Membranes
1. What does selective permeability mean and why is that important to cells?
Selective permeability means the cell can control what can come in and what can
exit the cell and the selective permeability also allows some substances to cross
more easily than others. This selective permeability is important to cells because it
allows for certain products important for the cell to survive to enter or exit, for
example, the selective permeability allows nutrients to be absorbed while
eliminating waste.
2. What is an amphipathic molecule?
An amphipathic molecule is a phospholipid which has both the hydrophilic and
hydrophobic region.
3. How is the fluidity of the cell membrane maintained?
Fluidity of the cell membrane is maintained by kinks that keep the unsaturated
hydrocarbon tails of phospholipids from packing together. The fluidity of the cell
membrane is also maintained in low temperatures by cholesterol found embedded
between the phospholipid molecules. This cholesterol prevents the solidification of
the membrane by interrupting the usual packing of phospholipids and at higher
temperatures; cholesterol stops the fluidity of the phospholipids.
4. Label the diagram below – for each structure – briefly list its function:
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Microfilaments of cytoskeleton- These can be bounded to the membrane proteins
and they help in stabilizing the location of certain membrane proteins and they also
play a role in maintaining the shape of the cell.
Integral protein- this protein informs the plasma membrane to do specific functions
and allows certain molecule to enter or exit the cell. .
Cholesterol- The cholesterol in the membrane acts like a “temperature buffer” and
it also plays a role in membrane fluidity and gives cell flexibility.
Peripheral protein- these proteins are loosely bound to the plasma membrane
Glycoprotein- One function of a glycoprotein is to serve as an identification card.
Carbohydrates-The carbohydrates bond either with lipids and proteins to form
glcolipids or glcoprotiens. Membrane carbohydrates also help to distinguish
between one cell from another.
Glycolipid- These also play a role in identification.
Fibers of extracellular matrix- Certain membrane proteins are attached to the fibers
of extracellular matrix and this allows for a stronger framework for the cell.
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 also attachment to the
extracellular matrix and cytoskeleton.
6. How do glycolipids and glycoproteins help in cell to cell recognition?
Glycolipids and glycoproteins help in cell to cell recognition because they are
composed of a certain carbohydrate and protein. This difference in carbohydrate
enables glycolipids and glycoproteins to play a role in cell to cell recognition.
7. Why is membrane sidedness an important concept in cell biology?
8.
What is diffusion and how does a concentration gradient relate to passive
transport?
Diffusion is the movement of anything but water from a high to a low concentration.
Passive transport is the diffusion of a substance across a membrane without the
use of energy. Concentration gradient and passive transport are related because
the concentration gradient represents a potential energy and passive transport
uses the concentration gradient during diffusion or osmosis.
9.
Why is free water concentration the “driving” force in osmosis?
Free water concentration is the “driving” force in osmosis because it is the
difference in the free water concentration that decides which way the water will
move.
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10. Why is water balance different for cells that have walls as compared to cells
without walls?
Water balance is different for cells that have walls as compared to cells without
walls because cells with walls are able to maintain water balance by expanding
and then exerting a pressure called turgor pressure and by doing so it stops any
further water uptake. While in cells without walls, in order to maintain balance the
cell would have to adapt to osmoregulation and by doing so control its water
uptake.
11. Label the diagram below:
12. What is the relationship between ion channels, gated channels, and facilitated
diffusion – write 1 -2 sentences using those terms correctly.
Ion channels and gated channels function in part with facilitated diffusion by
allowing passage of molecules that the lipid bilayer had stopped. The ion
channels, which function as gated channels allow the passage of ions. These
gated channels (which sometimes function as ion channels) are opened or closed
by either a chemical or electrical stimulus. Overall, the gated and ion channels are
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related to facilitated diffusion because they play a role in allowing the passages of
large molecules, or they help to speed up the process of diffusion.
13. How is ATP specifically used in active transport?
ATP is specifically used in active transport by supplying the energy needed for
active transport to move solutes against a concentration gradient.
14. Define and contrast the following terms: membrane potential, electrochemical
gradient, electrogenic pump and proton pump.
Membrane potential- the voltage across a membrane
Electrochemical gradient- the chemical force and the electrical force driving the
diffusion of ions across a membrane is known as electrochemical gradient
Electrogenic pump- this is a transport protein that generates voltage across a
membrane.
Proton pump-this is the main electrogenic pump of plants, fungi and bacteria that
actively transport hydrogen ions out of the cell.
The membrane potential favors the passive transport, but in an electrochemical
gradient, the ion must go down its electrochemical gradient and not its
concentration gradients, there by contradicting the membrane potential. Both the
electrogenic and proton pump are a means of transport, however the electrogenic
pump transports sodium-potassium, while the proton pump transports hydrogen
ions.
15. What is cotransport and why is an advantage in living systems?
Cotransport is a type of membrane traffic that is powered by a single ATP pump
that transports a specific solute and by doing so it indirectly drives the active
transport of several other solutes. This is an advantage in living systems because it
allows for substances to do work as they move back across a membrane by
diffusion and it allows proteins to transport in ways they couldn’t without the
cotransport.
16. What is a ligand?
Lipoproteins are particles composed of lipids and proteins. These particles act as
ligands, a term used to describe a molecule that binds specifically to a receptor site
on another molecule, by binding to LDL receptors and then entering the cell by the
use of endocytosis.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
In phagocytosis, a cell engulfs a particle by wrapping around it and packaging it
within a membrane-enclosed sac known as the vacuole. In pinocytosis, the cell
takes in droplets, with the use of a vesicle, of extracellular fluid and uses the
molecules dissolved in the droplet. In receptor-mediated endocytosis the cell can
acquire a bulk amount of specific substances. During this process, there are
proteins with specific receptor sites exposed to the extracellular fluid embedded in
the membrane. The receptor proteins are clustered in regions called the coated
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pits, and ligands come and bind to these receptors. Then the vesicle coated to
protein travels to its designated site and then the receptor are recycled.
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