement of substances across the membrane without the expenditure of
cellular energy. During this type of transport, materials move by simple diffusion or
by facilitated diffusion through the membrane, down their concentration gradient. Water
passes through the membrane in a diffusion process called osmosis. Osmosis is the diffusion
of water through a semi-permeable membrane down its concentration gradient. It occurs
when there is an imbalance of solutes outside of a cell versus inside the cell. The solution that
has the higher concentration of solutes is said to be hypertonic and the solution that has the
lower concentration of solutes is said to behypotonic. Water molecules will diffuse out of the
hypotonic solution and into the hypertonic solution (unless acted upon byhydrostatic forces).
Osmosis
Osmosis is the diffusion of water through a semipermeable membrane down its concentration gradient.
If a membrane is permeable to water, though not to a solute, water will equalize its own concentration
by diffusing to the side of lower water concentration (and thus the side of higher solute concentration). In
the beaker on the left, the solution on the right side of the membrane is hypertonic.
In contrast to passive transport, active (energy-requiring) transport is the movement of
substances across the membrane using energy from adenosine triphosphate (ATP). The
energy is expended to assist material movement across the membrane in a direction against
their concentration gradient. Active transport may take place with the help of protein pumps
or through the use of vesicles. Another form of this type of transport is endocytosis, where a
cell envelopes extracellularmaterials using its cell membrane. The opposite process is known
as exocytosis. This is where a cell exports material using vesicular transport.
Cytoplasm
The cell's plasma membrane also helps contain the cell's cytoplasm, which provides a gel-like
environment for the cell'sorganelles. The cytoplasm is the location for most cellular
processes, including metabolism, protein folding, and internal transportation.