Download Homework due 10/13/14 – KEY Your first exam will include many

BIOL 241 (section 0439.25)
October 13, 2014
Homework due 10/13/14 – KEY
Your first exam will include many questions that test your recall of facts, but some of these questions
will ask you to use these facts in new ways. For example, you may need to combine and compare
information from two or three different lectures, or you may need to apply what you know about a topic
to a subtopic that we didn't cover. The questions below will give you practice in thinking through the
material in this way, and may be similar to some questions that turn up on the exam.
1. Gonadotropin releasing hormone (GnRH) from the hypothalamus stimulates the release of follicle
stimulating hormone (FSH) and luteinizing hormone (LH) from the anterior pituitary gland. FSH and
LH stimulate the release of testosterone from the testes (among other effects). High testosterone
levels, in turn, inhibit the hypothamus’s release of GnRH and the pituitary’s release of FSH and
LH. Does this system use negative feedback? Explain.
Yes, this can be considered negative feedback. If we focus on testosterone, we see that high levels of
testosterone (presumably above the setpoint) cause changes in GnRH, LH, and FSH that cause
testosterone levels to fall back toward the setpoint. GnRH, LH, and FSH levels also have setpoints, so
similar reasoning can be applied to them.
2. Bilateral symmetry is a trait in which the right half of something is the mirror image of its left half.
Suppose that you wanted to judge the bilateral symmetry of a few human volunteers using an
imaging technique such as magnetic resonance imaging (MRI). What kinds of sections would you
take of their body: coronal, sagittal, or transverse?
This question turned out to be more open-ended than I had intended! A single sagittal slice would not
allow comparison of left and right halves; however, multiple slices would allow this (as explained by
James in class). I was thinking that coronal slices would be ideal, but a case for transverse slices could
also be made.
3. Describe the “journey” of a collagen protein from its initial synthesis to its final position in the
extracellular matrix. Which organelles are involved?
The protein is synthesized at a ribsome on rough endoplasmic reticulum (ER), shipped to the Golgi body,
packaged into vesicles, and released to the extracellular matrix via exocytosis.
4. Glutamate is a charged amino acid needed by most cells of the body. What are the possible ways in
which it might be able to cross the plasma membrane? If researchers identified a protein that helps
glutamate get through the plasma membrane, describe a simple experiment that would help you
figure out exactly what this protein is doing.
Many answers are possible here. For example, you could take an artificial bilayer, insert the protein, and
see whether glutamate transport increased (as was done for CFTR – see the previous homework
assignment). You could also adjust the glutamate concentrations inside and outside to see whether
glutamate can be transported against its gradient, and add or take away ATP to see whether the
transport is ATP-dependent. These experiments could help establish whether the protein transports
glutamate via facilitated diffusion or active transport.
5. In what sense can a cell’s DNA be considered its “central computer”?
BIOL 241 (section 0439.25)
October 13, 2014
The central computer is a machine that runs a building or a spaceship or something; it tells all of the
other parts what to do. Similarly, a cell’s DNA tells the rest of the cell what to do; its genes are
instructions for making proteins, which then do the work of the cell.
6. The genetic code is a triplet code. Explain what that means.
Every set of three DNA nucleotide bases (combinations of A, C, G, and T) – that is, every “triplet” of these
bases – specifies one amino acid of a protein.
7. You are studying cells that are being grown in a research lab. You add a drug that stops the nuclear
envelope (nuclear membrane) from disintegrating, thus stopping further progression through the
cell’s life cycle. If you add this drug to actively growing cells, in what stage of the life cycle will the
cells eventually be “frozen”? Will each of these cells have enough organelles for one cell, or for
two?
Since the nuclear membrane normally disintegrates during prophase of mitosis, cells would likely be
“frozen” at this stage. Each cell would have enough organelles for two cells after duplicating its
organelles in stage G1 of interphase.
8. Claudins are important proteins found in the structure of tight junctions. Imagine that a patient has
a mutation in her claudins, such that one amino acid has been changed. Might this mutation cause
problems in the patient’s trachea? Explain.
Problems MIGHT occur. A mutation in the protein could lead to weaker connections between cells,
allowing substances (bacteria, foreign particles trapped in mucus, etc.) to seep between the cells and
cause problems. However, as pointed out by Kelsey Q., a change in a single amino acid may not have
ANY effect! Some amino acids can be changed without affecting a protein’s function at all.
9. Explain how the structures of the following cells/tissues support their functions: (a) epithelium in
the mouth, (b) epithelium in the small intestine, (c) cardiac muscle cells, (d) adipocytes, (e) elastic
tissue.
(a) The epithelium in the mouth is highly exposed to the environment, so it needs to be
tough to protect the underlying tissues, and thus has many layers of squamous cells (i.e.,
the tissue is stratified squamous). [Some areas of the mouth are keratinized for added
protection – you didn’t need to know this.]
(b) The main jobs of these epithelial cells are to absorb nutrients and to keep food moving
along through the intestine. They have microvilli to increase surface area, thus
maximizing absorption, and goblet cells to secrete mucus, lubricating the intestine.
(c) Cardiac muscle cells is packed full of thick filaments (myosin) and thin filaments (actin)
that enable the cells to contract. In contracting, the heart performs its function of
pumping blood through the circulatory system. The structure of heart cells also
includes lots of mitochondria (for sustained ATP production to power the never-ending
contractions) and gap junctions (to allow electrical signals to spread from cell to cell).
BIOL 241 (section 0439.25)
October 13, 2014
(d) Adipocytes consist almost entirely of lipids. Lipids are good for two things: energy
storage (they can be broken down to make ATP) and soft cushioning of surrounding
tissues.
(e) Elastic tissue has lots of the protein elastin arranged into elastic fibers. These fibers can
easily expand and contract, allowing structures containing them (like hollow organs) to
expand and contract. As shown in the “Which is elastin, and which is collagen?” slide
from October 6, elastin proteins are hooked together in a way that each protein can be
pulled straight or relax into a twisted structure.
10. List two ways that you can tell the difference between epithelial cells lining the intestine and
epithelial cells lining the trachea.
Tracheal epithelial cells are ciliated, whereas intestinal epithelial cells have microvilli. Also, intestinal
epithelial cells are columnar, whereas tracheal epithelial cells are pseudostratified.
11. Epithelial cells experience lots of wear and tear, but individual epithelial cells do not usually break
away from the others. Why not?
Tight junctions and desmosomes (the latter have Cell Adhesion Molecules, or CAMs) keep cells together.
Also, cells are anchored to the basement membrane via hemidesmosomes, which help keep them
together as well.
12. How can you tell the difference between dense regular connective tissue and hyaline cartilage?
Different answers are possible. Among the clearest differences are that only regular connective tissue
has visible parallel extracellular protein fibers, while only hyaline cartilage has chondrocytes in lacunae
(regular connective tissue has scattered fibroblasts that aren’t in lacunae).
13. You have a sample of tissue that is either dense regular connective tissue or elastic tissue. What is
the easiest way to figure out which one it is – either by using a microscrope, or by doing something
else?
Just pull on it and see how elastic it is!
14. Consider the cross-section of an arteriole (a fairly large blood vessel). Where would the epithelial,
connective, and muscle tissues be found? What function does the muscle tissue perform? What
kind of muscle tissue is it?
Working our way from the inside of the blood vessel outward, we would first encounter epithelial tissue,
then connective tissue (always found next to epithelial tissue), and then smooth muscle. (And then even
more connective tissue, which you don’t have to mention.) The smooth muscle regulates the diameter
of the blood vessel, thus regulating how much blood can flow through it.
15. Explain why your elbow swells up after you scrape it.
The scraping injures the epithelial tissue and underlying connective tissue. Mast cells in the connective
tissue release histamine and other chemical signals that lead to an increase in blood flow to the affected
BIOL 241 (section 0439.25)
October 13, 2014
area. Increased blood flow causes some fluid in the blood vessels to leak out into the surrounding tissue,
which swells.