Download Oliver Bawmann week 6

Period 4
Membranes
What does selective permeability mean and why is that important to cells?
If a membrane is selectively permeable then this means that it a barrier that only allows
passage of specific things such as water, oxygen and nutrients. While some things pass
fairly easy others have difficulty and need facilitation proteins or simply cannot pass
through at all. By being selectively permeable a cell can control what enters and exists
the cell as everything must pass through this membrane. It also keeps all of the
contents of the cell inside it, while preventing harmful stuff from entering.
What is an amphipathic molecule?
An amphipathic molecule is a molecule that has both a hydrophilic and hydrophobic
region, such as the phospholipid bilayer of the plasma membrane which has a
hydrophilic head and hydrophobic tails.
How is the fluidity of cell’s membrane maintained?
The fluid membrane is held together by hydrophobic interactions that occur within the
cell membrane. Some of the proteins and all of the lipids can drastically move laterally
on the plane of the membrane. Temperatures affect the fluidity of the membrane,
however the affect of temperature depends on the phospholipids which compose the
membrane. Unsaturated hydrocarbons have kinks which prevent tight packing due to
the bonding that occurs. This the membrane more fluid, even at low temperatures.
Cholesterol has an effect on fluidity, acting as a buffer against change. It decreases
fluidity when the temperature is high, and increases it when it is low.
Label the diagram below – for each structure – briefly list it’s function:
extracellular matrix – surrounds and connects cells in living organisms
carbohydrate – attach to proteins or lipids to form glycoproteins or glycolipids which
act as
“ID tags” in cell recognition; they can also hold adjoining
cells together or act as sites where viruses or chemical messengers
such as hormones can attach.
glycoprotein – cell to cell recognition
cytoskeleton – cellular structural integrity for example shape and support
cholesterol – moderates fluidity of membrane at different temperatures by
decreasing it.
glycolipid – cell to cell recognition
integral protein – channels for transport of molecules, etc.
peripheral protein – cell recognition, enzymatic activity, etc.
List the six broad functions of membrane proteins.
Transport in and out of the cell, attachment to the cytoskeleton, signal transduction, cell
to cell recognition (glycoproteins), intercellular joining, the extracellular matrix and
enzymatic activity,
How do glycolipids and glycoproteins help in cell to cell recognition?
Glycolipids and glycoproteins function as ID tags during the process of cell to cell
recognition. Cells have specific, complementary sites that recognize the ID tags on
another cell and hence, recognize that cell which allow it to initiate the appropriate
response. When cells come in contact with each other they can tell through these
glycolipids or glycoproteins what type of cell they are dealing with.
Why is membrane sidedness an important concept in cell biology?
The two lipid layers can differ in specific lipid composition and each protein has a
directional orientation in the membrane. It is an important concept, because it has to so
with the processes of endo/exocytosis. The inside of the ER that becomes the inside of
a vesicle becomes the outside of the plasma membrane. This explains how certain
molecules end up on the extracellular face of the membrane as correspondence is
necessary.
What is diffusion and how does a concentration gradient relate to passive transport?
Diffusion is a form of transport in which molecules move along a concentration gradient
from high to low concentration. The concentration gradient determines weather
molecules should diffuse in or out of a cell through the cell membrane. Once again we
see the concept of selective permeability. If the molecules diffuse down the
concentration gradient then now ATP is used and the process is passive.
Why is free water concentration the “driving” force in osmosis?
Osmosis is similar to diffusion in that water diffuses from high to low concentration. The
free water concentration is the thing that causes the water to move across a membrane.
If it is high, the water moves to another area, if it is lower than the area where the water
is, then it moves to back to this area. Because free water concentration determines
where water will go it is the driving force.
Why is water balance different for cells that have walls as compared to cells without
walls?
Cells with a cell wall are turgid meaning the pressure of the elastic cell wall pushes back
against the cell to prevent further uptake of water, keeping the cell firm. A Cell without a
cell wall are healthiest in an isotonic solution, because in a hypertonic solution hey have
no elastic wall to counteract the intake of water which eventually causes the cell to burst
What is the relationship between ion channels, gated channels and facilitated diffusion.
Ion channels are constructed of proteins which establish a voltage concentration
gradient that allows the passage of specific ions. certain ion channels are gated which
mean they only open/ allow passage depending on the voltage gradient across the
membrane. This is all different to facilitated diffusion in which an integral protein allows
the passage of hydrophilic substances into the cell.
How is ATP specifically used in active transport?
A protein pump pushes molecules up and thus against its concentration gradient.
however to do this the proteins require energy in the form of none other than ATP.
Transporter proteins bind ATP directly and use the energy of its hydrolysis to drive
active transport. Others use the energy already stored in the gradient of a directlypumped ion. Direct active transport of the ion sets up a concentration gradient. When
the concentration gradient is eased through facilitated diffusion, the energy released
can be harnessed to the pumping of something else.
Define and contrast the following terms: membrane potential, electrochemical gradient,
electrogenic pump and proton pump.
Membrane potential is the difference in electrical potential in the fluid inside and outside
a cell. The change in voltage from outside to in or vice versa occurs mainly in the width
of the membrane itself. The electrochemical gradient is the concentration gradient of an
ion. It represents a type of potential energy that “accounts for both the concentration
difference of the ion across a membrane and its tendency to move relative to the
membrane potential.” An electrogenic pump is one that produces charge imbalance
across the cell membrane, and can contribute directly to the membrane potential. A
proton pump in an integral protein capable of maving ions across a membrane (against
its concentration gradient – hence the “pumping”). Membrane potential is the difference
in electtrical potential inside and outside of a cell, an electrogenic pump can contribute
to the membrane potential. A proton pump moves ions across a membrane, effecting
the electrocehemical gradient (concentration of an ion).
What is cotransport and why is an advantage in living systems?
Cotransport is the mechanism by which a single ATP-powered pump that transports a
specific solute can indirectly drive the active transport of several other solutes.
Contransport is advantageous, because it is basically getting two processes done for
the energy price of one. The process that requires ATP drives the transport of other
materials, allowing the cell to be as productive (all the processes that need to occur do)
and energy efficient as possible.
What is a ligand?
A ligand is a molecule that that binds specifically to a receptor site of another molecule.
Here it is a low-density lipoprotein, complexes of lipids and proteins, which cholesterol
travels in the blood stream in.
Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
In phagocytosis, a cell engulfs a particle by wrapping membrane (pseudopodia)
around it and packaging it in a sac large enough to be considered a vacuole. The
particle is digested when the vacuole fuses with a lysosome. In pinocytosis, the
cell “gulps” droplets of extracellular fluid into tiny vesicles, in order to get the
molecules dissolved in the droplets. Pinocytosis is nonspecific in the substances
it transports, because any and all included solutes are taken in with the
molecules. Receptor-mediated endocytosis is when cells use regions of the
membrane called coated pits (with a layer of coat proteins on the cytoplasmic
side) to acquire larger quantities of a substance, even if it is not very
concentrated in the extracellular fluid. Proteins are embedded in the membrane
that have specific receptor sites and these are exposed to the extracellular fluid,
and extracellular substances bind to these receptors, then the coated pit forms a
vesicle containing the ligand molecules, as well a some other molecules (in lower
quantities than the bound molecules). The material is “liberated” from the vesicle,
and the receptors are recycled to the plasma membrane again.
Page PAGE 1 of NUMPAGE \*Arabic 5