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October 1, 2015
Biology I
CSI Worksheet KEY 3
1. Describe what happens in a GRCP pathway as well as a phosphorylation cascade.
 When the appropriate signaling molecule binds to the extracellular side of the receptor,
the receptor is activated and changes shape. Its cytoplasmic side then binds and activates
a G protein. The activated G protein carries a GTP molecule. The activated G protein
leaves the receptor, diffuses along the membrane, and then binds to an enzyme, altering
the enzyme’s shape and activity. Once activated, the enzyme can trigger the next step
leading to a cellular response. Binding of signaling molecules is reversible. The activating
changes in the GPCR, as well as the changes in the G protein and enzyme, are only
temporary; these molecules soon become available for reuse.
 In phosphorylation cascade, a series of different molecules in a pathway are
phosphorylated in turn, each molecule adding a phosphate group to the next one in line.
Dephosphorylation returns the molecule to its inactive form.
2. Describe at least three ways in which membrane proteins can help substances cross a cell
membrane.
 Ion channels: they form pores, or openings, ions move through theses openings in a
predictable direction, from high concentration to low concentration. Ions need the protein
to transport through the protein because they are polar and the inner layer of the
membrane is not.
 Channel proteins: each protein has a structure that allows only a particular type of ion or
small molecule through it. Example: aquaporins, a channel protein that allows water to
cross the membrane.
 Gated channels: these protein membranes open and close in response to a signal, such as
binding or a particular molecule or a change in electrical voltage across a membrane.
Example: having potassium go through the membrane to change the charge of the cell.
3. You have a jelly sack with a semipermeable membrane that only allows water to flow through
it. Inside the sack you have a 5M NaCl (aq) solution. You place this sack in a beaker of 1M
NaCl (aq). Describe what you would see happen and why.
 You would see the bag swell and possibly rupture from the water flowing into it.
Molecules, ions, atoms flow from high concentration to lower concentration. There is a
higher concentration of water in the 1M NaCl solution compared to the 5M NaCl solution.
Since there is a lower concentration of water inside the bag, water will flow into the bag
until the beaker and bag reach equilibrium.
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4. Draw an unsaturated, saturated and cholesterol membrane. Describe how the differences in
the membrane structure affect its fluidity.
5. What are the three different types of cytoskeleton and where/when are each of them used?
Structure
Subunits
Functions
Actin Filaments
(Microfilaments)
Intermediate
filaments
Microtubules
Strand in a double
helix with a
positive and
negative
Fibers would into
thicker cables, nonpolar
A hollow tube of
13 protofilaments
arranging into a
circle
Actin
Keratins,
Lamins
α and β
tubulin
dimers
Cell shape by resisting tension; move
cells via muscle contractions; divides
cells into two; moves organelles and
creates a current in the cytoplasm.
Cell shape by resisting tension; anchors
nucleus and some other cells; makes up
skin, hair, nails
Cell shape by resisting compression;
flagella and cilia; moves chromosomes
during cell division; moves organelles;
provides track for intercellular
movement
6. Describe the four levels of protein structure and include the stabilizing bonds in each structure.
 Primary: the sequence of amino acids in a polypeptide; stabilized by peptide bonds
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
Secondary: the formation of α-helices and β-pleated sheets (β-sheets) in a
polypeptide; stabilized by hydrogen bonding between groups along the peptidebonded backbone, depends of primary structure

Tertiary: overall three-dimensional shape of a polypeptide; stabilized by bonds and
other interactions (i.e. polarity) between R-groups
Each colored section of the structure represents
one tertiary structure, so in the picture there are
four tertiary structures

Quaternary: shape produced by combinations of tertiary structures; stabilized by
bonds and other interactions (i.e. polarity) between R-groups
7. In what ways does RNA differ from DNA?
 The sugars in the nucleic acids differ. DNA is composed of deoxyribose while RNA
contains ribose. Additionally the nitrogenous bases differ. Both have C, G, and A.
However DNA contains T and RNA contains U. Finally, DNA is usually double stranded,
while RNA is usually single stranded.
8. Sketch a nucleotide, label its three basic parts, and identify the 2’, 3’, and 5’ carbons.
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9. Sketch an amino acid, and describe what the side chains are and how they affect the
molecule.
10. Sketch a monosaccharide α 1-4 linked to another monosaccharide. Draw a β 1-4 linked
disaccharide.
11. Sketch a phospholipid's molecular structure, and explain why phospholipids spontaneously
form bilayers in water.
 The hydrophobic hydrocarbon chains repel from water. In order to interact as little as
they can with water they match up with the other so that they are interacting with the
other hydrocarbon chains. This leaves the polar phosphate group on the outside of the
membrane facing the water.
COLORADO SCHOOL OF MINES
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12. Energetically, what makes a reaction nonspontaneous? Under what conditions can a
nonspontaneous reaction occur?
 A reaction being spontaneous means that it will favor going toward products over
reactants in standard conditions. Spontaneity depends on the Gibbs free energy of
the reaction as well as the entropy change. A nonspontaneous reaction will have a
positive Gibbs free energy and entropy decrease. This means that it will take energy
in order to go towards products. Also an entropy decrease means that the overall
system becomes more organized which is not favored.
 A few things can happen in order for a nonspontaneous reaction to occur. One
example is using ATP. This adds energy to the reaction to help the reaction go towards
productions. Additionally, catalysts made be used. They lower the activation energy,
or the Gibbs free energy. Finally things such as heat pressure or very low product
concentrations may be used. This is found in the citric acid cycle. A step in the cycle is
nonspontaneous, however when the product concentration is low, it pulls the reaction
forward towards products.
13. How do buffers work in balancing pH?
 A buffer is a substance that minimizes changes in the concentration of H+ and OH- in a
solution. It does so by accepting hydrogen ions from the solution when they are in excess
and donating hydrogen ions when to the solution when they have been depleted. Most
buffer solutions contain a weak acid with its corresponding base, which combine reversibly
with hydrogen ions.
 A buffer used in the human body is carbonic acid (H2CO3). This is formed when CO2 reacts
with water in blood plasma. If the H+ concentration in the blood begins to fall (if the pH
rises), the carbonic acid dissociates, replenishing the ion concentration. If the H+
concentration in the blood begins to rise (the pH lowers), the reaction goes back towards
carbonic acid by HCO3- accepting protons.
14. What is the difference in a polar bond, nonpolar bond, polar molecule, and nonpolar
molecule? Sketch an example of each.
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