Download Membranes Reading Guide

Leorah McGinnis
Period 4
Membranes
1. What does selective permeability mean and why is that important to cells?
Selective permeability means that it is a selective barrier, which allows passage
of certain materials (waste, water, oxygen, nutrients, etc.) but not others, or at
least some may pass more easily. It allows a cell to discriminate in its chemical
exchanges with its environment. This is important to cells because it keeps them
from spilling all their contents, keeping the bigger molecules, organelles, cytosol,
etc. inside, while allowing the cell to get rid of unnecessary and harmful
materials, and to take in other materials like water which the cell needs.
2. What is an amphipathic molecule?
An amphipathic molecule has both a hydrophilic and hydrophobic region, such as
a phospholipid.
3. How is the fluidity of cell’s membrane maintained?
The membrane is held together by hydrophobic interactions. The lipids and some
proteins can (rapidly) drift laterally, on the plane of the membrane. Temperatures
affect the fluidity of the membrane, but it depends on the phospholipids which
compose it. If the hydrocarbon tails are unsaturated, they have kinks which
prevent tight packing, making the membrane more fluid, even at relatively 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.
4. Label the diagram below – for each structure – briefly list it’s function:
Page 1 of 5
extracellular matrix – surrounds and connects cells in living organisms
carbohydrate – attach to proteins or lipids to form glycoproteins or glycolipidsact as
“ID tags” in cell recognition; they can also hold adjoining cells together
or act as sites where viruses or chemical messengers (like hormones)
can attach.
glycoprotein – cell to cell recognition
cytoskeleton – cellular structural integrity (shape and support
cholesterol – moderates fluidity of membrane at different temperatures
glycolipid – cell to cell recognition
integral protein – channels for transport of molecules, etc.
peripheral protein – cell recognition, enzymatic activity, etc.
5. List the six broad functions of membrane proteins.
Transport, enzymatic activity, signal transduction, cell to cell recognition,
intercellular joining, attachment to the cytoskeleton and extracellular matrix
6. How do glycolipids and glycoproteins help in cell to cell recognition?
Glycolipids and glycoproteins act as ID tags in cell to cell recognition, where cells
have specific, complementary sites(complementary carbohydrate binding
proteins) that recognize those tags on another cell and hence, recognize that cell
(i.e. “initiating the appropriate response). For instance a white blood cell, when it
comes in contact with another cell, can tell through these carbohydratelipids/proteins whether that cell belongs, or poses a threat.
Page 2 of 5
7. 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.
8. What is diffusion and how does a concentration gradient relate to passive
transport?
Diffusion is the movement of substances from high concentration to low. The
concentration gradient causes molecules to diffuse in or out of a cell via the cell
membrane. As long as the molecules are moving down the concentration
gradient, their natural tendency, no ATP is needed to pump them, hence passive
not active transport.
9. Why is free water concentration the “driving” force in osmosis?
Water moves from an area of high concentration to one of low concentration. The
free water concentration is what causes the water to move. If it is high, the water
moves to another area, if it is lower than the area where the water is, it moves to
back to the first area, hence causing or “driving” osmosis.
10. Why is water balance different for cells that have walls as compared to cells
without walls?
Cells with a cell wall are turgid, and usually healthiest in a hypotonic solution,
because the pressure of the elastic cell wall pushes back against the uptake of
water, keeping the cell firm. 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, so the cells burst.
11.
Label the diagram below:
Page 3 of 5
12. What is the relationship between ion channels, gated channels and facilitated
diffusion.
Facilitated diffusion of ions occurs when ions move down their concentration
gradient, with the assistance of transmembrane proteins. Ion channels are
proteins or series of proteins that form water filled channels for the ions to pass
through, down their concentration gradient. Some ion channels can open and
close in response to the binding of a signaling molecule (a "ligand"). These
channels are called gated channels. Facilitated diffusion is the process, ion
channels are the means, and gated channels are a common type of this channel.
13. How is ATP specifically used in active transport?
A protein pump pushes molecules against its concentration gradient. But in order
to physically do this, the proteins require energy in the form of ATP. Some
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
directly-pumped 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.
14. 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
Page 4 of 5
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).
15. 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.
16. 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.
17. 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 5 of 5