Grace Michaels
Period 4
Membranes
1. What does selective permeability mean and why is that important to cells?
It means that it allows some substances to cross it more easily than others. It is
important to cells because it gives cells the ability to discriminate in its chemical
exchanges with its environment
2. What is an amphipathic molecule?
An amphipathic molecule is a molecule that has both a hydrophilic region and a
hydrophobic region. Phospholipids are amphipathic molecules. Membranes are
composed of lipids and proteins with phospholipids being the most abundant of the
lipids.
3. How is the fluidity of cell’s membrane maintained?
Membranes are held together by primarily hydrophobic interactions which are weaker
than covalent bonds and thus allow lipids and some of the proteins to drift about
laterally and be “fluid”. A membrane remains fluid as temperature decreases until the
phospholipids settle into a closely packed arrangement and the membrane solidifies.
The membrane can be more fluid from when unsaturated hydrocarbons cannot pack
together as closely as saturated hydrocarbons. Cholesterol can make the membrane
less fluid by restraining the movement of phospholipids.
4. Label the diagram below – for each structure – briefly list it’s function:
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extracellular matrix – The substance in which animal tissue cells are embedded,
consisting of protein and polysaccharides.
carbohydrate – A sugar (monosaccharide) or one of its dimmers (disaccharides) or
polymers (polysaccharides).
glycoprotein – a protein covalently attached to a carbohydrate.
cytoskeleton –A network of microtubules, microfilaments, and intermediate filaments
that branch throughout the cytoplasm and serve a variety of mechanical and
transport functions.
cholesterol – A steroid that forms an essential component of animal cell membranes
and acts as a precursor molecule for the synthesis of other biologically important
steroids
glycolipid –A lipid covalently attached to a carbohydrate.
integral protein – typically a transmembrane protein with hydro phobic regions that
completely spans the hydrophobic interior of the membrane.
peripheral protein – protein appendage loosely bound to the surface of a membrane
and not embedded in the lipid bilayer.
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?
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Cells recognize each other by binding to surface molecules and, often carbohydrates,
on the plasma membrane. Some of these carbohydrates are covalently bonded to lipids
forming glycolipds, but most are bonded to proteins which make glycoproteins.
7. Why is membrane sidedness an important concept in cell biology?
Membrane sidedness is important because membranes have distinct inside and
outside faces that differ in specific lipid composition, and each protein has
directional orientation in the membrane. The outside layer becomes continuous
with the cytoplasmic layer of the plasma membrane after the vesicle fuses with it
which makes molecules that start out on the inside face of the ER end up on eht
outside face of the plasma membrane.
8. What is diffusion and how does a concentration gradient relate to passive
transport?
Diffusion is the movement of anything besides water from a high concentration to a low
concentration; any substance will diffuse down its concentration gradient. No work must
be done for this to happen; it’s a spontaneous process. Because of this diffusion across
a membrane is called passive transport because the cell does not have to expend
energy for it to happen.
9. Why is free water concentration the “driving” force in osmosis?
Some water in solutions are unavailable to cross the membrane because some water
molecules cluster around the hydrophilic solute molecules. Only the free water can
diffuse across the membrane in what is called osmosis.
10. Why is water balance different for cells that have walls as compared to cells
without walls?
A cell without rigid walls can tolerate neither excessive uptake nor excessive loss of
water. This water balance can be solved by living in isotonic surroundings. Cell walls
can help maintain the cell’s water balance.
11.
Label the diagram below:
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12. What is the relationship between ion channels, gated channels and facilitated
diffusion.
Facilitated diffusion is when many polar molecules and ions impeded by the lipid bilayer
of the membrane diffuse passively with the help of transport proteins that span the
membrane. Channel proteins provide corridors that allow a specific molecule or ion to
cross the membrane. Ion channels, many of which function as gated channels which a
stimulus causes them to open or close to facilitate certain molecules and ions.
13. How is ATP specifically used in active transport?
ATP supplies the energy for most active transport. It cane transfer its terminal
phosphate group directly to the transport protein. This could induce the protein to
change its conformation so that it can cross the membrane.
14. Define and contrast the following terms: membrane potential, electrochemical
gradient, electrogenic pump and proton pump.
Membrane potential is the voltage across a membrane and it ranges from -50 to -200
millivolts and it acts like a battery. It favors passive transport of cations into the cell and
anions out of the cell. This means that two forces drive the diffusion of ions across a
membrane: a chemical force and an electrical force. This combination of forces acting
on an ion is called the electrochemical gradient. A transport protein that generates
voltage across a membrane is a electrogenic pump and the main electrogenic pump of
plants, fungi, and bacteria is a proton pump.
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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 concentation gradient. It is an advantage in
living systems because it is used in photosynthesis.
16. What is a ligand?
A liagand is a molecule that binds specifically to a receptor site.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
Phagocytosis is a type of endocytosis involving large, particulate substances,
accomplished mainly by macrophages, neutrophils, and dendritic cells. Pinocytosis is
also a type of endocytosis, but it is when the cell ingests extracellular fluid and its
dissolved solutes. Receptor-mediated endosytosis is obviously also a type of
endosytosis but is the movement of specific molecules into a cell by the inward budding
of membraneous vesicles containing proteins with receptor sites specific to the
molecules being taken in. It enables a cell to aquire bulk quantities of specific
substances.
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