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Julia Sigman
October 4, 2009
Period 6
Membrane Reading Guide
1. What does selective permeability mean and why is that important to cells?
A selectively permeable membrane is a property of biological membranes that allows some
substances to cross more than others. It is fundamental to life that cells can discriminate in its
chemical exchange with the environment and it is the plasma membrane and its component
molecules that make this selectivity possible.
2. What is an amphipathic molecule?
An amphipathic molecule has both a hydrophilic and hydrophobic region, for example, a
phospholipid is a amphipathic molecule.
3. How is the fluidity of cell’s membrane maintained?
A membrane is held together by hydrophobic interactions, which are weaker than covalent
bonds. The lipids and some proteins can very quickly drift laterally, in the plane of the
membrane. Temperatures affect the fluidity of the membrane (the membrane will remain
fluid as temperature decreases until finally the phospholipids settle into a tight packed
arrangement), but it depends on the phospholipids which it is composed of. 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:
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extracellular matrix – microfilaments or other elements of the cytoskeleton may be bonded to
membrane proteins, a functions that helps maintain cell shape and
stabilizes the location of certain membrane proteins. Proteins that adhere
to the ECM can coordinate extra and intra cellular changes.
carbohydrate – attach to proteins or lipids to form glycoproteins or glycolipidsact as tags that
can be identified in cell recognition. They can also hold adjoining cells
together or act as sites where viruses or chemical messengers, for example,
hormones, can attach.
glycoprotein – cell to cell recognition
cytoskeleton – cellular structural integrity, for example shape and support
cholesterol – controls fluidity of membrane at different temperatures, and acts as a
temperature buffer for the membrane, resisting changes in membrane fluidity.
glycolipid – cell to cell recognition
integral protein – necessary proteins that penetrate the hydrophobic core of the lipid bilayer
but many are transmembrane proteins which completely span the membrane.
They can act as channels in facilitated diffusion
peripheral protein – appendages loosely bound to the surface of the membrane. They are used
in cell recognition, enzymatic activity, etc.
5. List the six broad functions of membrane proteins.
Six functions of membrane proteins are transportation, enzymatic activity, signal
transduction, cell to cell recognition, intercellular joining, the attachment to the cytoskeleton
and extracellular matrix
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6. How do glycolipids and glycoproteins help in cell to cell recognition?
Glycolipids and glycoproteins act as identification factors 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, for example, “initiating
the appropriate response. For instance a white blood cell, when it comes in contact with
another cell, can tell through glycoproteins and glycolipids whether that cell belongs, or
should be disposed of.
7. Why is membrane sidedness an important concept in cell biology?
The two lipid layers can vary in specific lipid make up and each protein has a directional
orientation in the membrane. The plasma membrane has distinct cytoplasmic and
extracellular sides, or faces, with the extracellular face arising from the inside face of the ER,
golgi, and vesicle membranes. It is an important concept, because it has to so with the
processes of endocytosis and exocytosis. The inside of the ER that becomes the inside of a
vesicle becomes the outside of the plasma membrane. This results in certain molecules
ending 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 to low concentration. The concentration
gradient causes molecules to diffuse in or out of a cell via the membrane of the cell. As long
as the molecules are moving down the concentration gradient, their natural tendency, no
energy or ATP is needed to pump them, thus passive is not a form of 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, much like in
diffusion. The free water concentration is the cause of the movement of water. If it is a high
concentration, the water moves to a area of lower concentration and if it is lower than the
area where the water is, it moves to back to the first area, thus causing osmosis.
10. Why is water balance different for cells that have walls as compared to cells without
walls?
Cells with a cell wall are very firm, and usually healthiest in a hypotonic solution, because
the pressure of the expandable cell wall pushes against the uptake of water, maintaining the
cell’s firmness. Cell without a cell wall are healthiest in an isotonic solution, because in a
hypertonic solution they do not have an expandable wall to work against the intake of water,
so the cells explode.
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11. Label the diagram below:
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 protein channels that allow for larger molecules. 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, also called a ligand. These channels are known as gated channels.
Facilitated diffusion is the process, ion channels are the materials used, and gated channels
are a common type of said channel.
13. How is ATP specifically used in active transport?
A protein pump pushes molecules against its concentration gradient, from low to high. 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 force active transport.
Others use the energy previously stored in the gradient of a directly-pumped ion. Direct
active transport of the ion creates a concentration gradient. When the concentration gradient
is eased through facilitated diffusion, the energy released can be attached to the propeling of
something else.
14. Define and contrast the following terms: membrane potential, electrochemical gradient,
electrogenic pump and proton pump.
Membrane potential is the charge difference between a cell’s cytoplasm and the extracellular
fluid, due to the differential distribution of ions. Membrane potential affects the activity of
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excitable cells and the transmembrane movement of all charged substances. The
electrochemical gradient is the diffusion gradient of an ion, representing a type of potential
energy that accounts for the bother concentration difference of the ion across a membrane
and its tendency to move relative to the membrane potential. An electrogenic pump is an ion
transport protein that generates voltage throughout the membrane. A proton pump is an
active transport mechanism in a cell that used AYP to force hydrogen ions out of a cell,
generating a membrane potential in the process.
15. What is cotransport and why is an advantage in living systems?
Cotransport is the coupling of the “downhill”diffusion of one substance to the “uphill”
transport of another against its own concentration gradient. Contransport is an advantage in
living systems because basically it is getting two processes done for the energy cost of one.
The process that requires ATP forces the transport of other materials, allowing the cell to be
as productive and energy resourceful as possible.
16. What is a ligand?
A ligand is a molecule that that binds specifically to a receptor site of another molecule.
Ligands bind to the low-density lipoprotein receptors on membranes and then enter the cell
via endocytosis.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
Phagocytosis is a type of endocytosis involving large, particular substances, accomplished
mainly by microphages, neutrophils, and dentritic cells. In phagocytosis, a cell engulfs a
particle by wrapping membrane 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.
Pinocytosis is a type of endocytosis in which the cell ingests extracellular fluid and its
dissolved solutes. In pinocytosis, the cell swallows 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 the movement of specific molecules into a cell
by the inward budding of membranous vesicles containing proteins with receptor sites
specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific
substances.
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