HOME >HOMEWORK #3 SOLUTION

1. Grading as follows for problem #1:
Seems like well thought out and read the paper thoroughly: 7 points.
Didn't miss major points from the article which related to their paper topic: 2 points.
Discretionary: 1 point.

2. When a baby is born, its retina is still developing. Their rods are just like the rods of adults, except that they are much shorter--baby's rods have only a dozen or so disks in their outer segments compared to several hundred in an adult's rods. Many parameters are the same for adult rods and baby rods, including: concentration of all proteins in the disk (rhodopsin, transducin, phosphodiesterase), properties of the inner segment, the density of channels in the outer segment, the concentration of cGMP, etc.

Below is a list of parameters we've talked about in class. Ignore the effects of calcium for this problem. For each of the parameters, do the following: (1) explain whether you'd expect it to be the same or different in baby rods compared to adult rods. (2) explain why it is the same or different. (3) if different, explain qualitatively (i.e. say bigger/smaller, faster/slower, etc) how the baby rod would be different & why.

(2 points each)

A: Probability of absorbing a photon of a given wavelength

According to Beer's Law, the probability of absorbing a photon is exponentially related to the length of the rod. Therefore, the probability of absorbing a photon is less for a baby rod.

B: # of thermal activations of rhodopsin/unit time/rod

The probability of a thermal activation of a single rhodopsin is endependent of the amount of other rhodopsins, so the number of activations is dependent on the number of rhodopsins. The baby rod has less rhodopsins, so the probability of absorbing a phonton is less.

C: Relative absorbance spectrum of the rod

The absorbance spectrum will be narrower than an adult rod. Look at the section in the handout for week #2 on Beer's law. Increasing the length of the rod broadens the absorbance spectrum.

D: Magnitude of the current flowing into the outer segment in the dark

The dark current is proportional to the number of ion channels in the outer segment. The density of these channels is the same for adults and babies. Baby rods are shorter, so they have less total channels, so they have less dark current.

E: Magnitude of the current flowing into the outer segment in bright light

Bright light should cause all of the channels to close. Therefore the bright light current induced should be the same in both (~zero). [In actuality, one might expect a little current due to noise in the channels, which would be larger in adults for the same reason as given in (d)].

F: # of transducin molecules activated by a single active rhodopsin

The # of transducin molecules activated by a single active rhodopsin is a result of properties intrinsic to rhodopsin and transducin and concentrations of each. These things are all the same in babies and adults.

G: Fraction of cGMP molecules in the rod outer segment destroyed as the result of a single active rhodopsin

Since adult rods are bigger, they have more total rhodopsin, transducin, PDE, cGMP, and ion channels. A single rhodopsin activation activates a fixed mean number of transducin molecules which, in turn, destroy the same number of cGMP in each. Since the number per rhodopsin molecule is fixed, but the total is greater in adults, a single activation will cause a smaller fraction of cGMP to be destroyed in adults.