Phil Than
Period 4
1. What does selective permeability mean and why is that important to cells?
Selective permeability means that the membranes of the cells allow certain things to go in
and out of the cells. It is important to cells because without the selectiveness, there will
be no control of what goes in and out of the cell, which would potentially harm the cell.
If waste could go inside the cell, there would be too much and the cell would die. But if
the cell needs something and it is allowed to go out, the cell would be harmed too.
2. What is an amphipathic molecule?
Amphipathic molecules have two ends. A hydrophilic and a hydrophobic end. An
example would be the phospholipids that make up the cell membrane. Tey function as
transporters of hydrophobic material in the hydrophilic environment of the body.
3. How is the fluidity of cell’s membrane maintained?
The fluid mosaic model of a cell membrane. It shows the various structures that are
included/imbedded in them membrane like proteins that are imbedded on and in the
membrane. Also, the phospholipids can move from side to side, allowing the membrane
to not be a static thing with molecules packed in a certain place. A membrane remains
fluid as temperature decreases, until the phospholipids settle into a closely packed
arrangement and the membrane solidifies. Cholesterol also aids in the fluidity of the
membrane.
4. Label the diagram below – for each structure – briefly list it’s function:
Integral Protein- imbedded in the cell membrane, completely spans the membrane. Channels
for transport of molecules.
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Peripheral Proteins- On surface of the membrane. Cell recognition, enzymatic activity.
Extracellular matrix – Connects the cell, surrounds it.
Carbohydrate – Short, branched chains that are covalently bonded to lipids, forming
molecules called glycolipids.
Glycoplipids- Cell-cell recognition
Glycoprotein – Glycolipids are covalently bonded to proteins. Cell-cell recognition
Cytoskeleton – Shape and support
Cholesterol – Fluidity of the cell
5. List the six broad functions of membrane proteins.
Transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular
joining, attachment to the cytoskeleton and extracellular matrix
6. How do glycolipids and glycoproteins help in cell to cell recognition?
Cells can recognize each other by bonding to surface molecules, mostly carbohydrates,
on the membrane. Glycolipids are carbs that are bonded to lipids, and glycoproteins are
carbs bonded to proteins. The diversity of the molecules and their location on the cell’s
surface enable membrane carbohydrates to function as markers that distinguish one cell
from another.
7. Why is membrane sidedness an important concept in cell biology?
Because the different sides of the cell is needed to maintain balance in intra/extracellular
processes
8. What is diffusion and how does a concentration gradient relate to passive transport?
Diffusion is one principle method of movement of substances within cells, as well as the
method for essential small molecules to cross the cell membrane. The substances move
across the concentration gradient, which is needed for passive transport to occur.
9. Why is free water concentration the “driving” force in osmosis?
Different concentrations of solute molecules leads to different concentrations of free
water molecules on either side of the membrane. On the side of the membrane with
higher free water concentration (i.e. a lower concentration of solute), more water
molecules will strike the pores in the membrane in a give interval of time. More strikes
equates to more molecules passing through the pores, which in turn results in net
diffusion of water from the compartment with high concentration of free water to that
with low concentration of free water.
10. Why is water balance different for cells that have walls as compared to cells without
walls?
A cell without a cell wall can tolerate neither excessive uptake nor excessive loss of
water.
11. Label the diagram below:
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When in a hypotonic solution, the water diffuses into the cell to balance out the
concentration and the cell has too much water, it bursts.
In an isotonic solution, the concentrations are the same so water goes in and out of the
cell at the same rate.
In a hypertonic solution, water has to diffuse out of the cell, and so it shrivels because it
doesn’t have enough water.
12. What is the relationship between ion channels, gated channels and facilitated diffusion?
In facilliated diffusion, substances need to be aided by channel proteins in order to move
across the membrane. An example of such protein, is an ion channel which allow ions to
move across the membrane. Sometimes ion channels are gated channels which means
that it needs a chemical or some kind of stimulation to ‘open’ or ‘close’ to allow ions to
go through, or disallow.
13. How is ATP specifically used in active transport?
Active transport requires the cell to spend energy, usually in the form of ATP. Examples
include transport of large molecules (non-lipid soluble) and the sodium-potassium pump.
14. Define and contrast the following terms: membrane potential, electrochemical gradient,
electrogenic pump and proton pump.
Membrane potential is the voltage difference (or electrical potrential difference) between
the interior and exterior of a cell. “Voltage across the membrane.”
An electrochemical gradient is a spatial variation of both electrical potential and chemical
concentration across a membrane
Electrogenic pump - Ion pump that generates net charge flow as a result of its activity
A proton pump is an integral membrane protein that is capable of moving protons across the
membrane of a cell, mitochondrion, or other subcellular structure.
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The first two terms explain the energy related to or across the membrane. While next two,
are terms that fall under the first two terms. They are pumps that use energy, while the first
two are related to energy.
15. What is cotransport and why is an advantage in living systems?
Cotransport is when a substance being diffused down its concentration gradient aids in
the up concentration of another substance. This is an advantage because it can kill two
birds with one stone, because less energy is needed. Energy is saved which is good.
16. What is a ligand?
A ligand is a molecule that binds specifically to the receptor site of another molecule.
17. Contrast the following terms: phagocytosis, pinocytosis and receptor-mediated
endocytosis.
Phagocytosis means ‘cell eating.’ So it’s the process of engulfing whole cells, like
bacteria, etc.
Pinocytosis means ‘cell drinking.’ And it’s the process of engulfing small things. Occurs
in almost all cells/continuously.
Some of the integral membrane proteins that a cell displays at its surface are receptors for
particular components of the ECF. Receptor-mediated endocytosis is many thousand
times more efficient than simple pinocytosis in enabling the cell to acquire the
macromolecules it needs.
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