Download CH - TeacherWeb

CH.7 Cell Membranes
Lipid bi-layers are formed from PHOSPHOLIPIDS. It is composed of a Glycerol
backbone and 2 fatty acid chains (similar to a Triglyceride). The fatty acids are
NONPOLAR, hence hydrophobic. The remaining Carbon from the glycerol is attached
to an alcohol group that forms hydrogen bonds with water – hydrophilic. The alcohol
then is attached to a phosphate group.
A lipid bi-layer is fluid causing them to move about within the membrane. Phospholipids
are highly viscous and changes due to temperature, as does oil. Just like fats and oils
the tails behave accordingly. If the tail of the phospholipid has too many double bonds
then it has too many kinks, more fluid (like oils).
In lipid bi-layers one can also find cholesterol, proteins, glycolipid, and cytoskeleton.
Lipid bi-layers offer flexibility to the cell and imposes a barrier to permeability.
Proteins can be found on the lipid bi-layer acting as a marker or message holder, and in
through the bi-layer as a pore. They are free to move around as the Phospholipids.
Supporting fiber can be proteins and/or cytoskeleton which help to give cells their shape
or hold certain proteins anchoring to a site on the membrane.
Glycolipids (glycoclayx) are derived from inside the cell (golgi body & ER). These are
sent to existing proteins on the outer membrane and act as cell markers.
There are six key classes of proteins found in the membrane:
 TRANSPORT channels – they are found throughout the membrane and allow
molecules to enter and leave the cell.
 ENZYMES – many are found in the interior of the membrane and carry out
chemical reactions.
 CELL SURFACE receptors – act as antennae that are sensitive to chemical
messages on the outer membrane.
 CELL SURFACE markers – these are found on the outer membrane and serve
as the cells ID.
(continued next page)
 CELL ADHESION proteins – they glue themselves to other proteins found on the
outer membrane.
 ATTACHMENTS to the cytoskeleton – surface proteins that interact with other
cells are often anchored to the cytoskeleton by linking proteins.
DIFFUSION – is a process where molecules and ions travel from areas of high
concentration to areas of low concentration. However, the plasma membrane is
selectively permeable, because the proteins allow certain substances to pass through
and others not.
ION channels allow the ions to pass through with no effort. Ions can be positively
(cations) or negatively (anions) charged. They can not pass through the cell membrane
because they would be repelled by the non-polar interior of the bi-layer so they have
special Ion channels filled with water that allows free passage thru the membrane.
1
Each channel is specific to an ion and the direction they move is determined by either
their concentration or voltage across the membrane.
Facilitated Diffusion – has substances going from an area of High concentration to an
area of Low concentration, but requires a CARRIER. These carriers are specific for a
certain type of solute and works by the solute binding on the protein carrier. The carrier
pulls the solute to the other side of the membrane – depending on concentration.
Carrier-mediated Processes Saturate – is that when the concentration gradient of
a substance is progressively increased, its rate of transport will also increase to a
certain point and then level off.
OSMOSIS – water is small enough that it can pass thru the lipid bi-layer. The passage
of water thru the plasma membrane is referred to as osmosis.
Osmotic concentration refers to the concentrations of solute on both sides of the
membrane.
Hyperosmotic or Hypertonic – high concentration of solute
Hyposmotic or Hypotonic– low concentration of solute
Isosmotic or Isotonic – concentrations are equal
Extrusion – there are contractile vacuoles that collects water from the cytoplasm and
transports to the membrane.
Isomotic solutions – some organisms have the ability to change the concentration of
solutes inside their cells.
Turgor – hydrostatic pressure. The pressure inside a plant cell.
ENDOCYTOSIS – is the process when the cell membrane extends outward and
envelops food particles. Ex. amoeba
There are 3 types of ENDOCYTOSIS:
Phagocytosis - when the cell takes in particulate matter or some fragment of organic
matter to large to bring in through the cell membrane.
Pinocytosis – when the cell takes in liquid matter.
Receptor-mediated endocytosis – specific molecules are often transported into
eukaryotic cells. These cells have indented pits on their outer plasma membrane,
coated with the protein clathrin. The substance that attaches to the membrane is then
engulfed by it, pinches off to form a vesicle and brings it into the cell.
EXOCYTOSIS – the reverse of endocytosis.
ACTIVE TRANSPORT – this process involves using energy but sends substances
against the gradient / from LOW to HIGH.
Na+ K+ pump – Most cells have a low internal concentration of Sodium and a high
internal concentration of Potassium. Cells are actively pumping out Na+ and bringing
the K+ into the cell. The energy to operate this pump comes from ATP.
Step 1 – 3 sodium ions bind to the cytoplasmic side of the protein, causing the
protein to change its conformation.
Continued next page
2
Step 2 – in its new conformation, the protein binds a molecule of ATP and
cleaves it into ADP. Phosphate group remains binded to the protein.
Step 3 – after this step the protein is termed “phosphorylated” which induces a
second conformational change. The change allows the 3 Na + ions to travel to the
exterior of the cell.
Step 4 – the new conformation has a high affinity for K+, two of which bind to the
extracellular side of the protein as soon as it is free of the Na+.
Step 5 – The binding of the K+ causes another conformational change in the
protein, this time resulting in the dissociation of the bound phosphate group.
Step 6 – freed from the phosphate group, the protein reverts to its original
conformation, exposing the two K+ to the cytoplasm.
3 sodium leave and 2 potassium enter in every cycle. This occurs fast and enables
each carrier to transport as many as 300 Na+ per second. This process is found in
nerve cells.
COUPLED channels – the process, called co-transport, uses sodium ions or protons
that are moving down the concentration gradient instead of ATP to power ions.
1. Establishing the down gradient. ATP is used to establish the sodium ion or
proton down gradient.
2. Traversing the up gradient. Co-transport channels carry the molecule and
either a sodium ion or a proton together across the membrane.
The cell sets up its own HIGH to LOW gradient by moving the ions around. The
movement of the sodium ion (binding to the channel) facilitates the movement of sugar
or amino acids up its concentration gradient.
Cell – Cell Interaction
Types of Cell signaling – cells communicate thru 4 basic mechanisms:
Direct contact – this is when cells have direct contact with one another. This is
one of the important roles in early development of the cell.
Paracrine signaling – as direct contact, this is also an important way that cell
interact in early development. Signal molecules are secreted by one cell and affects
neighboring cells.
Endocrine signaling – is when the signal molecules travel further than
neighboring cells and are swept via the circulatory system. These molecules are also
known as HORMONES.
Synaptic signaling – nerve cells secrete neurotransmitters that travel throughout
the nervous system.
Cell communication mechanisms
Intracellular Receptors (within the cell) – lipid soluble signal molecules persist in the
blood far longer than water soluble signals. Water soluble hormones break down within
minutes, and neurotransmitters within seconds.
3
Cell surface receptors – water soluble signals (neurotransmitters, peptide hormones
and “growth factors” proteins) can NOT diffuse through the cell membrane, so they must
signal the cells by the surface protein.
Chemically gated ion channels – a long protein chain weaves in and out of the
membrane to create a pore across the membrane. Ions will pass once a
neurotransmitter signals the protein to change shape and allow the ions across.
Enzymatic receptors – is when a signal molecule adheres to the protein receptor
on the exterior of the membrane and signals an enzyme to become active just inside the
cell.
G protein linked receptors – GTP (guanosine triphosphate binding protein) are
used to mediate passage of the signal form the membrane surface into the cell interior.
Scientists have discovered more than 100 different ones. These guys are known as
mediators, because they are the link between the receptor on the cell surface and signal
pathways within the cytoplasm.
Second messengers are small molecules or ions that carry out the message to a
particular target once the G protein is activated. cAMP & Calcium are the two most
widely used.
4