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Summary cellular and molecular biology 1- usmlerx bricks
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Cell MembranesBy ScholarRx
Updated September 8, 2022
access_time17 min
Learning Objectives (4)
After completing this brick, you will be able to:
● Discuss the structural organization and lipid composition of
eukaryotic cell membranes.
● 1
● Summarize the physiologic roles of eukaryotic cell membranes.
● 2
● Recall the structure and function of the major classes of membrane
proteins.
● 3
● Differentiate the structural and functional domains of eukaryotic
plasma membranes.
● 4
cableCASE CONNECTION
You are seeing CH, a 72-year-old man who was found in a snowbank
in midwinter. CH has Alzheimer disease and wandered away from his
home, unnoticed by his family. When brought to the emergency
department, CH is confused and irritable. His heart rate is 42/min,
respiratory rate 12/min, and core body temperature is 93°F. You begin
rewarming treatments.
What is happening at the level of the cell membrane throughout CH’s
body during this period of hypothermia? Consider your answer as you
read, and we’ll revisit at the end of the brick.
,GO TO CONCLUSION arrow_downward
What Are Cell Membranes?
Cell membranes are often thought of simply as the borders of cells,
when in fact they are so much more. Cells need to be able to maintain
homeostasis and interact with their extracellular environment. To do
this, the cell membrane contains elements that facilitate transport and
communication, allowing the cell to monitor and respond to the
extracellular environment. In this brick, you will learn about the
structure and function of these membranes.
Cell Membrane Structure and
Function
Eukaryotic cells are enveloped by a continuous asymmetrical lipid
bilayer that forms the cell membrane. This bilayer consists of an inner
and an outer leaflet that together separate the intracellular space from
the extracellular environment.
But their role doesn’t stop there. Cell membranes are needed to carry
out the following cellular functions:
● Envelope the cell and maintain structural integrity
● Protect intracellular organelles from the external environment
● Act as a semipermeable membrane that regulates the
intracellular environment by maintaining the traffic of ions and
macromolecules needed for proper cell function
● Participate in signal transduction of extracellular signals into
intracellular events
Not only do our cells have membranes, but so do the organelles that
reside within our cells. However, the structure of these two types of
membranes differs in several important ways. While the cell
,membrane and most organelle membranes are composed of a single
lipid bilayer (Figure 1), some organelles (eg, mitochondria) have a
membrane that consists of two lipid bilayers. Membrane-associated
proteins differ between the plasma membrane and organelles in order
to carry out specialized functions. Furthermore, the phospholipids that
make up the membranes differ between the plasma membrane and
organelle membranes.
What Are Membrane Lipids and What
Purpose Do They Serve?
The main structural components of cell membranes are membrane
lipids, along with embedded proteins, carbohydrates, glycoproteins,
and glycolipids. Phospholipids make up the majority of the cell
membrane, forming the two opposing sheets of the lipid bilayer.
Phospholipids are amphipathic, meaning they contain both
hydrophilic, “water-loving,” and hydrophobic, “water-hating,”
regions (Figure 2).
QUIZ
Figure 2
Credit: ©ScholarRx
The hydrophilic polar region consists of the phosphate head group
built on a carbon backbone that faces the aqueous intracellular or
extracellular space. The hydrophobic nonpolar region consists of two
fatty acid tails that intermingle within the center of the membrane,
, forming weak noncovalent bonds with one another to attach the two
leaflets together. This arrangement allows the hydrophilic heads to
interact with the aqueous environment both inside and outside the cell
while also shielding the neutral hydrophobic tails from these polar
environments.
Because of their amphipathic nature, phospholipids spontaneously
form lipid bilayers when placed in an aqueous environment so that the
hydrophobic tails are not exposed to the aqueous intracellular and
extracellular environments. If phospholipids have small tails, or a
single hydrocarbon chain, they may form a small, single-layered
sphere known as a micelle. If phospholipids have bulkier tails, or two
hydrophobic tails, they may form a hollow droplet known as a
liposome (Figure 3). However, cell membranes are more complex
than just a simple lipid bilayer and do not arise spontaneously.
Instead, they are formed from existing bilayers and maintained
through tightly regulated synthesis of new components, including
lipids and proteins.
Figure 3
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