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Elena Aragon
October 4, 2009
Membranes
1. What does selective permeability mean and why is that important to cells?
Plasma membranes have selective permeability, meaning certain substances can cross it more
easily than others. This process is fundamental to cell processes and is possible due to the
component molecules of the membranes. Selective permeability allows beneficial molecules to
get in and waste to get out.
2. What is an amphipathic molecule?
An amphipathic molecule has a hydrophilic region as well as a hydrophobic region. A
phospholipid is amphiapathic, as are most of the proteins of membranes.
3. How is the fluidity of cell’s membrane maintained?
The cell membrane maintains its fluidity by a mosaic of proteins attached to a double layer of
phospholipids. The membrane is mostly held together by hydrophobic interactions. Lipids and
proteins can drift in the plane of the membrane but it is only possible to transverse the membrane
when the hydrophilic part of the molecule crosses the hydrophobic part of the membrane.
Phospholipids change quite quickly positions. Proteins drift as well, although much more slowly,
and some are quite immobile. The membrane remains fluid until the phospholipids settle into an
arrangement at a cooler temperature. The temperature depends on the type of phospholipids.
4. Label the diagram below – for each structure – briefly list it’s function:
extracellular matrix – attaches to proteins that coordinate extracellular and intracellular changes
carbohydrate – added to proteins in the endoplasmic reticulum, which then transform into
glycoproteins
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glycoprotein – proteins that acquired carbohydrates, secreted from vesicles on the outside of the
plasma membrane
cytoskeleton – contains microfilaments that bond to membrane proteins to stabilize location of
certain membrane proteins as well as maintain cell shape
cholesterol – reduces membrane fluidity at moderate temperatures by reducing phospholipid
movement, at low temperatures it disrupts packing of phospholipids and therefore solidification
glycolipid – lipids that acquired carbohydrates, secreted by vesicles on the outside of the plasma
membrane
integral protein – penetrate the hydrophobic core of the lipid bilayer, many of which completely span
the membrane
peripheral protein – not embedded in the lipid bilayer, loosely bound to surface of the membrane,
often to exposed parts of the integral proteins
5. List the six broad functions of membrane proteins.
The six functions of membrane proteins are transport, enzymatic activity, signal transduction, cell
to cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular
matrix.
6. How do glycolipids and glycoproteins help in cell to cell recognition?
Glycolipids and glycoproteins aid in cell to cell recognition by being identification tags that are
specifically recognized by other cells. Glycolipids and glycoproteins are lipids and proteins
covalently bonded to a carbohydrate, respectively.
7. Why is membrane sidedness an important concept in cell biology?
Membrane sidedness is an important concept in cell biology. This is because the two lipid layers,
which differ in specific lipid composition and contain proteins with a directional orientation in the
membrane, is important for endocytosis and exocytosis. When a vesicle fuses with the plasma
membrane, the outside layer of the vesicle becomes a part of the cytoplasmic layer of the plasma
membrane. This means that molecules that start out on the inside face of the endoplasmic
reticulum end up on the outside face of the plasma membrane, due the membrane sidedness.
8. What is diffusion and how does a concentration gradient relate to passive transport?
Diffusion is the movement of molecules from high concentration to low concentration. A
concentration gradient relates to passive transport because a substance diffuses down its
concentration gradient and thus it is spontaneous and doesn’t require any ATP from the cell, thus
it is passive transport. The concentration gradient itself represents potential energy.
9. Why is free water concentration the “driving” force in osmosis?
Free water concentration is the driving force in osmosis because water moves from a high
concentration to a low concentration thus the concentration gradient decides in which direction
water will flow.
10. Why is water balance different for cells that have walls as compared to cells without
walls?
Water balance is different for cells with walls compared to cells without walls due to pressure.
Cells without walls that are immersed in an isotonic environment, there will be no net movement
of water across the plasma membrane, because water is flowing across the membrane at the
same rate in both directions. Thus, in an isotonic environment, the volume of a cell without walls
is stable. In a hypertonic solution, the cell will lose water to its environment and most likely die. In
a hypotonic solution, water will enter the cell at a faster rate than it leaves, thus the cell will burst.
Cells without walls thus must have special adaptations for osmoregulation, the control of water
balance. For cells with walls, the cell can only expand so much before it exerts a pressure that
opposes further uptake. In an isotonic solution, the cell becomes limp because there is no
tendency for water to enter. In a hypertonic solution, however, the cell wall is no benefit because
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the cell will lose all of its water either way.
11. Label the diagram below:
12. What is the relationship between ion channels, gated channels and facilitated diffusion?
Facilitated diffusion is the process of polar molecules and ions diffusing passively accrose the
membrane with the help of transport proteins. Ion channels, many of which are gated channels,
function by a stimulus that causes them to open or close, allowing facilitated diffusion to occur.
The stimulus may be either electrical or chemical, and is different than the substance being
transported.
13. How is ATP specifically used in active transport?
To pump a molecule across a membrane against its gradient, the cell must expend energy in the
form of ATP. When the cell expends energy it is called active transport. Carrier proteins rather
than channel proteins are used to enable the molecule to move against the gradient. ATIP
powers active transport by transferring its terminal phosphate group directly to the transport
protein. This induces the protein to change its conformation in a manner that translocates a solute
bound to the protein across the membrane. For example, the sodium-potassium pump exchanges
sodium for potassium across the plasma 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, acting like a battery that affects the traffic
of all charged substances across the membrane. It favors the passive transport of cations into the
cell and anions out of the cell because the inside of the cell is negative. Electrochemical gradient
is the combination of forces acting on an ion. Electrogenic pump is a transport protein that
generates voltage across a membrane, the sodium-potassium being the major one of animal
cells. Proton pump is the main electrogenic pump of plants, fungi, and bacteria, which works by
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actively transporting hydrogen ions out of the cell.
15. What is cotransport and why is an advantage in living systems?
Cotransport is a single ATP-powered pump that transports a specific solute, indirectly driving the
active transport of several other solutes. It is an advantage in living systems because a substance
that has been pumped across a membrane can do work as it moves back down across the
membrane by diffusion, being very efficient.
16. What is a ligand?
A ligand is a general term for any molecule that binds specifically to a receptor site of another
molecule.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
Phagocytosis is a type of endocytosis that involves large, particulate substances, accomplished
mainly by macrophages, neutrophils, and dendridic cells. Pinocytosis is another type of
endocytosis for smaller molecules in which the cell ingests extracellular fluid and its dissolved
solutes. 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, enabling a cell to acquire bulk quantities of specific substances.
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