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Membranes
1. What does selective permeability mean and why is that important to cells?
Selective permeability means that the membrane of a cell allows some materials
to pass through more easily than others. This allows cells to discriminate what it
allows into its body and what is pushed out. This is important because this way,
cells can let in the good materials, secrete the bad ones, and keep everything it
doesn’t want out.
2. What is an amphipathic molecule?
An amphipathic molecule is a molecule that has both a hydrophilic and
hydrophobic region
3. How is the fluidity of cell’s membrane maintained?
The fluidity of the membrane is maintained because the phospholipids move
laterally, and rarely flip, within the hydrophobic area of the membrane. These
movements occur very quickly.
4. Label the diagram below – for each structure – briefly list it’s function:
extracellular matrix – holds animal tissue cells, consist of proteins and
polysaccharides.
carbohydrate – attach to proteins or lipids to form glycoproteins or glycolipids act as
“ID tags” in cell recognition; they can also hold adjoining cells together
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or act as sites where viruses or chemical messengers (like hormones)
can attach.
glycoprotein – cell to cell recognition
cytoskeleton – maintains cell shape
cholesterol – moderates fluidity of membrane at different temperatures
glycolipid – cell to cell recognition
integral protein – channels for transport of molecules, etc.
peripheral protein – cell recognition, enzymatic activity, etc.
5. List the six broad functions of membrane proteins.
Transport, enzymatic activity, signal transduction, cell to cell recognition,
intercellular joining, attachment to cytoskeleton and extracellular matrix.
6. How do glycolipids and glycoproteins help in cell to cell recognition?
Cells recognize other cells by binding to surface molecules, which are typically
carbohydrates, mainly glycolipids and glycoproteins. These vary depending on
which cell they’re attached to, which makes cells able to recognize each cell
depending on the structure of their glycolipids and glycoproteins.
7. Why is membrane sidedness an important concept in cell biology?
Membrane sidedness is an important concept, because it has to do with the
processes of endo/exocytosis. The inside of the endoplasmic reticulum becomes
the inside of a vesicle then 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 in the passage of materials that aren’t water through the membrane
from a higher concentration to a lower concentration. A concentration gradient is
relative to passive transport because for spontaneous diffusion to occur, there
must be a difference in concentrations.
9. Why is free water concentration the “driving” force in osmosis?
In osmosis, water moves from an area of high concentration to an area of lower
concentration. The free water concentration is what causes the water to move. If
it is high, the water moves to another area where it is lower. If it is lower than the
area where the water is, it moves to back to the first area, hence causing or
“driving” osmosis.
10. Why is water balance different for cells that have walls as compared to cells
without walls?
Cells without walls cannot usually survive out of a isotonic solution or
environment because it will either lose or gain too much water without gaining or
losing water in return. Cells with walls can maintain water balance when put in a
hypotonic solution because the pressure of the cell wall pushes back, because of
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its elasticity, against the uptake of more water than the cell can hold, which
keeps the cell firm and healthy. When a cell with walls is put in a hypotonic or
isotonic solution, though, the cell will wither.
11.
Label the diagram below:
12. What is the relationship between ion channels, gated channels and facilitated
diffusion.
Facilitated diffusion occurs when materials other than water are transported
down a concentration gradient, spontaneously. Ion channels are a series of
proteins that form channels for the materials/ions to flow through, down
concentration gradients, for diffusion to occur. Gated channels are ion channels
that can open and close when a signaling molecule, ligand, binds to them.
Facilitated diffusion occurs through ion channels, which are often gated.
13. How is ATP specifically used in active transport?
Facilitated diffusion occurs without the use of energy because it is spontaneous
when materials travel down a concentration gradient. In active transport, the
materials are shifted up a concentration gradient, requiring energy to push
against the gradient.
14. Define and contrast the following terms: membrane potential, electrochemical
gradient, electrogenic pump and proton pump.
Membrane potential is the difference in charge between a cell’s cytoplasm and
the extracellular fluid, which occurs because the ions are not distributed evenly.
The electrochemical gradient is the concentration gradient of an ion. An
electrogenic pump produces charge imbalance across a cell membrane, and
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contributes to the membrane potential. A proton pump in a protein that can move
ions up a concentration gradient, across a membrane. The electrogenic pump
can contribute to the membrane potential. The proton pump can affect the
electrochemical gradient because it changes the concentrations of ions.
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.” It’s an
advantage in living systems because it allows cells to do two jobs, while only
exerting energy for one. It saves energy.
16. What is a ligand?
A ligand is a molecule that that binds specifically to a receptor site of another
molecule.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
Phagocytosis is when a cell “eats” large, damaged cells through endocytosis.
The cell engulfs it by creating a vacuole around it, and then it’s digested by a
lysosome. Pinocytosis is when extracellular fluid is “gulped” into vesicles, so that
the molecules can be dissolved, but unlike phagocytosis, pinocytosis is not
specific to the substances it dissolves. Receptor-mediated endocytosis is when
cells use an area of the membrane called coated pits, which are layers of coat
proteins, to gain/acquire large quantities of materials. To do this, there are
specific proteins in the membrane that have special receptor sites. These are
exposed to the extracellular fluid, and then extracellular substances bind to the
receptors. Then coated pits form vesicles, with ligand molecules, around them.
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