Section
5: Plasma Membrane and Cell Communication
I. The Plasma Membrane: Both Gate and
Gatekeeper
II. Water Requires a Cellular Balancing Act
III. Cell Membranes as Transport Luggage
IV. Cooperation among Cells in Multicellular
Organisms
V. Connections between Neighboring Cells
VI. Signaling Molecules in Cell Communication
I. The Plasma Membrane
v The
plasma
membrane separates the inside of a cell from the environment outside.
v According
to the Fluid Mosaic Model:
o
Hydrophobic
tails of phospholipids face inward, away from the water.
o
Hydrophilic
heads of phospholipids are on the outside where they interact with water
molecules in the fluid environment of the cell.
o
Proteins
are floating within the bilayer.
§ Some proteins span the entire
bilayer and may contain channels to allow passage of molecules through the
membrane (Integral proteins).
§ Some proteins are on the surface of
the lipid bilayer (Peripheral proteins).
o
Entire
membrane is fluid, with proteins freely moving within the bilayer.
§ Although
some biologically important molecules can pass through the phospholipid bilayer
of membrane, most cannot. The membrane
is selectively permeable; the materials that enter and leave the cell are
controlled.
v Why have a Plasma Membrane?
o
Keeps the cell’s molecules
of life (DNA, RNA, proteins) from dissipating away
o
Keeps out foreign molecules
that damage or destroy the cell’s components and molecules
§ However, cells DO have to communicate with
the environment constantly to monitor external conditions and adapt to them.
§ Cells depend on membrane proteins to gather
information about the environment.
v Cells
can move materials across the membrane with or without the expenditure of
energy.
o
Molecules move down concentration
gradients passively (passive transport).
§ Diffusion
is the movement of particles from an area of high concentration to an area of
low concentration.
§ Small
molecules can diffuse through the phospholipid bilayer
§ Chemical
proteins and passive carrier proteins allow molecules to cross the plasma membrane
passively.
·
Channel proteins provide means for
hydrophilic molecules of the right size and charge to move through the plasma
membrane from areas of high to low concentration.
·
Passive carrier proteins can bind
to a molecule that fits into the folds of the protein. Once bound to the molecule, the protein
undergoes a shape change, thus transferring the molecule from one side of the
plasma membrane to the other.
o
Molecules can move up
concentration gradients actively (active transport).
§ Only
active carrier proteins can move materials against a concentration gradient.
§ Active
carrier proteins use energy (such as that in ATP) to move molecules across the
plasma membrane, regardless of the concentration of molecules on either side of
the membrane.
v Active
and passive transport often work together.
o
Ex:
the sodium-potassium pump:
v Cell membrane separates two electrical
environments.
o
Intracellular and
extracellular compartments have slightly different charges.
o
Extracellular contains more
sodium and chloride ions, which are small and could potentially cross the
membrane.
o
Intracellular has
negatively charged molecules that cannot cross the membrane, which results in a
slight negative charge inside the cell.
§ This is a good thing and has to be
maintained!!!
§ The concentration differences are 10 -fold
for both ions.
§ The maintenance of these differences is
critical to maintain homeostasis within the cell.
o
Big differences in
concentration gradients cause huge concentration gradients for the two ions.
(They are yearning to diffuse!!!)
§ They could diffuse if there were many
transport channels to assist them.
§ In the image on screen, the Green channel (in
the middle) is a pore that is always open to allow K+ to pass. Purple channel
to right only lets Na+ pass.
§ Leaking is when ions diffuse through these
pores, causing an imbalance in the normal distribution of ions.
§ The leaking ions must be returned to their
appropriate sides on the membrane, which requires energy because the ions have
to be moved AGAINST their concentration gradients.
·
Requires protein pumps and
ATP
§ One pump simultaneously moves sodium ions
back out and potassium ions back in (Sodium-potassium pump).
§ There are less potassium ions being pumped in
than there are sodium ions being pumped out.
§ This maintains the negative charge within the
cell, which is required for the cell to function properly.
o
The gradient created on the
outside of the cell can be used by the cell as energy for other processes.
II. Water and the cellular balancing act
v The
environment inside and outside the cell is watery. Water moves into and out of cells by osmosis: the passive movement of water
across a selectively permeable membrane.
o
Too much or too little water
within the cell can be disastrous.
v Hypotonic
Solution: Outside medium is more watery (fewer solutes) than cytosol of cell,
resulting in more water flowing into the cell than out of it.
o
Hemolysis in red blood cells
v Hypertonic
Solution: Outside medium is less watery (more solutes) than cytosol, resulting
in more water flowing out of cell than into it.
o
Crenation in red blood cells
v Isotonic
Solution: Outside medium is the same as the cytosol of the cell, resulting in
equalization of water flow into and out of the cell.
III. Cell
membranes as transport luggage
v Many
substances are exported from and imported into cells by becoming wrapped in, or
unwrapped from, pieces of plasma membrane.
v Exocytosis:
release of substances by cells into surroundings
v Endocytosis:
section of plasma membrane bulges inward around substances outside of cell
o
Pinocytosis: Taking in of fluid
o
Phagocytosis: Taking in of larger
molecules: restricted to specialized cells
§ Ex:
white blood cells
o
Receptor-mediated endocytosis:
receptor proteins embedded in some areas of the membrane determine what
substances enter the cell
§ Ex:
liver cells
IV. Cooperation among cells in multicellular
organisms
v Principle
of cell specialization: cells found in a multicellular organism are not all the
same.
o
Ex: bone cells in vertebrates;
hollow cells in plants
v Principle
of cell communication: Cells depend on an ability to communicate with each
other
o
The means of communication between
cells is dependent on direct physical contact with each other and signaling
molecules when cells are not close together.
V. Connections between neighboring cells
v Cell
junctions: neighboring cells often share cell junctions that contribute to the
overall stability of cellular communities and allow neighboring cells to
communicate by providing passages for substances to move from one cell to
another.
v Plasmodesmata:
found in plants--tunnel-like channels in the cell wall
v Extracellular
matrix: secreted by animal cells—helps hold cells together
o
Tight junctions: hold cells
together with strands of protein arranged in a belt beneath the plasma
membrane
o
Anchoring junctions: act as
protein hooks between cells or between a cell and the extracellular matrix;
allow materials to pass in-between cells while still holding them together
o
Gap junctions: direct channel
protein connections between plasma membranes of two cells that allow passage of
ions and small molecules
VI. Cell Communication
v Multicellular
organisms use small molecules to transmit signals between cells. These “signaling molecules” are the language
of cellular communication and are received by target cells.
o
Some signaling molecules are fast
and bring about response in the target cell almost instantly.
o
Other signaling molecules must
travel long distances through the bloodstream and work more slowly to bring
about a response once they reach the target cell.
v When a signaling molecule reaches a
target cell, that cell must have a specific means of receiving it and acting on
its message. These tasks are the
responsibility of a class of proteins called receptors.
o
Receptors lie on the surface of
the target cell
o
Receptors can also be found in the
cytosol or inside the nucleus of the target cell.
v Hormones
are long-range signaling molecules
o
All living organisms use hormones
to coordinate the activities of different cells and tissues.
o
Hormones are produced by cells in
one part of the body and transmitted to target cells in another part of the
body.
o
Steroid hormones can cross cell
membranes.
§ Steroid
hormones are essential for many growth processes, including the normal
development of reproductive tissues in mammals.
§ Steroid
hormones are hydrophobic.
§ Steroid
molecules alter the production of specific proteins inside the target
cell.
·
The steroid and receptor protein
forms an active molecular complex that can enter the nucleus and interact with
the target cell’s DNA.
·
Ex: progesterone in target cells
in the uterus
§ Not
all steroids enter the cell as steroids do.
Certain hormones send their signals into the cell via cell surface
receptors.
v Interesting
Science: Tagging proteins