BTW, rocked the last test- 93%.
I'm loving this cell membrane unit. It makes sense to me, builds on what we've learned before. I find it fascinating how all the pieces work together and form a wall that holds all organelles in and anchors cytoskeleton.
In all things I love about biology, the plasma membrane
really interests me. It's a true work of scientific engineering. It’s made of
mostly of phospholipids and embedded proteins. These proteins can do a lot of
things, like help channel things in and out of the cell, help with cell
recognition or be enzymatic proteins. This is all covered later.
With all of this content, the structure of the fluid mosaic
membrane is still fluid, with a texture similar to light oil. The proteins are
scattered throughout the sea of phospholipids and create the mosaic texture
that earns the phospholipid bilayer the term the ‘fluid mosaic membrane’.
Remember our phospholipid. He has a polar head and nonpolar
tails. He’s made from two fatty acids on one phosphate molecule. These nonpolar
tails are hydrophobic while the head is hydrophilic. This is how the plasma
membrane spontaneously arranges itself with the heads pointing to the outsides
of the membrane and the tails facing each other.
Here is a generic picture that I take no credit for but really- I could have drawn it ;)
Within this membrane of phospholipids, a molecule similar to
a phospholipid resides. It’s a glycolipid. It is made of two hydrophobic tails
with a head made of sugars joined into a straight or branching carbohydrate
chain. Even though it reduces the
permeability of most biological molecules, cholesterol is also found in the
plasma membranes of animals. Plants have similar steroid molecules in their
membranes.
The cell isn’t symmetrical on the outside and inside. While
the phospholipid bilayer sounds like it would be symmetrical, the outside is
coated with carbohydrates from glycolipids and glycoproteins. As well, the
outside has proteins that anchor to an intracellular matrix.
The inside has
protein anchorings for the cytoskeletal filaments.
Proteins are found throughout the membrane.
Here are a few
kinds.
Integral proteins usually have a hydrophobic quality to them. Most of
these integral proteins are also glycoproteins meaning they have an attached
carbohydrate chain that floats out from the outside of the cell membrane.
Peripheral proteins hang out on the outside of the membrane (either the cytoplasmic
or external side) sometimes anchored by covalent bonds, other times, just
resting on the surface of the cell lightly tethered with non-covalent
interactions and are prone to shifting around when the cell moves or is shaken or even
has a pH shift.
Channel protein:
Shaped like a tube. It’s shaped to let particular kinds of
ions pass at will across the plasma membrane.
Carrier protein:Selectively interacts with specific molecules or ions so
they can cross the plasma membrane.
Cell recognition protein: The carbohydrate chains and glycolipids and glycoproteins
are the cell’s fingerprints. These vary from person to person and species to
species. These cells are why organ transplants are difficult to match since the
body’s immune system can sense that the cells of the foreign organ do not have
the same ‘fingerprint’ as the body’s original cells.
Receptor protein:
These are shaped in such a way that they can only bind to a
certain substance i.e. growth hormones.
Enzymatic protein: Catalyzes specific reactions for example, the protein
adenylate cyclase is involved in ATP metabolism.
The cell membrane is selectively [differentially] permeable.
Some things can move across the membrane and others can’t.
Ions and other charged molecules can’t cross
because they can’t get through the hydrophobic layer. Macromolecules are just
too big to pass through.
Oxygen, Carbon Dioxide and Water can easily pass
through because they are noncharged and small enough.
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