The document discusses cell membranes. It describes how cell membranes are made up of lipids like phospholipids and cholesterol that form a bilayer. Proteins are also embedded throughout membranes and help regulate transport of molecules. The membrane acts as a selective barrier, controlling what enters and exits the cell using transport proteins. Membranes surround not just cells but also internal organelles, and their compositions vary by location to support different functions.

1. Cell Membrane; its Molecular
Organization & Functional Role
By
ALI ZAIN
Lecturer
Department of Biotechnology

2. Cell Membrane
• The cell membrane (plasma membrane) is a thin
semi-permeable membrane that surrounds
the cytoplasm of a cell.
• Its function is to protect the integrity of the
interior of the cell by allowing certain substances
into the cell while keeping other substances out.
• It also serves as a base of attachment for
the cytoskeleton in some organisms and the cell
wall in others.
• Thus the cell membrane also serves to help
support the cell and help maintain its shape.​

3. • Another function of the membrane is to regulate cell
growth through the balance of endocytosis
and ​exocytosis.
• In endocytosis, lipids and proteins are removed from
the cell membrane as substances are internalized.
• In exocytosis, vesicles containing lipids and proteins
fuse with the cell membrane increasing cell size.
• Animal cells, plant cells, prokaryotic cells, and fungal
cells have plasma membranes.
• Internal organelles are also encased by membranes.

4. What Are Cellular Membranes Made
Of?
• With few exceptions, cellular membranes — including plasma
membranes and internal membranes — are made
of glycerophospholipids, molecules composed of glycerol, a
phosphate group, and two fatty acid chains.
• Glycerol is a three-carbon molecule that functions as the
backbone of these membrane lipids. Within an individual
glycerophospholipid, fatty acids are attached to the first and
second carbons, and the phosphate group is attached to the
third carbon of the glycerol backbone. Variable head groups
are attached to the phosphate.
• Space-filling models of these molecules reveal their cylindrical
shape, a geometry that allows glycerophospholipids to align
side-by-side to form broad sheets.

7. Glycerophospholipids
• Glycerophospholipids are by far the most abundant
lipids in cell membranes. Like all lipids, they are insoluble
in water, but their unique geometry causes them to
aggregate into bilayers without any energy input.
• This is because they are two-faced molecules, with
hydrophilic (water-loving) phosphate heads and
hydrophobic (water-fearing) hydrocarbon tails of fatty
acids.
• In water, these molecules spontaneously align — with
their heads facing outward and their tails lining up in the
bilayer's interior.
• Thus, the hydrophilic heads of the glycerophospholipids
in a cell's plasma membrane face both the water-based
cytoplasm and the exterior of the cell.

8. Cholesterol molecules
• Altogether, lipids account for about half the mass of cell membranes.
• Cholesterol molecules, although less abundant than glycerophospholipids,
account for about 20 percent of the lipids in animal cell plasma membranes.
However, cholesterol is not present in bacterial membranes or
mitochondrial membranes.
• Also, cholesterol helps regulate the stiffness of membranes, while other less
prominent lipids play roles in cell signaling and cell recognition.
• In addition to lipids, membranes are loaded with proteins. In fact, proteins
account for roughly half the mass of most cellular membranes.
• Many of these proteins are embedded into the membrane and stick out on
both sides; these are called transmembrane proteins. The portions of these
proteins that are nested amid the hydrocarbon tails have hydrophobic
surface characteristics, and the parts that stick out are hydrophilic.

10. • At physiological temperatures, cell membranes are
fluid; at cooler temperatures, they become gel-
like.
• Scientists who model membrane structure and
dynamics describe the membrane as a fluid
mosaic in which transmembrane proteins can
move laterally in the lipid bilayer.
• Therefore, the collection of lipids and proteins that
make up a cellular membrane relies on natural
biophysical properties to form and function.
• In living cells, however, many proteins are not free
to move. They are often anchored in place within
the membrane by tethers to proteins outside the
cell, cytoskeletal elements inside the cell, or both.

11. What Do Membranes Do?
• Cell membranes serve as barriers and gatekeepers. They
are semi-permeable, which means that some molecules
can diffuse across the lipid bilayer but others cannot.
• Small hydrophobic molecules and gases like oxygen and
carbon dioxide cross membranes rapidly.
• Small polar molecules, such as water and ethanol, can
also pass through membranes, but they do so more
slowly.
• On the other hand, cell membranes restrict diffusion of
highly charged molecules, such as ions, and large
molecules, such as sugars and amino acids.
• The passage of these molecules relies on specific
transport proteins embedded in the membrane.

13. • Membrane transport proteins are specific and selective for the molecules
they move, and they often use energy to catalyze passage.
• Also, these proteins transport some nutrients against the concentration
gradient, which requires additional energy.
• The ability to maintain concentration gradients and sometimes move
materials against them is vital to cell health and maintenance.
• Thanks to membrane barriers and transport proteins, the cell can
accumulate nutrients in higher concentrations than exist in the
environment and, conversely, dispose of waste products (Figure 3).
• Other transmembrane proteins have communication-related jobs. These
proteins bind signals, such as hormones or immune mediators, to their
extracellular portions.
• Binding causes a conformational change in the protein that transmits a
signal to intracellular messenger molecules.
• Like transport proteins, receptor proteins are specific and selective for
the molecules they bind.

15. • Peripheral membrane proteins are associated
with the membrane but are not inserted into the
bilayer.
• Rather, they are usually bound to other proteins
in the membrane.
• Some peripheral proteins form a filamentous
network just under the membrane that provides
attachment sites for transmembrane proteins.
• Other peripheral proteins are secreted by the cell
and form an extracellular matrix that functions in
cell recognition.

16. Cell Membrane Structure
• The cell membrane is primarily composed of a
mix of proteins and lipids.
• Depending on the membrane’s location and role
in the body, lipids can make up anywhere from 20
to 80 percent of the membrane, with the
remainder being proteins.
• While lipids help to give membranes their
flexibility, proteins monitor and maintain the
cell's chemical climate and assist in the transfer of
molecules across the membrane.

17. Cell Membrane Lipids

18. • Phospholipids are a major component of cell
membranes. Phospholipids form a lipid bilayer in which
their hydrophilic (attracted to water) head areas spontaneously arrange to
face the aqueous cytosol and the extracellular fluid, while their hydrophobic
(repelled by water) tail areas face away from the cytosol and extracellular
fluid. The lipid bilayer is semi-permeable, allowing only certain molecules
to diffuse across the membrane.
• Cholesterol is another lipid component of animal cell membranes.
Cholesterol molecules are selectively dispersed between membrane
phospholipids. This helps to keep cell membranes from becoming stiff by
preventing phospholipids from being too closely packed together. Cholesterol
is not found in the membranes of plant cells.
• Glycolipids are located on cell membrane surfaces and have
a carbohydrate sugar chain attached to them. They help the cell to recognize
other cells of the body.

19. Cell Membrane Proteins

20. • The cell membrane contains two types of associated proteins.
1. Peripheral membrane proteins are exterior to and connected
to the membrane by interactions with other proteins.
2. Integral membrane proteins are inserted into the membrane
and most pass through the membrane. Portions of these
transmembrane proteins are exposed on both sides of the
membrane. Cell membrane proteins have a number of different
functions.
• Structural proteins help to give the cell support and shape.
• Cell membrane receptor proteins help cells communicate with
their external environment through the use of hormones,
neurotransmitters, and other signaling molecules.
• Transport proteins, such as globular proteins, transport
molecules across cell membranes through facilitated diffusion.
• Glycoproteins have a carbohydrate chain attached to them. They
are embedded in the cell membrane and help in cell to cell
communications and molecule transport across the membrane.

21. How Diverse Are Cell Membranes?
• In contrast to prokaryotes, eukaryotic cells have not only a plasma membrane that
encases the entire cell, but also intracellular membranes that surround various
organelles.
• In such cells, the plasma membrane is part of an extensive endomembrane
system that includes the endoplasmic reticulum (ER), the nuclear membrane,
the Golgi apparatus, and lysosomes. Membrane components are exchanged
throughout the endomembrane system in an organized fashion.
• For instance, the membranes of the ER and the Golgi apparatus have different
compositions, and the proteins that are found in these membranes contain sorting
signals, which are like molecular zip codes that specify their final destination.
• Mitochondria and chloroplasts are also surrounded by membranes, but they have
unusual membrane structures — specifically, each of these organelles has two
surrounding membranes instead of just one.
• The outer membrane of mitochondria and chloroplasts has pores that allow small
molecules to pass easily. The inner membrane is loaded with the proteins that
make up the electron transport chain and help generate energy for the cell.
• The double membrane enclosures of mitochondria and chloroplasts are similar to
certain modern-day prokaryotes and are thought to reflect these organelles'
evolutionary origins.

22. Organelle Membranes
• Some cell organelles are also surrounded by protective
membranes. The nucleus, endoplasmic
reticulum, vacuoles, lysosomes, and Golgi
apparatus are examples of membrane-bound
organelles. Mitochondria and chloroplasts are bound
by a double membrane.
• The membranes of the different organelles vary in
molecular composition and are well suited for the
functions they perform.
• Organelle membranes are important to several vital
cell functions including protein
synthesis, lipid production, and cellular respiration.

23. Eukaryotic Cell Structures
• The cell membrane is only one component of a cell. The following
cell structures can also be found in a typical animal eukaryotic cell:
• Centrioles—help to organize the assembly of microtubules.
• Chromosomes—house cellular DNA.
• Cilia and Flagella—aid in cellular locomotion.
• Endoplasmic Reticulum—synthesizes carbohydrates and lipids.
• Golgi Apparatus—manufactures, stores and ships certain cellular
products.
• Lysosomes—digest cellular macromolecules.
• Mitochondria—provide energy for the cell.
• Nucleus—controls cell growth and reproduction.
• Peroxisomes—detoxify alcohol, form bile acid, and use oxygen to
break down fats.
• Ribosomes—responsible for protein production via translation.

24. Conclusion
• Membranes are made of lipids and proteins,
and they serve a variety of barrier functions
for cells and intracellular organelles.
Membranes keep the outside "out" and the
inside "in," allowing only certain molecules to
cross and relaying messages via a chain of
molecular events