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#Kathy Bui
Period 4
Membranes
1. What does selective permeability mean and why is that important to cells?
Selective permeability is a term used to describe the barrier that is present
allowing only specific molecules to pass. When referring to cells only smaller,
polar molecules can diffuse across the cell membrance such as waste, oxygen,
nutrients, water etc. This allows regulation of the cell to occur. The membrane
regulates what comes into the cell and what is allowed to go out; such as keeping
the organelles inside the cell and allowing all the waste and harmful substances to
leave the cell.
This is important to cells because it can regulate what goes inside and outside of a
cell; organelles and proteins as an example can stay inside the cell and harmful
substances and waste can leave the cell.
2. What is an amphipathic molecule?
A amphipathic molecule has both polar and non-polar regions, making it partly
hydrophobic and partly hydrophilic, such as phospholipids.
3. How is the fluidity of cell’s membrane maintained?
The membrane is held together by hydrophobic interactions. Cholesterol is the
cell’s main method of maintaining the fluidity at a specific point even if the
temperature fluctuates. Cholesterol just gets in the way of the phospholipids. If it
is cold and the phospholipids try to pack together to solidify, then cholesterol gets
in the way. If it is warm and the phospholipids are darting about too fast,
cholesterol gets in the way again and holds the membrane fluidity down. If the
hydrocarbon tails are unsaturated, they have kinks which prevent tight packing,
making the membrane more fluid, even at relatively low temperatures.
4. Label the diagram below – for each structure – briefly list it’s function:
extracellular matrix – connects and fastens cells to one another.
carbohydrate – messaging part of a glycoprotein or a glycolipid. Used in cell to cell
communication.
glycoprotein – cell to cell recognition
cytoskeleton – holds the cell together, provides its shape.
cholesterol – moderates membrane fluidity
glycolipid – cell to cell recognition
integral protein – proteins that stretch all the way through the membrane, for
functions necessary to that structure. Channel proteins, some enzymes etc.
peripheral protein – Only on one side of the membrane, and have a function necessary
to that structure, such as cell communication, enzymes, 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 Glycoprotiens are used in cell to cell recognition. The
carbohydrate chains act as the identification in cell to cell recognition, with only a
correct chain being accepted. If a chain is incorrect such as a bacteria or virus,
then when a white blood cell sees that the carbohydrate chain is wrong, it will
destroy the offending cell.
7. Why is membrane sidedness an important concept in cell biology?
Lipids proteins in/on the membrane differentiate in composition on each side of
the membrane and proteins, making up the membrane sidedness. Sidedness is an
important concept in cell biology, because it helps explain endocytosis and
exocytosis and the flip-flop movement of membrane components. 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 where solutes move down the concentration gradient, from high
concentration to low. In passive transport, ATP is not used, so there must be a
concentration gradient present to help function the passive transport from high
concentrations to low.
9. Why is free water concentration the “driving” force in osmosis?
Osmosis is the movement of water from an area of high concentration to low
concentration. Free water is basically the water molecules able to diffuse and
move. Their concentration is what causes/drives osmosis. If it is higher than
another given area, the water moves to an area of low concentration. If it is lower,
water will move toward that area.
10. Why is water balance different for cells that have walls as compared to cells
without walls?
Cells with a cell wall, when placed in a hypotonic solution, expand to their
maximum size and then can’t expand any further (turgid), because of the cell
wall. So a cell with a cell wall prefers a hypotonic water balance. Whereas, cells
without a cell wall can burst when they get put in a hypotonic solution, so it is
best for them to be in a isotonic solution.
11.
Label the diagram below:
Animal Cell (Hypotonic solution) – The cell has taken in too much water from
osmosis in the hypotonic solution and plasmalyzed, or bursted.
Animal Cell (Isotonic solution) – An equilibrium has been met because there is
an equal amount of water going into and leaving the cell.
Animal Cell (Hypertonic solution) – The cell has shriveled up because too much
water to diffused to the hypertonic solution.
Plant Cell (Hypertonic solution) – The cell is balancing the water intake and is
keeping the cell turgid.
Plant Cell (Isotonic solution) – Equilibrium is met; water is moving in and out
the cell like normal.
Plant Cell [Hypertonic] – Too much water has diffused out of the cell.
12. What is the relationship between ion channels, gated channels and
facilitated diffusion.
Facilitated diffusion allows some molecule that usually can’t diffuse through a
membrane to pass across the membrane, down its concentration gradient. A ion
channel is one of the types of integral proteins that allows this, with the main
molecule allows through being an ion. A gated channel is a channel protein that is
able to close the connection upon reception of a signal molecule.
13. How is ATP specifically used in active transport?
ATP is the energy source for active transport. It powers a protein pumps to allow
them to pump molecules against the concentration gradient. In most cases the
terminal phosphorous of the ATP will attach to the protein, both releasing energy
and possibly changing the shape of the protein.
14. Define and contrast the following terms: membrane potential,
electrochemical gradient, electrogenic pump and proton pump.
Membrane potential is the energy stored by having more of a certain charge inside
the molecule than outside the molecule.
Electrochemical gradient is the combination of the two forces that want to correct
the membrane potential. These two forces are the concentration gradient and the
electrical potential.
Electrogenic pump is how a cell establishes a membrane potential, by pumping
ions across a membrane using ATP.
Proton pump is a specific type of a electrogenic pump, that only transports H+
ions, protons.
15. What is cotransport and why is an advantage in living systems?
Cotransport is one way a diffusion gradient can be used for energy. In this case,
an ATP powered pump creates a concentration gradient, and another specialized
transport enzyme allows the molecule of the first gradient to pass through and
harvests the energy released to pump a different molecule up its gradient. This is
an advantageous process because it allows one energy source to power many
different functions. Instead of making each of the proteins use ATP, they all draw
from a commen energy source, a gradient. It is very efficient.
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.
Phagocytosis the cell membrane extends itself out to engulf something, food or a
bacterium or something.
Pinocystosis the membrane folds in and “gulps” fluid into itself, to get the solutes
in the liquid. Receptor-mediated Endocytosis, a ligand binds to a receptor
protein, signaling the membrane to fold up and make a vesicle, carrying the
ligands with it.